model 2704 controller installation and operation handbook - Soliton

2704 Controller Loop Set Up
9.6. OVERRIDE CONTROL
9.6.1. Introduction:
Override Control allows a secondary control loop to override the main control output in order
to prevent an undesirable operating condition. The override function can be configured to
operate in either minimum, maximum or select mode.
A typical example can be implemented in a heat treatment furnace with one thermocouple
attached to the workpiece, and another situated close to the heating elements. Control of the
furnace during the heating up period is regulated by the override (heating element)
temperature controller which provides a safeguard against overheating. Control of the
furnace will switch over to the workpiece temperature controller at some point when the
temperature is near to its target setpoint. The exact point of switchover is determined
automatically by the controller, and will be dependent on the selected PID terms.
9.6.2. Simple Override
Override control is available with analogue, time proportioning and ON/OFF control outputs.
It is not available with valve position outputs. Figure 9.3 shows a simple override control
loop. The main and override controller outputs are fed to a low signal selector. The override
controller setpoint is set to a value somewhere above the normal operating setpoint, but
below any safety interlocks.
There is one Auto Manual switch for both loops. In manual mode the control outputs of both
loops track the actual output therefore ensuring bumpless transfer when auto is selected.
Main SP
Main PV
Override SP
Main
Control
Loop
Main OP
Min select
Control
Output
Override PV
Override
Control
Loop
Override OP
Figure 9-3: Simple Override Control
Installation and Operation Handbook. Part No HA026502 Issue 1.0 Apr-00 9-13