Integral Control; Derivative Control - Honeywell CVL4022ASVAV1 Engineering Manual

INTEGRAL CONTROL

The purpose of the integral function is to eliminate the offset
inherent in proportional control. Integral control functions to
hold the input sensor value at setpoint. See Fig. 14
Fig. 36. Proportional Integral control
The integral function is a result of proportional error and time.
When the proportional error is greater than 0, the integral
error is calculated and added to the proportional error to
determine the control loop output. The integral error is
cumulative and continues to increase as long as the
proportional error is greater than 0. The increase in the output
signal drives the controlled device further open and the
controlled medium is brought closer to the setpoint. While the
proportional error is reduced, the integral error continues to
increase until the proportional error is eliminated. When the
proportional error equals 0, the calculated integral error is no
longer increasing or decreasing and no change is made to the
output. When the proportional error is less than 0, the integral
error decreases in value.
The integral time value is set in seconds based on the lag time
of the controlled process. A slow process such as space
temperature control requires a long integral time (600 seconds
or more), while a fast process such as static pressure control
requires a short integral time. An integral time of 0 (default)
eliminates the integral function for the control loop.
Stability of the PI control loop is a balance of the throttling
range and the integral time. If a PI control loop is unstable,
increase the throttling range and/or increase the integral time.
Generally the throttling range required for PI control is greater
than what is used for proportional control only. PI control
should only be used in closed loop applications. Without
feedback from the controlled medium, integral windup occurs.
Integral windup is a run away condition in which the integral
error continues to increase due to the lack of proportional
corrections. Plan the control strategy to insure integral windup
does not occur or cause problems in the system performance.

DERIVATIVE CONTROL

The purpose of derivative control is to reduce ringing or
severe overshoot and undershoot when there is a significant
load change in a short period of time. See Fig. 37 through 39.
31-00083—02
Fig. 37. Proportional control
Fig. 38. Proportional integral control
Fig. 39. Proportional integral derivative control
NOTE: Few applications in HVAC require the use of
derivative control. The loop stability and time
associated with tuning derivative loops dictates
that derivative not be used unless absolutely
necessary.
Applications that are candidates for derivative control are very
fast responding and subject to sudden large changes in
external load conditions. These include static pressure control
of fan systems, direct fired gas units, oversized controlled
equipment and in some chilled water temperature control
cases. Each application should be carefully examined before
applying derivative control.
30