Emerson Rosemount 56, Instruction Manual

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Instruction Manual Section 8: Time Proportional Control
LIQ-MAN-56 April 2017
Time Proportional Control 63
Section 8.0 Time Proportional Control
8.1 Introduction
8.1.1 Time Proportional Control
Time Proportional Control is more commonly known as Duty Cycle or Pulse Width Modulation.
It applies PID control to the activation of a relay rather than using an analog output.
The TPC output is defined as the percent of time that a relay is on (% On Time), during a user
selected time period (Time Period). As the control output increases the on time increases:
The proportional, integral, and derivative are defined the same as analog PID control, but
use % On Time instead of % Output:
Proportional On Time (%) = [Gain
×
(PV – SP) x 100 / (URV – LRV)] + BIAS
Proportional and Integral Control:
% On Time = [Gain
×
100 / (URV – LRV)] + [(PV – SP) + 1/I
Σ
(PV
t
– SP)
Δ
t]
Derivative Mode:
% On Time = [Gain
×
100 / (URV – LRV)]
×
D
×
[(PV
t
– SP)
−
(PV
t-1
– SP)] /
Δ
t
As with analog PID control, TPC can be direct acting (URV > LRV) or reverse acting (LRV >
URV). For more detail see Section X.1, PID Control Introduction.
Fig. 8-1 TPC Periods and On Time