The term PID is short for proportional, integral and derivative. The AirLogix® controller uses a total of four PID loops to optimize the performance of a centrifugal air compressor. A PID loop is a complex method of controlling a process (in this case air pressure) using feedback. Simply put, the PID is constantly looking at a process variable (air pressure, motor current, etc.) and comparing the process variable (PV) to the process set point (SP). If the PID sees a mathematical difference between the two (%error), it will adjust the control variable (CV) or the inlet or bypass valve, to try and meet the process set point.
Each element of the PID controller refers to a particular action taken on the error.
Proportional
With the Allen-Bradley CompactLogix and many other controllers, this term is referred to as ‘gain.’ This element is the amplifier of the control loop and increases rise time. It takes the %error multiplied by the gain constant and directly applies it to the CV. Gain affects the amount of valve movement each time the PID updates. A gain too large will cause overshoot and PV oscillation, and a gain too small will cause a slow, sluggish response—possibly not allowing the controller to ever stabilize.
Integral
This element is referred to as ‘reset.’ It takes the responsibility of reducing overshoot and attempts to allow the gain to affect the PV before any other action is taken. Reset is a time constant measured in seconds. Simply put, when %error is calculated and the CV is adjusted accordingly, the reset determines the time span before another CV change is made (time elapsed per repeat). With the AirLogix® controller, the lower the integral value, the more frequently the CV will be updated. A value too low will result in overshoot because the controller did not allow for the CV to affect the PV. A value too high will make the process sluggish because too much time was allowed to pass before making another change.
Derivative
This element—referred to as ‘rate’—attempts to reduce any lag that occurs in the process variable. It is also a time constant like the integral element, but this one looks to predict what is going to happen next by looking at the rate at which the PV is changing in time due to a change in the CV. It sounds very useful but can cause much grief and is commonly referred to as the villain of PID loop control. You can think of rate as a weather forecaster, with respect to the fact that it isn’t always correct. Rate works fairly well when the fluctuations in demand amount are always the same (very rare), and when process variable lag times are relatively long. Good candidates might be temperature and level control. With compressed air, demand amounts are usually changing, and lag times are short. This reduces the need for any derivative action. In most situations, if not all, a properly-set gain and reset will produce adequate results.
Interested in learning more about AirLogix® controllers or any of the other compressed air system solutions we offer at Case Controls? Feel free to give us a call or contact us online today to speak with a representative!