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Bias (Manual Reset)
Used to manually bias the proportional output(s) to compensate for process load variations.
Bias is expressed as a percentage of output power and is adjustable in the range 0% to
100% (for Primary Output alone) or -100% to +100% (for both Primary and Secondary
Outputs). This parameter is not applicable if the Primary output is set to ON/OFF control
mode. If the process settles below setpoint use a higher Bias value to remove the error, if the
process variable settles above the setpoint use a lower Bias value. Lower Bias values will
also help to reduce overshoot at process start up.
Display code = 6<5: , default value = 25%.
Also refer to ON/OFF Control and PID.
Bumpless Transfer
A method used prevent sudden changes to the output power level when switching between
Automatic and Manual control modes. During a transition from Automatic to Manual, the
initial Manual Power value will be set to equal the previous automatic mode value. The
operator can then adjust the value as required. During a transition from Manual to Automatic,
the initial Automatic Power value will be set to equal the previous manual mode value. The
correct power level will gradually applied by the control algorithm.
Also refer to Manual Mode.
Cascade Control
Applications with two or more capacities (such as heated jackets) are inherently difficult for a
single instrument to control, due to large overshoots and unacceptable lags. The solution is
to cascade two or more controllers, each with its own input, in series forming a single
regulating device. The product setpoint temperature is set on the master controller. This is
compared to the product temperature, and the master's PID output (mA or VDC) is fed into a
remote setpoint input on the slave. The RSP is scaled to suit any expected temperature. The
slave loop's natural response time should ideally be at least 5 times faster than the master.
As the jacket temperature rises, the slave's heater output falls. The product temperature also
rises at a rate dependant on the transfer lag between the jacket and product. This causes the
master's PID output to decrease, reducing the 'jacket' setpoint on the slave, effectively
reducing the output to the heater. This continues until the system becomes balanced.
When tuning, first set the master to manual mode. Tune the slave controller using
proportional control only (I & D are not normally required) then return the master to automatic
mode before tuning the master. The result is quicker, smoother control with minimum
overshoot and the ability to cope with load changes, whilst keeping the jacket temperature
within acceptable tolerances.
Also refer to Manual Mode, Master & Slave, PID, Remote Setpoint, Remote Setpoint Lower
Limit, Remote Setpoint Upper Limit, Setpoint, Setpoint Select and Tuning.
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Glossary
- DIN Controllers & Indicators - Product Manual
Type: Controller Tuning Parameter
In the example, the maximum input
represents 400ºC, thus restricting the jacket
temperature. At start-up the master
compares the product temperature
(ambient) to its setpoint (300ºC) and gives
maximum output. This sets the maximum
(400ºC) setpoint on the slave, which is
compared to the jacket temperature
(ambient) giving maximum heater output.
Type: Controller Definition
Type: Controller Definition
59305, Issue 5 – March 2005
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