56 | P a g e
A well-tuned PI controller will result in smoother pressure control but may not be required. When the
controller is in simple mode the only parameters which affect the pressure control loop are the psi
SETPOINT, STARTUP RAMP, SHUTDOWN RAMP and BOOST AMOUNT, which function in the same
way as they do in the PI control mode. Adjusting ramp times may smooth out any oscillation or
overshoot in simple mode. If pressure is not adequately controlled, switch to PI control by increasing
parameter PROPORTIONAL GAIN.
Using PI Control in Constant Pressure
When using an analog pressure transducer for control in CP water systems, it may be desirable to use
a proportional-integral (PI) controller in the feedback loop. This type of controller has a proportional
gain and integral gain which can be tuned by the user to obtain optimum performance for each
particular application. Increase parameter PROPORTIONAL GAIN to switch control from Simple Mode
to PI control. It is recommended to start with a parameter value of 5. A proportional gain value too low
will result in slow response time to reach the psi SETPOINT. A value too high will result in overshoot of
the psi SETPOINT and may create excessive oscillation of pressure.
First, attempt to control pressure by adjusting parameter PROPORTIONAL GAIN, leaving parameter
INTEGRAL GAIN at the default value of 15. If adequate control cannot be obtained by adjusting
proportional gain, set proportional gain at the value that gives the best control, and then adjust integral
gain to improve pressure control.
For difficult to control systems, a derivative term can be introduced for PID control, which can help
control pressure oscillation and overshoot. Increase the value of parameter DERIVATIVE GAIN to a
value greater than zero to enable PID control. This parameter should be used only when necessary as
it tends to amplify noise in the transducer signal. The system may become unstable. The parameters
PID FILTER GAIN and PID FILTER TIME help to prevent overshoot. It may be advisable to contact the
factory for assistance when using PID control for the first time.
In conceptual terms, proportional gain affects how quickly the system responds to pressure changes
and integral gain affects the accuracy of pressure tracking. Adjusting ramp times can also be
considered. Increasing ramp time will damp response to pressure changes, while decreasing ramp time
will quicken the response.
CAUTION: Long ramp times can interfere with PI control of constant pressure. It is advisable to
start with factory default ramp times.
Pressure Control at Minimum Speed
There is a possibility of conflict between the minimum pump speed setting, controlled by the parameter
MIN FREQUENCY, and the pressure setting of the transducer. That is, if under no-flow conditions the
pump at its minimum speed setting produces a pressure greater than the desired set-point, either the
minimum speed will have to be reduced (not an option with submersible pumps) or the pressure set-
point will have to be increased. Most pumps should not produce enough pressure head at 30Hz for this
to be an issue.
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