Figure 7-23 PID Function Block Diagram for Cascade Control Example
7.12.12 Cascade Control with Override
You can use the PID function block with other function blocks for complex control strategies.
Figure 7-24 illustrates the function block diagram for cascade control with override.
When configured for cascade control with override, if one of the PID function blocks connected
to the selector inputs is deselected, that PID block filters the integral value to the selected value
(the value at its BKCAL_IN). The selected PID block behaves normally and the deselected
controller never winds up. At steady state, the deselected PID block offsets its OUT value from
the selected value by the proportional term. When the selected block becomes output-limited,
it prevents the integral term from winding further into the limited region.
When the cascade between the slave PID block and the Control Selector block is open, the open
cascade status is passed to the Control Selector block and through to the PID blocks supplying
input to it. The Control Selector block and the upstream (master) PID blocks have an actual
mode of IMan.
If the instrument connected to the AI block fails, you can place the AI block in Manual mode and
set the output to some nominal value for use in the Integrator function block. In this case, IN at
the slave PID block is constant and prevents the integral term from increasing or decreasing.
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