Importance

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Abstract

A 12-step commutation in a brushless DC motor is achieved by six shaft-mounted commutation sensors and circuitry which permits inexpensive shaft-angle decoding for coil excitation. The commutation signal devices are Hall sensors arranged in two sets. A first set is arranged to produce the desired switching signals every 120 electrical degrees; a second set of three Hall sensors is similarly arranged equally spaced between the electrical switch points of the first set, however, the second set of sensors is wired together through the process of EXCLUSIVE-ORing to form a single, fourth output. Thus, the six Hall sensors are configured to produce four output signals which may be inexpensively decoded using a standard 32.times.8 PROM. A further refinement includes current modulation to the field coils in order to equalize the torque output of alternate steps. In one embodiment, this is accomplished by reducing the gain of the motor's servo amplifier so that the desired current shift is achieved by changes in the motor's load impedance.

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First Claim