Brushless dc motor including a variable attenuator fed commutator

Claim 1

A brushless DC motor comprising a permanent magnet rotor rotatable about an axis, said rotor containing a pair of magnetic poles; a plurality of three stator coils spaced at equiangular intervals around said axis, each of said coils being energizable to attract the same specified magnetic pole of said rotor; a stationary light source; a stationary photosensor; cylindrical attenuating means rotatable with said rotor for limiting the level of energy reaching said photosensor from said source; said cylindrical attenuating means being mounted coaxially on said rotor and positioned so that some portion of its cylindrical surface is in the path of the energy propagating between said source and said photosensor; a plurality of three arcuate segments disposed around the periphery of the cylindrical attenuating means, each segment corresponding to a given stator coil and providing low, intermediate and high degrees of attenuation respectively, each of said segments being positioned so that successive segments are exposed to the energy from said source as the rotor rotates, said segments being further positioned so that a change in attenuation is accomplished just before said specified magnetic poles become aligned with the axis of a stator coil; level detection means including first and second trigger means, said first trigger means being adjusted to produce a switching signal when the sensor receives light from any but said high attenuation arcuate section, said second trigger means being adjusted to produce a switching signal only when the sensor receives light from said low attenuation arcuate section; and switching means including logic means for energizing the first of the stator coils when neither trigger means produces a switching signal, the second of the stator coils when only the first trigger means produces a switching signal, and the third of the stator coils when both trigger means produce switching signals.