Introduction to Control Engineering and PID Control

One dictionary definition of the word control is:

–verb (used with object)
to exercise restraint or direction over; dominate; command.

Engineers usually use the word to mean something along the lines of:

The applied science of changing the dynamic behaviour
of a physical process to get a desired outcome.

Examples of modern control systems in use every day are:

Aircraft Autopilot Systems
Car Engine Management Systems
Hot water thermostats

A control system is the combination of a controller with a physical process. The control system will also need a sensor and an actuator.

The sensor senses the physical conditions of the process, and relays this information to the controller. The controller uses this information to decide what to do next to reach it’s progeammed, desired objective and sends a command to the actuator to alter the process behaviour. Hopefully in a way that makes the process get closer to its objective. e.g. get the jumbo nearer to the runway!

Control systems can be classed as:

Open Loop, or
Closed Loop

Open Loop Control

In an open loop system, the controller is told what the desired process output is, and the controller sends a command to the actuator to try and achieve this. The process is shown below as a block diagram:

an open loop control system (block diagram)

An example of open loop control is a cheap electric heater.

Referring to the block diagram above - our Desired Process Response is a comfortable temperature in the room.

The controller is the person in the room who switches the heater on when he feels a bit chilly.

The actuator is the heater itself.

The plant is the thermodynamic behaviour of the room. Which is a fancy way of saying ‘how much heat the room loses’.

If there is no further intervention once the fire has been switched on, then the process-output (the room temperature) will eventually settle at a constant temperature. This temperature is dependent on two things:

The amount of heat gnerated by the fire
The heat losses from the room

These 2 factors are the only things that will affect the final temperature. For example: If the heater is too powerful and/or the room is super-insulated, we will get too hot.

Closed Loop Control

Of course, what happens in the real world is that if the person in the room gets too hot, she switches the heater off.

If she does this then she has just upgraded herself to a closed loop controller.

She sensed the temperature was too hot, and processed a simple control algorithm with the rules:

If temperature is more than comfortable
then
switch heater off.

This is the simplest of closed loop control systems and is shown in the block diagram below: