PID function block diagram for cascade control exampleFigure 7-22:
7.9.12 Cascade control with override
You can use the PID function block with other function blocks for complex control
strategies.
Figure 7-23 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 input 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 block
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.
Foundation Fieldbus
Reference Manual 169
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