Proportional-only control adjusts the controller output, proportional to the proximity of PV to
SV. The closer to setpoint, the lower the output %. The further from setpoint, the higher the
output %.
Proportional-only control, however, has a side effect (known as droop or offset) where the
process is brought to equilibrium, but not exactly at setpoint. This is where Integral control is
useful, as it checks for droop as often as you specify in the ‘ti’ parameter (Menu 3, PID).
Proportional-Integral (PI) control is fine for systems where there are no sudden process
changes expected. In cases where sudden changes to process are possible (or even frequent),
then Derivative control can be used.
Derivative control (‘td’ in Menu 3, PID) is known as anticipatory control, as it looks at the rate
of change in a process and makes output adjustments to prevent process disturbances.
It calculates where the process will be at some point in the future, given the present rate of
change, and uses that information to determine the output adjustment needed.
6.10 PID ‘Time Proportioning’ Control
PID control is easy to picture with a linear analog output, where a 4 to 20mA output, for
example, responds linearly to the 0 to 100% control output variations. 0% power = 4ma, 50%
power = 12ma, 100% = 20mA, and so on.
A relay, however, is very different from a linear analog output in that it has only two states, ON
and OFF, where the 4 to 20 signal is continuous throughout its range. So, to apply PID control
to a relay, Time Proportioning PID control is used.
Time Proportioning adjusts the ratio of relay ON-time to relay OFF-time, over an adjustable
time duration, known as the Cycle Time (‘ct’ parameter in Menu 3, PID).
The controller ‘throttles’ the output relay, keeping the relay ON longer, when the PV is further
from SV, and OFF longer, when the PV is close to the SV.
The ratio of relay ON-time to relay OFF-time is automatically adjusted at the start of each Cycle
Time window, based on PV/SV proximity.
Time Proportioning control is active when the Proportional Band ‘Pb’ (or Cooling Proportional
Band ‘CPb’) parameter is set to some value other than ‘0’ in Menu 3, PID.
The examples below illustrate three stages in a process where the output % is changed, based
on process conditions.
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