Brushless DC motor control method and apparatus for reduced commutation noise

• US 6,002,226 A
• Filed: 06/17/1998
• Issued: 12/14/1999
• Est. Priority Date: 06/17/1998
• Status: Expired due to Term

• Current Assignee: GM Global Technology Operations Incorporated (General Motors Company), Steering Solutions IP Holding Corporation (Nexteer Automotive Group Ltd.)
• Original Assignee: General Motors Corporation (General Motors Company)
• Inventors: Skellenger, Dennis B., Collier-Hallman, Steven James, Kleinau, Julie Ann
• Primary Examiner(s): Masih, Karen
• Application Number:
  US09/099,007
• Time in Patent Office: 545 Days
• Field of Search: 318/254, 318/138, 318/439, 318/139, 318/599, 318/432
• US Class Current: 318/400.23
• CPC Class Codes: H02P 6/10 Arrangements for controlling torque ripple, e.g. providing reduced torque ripple

Abstract

A brushless DC motor modifies motor commutation events to reduce noise by providing overlapping activation of the switch being deactivated with the switch being newly activated so that three switches are temporarily activated. The newly activated switch is pulse width modulated with a motor torque controlling duty cycle, and the switch being deactivated is pulse width modulated synchronously with the newly activated switch but with a duty cycle decreasing from the torque controlling duty cycle. The decreasing modulation of the switch being deactivated allows a slower current decrease in the phase turning off to reduce or eliminate the disturbances, and thus the noise. The decreasing modulation is preferably exponential in manner, with a time constant varying inversely with motor speed so that the duty cycle reaches a predetermined minimum in a predetermined maximum number of motor electrical degrees. The decreasing modulation may be switched on at low motor speeds where its noise reduction is required and switched off at higher motor speeds where the potential for interference with commutation is greater. The decreasing modulation is applicable in a dual switching mode in which each activated switch, except for the switch being deactivated in commutation, is pulse width modulated at the torque controlling duty cycle, or in a modified single switching mode in which the activated switch of one of the upper or lower groups is always closed continuously during its activation, except for commutation, during which the switch remaining on, which alternates between the groups, is closed continuously. Each mode of operation is applied in the motor speed/torque region to which it is best suited.

Claims

1. A method for providing activation with reduced commutation noise of a brushless DC motor having a stator with three phase armature windings and a permanent magnet rotor, the activation using a bridge switching circuit comprising first and second groups of three switches each, each switch of the first group being connected in series with a different one of the switches of the second group across a source of electric power with a junction of each pair of series connected switches also being connected to a different one of the three phase armature windings, each of the switches being a semiconductor switch having a parallel fly-back diode, the method comprising:

repeatedly activating selected switches of the first group with selected non-series connected switches of the second group to connect selected pairs of the three phase windings in series with the electric power source in a predetermined commutation pattern to provide torque producing phase currents in the windings of the motor, the commutation pattern defining commutation events in which a switch of one of the first and second groups is deactivated, a new switch of the one of the first and second groups is activated and a switch of the other of the first and second groups remains activated, the newly activated switch alternating in consecutive commutation events between the first group and the second group; and

in each commutation event, providing overlapping activation of the switch being deactivated with the switch being newly activated and the switch remaining activated for the duration of the commutation, the newly activated switch being pulse width modulated with a motor torque controlling duty cycle and the switch being deactivated being pulse width modulated with pulses initiated synchronously with those of the newly activated switch but with a duty cycle decreasing from the torque controlling duty cycle.

2. The method of claim 1 in which the switch remaining activated is pulse width modulated synchronously during each commutation event with the switch being newly activated at the motor torque controlling duty cycle.

3. The method of claim 1 in which the switch remaining activated is closed continuously during each commutation event.

4. The method of claim 1 in which, except during commutation events, the activated switches of the first group are pulse width modulated at the torque controlling duty cycle and the activated switches of the second group are continuously closed.

