Sensors andTransducers

Dr. Kurtuluş Erinç Akdoğan

kurtuluserinc@cankaya.edu.tr

INTRODUCTION

Much of biomechatronic engineering involves the measurement of physical processes stemming either from the biological organism or from an associated prosthesis.

These measurements can include voltage, chemical concentration, pressure, position, displacement, and rate.

They are processed in some way and then used to apply some form of actuation such as the motion of a prosthetic arm, the contraction of an artificial heart, or the electrical stimulation of the cochlea.

In most cases the measured parameter is converted to an electrical signal for analysis or processing, though a number of devices such as the sphygmomanometer, used to measure blood pressure, often still use pneumatics.

Since the introduction of low-cost lightweight, and reliable electronics in the last 60 years, and particularly since the invention of the microprocessor, far more complex analysis and processing can be undertaken.

This lecture examines some of the more common sensors and transducers that are used in biomechatronic engineering.

SWITCHES

The fundamental electrical building block is the switch, of which a number of examples are shown in Figure 2-1.

These are devices that control the flow of current in a binary (on–off) manner, using either a mechanical contact or an electronic device such as a transistor.

Switches are commonly used to isolate the supply of current to a device, are used as selectors, or, often in mechatronic devices, are used to identify the limits of travel of some mechanical structure.

Toggle Push button Rotary

Toggle Switches Toggle switches are available in a number of different configurations depending on the

application and are characterized by the number of switching contacts (or poles) and the number of terminals.

For example, a single on–off switch shown in Figure 2-2a is referred to as a single pole single throw (SPST) device.

In the case where the switch can connect to a pair of terminals, as shown in Figure 2-2b, it is referred to as a single pole double throw (SPDT) device.

Also common are multipole switches, in which individual switching elements are ganged together to form double, triple, and even quadruple devices. The nomenclature follows that of the previous cases, so a double pole double throw switch would be referred to as DPDT.

All double throw switches are break-before-make in that the pole never contacts both of the terminals simultaneously. This is important to ensure proper isolation and to avoid short circuits.

Singel pole single throw Single pole double throw Double pole double throw

Singel Pole Single Throw

You can see that a SPST toggle switch only has 2 terminals. 1 terminal is for the input. The other terminal is for the output.

SPST toggle switches function as simple ON-OFF switches. When open, they disconnect the circuit so that current cannot flow to the load. When closed, current can flow and power the load.

You can see that this circuit functions simply as an ON-OFF switch to turn on or shut off the DC motor.

Single Pole Double Throw

A SPDT toggle switch has 3 terminals. Terminal 1 can connect up to any load to power a certain device. And terminal 3 can connect to any load to power any device. Terminal 2 is the terminal which receives the power necessary so that the loads on terminals 1 and 3 can be powered.

So a SPDT switch can power either one of 2 circuits. It can flip between the 2 circuits so that just by the flip of the switch, different circuits, or devices, can be powered.

In this circuit, we connect our 9-volt DC power source to terminal 2. Terminal 2 represents the toggle switch which we can flip between terminals 1 and 3. Terminal 1 is connected to a fan. When we flip the switch to the left (terminal 1), the DC fan runs while the DC motor does not. When we flip the switch to the right (terminal 3), the DC motor runs while the fan does not.

Double Pole Double Throw A DPDT toggle switch has 6 terminals.

Terminals 3 and 4 represent the toggle switch. These terminals receive the power necessary to drive the loads on terminals 1 and 5 and 2 and 6. Terminals 3 can flip between terminals 1 and 5. So if a fan is connected to terminal 1 and a motor is connected to terminal 5, terminal 3, representing the toggle switch, can switch between running the fan and running the motor. The same condition is true for terminal 4. Terminal 4 can flip between terminals 2 and 6. So if a heater is connected to terminal 2 and a blower is connected to terminal 6, terminal 4, representing the toggle switch, can switch between the heater and the blower. A DPDT switch has 2 input switches which can each connect to one of 2 terminals. Therefore, it can control 4 different circuits, or devices, with 2 switches.

Push-button switches

Push-button switches are momentary contact devices because they operate only when force is applied.

They come in two varieties, with a normally open (NO) or a normally closed (NC) contact for single throw devices,

But are often also configured as double throw devices as shown in Figure 2-3c

Normally open Normally closed Double throw.

Normally Open

A Normally Open (NO) Push Button is a push button that, in its default state, makes no electrical contact with the circuit. Only when the button is pressed down does it make electrical contact with the circuit.

Normally Closed

A Normally Closed (NC) Push Button is a push button that, in its default state, makes electrical contact with the circuit.

2.2.3 Limit Switches

Mechanical limit switches are a variation of the standard push-button configuration fitted with a lever arm. These are often known as microswitchesbecause of their small size

Limit Switches are electro-mechanical devices that consist of an actuator mechanically linked to a set of contacts. When an object comes into contact with the actuator, the device operates the contacts to make or break an electrical connection.

In the interests of reliability, mechanical limit switches have mostly been superseded by optical or hall switches (discussed later), which have no moving parts and are generally easier to interface. Figure 2-4 shows photos of some examples of microswitches and their electronic equivalents.

Micro-switch Optical Reed Hall device

Micro Switch A miniature snap-action switch, also trademarked and frequently known as

a micro switch, is an electric switch that is actuated by very little physical force, through the use of a tipping-point mechanism, sometimes called an "over-center" mechanism.

Optical Switch

Figure a shows a slotted optical switch. An LED is mounted in a plastic housing, facing a phototransistor, but separated by a gap. If something moves into the gap, it blocks the light path between the LED and the phototransistor.

Figure b shows a reflective sensor, which works similarly. The phototransistor in a reflective sensor picks up reflected light from whatever is in front of the switch.

Reed Switch

The basic reed switch consists of two identical flattened ferromagnetic reeds, hermetically sealed in a dry inert-gas atmosphere within a glass capsule, thereby protecting the contact from contamination. The reeds are sealed in the capsule in cantilever form so that their free ends overlap and are separated by a small air gap.

Hall Switch

A Hall IC switch is OFF with no magnetic field and ON in the presence of a magnetic field.

Comparison of Limit Switches

Limit switches can be mechanical, magnetic, optical, or a combination.

There are pros and cons to each.

For example, mechanical are the easiest to setup, test, and adjust; but they are also physically moved and therefore can fail or get in the way.

Optical sensors don't require contact so the potential for damage is reduced, but calibration can be difficult. Dust and dirt can degrade reliability run-after-run.

Magnetic sensors, such as Hall Effect sensors, are a great option as they only need to be calibrated once and are very reliable even if dirty.

2.2.4 Rotary Switch

Rotary switches consist of a number of circular wafers containing many poles operatedby a spindle that passes through the center of each wafer.

There are many types, including both shorting (make-before-break) and nonshorting(break-before-make).

However, as with most other mechanical switches, these are not particularly reliable and are being replaced by their electronic counterparts.

2.2.7 Relays Relays are electrically controlled switches. In their mechanical embodiment they consist of an

electromagnet that controls a mechanical lever arm to operate a switching mechanism, as illustrated in Figure 2-5.

As with toggle switches, many switching configurations are possible.

Electronic switches have replaced their mechanical counterparts in many applications both for switching high-voltage mains and low-voltage control signals.

Solid-state relays (SSRs) generally have an opto-isolated low-voltage input that controls highcurrent-handling transistors (usually metal–oxide–semiconductor field-effect transistors[MOSFETs]) or thyristors. When designed for lower current applications, these devices are usually called analog switches. Some examples of these devices are shown in Figure 2-6.

In biomechatronic applications, relays are often used to provide the mandatory electricalisolation between the various parts of a system to ensure patient safety. Solid–state relays are also quite common where gross on–off control of electric motors is required in primitive prostheses.

Mechanical relay SPDT switching configuration Solid-state relays Analog switch

2.3 POWER SUPPLIES

Medical and biomechatronic devices generally run from mains power if they are static and from batteries if they need to be portable.

In most cases this supply voltage will need to be converted to one or a number of other voltages to power the various modules within any device. For example, a prosthetic arm may require 12 V to power the motors, +/−5 V for the analog electronics, and 3.3 V for the signal processor.

It is now common to provide power to embedded systems such as cochlear devices and artificial hearts using electromagnetic induction through the unbroken skin, as this eliminates a common source of infection.

An alternative that is becoming feasible as the efficiency of implants improves is to scavenge power from flexing muscles or changes in pressure driven by the heartbeat.

There are two basic forms of power supply used in electronics equipment:

Unregulated: This form of power supply was the only type used many years ago. It simply consisted of a rectifier section and this was followed by capacitor or capacitor and inductor smoothing. There was no regulation to steady the voltage. If a large current was drawn the voltage would fall as a result of the resistive losses, and also the smoothing would not be as effective and the level of hum would rise.

Voltage regulated: As transistor circuitry became more commonplace, regulated power supplies became more common. Today they are almost universally used. They typically incorporate a voltage reference, and the output voltage is compared to this and altered accordingly by control circuitry within the regulated power supply.

In addition to this, regulated power supplies may be further subdivided:

Linear regulated power supply: Linear regulated power supplies use an analogue approach. A series element - a semiconductor transistor or FET - is controlled allow the correct voltage at the output for any current within the operating range.

