What’s going on this diagram?
The Setpoint (SP) is the value that we want the process to be.
For example, the temperature control system in our house may have a SP of 22°C. This means that
“we want the heating and cooling process in our house to achieve a steady temperature of as close to 22°C as possible”
The PID controller looks at the setpoint and compares it with the actual value of the Process Variable (PV). Back in our house, the box of electronics that is the PID controller in our Heating and Cooling system looks at the value of the temperature sensor in the room and sees how close it is to 22°C.
If the SP and the PV are the same – then the controller is a very happy little box. It doesn’t have to do anything, it will set its output to zero.
However, if there is a disparity between the SP and the PV we have an error and corrective action is needed. In our house this will either be cooling or heating depending on whether the PV is higher or lower than the SP respectively.
Let’s imagine the temperature PV in our house is higher than the SP. It is too hot. The air-con is switched on and the temperature drops.
The sensor picks up the lower temperature, feeds that back to the controller, the controller sees that the “temperature error” is not as great because the PV (temperature) has dropped and the air con is turned down a little.
This process is repeated until the house has cooled down to 22°C and there is no error.
Then a disturbance hits the system and the controller has to kick in again.
In our house the disturbance may be the sun beating down on the roof, raising the temperature of the air inside.
So that’s a really, really basic overview of a simple feedback control system. Sounds dead simple eh?