5. The method of claim 1 in which the motor is operated alternatively in a single switching mode in which the switch remaining activated is closed continuously during activation or a dual switching mode in which the switch remaining activated is pulse width modulated synchronously with the switch being newly activated at the motor torque controlling duty cycle and in which the motor is changed between the single switching mode and the dual switching mode in response to a signal varying with motor torque.

6. The method of claim 5 in which the motor is changed between the single switching mode and the dual switching mode in further response to a signal varying with motor speed.

7. The method of claim 1 in which the duty cycle of the switch being deactivated is decreased continuously through the commutation event from the torque controlling duty cycle to a predetermined minimum duty cycle in a predetermined number of electrical degrees of the motor.

8. The method of claim 1 in which the duty cycle of the switch being deactivated is decreased in an exponential manner.

9. The method of claim 8 in which the time constant of the exponential decrease is varied inversely with vehicle speed to produce a predetermined minimum duty cycle over a predetermined maximum number of electrical degrees of the motor.

10. The method of claim 1 in which the overlapping activation of the switch being deactivated with the switch being newly activated and the switch remaining activated for the duration of the commutation event is provided at low motor speeds where the commutation noise is most audible and not provided at high motor speeds where it may interfere with commutation.

11. An electromechanical activation apparatus comprising, in combination:

A source of DC electric power;

a brushless DC motor having a stator with three phase armature windings and a permanent magnet rotor;

a bridge switching circuit comprising first and second groups of three switches each, each switch of the first group being connected in series with a different one of the switches of the second group across the source of electric power with a junction of each pair of series connected switches also being connected to a different one of the three phase armature windings, each of the switches being a semiconductor switch having a parallel fly-back diode;

first control means for repeatedly activating selected switches of the first group with selected non-series connected switches of the second group to connect selected pairs of the three phase windings in series with the electric power source in a predetermined commutation pattern to provide torque producing phase currents in the windings of the motor, the commutation pattern defining commutation events in which a switch of one of the first and second groups is deactivated, a new switch of the one of the first and second groups is activated and a switch of the other of the first and second groups remains activated, the newly activated switch alternating in consecutive commutation events between the first group and the second group; and

second control means for, in each commutation event, providing overlapping activation of the switch being deactivated with the switch being newly activated and the switch remaining activated for the duration of the commutation, the newly activated switch being pulse width modulated with a motor torque controlling duty cycle and the switch being deactivated being pulse width modulated with pulses initiated synchronously with those of the newly activated switch but with a duty cycle decreasing from the torque controlling duty cycle.

12. A method for providing activation with reduced commutation noise of a brushless DC motor having a stator with three phase armature windings and a permanent magnet rotor, the activation using a bridge switching circuit comprising first and second groups of three switches each, each switch of the first group being connected in series with a different one of the switches of the second group across a source of electric power with a junction of each pair of series connected switches also being connected to a different one of the three phase armature windings, each of the switches being a semiconductor switch having a parallel fly-back diode, the method comprising:

repeatedly activating selected switches of the first group with selected non-series connected switches of the second group to connect selected pairs of the three phase windings in series with the electric power source in a predetermined commutation pattern to provide torque producing phase currents in the windings of the motor, the commutation pattern providing periods in which a switch of the first group and a switch of the second group are simultaneously activated separated by commutation events in which a switch of one of the first and second groups is deactivated in a controlled manner, a new switch of the one of the first and second groups is activated and a switch of the other of the first and second groups remains activated, the newly activated switch alternating in consecutive commutation events between the first group and the second group;

between commutation events, pulse width modulating the activated switch of the first group while maintaining the activated switch of the second group continuously closed;

in each commutation event, regardless of whether an activated switch is in the first group or the second group, pulse width modulating the newly activated switch with a motor torque controlling duty cycle, maintaining the switch remaining activated in a continuously closed state and controlling the switch being deactivated in a predetermined, commutation noise reducing manner for the duration of the commutation event.