2.8.4 Control Structure Closed Loop
The 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 set-point 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 set-point 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 set-point reference, the drive will slow down to reduce the
pressure. In a similar way, if the pipe pressure is lower than the set-point reference, the drive will automatically speed up to increase the pressure provided
by the pump.
NB!
While the default values for the drive’s Closed Loop controller will often provide satisfactory 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.
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.
2 Introduction to VLT Automation VT Drive
Automation VT Drive FC322 Design Guide
26
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