Switching regulator power supply: The switching regulator format for a power supply uses a large output reservoir capacitor. A series element - a transistor or FET -is switched on and off to keep the voltage on the capacitor within the required limits.

2.3.1 Linear Power Supplies Linear power supplies gain their name from the fact

that they use linear, i.e. non-switching techniques to regulate the voltage output from the power supply. The term linear power supply implies that the power supply is regulated to provide the correct voltage at the output.

The main elements of the linear power supply are:

Input transformer: As many power supplies take their source power from an AC mains input, it is common for linear power supplies to have a step down or occasionally a step up transformer.

Rectifier: As the input from an AC supply is alternating, this needs to be converted to a DC format.

Smoothing: Once rectified from an AC signal, the DC needs to be smoothed to remove the varying voltage level.

Linear regulator: Linear regulators are active control systems that compare the output voltage with a fixed reference and use the error to adjust a series transistor to keep the output constantirrespective of changes in the input voltage or the load.

2.3.2 Switch-Mode Power Supplies

Switch-mode power supplies use a different method to convert AC to DC.

First, the 60 Hz AC line voltage is rectified and filtered using diodes and capacitors resulting in DC high voltage.

Inverter has power transistors, typically switching at a preset frequency anywhere from 20 kHz to 500 kHz, convert the high voltage to a higher frequency AC.

The high frequency AC is then reduced to a lower voltage using a relatively small, lightweight transformer.

Finally, the voltage is converted into the desired DC output voltage by another set of diodes, inductors, and capacitors.

2.3.2 Switch-Mode Power Supplies

Another method of generating a regulated DC voltage is to use a switching regulator.

The basic concept behind a switch mode power supply or SMPS is the fact that the regulation is undertaken by using a switching regulator. This uses a series switching element that turns the current supply to a smoothing capacitor on an off.

The time the series element is turned on is controlled by the voltage on the capacitor. If it is higher than required, the series switching element is turned off, if it is lower than required, it is turned on. In this way the voltage on the smoothing or reservoir capacitor is maintained at the required level.

Because the control element is either on or off, there isvery little power dissipation within the device, which makes the conversion efficiency very good.

The main problems with switch-mode power supplies are residual switching noise on the output and also noise fed back onto the input lines.

Linear PSU Advantages and Disadvantages

Linear PSU advantages

Low noise: The use of the linear technology without any switching element means that noise is kept to a minimum and the annoying spikes found in switching power supplies are now found.

Established technology: Linear power supplies have been in widespread use for many years and their technology is well established and understood.

Linear PSU disadvantages

Efficiency: In view of the fact that a linear power supply uses linear technology, it is not particularly efficient. Efficiencies of around 50% are not uncommon, and under some conditions they may offer much lower levels.

Size: The use of linear technology means that the size of a linear power supply tends to be larger than other forms of power supply.

Heat dissipation: The use of a series or parallel (less common) regulating element means that significant amounts of heat are dissipated and this needs to be removed.

SMPS Advantages and Disadvantages

SMPS advantages

High efficiency: The switching action means the series regulator element is either on or off and therefore little energy is dissipated as heat and very high efficiency levels can be achieved.

Compact: As a result of the high efficiency and low levels of heat dissipation, the switch mode power supplies can be made more compact. It weighs 2.2kilos while linear supply weighs 7kilos.

SMPS Disadvantages

Noise: The transient spikes that occur from the switching action on switch mode power supplies are one of the largest problems. The spikes or transients can cause electromagnetic or RF interference which can affect other nearby items of electronic equipment, particularly if they receive radio signals.

External components: The most obvious is the reservoir capacitor, but filter components are also needed.

Expert design required: To ensure that it performs to the required specification can be more difficult. Ensuring the ripple and interference levels are maintained can be particularly tricky.

2.3.3 Batteries

Many of the biomechatronic systems discussed in this course are portable and therefore need to operate using battery power.

Where the power consumption is significant and the battery pack is integral to the device for reliability and sealing issues or where access to the device is difficult, rechargeable batteries (secondary batteries) are used.

Such devices include prosthetic limbs and artificial hearts.

In low-power applications where battery life can be months and even years, then it is more practical to use primary batteries.

The oldest form of rechargeable battery still in use is the lead-acid battery. These are wet cell devices and mostly need to be kept upright and placed in well-ventilated areas as they generate hydrogen if overcharged.

One convenient alternative form is the gel cell, which contains a semisolid electrolyte that prevents spillage.

Rechargable Battery Types

Most portable rechargeable batteries are dry cell types, which are hermeticallysealed.

nickel cadmium,

nickel metal hydride,

lithium types.

Selecting batteries for a specific application:

Nickel cadmium (NiCd): These are good for devices that include motors and otherhigh-discharge requirements. They can accommodate heavy drain rates, but theirmAh rating is lower than more modern rechargeables and they also have a strong memory effect.

Nickel metal hydride (NiMH): These batteries have a high mAh rating and cansustain moderate to high current drain.

Lithium ion (Li-Ion): These have a very long shelf life and are excellent for moderate to low-power applications.

Lithium polymer (Li-Po): These have similar chemistry to Li-Ion, but becausethey are manufactured in flat sheets rather than cylinders they have a higher power density.

Primary Battery Types

A large number of primary battery types are available for different applications:

Zinc carbon: This is a low-cost battery good for light current drain applications.

Zinc chloride: This is similar to the zinc carbon but with a slightly higher powerdensity.

Alkaline: These are long-life batteries and are suitable for low and high currentdrain applications. Their energy density is significantly higher than that of zinccarbon types.

Silver oxide: This type is commonly used for hearing aids and watches where the current drain is low.

Mercury: This type was formerly used in a wide range of devices but is seldomused today because of toxicity issues.

Zinc air: These are generally used in hearing aids.

The main advantages of primary batteries are their high energy density, long storage life, and ease of disposal (most primary batteries contain little toxic material).

A regular alkaline battery provides 50% more power than a Li-Ion, and a primary lithium battery has three times the energy of a similar sized Li-Ion battery.

A watt (W) is a unit of power, and power is the rate at which energy is produced.

A watt-hour (Wh) is a unit of energy; it’s a way to measure the amount of work performed or generated.

If left on for two hours, then the 60 W light bulb will have used 120Wh of energy.

Watt-hour per kilogram is a unit specific energy, or gravimetric energy density, defines battery capacity in weight (Wh/kg); energy density, or volumetric energy density, reflects volume in liters (Wh/l).

What is C-rate?

Charge and discharge rates of a battery are governed by C-rates. The capacity of a battery is commonly rated at 1C, meaning that a fully charged battery rated at 1Ah should provide 1A for one hour.

Optimal discharge rate:

When discharging a 1Ah battery at the faster 2C-rate, or 2A, the battery should ideally deliver the full capacity in 30 minutes. In reality, internal losses turn some of the energy into heat and lower the resulting capacity to about 95 percent or less.

The energy density ratings at the optimum discharge rate for that particular cell typeare shown in the first figure.

In the second figure, energy density ratingsfor a high-current mode application such as a prosthetic limb are shown.

It can be seen that although the alkaline battery is good for low discharge rates itsperformance in this case is poor.

It can also be seen that the performance of the Li-Ion secondary battery is approaching that of the primary lithium type.

Optimum Discharging

High Current Mode

A comparison of the energy densities of a number of rechargeable batteries is shown in Figure

Rechargeable batteries have a very low internal resistance. This allows high current to be drawn on demand, which is essential for many digital devices, and to drive switch-mode power supplies, which have high inrush currents.

Their main disadvantages include a limited shelf life and high self-discharge. Whereas a primary battery may have a shelf life of 10 years, lithium-based batteries are good for 2–3 years in normal use, though cool storage at moderate charge prolongs longevity.

Nickel-based batteries have a shelf life of up to 5 years but require careful priming (Full performance is reached after the battery has been primed through several charge/discharge cycles, either with a battery analyzer or through normal use.) to regain performance after long storage.

Nickel-based batteries exhibit a 10–20% self-discharge per month compared with a 5–10% value for lithium- and lead-based batteries. This rate increases at high temperature.

Finally, secondary batteries provide a limited number of charge–discharge cycles.

The number that can be achieved is determined by depth of discharge,

Environmental conditions,

charge methods,

maintenance.

Nickel-based batteries need to be deep discharged periodically to reverse crystal growth (memory effects)

Lithium- and lead-based batteries have no memory and can therefore be operated in shallow-cycle applications, with only an occasional deep discharge to verify performance.

2.3.4 Energy Scavenging

Energy scavenging (also known as power harvesting or energy harvesting) is the process in which energy is captured from a system's environment and converted into usable electric power.

Energy scavenging from a biomechatronic perspective can include various sources of power including gross body motion,

vibration,

changes in shape or volume and pressure,

temperature gradient,

and even fuel cells that oxidize blood glucose.

Energy Production from Body Heat

A naked human being radiates about 150 W of infrared at 17C

Neck comprising 1% of the total surface area of the body

Covering neck by thermoelectric generators with an efficiency of 1%

150*0.01*0.01=0.015W=15 mW of power could be harvested.

Thermoelectric generator

A body heat powered music player (Skinny Player MP3)

Energy Production from Respiration

A face mask housing a small turbine driven generator can capture energy from breathe

AIRE, a concept mask that converts wind energy (provided by the wearer's breath) into electricity for the recharging of small electronic devices.

