Instrumentation System Design – part 1 Chapter6:

Instrumentation System Design –

part 1

Chapter6:

Think about System Design…(1)

SYSTEM SYSTEM DESIGNDESIGN

Think about System Design…(2)

SYSTEM SYSTEM DESIGNDESIGN

SKILLS/MANPOWER

INSTRUMENT/DEVICES

PERFORMANCES

COST$$$

Instrumentation System Design

Instrumentation defined as the art and science of measurement and control of process variables within a production, or manufacturing area

System refers to all of the parts and interactions between parts of a complex project

Design refers to a plan or convention for the construction of an object or a system

What is Instrumentation System Design???

What to choose for Instrumentation System Design? (1)

What to choose?What to choose?

Task: Design pick and place robot for IC industry use.

What to choose for Instrumentation System Design? (2)

What to choose?What to choose?

DAQ hardware

Computer bus

Computer system

Sensor

Analysis tools

Many different sensors on the market today to measure all type of natural phenomena.

Most common sensors for measuring seven of these phenomena to help you choose the best option for your application. 1. Temperature – thermocouples, thermistors and RTDs

(resistance temperature detector)2. Strain – axial, bending, torsional and shear3. Sound – microphones (condenser, piezoelectric,

magnetic)4. Vibration – ceramic piezoelectric, VRVS5. Position and displacement – optical encoders, half

effect sensor, potentiometers, eddy current sensor6. Pressure – gauge, vacuum7. Force – beam style, S Beam

1- Choosing a Sensor (1)


Sensor takes an input quantity and converts it to an output quantity. May be simple physical measurement systems, or complex electronic devices requiring sophisticated DAQ systems. No matter the type of sensor, input type, or output type, every sensor has inherent characteristics that allow the user to select the right sensor for the task at hand.


Some sensor characteristics include: Input Range Output Range Accuracy Repeatability Resolution

Input Range the maximum measureable range that the

sensor can accurately measure.Output Range

refers to electronic sensors, and is the range of electrical output signal that the sensor returns.

Accuracy refers to the amount of error, or inaccuracy,

that may be present in a sensor. Accuracy can be stated as a unit of

measurement.


Repeatability refers to how often a sensor under the

same input conditions will return the same value.

If a sensor is designed to be used over and over again, it is important that the output value is accurate over every measurement cycle for the life of the sensor.

Repeatability is determined by calibration testing of the sensor using known inputs.


Resolution the smallest unit of measurement that the

sensor can accurately measure. Some transducers return output signals in

discrete steps, and therefore the resolution is easily defined.

Resolution can be stated as a unit of measurement or as a percentage.

For electronic sensors, resolution is also dictated by the resolution of the signal conditioning hardware or software.


DAQ systems are used to collect data about the behavior of objects or systems. DAQ is the practice of collecting and storing data from sensors or other measurements equipment. Technically, DAQ techniques could include manual monitoring and recording methods.

2- Choosing a DAQ Hardware (1)

QuestionsQuestions before choosing DAQ devices:

1. What type of signals do I need to measure/generate?

2. Do I need signal conditioning?3. How fast do I need to acquire/

generate samples of signal?4. What is the smallest change in the

signal that I need to detect?5. How much measurement error does

my application allow?


Q1: Q1: What type of signals do I need to measure/ generate? Different types of signals need to be measured or generated in different ways. Sensor – device that converts a physical phenomenon into a measureable electrical signal, such as voltage or current Based on the signals in your application, you can start consider which DAQ to use. Functions of DAQ devices:

analog inputs measure analog signals analog outputs generate analog signals digital I/Os measure and generate digital signal counters/timers count digital events or generate

digital pulses/signals


devices support just one function or multifunction devices.

consider purchasing a device with more channels and has capability upgraded to new applications in future.

multifunction devices support different types of I/O, which gives you the ability to address many different applications that a single function DAQ device would not

another option is a modular platform that you can customize to your exact requirement.

advantage modular system: select different modules and also, the ability to select the number of slots.


Modular Platform

Q2: Q2: Do I need signal conditioning (SC)? general-purpose DAQ device can

measure/ generate +/-5V or +/-10V. some sensors generate signals to

difficult or dangerous to measure directly. most require signal conditioning, such as

amplification or filtering, before they can effectively and accurately measure a signal


Q2: Q2: Do I need signal conditioning (SC)? Example:

Thermocouples o/p signals in mV range that require amplification to optimize the limits of ADCs.

