POLYTECHNIC UNIVERSITY OF THE PHILIPPINESCOLLEGE OF
ENGINEERINGELECTRONICS ENGINEERING DEPARTMENT
PRESENTERS:BERGANIO, JASMIN F.CASTRO, JAMELO B.ESCOTE, JIMMY
M.ESTANGCO, MARY ANGELINE L.FRIAS, MARA B.GANZALINO, MARK PAUL
S.SORIANO, PATRICK JIM O.BS ECE 4-4Presenter #1: BERGANIO, Jasmin
F.
1. Presented topic:
Introduction to Analog Process Control and Sensors
A. Objectives
The goal of this presentation is to provide a motivation for,
and an introduction to, process control and sensors. After studying
this material, the reader, should be able to grasp and be able to
recognize the concept behind the topic Introduction to Process
Control and Sensors and be able to do the following:
Will be able to identify and discuss each stage in a process
control system. Assess the importance of process control from
safety, environmental, and economic points of view Understand the
basic idea of actuators, control valves, control sensors,
transmitters & transducers and process sensors.
The major sections of this presentation are as follows:I.
Introduction to APCS a. Define Process Control systemb. Illustrate
and discuss the Process Control Flowc. Describe the Basic Process
Control componentsd. Application of APCS
B. Discussion
Sensors to measure process conditions and valves to influence
process operations are essential for all aspects of engineering
practice. Engineers want to design and operate processes that
remain in safe conditions, produce the desired amounts of high
quality products and are profitable. Therefore, engineers must
provide measuring devices for key variables and valves (or other
devices, such as variable speed electric motors) to influence of
steer the process. This discussion provides educational material on
sensors and valves for use in the analog process control
industries. While sensors and valves are important in all aspects
of engineering, they assume greatest importance in the study of
automatic control, which is termed process control when applied in
the process industries.
Definition of Process control system
Process control is the study of automatic control. It deals with
the regulation of processes by applying the feedback principle
using various computing devices, principally digital computation.
Process control requires sensors for measuring variables and valves
for implementing decisions. Therefore, the presentation of this
material is designed to complement other learning topics in process
control.
The Process Control Flow
The Figure shows basic elements of a feedback control system as
represented by a block diagram. The functional relationships
between these elements are easily seen. An important factor to
remember is that the block diagram represents flow paths of control
signals.
Below are several terms associated with the closed-loop block
diagram:
TheLogical Signal is an external signal applied to the summing
point of the control system to cause the plant to produce a
specified action. This signal represents the desired value of a
controlled variable and is also called the setpoint. The Signal
Processing and Amplification is where the input signals where
processing and amplified enabling the signal to be transferred to
the preceding elements of the process control flow. Theactuator
sensoris a function of the output signal. It is sent to the summing
point and algebraically added to the reference input signal to
obtain the actuating signal. The mechanism block is where the
control valves are located. These control are valves used to
control conditions such as flow, pressure, temperature, and liquid
level by fully or partially opening or closing in response to
signals received from controllers that compare a "setpoint" to a
"process variable" whose value is provided by sensors that monitor
changes in such conditions.
Basic Process Control componentsThese are the basic process
control components to be discussed: An actuator is a type of motor
that is responsible for moving or controlling a mechanism or
system. It is operated by a source of energy, typically electric
current, hydraulic fluid pressure, or pneumatic pressure, and
converts that energy into motion. Control valves are valves used to
control conditions such as flow, pressure, temperature, and liquid
level by fully or partially opening or closing in response to
signals received from controllers that compare a "setpoint" to a
"process variable" whose value is provided by sensors that monitor
changes in such conditions.[1] Control Valve is also termed as the
Final Control Element. A sensor is a device that detects and
responds to some type of input from the physical environment. The
specific input could be light, heat, motion, moisture, pressure, or
any one of a great number of other environmental phenomena.
Transducers are voltage-output devices that can be used with simple
signal conditioning but are more sensitive to electromagnetic
interference. Transmitters are current-output devices and may have
two or three wires. Where two wires are used to both receive power
and transmit an output signal, significant cost savings can be made
where long cables are needed.
Application
As an example for Analog Process Control system heres a shower
flow diagram. The cold water and hot water serves as the input
signal on the control system. The valve signifies the use of
control valves which opens the outlet in the shower which opens or
closes the shower valve. The actuator on this diagram is the
individual in the shower room which turns on or off the control
valve.
Presenter #2: CASTRO, Jaimelo B.
1. Presented topic:
Process Actuators and Output Devices
A. Objectives
This presentation aims to provide in-depth understanding about
process actuators and output devices. After studying this topic,
the reader, should be able to
Define what process actuators and output devices are. Know how
process actuators and output devices works. Know the common and
industrial types of process actuators. Know the different types of
output devices. Know the applications of process actuators.
The major sections of this presentation are as follows:I.Process
Actuators and Output Devicesa.Introduction to Process
Actuatorsb.Common Types of Actuatorsc.Industrial
Actuatorsd.Introduction to Output Devicese. Types of Output
devices
B. Discussion
Actuators are the final elements in a control system. They
receive a low power command signal and energy input to amplify the
command signal as appropriate to produce the required output.
