Lenze MCH Series - Manual (Page 76)

72 13435744_EDBMH01_v13
Example 1: A 0-100 psi transducer outputs a 4 mA signal at 0 psi and 20 mA at 100
psi. Program Parameter 75 to 0.0 PSI and Parameter 76 to 100.0 PSI (this
assumes that Parameter 31 - UNITS is set to pid: PSI and Parameter 33 -
UNITS DECIMAL is set to XXX.X).
For a reverse acting transducer, set Parameter 75 - FEEDBACK @ MIN to the maximum
process variable feedback value, and set Parameter 76 - FEEDBACK @ MAX to the
minimum process variable feedback value. In other words, Parameter 75 is set higher
than Parameter 76. Refer to Example 2.
Example 2: A 0-100 psi transducer outputs a 20 mA signal at 0 psi and 4 mA at 100
psi. Program Parameter 75 to 100.0 PSI and Parameter 76 to 0.0 PSI
(This assumes that Parameter 31 - UNITS is set to pid: PSI, and Parameter
33 - UNITS DECIMAL is set to XXX.X).
19.2 THE SYSTEM - DIRECT AND REVERSE ACTING
The entire system can also be direct or reverse acting. In a direct acting system, an
increase in motor speed causes an increase in the process variable. This is equivalent
to requiring an increase in motor speed in response to a decreasing process variable
feedback.
In a reverse acting system, an increase in motor speed causes a decrease in the
process variable. This is equivalent to requiring an increase in motor speed in response
to an increasing process variable feedback. Examples of direct and reverse acting
systems are described in the next section.
19.3 PID CONTROL - DIRECT AND REVERSE ACTING
The PID Setpoint Control software can be direct or reverse acting, and must be set to
match the system. This is accomplished with Parameter 70 - PID MODE. When set to
NORMAL (for direct acting systems), the PID unit will command an increase in motor
speed if the process variable feedback decreases, and a decrease in motor speed if
the process variable feedback increases. When set to REVERSE (for reverse acting
systems), the opposite is true.
NOTE:
Do not use the REVERSE setting for Parameter 70 - PID MODE to
compensate for reverse acting feedback devices. If a reverse acting
feedback device is used, program Parameters 75 and 76 as described
above in Section 19.1.
An example of a direct acting system is one that maintains duct pressure. Duct pressure
is the process variable that is monitored by the feedback device. As duct pressure
rises, motor speed needs to be decreased to maintain the pressure. A direct acting PID
control will command a decrease in motor speed in response to an increasing process
variable feedback.
An example of a reverse acting system is one that maintains water temperature in
a cooling tower. Water temperature is the process variable that is monitored by the
feedback device. As the water temperature rises, motor speed needs to be increased
to lower the temperature of the water. A reverse acting PID control will command an
increase in motor speed in response to an increasing process variable feedback.