Additionally, thermocouples measurement benefits from LPF to remove high frequency.

SC provides distinct advantage over DAQ device alone because it enhance performance and accuracy of DAQ.


Q3: Q3: How fast do I need to acquire/ generate samples of signal? sampling rate – speed at which the DAQ device’s ADC takes sample of a signal. sampling rate depends on maximum frequency components of the signal that you are trying to measure or generate. Try to relate with Nyquist Theorem! Choosing a DAQ device with a sample rate at least 10x the frequency of your signal ensures that you measure or generate a more accurate representation of your signal.



Example: suppose in your application you want to measure a sine wave that has a frequency of 1kHz. According to Nyquist T., you must sample at 2kHz at least but it is highly recommended to sample at 10kHz to measure or generate a more accurate representation of your signal.

10kHz vs. 2kHz representation

of a 1kHzSine Wave

Q4: Q4: What is the smallest change in the signal that I need to detect?

Smallest detectable change in signal determines the resolution that is required of your DAQ device.

Resolution refers to the number of binary levels an ADC use to represent a signal

Try to imagine that how a sine wave would be represented if it were passed through an ADC with different resolution.


a 3-bit ADC can represent EIGHT discrete voltage levels while a 16-bit ADC can represent 65,536 discrete voltage levels

Representation of the sine wave with a 3-bit resolution looks more like a step function than a sine wave where the 16-bit ADC provides a clean-looking sine wave.


16-Bit resolution vs. 3-Bit resolution chart of a sine wave

Q5: How much measurement error does my application allow? accuracy – a measure of the capability of an instrument to faithfully indicate the value of a measured signal. Not related to resolution. how you specify the accuracy of your measurement depends on the type of measurement device. factors of uncertainty value – system noise, gain error, offset error, and nonlinearity. uncertainty absolute accuracy absolute accuracy = [(reading*gain) + (voltage range *offset) + noise uncertainty]


Data Acquisition Terminology DAQ systems and techniques have some

unique terminology. Some of the more prominent terms include:i. Excitation – A voltage or current input

into a sensor. The sensor reacts in some way to the input and produces an output voltage or current that is measured by the DAQ system.

ii. Gain – A gain circuit boosts the voltage of very low level signals so that they can be separated from background noise.


Data Acquisition Terminology (cont.)iii. Sample Rate – The number of data points

collected per unit of time. A fast phenomenon such as automobile collision testing requires very high sample rates to properly characterize the phenomena.

iv. Signal Conditioning – Additional processing that is performed on a data signal to improve its quality.


Data Acquisition Terminology (cont.) v. Signal-to-Noise Ratio – This ratio is the comparison of the amplitude of a data signal over the amplitude of the background noise in the data. The larger the ratio, the less likely the data will be lost within the noise.vi. Transducer – A transducer is another name for a sensor

** DAQ is a vital tool for engineers to understand how systems functionsystems function, and is an important part of system monitoring system monitoring and safety testingsafety testing **


a Bus - is a set of physical connections (cables, printed circuits, etc.) which can be shared by multiple hardware components in order to communicate with one another.

purpose - to reduce the number of "pathways" needed for communication between the components, by carrying out all communications over a single data channel.

If only 2 hardware components communicate over the line, it is called a hardware port (such as a serial port or parallel port).

3- Choosing a Computer Bus (1)

http://en.kioskea.net/contents/pc/serie.php3

http://en.kioskea.net/contents/pc/serie.php3


Computer System Bus Diagram


control bus transports orders and synchronization signals coming from the control unit and travelling to all other hardware components. it also transmits response signals from the hardware.

address bus : transports memory addresses which the processor wants to access in order to R/W data.

data bus transfers instructions coming from/going to the processor.

Characteristics of a bus: characterized by the amount of information that

can be transmitted at once. expressed in bits, corresponds to the number of

physical lines over which data is sent simultaneously.

Eg: A 32-wire ribbon cable can transmit 32 bits in parallel. The term "width" is used to refer to the number of bits that a bus can transmit at once.

