Then the resulting “error x control actions” are added together and sent to the controller output. The error is simply multiplied by one, two or all of the calculated P, I and D actions (depending which ones are turned on).
The PV is subtracted from the SP to create the Error. Here’s the Block Diagram we used before, with the labels changed to represent the car-on-windy-freeway control loop. We are the driver, and therefore the controller of the process of changing the car’s position. We will be using the analogy of changing lanes on a freeway on a windy day. When we have gone through these thought experiments we will appreciate why a PID algorithm is needed and why/how it works to control the process. To understand why, we will be doing some “thought experiments” where we are the controller. Unfortunately, in the real world we need a controller that is a bit more complicated than the one described above, if we want top performance from our loops. Sounds dead simple eh? Understanding the controller So that’s a really, really basic overview of a simple feedback control system. In our house the disturbance may be the sun beating down on the roof, raising the temperature of the air inside. Then a disturbance hits the system and the controller has to kick in again. This process is repeated until the house has cooled down to 22☌ and there is no error. 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.
The air-con is switched on and the temperature drops. Let’s imagine the temperature PV in our house is higher than the SP. In our house this will either be cooling or heating depending on whether the PV is higher or lower than the SP respectively.
However, if there is a disparity between the SP and the PV we have an error and corrective action is needed. It doesn’t have to do anything, it will set its output to zero. If the SP and the PV are the same – then the controller is a very happy little box.
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