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In practice, it performs PID computation based on the deviation between the setpoint set
in the actual mode and the PV, and generates a value of its output OUT so as to decrease
the deviation. The PID block works with other function blocks such as the AI and AO
blocks connected to it. The major functions of the PID block include (Figure 71):
❑
Filtering
❑
Setpoint limiters - both for the value and rate of change
❑
Scaling of process variable (PV), setpoint (SP), and output (OUT)
❑
PID control computation
❑
Control action bypass
❑
Feed-forward
❑
External-output tracking
❑
Measured-value tracking
❑
Output limiters
❑
Mode shedding upon computer failure
❑
Alarm generation
Modes
The target mode for the PID function block can be set from five block modes: ROut,
RCas, Cas, Auto, Man, and O/S. Regardless of the target mode, the PID block
automatically enters the IMan or LO mode when a specified condition is met (such as
when another function block enters a specific status), depending on the parameter
settings.
Figure 71 Function Diagram of PID Function Block
FVP 110 Fieldbus Valve Positioner Manual
158
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