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The controller generates an output signal
that represents the demanded position. This signal is sent to
the positioner. Externally, the positioner consists of an
input connection for the control signal, a supply air input
connection, a supply air output connection, a supply air vent
connection, and a feedback linkage. Internally, it contains
an intricate network of electrical transducers, air lines,
valves, linkages, and necessary adjustments. Other
positioners may also provide controls for local valve
positioning and gauges to indicate supply air pressure and
control air pressure (for pneumatic controllers). From an
operator’s viewpoint, a description of complex internal
workings of a positioner is not needed. Therefore, this
discussion will be limited to inputs to and outputs from the
positioner.
In Figure F2, the controller responds to a
deviation of a controlled variable from setpoint and varies
the control output signal accordingly to correct the
deviation. The control output signal is sent to the
positioner, which responds by increasing or decreasing the
supply air to the actuator. Positioning of the actuator and
control valve is fed back to the positioner through the
feedback linkage. When the valve has reached the position
demanded by the controller, the positioner stops the change
in supply air pressure and holds the valve at the new
position. This, in turn, corrects the controlled variable’s
deviation from setpoint.
Figure F2
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