Continuous positioners

Figure 8-74b is an example of a pneumatic positioner/actuator. The input signal is a pneumatic pressure that (1) moves the summing beam, w ch (2) operates the spool valve amplifier, which (3) provides flow to and from the piston actuator, which (4) causes the ac tuator to move and continue moving until (5) the feedback force returns the beam to its original position and stops valve travel at a new position. Typical positioner operation is thereby achieved. 

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