# What is the output of a PID controller (or any controller in general)?

I'm new to Simulink and I'm trying to get my hands a little dirty there to get used to using it. I'm working on a speed control of a switched reluctance motor. The input signal is to be fed to a converter (IGBT's gate) to control the speed of the motor.

Consider the general diagram below:

So my controller is a PID , my input is the reference speed of motor , my output is the measured (actual speed) of the motor and the plant is the "converter-motor" set up.

(1) What is the actuating signal obtained from the controller? Is it the control signal to be fed to the IGBTs?

(2) What if the plant input is a voltage (we are directly feeding voltage to the terminals of the motor). Isn't the PID output pretty much the same as the case of IGBTs? The Simulink controller produces a control signal how should I use it to feed voltage instead ?

Thanks ahead

## 2 Answers

What is the actuating signal obtained from the controller?

It is usually an analog signal like 0 to 10 volts with 20 mA maximum load current or 4 to 20 mA with 24 volts maximum load voltage.

Is it the control signal to be fed to the IGBTs?

No. For a switched reluctance motor, The IGBTs would be fed gate signals that would have a timing sequence determined by the reluctance motor controller. That controller could receive an analog input signal from a PID controller.

What if the plant input is a voltage (we are directly feeding voltage to the terminals of the motor).

A DC motor with a commutator could be fed voltage directly. In that case, there would be a control amplifier between the PID controller and the motor. The amplifier simply converts the output of the PID controller to a higher voltage and current.

The Simulink controller produces a control signal how should I use it to feed voltage instead?

You would need to add a power amplifier between the PID and motor.

The same as with any math equation representing reality, you have to keep track of your units. The output of a PID will be in whatever set of units you design it to be.

However, although this is often ignored, PIDs represent continuous systems and Nyquist theorem still applies. Discrete-time, quantized, or saturated-variables PIDs introduce other considerations that will affect behavior. Some times for the better other times for the worse.

Any discontinuous or quantized element, such as an IGBT, requires this behavior to be modeled and, very likely, designed-around so that the PID can remain (mostly) continuous and common design heuristics can still apply.