Instrumentation and Control Technician exam: PID control

Key points

• Proportional responds to the size of the error. Too much gain causes oscillation.
• Integral removes steady-state offset over time. Too much causes overshoot and windup.
• Derivative reacts to how fast the error changes. It is sensitive to noise.
• Direct acting: output rises as the measurement rises. Reverse acting: the opposite.
• Flow loops are fast and noisy, so they rarely use derivative.

Study tip

If a loop settles with a steady offset, the missing piece is integral action.

Try these

1. Which mode removes steady-state offset?

   1. Proportional
   2. Integral
   3. Derivative
   4. On/off
   Show answer

   B. Integral Integral keeps acting until error is zero.

2. A loop oscillates after raising gain. You should:

   1. Raise gain more
   2. Lower gain
   3. Add derivative on a flow loop
   4. Remove integral only
   Show answer

   B. Lower gain Too much gain causes oscillation.

3. Derivative action is often left out of flow loops because:

   1. They are slow
   2. Flow signals are noisy
   3. It removes offset
   4. It is not allowed
   Show answer

   B. Flow signals are noisy Noise makes derivative jumpy.

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