Pid - Danfoss VLT Series Design Manual

2. Introduction to VLT HVAC Drive

2.9. PID

2.9.1. Closed Loop (PID) Controller
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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 ventilation system where the speed of the supply fan is to be controlled
so that the static pressure in the duct 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 duct 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 duct pressure is lower than the setpoint reference, the drive will automatically speed up to
increase the pressure provided by the fan.
The figure is a block diagram of the drive's
Closed Loop Controller. The details of the Ref-
erence Handling block and Feedback Handling
block are described in their respective sections
below.
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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 ad-
justing some of the Closed Loop Controller's parameters.
MG.11.B2.02 - VLT
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is a registered Danfoss trademark
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VLT HVAC Drive Design Guide