C4-4
IM 34M06H62-02E 2nd Edition : June 2008-00
C4.3 Limiting the Set Point
You can set upper limit and lower limit for the control set point (CSP).
This feature is useful in situations where it is desirable to limit the control set point within
a certain temperature range, depending on the characteristics of equipment and
materials involved.
- Set the SP Upper Limit (SPH) parameter to the desired upper limit and the SP Lower
Limit (SPL) parameter to the desired lower limit.
The SP limit function is always enabled, regardless of the remote/local selection.
It is always enabled for the secondary loop in cascade control, regardless of the
Automatic/manual/cascade selection.
Table C4.3 Parameters Related to the Set Point Limit Function
Data Position Number
Symbol Description Unit Data Range
Default
Value
Attribute
Stored
Loop1 Loop2 Loop3 Loop4
103 303 503 703 CSP Control set point
Industrial
unit
PRL to PRH — RO —
164 364 564 764 SPH Upper SP limit
Industrial
unit
PRL to PRH If SPL<SPH.
If SPL ≥ SPH, CSP is always
equal to SPL.
PRH RW
165 365 565 765 SPL Lower SP limit PRL RW
Set the SP limits such that SPL < SPH.
If SPL ≥ SPH, then the control set point (CSP) will always be equal to SPL.
If SPH and SPL are used in the secondary loop in cascade control, align the Upper
Output Limit (OH) and Lower Output Limit (OL) of the primary loop with SPH and SPL of
the secondary loop respectively. Otherwise, the control set point of the secondary loop
may be constrained by SPH or SPL of the secondary loop, resulting in a large PV
overshoot in the primary loop. For instance, if the SPH of the secondary loop is 50% of
the PV range between PRH and PRL, it is recommended that OH of the primary loop be
set to 50%.
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