Open and Closed-Loop Feedback Systems:
How Fast Controllers Will Change The Output

All controllers (pneumatic, op amp, and microprocessor) have some method of determining the speed at which they will adjust the output signal (between 0-100%) when some amount of error is detected. This is usually accomplished by circuits that control the gain, reset, and rate for the controller. In the older pneumatic controllers these factors were controlled by adding springs and levers to allow more or less air to be sent to the output device. If the full amount of air (15 psi) is sent to the output it will operate at 100%, and if the minimum amount of air (3 psi) is sent to the output, the output will operate at 0%. The op amp controller uses capacitors to provide some amount of time delay for the functions of gain, reset, and rate, which will determine the speed at which output changes.

Microprocessor controllers use mathematical calculations called algorithms to determine the amount of adjustment that must be made to the output signal when it receives a change in the error signal. The error signal is determined by comparing the SP to the PV signal from the sensor.

It may be easier to understand why a system must adjust the speed response of the output by using an example of a simple heating system. In this example the controller is trying to control the temperature in a laboratory testing furnace. The temperature may not vary too much unless someone opens the door to the furnace to add a new test specimen. If the furnace is operating at 450°F and the door is opened, the temperature may drop 15°F while the new test sample is placed inside. This drop in temperature will be seen as error and the controller will change the output accordingly to try and get the temperature back to 450°F. The technician can set the gain, reset, and rate variables to make the controller change the output immediately or gradually, depending on the type of sample that's being tested. Some types of materials may have limitations that require that their temperature may be increased at a rate of 1 degree per minute, so the controller response can be set by the gain value to regain the lost 15°F slowly over 15 minutes.

Another type of material may be able to absorb the change at a rate of 10°F per minute, and the gain can be changed to regain the lost temperature in 1.5 minutes. The good thing about the microprocessor is that different gain values can be programmed in for each different type of sample that's being tested.

PREV: Valve Positioning System To Control Water Level In A Tank

NEXT: Understanding Gain, Reset, and Rate

Top of page