Energy Harvesting From Motion

Activities specifically designed to generate electricity such as shake-driven torches or wind-up radios and cell phone chargers can be reasonably efficient.

K-Tor Pocket Socket 2 is an hand crank generator which provides 120V at 10W power rate.

A good example of one of the larger devices is a knee brace design.

The 1.5 kg device consists of a flexible joint that converts knee flexion into rotary motion that drives a generator.

It is capable of producing 5 W of electrical power.

Piezoelectric materials produce an electrical charge when mechanically stressed.

The most common piezoelectric material is quartz. Certain ceramics, Rochelle salts, and various other solids also exhibit this effect.

The superior flexibility of PVDF (polyvinylidene fluoride) film makes it a better candidate for power extraction.

A 16-layer bimorph PVDF insole developed at the MIT Media Lab produced peak powers of about 15 mW at heel-up into a matched resistive load

The design is a self-powered, active radio frequency (RF) tag that transmits a short-range, 12-bit wireless identification (ID) code while the bearer walks.

In the last few years, research has focused on using PZT or PVDF nanofibers to produce smaller devices, as shown in Figure.

The ultimate goal is to weave these fibers into normal fabrics so that they can be used to produce power directly from a vest or shirt

2.3.4.2 Internal Devices

Energy harvesting from the breath and heart beat of a live rat using a single wire generator (SWG). A SWG attached to a live rat's diaphragm (a) and its heart (b), which drives the SWG to periodically bend and produce an AC power output. c) I–V characteristics of the SWG. The inset illustrates the schematic of the SWG and its connection configuration in reference to the measurement system. d) Typical current output recorded from a SGW under in vivo conditions [from the set-up in (a)].

2.4 SENSORS AND TRANSDUCERS

STRAIN GAGES

R of a wire with resistivity (ohm.meter), length L

(meters), and cross-sectional area A (meter squared) is given by:

The total differential change in R is found by summingthe differential changes resulted in R due to differentialchanges in , L and A.

Divide 2.2 by 2.1 results in

(2.1)

(2.2)

(2.3)

STRAIN GAGES

Poisson's ratio relates the change in diameter D to the change in length L:

Substitute this formula in (2.3)

the change in resistance is a function of changes in dimensionlength,

plus the change in resistivity due to strain-induced changes in the lattice structure of the material,

(2.4)

STRAIN GAGES

The gage factor G, found by dividing (2.4)by L/L, is useful in comparing various strain-gage materials.

The relative sensitivity of a strain gauge is given by the gauge factor, G, which is defined as

Typical strain gauges have R120 and G 2

L

Laxial

axialaxial

d

R

dRG

121

1

axial

RRG

/

Example

If a 120 strain gauge with a gauge factor G=2.0 measures a strain of L/L =50 με(50×10−6) (read as “50 microstrain”), what is the change in resistance from the unloaded to the loaded state?

Answer:

R= 120 × 50 × 10−6 × 2 = 0.012

GL

LRR

GLL

RR

/

/

Measuring Resistance with a Wheatstone Bridge

To determine strain applied, change in resistance must be measured.

Because the change in resistance is small, measurement can be quite challenging.

Because of its outstanding sensitivity, the Wheatstone bridge circuit is the most frequently used circuit for static strain measurement.

Static Balanced Mode

In the static balanced mode, R1 and R2 are precision resistors while R3 is a precision potentiometer with an accurate scale displaying its resistance scale. Rs is the resistance of the strain gauge in the circuit

To balance the bridge, the potentiometer is adjusted until the voltage between nodes A and

B is zero

Static Balanced Mode - Result

As R1 and R2 are known, and R3 can be determined accurately from the angular displacement of the potentiometer, then Rs can be determined.

As strain is appled to the gage, the change in Rs due to the strain will unbalance the bridge and VOUT will become nonzero.

If we adjust the value of R3 to once again balance the bridge, the amount of the change required in resistance R3 will equal the change in RS due to the strain.

Note that this result is independent of the excitation voltage.

2

31

R

RRRs

Dynamic Deflection Mode

Under the strain applied to strain gage, instead of rebalancing the bridge, we could install an indicator, calibrated in micro-strain, that responds to VOUT.

VOUT is directly related with the change in resistance of strain gage.

The bridge is first balanced under no-load conditions .

The measured output voltage can be used to determine the strain gauge

resistance as a load is applied.

Dynamic Deflection Mode - Result

The relationship between the relative change in resistance and the measured output voltage

where

1

1

3

ss

s

RA

AR

R

R

21

1

RR

R

V

VA o

Temperature Compensation

It should be noted that temperature changes can result in resistance changes of the same order as those caused by strain and so temperature compensation is essential if good accuracy is required

This is achieved by using a pair, or even 4 strain gauges in the bridge

Application – Beam Deflection

The strain gauges are attached to the upper and lower faces of a cantilever beam.

When the beam deflection is downward, the length of the strain gauges on

the upper surface increases and those on the lower decreases

Strain Gauge

Resistive Displacement Sensors: Potentiometers

Large displacements can be measured using precision potentiometers.

These come in three types:

Rotary

Linear-motion

String pot (cable pots, yo-yo pots or draw-wire transducers

Generally implemented as part of a Wheatstone bridge to minimise loading the resistance

Linear and Rotary Potentiometers

Rotary pots are single or multi turn (commonly 3, 5 or 10 turns)

Linear motion pots are available with maximum strokes

ranging from 5mm to over 4m

http://spectrum.ieee.org/robotics/medical-robots/exoskeletons-are-on-the-march

Yoyo Potentiometer

Measure the extended length of a spring-loaded cable as shown in the diagram

The rotating shaft creates an electrical signal proportional to the cable's linear extension or velocity.

Measurement range:

The smallest pots measure 0 - 50mm

The largest pots measure 0 -100m

http://www.pc-control.co.uk/string_encoders.htm

Linearity

For displacement measurements, most pots output varies linearly with wiper motion.

Linearity is defined as the maximum deviation of the output function from a straight line.

With laser trimming, overall linearities of better than 0.1% are routinely achieved.

Linear variable differential transformer (LVDT).

Probably the most common inductive device for measuring linear displacement is the linear variable differential transformer (LVDT).

Consists of a primary and two secondary cores mounted axially over a movable iron core as shown

The two secondary coils can be connected in series-opposing configuration so that their combined output describes both the magnitude and direction of the core motion when the primary coil is excited by an AC signal.

http://metrolog.net/transdutores/

How LVDT’s Work

When the core is positioned midpoint between the windings, each of the secondaries provides signals with the same amplitude, but phase shifted by 180 with the result that the output, Vo = 0

As the core is moved from the balance point, the induced voltage in one of the secondary windings will increase, and that in the other will decrease

The result that the magnitude of the output signal will grow in a

fairly linear fashion

LVDT Demodulation

Direction of core displacement requires that the phase of the output be known.

This can be determined using:

phase sensitive detector

synchronous detector

Full-wave bridge rectifier shown below

LVDT Specifications

Commercial LVDTs are available with different diameters, lengths and strokes.

Internal electronics provides the AC signal and performs the demodulation, with a DC voltage output.

Good accuracy over the linear range

Output large enough not to require amplification.

Nonlinearities are typically 0.25% with precision units being as low as 0.05%.

Insensitive to variations in temperature

Main disadvantage is limited frequency response. Determined by: Inertial effects associated with the core’s mass

Choice of excitation frequency

Bandwidth of the low-pass filter at the output

Magnetic Displacement Sensors

Magnetic field strength can best be measured using the following:

Hall effect

Magnetoresistance

Displacement Sensor

http://mustusepowers.blogspot.com.au/2010/04/todays-science-is-yesterdays-science.html

Hall Effect

A voltage, VH, appears across a conductor when a magnetic field is applied at right angles to the current flow. The magnitude of the Hall voltage, VH (V), is proportional to both the magnetic flux density, (Gauss) and the current, I (A). KH is the hall constant and z (m) is the thickness of the conductor

z

IKV H

H

Magnetoresistance (MR)

A property of most magnetic materials

A decrease in electrical resistance occurs when a magnetic field is applied perpendicular to the direction of current flow

The resistance decreases as the magnetic flux density increases until the material reaches magnetic saturation

The total change in resistance is about

0.3% for iron and 2% in nickel.

Giant Magnetoresistance (GMR)

Giant Magnetoresistance (GMR) was discovered in 1988

It is the phenomenon where the resistance of certain layered materials drops dramatically as a magnetic field is applied

It is described as Giant since it is a much larger effect than had ever been previously seen in metals.

With GMR, changes in resistance of up to 10% have been measured

Implementation

MR and GMR sensors are often incorporated into a Wheatstone bridge configuration to reduce temperature dependence and to increase sensitivity.

Two of the elements are shielded from the applied magnetic field and the opposite pair exposed to it

Commercial sensors are available with outputs that vary by more than 5% of the

applied voltage

Sensing Small Displacements

Magnetic linear position sensor can be used to sense shortdistances.

A pair of opposing magnets mounted on a movable, nonmagnetic yoke creates a magnetic null point halfway between the two magnets.

Apart from other magnetic distance sensing schemes this sensor

has a stable reference point

offers fairly linear function between the sensed field anddisplacement.

