Specify Scaling; Use Pid Instructions - Allen-Bradley Logix5000 Reference Manual

Chapter 13 Special Instructions (FBC, DDT, DTR, PID)
In this field
PV unscaled maximum
PV unscaled minimum
PV engineering units maximum Enter the maximum engineering units corresponding to .MAXI (.MAXS)
PV engineering units minimum Enter the minimum engineering units corresponding to .MINI (.MINS)
CV maximum
CV minimum
Tieback maximum
Tieback minimum
PID Initialized
(1) When using the ladder-based PID instruction, if you set MAXO = MINO, the PID instruction will reset these values
to default. MAXO = 100.0 and MINO = 0.0

Use PID Instructions

518

Specify Scaling

Select the Scaling tab. You must click OK or Apply for any changes to
take effect.
Specify
Enter a maximum PV value (.MAXI) that equals the maximum unscaled value received from the
analog input channel for the PV value.
Enter a minimum PV value (.MINI) that equals the minimum unscaled value received from the
analog input channel for the PV value.
Enter a maximum CV value corresponding to 100% (.MAXCV).
Enter a minimum CV value corresponding to 0% (.MINCV).
Enter a maximum tieback value (.MAXTIE) that equals the maximum unscaled value received from
the analog input channel for the tieback value.
Enter a minimum tieback value (.MINTIE) that equals the minimum unscaled value received from
the analog input channel for the tieback value.
If you change scaling constants during Run mode, turn this off to reinitialize internal descaling
values (.INI).
PID closed-loop control holds a process variable at a desired set point. The
illustration shows an example of a flow-rate/fluid level.
Setpoint
Flow Rate
In the above example, the level in the tank is compared against the setpoint. If
the level is higher than the setpoint, the PID equation increases the control
variable and causes the outlet valve from the tank to open; thereby decreasing
the level in the tank.
The PID equation used in the PID instruction is a positional form equation
with the option of using either independent gains or dependent gains. When
using independent gains, the proportional, integral, and derivative gains affect
only their specific proportional, integral, or derivative terms respectively.
Rockwell Automation Publication 1756-RM003N-EN-P - October 2011
Error
-
+
Process Variable
Level Detector
(1)
(1)
PID Equation
Control Variable
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