Liebert TM-10098 service manual Proportional + Integral + Derivative PID Control

iCOM Control Training and Service Manual

The integral (I) term is determined by two things: the difference between the return air temperature and control set point and the amount of time this difference has existed. This term is expressed in % cooling (heating) desired for each minute and degree above (below) the set point. It is adjustable from 0% - 100% per degree/minute. The purpose of this term is to force the control to maintain the temperature around the set point by slowly but continuously adding (subtracting) a small amount of cooling (heating) to the total control output until the temperature is at the set point.

Proportional + Integral + Derivative (PID) Control

The PID control combines three (3) individual terms to determine the control output for a given set of conditions. Note that PID control is used only for temperature. If PID control is selected, the humidity control will be in percent relative humidity (%RH).

The proportional (P) term is determined by the difference between the current temperature and the control set point. This term is expressed in % cooling (heating desired for each degree above (below) the set point. It is adjustable from 0% to 100% per degree. The purpose of this term is to adjust the control output for any deviation between the current temperature and the control set point.

The integral (I) term is determined by two things: the difference between the return air temperature and control set point and the amount of time this difference has existed. This term is expressed in % cooling (heating) desired for each minute and degree above (below) the set point. It is adjustable from 0% - 100% per degree/minute. The purpose of this term is to force the control to maintain the temperature around the set point by slowly but continuously adding (subtracting) a small amount of cooling (heating) to the total control output until the temperature is at the set point.

The derivative (D) term is determined by the rate of change of temperature. This term is expressed in % cooling (heating) desired for each degree per minute rise (fall) in temperature. It is adjustable from 0% to 100% per degree/minute. The purpose of this term is to adjust the control output for quickly changing temperatures, thus providing an anticipation control.

All three terms are adjusted by selecting the “Setpoints” icon in either the USER or SERVICE Menu screen. If PID control is selected, the temperature proportional band value (and optional temperature dead band value) is not used by the control. For optimum performance, a PID control must be adjusted or tuned according to the characteristics of the particular space and load to be controlled. Improper tuning can cause the control to exhibit poor response and/ or hunting. The characteristics of the space and load may change seasonally, so occasional returning is required for optimum performance.

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