Sensing Small Angles

A rotary sensor can be made using a plastic yoke formed into a ring supporting a pair of magnets as shown.

When the magnets are rotated around the sensor, it sees a sinusoidally varying field with good linearity available over +/-30.

Magnetic Incremental Encoders

Magnetic encoders use magnets with multiple poles to extend the maximum linear displacement or improve the resolution of angular encoders

Two magnetic sensors are used to enable incremental encoders to determine the direction of travel.

One is displaced from the other along the axis of travel by 90 so that the outputs are in quadrature.

Magnetic Incremental Encoders

Incremental encoders include only a single bit.

They measure relative position by counting starting from a refernce point.

Incremental encoders are simpler and cheaper thanabsolute encoders.

But if power is lost then position information is lost and it is necessary to return to the reference point.

Linear Magnetic Encoder

By “unwinding” the ring magnet, a device that can measure linear motion is produced.

While a rigid rod magnet could be used to provide the required alternating pole pattern, flexible magnetic materials have made this sensor much easier to implement, especially for long linear runs.

For low-resolution applications where the sensor can be very close to the magnetic strip, flexible ferrite materials offer good resolution.

For more demanding applications, either in terms of number of poles per mm or working distance from sensor to magnet strip, rare-earth materials (neodymium-iron-boron or samarium-cobalt) mixed with plastic binders can be used.

LM10 linear magnetic encoder

Optical Displacement Sensors

Digital optical encoders are probably the most common of all the sensors used for angle and angular rate measurement.

They consist of a light source and a number of photo transistors separated by a rotating mask made from transparent and opaque bands.

Come in two types: Absolute encoders, where a unique digital word

corresponds to every rotational position (within the quantisation level),

Incremental encoders which produce a sequence of digital pulses as the shaft rotates, allowing measurement of the relative displacement of the shaft.

Incremental Encoder

Discs can be made from glass or plastic onto which has been deposited a radial pattern organised into tracks

For more robust applications, a metal disc through which holes have been cut.

http://www.globalspec.com/industrial-directory/1_channel_encoder

Incremental Encoder Disc

Consists of a double track of evenly spaced clear and opaque segments displaced by 90 electrical as shown

Resolution is dictated by the number of clear and opaque lines on the disc.

An incremental encoder with a resolution of 256 pulses per revolution (ppr) would have 256 distinct lines on the disc.

An index marker marks the zero reference to allow for absolute positioning

To measure direction, the controller’s electronics monitor the A and B channels to determine which channel arrives first.

Distance is measured by assigning each pulse interval a distance value, and counting the number of pulses with an up–down counter

Schematic of an unrolled incremental encoder disc

Decoding

Flip-flop logic determines which channel pulse is leading

Output to cascaded up/down counters to provide a binary count

Index pulse resets the counters

Counts per Revolution

In standard mode, an encoder counts the leading edge of one square wave signal.

By counting both the leading and trailing edge of the signal, resolution is doubled, a process called 2 times interpolation.

By counting both edges on the A and B channels simultaneously, output increases to four times the ppr, also known as 4 times interpolation.

Interpolating a quadrature output by a factor of four, a 256-ppr encoder can offer 1024 ppr.

With this technology, resolutions of 40,000 ppr are possible.

For even higher resolution, some encoders replace square wave signals with sine wave outputs that allow for interpolation factors as high as 10 times

Absolute Encoders

The optical disk of an absolute encoder is designed to produce a digital word that distinguishes N distinct positions on the shaft.

If there are eight tracks, then the encoder is capable of measuring 28 = 256 distinct angles each corresponding to an angular resolution of 360/256 = 1.406.

The optical pattern is either binary or gray encoded

5 Bit Binary Encoded Disc

The encoder’s inner rings that encode for the more significant bits being smaller.

However, because the transitions are aligned along the radial and the individual read heads are smaller than the size of a bit on the innermost ring, the readout should be accurate.

Problems with Binary Codes

If the level transitions are not simultaneous, then large errors will occur in the digital output.

For example in the transition at 180, all four bits must change simultaneously, otherwise any output between 1 and 14 could occur for a brief period during the transition.

Schematic of unrolled binary absolute encoder

Grey Codes

The gray code is designed so that only one bit changes state for each count transition

The largest error can be a single count

Gray code to binary conversion is required

Exercise – derive an algorithm to convert from Gray to Binary

Exercise – derive an algorithm to convert from Gray to Binary Use xor truth table to convert from gray to binary for the

following configuration.

XOR Truth table

Limits of Resolution

Like single-turn absolute encoders, multi-turns have a unique code for each position within 360° of rotation, dependent on the encoder's resolution, but provide unique codes for each revolution.

An absolute multi-turn encoder will not lose its revolution count or angular position if power fails.

The newest absolute multi-turn encoders are available with 36-bit resolution—18 bits over 360° and another 18 bits for counting revolutions.

With this resolution, multi-turns can offer 262,144 positions over 262,144 revolutions, allowing the encoder to track 68,719,476,736 unique angular positions!

Ranging Sensors

Noncontact distance measurement methods can be classified into three categories: (1) interferometry;

(2) triangulation;

(3) time of flight.

The method used by a particular sensor usually depends on the maximum range and the measurement accuracy required. Interferometric methods can be extremely accurate but are

prone to range ambiguity,

whereas time-of-flight methods operate at longer ranges with poorer accuracy;

triangulation sits somewhere in the middle.

Measuring Displacement -Interferometry

Optical interferometry operates using the superposition of two monochromatic light beams so that extremely small displacements can be measured.

Typically, an incoming beam of light will be split into two identical beams by a grating or a partial mirror. Each of these beams will travel a different path before they are recombined at a detector. The difference in the distance traveled by each beam creates a phase difference between them. This introduced phase difference creates the interference pattern between the initially identical waves. If one of the paths is held constant as a reference, then any change in the distance to the mirror in the measurement path will result in a change in the relative phase of the two beams back at the detector.

Interferometers are ambiguous over distances of /2

Count can be made of the number of electrical cycles out of the detector – as with any incremental encoder

Triangulation Sensors

A LED or laser source emits a narrow beam of infrared light in the direction of the target.

The reflected light is focussed onto the sensitive surface of the position sensitive detector (PSD) which generates two output currents, IA and IB, that are each

proportional to the displacement of the light spot from the centre of the device

The relationship between the range to the target, Lo, and the distance between the transmit and receive apertures, LB, is a function of the focal length, f , and the displacement, x, from the center of the PSD.

Dx

f

LB

Target

Lo

LED

PSD

Lens

Lens

IA

IB

Si substrate

p layer

N+ layer

A B

C

x

Light Beam

Range Measurement

Though the current generated by the PSD depends on the intensity of the incident radiation, the ratio of the two currents does not, and so the distance to the target can be determined with good accuracy.

The distance between the upper electrodes is D, and the corresponding resistance is RD.

If the beam strikes the PSD at a distance x from electrode A, the resistance between that point and the electrode is Rx , and a photocurrent, Io, proportional to the intensity of the light, will flow.

The amount that flows to the electrodes A and B will be proportional to the relative distances from the incident beam; therefore,

Taking the ratio of the two currents IA and IB

Then

Where S is the ratio between the two currents and k is the module geometrical constant

Triangulation ICs

A common IC that operates using this principle is the Sharp Electronics GP2Y0A21YF which has an operational range between 10cm and 80cm with the characteristics shown

PSD

SignalProcessor

LEDDriver

OutputAmp

Oscillator

VoltageRegulator

LED

OutputVoltage

+5VGnd

807060504030201000

0.5

1

1.5

2

2.5

3

3.5

Range L (cm)

Outp

ut (

V)

Application

Figure shows a diagram of a sensor developed for 2-D scanning.

Line-scanned triangulation-based line scanner.

Time of Flight Sensors

The basic principles of active non-contact range-finding are similar for electromagnetic (radar, laser etc.) and active acoustic sensing.

A signal is radiated toward a target of interest,

The reflected signal is detected by a receiver and used to

determine the range

2

TvR

where R is the range (m), v is the propagation velocity (m/s), and T is the round-trip time (s).

Measuring Rate and Angular Rate

Many techniques for measuring rate are available:

Pulse frequency out of incremental encoders

Tacho generators

Rate gyros

Doppler

Rate from Incremental Encoder

Good estimates of the rate can be obtained by measuring the frequency over a reasonable interval by counting cycles.

If the speed is low, then better estimates are made by running a high-speed clock and measuring the interval

between pulses

Estimating rate fromthe output of anincremental encoderfor (a) high speedsand (b) low speeds.

Rate from Tacho-Generators

Tacho-generators are small AC or DC generators that output a voltage in proportion to the rotational speed of a shaft.

They are capable of measuring speed and direction of rotation but not position.

Direction is determined by the polarity in the case of DC generators while

in the AC case, it is the relationship between phases that changes.

They are mounted on or in the same housing as a servo motor.

Generated voltage is measured in volts (V) per revolution per minute (rpm)

Motor

Tacho-generator

http://www.servotek.com/

Rate Gyros

Rate gyroscopes, commonly called rate gyros use the conservation of angular momentum to keep one or more inertial axes pointed in one direction as the external frame translates and rotates

Many possible mechanisms can be used including

Rotating mass

Vibrating mass

Rotating Mass Rate Gyros

If the gyro's plane of rotor spin is changed by rotating the case about the input axis, the gyro will precess because turning the gyro case has the same effect as applying a torque on the spin axis, Y.

