Overview

Vocabulary

Plant: System we want to control

Actuating Signal: input into plant

Controlled Variable: output of the plant

Commanded Variable: Desired output of the plant

Feedback Path: output of the system is fed back and compared to command

Error Term: Difference between Controlled and Commanded Variable (Desired Value is 0)

PID Controller: Controller used to combat error term and drive it to 0

Basic Diagram

Diagram

PID Meaning

Proportion

  • Used to reach an error term of 0 by multiplication of a constant factor

  • Gives a magnitude to combat error and approach commanded variable

  • In many cases solely a proportional controller works

  • Sometimes leaves an error (known as steady state error) which requires the use of an integrator

Integral

  • Used to combat steady state error

  • Keeps track of past information and has a memory of sorts

  • Its job is to combat the error by summing the past errors

  • A steady state error is a non zero constant value, and when that is integrated, a linear term appears for the integrator

  • Once a steady state error is reached, the proportional path does nothing

  • The path may cause oscillation, which requires the use of a derivative path

Derivative

  • Used to combat oscillation

  • Measures the rate of change of the error and determines how we are approaching the goal

  • Prematurely alters the rate of change to prevent overshooting