Brushless DC motors are controlled by sensing the position of the rotor and then switching the power to the stator windings through an inverter in a manner which is a function of the position of the rotor. Circuits are well known for sensing the position of the rotor such as Hall effect position sensors which produce pulse trains having a duty cycle of 50%. These position signals are used to control the precise switching point at which power is applied to the stator windings. The torque produced by the rotor may be changed by varying the angle at which the windings of the stator are activated with respect to the position of the rotor.
FIG. 1 illustrates a prior art torque control for a variable speed brushless DC motor. The rotor (not illustrated) of brushless DC motor 10 is mechanically coupled to a resolver 12 which produces a pair of analog output signals which are in phase quadrature to each other. The phase quadrature signals are applied to a resolver to digital converter 14 which functions to convert the analog phase quadrature signals into a digital counterpart. The output digital phase quadrature signals are applied to an arithmetic logic unit (ALU) 16 which functions to add or subtract a digital word proportional to the desired phase advance or delay of the rotor position signals produced by the resolver 12 to control the torque produced by the motor. The resultant digitally encoded phase quadrature signals produced by the ALU 16 which reflect a desired amount of phase advance or delay, are applied to a decoder 18 which decodes the signals to produce suitable switching signals applied to inverter 20 having an output applied to stator windings (not illustrated) of the brushless DC motor. While the foregoing system functions to accurately control the torque of a brushless DC motor, it suffers from the disadvantage that the circuitry in the resolver to digital converter 14, which functions to convert the analog resolver signals into a digital counterpart is too slow to permit torque control for DC motors having high speed applications. Thus in situations where rotor velocities of 30,000 or more revolutions per minute are required, the processing speed of the electronics in the resolver to digital converter 14 is insufficient to produce precise torque control of the brushless DC motor 10 by varying the angular position of the control signals applied to the inverter 20 from the angular position at which the control signals would be applied to the inverter without phase compensation.
U.S. Pat. Nos. 4,546,293, 4,584,505, 4,572,999, 4,692,674, 4,680,515 and 4,697,125 disclose examples of controls for brushless DC motors. U.S. Pat. Nos. 4,072,884 and 4,447,771 each disclose examples of brushless DC motor controls utilizing analog signal processing which, as described above, may present difficulty in control where high speed applications are required. U.S. Pat. No. 4,584,505 discloses a variable torque brushless DC motor control in which rotor position signals are applied to a pulse delay circuit 7 which produces delayed control pulses which are used to control the switching of the stator windings of the brushless DC motor. The degree of delay produces a controllable reaction torque such that the greater the delay the smaller the net torque applied to the rotor. None of the foregoing patents discloses a variable torque control for a variable velocity brushless DC motor based upon the ratio of a fixed frequency signal and a variable frequency signal whose frequency is proportional to the desired degree of phase delay of control signals applied to an excitation circuit of the stator windings of a brushless DC motor which ratio is independent of rotor velocity.