The amount of precession is proportional to the rate at which the gyro base is turned

As the gyro precesses, it exerts a precessionalforce against the restraining springs that is proportional to the momentum of the spinning wheel.

This force will result in the precession increasing until it is just balanced by the force on the springs, and it will remain in the precessed position as long as the gyro base is rotated at the same constant speed

A low-friction device to measure the precession angle produces a voltage proportional to the rate

Neets. (2000, June). Module 15-Principles of Synchros, Servos, and Gyros. Available: http://www.tpub.com/content/neets/14187/

MEMS Coriolis Gyros

A MEMS structure is caused to vibrate in one plane. If the structure is also rotating, then the Coriolis force will be generated at right angles to both the direction of vibration and the axis of rotation

If V (m/s) is the instantaneous velocity of the vibrating structure of mass m (kg), which is rotating at an angular rate (rad/s) then the Coriolis force is given by

As the velocity of the vibrating element is sinusoidal, the Coriolisforce will introduce a lateral vibration at the same frequency

VmF 2

Implementation

A proof mass attached to springs is forced to oscillate in the horizontal plane – this is manufactured as the top plate of a capacitor

When rotating, the Coriolis force introduces vertical vibration that changes the gap between the capacitor plates

This changes the capacitance which is measured as an AC signal output

Packaged MEMS rate-gyrohttp://www.robotshop.com/sensors-gyroscopes.html

Fibre Optic Gyro (FOG)

Two beams from a laser are injected into the same fibre but in opposite directions.

If the ring is static, then the path lengths back to the detector are identical.

If the ring is rotating, then due to the Sagnac effect, the beam travelling against the rotation experiences a slightly shorter path delay than the other beam.

The resulting differential phase shift is proportional to the angular velocity.

Sagnac Phase Shift

The Sagnac phase shift, S, is directly proportional to the angular rate

A (m2) is the cross sectional area enclosed by the fibre optic coil

n is the number of turns of fibre optic cable around the ring

(rad/s) is the angular rate at which the ring is rotating

c (m/s) is the speed of light

(m) is the wavelength of the light

c

nAS

8

Accelerometers

Accelerometers are sensors designed to measure continuous mechanical vibration such as bearing vibration, transitory vibration from blasts or impacts, or the lower frequency acceleration generated by bodies in motion.

These devices are generally bonded firmly to a structure and aligned in the direction of interest as they are sensitive to motion

along one axis only.

Implementation

All accelerometers are based on the inertial effects associated with a mass connected to a moving object through a spring, damper and a displacement sensor

The important characteristics of an accelerometer are:

sensitivity

dynamic range

frequency response

Frequency Response

Assume that,

The accelerometer transfer function can be derived from the solution of a 2nd order differential equation

If the accelerometer is designed so that Ha() = 1 over a

large frequency range, then the input acceleration amplitude is given by the relative displacement of the mass scaled by a constant factor n

2

2/1

2

2

22

2

2

41

1

nn

i

nra

X

XH

Frequency Response & Damping

•the most linear response occurs with a damping ratio ξ = 0.707 and the natural frequency ωn as large as possible.

•ωn can be made large by choosing a small seismic mass and a large spring constant.

Piezoelectric Accelerometers

Piezoelectric accelerometers use piezoelectric crystals to measure the force exerted by the seismic mass.

Deformation of these crystals results in the generation of a charge across the opposite faces

Because the crystal is bonded firmly to the mass, the frequency response is flat and the damping ratio is high

In general, piezoelectric accelerometers cannot measure constant or slowly changing accelerations because the crystal can measure changes only in strain.

Low-frequency response is typically a few hertz, while the high-frequency response is determined by the mechanical characteristics of the accelerometer and the mounting stiffness

PiezoresistiveAccelerometers

Piezoresistive accelerometers that can operate down to DC and are therefore useful for measuring static acceleration (gravity).

The sensing element of this accelerometer type incorporates strain piezoresistive gauges which measure strain in the mass supporting springs.

The strain can be directly correlated with the magnitude and rate of mass displacement, and hence with acceleration.

These are robust sensors with a wide bandwidth (dc to 13kHz)

J. Fraden, Handbook of Modern Sensors: Physics, Designs and Applications, 3 ed.: Springer Verlag, 2003.

Capacitive Accelerometers A capacitive accelerometer contains a sprung mass sandwiched

between a cap and the base (all three components are micro-machined in silicon).

As the mass moves upwards with respect to the housing, the distance between the mass and the cap, d1 decreases while simultaneously the distance to the base, d2 increases.

The displacement is equal to the force Fm acting on the mass divided by the spring constant k of the silicon springs =Fm/k.

If the electrostatic forces are sufficiently small to be ignored then the capacitance, which can be measured, is a linear function of Fm

J. Fraden, Handbook of Modern Sensors: Physics, Designs and Applications, 3 ed.: Springer Verlag, 2003.

Tilt Sensors Tilt sensors can be manufactured in various ways:

Mercury tilt switch

One end of the cavity has two conductive elements (poles). When the sensor is oriented so that that end is downwards, the mass rolls onto the poles and shorts them, acting as a switch throw.

3 axis piezoresistive accelerometer

Electrolytic bubble level

Mercury tilt switchesElectrolytic tilt sensorshttp://cotf-nie.blogspot.com.au/2008/08/accelerometers-on-pdas.html

Bubble Level Tilt Sensor More sophisticated tilt sensors can be manufactured from

accelerometers mounted on three orthogonal axes and equating the relative gravity vector in each.

However, most low-cost tilt sensors exploit the bubble-level principle.

The fluid is electrically conductive, and the conductivity between the two electrodes is proportional to the length of electrode immersed in the fluid.

At the angle shown, for example, the conductivity between pins a and b would be greater than that between b and c.

Bubble Level Tilt Sensor Electrically, the sensor is similar to a potentiometer, with

resistance changing in proportion to tilt angle and therefore the output can be determined using a Wheatstone bridge in its dynamic unbalanced configuration.

For small angles, typically less than 20, the output voltage is proportional to the tangent of the tilt angle

For larger angles, nonlinearities become more pronounced, and more sophisticated microcontrollers are required to apply corrections to the measurements

To prevent electrolysis, AC source must be used to minimiseelectrolytic activity within the cell

Pressure Measurement Pressure is defined as the normal force per unit area

exerted by a fluid (liquid or gas) on any surface.

Only the force normal to the surface needs to be considered

Three types of pressure measurements are commonly performed:

Absolute pressure

Gauge pressure

Differential pressure

Absolute pressure: Represents the

pressure difference between the point

of measurement and a perfect vacuum

where the pressure is zero.

Gauge pressure: This is the pressure

difference between the point of

measurement and the ambient

(atmospheric) pressure.

Differential pressure: The difference in

the pressure between two points, one

of which has been chosen to be the

reference.

Hydrostatic Measurements A column of fluid is commonly used as to measure

pressure because there is a simple relationshipbetween the pressure, P (Pascal), the density of thefluid, (kg/m3), acceleration due to gravity, g (m/s2),and the height of the column, h (m).

Biomedical pressure sensors are used mostly tomeasure blood or air pressure. They can also be usedto measure the pressure in pneumatic and hydraulicactuators for control or monitoring purposes.

ghP

http://www.hzproduct.com/

Hydrostatic Measurements The conventional sphygmomanometer is an

indirect method of measuring blood pressure.

Conventional sphygmomanometers consistingof an inflatable cuff and a mercury manometerare considered to be the “gold standard” forblood pressure measurement because theycannot be in error if operated correctly.

Its main disadvantage is that it is unable toprovide a continuous reading of pressurevariations, and its update rate is limited. Inaddition, only the systolic and diastolic arterialpressures can be measured.

http://www.hzproduct.com/

Hydrostatic Measurements The operational principles of the

sphygmomanometer are quite simple.

A cuff is placed around the upper arm andinflated, and arterial blood can flow past it onlywhen the arterial pressure exceeds that of thecuff.

At this stage, blood squeezing through thereduced aperture of the brachial artery generatesturbulence that can be heard by a practitioner witha stethoscope.

As the pressure in the cuff is reduced andmonitored using a manometer, the first indicationof this sound (called Korotkoff sounds) gives anindication of the systolic pressure.

This reduction in cuff pressure continues untilKorotkoff sounds disappear, and that is anindication of the diastolic pressure.

http://www.hzproduct.com/

Pressure Sensors Consist of a chamber with a flexible diaphragm making up a

portion of one wall, with the other side of the diaphragm at

atmospheric pressure.

A pressure differential across the diaphragm will cause a

deflection that can be measured using a displacement sensor

Modern pressure transducers are manufactured onto a single

MEMS silicon chip where a portion of the chip is formed into a

diaphragm and semiconductor strain gauges

http://www.a-tech.ca

Total force F (N), applied to the membrane is equal to the product of

differential pressure, ∆P (Pa), and the area of the membrane, A (m2)

F = ∆P.A

The stress -> F -> ∆R change in resistors

R is the initial resistance, αl and αt are the piezoresistive coefficients in the

longitudinal and transverse directions, and σl and σt are the stresses in

these directions, respectively.

