West Control Solutions Pro-EC44 - Manual (Page 85)

Pro-EC44 2-Loop Graphical Profile Controller & Recorder
Pro-EC44 Product Manual - 59540-1 October 2013 Page 78
11 Cascade Control
Applications with long time lags (e.g. with two or more capacities such as heated jackets) can
be difficult to control with a single control loop. The solution is to split the process into two or
more cascaded loops consisting of a Master and Slave(s) acting on a common actuator.
Ideally, the slave loop’s natural response time should be at least 5 times faster than the
master.
The master controller measures the process temperature and compares it to the desired
product setpoint. Its
correcting variable (0 to 100% PID output) becomes the slave’s effective
setpoint (scaled to suit the process). This setpoint is compared to the slave’s process input,
and the controlling actuator is adjusted accordingly.
Note: Cascade control is only available on models fitted with the 2nd control
loop. The master loop uses input 1; and the slave loop uses input 2.
Example Cascade Application
In this example the controlling actuator is a heater, indirectly heating the product via an oil
jacket. The maximum input to the slave represents 300ºC, thus restricting the jacket
temperature. At start-up the master compares the product temperature (ambient) to its
setpoint (250ºC) and gives 100%. This sets the maximum slave setpoint (300ºC), which is
compared to the oil temperature (ambient) and the slave requests maximum heater output.
Figure 47. Cascade example
As the oil temperature rises towards the slave setpoint, its output falls. Gradually, the product
temperature will also begin rising, at a rate dependant on the transfer rate/lag between the oil
jacket and the product. Eventually this
causes the master’s PID output to decrease, reducing
PRODUCT
0-300°C
Slave SP
0-100%
Output
SLAVE SENSOR
MASTER SENSOR
250°C Master Setpoint
HEATER
O
IL
J
AC
KET
MASTER
IP1
OP SP
SLAVE
IP2
OP SP