Preset Frequency; Process Pi Loop - Allen-Bradley PowerFlex 70 Reference Manual

Preset Frequency

Process PI Loop

There are 7 Preset Frequency parameters that are used to store a discrete
frequency value. This value can be used for a speed reference or PI Reference.
When used as a speed reference, they are accessed via manipulation of the digital
inputs or the DPI reference command. Preset frequencies have a range of plus/
minus [Maximum Speed].
[PI Config]
[PI Control]
[PI Reference Sel]
[PI Setpoint]
[PI Feedback Sel]
[PI Integral Time]
[PI Prop Gain]
[PI Upper/Lower Limit]
[PI Preload]
[PI Status]
[PI Ref Meter]
[PI Feedback Meter]
[PI Error Meter]
[PI Output Meter]
The internal PI function provides closed loop process control with proportional
and integral control action. The function is designed to be used in applications
that require simple control of a process without external control devices. The PI
function allows the microprocessor to follow a single process control loop.
The PI function reads a process variable input to the drive and compares it to a
desired setpoint stored in the drive. The algorithm will then adjust the output of
the PI regulator, changing drive output frequency to try and make the process
variable equal the setpoint.
Proportional control (P) adjusts output based on size of the error (larger error =
proportionally larger correction). If the error is doubled, then the output of the
proportional control is doubled and, conversely, if the error is cut in half then the
output of the proportional output will be cut in half. With proportional control
there is always an error, so the feedback and the reference are never equal.
Integral control (I) adjusts the output based on the duration of the error. (The
longer the error is present, the harder it tries to correct). The integral control by
itself is a ramp output correction. This type of control gives a smoothing effect to
the output and will continue to integrate until zero error is achieved. By itself,
integral control is slower than many applications require and therefore is
combined with proportional control (PI).
Derivative Control (D) adjusts the output based on the rate of change of the error
and, by itself, tends to be unstable. The faster that the error is changing, the larger
change to the output. Derivative control is generally not required and, when it is
used, is almost always combined with proportional and integral control (PID).
Rockwell Automation Publication PFLEX-RM001H-EN-P - June 2013
Preset Frequency
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