System 450™ Series Modular Control Systems with Standard Control Modules Technical Bulletin
20
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 7).
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 7). 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.
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 7). 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.
The integration constant establishes the rate at which the control readjusts the analog output signal. The faster the
integration constant, the faster the control readjusts the output signal and the faster the recovery rate of a properly
sized and setup control loop.
Note: PI control is not suitable for all controlled systems. Improperly applied PI control loops are unstable and
can overshoot setpoint, resulting in control loop oscillation. Also, with PI control, the proportional band
(throttling range) and the integration constant are interdependent and you must properly set up these
values in relation to each other. You must also properly size the system equipment to handle the maximum
load. Close observation over several cycles and under different load condition is required to properly set up
a PI control loop. On a properly sized system, a PI control loop can drive the system condition much closer
to setpoint than proportional-only control.
Figure 7: Proportional-Only Control Versus Proportional Plus Integral Control
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