Pid - Danfoss VLT AQUA Drive Design Manual

2. Introduction to the VLT AQUA Drive
Reference Handling - Local Reference
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2.9. PID

2.9.1. Closed-loop (PID) Controller
The drive's closed-loop controller allows the drive to become an integral part of the controlled
system. The drive receives a feedback signal from a sensor in the system. It then compares this
feedback to a setpoint reference value and determines the error, if any, between these two signals.
It then adjusts the speed of the motor to correct this error.
For example, consider a pump application where the speed of a pump is to be controlled so that
the static pressure in a pipe is constant. The desired static pressure value is supplied to the drive
as the setpoint reference. A static pressure sensor measures the actual static pressure in the pipe
and supplies this to the drive as a feedback signal. If the feedback signal is greater than the
setpoint reference, the drive will slow down to reduce the pressure. In a similar way, if the pipe
pressure is lower than the setpoint reference, the drive will automatically speed up to increase
the pressure provided by the pump.
The figure is a block diagram of the drive's closed-loop controller. The details of the reference
handling block and feedback handling block are described in their respective sections below.
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NOTE
While the default values for the drive's closed-loop controller will often provide sat-
isfactory performance, the control of the system can often be optimized by adjusting
some of the closed-loop controller's parameters. It is also possible to autotune the
PI constants.
MG.20.N2.22 - VLT ® is a registered Danfoss trademark.
VLT ® AQUA Drive Design Guide