user manual

DCP200 Profile Controller & Recorder - Product Manual
51-52-25-150, Issue 1 – April 2009 Glossary Page 137
Bar Graphs
The instrument displays a bar-graph in the base operation mode screen. These can show
PID Power Output (single control = uni-directional, dual control = bi-directional), Control
Deviation (bi-directional) or for Data Recorder version %Memory Used (uni-directional). Bar-
graphs are uni-directional or bi-directional depending on the information to be displayed.
Also refer to: Control Deviation, Data Recorder, Display Configuration, Operation Mode, Main
Menu and PID
Bias (Manual Reset)
Used to manually bias proportional output(s) to compensate for control deviation errors due
to process load variations. Bias is expressed as a percentage of output power. This
parameter is not applicable if the Primary output is set to ON-OFF control. If the process
variable 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. Integral action performs a similar
function automatically when using PI or PID control.
Lower Bias values will also help to reduce overshoot at process start up.
Settings = 0 to 100% (-100% to +100% for dual control). Default value = 25%.
Also refer to: Control Deviation, Integral Action, ON/OFF Control, PI Control, PID,
Proportional Control, Process Variable, and Setpoint.
Bumpless Transfer
A method used to prevent sudden changes to the correcting variable, when switching
between automatic PI or PID and Manual control modes. During a transition from PI or PID to
Manual control, the initial Manual Power value is set to the previous automatic mode value.
The operator then adjusts the value as required. During a transition from Manual control to PI
or PID, the initial automatic value is set to the previous Manual mode value. The correcting
variable level will gradually adjusted by the control algorithm at a rate dependant on the
integral action resulting from the Integral Time Constant value. Since integral action is
essential to Bumpless Transfer, this feature is not available if Integral is turned off.
Also refer to: Correcting Variable, Integral Action, Manual Mode, PI and PID.
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 to form 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
the auxiliary input of the slave controller as a remote setpoint input. 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.