Technical Bulletin—TC-9100 Universal Controller 25
ON
OCM = Output Control Module
DIF = Differential
WSP = Working Set Point
PV = Process Variable
DIF
WSP
OFF
0
100
Output
(OCM)
Input (PV)
Figure 7: Reverse-acting (Heating) On/Off Controller
ON
OFF
WSP
DIF
0
100
Output
(OCM)
Input (PV)
Figure 8: Direct-acting (Cooling) On/Off Controller
Proportional/Integral (PI) Controller
When configured as a PI controller, the programmable module
continuously compares its Input I1, known as the process variable (PV),
with its Working Set Point (WSP), and linearly increases its output (OCM)
from 0% when the PV is equal to the WSP, to 100% when the PV exceeds
the WSP by a value equivalent to the proportional band (PB) for a direct-
acting controller, or from 0% when the PV is equal to the WSP, to 100%
when the PV is less than the WSP by a value equivalent to the Proportional
Band (PB) for a reverse-acting controller.
The proportional band is defined as a percentage of the range of the process
variable. For an analog input, the range is defined as the difference between
the High Range Input (HRI) and the Low Range Input (LRI). See Analog
Input Configuration for details. For example, if HRI = 40 and LRI = 10, the
range of the process variable is (40 - 10) = 30. A proportional band of
100% is then equivalent to 30 units of the process variable, and a PB of
10% is equivalent to 3 units of the PV. If the process variable is the output
of a programmable module, the range is fixed at 100 and the proportional
band is in units of the output signal.
An Integral Action Time (TI) in “repeats per minute” may be defined for a
direct-acting or reverse-acting controller. When TI is set to zero, the
integral action is disabled. The integral action creates an “integral term,”
which is added to the “proportional term” created by the proportional
action.
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