Control Modes & Algorithms 90
Once enabled, and within the Deviation, the duration of the activated control is proportional to the variance; as
the measurement approaches set point, the On period (relay energized) diminishes.
Note: When configuring the Setup values for this control type, it is important to understand the dynamics of
the process. This can be determined by manually adding chemicals to the process and seeing how long they
take to react. The control period should be approximately 1½ times it takes the system to react. If this time
is too short an additional dose causes overshooting the desired Set point, if it is too long, the Set point may
never be reached.
SP
Relay
Control
period
On
Off
Control Low for HI520 base-dosing solution
75% 50% 25% 0%100%
Figure 30: Control Low with Relay On, Set Point and Deviation
On
Off
Control High for HI520 acid-dosing solution
0%
Control
75%
50%
25%
Relay
100%
period
SP
Figure 31: Control High with Relay On, Set Point and Deviation
Following graphs exemplify how the input parameters work.
Relay On time is proportional with Setpoint variance over Control period.
SP
Control period
Control mode
High
Control mode
Low
Deviation
Process value
y out
y out
On
On
Off
Off
Figure 32: Proportional Control, Relay Out ‑ Control Mode High/Low
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