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CONTROL MODES
14.1.2. 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 (relay
energized) time is decreased. This type of control can provide 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.
Control Period
is the time interval required for updating control output. High dynamic processes
require frequent control updates, meaning shorter Control periods.
Dead Band
represents an area where the error between Set point and process value is considered 0.
Dead Band area is unidirectional, for Control mode Low is below the Set point, for control mode High
is above the Set point.
Outputs
Proportional control block
Set point
Control mode
Deviation
Control period
Minimum On time
Overtime
Control output
Control enabled
Dead Band
• Control output as 0 to 100%
Update rate
= Control period
Enabled by
• Settings
• Controller status
Figure 26:
Proportional Control Block
