What is Cascade Control?
Applications with two or more capacities (such as heated jackets) are inherently difficult to control with a single control loop due to large overshoots and unacceptable lags. The solution is a cascade of two or more control loops, each with its own input, in series forming a single regulating device.
The product setpoint temperature is set on the master control loop. This is compared to the product temperature, and the master’s PID output is used to set the remote setpoint of the slave. This 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.
Historically Cascade Control has commonly been achieved by using 2 or more individual controllers, however it is possible to achieve this using a dual or multi loop controller that offers cascade control functionality.
Tuning for Cascade Control
Example of a Cascade Control Application
In the example below a product temperature is being controlled via a heated oil jacket. 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 towards setpoint, the slave’s heater output falls. The product’s temperature will also have begun rising at a rate dependant on the transfer lag between the jacket and product. This causes the masters 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.
The result is quicker, smoother control with minimum overshoot and the ability to cope with changes in the load, whilst keeping the jacket temperature within acceptable tolerances.