Lo

ng

itu

din

al

Transverse

Measuring Blood Pressure The chamber containing the diaphragm is coupled via

a thin plastic catheter to an artery.

The catheter is filled with saline solution so that the

arterial blood pressure is coupled to the diaphragm

Important considerations:

Both ends of the catheter must be at the same height to

avoid hydrostatic effects,

The tube must be sufficiently stiff to minimise compliance

effects on the frequency response of the sensor

Air bubbles in the catheter and obstructions due to clotted

blood can introduce distortions into the measured

waveform

Miniature Pressure Sensors

It is now possible to obtain miniature pressure

sensors that are located at the tip of the catheter

and measure pressure within the blood vessel

www.fiso.com

Fiber optic

pressure

sensor from

FISO

Technologies

employs a

MEMS-based

sensing catheter

for

in vivo

physiological

measurements.

Long-term stability of pressure monitors is a problem

Drift, particularly when measuring low pressures such as venous blood or

cerebrospinal fluid, can result in significant errors.

Pressure transducers need regular recalibration, which can pose significant

problems, particularly if the sensor is implanted.

Measuring Air Pressure for

Respiration

Sensors for measuring air pressure in positive ventilation devices need not be miniaturised, but must be capable of measuring pressures from less than 2cm of H2O(≈196pa) up to 40cm H2O (=3920Pa)

The piezoresistive bridge is only the beginning of the pressure measurement process. The bridge output is filtered and then digitised along with the

temperature and a band-gap reference voltage.

These parameters are then processed by the onboard DSP to correct for pressure and temperature nonlinearities

The pressure is then converted back to an analog voltage and output

Inside a Pressure Sensor

ca.digikey.com

Flow Measurement Applications

The most common medical applications of flow

measurement include the following:

Blood flow through arteries and veins

Airflow into and out of the lungs

Measurement of drug dispensing through catheters

Gas flow during anaesthetic administration

http://ki.se/ki/jsp/polopoly.jsp?d=34984&l=en

Flow Measurement Methods

The most common methods to measure flow

include the following:

Pressure drop across a restriction

Temperature correlation

Doppler Ultrasound

Correlation Ultrasound

Turbine flow meters

Impeller flow meters

Bernouli’s Equation

The Bernoulli Equation

can be considered to be

a statement of the

conservation of energy

principle appropriate for

flowing fluids. The

qualitative behavior that

is usually labeled with

the term "Bernoulli effect"

is the lowering of fluid

pressure in regions

where the flow velocity is

increased

Measuring Pressure Using a

Restriction in the Pipe If there is a restriction in a piece of

pipe and h1=h2

Bernoulli’s equation can be rewritten

as

the conservation of mass requires

that

The volumetric flow rate, Q, in terms

of the drop in pressure across the

restriction in the pipeline

Common Differential

Pressure Flowmeters The Venturi tube, (a) , is the oldest type of

differential pressure flowmeter. Its major

disadvantages are the lower differential

pressure for a given diameter ratio and the

expense of manufacture.

The orifice plate (b) , is the simplest and

cheapest type of differential pressure

flowmeter. It is simply a plate with a hole

of the specified diameter that is clamped

into the pipe. But correction is required.

The nozzle method (c ) combines the

best features of the other two techniques,

making it both reasonably cheap to

manufacture and accurate.

Temperature Flow meter The sensor consists of a small isolated heating element immersed in

the fluid between two similarly immersed temperature probes

If the medium is not flowing, then diffusion will result in the two probes reading the same temperature

If there is flow, then the downstream probe will be warmer than the upstream one

The temperature differential can be calibrated to measure the flow rate for both laminar and turbulent flow

Ultrasound Flow Meter Two transducers are placed on either side of a pipe directed through

the fluid towards each other at an angle

If a high frequency pulse (f > 3MHz) is transmitted from the one then the time, T, taken to reach the other will be

If the fluid is flowing then the propagation speed is altered by the flow and equation becomes

where vc is the average fluid velocity

and the +/- refers to the direction of

flow relative to the direction of the

ultrasound signal

cDT /

coscvc

DT

Flow Meter Processing By taking the time difference, T, between the downstream and

upstream measurements, the flow velocity, vc, can be determined

For c >> vccos

2222

cos2

cos

cos2

c

Dv

vc

DvT c

c

c

Turbine Flow Meters

Medical appliances still use the basic turbine or

vane to measure flow because, if properly

installed and calibrated, they provide the highest

accuracies attainable for any currently available

flow meter for both liquids and gas

Implementation A multiple-bladed rotor is mounted with a pipe, perpendicular to the liquid flow

The rotor spins as the liquid passes through the blades

The rotational speed is a direct function of flow rate and can be sensed by magnetic pick-up.

A magnetic pickup is essentially a coil wound around a permanently magnetized probe. When discrete ferromagnetic objects—such as turbine rotor blades are passed through the probe's magnetic field, the flux density is modulated. This induces AC voltages in the coil. One complete cycle of voltage is generated for each object passed.

If the objects are evenly spaced on a rotating shaft, the total number of cycles will be a measure of the total rotation, and the frequency of the AC voltage will be directly proportional to the rotational speed of the shaft.

Impeller Flow Meters Impeller flow meters come in two

varieties

In-line meters are constructed as a unit

which includes inlet and outlet orifices

insertion meters, can be installed into

existing pipes through a round hole

Impeller meters are more sensitive

than axial turbine flow meters at low

flow rates because the blade

incidence angle is much larger.

Impeller meters also relatively

insensitive to the flow regime

(laminar or turbulelent)

Temperature Sensing The measurement of temperature is one of the

most common sensing requirements in the medical field

The human organism can only function effectively over a small range of temperatures, so almost all processes associated with the organism are temperature controlled

These include: Heart-lung machines

Incubators for premature babies

respirators.

http://www.biosulf.org

How to Measure Temperature

Temperature is an expression for the kinetic

energy of vibrating atoms and molecules of a

matter.

This energy can be measured by various

secondary phenomena, e.g.,

change of length, volume or pressure,

electrical resistance,

electromagnetic force,

electron surface charge, or

emission of electromagnetic radiation.

Temperature Sensors

Temperature can be measured via a diverse array of

sensors. All of them infer temperature by sensing some

change in a physical characteristic of the device. The

types with which an engineer is likely to come into

contact are:

Thermocouples,

Resistance temperature devices (RTD’s and thermistors),

Infrared radiators,

I.C. sensors,

Bimetallic devices,

Liquid expansion devices, and

Change-of-state devices

Bimetallic Strips The radius of curvature of a uniformly heated

bimetallic strip will be R with a temperature

change from To to T, if the strip starts out flat

where 1 and 2 are the coefficients of

expansion of the two materials 1 > 2 and t

is the strip thickness

t

TT

R

012

2

31

How a bimetallic

thermostat switches

on and off

Resistance Temperature

Detection (RTD) Thermometers

Possibly the most accurate of temperature sensors are resistance temperature detectors (RTD).

Standard platinum RTDs can be manufactured with accuracies of +/-0.0001C, while simple industrial RTDs are generally accurate to about +/-0.1C.

Resistance temperature detection capitalize on the fact that the electrical resistance of a material changes as its temperature changes

Where R0 is the resistance at T=T0 and α is the temperature coefficient of the device.

Figure: Resistance-Temperature Curve for a

100 Ω Platinum RTD, α = 0.00385

The temperature coefficient of

resistance is defined as the

change in resistance per degree

C per ohm over the range from

0C to 100C

R0=100Ω Resistance at T0=0C

R=138.5Ω at T=100C

Substitute in equation these

values then α=0.00385

Resistance-Temperature Curve

for a 100 Ω Platinum RTD

Copper RTD – Theory and

Measured Data

Implementation The sensitive portion of an RTD is a coil of high-purity wire

or a thin film deposited onto a ceramic substrate

The metal used is usually platinum, copper or nickel depending on the temperature range and accuracy.

Platinum is the material of choice because it doesn’t oxidise even when subjected to very high temperatures.

In operation, a constant current (0.8mA to 1mA typically) is passed through the coil.

As the temperature increases, the resistance will increase (the temperature coefficient is positive), and the voltage that is developed across the coil will reflect this.

An accurate voltmeter calibrated to measure temperature can be used to display this value

Performance Considerations Copper – α=0.00427, Good linearity, Limited temperature range - oxidises

at 150C

nickel – α=0.00672, Best sensitivity, Very nonlinear above 300C

platinum – α=0.003902, Widest temperature range -184C to +649C, Good linearity Good repeatability Long life

Ro is the resistance at 0C

RTD

A resistance temperature device is

placed on the isothermal block.

R0 = 100Ω at T0=0C,α=4x10-4/C.

Calculate RT and its sensitivity to T at

the T=25C.

Assume RT is placed into one arm of the

Wheatstone bridge as shown in the

figure. Calculate the bridge voltage at

0C and 25 C.

Thermistors Like the RTD, the thermistor is also a

temperature sensitive resistor.

Thermistors are generally semiconductor materials

Exhibit a higher temperature coefficient of resistance than pure metals

Linearity is generally worse.

Come in two kinds: Silicon positive temperature coefficient

(PTC) thermistors rely on the bulk properties of doped silicon and have temperature coefficients of between 0.07 and 0.08.

