Patent GB-819790-A (GB)
Title: Improvements in magnetic recording apparatus
IPC: G05D3/14
Filed: 1956-12-20
Abstract: 819,790. Magnetic recording and reproducing apparatus. INTERNATIONAL BUSINESS MACHINES CORPORATION. Dec. 20, 1956 [Dec. 23, 1955], No. 38807/56. Class 40 (2). A device for selectively positioning a transducer includes mechanism for moving the transducer to selected positions along a predetermined path and means for differentially damping the operation of the moving mechanism. The embodiment described is directed to the mechanism and electrical circuits for moving a magnetic transducer 13 to any selected track on any selected record 11 of a stack mounted on a rotatable shaft 12. The transducer 13 is mounted at the end of a bar 14 which is horizontally movable in a carriage 15 vertically movable on an upright way 16. Both motions are effected by a cable 17 driven in either direction by a motor 31 via magnetic clutches 32 or 33 according to the direction of movement required. The cable 17 is connected at one end 21 of bar 14, passes over a pulley 23 on the carriage 15, over top pulley 24 in upright 16, down round a drive pulley 25, up over a further pulley 29 on carriage 15, and finally to the transducer end of bar 14 where it is secured. It will be seen that locking the carriage 15 against vertical movement causes the cable 17 to drive the bar 14, while locking the bar 14 (in its outermost position as shown in Fig. 1) causes the cable 17 to lift or lower the carriage 15. The carriage 15 may be locked, by operating means not shown, in any vertical position by a conical member 45 which is engageable with any one of a number of apertures 46 in the upright 16, and when such engagement is made a tongue 43 is withdrawn from the end of a bar 48 on bar 14 to permit horizontal movement thereof. Until the bar 14 returns to its initial position, the tongue 43 is held back by bar 48 to prevent the withdrawal of member 45. The movements of the carriage 15 and bar 14 are controlled by tapped potentiometers 51, 58, respectively, Fig. 5, connected across floating voltage sources. The potentiometer 51 comprises a straight resistance element mounted on the upright 16 and having tappings corresponding to the positions of the record discs. Contact brushes 52, 53 on the carriage 15 correspond to the adjustable contact 65 in Fig. 5. Potentiometer 58 has a circular tapped element and is mounted on the carriage 15. Its movable contact, corresponding to contact 63 in Fig. 5, is coupled to the bar 14 by a rack 64 and pinion 62. The carriage 15 and slide bar 14 are driven to desired positions by the settings of switch arms 57, 61, respectively, on potentiometers 51, 58. If initially when power is switched on the bar 14 is in the withdrawn position shown in Fig. 1, the contacts a and b of a relay 67 will be in the normal position shown in Fig. 5, bar 14 will be looked, and carriage 15 will be lifted or lowered in dependence on the polarity of the voltage between contact 65 and switch 57. The error voltage so derived is fed to one of the input grids 68 of a difference amplifier, the other input grid 71 of which is connected to a potentiometer 72 across the output of a tachometer 73 coupled to driving pulley 25. Initially the tachometer voltage will be zero and thus which magnetic clutch 32, 33 is operated will be in dependence on the polarity of the error voltage. The tachometer voltage rises when drive is established, and remains constant when maximum velocity is reached until it exceeds the error voltage. This tends to reverse the direction of drive of pulley 25, but owing to inertia the effect is merely to brake, and drive ceases when the error voltage is zero. The tachometer in effect anticipates the destination determined by the. position of switch 57 so that braking causes the carriage to come to rest at its destination with a minimum of hunting. The optimum voltage from the tachometer to accomplish this is effected by adjustment of the potentiometer 72. As the carriage 15 comes to rest and is locked in position by member 45 bar 14 is unlocked and relay 67 is energized whereby contacts a and b are moved to the opposite positions to those shown in Fig. 5 and drive is restored through the medium of an error voltage derived from the setting of potentiometer 58 and the slide bar 14 driven to its selected position, damping being in this case derived from a potentiometer 78 across the tachometer 73. Additional damping is provided, according to the length and direction of movement of carriage 15 or bar 14, by means of a diode 84 or 85 which provide voltages which are a function of the error signal, i.e. a function of the length of travel, adjustment for optimum value being made by means of rheostats 86 and 89. Specification 819,789 is referred to.