Patent GB-517423-A (GB)
Title: Improvements in electrical measuring systems
IPC: G08C19/02
Filed: 1938-07-23
Abstract: 517,423. Potentiometers. KENT, Ltd., G., and SAVAGE, W. July 23, 1938, No. 21937. [Class 37] [Also in Groups XXIV and XXXVIII] A self-balancing network, such as a potentiometer to which thermo-couples are connected in turn for recording a number of temperatures, has a coarse adjustment operative when the galvanometer deflection exceeds a certain value and a fine adjustment which comes into operation when the deflection is be1ow that value The galvanometer pointer 1, Fig. 3, swings beneath a fixed abutment 6, below which is a frame 7 which is raised and lowered at regular intervals by a cam. In this frame are mounted two detector rods 12 so as to be free to slide vertically. Their lower ends engage inclined surfaces 13 on a carriage 14 which is supported on rollers 15 so as to be movable horizontally. When the pointer 1 is deflected to such an extent as to come between the upper face of one of the rods 12 and the abutment 6, this rod will be depressed as the frame 7 moves upwards and the carriage 14 will be moved to the right or left according to which of the rods 12 has been depressed. A lever 16 is thereby rocked about its pivot 17 and rocks a beam 23, Fig. 4, through a pin-and-slot connection 25. One of two abutments 22 is thus brought into engagement with a corresponding cam 20 on a block 19 fixed to a rotating shaft 10. This is moved axially so as to cause one of two clutches to engage and drive a shaft 26 in one direction or the other through bevel gearing 31, 32, so as to effect adjustment of the potentiometer slide wire in the appropriate direction. Each clutch consists of a driving part consisting of ratchet teeth 29, Fig. 5, formed at the end of the cam block 19 and a driven part 30 consisting of a ring having ratchet teeth 30<1> and fast with a bevel-wheel 31 loosely mounted on the shaft 10. The driven part 30 has only two teeth 30<1>, which are formed on a member 60 mounted to rock about a pin 61. The teeth 30' are spaced 180 degrees apart less half the pitch of a ratchet tooth so that if one of the teeth 30<1> meets one of the teeth 29 end on, the member 60 rocks the causes the other tooth 30<1> to project into the space between two teeth 29. A gate 33, Fig. 3, is operated by the beam 23 so as to be raised into the path of the pointer 1 while the beam 23 is deflected and the coarse adjustment is in operation. When the pointer has returned so far towards its zero position that it meets the edge of the gate 33 it is in the path of the upper face of one or other of two additional slidable rods 34, the lower ends of which engage inclined faces 35 on the carriage 14. When the frame 7 is next raised the rod 34 is depressed and co-operates with the surface 35 to move the carriage 14 back to its original position thereby rocking the beam 23 back to its central position. The clutches 29, 30 are thereby disengaged and the coarse adjustment ceases to operate, the balancing of the potentiometer being completed by the normal fine adjustment. During the operation of the coarse adjustment the fine adjustment is disengaged. The shaft 2 that drives the fine adjustment mechanism is driven through a clutch comprising a driving part 37, Fig. 7, and a driven part 42 fast with the shaft 2. These parts are coupled by a spring-pressed plunger 43 sliding in the part 42 and engaging an aperture in the part 37. The plunger 43 is engaged by a pin 46 on an arm 47 pivoted at 48 on the part 42. A lever 49 is pivoted at 50 and rocked by an arm 51 bearing on a block 52 having a groove 53. This block is coupled with the shaft 10 so that when this shaft is displaced axially the arm 51 is lifted and the end of the lever 49 moves into the path of the arm 47 and causes the plunger 43 to be disengaged from the clutch member 37 so that the fine adjustment drive is interrupted. When the shaft 10 is returned to its normal position the groove 53 registers with the arm 51 which drops into the groove and removes the lever 49 from the path of the arm 47. The plunger 43 thereupon engages the aperture in the clutch part 37 and the fine adjustment drive is set in operation. In order to prevent the coarse and fine adjustments from operating together, a further gate is controlled by a cam on the fine-adjustment shaft so as to prevent the pointer from moving to its full deviation position until the fine adjustment mechanism is in a suitable position. Specification 486,446, [Group XXXVIII], is referred to.