Burkert Type 8620 - Manual (Page 87)

mxCONTROL Type 8620
Page 87
Set value limiter (SP Limit)
The set point limiter is used to limit the adjustable set point through a minimum and maximum value in
order to prevent the definition of wrong set points, that is, beyond this valid range by the
user/operator.
In case of control with external set point or "cascade control" internal set point, the set point limiter can
also be used to limit the min. and max. set point.
Ramp function for set point setting (SP Ramp)
The ramp function serves to attenuate sudden changes of the set point and can be parameterized
separately for positive and negative set point jumps. Possible instability problems can thereby be
avoided which may occur with major set point changes in purely tuned systems.
SP
Figure 28: Slope of the ramp
If the ramp function is enabled and a sudden change of the set point occurs which is greater than the
rise of the ramp, the new set point is not immediately specified to the controller. Over a period of
time defined by the slope and the set point change, small changes will be fed to the controller until
the desired new set point has been reached.
If the operator switches from manual to automatic mode while the ramp is activated, the set point
will be increased/decreased from the last process value in manual mode to the set point in automatic
mode according to the defined slope.
In manual mode the set point after ramp is set to the current process value.
The ramp function is parameterized via the separate specification of the rise for positive and negative
set point changes. Each of the max. permitted positive (D+) and negative (D-) set point change must
therefore be parameterized per minute.
PID Controller
The Proportional Integral Differential (PID) Controller as well as the PI Controller is used to control the
process values, e.g. with proportional actuators (proportional valves or pumps) - by outputting the
control output 0 … 100 % as:
• Pulse Width modulated (PWM) signal or
• Pulse Frequency modulated (PFM) signal or
• Analog (4…20 mA) signal.
Controller output limitation:
The controller output is limited only in automatic mode by the parameterized lower and upper output
limit (Lim-, Lim+).
Cycle time Tsample:
The cycle time of the controller can be set with the controller parameter “Tsample“ (in seconds). In that
cycle time (in automatic mode) the current and the set point values will be regularly compared and a
new control output will be calculated.