30.3 CONTROL SYSTEMS

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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

30.3.1 PID Control Systems

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.

30.3.1.1 - PID Control With a PLC

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.