36
Parameter
Meaning or Function
Proportional
Band
The proportional term, in display unit or %, delivers and output which
is proportional to the size of the error signal.
Integral Time
Removes steady state control offsets by ramping the output up or
down in proportion to the amplitude and duration of the error.
Derivative Time
Determines how strongly the controller will react to the rate of change
in the measured value. It is used to prevent overshoot and
undershoot and to restore the PV rapidly if there is a sudden change
in demand.
High Cutback
The numbers of display units, above set point, at which the controller
will increase the output power, in order to prevent undershoot on cool
down.
Low Cutback
The number display units, below set point, at which the controller will
cut back the output power, in order to prevent overshoot on heat up.
Relative Cool
Gain
Only present if cooling has been configured. Sets the cooling
proportional band, which equals the heat proportional band value
divided by the cool gain value.
The controller uses a one shot tuner which automatically sets up the initial values of the
parameters listed in the table.
The ‘one-shot’ tuner works by switching the output on and off to induce an oscillation in the
measure value. From the amplitude and period of the oscillation, it calculates the tuning parameter
values.
If the process cannot tolerate full heating or cooling being applied, then the levels can be restricted
by setting the high power limit (‘O P .HI’) and low power limit (‘O P .LO’). However, the measured
value must oscillate to some degree for the tuner to be able to calculate values.
A One-Shot tune can be performed at any time, but normally it is performed only once during the
initial commissioning of the process. However, if the process under control subsequently becomes
unstable (because its characteristics have changed), you can re-turn again for the new conditions.
It is best to start tuning with the process at ambient conditions and with the SP close to the normal
operating level. This allows the tuner to calculate more accurately the low cutback and high
cutback values that restrict the amount of overshoot or undershoot.
How to Tune:
Set the setpoint to the value at which you will normally operate the process.
Operator level 1 list, press,
until A.Tune appears. Set to ON.
Press the Page and Scroll buttons together to return to the Home display. The display will
flash [tunE] to indicate that tuning is in progress.
After one (1) minute to determine steady state conditions the controller will induce an
oscillation in the temperature by turning the output on and then off. The first cycle will not
complete until the measured value has reached the required setpoint.
