• Control systems use some output state of a system and a desired state to make control decisions.
• In general we use negative feedback systems because,
- they typically become more stable
- they become less sensitive to variation in component values
- it makes systems more immune to noise
• Consider the system below, and how it is enhanced by the addition of a control system.
• Some of the things we do naturally (like the rules above) can be done with mathematics
• The basic equation for a PID controller is shown below. This function will try to compensate for error in a controlled system (the difference between desired and actual output values).
• The figure below shows a basic PID controller in block diagram form.
• The PID controller is the most common controller on the market.
• The PID calculation is effectively a calculation in the PLC. One basic method of PID control is i) read voltage, ii) do PID calculation, iii) set output voltage. (Note: it is also common to get a self contained PID card for the PLC that deals with all inputs and outputs). The ladder logic below shows a PID control function.
