Super Systems 3 Series - Manual (Page 51)

Series 3 Operations Manual
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• Quick response to deviations from the setpoint caused by external disturbances, thereby rapidly restoring the PV to the
setpoint value.
Tuning involves setting the following parameters:
Proportional Band ‘PB’, Integral Time ‘TI’, Derivative Time ‘TD’, Cutback High ‘CBHI’, Cutback Low ‘CBLO’, and Relative Cool
Gain ‘R2G’ (applicable to heat/cool systems only).
The controller is shipped with these parameters set to either the customer’s specifications or default values. In many cases the
default values will give adequate stable straight line control, however, the response of the loop may not be ideal. Because the
process characteristics are fixed by the design of the process it is necessary to adjust the parameters in the controller to achieve
best control. To determine the optimum values for any particular loop or process, it is necessary to carry out a procedure called
loop tuning. If significant changes are later made to the process that affect the way in which it responds, it may be necessary to
retune the loop.
Users have the choice of tuning the loop automatically or manually. Both procedures require the loop to oscillate and both are
described in the following sections.
11.3.1 Loop Response
If we ignore the situation of loop oscillation, there are three categories of loop performance:
Under Damped - In this situation the terms are set to prevent oscillation but do lead to an overshoot of the Process Value
followed by decaying oscillation to finally settle at the Setpoint. This type of response can give a minimum time to Setpoint but
overshoot may cause problems in certain situations and the loop may be sensitive to sudden changes in Process Value. This will
result in further decaying oscillations before settling once again.
Critically Damped - This represents an ideal situation where overshoot to small step changes does not occur and the process
responds to changes in a controlled, non oscillatory manner.
Over Damped
The balancing of the P, I and D terms depends totally upon the nature of the process to be controlled.
- In this situation the loop responds in a controlled but sluggish manner which will result in a loop performance
which is non ideal and unnecessarily slow.
In a plastics extruder, for example, a barrel zone will have a different response to a die, casting roll, drive loop, thickness control
loop or pressure loop. In order to achieve the best performance from an extrusion line, all loop tuning parameters must be set to
their optimum values.
11.3.2 Initial Settings
In addition to the tuning parameters listed above, there are a number of other parameters which can have an effect on the way in
which the loop responds. Ensure that these are set before either manual or automatic tuning is initiated. Parameters include, but
are not limited to:
Setpoint. Set this as closely as practicable to the actual setpoint in normal operation.
Load Conditions. Set the load conditions as closely as possible to those which will be met in practice. For example, in a furnace
or oven application a representative load should be included, an extruder should be running, etc.
Heat/Cool Limits. The minimum and maximum power delivered to the process may be limited by the parameters ‘OUTPUT
LOW’ and ‘OUTPUT HIGH’ both of which are found in the Control list. For a heat only controller the default values are 0 and
100%. For a heat/cool controller the defaults are -100 and 100%. Although it is expected that most processes will be designed to
work between these limits there may be instances where it is desirable to limit the power delivered to the process. For example, if
driving a 220V heater from a 240V source the heat limit may be set 80% to ensure that the heater does not dissipate more than its
maximum power.
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The measured value must oscillate to some degree for the tuner to be able to calculate values. The limits must be set to
allow oscillation about the setpoint.