Hanna Instruments HI520 - Page 87

87 Control Modes & Algorithms
By setting hysteresis, an upper and lower control limit is created. The switching around the Set point is thus reduced.
SP
Control mode High
Control mode Low
Hysteresis
Process value
Control output
Control output
On
Off
On
Off
Figure 26: On/Off Control with Hysteresis
Running control On continuously for an extended period of time is prevented by Overtime control action.
SP
Control mode High
Overtime
Hysteresis
Process value
Control output
Alarm condition
On
Off
Figure 27: On/Off Control, Overtime Control Action
Relay On time has a guaranteed minimum to prevent stressing the actuators electrically or mechanically.
SP
Control mode High
Minimum On time
Hysteresis
Process value
Control output
Figure 28: On/Off Control, Minimum On Time