This document discusses error detectors and two-position controllers in electronic process instrumentation. It describes how an error detector takes the difference between a process signal voltage and a setpoint voltage to calculate an error signal. It also explains how a two-position controller uses an operational amplifier comparator circuit to provide ON/OFF control output based on adjustable high and low trip points for the input voltage. The document is for an instrumentation and control course taught by Dr. S. Meenatchisundaram at MIT Manipal from August to November 2015.

1. ICE401: PROCESS INSTRUMENTATION
AND CONTROL
Class 23
Electronic Controllers – Error Detector, Two
Position Controller
Dr. S. Meenatchisundaram
Email: meenasundar@gmail.com
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
controllers by taking the difference between voltages.
• One voltage is generated by the process signal current passed
through a resistor. The second voltage represents the setpoint.
• This is usually generated by a voltage divider using a constant
voltage as a source. An example is shown in Figure.

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
signal current.
• The signal current is used to produce a voltage, IR, across the
resistor, R. This is placed in series opposition to a voltage, Vsp,
tapped from a variable resistor, Rsp, connected to a constant
positive source, V0.
• The result is an error voltage, Ve = Vsp – IR. This is then used
in the process controller to calculate controller output.
• An error detector also can be made from a differential
amplifier. Such a system can be used only if the current from
the transducer is referenced to ground.

4. Error Detector:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
• A typical configuration is shown in the figure above.
• The sensor signal current passes to ground through RL,
providing a signal voltage, Vm=IRL. The differential amplifier
then subtracts this from the set point voltage.

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
adjustable neutral zone is given in Figure.
• For this circuit, we assume that if the controller input voltage, V
reaches a value VH, then the comparator output should go to the ON
state, which is defined as some voltage, V0.

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

7. Single Mode:
Process Instrumentation and Control (ICE 401)
Dr. S.Meenatchisundaram, MIT, Manipal, Aug – Nov 2015
• When the input voltage falls below a value VL, the comparator
output should switch to the OFF state, which is defined as 0 V.
• The comparator output switches states when the voltage on its
input, V1, is equal to the setpoint value, Vsp.
• Analysis of this circuit shows that the high (ON) switch voltage is
VH = Vsp
• and the low (OFF) switching voltage is VL = Vsp – (R1/R2)V0
• The inverter resistance in Figure can be chosen as any convenient
value. Typically, it is in the 1- to 100-k range.

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