De-mystifying PID feedback loop control

If you were to call a PID loop a compensator instead, nobody apart from the most pedantic academic could take issue with you. OK, some engineers can get quite technical and start talking about lead-lag compensation and suchlike, but the principle is the same.

A PID controller examines signals from sensors placed in a process, called feedback signals. When a feedback signal is received, it is compared with the desired value, or set-value, and a calculation is made of what the necessary response is in order to make the feedback signal, also known as the ‘error’ signal because of its deviation from the set-value, match the set-value.

Process controllers generally work on the principle of a ‘closed-loop’. Taking a typical application like an oven, the measured temperature, referred to as the process value (PV), is fed into the controller and compared to the users set value (SV), which is the desired final temperature value. As the temperature is seen to rise, the power to the oven is reduced by the controller until a power level which can maintain the desired temperature is reached. By continuing to monitor and adjust the power level, accurate control is achieved.

Three-term control

Most industrial processes such as plastic extrusion require a stable 'straight-line' control of the temperature. The PID control algorithm, referred to as 'three-term' control provides exactly that. The output of the controller is the sum of the three terms. The combined output is a function of the magnitude and duration of the error signal and the rate of change of the PV.

The PID controller will automatically control process variables such as temperature, pressure and flow – in fact almost any physical variable that can be represented as an analogue signal.

Integral, also called ‘reset’, has one primary function, to eliminate offset. Reset pushes the actual temperature (PV) towards the set-value (SV) temperature until both are equal. This eliminates the temperature offset condition caused by proportional control on system start-up. To reduce or eliminate overshoot, we must use the D or derivative term of PID control.

In many thermal systems, overshoot (or undershoot) of the set-value temperature is perfectly acceptable. However, in some systems this can produce poor quality products or perhaps even damage expensive equipment. So derivative has one main job – to prevent or greatly reduce overshoot and undershoot. It does this by measuring the rate of temperature change, that is how fast the temperature is rising or falling. If the temperature rise is too fast, it will begin switching the heater off to prevent overshoot. If the temperature is falling too fast, it will begin switching the heater on longer to prevent undershoot.

Luckily for us, most PID controllers come with automatic PID tuning but if you’re ever feeling brave and want to have a go, remember the following:

• Increasing the PB will result in tighter control but give a slower response, taking longer to reach the set value.
• A narrow PB generates a smaller offset than a wide PB.
• Too fast a reset (integral) causes overshoot, while too slow a reset delays recovery.
• A long rate (derivative) time may cause overshoot, while a short rate time may cause a delay in recovery.

Failing that, give us a call – it’s far easier than messing around with spaghetti!