PID Control Technical Notes
PID (Proportional-Integral-Derivative) control action allow the process control to accurately maintain setpoint by adjusting the control outputs. In this technical note we have attempted to explain what PID is in practical terms. We have available further technical references for our customers.
ISE has a complete line of PID controls suitable for virtually an application. We also have numerous tools (such as software, data loggers and recorders) to help to optimize any control application. Our application engineers have extensive practical knowledge in the tuning of PID controls to all types of applications.
While controls can be used for many different process variables for clarity we have chosen to use temperature as the process variable throughout these notes. Other processes can utilize these control concepts and the effects will be the same.
PROPORTIONAL & PID CONTROL ACTION
Proportioning control continuously adjusts the output dependent on the relative positions of the process temperature and the setpoint. PID (Proportioning/Integral/Derivative) are control functions commonly used together in today's controls. These functions when used properly allow for the precise control of difficult processes.
1) Proportioning Band: is the area around the setpoint where the controller is actually controlling the process; The output is at some level other than 100% or 0%. The band is generally centered around the setpoint (on single output controls) causing the output to be at 50% when the setpoint and the temperature are equal.
On (2) two output controls (i.e.: heat/cool) there are two proportioning bands. One is for heating and one is for cooling. In this case the bands generally end at the setpoint as shown below.
Proportioning bands are normally expressed in one of three ways:
If the proportioning band is too narrow an oscillation around the setpoint will result. If the proportioning band is too wide the control will respond in a sluggish manner, could take a long time to settle out at set point and may not respond adequately to upsets.
Manual Reset: Virtually no process requires precisely 50% output on single output controls or 0% output on two output controls. Because of this many older control designs incorporated an adjustment called manual reset (also called offset on some controls). This adjustment allows the user to redefine the output requirement at the setpoint. A proportioning control without manual or automatic reset (defined below) will settle out somewhere within the proportioning band but likely not on the setpoint. Some newer controls are using manual reset (as a digital user programmable value) in conjunction with automatic reset. This allows the user to preprogram the approximate output requirement at the setpoint to allow for quicker settling at setpoint.
Automatic Reset (Integral): Corrects for any offset (between setpoint and process variable) automatically over time by shifting the proportioning band. Reset redefines the output requirements at the setpoint until the process variable (temperature) and the setpoint are equal. Most current controls allow the user to adjust how fast reset attempts to correct for the temperature offset. Control manufacturers display the reset value as minutes, minutes/repeat (m/r) or repeats per minute (r/m). This difference is extremely important to note for repeats/ minute is the inverse of minutes or minutes/ repeat). The reset time constant must be greater (slower larger number m/r smaller number r/m) than the process responds. If the reset value (in minutes/repeat) is too small a continuous oscillation will result (reset will over respond to any offset causing this oscillation). If the reset value is too long (in minutes/ repeat) the process will take too long to settle out at setpoint. Automatic reset is disabled any time the temperature is outside the proportioning band to prevent problems during startup.
Below is an example of a single output (heat only temperature control) with a 10% proportioning band and a setpoint of 400. Note how reset shifts the proportioning band when the temperature (PV) enters the proportioning band.
Reset stops moving the proportioning band as soon as the setpoint and PV are equal. In the above example reset determined approximately 38% output is required to maintain setpoint. Stable control is achieved and the temperature matches the setpoint of 400.
Rate (Derivative): Shifts the proportioning band on a slope change of the process variable. Rate in effect applies the "brakes" in an attempt to prevent overshoot (or undershoot) on process upsets or startup. Unlike reset rate operates anywhere within the range of the instrument. Rate usually has an adjustable time constant and should be set much shorter than reset. The larger the time constant the more effect rate will have. Too large of a rate time constant will cause the process to heat too slowly. Too short and the control will be slow to respond to upsets. The time constant is the amount of time any effects caused by rate will be in effect when rate is activated due to a slope change.
Self Tuning /Adaptive Tuning / Pre-Tuning
Many control manufactures provide various facilities in their controls that allow the user to more easily tune (adjust) the PID parameters to their process. Below is a description of same.
Tuning On Demand with Upset: This facility typically determines the PID parameters by inducing an upset in the process. The controls proportioning is shut off (on-off mode) and the control is allowed to oscillate around a setpoint. This allows the control to measure the response of the process when heat is applied and removed (or cooling is applied). From this data the control can calculate and load appropriate PID parameters. Some manufactures perform this procedure at setpoint while others perform it at other values. Caution must be excersized for substantial swings in the process variable values will likely occur while the control is in this mode.
Adaptive Tuning: This mode tunes the PID parameters without introducing any upsets. When a control is utilizing this function it is constantly monitoring the process variable for any oscillation around the setpoint. If there is an oscillation the control adjusts the PID parameters in an attempt to eliminate them. This type of tuning is ideal for processes where load characteristics change drastically while the process is running. It cannot be used effectively if the process has externally induced upsets for which the control could not possibly tune out. For example: A press where a cold mold is inserted at some cyclic rate could cause the PID parameters to be adjusted to the point where control would be totally unacceptable.
Some manufactures call Tuning on demand Self Tune, Auto Tune or Pre-Tune. Adaptive tuning is sometimes called Self Tune, Auto Tune or Adaptive Tune. Since there is no standardization in the naming of these features questions must be asked to determine how they operate.
General Control Types
ON-OFF CONTROL ACTION
On-Off control is the most basic form of temperature control.
Time Proportioning Controls
Linear Output Controls
Closed Loop Valve Motor Controls
Open Loop Valve Motor Controls
High & Low Limit Controls
Go to ISE's Process Control Page.
Go to ISE's Thermocouple Technical Reference Page.
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