PID

Proportional Integral Derivative Control

PID, proportional integral derivative control, is a powerful and popular method of regulating systems. Typical applications include speed control and position control. PID control is popular with manufacturers of electronics, often less popular with those who have to tune the system. HCT uses this method as we have not found an easier way for the user to tune system performance.

Closed loop control of systems is used to cause the system to arrive at a commanded state exactly. This will automatically adjust out many non-ideal characteristics of your system, but at a price. There must be feedback from the system to tell the closed loop controller where the system is so that it will know when it has arrived where it should be. There must be a command from the user to tell the controller where the system should be. When the command does not match the feedback there is an error. So just correct it right? The problem is inertia, inductance and delay in the system, sensors and electronics. These effects can cause over correction by the controller. If you try to point a satellite mini dish by having someone yell STOP when the signal is good, you will experience system delay and over shoot. Over shoot is going past the desired setting because of a delay in getting the feedback, or a delay in stopping the change in the system. If you see that you have gone past the commanded system state and backup, you can then over shoot in the opposite direction. Multiple over shoots are called oscillation. One solution to the delay problem is to slow down the system's rate of change so much that the delays and inertia are insignificant. This is usually not a popular solution as most designers want quick response from the machine. How the system responds to the error is tunable by setting the PID parameters. These settings will determine the speed of response, degree of over shoot, maximum error and final error. The proportional term will drive the output proportionally to the error, causing the system to change more rapidly as the error increases. As the error approaches zero, the proportional term also approaches zero and will always get there before the system does. Proportional control alone is the simplest to tune, but will result in some steady state error. Increasing the proportional term enough to limit the steady state error to a small value will likely cause oscillation. The integral term adds up the error as a function of time and drives the output harder as the error increases and as time in error increases. The integral of the flow rate of water is the depth in the bucket. The integral error bucket can be emptied, or go negative, if the error changes direction. This term is typically is slower to respond, but will cause an extremely small final error as any detectable error will continue to add up till the output changes in the direction to correct it. This term can also cause oscillation if set too high, and will need the help of the P term for fast machine operation. The derivative term reacts to the speed of change in error. This term is primarily used to slow down the change in system output as it nears the correct state, to limit over shoot. HCT does not include this term in some controllers that we do not believe will benefit from it. D does add to the complexity of the tuning process while some times allowing more aggressive tuning.

PID tuning can be done by most anyone, but can be very difficult if you do not have a computer interface with numbers that can be typed in and graphs to show the effects of the tuning. We do not sell adjustable PID controllers without these features. If you are using ten turn pots and a stop watch we wish you the best of luck. It is easier to tune a PID loop if you understand the basic concepts first.

This is intended to be a brief over view of a very complicated subject. See HCT manuals for more details on how to tune our products.