538,413. Fluid-pressure servomotors : hydraulic control mechanism. VICKERS Inc. Feb. 1, 1940, No. 1950, Convention date, Feb. 7, 1939. [Classes 69 (ii) and 135] In the operation of steering and like gear by a power unit 9 which may be a Selsyn unit or a hydraulic follow-up unit such as described in Specification 391, 775, [Group XXXI], mechanism is provided for minimizing the lag which may occur between the signal unit 5 and the output shaft 7, which operates the final controlled member. As shown in Fig. 1, the control member 10 of the power member 9 is driven by differential gearing 12 - 15 from the shaft 6 of the signal member which is also connected by gears 21, 22, 23, to the bevel gear 17 of a second differential gear 16 - 20. The gear 18 of this differential is connected by gears 24, 25, 26 to the output shaft 7, and the ring gears 15, 20 of the two differentials. The arrangement is such that as long as the shaft 7 lags behind the shaft 6 rotation of ring gear 20 takes place in such direction that the control member 16 is accelerated to a greater speed than the shaft 6 so that any lag of the shaft 7 is soon overcome, and the motion is synchronized. The gears 15, 20 then remain stationary. In a modification, Fig. 2, the output shaft 7 is driven by a constant displacement hydraulic motor 27 driven by a variable displacement pump 28 operated by a motor 29 and controlled by a tilting block 30 connected to a piston 32 in a cylinder 33 formed in a housing 34 formed with a cylinder 35 in which a piston 36 operates. Pipes 37, 38 supply the cylinders with hydraulic fluid and operation of the control shaft 6 actuates a gear pump 39 which forces the fluid into the housing 34. A differential gear 49 is arranged between the shaft 6 and bevel gears 45, 46, 47, by means of which a second gear pump 40 is driven. If the shaft 6 is moving faster than the shaft 7 the ring gear of the differential rotates slightly and by means of a shaft 54, lever 55, and rod 56 depresses the piston 36 which causes the piston 32 to rise and operate the arm 30 of the pump 28, 80 as to increase the speed of the motor 27 and shaft 7. In a further modification the piston 32 operates in a single cylinder and is normally controlled by a centring spring, and a third gear pump is substituted for the cylinder 35 and piston 36, and this applies the correction factor to the piston 32. The mechanism also operates upon deceleration to correct the lead of the follow-up mechanism.
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