Applications range from simple low power switches to high power
hydraulic devices operating flaps and control surfaces on aircraft;
valves, car steering, process plant automation, etc.
An Actuator converts the command signal from controllers or
higher-level components into physical adjustment in adjustable
process variable. Actuators drive motions in mechanical systems.
Most often this is by converting electrical energy into some form
of mechanical motion.
Figure 1.2Process flow in an actuator
Basic actuators are used to move valves to either fully opened
or fully closed positions. Actuators for control or position
regulating valves are given a positioning signal to move to any
intermediate position with a high degree of accuracy. Although the
most common and important use of an actuator is to open and close
valves, current actuator designs go far beyond the basic open and
close function. The valve actuator can be packaged together with
position sensing equipment, torque sensing, motor protection, logic
control, digital communication capacity and even PID control all in
a compact environmentally protected enclosure.
Common Types of Actuators Manual actuators do not require an
outside power source to move a valve to a desired position.
Instead, they use a hand wheel, chain wheel, lever, or declutchable
mechanism to drive a series of gears whose ratio results in a
higher output torque compared to the input (manual) torque. Most
manual actuators use worm gears, mechanical devices that transmit
motion between non-intersecting right-angle axes. Electric actuator
has a motor drive that provides torque to operate a valve. Electric
actuators are frequently used on multi-turn valves such as gate or
globe valves. With the addition of a quarter-turn gearbox, they can
be utilized on ball, plug, or other quarter-turn valves.
Electromagnetic Actuators exploits the mutual attraction of soft
ferrous materials in a magnetic field. The device has one coil
which provides the field energy and the energy to be transformed.
The attractive force is unidirectional such that the return device
of some type is needed, often a spring. Relays or solenoids are
based on this principle which is widely used in cars to switch a
range of electrical equipment with a current demand of more than
about 10Amps examples include in fans, head lights, horn, and
wipers. Electrodynamic Actuators is based on the (Lorenz) force
generated when a current carrying conductor (often in the form of a
coil) is held in a magnetic field. DC motors are frequently used as
part of an actuator system. Hydraulic and Pneumatic actuators are
often simple devices with a minimum of mechanical parts, used on
linear or quarter-turn valves. Sufficient air or fluid pressure
acts on a piston to provide thrust in a linear motion for gate or
globe valves. Alternatively, the thrust may be mechanically
converted to rotary motion to operate a quarter-turn valve. Most
types of fluid power actuators can be supplied with fail-safe
features to close or open a valve under emergency
circumstances.
Industrial Actuators Solenoids Solenoids are the most common
actuator components. The basic principle of operation is that,
there is a moving ferrous core (a piston) that will move inside
wire coil as shown in Figure 3.1. Normally the piston is held
outside the coil by a spring. When a voltage is applied to the coil
and current flows, the coil builds up a magnetic field that
attracts the piston and pulls it into the center of the coil. The
piston can be used to supply a linear force. Well known
applications of these include in pneumatic values and car door
openers.
Figure 3.1Operation of Solenoid Actuators
Piston and Cylinder Actuators A cylinder uses pressurized fluid
or air to create a linear force/motion. A fluid is pumped into one
side of the cylinder under pressure causing that side of the
cylinder to expand, and advancing the piston. The fluid on the
other side of the piston must be allowed to escape freely - if the
incompressible fluid was trapped the cylinder could not advance.
The force the cylinder can exert is proportional to the cross
sectional area of the cylinder.Output DevicesAnoutput deviceis any
piece ofcomputer hardwareequipment used to communicate the results
ofdata processingcarried out by aninformation processing
system(such as acomputer) which converts the electronically
generated information into human-readable form.Types of Output
Devices Graphics (Visual) Adigital imageis a numeric representation
of an image stored on a computer. They don't have any physical size
until they are displayed on a screen or printed on paper. Until
that point, they are just a collection of numbers on the computer's
hard drive that describe the individual elements of a picture and
how they are arranged.
Tactile Atactile feedbacktechnology takes advantage of the sense
of touch by applying forces, vibrations, or motions to the
user.Several printersand wax jet printers have the capability of
producing raised line drawings. There are also handheld devices
that use an array of vibrating pins to present a tactile outline of
the characters or text under the viewing window of the device.
Audio Speech output systems can be used to read screen text to
computer users. Special software programs calledscreen
readersattempt to identify and interpret what is being displayed on
the screen and speech synthesizers convert data to vocalized sounds
or text.
Presenter #3: ESTANGCO, Mary Angeline L.
1. Presented topic:
Control Valves
A. Objectives
The aim of this presentation is to provide important information
about control valves and its relevance in the industry and in our
daily existence. After studying, this material will let the reader
to be familiar with: The fundamental nature of control valves and
how it works in a system. Distinguishing the relevance and
application of control valves. Identifying the things to be
considered in selecting control valves.
The major sections of this presentation are as follows:I.
Introduction to Control Valvesa. Definition of control valvesb.
Explanation of each parts of control valvesc. Types of control
valvesd. Classifications of control valves
B. Discussion
Control ValvesThe most common method for influencing the
behavior of chemical processes is through the flow rate of process
streams. Usually, avariable resistancein the closed conduit or pipe
is manipulated to influence the flow rate and achieve the desired
process behavior. A valve with a variable opening for flow is the
standard equipment used to introduce this variable resistance. The
valve is selected because it is simple, reliable, relatively low
cost and available for a wide range of process applications. In
some cases the valve resistance is set by a person adjusting the
opening, like a home faucet. In many cases, the valve resistance is
determined by an automatic controller, with the valve designed to
accept and implement the signal sent from the controller.These are
control valves.
