Proportional Plus Integral Control And Integration Constants - Johnson Controls Penn System 450 Series Technical Bulletin

Proportional Plus Integral Control and Integration Constants

In addition to standard proportional control, System 450 provides Proportional
plus Integral (PI) control capability. The addition of integral control enables a
properly set up analog output to drive a controlled condition closer to Setpoint
(Figure 8).
Standard proportional-only controls continuously adjust the output in proportion
to the difference (offset error) between the Setpoint value and the sensor value. As
the load on the system increases, the offset error increases. A proportional-only
control responds to the increased offset error by changing the output signal, which
drives the controlled equipment to compensate for the load change (Figure 8).
Proportional-only control loops are relatively easy to set up and adjust.
Typically, under constant system load, proportional-only control loops do not drive
a system to the selected Setpoint. Instead, the controlled system is maintained at a
control point within the proportional band (throttling range) between setpoint and
end point. The larger the load on the system, the further the control point drifts
from setpoint. Still, for many applications, proportional-only control is the best
choice for analog output control.
Offset Error
Zero
Offset
Error
Figure 8: Proportional Only Control Versus Proportional Plus Integral Control
Proportional plus Integral (PI) control incorporates a time-integral control action
with proportional control action and, if properly set up, a PI control loop can
effectively eliminate offset error and enable a controlled system to drive to setpoint
even under large constant loads (Figure 8). On a properly sized system with
predictable loads, PI control can maintain the controlled system very close to
setpoint.
A system's output capacity, the size of the load on the system, and the integration
constant selected determine the speed (recovery rate) at which the PI control drives
the system to setpoint.
System 450™ Series Modular Control Systems with Standard Control Modules Technical Bulletin
Proportional Only Control
Control point and output
follows the system load.
Integral control adjusts the output
to bring the system condition to
Setpoint regardless of system load.
End Point
Offset Error
End Point
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