Control Modes & Algorithms 88
On/Off Control Interaction with Controller Status
Function / Mode Control Output
Relay assigned to
Setpoint Control Output
Analog Output assigned to
Setpoint Control Output
Control
Measure 0 or 100 % Off or On Scaled value of control output
Start up 0 % Off Scaled value of control output
Hold 0 % Off
Scaled value of last control output
or a fixed value of control output
Alarm* 0 % Off
Scaled value of control output or
22 mA, if option configured
Cleaning 0 % Off
Scaled value of last control output
or a fixed value of control output
Edit 0 % Off
Scaled value of last control output
or a fixed value of control output
Calibration 0 % Off
Scaled value of control output
or a fixed value of control output
Manual 0 % On or Off Any value in range 0 to 22 mA
Error 0 % Off Scaled value of control output
* Controlled parameter alarms, overtime control alarms, probe disconnected
18.2.2. Proportional Control Algorithm
With proportional Control (Proportion) the controller drives the relay from continuous On to Off in a defined control
period. The Relay On time of the activated control is proportional to the “deviation value”, a variance from the
Set point. At the full deviation the relay is fully On with the maximum output occurring. As the measurement
approaches the Set point through the deviation, the On time (relay energized) is decreased. This provides tighter
control of a process variable compared to On / Off control.
It is best used in batch or recirculating systems that retain the solution for a period of time.
Inputs
• Set point as an absolute controlled parameter value
• Control mode as High or Low
• Deviation as a relative parameter
• Control period as time
• Dead Band as a relative parameter value
Where:
Deviation is the interval aligned with the Set point where control output can take values from 0 to 100 %.
0 % indicates no action and 100 % indicates full control output action. If control output is assigned to a relay,
0 % control output will keep relay Off during control time, while 100 % will drive relay On for this entire time.
A low value for this parameter is suitable for low latency processes, allowing the control system to react quickly
and strongly.
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