It is a power-operated device used to modify the fluid or
substance flow rate in a process system.The control valve
manipulates a flowing fluid, such as gas, steam, water, or chemical
compounds, to compensate for the load disturbance and keep the
regulated process variable as close as possible to the desired set
point.Parts of Control Valves GlobeThe name "globe" refers to the
external shape of the valve, not the internal flow area.A typical
globe valve has a stem that is adjusted linearly (up and down) to
change the position of the plug. As the plug changes, the area for
flow between the plug and seat (opening) changes.Many different
seat and plug designs are available to achieve desired
relationships between the stem position and flow rate; see the
discussion on valve characteristic below.The standard plug must
oppose the pressure drop across the valve, which is acceptable for
small pressure drops. BallThe restriction for this body is a solid
ball which has some part of the ball removed to provide an
adjustable area for flow.The ball is rotated to influence the
amount of flow.The example ball valve displayed through the link
below has a tunnel through the ball, and the ball is rotated to
adjust the fraction of the tunnel opening available for flow.Other
types of ball valves have different sections removed from the ball
to give desired properties. ButterflyThe butterfly valve provides a
damper that is rotated to adjust the resistance to flow.This valve
provides a small pressure drop for gas flows. DiaphragmThe
diaphragm valve has one surface which is deformed by the force from
the valve stem to vary the resistance to flow. GateThese valves
have a flat barrier that is adjusted to influence the area for
flow.These bodies are used primary for hand-operated valves and
valves automated for emergency shutoff.
Types of Control Valves Rotary-motion Control ValveRotary type
valves, often called quarter-turn valves, include plug valves, ball
valves and butterfly valves. All require a rotary motion to open
and close, and can easily be fitted with actuators. Linear-motion
Control ValveLinear types include globe valves and slide
valves.
Classifications of Control Valves The two-port valves which
throttle or restrict the fluid passing through them.
a. Globe valvesGlobe valves are frequently used for control
applications because of their suitability for throttling flow and
the ease with which they can be given a specific 'characteristic',
relating valve opening to flow.
b. Slide valvesSlide valves tend to come in two different
designs; wedge gate type and parallel slide type. Both types are
well suited for isolating fluid flow, as they give a tight shut-off
and, when open, the pressure drop across them is very small. c.
Eccentric plug valvesThese valves are normally installed with the
plug spindle horizontal as shown, and the attached actuator
situated alongside the valved. Ball valvesThe ball has a hole
allowing fluid to pass through. When aligned with the pipe ends,
this gives either full bore or nearly full bore flow with very
little pressure drop. Rotating the ball through 90 opens and closes
the flow passage. e. Butterfly valvesIn the open position the disc
is parallel to the pipe wall, allowing full flow through the valve.
In the closed position it is rotated against a seat, and
perpendicular to the pipe wall.
The three-port valves which can be used to mix or divert liquid
passing through them.
a) Piston valve typeThis type of valve has a hollow piston which
is moved up and down by the actuator, covering and correspondingly
uncovering the two-port A and B. b) Globe plug typeThe actuator
pushes a disc or pair of valve plugs between two seats increasing
or decreasing the flow through ports A and B in a corresponding
manner.
c) Rotating shoe typeThis type of valve employs a rotating shoe,
which shuttles across the port faces.
Issues in Selecting Control Valves CapacityThe maximum flow rate
through the flow system (pipes, valves, and process equipment) must
meet operating requirements.Guidelines are available for
calculating the pipe diameter for a desired flow rate, and
guidelines are given here for the percentage of the system pressure
drop contributed by the valve. RangeThe range indicates the extent
of flow values that the valve can reliably regulate; very small and
large flows cannot be maintained at desired values. Pressure
dropThe purpose of the valve is to create a variable pressure drop
in the flow system.However, a large pressure drop wastes energy.In
some systems, the energy costs for pumping or compressing can be
very high, and the pressure drop introduced by the valve should be
as small a practically possible. PrecisionIdeally, the valve would
move to exactly the position indicated by the signal to the valve,
which is usually a controller output.However, the valve is a real
physical device that does not perform ideally. DynamicsThe valve is
part of the feedback system, and any delay due to the valve slows
the feedback correction and degrades control performance.Therefore,
the valve should achieve the desired opening rapidly. CostEngineers
must always consider cost when making design and operations
decisions.Valves involve costs and when selected properly, provide
benefits.These must be quantified and a profitability analysis
performed. In some cases, a valve can affect the operating costs of
the process, where the pumping (or compression) costs can be high,
and the pressure drop occurring because of the valve can
significantly increase the pumping costs.In such situations, a
valve with a low (non-recoverable) pressure drop is
selected.Presenter #4: GANZALINO, Mark Paul S.
1. Presented topic:
Introduction to Control Sensors
A. Objectives
The aim of this presentation is to broaden each and everyones
knowledge on the topic Introduction to Control Sensors which will
educate us about its significance not just in the industry but also
in our daily lives. This presentation also intends to let the
readers:
Understand the essence of sensors in any control and automation
system. Apprehend the main parameters that are being measured and
controlled by the control sensors Perceive the basic knowledge on
the issues that they might encounter in using and/ or when choosing
the appropriate control sensors
The major sections of this presentation are as
follows:I.Introduction to Control Sensorsa.Define and illustrate
purpose of Control Sensorsb.Discuss the main parameters measure and
controlled by Control Sensorsc.Enumerate and explain the issues
encountered in sensor selection
B. Discussion
Control SensorsSensors play a major role in any control and
automation system. Without sensors the apparatus is essentially
running blind. The sensors convey details of the status of the real
world in a more detailed way than any human senses can. They exist
to cover most measurable quantities of the real world and include
motion, temperature, proximity, light, sound, moisture and many
more.
Control Sensors are used for process monitoring and for process
control.These are essential elements of safe and profitable plant
operation that can be achieved only if the proper sensors are
selected and installed in the correct locations.While sensors
differ greatly in their physical principles, their selection can be
guided by the analysis of a small set of issues.
Many types of sensors have been developed during the past
several years. Some those are being used in the military, medicine,
automotive applications, and others but it mostly used for
INDUSTRIAL PROCESS CONTROL. PROCESS CONTROL SENSORS plays a
significant role in improving productivity, qualitatively and
quantitatively.
ParametersThere are main parameters to be measured and
controlled in industrial plants. Those are temperature,
displacement, force, pressure, fluid level, and flow. In addition,
detectors for leakage of explosives or combustible gases and oils
are important for accident prevention. And, from these parameters
different types of sensors are being made.
TEMPERATURE Temperature control is important for separation and
reaction processes, and temperature must be maintained within
limits to ensure safe and reliable operation of process
equipment.
PRESSURE Most liquid and all gaseous materials in the process
industries are contained within closed vessels. For the safety of
plant personnel and protection of the vessel, pressure in the
vessel is controlled. In addition, pressured is controlled because
it influences key process operations like vapor-liquid equilibrium,
chemical reaction rate, and fluid flow.
FLOW Flow measurement is critical to determine the amount of
material purchased and sold, and in these applications, very
accurate flow measurement is required. In addition, flows
throughout the process should the regulated near their desired
values with small variability; in these applications, good
reproducibility is usually sufficient. Flowing systems require
energy, typically provided by pumps and compressors, to produce a
pressure difference as the driving force, and flow sensors should
introduce a small flow resistance, increasing the process energy
consumption as little as possible.
LEVEL Level of liquid in a vessel should be maintained above the
exit pipe because if the vessel empties the exit flow will become
zero, a situation that would upset downstream processes and could
damage pumping equipment that requires liquid. Also, the level
should not overflow an open vessel nor should it exit through a
vapor line of a closed vessel, which could disturb a process
designed for vapor.
POSITION Displacement sensors measure the distance an object
moves and they can also be used to measure object height and width.
IssuesFrom the main parameters, the major issues in sensor
selection are summarized in the following: Accuracy-Accuracy is the
degree of conformity of the measured value with the accepted
standard or ideal value, which we can take as the true physical
variable. Repeatability The closeness of agreement among a number
of consecutive measurements of the same variable (value) under the
same operating conditions, approaching in the same direction.
Reproducibility The closeness of agreement among a number of
consecutive measurements of the same variable (value) under the
same operating conditions over a period of time, approaching from
both directions. Range/Span - Most sensors have a limited range
over which a process variable can be measured, defined by the lower
and upper range values. Reliability Reliability is the probability
that a device will adequately perform (as specified) for a period
of time under specified operating conditions. Linearity -This is
the closeness to a straight line of the relationship between the
true process variable and the measurement. Maintenance - Sensors
require occasional testing and replacement of selected components
that can wear.Engineers must know the maintenance requirements so
that they can provide adequate spare parts and personnel time.
Dynamics - The use of the sensor dictates the allowable delay in
the sensor response.When the measured value is used for control,
sensor delays should be minimized, while sensors used for
monitoring longer-term trends can have some delay. Safety -The
sensor and transmitteroften require electrical power.Since the
sensor is located at the process equipment, the environment could
contain flammable gases, which could explode when a spark occurs.
Cost- Engineers must always consider cost when making design and
operations decisions.Sensors involve costs and when selected
properly, provide benefits.These must be quantified and a
profitability analysis performed. The relative importance of each
issue depends upon the specific application; for example, one
application might require excellent accuracy, while another might
require only moderate accuracy, but high reliability.Generally, we
find that the greater the requirements for good performance, the
higher the cost for purchase and maintenance. Therefore, we must
find the proper balance of performance and cost, rather than always
specify the best performing sensor.
Presenter #5: SORIANO, Patrick Jim O.
1. Presented topic:
Transducers and Transmitters
A. Objectives
The intent of this presentation is to provide the fundamental
knowledge on the topic Transducers and Transmitters. This
presentation also wants to:
Discuss and define transmitters and transducers Illustrate how
do transmitters and transducers operates Explain the differences
between transmitters and transducers
The major sections of this presentation are as
follows:I.Transducers and Transmittersa.Definition and functions of
Transducersb.Effectiveness factor of a transducerc.Classification
of Transducersd. Definition and functions of Transmitters
B. Discussion
Transducers are simply the transformation of a form of energy
into another basically electric. These parameters often contain
voltage levels at which the transducer system practically read at
levels linear to parameter change. There is also inverse transducer
which would read the electrical signals into a form desired by the
user as another energy type. Inverse transducers are often called
indicators. Transmitters, on the other hand serves as an indicator
that reads electrical signal but forms into electrical signal
capable of transmission. This signal is then receive by a inverse
transducer capable of decoding information and transform into
another energy form.
Transducer It is a device used to convert one form of energy
into electrical signals. Also, it is capable of switching a
physical quantity into an electrical quantity such as voltage or
current.
Transducers are being used to convert a physical occurrence into
an equivalent electrical output. Transducer devices have two basic
parts the sensing element and the transducer element. Sensing
element functions as the physically component that reads the
physical signals in different parameters, such as pressure,
temperature, position and etc. The transducer element functions as
the reactive component to the input signal and converts it into an
equivalent intensity of reaction.
Effectiveness factor of a Transducer Sensitivity device must be
sensitive enough to produce readable voltage or current output.
Operating range must reach deep minimum and high maximum working
intensity. Accuracy must perform according physical interface.
Error must prevent high percent errors. Operating Principle it must
be used depending on the needed specifications and parameters
present in the environment. Transient and frequency response should
meet desired time domain specification such as V overshoot, rise
time, setting time and small dynamic errors. Loading effect should
have a high output impedance and low input impedance to avoid
loading effects. Environmental Compatibility should be assured that
the transducer being used in a certain environmental condition will
not be affected by other environmental phenomenon. Insensitivity to
Unwanted signal it should minimize capture of unwanted signal and
maintain good reception of wanted signals.
Classification of Transducers 1) Analog and DigitalAnalog -
converts input into output into continuous time function. Digital
converts input into output into form of time based pulses.
2) Primary and SecondaryPrimary when signal is sensed by
transducer and converts it into electrical signal directly.
Secondary when signal that serves as input is not a direct physical
parameter, in other words came from the primary transducer, it is
classified as secondary.
3) Transducers and Inverse TransducersTransducers Converts
non-electrical quantity into an electrical quantity.Inverse
Converts an electrical quantity into a non-electrical quantity. 4)
Active and PassiveActive if the transducer operates if and only if
there is a voltage source applied in the system. Passive
transducers that operates even without the use of voltage
source.
Transmitters It is similar to the transducers but it is a device
with a current output. Trasmitters are electronics that would allow
the transducers millivolt signal output to be processed and
converted to current for transmission over long distances. Since
current are virtually unaffected by radio frequency interferences
unlike voltage which are sensitive and to it and would add noise to
its reading in addition to being intolerant to resistance, usually
a two-wire transmitter is used with 4-20mA output.
Presenter #6: ESCOTE, Jimmy M.
1. Presented topic:
Process Sensors: Temperature, Pressure and Flow
A. Objectives
This topic aims to give knowledge about different types of
sensors and how they work. It will educate students about the basic
functions of sensors and will broaden their thinking about it which
will led in understanding the use of sensors in our industry. The
readers were expected to learn the following:
Understand the use of sensors in machinery, factories and in
homes. Have knowledge in selecting sensors that they will use for a
project. Different parameters and factors that may affect the
functions of sensors
The major sections of this presentation are as
follows:I.Different Process Sensorsa.Defining temperature, pressure
and flow sensorsb.Comparing the three sensors c.Application of the
sensors in industry
B. Discussion
Temperature SensorsTemperature control is important for
separation and reaction processes, and temperature must be
maintained within limits to ensure safe and reliable operation of
process equipment. Temperature can be measured by many methods;
several of the more common are described in this subsection. You
should understand the strengths and limitations of each sensor, so
that you can select the best sensor for each application. In nearly
all cases, the temperature sensor is protected from the process
materials to prevent interference with proper sensing and to
eliminate damage to the sensor. Thus, some physically strong,
chemically resistant barrier exists between the process and sensor;
often, this barrier is termed a sheath orthermowell, especially for
thermocouple sensors. An additional advantage of such a barrier is
the ability to remove, replace, and calibrate the sensor without
disrupting the process operation.
Types of Temperature Sensors Thermocouples:When the junctions of
two dissimilar metals are at different temperatures, an
electromotive force (emf) is developed. The cold junction, referred
to as the reference, is maintained at a known temperature, and the
measuring junction is located where the temperature is to be
determined. The temperature difference can be determined from the
measured emf. ResistanceTemperature Detectors (RTD):The electrical
resistance of many metals changes with temperature; metals for
which resistance increases with temperature are used in RTDs.
Thermistor:This sensor is similar to an RTD, but applies metals for
which the resistance decreases with increasing temperature. The
relationship is often very nonlinear, but thermistors can provide
very accurate temperature measurements for small spans and low
temperatures Bimetallic:Metals expand with increasing temperature,
and the rate of expansion differs among metals. A spiral
constructed of two bonded metal strips will coil (uncoil) as the
temperature changes. The changing position of the coil can be
detected and used to determine the temperature. This provides a
rugged, low cost sensor that is often used for local displays and
for on-off temperature control, i.e., a thermostat. Filled
systems:A fluid expands with increasing temperature and exerts a
varying pressure on the containing vessel. When the vessel is
similar to a bourbon tube, the varying pressure causes a
deformation that changes the position detected to determine the
temperature.
Pressure SensorsMost liquid and all gaseous materials in the
process industries are contained within closed vessels. For the
safety of plant personnel and protection of the vessel, pressure in
the vessel is controlled. In addition, pressured is controlled
because it influences key process operations like vapor-liquid
equilibrium, chemical reaction rate, and fluid flow.Types of
Pressure Sensorsa. Based on Mechanical Principles Bourdon : A
bourbon tube is a curved, hollow tube with the process pressure
applied to the fluid in the tube. The pressure in the tube causes
the tube to deform or uncoil. The pressure can be determined from
the mechanical displacement of the pointer connected to the Bourdon
tube. Typical shapes for the tube are C (normally for local
display), spiral and helical. Bellows: A bellows is a closed vessel
with sides that can expand and contract, like an accordion. The
position of the bellows without pressure can be determined by the
bellows itself or a spring. The pressure is applied to the face of
the bellows, and its deformation and its position depend upon the
pressure. Diaphragm:A diaphragm is typically constructed of two
flexible disks, and when a pressure is applied to one face of the
diaphragm, the position of the disk face changes due to
deformation. The position can be related to pressure. b. Based on
Electrical Principles Capacitive or inductance:The movement
associated with one of the mechanical sensors already described can
be used to influence an electrical property such as capacitance
affecting a measured signal. For example, under changing pressure a
diaphragm causes a change in capacitance or inductance. Resistive,
strain gauge:The electrical resistance of a metal wire depends on
the strain applied to the wire. Deflection of the diaphragm due to
the applied pressure causes strain in the wire, and the electrical
resistance can be measured and related to pressure. Piezoelectric :
A piezoelectric material, such as quartz, generates a voltage
output when pressure is applied on it. Force can be applied by the
diaphragm to a quartz crystal disk that is deflected by process
pressure.
Flow SensorsFlow measurement is critical to determine the amount
of material purchased and sold, and in these applications, very
accurate flow measurement is required. Flows throughout the process
should the regulated near their desired values with small
variability; in these applications, good reproducibility is usually
sufficient. Flowing systems require energy, typically provided by
pumps and compressors, to produce a pressure difference as the
driving force, and flow sensors should introduce a small flow
resistance, increasing the process energy consumption as little as
possible.
Types of Flow Sensors Orifice: An orifice plate is a restriction
with an opening smaller than the pipe diameter which is inserted in
the pipe; the typical orifice plate has a concentric, sharp edged
opening. Venturi Tube: The venturi tube shown in Figure 2 is
similar to an orifice meter, but it is designed to nearly eliminate
boundary layer separation, and thus form drag. The change in
cross-sectional area in the venturi tube causes a pressure change
between the convergent section and the throat, and the flow rate
can be determined from this pressure drop. Flow Nozzle: A flow
nozzle consists of a restriction with an elliptical contour
approach section that terminates in a cylindrical throat section.
Pressure drop between the locations one pipe diameter upstream and
one-half pipe diameter downstream is measured. Elbow meter: A
differential pressure exists when a flowing fluid changes direction
due to a pipe turn or elbow. The pressure difference results from
the centrifugal force Pitot tube and annubar:The pitot tube, shown
in Figure 4 below, measures the static and dynamic pressures of the
fluid atone pointin the pipe. The flow rate can be determined from
the difference between the static and dynamic pressures which is
the velocity head of the fluid flow. Turbine: As fluid flows
through the turbine, it causes the turbine to rotate with an
angular velocity that is proportional to the fluid flow rate. The
frequency of rotation can be measured and used to determine flow.
Vortex shedding: Fluid vortices are formed against the body
introduced in the pipe. These vortices are produced from the
downstream face in a oscillatory manner. The shedding is sensed
using a thermistor and the frequency of shedding is proportional to
volumetric flow rate. Positive displacement: In these sensors, the
fluid is separated into individual volumetric elements and the
number of elements per unit time is measured. These sensors provide
high accuracy over a large range. An example is a wet test
meter.
APPLICATIONSa. Temperature Sensor T-GAGE M18T Series Temperature
Sensors
b. Pressure Sensor Digital air pressure sensor
c. Flow Sensor Water ManagementOne of our most precious
resources needs to be managed responsibly and within a clearly
reported framework. We understand the increasing pressures that
exist within this industry and have a great number of specialized
tools for this purpose.
Presenter #7: FRIAS, Mara B.
1. Presented topic:
Process Sensors: Level and Position
A. Objectives
The presentation aims to discuss further about sensors
specifically level and position parameters. The topic also deals
with:
Different means of parameter changes and indications Functions
of different types of position and level sensors
The major sections of this presentation are as
follows:I.Different Process Sensorsa.Defining position and level
sensorsb.Discussing the types of position and level sensors
B. Discussion
Sensors is atransducerwhose purpose is tosense(that is,
todetect) some characteristic of its environments. It detects
events or changes in quantities and provides a corresponding
output, generally as an electrical or optical signal; for example,
athermocoupleconverts temperature to an output voltage.
Position SensorsBasically it is a sensor that reacts from an
input of position or mechanism. Position sensors are basically
sensors for measuring the distance traveled by the body starting
from its reference position. How far the body has moved from its
reference or initial position is sensed by the position sensors and
often the output is given as a fed back to the control system which
takes the appropriate action. Motion of the body can be rectilinear
or curvilinear; accordingly, position sensors are called linear
position sensors or angular position sensors.A ball mouse is an
example of rotary position sensor.Types of Position SensorsPosition
sensors use different sensing principles to sense the displacement
of a body. Depending upon the different sensing principles used for
position sensors, they can be classified as follows:1.
POTENTIOMETRIC POSITION SENSOR Uses resistive effect as the sensing
principle. The sensing element is simply a resistive (or
conductive) track. A wiper is attached to the body or part of the
body whose displacement is to be measured. The wiper is in contact
with the track. As the wiper (with the body or its part) moves, the
resistance between one end of the track and the wiper changes.
Thus, the resistance becomes a function of the wiper position. The
change in resistance per unit change in wiper position is linear.2.
CAPACITIVE SENSORS Uses an alternating voltage which causes the
charges to continually reverse their positions. The moving of the
charges creates an alternating electric current which is detected
by the sensor. The amount of current flow is determined by the
capacitance, and the capacitance is determined by the area and
proximity of the conductive objects. Larger and closer objects
cause greater current than smaller and more distant objects. The
capacitance is also affected by the type of nonconductive material
in the gap between the objects.3. LVDT DISPLACEMENT TRANSDUCER
comprises 3 coils; a primary and two secondaries. The transfer of
current between the primary and the secondaries of the LVDT
displacement transducer is controlled by the position of a magnetic
core called an armature. At the centre of the position measurement
stroke, the two secondary voltages of the displacement transducer
are equal but because they are connected in opposition the
resulting output from the sensor is zero. As the LVDTs armature
moves away from centre, the result is an increase in one of the
position sensor secondaries and a decrease in the other. This
results in an output from the measurement sensor.4.
MAGNETOSTRICTION Transducer can be used as a sensor and also for
actuation as thetransducercharacteristics is very high due to the
bi-directional coupling between mechanical and magnetic states of
the material. By measuring the amount of time between the
electronic pulse's launch and the strain pulse's arrival, a
magnetostrictive sensor can determine the precise positional
difference.5. EDDY-CURRENT SENSORS Operates with magnetic fields.
The driver creates an alternating current in the sensing coil in
the end of the probe. This creates an alternating magnetic field
with induces small currents in the target material; these currents
are called eddy currents. The eddy currents create an opposing
magnetic field which resists the field being generated by the probe
coil. The interaction of the magnetic fields is dependent on the
distance between the probe and the target. As the distance changes,
the electronics sense the change in the field interaction and
produce a voltage output which is proportional to the change in
distance between the probe and target. The target surface must be
at least three times larger than the probe diameter for normal,
calibrated operation; otherwise, special calibration may be
required.6. HALL EFFECT SENSORS are devices which are activated by
an external magnetic field. We know that a magnetic field has two
important characteristics flux density, (B) and polarity (North and
South Poles). The output signal from a Hall effect sensor is the
function of magnetic field density around the device. When the
magnetic flux density around the sensor exceeds a certain pre-set
threshold, the sensor detects it and generates an output voltage
called theHall Voltage, VH. 7. OPTICAL SENSORS Can measure changes
from several light sources at once, as in digital and electronic
relays, making them common in automated industrial processes.
Optical sensors can be either internal or external. External
sensors gather and transmit a required quantity of light, while
internal sensors are more often used to measure bends and other
small changes in direction. For optical sensors to work
effectively, they must be the correct type for the application, so
that they retain their sensitivity to the property they measure. In
addition they must not be able to measure any other property.
Optical sensors are integral parts of many common devices,
including computers, copy machines and light fixtures that turn on
automatically in the dark. Common applications include alarm
systems, synchros for photographic flashes and systems that can
detect the presence of objects.8. FIBER OPTICAL CABLES May be used
in intrinsic source and reacts to pressure from material enclosure
relevant to positioning.
Level SensorA level sensor detect thelevelof liquids and
otherfluidsand fluidized solids,
includingslurries,granularmaterials, andpowders that exhibit an
upperfree surface.
The level measurement can be either continuous or point
values.Continuous level sensors measure level within a specified
range and determine the exact amount of substance in a certain
placePoint-level sensors only indicate whether the substance is
above or below the sensing point.
Types of Level Sensor1. OPTICAL LEVEL SENSOR optical liquid
level sensors have no moving parts and are ideal for point level
sensing of water and other liquids. These optical level sensors
provide a switched output that can sense the presence or absence of
fluid.
2. CONDUCTIVE LEVEL SENSOR Conductivity level control systems
rely on the liquid level sensing capability of probes (electrodes)
that are housed inside fittings designed to accommodate one or more
probes.3. INDUCTIVE LEVEL SENSOR Are non-contact proximity devices
that set up a radio frequency field with an oscillator and a coil.
