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DCS Distributed Control System Interview Questions 1 contains interview Questions about Distributed Control System, I/p Converter, Thermocouple , Hart Patch Card, Field Terminal Assembly, Difference Between DCS And PLC,  Proportional, Integral And Derivative (PID).

  1. What Is A Distributed Control System?
  2. In Split Range Control, Whether The Signal Is Splitted Through I/p Converter Or The Converter Itself?
  3. What If Thermocouple Wire Is Opened In The Field? What Signal Goes To DCS?
  4. What If the Power Supply Connections to a Two-wire Transmitter Get Interchanged? What Signal Will Go To DCS?
  5. What Is The Difference Between A Hart Patch Card And A Field Terminal Assembly?
  6. What Is The Difference Between DCS And PLC?
  7. When Proportional, Integral And Derivative (PID) Controller Is Used?
  8. When Proportional (p) Controller Is Used?
  9. When Proportional (p) Controller Is Used?
  10. When Integral (i) Controller Is Used?
  11. When Derivative (d) Controller Is Used?
  12. When Proportional plus Integral (pi) Controller Is Used?
  13. When Proportional And Derivative (pd) Controller Is Used ?

1. What Is A Distributed Control System?

A distributed control system (DCS) is a computerized control system for a process or plant, in which autonomous controllers are distributed throughout the system, but there is central operator supervisory control.

2. In Split Range Control, Whether The Signal Is Splitted Through I/p Converter Or The Converter Itself?

This can be typically achieved by two ways:

  • By connecting o/p of one I/P converter to two positioners adjusted suitably for split range operation of control valves.
  • Taking two AO from DCS. Split range to be defined in DCS. Both I/P converters and positioners to be calibrated with input as 4to20 ma dc and 3to15 psi respectively.

3. What If Thermocouple Wire Is Opened In The Field? What Signal Goes To DCS?

In most modern instruments the signal may be programmed to go to either maximum or minimum depending upon end user’s requirement.

4. What If The Power Supply Connections To A Two-wire Transmitter Get Interchanged? What Signal Will Go To DCS?

Usually there is a blocking diode to protect the transmitter against supply reversal and almost zero current signal should be transmitted.

5. What Is The Difference Between A Hart Patch Card And A Field Terminal Assembly?

A field terminal assembly is a DCS component where field signals are terminated. 

In a DCS using analog transmitter signals but smart remotely configured transmitters field signals from barriers are first brought to a HART patch panel. 

From here the analog signals are wired to the DCS field termination assembly or I/O card. 

The digital signals are wired to the HART multiplexes who extract the digital information and provide two way communication path between smart instruments and a computer running suitable software.

6. What Is The Difference Between DCS And PLC?

Distributed control system (DCS) and Programmable logic controller. These are the control systems which handles fields I/Os. 

Basic difference between DCS & PLC is

  • DCS handles more nos of I/Os rather than PLC.
  • PLC is faster system than DCS.
  • DCS can handles handsome quantity of I/Os so that can be used for total plant automation. Where as PLC has own limitations so it generally used for small but for important(Safety point of view) units, like boiler automation, Make-up compressor automation Etc.
  • In the above mentioned case the these PLC’s can be get connected with the
  • DCS with the help of soft link. Generally this is used to make alert to both the operator.
  • As I heard the PLC used to handle the DI/DO signals so it can take fast actions. Some of the time it is used to handle few nos of AI/AO.
  • DCS & PLC’s speed depends on the scan rate of I/Os.
  • For both the system Marshalling panels, Consoles and other faculties of Ethernet Etc can be used according to the need.
  • According to the Cause and effects diagrams the System programmer assigns the control action block into the system, we can call them as memory assigning.

7. When Proportional, Integral And Derivative (PID) Controller Is Used?

PID controller is used when system requires:

  • System changes are small.
  • Offset must be eliminated.
  • Fast recovery time.

8. When Proportional (p) Controller Is Used ?

Proportional (P) Controller is used during the following conditions:

  • Load Changes are small.
  • Offset (error exist due to difference between steady state value and desired value) can be tolerated.
  • The process reaction rate is such as to permit a narrow proportional band.
  • Since this reduces the amount of the offset.

9. When Proportional (p) Controller Is Used ?

Proportional (P) Controller is used during the following conditions:

  • Load Changes are small.
  • Offset (error exist due to difference between steady state value and desired value) can be tolerated.
  • The process reaction rate is such as to permit a narrow proportional band.
  • Since this reduces the amount of the offset.

10. When Integral (i) Controller Is Used ?

Integral (I) controller is used during:

  • When the offset must be eliminated.
  • Integral saturation due to sustained deviation is not objectionable.

11. When Derivative (d) Controller Is Used ?

  • Large transfer or distance velocity lags are present.
  • It is necessary to minimize the amount of deviation caused by the system changes.

12. When Proportional Plus Integral (pi) Controller Is Used?

Proportional and Integral action is employed when

  • System changes is small.
  • Offset must be eliminated.
  • No objection on the recovery duration.

13. When Proportional And Derivative (pd) Controller Is Used ?

PD action is employed when:

  • When the system changes are small.
  • Improve the recovery time.
  • When system lags are high.

Thanks for reading - DCS Distributed Control System Interview Questions 1
Naitik Patel
Industrial Guide

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