Patent GB-850099-A (GB)
Title: Mechanical servo amplifier
IPC: G05G19/00
Filed: 1957-11-12
Abstract: 850,099. Cam mechanisms; screw and nut gearing. CURTIS - WRIGHT CORPORATION. Nov. 12, 1957 [Jan. 18, 1957], No. 35200/57. Class 80 (3). A cam mechanism functioning as a mechanical servo device provides an output movement derived from a source of power which is proportional to, or the integral of, or a combination of both, the movement of a control element. In its simplest form, Fig. 1, for proportional control a waved cam 17 is rotated steadily by a source of power at an input gear 19 to reciprocate through balls 20 a member 24 prevented from rotation by a key connection 26 with a casing 12. A rotatable control member 28 is arranged above the member 24 with a clearance 52 so that it is reciprocated by the member 24 but with periods of rest. The control member 28 has an upper waved cam surface 30 opposite a similar surface on an output member 36 between which are balls 32 in a cage 34. The members 28, 36 and 34 are coupled by a linkage 53 for a limited relative movement only and the member 36 which carries an output gear 44 is pressed down by springs 40 on to a stop 50 on the casing 12. In operation, if no control movement is imparted to member 28 the balls 32 remain at the bottoms of their cam surfaces and reciprocation only is imparted to member 36. If member 28 is moved in one direction or the other the balls 32 are displaced from their neutral positions to move up opposite cam slopes of members 38 and 36. Thus, during an upward movement of member 24 the member 28 is locked from rotating by frictional engagement with member 24 and a turning torque is exerted through the balls 32 and cam slopes on the output member 36 to restore the neutral positions of the balls and their cam slopes. The output member 36 is thereby rotated in proportion to the control movement with an amplification of torque. Fig. 2 shows an integral response mechanism in which movement of the output member continues as long as a control movement from a neutral position is effective. The lower part of this mechanism is similar to the Fig. 1 device but a member 57 is substituted for the member 36. This member 57 terminates in an upper face 58 and is restrained to a limited rotation by a centring spring 60. Above the face 58 is an output member 66 spring-pressed against an abutment 72. In operation, on a displacement of member 28 in one direction or the other the balls 32 are displaced as before and on the upward movement of member 24 a rotation is imparted to member 57 as well as an upward movement against the centring spring 60. During this movement the surface 58 engages and rotates the output member 66. On the subsequent downward movement of member 24 the surface 58 disengages from member 66 leaving it in a new position of rotation and member 57 is restored to its original position by spring 60. The balls 32 thus again assume their displaced position so that the above operation is repeated on the next upstroke of member 24 and the actuation of output member 66 continues as long as the control member 28 is displaced. In another arrangement the devices of Figs. 1 and 2 are combined, the proportional response being added to the integral response by means of ball type differential. In a further modification the proportional response is continued in diminishing amounts over a number of pulses of the system instead of only one. A similar result can be provided by a constant rate integral response device combined with a variable rate integral response device, the latter being progressively diminished until it becomes equal and opposite to the constant rate response. The application of the invention to the control of a load through a pair of nuts co-operating with a severed shaft is also described.