Watlow CLS - Manual (Page 155)

Appendix B: Enhanced Process Control
147
Ratio control
Ratio control allows you to specify a process variable of one loop,
(Master loop), multiplied by a Ratio, to be the SP of another loop (Ratio
loop). A single loop can be either set up as Cascade or Ratio control, but
not both. You can assign any Process Variable to determine the SP of a
Ratio loop.
Setting Up Ratio Control
In order to set up Ratio control, you must configure the following
variables:
1) Ratio Process Variable assignment. The Master loop PV which will
provide the output to the internal controller SP calculation for the
Ratio loop SP.
2) Minimum SP. The lowest allowable value of the Ratio loop SP. The
minimum SP overrides any calculation caused by the Ratio calcula-
tion calling for a lower SP. This value is expressed in the same engi-
neering units as the Ratio loop PV, adjustable from the minimum
reading to the maximum reading.
3) Maximum SP. The highest allowable value of the Ratio loop SP. The
maximum SP overrides any calculation caused by the Ratio calcula-
tion calling for a higher SP. This value is expressed in the same engi-
neering units as the Ratio loop PV, adjustable from the minimum
reading to the maximum reading.
4) Control Ratio. The multiplier which is applied to the Master loop PV.
5) SP Differential. The amount to be added or subtracted from the Ratio
loop SP calculation before it is used as a SP. This value is expressed
in the same engineering units as the Ratio loop PV, adjustable from
the minimum reading to the maximum reading.
By adjusting the Ratio control parameters, you can adjust the influence
the Master loop PV has on the SP of the Ratio loop.
Calculation of new Ratio loop SP
SP = (SP Differential) + (Master PV)*(Control Ratio)
Master loop Process Variable
Ratio Loop
Min
setpoint
Max
setpoint