Lenze MCH Series - Manual (Page 75)

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19 MCH PID SETPOINT CONTROL
PID Setpoint Control allows the MCH drive to maintain a process setpoint, such as PSI
or CFM, without using an external controller. When PID is activated (and the drive is in
AUTO mode), the MCH will operate in a closed-loop fashion, automatically adjusting the
motor speed to maintain the setpoint.
PID setpoint control requires feedback from the process in order to compare the process
variable “value” to the setpoint. The difference between the process variable “value” and
the setpoint is called the error. The MCH will increase or decrease the motor speed
in an attempt to minimize the error. By constantly adjusting the motor speed, the PID
control will drive the process toward the setpoint. Refer to the PID block diagram below.
SETPOINT
Process Variable
Feedback (transducer)
Motor
Error Speed
Command
P
I
D
Process
19.1 FEEDBACK DEVICES
A transducer or transmitter is required to monitor the process variable and provide
feedback to the PID unit in order to compare the process variable feedback to the
setpoint. A transducer outputs a signal corresponding to a fixed range of the process
variable. A transmitter provides offset and gain adjustments to allow the output signal
to be adjusted to correspond to different ranges of the process variable. Typical output
signals for transducers and transmitters are: 0-5 VDC, 0-10 VDC, or 4-20 mA. The
feedback device must be externally powered, as the drive does not have a power supply
for such devices. Program Parameter 74 - PID FEEDBACK for the appropriate terminal
(TB-5A or TB-5B), and connect the feedback device as described below:
POT
The positive signal wire (wiper) is connected to TB-5A, and the “high”
lead is connected to TB-6.
0-5, 0-10 VDC Connect the positive signal wire to TB-5A.
4-20 mA Connect the positive signal wire to TB-5B.
The common, or negative signal wire, is connected to TB-2 (circuit common).
Feedback devices can be direct or reverse acting. A direct acting device outputs a
signal that increases as the process variable increases. A reverse acting device
outputs a signal that decreases as the process variable increases. The programming of
Parameters 75 - FEEDBACK @ MIN and 76 - FEEDBACK @ MAX depend on the type
of feedback device being used.
When using a direct acting transducer, Parameter 75 - FEEDBACK @ MIN should
be set to the value of the process variable feedback corresponding to the minimum
feedback signal (0 VDC or 4 mA), and Parameter 76 - FEEDBACK @ MAX should
be set to the value of the process variable feedback corresponding to the maximum
feedback signal (5 or 10 VDC, or 20 mA). Refer to the example herein.