GB1485934A - Indicating instruments - Google Patents

Abstract

1485934 Speedometers; tachometers; odometer FORD MOTOR CO Ltd 10 Jan 1976 00941/76 Heading G1K An indicating instrument comprises an indicating member 25 movable relative to a fixed datum, a constant speed motor 10 drivingly connected to the indicating member by an electromagnetic coupling 27, and a drive circuit 30 which controls the coupling 27 in dependence on an input signal representing a parameter to be displayed. As shown, the motor 10 is driven by an oscillator 12 whose frequency is controlled by a crystal 14, via a divider circuit 16 and a shaping circuit 18. In a motor vehicle instrument system the motor 10 drives a clock 20 directly and a speedometer 22, tachometer 46, and odometer 56 via gearing 26 and respective electromagnetic couplings. The speedometer receives pulses from a sensor 28 at a frequency representing road speed. The drive circuit 30 supplies fixed length pulses at said frequency to a fixed coil 40 of the coupling 27. The motor 10 drives a rotor 38 with magnetic poles, and an electrically conductive disc 42 between the rotor 38 and coil 40 experiences a torque dependent on the frequency of the signal to the coil 40. The indicating member 25 is a pointer fixed to the disc 42 and having a spring 44 opposing the applied torque. The tachometer is similar to the speedometer, but receives a signal dependent on engine speed. The odometer 56 has an input shaft 58 carrying a gear-wheel 60 which has a portion, e.g. 5% without teeth. A leaf-spring 62 on the gear-wheel 60 is aligned with the toothless portion and can be attracted by an electromagnet 64 into engagement with the teeth of the adjacent gear 26. A switching circuit 66 of the drive circuit 30 energizes the magnet 64 as each consecutive unit of distance is completed, allowing the gear-wheel 60 to be rotated one turn, to advance the odometer counter.