When the input pressure applied to the SIG port of the
positioner increases, bellows q press balance lever w to the
left. As this movement moves flapper r to the left through
connecting spring e, the gap between nozzle t and flapper
r widens, and the nozzle back pressure of pilot valve y
drops. As a result, the pressure balance in the constant
pressure chamber is broken, and exhaust valve !4 presses
inlet valve B !5 to the right. Then, inlet port B opens, and
output pressure OUT1 increases.
In the meantime, the movement of exhaust valve !4 to the right
opens exhaust port A, and output pressure OUT2 decreases.
Therefore, pressure difference is generated between pressure
chamber 1 and pressure chamber 2 of oscillating actuator u,
and actuator shaft i turns in the direction of the arrow. The
movement of actuator shaft i deflects feedback arm !2 to the
right through feedback shaft o, cam !0, and bearing !1. Such
deflection increases the tension of feedback spring !3 and acts
on balance lever w.
Since oscillating actuator u moves until the tensile force of
feedback spring !3 and the force generated by bellows q
balance, it is always set in the position proportional to the input
pressure. When the signal air pressure decreases, the
operation is reversed.