West Control Solutions 8700+ - Manual (Page 142)

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- DIN Controllers & Indicators - Product Manual
59305, Issue 7
– March 2014 Glossary Page 137
Bias (Manual Reset) Type: Controller Tuning Parameter
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 =

, default value = 25%.
Also refer to ON/OFF Control and PID.
Bumpless Transfer Type: Controller Definition
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 at a rate dependant on the
integral action resulting from the Automatic Reset time. Since integral action is essential to
Bumpless Transfer, this feature is not available if Automatic Reset is turned off.
Also refer to Automatic Rest and Manual Mode
Boundless VMD Control Type: VMD Controller Definition
- Refer to Open Loop VMD.
Cascade Control Type: Controller Definition
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 pr
oduct 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.
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.
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.