Class 23 electronic controllers

Electronic Controllers

1. ICE401: PROCESS INSTRUMENTATION 
AND CONTROL
Class 23
Electronic Controllers – Error Detector, Two 
Position Controller
Dr. S. Meenatchisundaram
Email: [email protected]
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug
–
Nov 2015

2. Error Detector:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug
–
Nov 2015
•The detection of an error signal is accomplished in electronic
controllersbytakingthedifferencebetweenvoltages.
•One voltage is generated by the process signal current passed
througharesistor.Thesecondvoltagerepresentsthesetpoint.
•Thisisusuallygeneratedbyavoltagedividerusingaconstant
voltageasasource.AnexampleisshowninFigure.

3. Error Detector:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug
–
Nov 2015
•We assume a two-wire system is in use so that the current
drawn from the floating power supply is the 4- to 20-mA
signalcurrent.
•Thesignalcurrentisusedtoproduceavoltage,IR,acrossthe
resistor,R.Thisisplacedinseriesoppositiontoavoltage,V
sp,
tapped from a variable resistor, R
sp, connected to a constant
positivesource,V
0.
•Theresultisanerrorvoltage,V
e=V
sp–IR.Thisisthenused
intheprocesscontrollertocalculatecontrolleroutput.
•An error detector also can be made from a differential
amplifier. Such a system can be used only if the current from
thetransducerisreferencedtoground.

4. Error Detector:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug
–
Nov 2015
•Atypicalconfigurationisshowninthefigureabove.
•The sensor signal current passes to ground throughR
L,
providing a signal voltage, V
m=IR
L. The differential amplifier
thensubtractsthisfromthesetpointvoltage.

5. Single Mode:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug
–
Nov 2015
Two-Position:
•A method using op amp implementation of ON/OFF control with
adjustableneutralzoneisgiveninFigure.
•For this circuit, we assume that if the controller input volt age,V
reachesavalueV
H,thenthecomparatoroutputshouldgototheON
state,whichisdefinedassomevoltage,V
0.

6. Single Mode:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug
–
Nov 2015
ComparatorOutput:

7. Single Mode:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug
–
Nov 2015
•When the input voltage falls below a valueV
L, the comparator
outputshouldswitchtotheOFFstate,whichisdefinedas0V.
•The comparator output switches states when the voltage on it s
input,V
1,isequaltothesetpointvalue,V
sp.
•Analysisofthiscircuitshowsthatthehigh(ON)switchvoltageis
V
H= V
sp
•andthelow(OFF)switchingvoltageisV
L=V
sp–(R
1/R
2)V
0
•The inverter resistance in Figure can be chosen as any conven ient
value.Typically,itisinthe1-to100-kIrange.

8. References:
• Process Control Instrumentation Technology, by Curtis D.
Johnson, Eighth Edition, Pearson Education Limited.
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug
–
Nov 2015