Negative temperature coefficient (NTC) thermistors are made from metal oxides, and they exhibit a monotonic decrease in resistance with increasing temperature

High negative temperature coefficient (TC)

allows the thermistor circuit to detect

extremely small changes in temperature,

which could not be observed with an RTD,

or thermocouple circuit.

The thermistor is the most sensitive

temperature transducer. Of the three

major categories of sensors shown in

Figure, the thermistor exhibits by far the

largest parameter change with

temperature.

The price we pay for this increased

sensitivity is loss of linearity.

NTC Thermistor response The standard formula that describes the resistance of a NTC

thermistor as a function of temperature is

where R25 is the resistance at 25C, is the thermistor’s material constant (K) and T is the actual temperature of the thermistor (C)

R25 and are generally published in the manufacturer’s data sheet

R25 can range from 22 to 500k

typically ranges from 2500 to 5000K.

The temperature coefficient α

298

1

273

1

25

T

T eRR

Response Graph Resistance characteristics of a typical NTC thermistor,

R25 = 10k, = 3965K

Linearisation of this function can be achieved using resistance or voltage mode techniques

Theory for R25 = 10k Measured for R25 = 47k

Example

A thermistor is placed in 100 oC environment, and its

measured resistance is 20 KΩ; the thermal constant is

β=3650. If the thermistor is used to measure a particular

temperature; and its resistance is measured as 500 KΩ,

determine the thermistor temperature.

298

1

273

1

25

T

T eRR

Resistance Mode Linearisation Resistance mode linearisation involves placing a normal

resistor in parallel to the thermistor. If the resistance is

chosen to equal R25 then the region of relatively linear

resistance operation will be symmetrical around room

temperature

Theory Measured

Voltage Mode Linearisation The thermistor is connected in series with a normal

resistor to form a voltage divider. For a fixed bias voltage

and a resistor equal to R25, the region of linear voltage

will be symmetrical around room temperature

Thermocouple

A thermocouple consists of a combination of two

different materials bonded together which will

generate a potential difference, V, proportional

to the temperature difference, T, between the

hot and cold terminals. This characteristic is

known as the Seebeck effect.

THERMOCOUPLES When a temperature difference is applied between the two ends of a single Ni wire, a

voltage drop is developed across the wire itself. The end of the wire at the highest

temperature, T2, is called hot end, while the end at the lowest temperature, T1, is

called cold end.

When a voltmeter, with Cu connection wires, is used to measure the voltage drop

across the Ni wire, the measured voltage is in reality the voltage drop along the Ni

wire plus the voltage drop along the Cu wire.

The Emf along a single thermoelement cannot be measured: the Emf measured at

the tail end in previous figure is the sum of the voltage drop along each of the

thermoelements. As two thermoelements are needed, the temperature measurement

with thermocuoples is a differential measurement.

Note: if the wire in figure was a Cu wire a null voltage would have been measured at

the voltmeter.

THERMOCOUPLES

A thermocouple is a device made by two different wires joined at one end,

called junction end or measuring end. The two wires are

called thermoelements or legs of the thermocouple: the two

thermoelements are distinguished as positive and negative ones. The other

end of the thermocouple is called tail end or reference end (Figure). The

junction end is immersed in the enviroment whose temperature T2 has to be

measured, which can be for instance the temperature of a furnace at about

500°C, while the tail end is held at a different temperature T1, e.g. at

ambient temperature.

Because of the temperature difference between junction end and tail end a

voltage difference can be measured between the two thermoelements at the

tail end: so the thermocouple is a temperature-voltage transducer.

The temperature vs voltage relationship is given by:

where Emf is the Electro-Motive Force or Voltage produced by the thermocople at the tail end,

T1 and T2 are the temperatures of reference and measuring end respectively, S12 is called

Seebeck coefficient of the thermocouple and S1 and S2 are the Seebeck coefficient of the two

thermoelements; the Seebeck coefficient depends on the material the thermoelement is made

of.

Looking at equation it can be noticed that:

a null voltage is measured if the two thermoelements are made of the same materials:

different materials are needed to make a temperature sensing device,

a null voltage is measured if no temperature difference exists between the tail end and the

junction end: a temperature difference is needed to operate the thermocouple,

the Seebeck coefficient is temperature dependent.

The temperature measurement with thermocouples is also a differential measurement because

two different temperatures, T1 and T2, are involved. The desired temperature is the one at the

junction end, T2. In order to have a useful transducer for measurement, a monotonic “Emf versus

junction end temperature T2 relationship” is needed, so that for each temperature at the junction

end, a unique voltage is produced at the tail end.

However, from the integral in Equation1 it can be understood that the Emf depends on both

T1 and T2: as T1 and T2 can change independently, a monotonic “Emf vs T2 relationship” cannot be

defined if the tail end temperature is not constant. For this reason the tail end is maintained in an

ice bath made by crushed ice and water in a Dewar flask: this produces a reference temperature

of 0°C. All the voltage versus temperature relationships for thermocouples are referenced to 0°C.

The ice bath is usually replaced in industrial application with an

integrated circuit called cold junction compensator: in this case

the tail end is at ambient temperature and the temperature

fluctuations at the tail end are tolerated; in fact the cold junction

compensator produces a voltage equal to the thermocouple

voltage between 0°C and ambient temperature, which can be

added to the voltage of the thermocouple at the tail end to reproduce

the voltage versus temperature relationship of the thermocouple.

It should be underlined that the cold junction compensation cannot

reproduce exactly the voltage versus temperature relationship of the

thermocouple, but can only approximate it: for this reason the cold

junction compensation introduces an error in the temperature

measurement.

Figure shows also the filtering and amplification of the thermocouple.

Being the thermocouple voltage a DC signal, removal of AC noise

through filtering is beneficial; furthermore the thermocouples

produce voltage of few tens of mV and for this reason amplification

is required.

The small voltage range for some of the most common thermocouples (letter designated

thermocouples) is shown in Figure, where their voltage versus temperature relationship is

reported.

All the voltage-temperature relationships of the letter designated thermocouples are monotonic,

but not linear. For instance the type N thermocouple voltage output is defined by the following 10

degree polynomials

Implementation A typical thermocouple consists of wires made from the

two materials and a method of measuring the potential

difference

The reference temperature Tref is usually supplied by an

ice-water bath or in the field the ambient temperature is

used

where SA and SB, are the

thermoelectric coefficients of

the two materials

SBSA

outreftip

VTT

Material Thermoelectri

c Coefficient,

S (VK-1)

Material Thermoelectri

c Coefficient,

S (VK-1)

Aluminium 3.5 Nichrome 25

Antimony 47 Nickel -15

Bismuth -72 Platinum 0

Cadmium 7.5 Potassium -9

Carbon 3.0 Rhodium 6

Constantan -35 Selenium 900

Copper 6.5 Silicon 440

Germanium 300 Silver 6.5

Gold 6.5 Sodium -2

Iron 19 Tantalum 4.5

Lead 4 Tellurium 500

Mercury 0.6 Tungsten 7.5

Materials should have a high thermoelectric coefficient, low thermal conductivity,

and low resistivity. Unfortunately, as shown in below table, materials like gold, silver, and

copper that have low resistivity also have poor thermoelectric coefficients, whereas those

with high thermoelectric coefficients, like bismuth (Bi) and antimony (Sb), have high

resistivities

Example

It is required to measure

temperature by means of a

thermocouple having a sensitivity of α=αSA-αSB=50 μV/ C.

The reference temperature T0=0C.

Find the temperature for an output

of 2.5mV and 10mV.

SBSA

outreftip

VTT

Integrated Circuit (IC)

Semiconductor Thermal Sensors A semiconductor PN junction of a diode or transistor exhibits

a strong temperature dependence.

If the junction is connected to a constant current source, the

voltage becomes a measure of the junction temperature.

This relationship is very linear, and is therefore used for

accurate three terminal temperature sensing integrated

circuits

The LM35Z temperature sensor has a linear output internally

trimmed for the Celsius scale with a sensitivity of 10mV per

C and a nonlinearity error confined within +/-0.1C.

TVVout 0

Example

Using the TMP36 is easy, simply connect

the left pin to power (2.7-5.5V) and the right

pin to ground.

Then the middle pin will have an analog

voltage that is directly proportional (linear)

to the temperature.

The analog voltage is independent of the

power supply.

To convert the voltage to temperature,

simply use the basic formula:

Temp (Celsius) = [(Vout (mV)) - 500] / 10

if the voltage out is 1V that means that the

temperature is

((1000 mV - 500) / 10) = 50C

Comparison of Temperature Sensors

Tactile Sensing Tactile feedback is one of the critically important aspects

of any successful hand prosthesis. In addition, it also

plays a role in improving haptic feedback in remotely

monitored medical examinations and surgery.

Tactile feedback relies on contact-based effects

including contact stresses,

slippage,

heat transfer,

and hardness.

These properties, in a grasped object, can be classified into geometric and

dynamometric types (Webster, 1999). Among the geometric properties are presence,

location in relation to the end-effector,

shape,

dimensions,

and surface conditions.

Among the dynamometric parameters associated with grasping are force distribution,

slippage,

elasticity,

and hardness

as well as friction.

Tactile Sensing - Interaction Tactile sensing is also reliant on the processes through

which the device interacts with the explored object. These include controlling contact force and end-effector position and orientation.

