Technical Bulletin—TC-9100 Universal Controller 27
Working Set Point (WSP)
The working set point of a controller is calculated from the internal set
point of the controller called the Local Set Point (LSP), Input I2 known
as the Remote Set Point (RS), and Input I3 known as the Reference
Variable (RV). When the controller is in STANDBY mode or NIGHT
mode, an additional parameter is used in the calculation, known as the
“Set Point Bias During Standby” (BSB), or the “Set Point Bias
During Night” (BNT). These set point bias values will normally be
negative numbers for heating controllers to reduce the WSP during
standby or night periods, and positive numbers for cooling controllers to
increase the WSP during standby and night periods. The working set
point of a controller is given by the following equations:
In COMFORT mode:
WSP = (LSP + RS) × RV
In STANDBY mode:
WSP = (LSP + RS) × RV + BSB
In NIGHT mode:
WSP = (LSP + RS) × RV + BNT
Note: In OFF mode, the control module is calculated as in NIGHT mode
but the control module will only operate if the OFF mode override
for that control module has not been enabled.
The working set point is also shifted when the Winter or Summer
Compensation function is enabled for the controller concerned. Refer to
Summer/Winter Compensation for further details.
The working set points of programmable modules configured as
controllers can be read by a supervisory system. Refer to Supervisory
Functions for further details.
Error Dead Band in PI Controller
The error dead-band in a PI controller is defined by configuration as 1%
or 0.1% of the proportional band. When the control error (PV - WSP) is
less than the error dead-band, as a positive or negative number, the
integral action of the controller is suspended and the integral term is not
changed. This feature prevents unnecessary cycling of the output when
the process variable is very close to the working set point, due to the slow
increase of the integral term over time.
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