Lenze MC3000 Series - Manual (Page 72)

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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 DP is set to XXX .X) .
For a reverse acting transducer, set Parameter 75 - FB @ MIN to the maximum process
variable feedback value, and set Parameter 76 - FB @ MAX to the minimum process
variable feedback value . In other words, Parameter 75 is set higher than Parameter 76 .
See the example below:
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 DP 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:
The REVERSE setting for Parameter 70 - PID MODE is not used to
compensate for reverse acting feedback devices . If a reverse acting feedback
device is used, program Parameters 75 and 76 as described above .
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 .
19 .4 SETPOINT REFERENCE SOURCES
The setpoint reference input can be one of the following:
1 .
keypad
2 . 4-20 mA signal at TB-5B
3 . 0-10 VDC signal at TB-5A
4 . Potentiometer signal at TB-5A