Effect of control action, hysteresis and deadband – Super Systems 3 Series User Manual
Page 61
Series 3
Operations Manual
61
11.5.1
Effect of Control Action, Hysteresis and Deadband
For temperature control ‘CONTROL ACTION’ will be set to ‘
rev
’
. For a PID controller this means that the heater power decreases
as the PV increases. For an on/off controller output 1 (usually heat) will be on (100%) when PV is below the setpoint and output 2
(usually cool) will be on when PV is above the setpoint
Hysteresis applies to on/off control only. It defines the difference in temperature between the output switching off and
switching back on again. The examples below shows the effect in a heat/cool controller.
Deadband can operate on both on/off control or PID control where it has the effect of widening the period when no heating or
cooling is applied. However, in PID control its effect is modified by both the integral and derivative terms. Deadband might be
used in PID control, for example, where actuators take time to complete their cycle thus ensuring that heating and cooling are not
being applied at the same time. Deadband is likely to be used, therefore, in on/off control only. The second example below adds
a deadband of 20 to the above example.
In an on/off controller, if CONTROL ACTION = rev then OP2 will be on when PV is below SP. OP1 will be on when the PV is above
SP. The outputs are, therefore, reversed in the above example.
Deadband OFF
Deadband ON
SP 300
o
C
HYST.C
HYST.H
OP1 On Heating
100%
No OP
OP2 On Cooling
100%
Heating and Cooling Type both on/off
Setpoint = 300
o
C
Control Action = reverse
Heating Hysteresis = 8
o
C
Cooling Hysteresis = 10
o
C
Deadband = OFF
Heating off
at SP
(300
o
C)
Cooling on at
SP + HYST.C
(310
o
C)
Cooling off
at SP
(300
o
C)
Heating on at
SP – HYST.H
(292
o
C)
SP 300
o
C
HYST.C
HYST.H
Heating and Cooling Type
both on/off
Setpoint = 300
o
C
Control Action = reverse
Heating Hysteresis = 8
o
C
Cooling Hysteresis = 10
o
C
Deadband 50% of cooling hysteresis = 5
o
C
Heating off
at SP
(300
o
C)
Cooling on at
SP + HYST.C
(310
o
C)
Cooling off at
D.BAND
(305
o
C)
Heating on at
SP – HYST.H
(292
o
C)
D.BAND
OP1 On Heating
100%
No OP
OP2 On Cooling
100%
Power deadband