model 2704 controller installation and operation handbook - Soliton

2704 Controller Loop Set Up
9.4. CASCADE CONTROL
9.4.1. Overview
Cascade control is a technique used to enable processes with long time constants to be
controlled with the fastest possible response to process disturbances, including setpoint
changes, whilst still minimising the potential for overshoot. It is a combination of two PID
controllers, where the output signal from one (the master) forms the setpoint for the other (the
slave). For cascade control to be effective the slave loop should be more responsive than the
master.
9.4.2. Simple Cascade
The main process is controlled using the master PID loop, the output of which is used to
determine the setpoint of the slave. The implementation of cascade control in the 2704 is
available as a standard option. ie ; it is not necessary to order a dual loop controller to
perform cascade control.
9.4.3. Cascade with Feedforward
An available option with cascade control is feedforward. It allows either the master PV,
master SP or user defined variable to be fed forward so that it directly influences the slave
setpoint. The master PID output contribution of the slave setpoint is limited by the trim limit
set in engineering units. This parameter also adjusts the gain of the feedforward path.
A typical application for SP feedforward could be in a heat treatment furnace, where it can be
used to extend the life of heating elements by limiting their maximum operating temperature.
An application using PV feedforward could be in autoclaves or reactor vessels where it is
sometimes required to protect the product from excessive temperature gradients (also referred
to as Delta T Control).
9.4.3.1. Standard Feedforward
Standard feedforward is used if there is a requirement for some additional parameter, for
example an analogue input, to trim the master PID output value before the slave setpoint is
applied. An application may be a liquid temperature control system using cascade control of
heater temperature where variations in control rate can be directly fed forward into the slave
loop, modifying heater temperature and giving rapid compensation
9.4.4. Auto/Manual Operation
Auto/Manual operates on both master and slave loops.
When the controller is placed in manual the slave working setpoint will track the value of the
slave process value continually, therefore ensuring bumpless transfer.
When cascade is deactivated the master loop will monitor the setpoint of the slave loop and
provide a smooth transition of output power when the loop moves back to cascade mode.
Installation and Operation Handbook. Part No HA026502 Issue 1.0 Apr-00 9-9