Patent GB-741878-A (GB)
Title: Improved apparatus for calibrating electric wire-wound resistance elements
Inventor: PEARSE WILFRED HERBERT
IPC: H01C17/245
Filed: 1953-01-16
Abstract: 741,878. Calibrating wire-wound resistance elements. ELLIOTT BROS. (LONDON), Ltd. March 17, 1954 [Jan. 16, 1953], No. 2239/53. Class 37. [Also in Group XXIII] A machine for calibrating wire-wound potentiometer elements of rod-like form comprises means for moving a potentiometer element axially to-and-fro through a fixed location at which is disposed a buffing or abrading member adapted to remove metal from the winding of the element. An electro-mechanical transducer continuously controlled by prevailing winding resistance value automatically operates to bring the buffing or abrading member into contact with the winding. In Figs. 1 and 2 a carriage 5 as it travels to-and-fro between brackets 2 on the machine base 1 moves a potentiometer element 6 in engagement with two spaced-apart fixed contacts 52 and as the resistance between these contacts deviates from a standard or comparison resistance value, a transducer 58 responds to the sense of the deviation and urges a grinding wheel 68 into or away from metal-removing contact with the potentiometer element 6. The carriage 5 has end plates 17, 18 comprising spindles 21, 19 and the resistance element 6 is spring-releasably held between these spindles 21, 19. The two brackets 2 support a rotatable shaft 24 and as the carriage 5 approaches the right-hand bracket 2 after the length of the resistance element has passed the grinding-wheel 68 a conical pin 33 on the carriage 5 displaces a roller 35 (see Fig. 5) to operate ratchet pawl 28/30 so that through friction clutch 29 the shaft 24 is advanced one step and the two spindles 21, 19 and therefore the potentiometer element 6 likewise via gearing 36, 37 whereby the potentiometer element 6 is attacked by grinding wheel 68 along a different line throughout the next to-and-fro travel of the carriage 5. As the shaft 24 turns an annular member 51 carrying the two contacts 52 is also advanced one step by gearing 53, 54. The carriage 5 is cable-driven at 7, 8 to run by rollers 4 on guide-rods 3 and the motor driving the cable drum 8 is automatically reversed by limit switches 13/15 and 14/16 at each end of travel of carriage 5. When the shaft 24 has made a predetermined number of stepwise angular advances a limit switch 46, 47 operates to stop the motor driving the cable drum 8 and enable the element 6 to be replaced by another one to be calibrated. The transducer 58 is contained in a pivoted box 57 and the box table 59 has mounted thereon a rotatable L-shaped bracket 65, 66 whose upstanding arm 65 carries a short spindle 67 to which is secured the grinding wheel 68 which bites into the wire of element 6 to an extent determined by micrometer 71 adjustable jewel stop 69, 70 (see Fig. 4). The forwardly extending bracket arm 66 co-operates with a leaf-spring 76 and a flexible link 74 secured to the output arm of transducer 58 (see Fig. 5). According as the sense of the transducer 58 output varies (i) the flexible link 74 pulls downwards bracket arm 66 to swing away the grinding wheel 68 from element 6 or (ii) a thrust is applied through the flexible link 74 and spring blade 76 to urge the grinding wheel 68 into engagement with element 6. The grinding wheel 68 via a flexible cable 77 and belt/pulleys 80-82 is driven by a motor 84 and the flexible cable 77 extends through a hollow shaft 78 rotatably mounted on the shaft 64 for the bracket 65, 66 so that movement of the bracket 65, 66 does not interfere with the drive of the grinding wheel 68. As the carriage 5 reaches either end of travel the grinding wheel 68 is swung away by rollers 85 co-operating with cam faces 86 and the contacts 52 are displaced by cam surfaces 88 on the sockets 20, 22 on the ends of spindles 19, 21. If desired only one contact 52 may run on element 6 w th another contact made to one end of element 6 so that the rate of change of resistance may be measured and compared with a fixed resistance value representing the required rate of change. Also the adjustment of the resistance of the element 6 may be such as to produce variation in accordance with a selected non- linear law.