OE-5.3 Process Controllers & Controller Hardware

Work through these

• What a controller is and what controllers are used for

  What sits between the measurement and the actuator, and what it is for. Everything else in this topic is one kind of it.

• Open-loop and closed-loop control

  The distinction the whole subject rests on: does the controller look at the result or not. If only one idea from this topic survives, make it this one.

• Proportional control; analog and digital methods of control

  The simplest useful control law, and its characteristic flaw of leaving a steady error. The two implementation styles are here because both are still in service.

• Analog and digital controller hardware

  The physical realisation of the item above, in both styles. Read for the trade between them rather than the circuit detail.

• Pneumatic controllers

  Control with air rather than electronics, which is still used where a spark would be dangerous. A good reminder that the newest technology is not always the permitted one.

• Integral and derivative action

  The two terms that fix what proportional control alone cannot: one removes the steady error, the other anticipates. Knowing which does which is the practical skill.

• PI, PD and PID controllers

  The combinations actually used, and when each is appropriate. Tuning is the part that takes practice, and knowing why the derivative term is often left out is worth more than the formula.