Tactile sensing generally involves the interaction of a rigid object with the compliant cover layer of the tactile sensor. For example: A robot arm discovering its environment.

Analysed from two distinct perspectives: Measurement of the contact stresses (force distribution) in the layer,

which is relevant to controlling manipulation tasks

The second is the deflection profile of the layer which is important in recognising geometrical features of the object

Tactile Sensor Requirements

Spatial resolution 1mm – 2mm

Array sizes of 510 to 1020 elements

Sensitivity between 0.510-2 N and 110-2 N for

each sensing element

Dynamic range 1000:1

Stable behaviour with no hysteresis

Monotonic response, but not necessarily linear

Compliant interface, rugged and inexpensive

Resistive Sensors These sensors rely on materials whose resistance changes with increases in applied force.

Conductive elastomers manufactured by embedding conductive particles in natural or

silicone rubber were among the first. Piezoresistive materials and embedded pressure

sensors can be employed either.

A typical array will include conductive strips connecting the rows and columns of the grid so

that the individual element resistances may be sensed

A binary address selection connects a voltage, Vi , to a single row of resistive elements,

while simultaneously the address controls a multiplexer that connects a single column to an

op amp inverter.

i

RC

f

o VR

RV

Capacitive Sensors Row and column conductive strips are placed on

either side of a compliant dielectric material.

The intersection of each forms a tiny capacitor.

An applied force that reduces the distance between the plates will result in an increase in capacitance

capacitance of a parallel plate capacitor is directly proportional to product of the plate area, A, and the dielectric constant, , and inversely proportional to the distance between them, h

h

AC

Implementation

A capacitive divider is formed using an AC drive

signal and a load capacitor Cd

The output voltage is a function of the

capacitance of the sensor

i

RCd

RCo V

CC

CV

Piezoelectric Tactile Sensors A piezoelectric material is one that will develop a charge, Q, across

opposite faces when subjected to a force or deformation.

Because the material is an insulator, and a dielectric, it also forms a

capacitor, and so a voltage can be measured across each element

PVF2 has a strong piezoelectric effect, therefore it can be made

very sensitive. It is also flexible and can be made into small sensor

elements. However it is sensitive to temperature and response does not

extend right down to DC.

Sliding force sensors can be constructed using thin slivers of

piezoelectric materials orientated in different directions.

These are situated within a silicone rubber skin and will respond

differently as the sensor is moved in different directions across a

rough surface.

They are capable of detecting surface discontinuities or bumps as

small as 50 μm high.

Exotic materials such as carbon

nanotubes and carbon microcoils

(CMCs) can also be embedded into a

rubber matrix to make tactile sensors.

Unlike normal resistive sensors,

because of the coiled nature of the

conductive material a significant

inductive component also exists.

These sensors are therefore excited

by an AC signal at a frequency of

between 100 kHz and 400 kHz to give

the best response.

In addition, the sensitivity can be

controlled by adjusting the

concentration of the CMC material

compared with the silicone rubber,

Carbon Microcoils

Chemical Sensors Chemical sensors are sensitive to stimuli produced by various chemical

compounds or elements. Their most important property is selectivity (the

ability to discriminate between types), while a secondary one is their

relationship to concentration.

Chemical sensors are very important in medical applications; for instance,

the ability to monitor oxygen concentration in the air or in solution is crucial

during surgery.

Biosensors are considered a special class of chemical sensors because

they have a much greater selectivity and sensitivity than other chemical

sensors. Man-made biosensors often use enzymes that have evolved over

millions of years to be extremely sensitive to specific molecules.

Enzyme and Catalytic

Electrochemical

Resistive

Oscillating

Optical

Enzyme and Catalytic Sensors The enzyme is immobilised into a

hydrogel into which the chemical diffuses. The enzyme assisted reaction alters the characteristics of the hydrogel in a manner that can be detected

Catalytic sensors are a subset of these in which the calalytic reaction releases heat, and the temperature change can be measured by the sensor

Thermistor based sensors are usually part of a Wheatstone bridge

These sensors have beendeveloped specifically to measure low concentrations of inflammable gases (particularly in mines).

Electrochemical Sensors The most versatile and best developed of all of the

chemical sensor types.

Measure voltage, current or resistivity and generally consist of a pair of electrodes as part of a closed circuit.

In voltage based sensors, a redox reaction (a type of chemical reaction that involves a transfer of electrons between two species) at the electrode-electrolyte interface of the electrodes results in a half-cell potential developing at each. One of the reactions involves the molecule of interest, while at

the other, a known reaction occurs.

To maintain equilibrium, the current flow should be minimised, so a very high-impedance amplifier measures the potential difference across the two electrodes.

This potential difference is a function of the concentration of the molecule of interest

CHEMFET Sensors The gate of the transistor is coated with an appropriately

sensitive gate insulator material which is, in turn,

exposed to the electrolyte.

As the charge around the sensing area changes, the

conductivity of the FET is altered in a measurable way.

Ion selective membranes can be deposited on top of the

gate insulator to provide a large selection of different

chemical sensors

Oscillating Sensors A piezoelectric crystal cut to oscillate at ultrasonic

frequencies will have a resonant mode that is a function of its mass.

The surface of the crystal is covered by a thin layer of material that has an affinity for the molecule of interest,

If any is present, it will attach to the surface and alter the mass – and consequently the resonant frequency

Oscillating sensors can be extremely sensitive Typically 5MHz cm2/kg which means 1Hz frequency shift

corresponds to about 17ng/cm2 added weight.

The response is very linear with mass and the dynamic range is quite broad – up to 20g/cm2

Surface Acoustic Wave (SAW)

Oscillating Sensor These sensors are comprised of SAW transmission line (membrane)

covered by a chemically sensitive coating situated between an acoustic launcher and a receiver.

1. An oscillator drives the launcher -> 2.Produces a mechanical wave ->

3. Travels along the transmission line -> 4. Excites the receiver -> 5.Converted back to an electrical signal for analysis

Oscillation is maintained by feedback from the receiver which will be a function of the transmission time. This is in turn sensitive to the mass of the membrane which depends on the concentration of the molecule of interest.

The theoretical sensitivity of such sensors operating at 2.6 MHz is of the order of 900 cm2/g, so if a sensor with a sensitive area of 0.2 cm2 captures 10 ng the oscillator frequency is shifted by

Microbalance Sensors Odor sensors are gas sensors that have sensitivities approaching that of the

human nose.

Odour sensors use a composite sensor made from poly vinyl chloride (PVC), a plasticiser and a synthetic lipid

A 200m thick membrane of this polymer composite is spread on one side of the quartz crystal oscillator which is designed, along with the membrane blend, to have a resonant quality factor, Q > 5104.

Odour molecules embed into the membrane and alter the resonant frequency by increasing the mass.

Experimental results indicate a sensitivity starting at 1ppm (paarts-per-million) (approximately the human threshold) and extending in a linear fashion up to 3000ppm

Amylacetate

Relationship between gas

concentration and frequency change.

It has a scent similar to

bananas and apples.

Optical Chemical Sensors

Optical methods are among the

oldest techniques for sensing

biochemical concentration.

In principle, these sensors

consist of a light source tuned

to a frequency that will interact

with the molecule of choice, a

method of directing the light

into the medium, and a

photodetector for processing

the optical signal.

Optical Chemical Sensors Optical sensors are usually based on optical fibres or

planar waveguides

Three main methods of sensing: The molecule directly affects the optical properties of the

waveguide such as evanescent waves or surface plasmons

An optical fibre is used to convey light to and from the sample. Changes in the optical properties of the medium containing the molecule of interest are sensed by an external spectrophotometer

An indicator or chemical reagent placed near the tip of the optical fibre reacts with the molecule of interest to produce an optical signature that can be detected. These include absorption spectroscopy and fluorimetry.

Evanescent Waves: Electromagnetic waves generated in the medium outside the optical waveguide when light is reflected from within

Surface Plasmons: Resonances induced by an evanescent wave in a thin film deposited on a waveguide surface

Optical Fibre Sensors Optical fibres are small and low cost.

Nonconductive so no electrical risk to the patient

No interference from electric and magnetic fields

Measurements are based on spectral or absorption changes in the medium determined by the active molecule: Directly

Through an indicator mediated reaction

Most chemicals of medical interest such as hydrogen, oxygen, carbon dioxide and glucose require the use of reagents.

Optical Fibre Sensors Light travels efficiently to the end of the fibre

It exits into the medium

Interacts with the molecule or reagent

Returns via the same or a different optical fibre to a detector for analysis and interpretation

Measuring Blood Oxygen Level Known as oximetry

Generally relies on a colour change in the blood as the amount of deoxyhaemoglobin (Hb) and oxyhaemoglobin (HbO2) vary

Measurements are performed at two specific wavelengths At 660nm, where there is a large difference between the relative

absorption of the two molecules

At 805nm where the absorption is independent of blood oxygenation.

The oxygen saturation, O2sat, can then be determined from the ratio of the two absorption levels, 1 , and 1

2

12

baO sat a and b are sensor

dependent constants

Some OximetersThere are two types of oximeters such as in vivo fiber optic oximeters which can

be manufactured within catheters that are inserted into a vein or artery or as

noninvasive sensors clamped to the finger or ear where bloodflow is close to the

surface.