784,506. Fluid-pressure servomotor-control systems; valves. MINNEAPOLIS-HONEYWELL REGULATOR CO. Nov. 19, 1953 [Nov. 19, 1952], No. 32089/53. Class 135. In an automatic process controller, the variations of the process variable are measured by a measuring unit which is coupled either directly or through a differential of invariable ratio to a control unit, e.g. a nozzle-baffle arrangement; the output of the control unit, besides being applied to a regulating unit, e.g. a diaphragm-operated valve, is applied through two series-connected restrictions of adjustable ratio to a capacity in which the pressure is either kept constant or becomes equal to the output pressure as the controller stabilizes, and a negative feedback chamber having a total volume equal to or smaller than the capacity is connected to the junction of the two restrictions. In an embodiment, the variable is measured by a Bourdon tube 30, Fig. 1, and the movement of the latter is applied through a floating lever 40 and lever 50 to a hinged flapper F controlling the escape of fluid from a nozzle N which is supplied with pressure air from a source F.A.S. past a restriction Nr. The variable back pressure produced at the nozzle is transmitted at Po to the process regulating unit and is also applied through a restriction Rb to a bellows 62 where it can act on the lever 40 as a negative feedback signal giving derivative control. The desired value of the variable is set by adjusting a handle 120 connected to one end of the Bourdon tube. On a deviation of the variable from the desired value, a flow takes place through restrictions Rb, Ra in series and into or out of capacities 70, 71, and the negative feedback bellows. It is stated that, by adjustment of the ratio of restrictions Ra, Rb, the proportional band of the controller can be varied. A positive feedback giving integral control is obtained by applying the nozzle back pressure through a separate restriction to a pressure responsive device acting on lever 40 in opposition to bellows 62. In this modification, the capacity 70 is provided with connections to a pressure air supply and to exhaust, and the exhaust connection is controlled from the pressure in the integral bellows. Numerous modifications of the embodiment shown in Fig. 1 are described. In another embodiment, Fig. 7, the actual and desired values of the variable, and the derivative and integral control effects are applied as pressures to the separate chambers Pv, Ps, Pn, Pr, respectively, of a single stack. The various chambers are bounded by diaphragms connected to a single spindle which acts on the flapper F of a nozzleflapper combination. The nozzle is supplied with pressure air from a supply F.A.S., past a restriction Nr, but, in this embodiment, the restrictions Rb, Ra are fed with the nozzle back pressure signal through an amplifier PVR, the final output signal Po being similarly amplified. The flow path through the restrictions takes place now to and from the chamber Pr. In the amplifier, the nozzle back pressure is applied to a bellows 2 which, through a stem 3, acts on a hinged plate 9 controlling the admission of pressure air from a line 7 to a chamber 6 in communication with the outlet conduit 11. When the pressure in chamber 6 rises sufficiently, it acts on a further bellows 5 also connected to the stem 3 to lift the latter and allow the inlet valve 9 to close to shut off the pressure supply. Further lifting of the stem clear of the plate 9 frees an exhaust passage through the stem to vent chamber 6, thus giving a proportional action to the amplifier. Numerous modifications of this form of the controller are also described.