Additionally, the bus speed is also defined by its frequency (expressed in Hertz), the number of data packets sent or received per second. Each time that data is sent or received is called a cycle.


http://en.kioskea.net/contents/base/binaire.php3

Characteristics of a bus (cont.): This way, it is possible to find the

maximum transfer speed of the bus, the amount of data which it can transport per unit of time, by multiplying its width by its frequency.

What is a transfer speed of a bus with a width of 16 bits and a frequency of 133 MHz?


Characteristics of a bus (cont.): What is a transfer speed of a bus with a

width of 16 bits and a frequency of 133 MHz? 16 * 133.106 = 2128*106 bit/s or 2128*106/8 = 266*106 bytes/s or

266*106 /1000 = 266*103 KB/s or 259.7*103 /1000 = 266 MB/s


When you have hundreds of different data acquisition (DAQ) devices to choose from on a wide variety of buses, it can be difficult to select the right bus for your application needs.

Each bus has different advantages and is optimized for throughput, latency, portability, or distance from a host.


Questions to be considered when choosing the right bus for your measurement system:1) How much data will I be streaming across

the bus?2) What are my single point I/O

requirement?3) Do I need to synchronize multiple

devices? 4) How portable should this system be?5) How far will my measurement be from

my computer?


Q1: How much data will I be streaming across the bus?

All PC buses have a limit to the amount of data that can be transferred in a certain period of time.

This is known as the bus bandwidth and is often specified in megabytes per second (MB/s).

Example: PCI bus has a theoretical bandwidth of 132

MB/s that is shared among all PCI boards in the computer.

Gigabit Ethernet offers 125 MB/s shared across devices on a subnet or network


Q1: How much data will I be streaming across the bus?

minimum required bandwidth = number of bytes per sample x sampling speed x number of channels.

Question: What is a bandwidth required of a 16-bit

device sampling at 4 MS/s on four channels?


Answer = 32MB/s

Actual observed bandwidth depends on the number of devices in a system and any additional bus traffic from overhead.

If you need to stream a lot of data on a large number of channels, bandwidth may be the most important consideration when choosing your DAQ bus.


Q2: What are my single point I/O requirement?

Applications that require single-point R/W are often dependent on I/O values to be updated immediately and consistently.

Based on how bus architectures are implemented in both hardware and software, single-point I/O requirements could be the determining factor for the bus that you choose.


Q2: What are my single point I/O requirement?

Bus latency is the responsiveness of I/O. Depending on the bus you choose, this delay could range from less than a microsecond to a few milliseconds.

Example: In PID control system, this bus latency can directly impact the maximum speed of the control loop.


Q3: Do I need to synchronize multiple devices?

Many measurement systems have complex synchronization needs, whether it is synchronizing hundreds of input channels or multiple types of instruments.

The simplest way to synchronize measurements across multiple devices is to share a clock and a trigger.

Many DAQ devices offer programmable digital lines for importing and exporting both clocks and triggers.


Q4: How portable should this system be?Portability is an important factor for many

applications, and could easily be the primary reason to choose one bus over another.

In-vehicle DAQ applications, benefit from hardware that is compact and easily transported. External buses like USB and Ethernet are particularly good for portable DAQ systems because of quick hardware installation and compatibility with laptop computers.


Q4: How portable should this system be? Bus-powered USB devices offer additional

convenience because they do not require a separate power supply to be present, and are powered off the USB port.

Using wireless data transfer buses is another good option for portability because the measurement hardware itself becomes portable while the computer can remain stationary.


Q5: How far will my measurement be from my computer?

To achieve the best signal integrity and measurement accuracy, you should place your DAQ hardware as close to the signal source as possible.

This can be a challenge for large distributed measurements like those used for structural health monitoring or environmental monitoring.


Q5: How far will my measurement be from my computer?

Running long cables across a bridge or factory floor is costly and can result in noisy signals.

One solution to this problem is to use a portable computing platform to move the entire system closer to the signal source.

With wireless technology, the physical connection between the computer and the measurement hardware is removed altogether, and you can take distributed measurements and send the data back to a central location.



There are many different buses and form factors to choose from.

You can choose based on your DAQ requirements

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Instrumentation System Design – part 1 Chapter6: