lecture #2 (instrumentation system elements) v1

EBGE 2061: Process Control and Instrumentation

EBGE 2061:Process Control and Instrumentation

Lecture #2

Instrumentation System Elements

Outline Topics

Introduction

Sensor selection

Displacement Sensors

Speed sensors

Fluid Pressure sensors

Temperature sensor

Fluid flow and liquid level sensor

Signal processing

Data presentation elements

Smart Systems

Introduction

To be useful, systems must interact with their environment. To do this they use sensors and actuators

Almost any physical property of a material that changes in response to some excitation can be used to produce a sensor

widely used sensors include those that are:

resistive

inductive

capacitive

piezoelectric

photoresistive

elastic

thermal.

Sensor Selection

Range

maximum and minimum values that can be measured

Resolution or discrimination

smallest discernible change in the measured value

Error

difference between the measured and actual values

random errors

systematic errors

Accuracy, inaccuracy, uncertainty

accuracy is a measure of the maximum expected error

Sensor Selection

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.

Naturally, the maintenance costs must be included in the economic analysis of a design.

Sensor Selection

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.


1) Potentiometers

resistive potentiometers are one of the most widely used forms of position sensor

can be angular or linear

consists of a length of resistive material with a sliding contact onto the resistive track

when used as a position transducer a potential is placed across the two end terminals, the voltage on the sliding contact is then proportional to its position

an inexpensive and easy to use sensor


Potentiometers Construction

The output signal (Vout) from the potentiometer is taken from the centre wiper connection as it moves along the resistive track, and is proportional to the angular position of the shaft.


2) Inductive proximity sensors

Also known as Eddy current sensor.

Coil inductance is greatly affected by the presence of ferromagnetic materials

Here the proximity of a ferromagnetic plate is determined by measuring the inductance of a coil


Inductive Proximity Sensor how it works?

An inductive proximity sensor has four main components; The oscillator which produces the electromagnetic field, the coil which generates the magnetic field, the detection circuit which detects any change in the field when an object enters it and the output circuit which produces the output signal.


Inductive Proximity Sensor application

As well as industrial applications, inductive proximity sensors are also used to control the changing of traffic lights at junctions and cross roads. Rectangular inductive loops of wire are buried into the tarmac road surface and when a car or other road vehicle passes over the loop, the metallic body of the vehicle changes the loops inductance and activates the sensor thereby alerting the traffic lights controller that there is a vehicle waiting.


3) Switches

simplest form of digital displacement sensor

many forms: lever or push-rod operated micro-switches; float switches; pressure switches; etc.

A limit switch

A float switch


4) Opto-switches

consist of a light source and a light sensor within a single unit

2 common forms are the reflective and slotted types

A reflective opto-switch

A slotted opto-switch


5) Absolute position encoders

a pattern of light and dark strips is printed on to a strip and is detected by a sensor that moves along it

the pattern takes the form of a series of lines as shown below

it is arranged so that the combination is unique at each point

sensor is an array of photodiodes


Absolute position encoders

One main advantage of an absolute encoder is its non-volatile memory which retains the exact position of the encoder without the need to return to a "home" position if the power fails

Pressure Sensors

1) Strain gauge

stretching in one direction increases the resistance of the device, while stretching in the other direction has little effect

can be bonded to a surface to measure strain

used within load cells and pressure sensors

A strain gauge

Direction of sensitivity

2) Capacitive Sensors

Work based on measurement of capacitance from two parallel plates.

C = A/d , A = area of plates d = distance between.

This implies that the response of a capacitive sensor is inherently non-linear. Worsened by diaphragm deflection.

Must use external processor to compensate for non-linearity

Pressure Sensors

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3) Piezoresistive Sensors

Work based on the piezoresistive properties of silicon and other materials.

Piezoresistivity is a response to stress.

Some piezoresistive materials are Si, Ge, metals.

In semiconductors, piezoresistivity is caused by 2 factors: geometry deformation and resistivity changes.

Pressure Sensors

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There are two temperature sensing methods:

Requires direct physical contact with the object or media that is being sensed.

It can be used with solids, liquids or gases.

Does not require any physical contact with the object or media that is being sensed.

Used to sense the temperature of solids and liquids.

Contact

Non Contact

Temperature sensors

Contact

Non Contact

Thermocouples

Resistance Temperature Detectors (RTDs)

Full System Thermometers

Bimetallic Thermometers

Thermistors

Thermostat

Pyrometer

Temperature sensors

Temperature sensors

1) Resistive thermometers

typical devices use platinum wire (such a device is called a platinum resistance thermometers or PRT)

linear but has poor sensitivity

A typical PRT element A sheathed PRT

Temperature sensors

2) Thermistors

use materials with a high thermal coefficient of resistance

sensitive but highly non-linear

Easy to use and adaptable.

A disc and threaded thermistor

Temperature sensors

3) PN junctions

a semiconductor device with theproperties of a diode (we willconsider semiconductors anddiodes later)

inexpensive, linear and easy to use

limited temperature range (perhaps -50C to 150 C) due to nature ofsemiconductor material

pn-junction sensor

Temperature sensors

4) Thermocouple

A thermocouple is a junction formed from two dissimilar metals.

It is a pair of junctions. (One at a reference and the other junction at the temperature to be measured.)

Inexpensive, Rugged, Reliable

Can be used over a wide temperature range

Temperature sensors

5) Resistance Temperature Detectors (RTDs)

Sensors used to measure temperature by correlating the resistance of the RTD element with temperature.

Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core.

The most accurate temperature sensors.

Excellent stability and repeatability.

Immune to electrical noise

Contact

Non Contact

Advantages:-

Relatively rugged

Economical

Wide application range

Relatively accurate

Simple to apply

Advantages:-

Ideal for measuring objects in motion

Does not interfere with process

Faster response (milliseconds compared to seconds for contact sensing)

Can sense temperature of irregular shaped objects

Will not deface or contaminate

Will not act as a heatsink

Temperature sensors

Contact

Non Contact

Disadvantages:-

Requires physical contact, may damage or contaminate.

Can cause wear on rotary components.

Slow to respond relative to non-contact sensing

Acts as a heatsink, alters readings on small objects

Disadvantages:-

Will not measure gas temperatures

Emissivity variations

Field-of-view (spot size) restrictions

Ambient temperature restrictions

Indicated temperature affected by environmental conditions (dust, smoke, etc.)

Temperature sensors

Flow sensors

Important variable in plant operation

Measured primarily for determining the amount of fluid flowing

Flow measured as a quantity or rate of flow in terms of weight or flow.

Most popular type is the head type flow meter.

Produce a pressure difference when fluid flow is maintained through them

Difference Pressure proportional to square of flow rate

Uses Bernoullis theorem

Flow sensors

Head type flow meter

orifice

venturi

Flow nozzle

Pitot tube

Flow sensors

Orifice meters

used for large & medium pipes

Orifice plate- inserted to pipe to create a partial restriction to flow

Pressure before orifice plate rises and pressure after it reduces but velocity increases.

Simple but poor accuracy, poor calibration and a lot of maintenance problems

Flow sensors

Orifice plates are arranged vertically in the pipeline.

Flow sensors

Position where velocity is maximum & static pressure is min is known as vena contracta.

Flow sensors

Calculation of the flow:
qvVolumetric flow rate (m/s)C Coefficient of discharge (dimensionless)A Area of orifice (m)g Gravitational constant (9.8 m/s)h Differential pressure (m)

Flow sensors

2) Venturi meter

Converging conical section at up stream

Cylindrical throat- provides a panel for measurement- pressure decreased- flow rate steady

Diverging recovery outlet

Venturi meters are most commonly used for liquids, especially water.

Flow sensors

In the Venturi meter velocity is increased and the pressure decreased in the upstream cone.

The pressure drop from points F to I can be used to measure the rate of flow through the meter.

Flow sensors

Flow sensors

Calculation of the flow:

Q = is the flow rate through the pipe and through the meter (m3/s )

Cd= is the discharge coefficient, which is dimensionless

Ao= is the constricted area perpendicular to flow (m2)

P1= is the undisturbed upstream pressure in the pipe (N/m2)

P2= is the pressure in the pipe at the constricted area, Ao(N/m2)

= D2/D1= (diameter at A2/pipe diameter), which is dimensionless

= is the fluid density (kg/m3)

3) Flow nozzle

Principle- Bernoullis theorem

Simple

Used in higher velocities, difficult to maintain, costly

Used in gases

Flow sensors

Flow sensors

4) Pitot tube

Cylindrical probe inserted into fluid

Velocity head converted into impact pressure

Differences between static pressure & impact pressure- proportional to flow

Flow sensors

Easy to install

No pressure loss

Sensitive to up stream disturbance

Not used for sticky and dirty fluids

Flow sensors

Level Measurement

Two methods of Level Measurement

Direct Measurement

Direct Measurement uses methods such as

visual Inspection

displacement of float

Tuning fork etc

Inferential measurement

Inferential measurement uses methods such as

pressure head

time of flight etc

attenuation of radiation

Change in Capacitance of material

Photoelectric

Level Measurement (Direct)

1) Level Gauge

This is a visual method of measuring the level of fluid in a vessel.

A graduated pipe is connected to a vessel. As the level of the fluid inside the vessel increases the level inside the tube will increase.

A visual inspection of the tube ill provide an indication of the level inside the vessel.

Vessel


2) Reflex / transparent level indicator

Reflex level indicator

Transparent level


3) Magnetic floating detector

Containing a magnet rises and falls with the liquid

As the float moves, this information is transferred to the indication rail mounted on the outside of the tube.

The white and red indication flaps represent air and liquid level respectively.


Each of the colored flap contains a small magnet which rotates through 1800 when passed by the bar magnet within the float.

The bar magnet design does not lose magnet field strength even at temperatures of 4500C guaranteeing continuous operation in the most extreme applications.


4) Float type detectors

Most Float type detectors use the principle of loss in weight of a buoyant float to indicate the level of the fluid.

The float is selected such that it is lighter than the fluid. As the level of the fluid rises the float rises. This is sensed by the electronics assembly to indicate the level.


Advantages

Simple & proven techniques

Unlimited tank height

Better accuracy (depending on the float type)

Low capital, maintenance cost

Disadvantages

Subject to wear, corrosion, mechanical failure

Getting stuck due to material clogging, deposition, material buildup


5) Vibrating Element

The oscillations of a member (paddle) are damped when it is immersed in the liquid.

The attenuation of oscillations indicates that the liquid has reached the measured level.

The oscillations are stimulated and sensed by electronic means.

Level Measurement (Indirect)

1) Differential Pressure

Level can be inferred from the differential pressure (head) of the fluid inside the tank.


Options on Differential pressure based level measurement :

Purge type instruments

The nozzle on the vessel is continuously purged by a medium acceptable to process & back pressure is measured at HP & LP tapping.

Used for fluid with suspended particles or having crystallizing nature

Chemical seal type instruments

The nozzle on the vessel is flange type instead of threaded type.

Used when medium can chock the tapping

Cost of instrument is three times the conventional inst.


Advantages

economical and easy to install

Online checking & maintenance possible

Disadvantages

Solid level measurement not possible

Only clean fluid can be measured

Density variation gives error

Choking of impulse tubes possible


2) Ultrasonic

Ultrasonic instruments determine level by measuring the length of time it takes for a sound pulse to return to a piezoelectric transducer after bouncing off the process material.


The sensor uses high-performance Piezoelectric crystals to generate short ultrasonic pulses in the form of sound waves.

These pulses are directed toward a specific target from where they get reflected back to the transducer which acts as transmitter/receiver.

The transit time taken to receive the reflected pulse is measured by the electronics.

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3) Radar

Radar-based devices beam microwaves at the process material's surface.

A portion of that energy is reflected back and detected by the sensor.

Time for the signal's return determines the level.


Radar provides a non contact sensor that is virtually unaffected by changes in process temperature, pressure or the gas and vapor composition within a vessel.

In addition, the measurement accuracy is unaffected by changes in density, conductivity and dielectric constant of the product or by the air movement above the product.


4) Capacitive level sensor

Radio frequency (RF), based on capacitance or admittance, can handle wide range of process conditions.

Level transmitters of this type sense the change of electrical impedance that occurs with the change of level on the sensor.

RF devices ignore material buildup on sensor and work with all types of process material.

Signal Conditioning

Signal from detector/sensing stage has to be modified (conditioned) in order to make it more usable.

This signals is then to be use in later stage of system that may consist of processing elements.

Proper selection of signal conditioning circuit can improve the quality and system performance.

Types of Signal Conditioning

Can be classified into two types:

Analog Signal Conditioning the conditioned output is still an analog representation

Digital Signal Conditioning the conditioned output is in digital format

Analog signal conditioning :

Passive circuits using passive elements like resistor, capacitor, inductor

Active Circuit using IC like operational amplifier.

Functions of Signal Conditioners

There are many possible functions of signal conditioning stage:

Amplification

Attenuation

Filtering

Differentiation

Integration

Linearization

Converting a resistance to voltage signal

Converting a current signal to voltage signal

Etc

Analog Signal ConditioningBridge Circuit

Wheatstone Bridge - the most common bridge circuit used.

In signal conditioning it is used to convert impedance variations into voltage variations.

Voltage detector

Assume the detector impedance is infinite open circuit.

For null detector ,the resistors value must satisfy this equation:

Analog Signal ConditioningBridge Circuit

Analog signal ConditioningFiltering

Filter is a circuit/device that passes electrical signals at certain frequencies while attenuates or rejects others.

Consist at least one pass band - range of frequencies that are allowed to pass through the filter

Cut-off frequencies, fc - is the frequencies for which the output is attenuated by 3 dB or Vout / Vin = 0.707

Signal Filtering

Types of Filter

Both of passive and active filters can be classified as follows:

Low Pass Filter: deliver low frequencies and eliminate high frequencies

High Pass Filter: send on high frequencies and reject low frequencies

Band Pass Filter: pass some particular range of frequencies, discard other frequencies outside that band

Band Stop Filter: stop a range of frequencies and pass all other frequencies

Types of Filter

Categories of electrical filters:

(a) lowpass;

(b) highpass;

(c) bandpass;

(d) bandstop.

Low Pass Filter

Blocks high frequencies and passes low frequencies

In terms of resistor and capacitor, cut-off frequency, fc is given by:

Gain is also given by;

signals above cut-off frequency, fc are simply rejected.

the Phase Angle () of the output signal lags behind that of the input and at the -3dB cut-off frequency (fc) it is -45o out of phase

Low Pass Filter

High Pass Filter

to offer easy passage of a high-frequency signal and difficult passage to a low-frequency signal.

In terms of resistor and capacitor, cut-off frequency, fc is given by:

Gain is given by;

The signal is attenuated or damped at low frequencies

The phase angle () of the output signal leads that of the input and is equal to +45o at frequency c.

High Pass Filter

Band Pass Filter

Passes all signals lying within a band between a lower frequency limit and an upper-frequency limit

Rejects all other frequencies that are outside the specified band

combining the properties of low-pass and high-pass into a single filter

Band Pass Filter

Lower cut-off freq:

Upper cut-off freq:

In terms of gain and frequency;

High pass gain :

Low pass gain:

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Band Stop Filter

Frequencies within a certain bandwidth are rejected and

frequencies outside the bandwidth are passed.

At the very low and high frequencies the gain is almost unity, but between the two there is a frequency where the gain (Vout/Vin) become zero.

The frequency is known as Notch Frequency, fo.

Band Stop Filter

can be made out of a low-pass and a high-pass filter

the two filter sections in parallel with each other

commonly known as a Twin-T filter

The low-pass filter section is comprised of R1, R2, and C1 in a T configuration.

The high-pass filter section is comprised of C2, C3, and R3 in a T configuration as well

Digital Signal Conditioning

Memory

Input Stage

INPUTS: All sensors produce are a voltage signal of some type

Some Inputs are conditioned before going to the microprocessor.

Amplification

A/D Conversion

Input Conditioning

Microprocessor can only process some types of signals

Must amplify some signals

Must convert analog signals to digital signals

Processing Operation

Digital signal compared to lookup tables

Information is sent to microprocessor

Microprocessor decides what to do

Issues a command to output actuator

Output Stage

Microprocessor issues commands in the form of voltages

Can display information on a Scanner or Digital dash

Can control hydraulic, vacuum or electrical components.

Data Presentation

Local display

A sensor can display the measurement at the point where the sensor is located.

This information can be used by the people when monitoring or working on the equipment.

A measurement that has only local display involves the lowest cost, because the cost of transmission and interfacing to a digital system are not required.

Note that no history of these measurements is available unless people record the values periodically.

Data Presentation

2) Local panel display

Some equipment is operated from a local panel, where sensors associated with a unit are collected.

This enables a person to start up, shutdown and maintain the unit locally.

This must be provided for units that require manual actions at the process during normal operation (loading feed materials, cleaning filters, etc.).

Usually, the values displayed at a local panel are also displayed at a centralized control room.

Data Presentation

3) Centralized control room

Many processes are operated from a centralized control room that can be located a significant distance (e.g., hundreds of meters) from the process.

The measurement must be converted to a signal (usually electronic) for transmission and be converted to a digital number when interfaced with the control system.

A centralized control system facilitates the analysis and control of the integrated plant.

Data Presentation

4) Remote monitoring

In a few cases, processes can be operated without a human operator at the location.

In these situations, the measurements are transmitted by radio frequency signals to a centralized location where a person can monitor the behaviour of many plants.

Typical examples are remote oil production sites and small, safe chemical plants, such as air separation units.

Smart System

Digital conversion and transmission- The sensor can transmit additional information, including diagnostics and corrected estimates of a variable based on multiple sensors, e.g., orifice pressures and density.All values can be transmitted digitally, which allows many sensor values to be sent by the same cabling, which reduces the cost of an individual cable for each measurement, as required with analog transmission.

Diagnostics- The sensor can provide sophisticated diagnostics of its performance and warn when a measurement might be unreliable.

Configuration- The range of a sensor can be changed quickly to accommodate changes in process operating conditions.

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lecture #2 (instrumentation system elements) v1

Documents

sensor selectionsafety

sensor selectionrangemaximum

eddy current sensor

process control

coil inductance

output circuit

angular position

instrumentation ebge