Instrumentation

Modes of PID Controller

In this lesson, we will study modes of PID Controller and PID Control Action.

Modes of PID Controller

PID Controller has 2 modes.

Auto Mode:

In Auto mode, a particular setpoint is set and according to this setpoint, PID Controller adjusts its process variable automatically.

Manual Mode:

In manual mode, no set point is set and we manually open the control valve by adjusting the control variable. This mode is selected mostly for checking the control valve opening.

PID Control Action

PID Controller has 2 Actions.

1. Direct Action:

     A PID is said to be direct action PID if After SetPoint (SP) on Increasing PV if the CV is Increases. For Direct Action PID Kp is +ive.

     Example:

       A Temperature Reducing Valve. After SP when temperature increases, CV is increased     

  1. Reverse Action:

A PID Is said to be reverse Action PID if, After SetPoint (SP) On increasing PV, CV Decreases. For Reverse Action, PID Kp Is  –ive.

Example:

      A Pressure Reducing Valve. After SP on increasing PV, CV decreases.

  • As shown in the figure process variable (PV) comes from field-side i.e. from any transmitter or sensor. If the signal coming from the transmitter the signal is 4 – 20 m Amp. The controller has an indicator in it also.
  • In this indicator zero and span (Range)are set. According to these zero and span, this indicator shows the process variable.
  • Using Zero and Span range is set according to 4 and 20 mill amperes.
  • Now, this process variable is compared with the set point. An error signal is then generated, PID Controller tries to minimize this error.
  • Now by using these P, I and D values PID Controller use this Control Variable (CV) to open the control valve to minimize error or to zero this error.
  • This 4 – 20 m amp output is used to open the control valve.
  • Milli  Amp Control valve opening
  •          4 m Amp                0%      
  •          8 m Amp               25%      
  •       →   12 m Amp            50%          16 m Amp           75%

                    20 m Amp          100%

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