Tachometer

Claim 1

A tachometer comprising: a detection signal source operative to generate a detection signal having a frequency corresponding to a rotational frequency to be measured; a frequency-voltage converter coupled to said detection signal source, for converting said detection signal into a voltage signal having a voltage value corresponding to said frequency; a sine-cosine current generator connected to said frequency-voltage converter for generating a sine-wave current signal having a substantially sine value corresponding to the voltage value of said voltage signal and for generating a cosine-wave current signal having a substantially cosine value corresponding to the voltage value of said voltage signal, one cycle of each of the sine-wave and cosine-wave current signals corresponding to the range from minimum to maximum voltage values of said voltage signal; a cross-coil type indicator comprising first and second coils connected to the output side of said sine-cosine current generator, said coils being disposed with their axes substantially perpendicular intersecting each other and said coils being supplied with said sine-wave current signal and said cosine-wave current signal, respectively, said indicator including a rotary magnet rotatably disposed in a composite magnetic field of the magnetic fields set up by currents flowing through the first and second coils, a pointer being mounted on the rotary shaft of the rotary magnet and a dial being provided adjacent said pointer for indicating the rotational angular position of the pointer as a rotational frequency; and delay means connected between said frequency-voltage converter, said sine-cosine current generator and a power source, for delaying interruption of the operation of the frequency-voltage converter and the sine-cosine current generator when the supply of their operating voltages is cut off; said cross-coil type indicator including a square-shaped frame of a synthetic resinous material in which said rotary magnet is mounted and on which said first and second coils are wound with their axes perpendicular to said rotary shaft of said rotary magnet, said frame having a plurality of depressions formed in one outer face thereof and arranged around said rotary shaft, and a zero-resetting magnet fitted in one of said depressions in opposing relation to a marginal portion of said rotary magnet so that when said pointer is brought near to a zero-point of said dial, said zero-resetting magnet magnetically interacts with said rotary magnet to force said pointer to move to said zero-point.