The presence of an object alters this field and the sensor is able
to detect this alteration.4. HYDRO STATIC LEVEL SENSOR The level
can be read as the function of the head pressure with calibration
in inches of water column or psi taking into account the specific
gravity of the fluid.
C. Questions with Answers
C.1 Definition of Terms:1. __________ is the study of automatic
control. It deals with the regulation of processes by applying the
feedback principle using various computing devices, principally
digital computation. ANS: Process Control
2. __________ conveys details of the status of the real world in
a more detailed way than any human senses can.ANS: Sensors
3. It refers to the degree of conformity of the measured value
with the accepted standard or ideal value, which we can take as the
true physical variable.ANS: Accuracy
4. It converts the command signal from controllers or
higher-level components into physical adjustment in adjustable
process variable.ANS: Actuator
5. Rotary-motion Control Valve is a type of control valve that
is often called.ANS: Quarter-Turn Valves
6. _______ is one of the classifications of valves which can be
used to mix or divert liquid passing through them.ANS: Three-port
Valves
7. A device or object that transforms energy into another
form.ANS: Transducers
8. It refers to transducer that operates even without the
presence of voltage source.ANS: Passive Transducer
9. A sensor that reacts from input of position or mechanism.ANS:
Position Sensor
10. A sensor that detects thelevelof liquids and otherfluidsand
fluidized solids, includingslurries,granularmaterials, andpowders
that exhibit an upperfree surface.ANS: Level Sensor
C.2 Enumeration:1. List all (5) the parameters that are being
measure and controlled by control sensors.ANS: Temperature, Flow,
Pressure, Level and Position
2. Enumerate the five (5) common types of actuators.ANS: Manual
Actuators, Electric Actuators, Electromagnetic Actuators,
Electrodynamic Actuators and Hydraulic and Pneumatic Actuators
3. Enumerate the types of control valves.ANS: Rotary-motion
Control Valve, Linear-motion Control Valve
4. Enumerate at least 3 stages in a process control system.ANS:
Logical Signal, Signal Processing and amplification, Control
element stage, Actuator sensor
5. Enumerate all the process sensors.ANS: Temperature Sensor,
Pressure Sensor, Flow Sensor, Position Sensor and Level Sensor
C.3 Multiple Choices:1. These are used for process monitoring
and for process control.These are essential elements of safe and
profitable plant operation that can be achieved only if the proper
sensors are selected and installed in the correct locations.a.
control Valvesb. control Sensorsc. Control Actuatorsd. none of the
aboveANS: C
2. A_______________ takes advantage of the sense of touch by
applying forces, vibrations, or motions to the user.a. touch
feedback technologyb. touch feedback devicesc. tactile feedback
devicesd. Tactile feedbacktechnologyANS: D
3. Actuators are the _______________ in a control system.a.
first elementb. Final elementc. first processd. final processANS:
B
4. _______ is a power-operated device used to modify the fluid
or substance flow rate in a process system.a. process Controlb.
sensorc. Control Valved. all of the aboveANS: C
5. _______ is a type of control valve that is often called
quarter-turn valves.a. Rotary-motion Control Valveb. linear-motion
Control Valvec. vertical-motion Control Valvesd. horizontal-motion
Control ValvesANS: A
6. _______ is one of the issues in selecting control valves
which indicates the extent of flow values that the valve can
reliably regulate; very small and large flows cannot be maintained
at desired values.a. Rangeb. capacityc. precisiond. pressure
dropANS: A
7. Conversion of Light to electricity is often termed as.a.
Photonelectricity conversionb. photoelectric effectc. luminosity
volt leveld. light Energy generatorANS: A
8. Device use to read sound response.a. microphoneb. speakerc.
piezo crystald. Both a and cANS: D
9. This sensor is similar to an RTD, but applies metals for
which the resistance decreases with increasing temperature.a.
thermocoupleb. Thermistorc. bimetallic d. both a and cANS: B
10. It is consists of a restriction with an elliptical contour
approach section that terminates in a cylindrical throat section.a.
orifice plateb. venturi tubec. Flow nozzled. elbow meterANS: C
C.4 Real Life Engineering Problem:1. As automation is adopted in
more facilities, physical work is being replaced by machines and
their automatic controls. The need for valve actuators to provide
the interface between the control intelligence and the physical
movement of a valve has grown. There is an important need for the
increased working safety and the environmental protection that
valve actuators can provide. Some areas are hazardous or hostile to
human beings. In these circumstances an automated actuation device
can reduce the risk to the individuals. Certain critical valves
need to be opened or closed rapidly in the event of emergency
circumstances. Hence, if you are assigned to design a generic
process control systems how will you designed it in response to
needs as said earlier? Note: Use a block diagram to design a
generic process control system. You are free to use any type of
actuators or control valves.ANS:
2. Generally, we find that the greater the requirements for good
performance, the higher the cost for purchase and maintenance. How
can an individual/engineer select a good and suitable control valve
for a specific control system?ANS: We must find the proper balance
of performance and cost rather than always specify the best
performing valve. An individual/engineer should refer to accepted
standards provided such as the capacity, range, pressure drop,
precision, dynamics and cost.
3. Site a real life application of analog process control system
and identify its stages related to process control
system.ANS:Shower flow diagram example. or (Any example with
actuators, control valve, sensors, input signal etc.)D.
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