Extech Instruments 48VTR User Manual page 9

Time Proportioning Control
To overcome the limitations of ON-OFF control (setpoint overshoot, wide process oscillations,
'droop', etc.), Time Proportioning can be used.
Time Proportioning control is active when Proportional Band 'Pb' (2
other than '0'. Proportional Band is a key tuning parameter as discussed below.
When Time Proportioning is active, the controller 'throttles' the output relay, keeping the
relay on for longer periods when the PV is far from SV, and vice versa. The ratio of output ON
time to output OFF time is automatically adjusted for each 'Cycle Time'.
At the start of each time cycle, the controller re-adjusts the % output, based on PV/SV
proximity. The examples below show three points in a typical process where the output % is
changed, based on process conditions.
DIAGRAM 1
Diagram 1: The controller is calling for 80% power since the PV is far from the SV.
Diagram 2: The PV is closer to SV, so the controller calls for 50% power.
Diagram 3: The PV is very close to SV, so the output is at 10% power.
The controller must be tuned, and the Cycle Time set accurately, before Time Proportioning
can be effective. See the Auto Tune section for tuning details. Cycle Time should be set to
DIAGRAM 3
9
nd Menu) is set to any value
DIAGRAM 2
48VTR User Manual v6.0 10/1/20