Brushless motor control circuitry with optimum current vector control

• US 4,651,068 A
• Filed: 10/01/1984
• Issued: 03/17/1987
• Est. Priority Date: 10/01/1984
• Status: Expired due to Fees

• Current Assignee: E.C. Motors, Inc. (National Oilwell Varco Incorporated)
• Original Assignee: E.C. Motors, Inc. (National Oilwell Varco Incorporated)
• Inventors: Meshkat-Razavi, Saeid
• Primary Examiner(s): Shoop, Jr., William M.
• Assistant Examiner(s): Ro, Bentsu
• Application Number:
  US06/656,459
• Time in Patent Office: 897 Days
• Field of Search: 318/138, 318/254, 318/439, 318/661, 318/722, 318/314
• US Class Current: 318/400.02
• CPC Class Codes: H02P 2209/07 Trapezoidal waveform
• CPC Class Codes: H02P 6/06 Arrangements for speed regulation of a single motor wherein the motor speed is measured and compared with a given physical value so as to adjust the motor speed
• CPC Class Codes: H02P 6/153 wherein the commutation is advanced from position signals phase in function of the speed
• CPC Class Codes: H02P 6/28 Arrangements for controlling current H02P6/10 takes precedence

Abstract

A drive circuit for a brushless motor utilizing both phase shift and current shaping to provide for efficient operation of a brushless motor in a servo control system. The circuit includes circuitry for altering the phase of the drive signal as a function of motor velocity to compensate for increased current phase lag due to inductance of the motor at high commutation frequencies corresponding to high motor velocities. In addition to altering the phase shift of the drive signal, the circuitry also alters the commutation signal waveform to compensate for a non-sinusoidal field distribution to produce a nearly sinusoidal torque from each of the brushless motor windings. The compensation of both phase shift and commutation signal waveform is accomplished using digital circuitry. The phase commutation is a non-linear function of motor velocity. The waveform compensation is selected to compensate for a trapezoidal magnetic field distribution in the motor air gap.

Claims

1. A motor control system for driving at a constant speed a multiple phase brushless DC motor, including at least two stator windings, a permanent magnet rotor and a position indicator for indicating the angular position of the rotor, the system comprising:

(a) excitation current generating circuit means connected for providing excitation current to the stator windings in response to a current command vector signal;

(b) computing means coupled to the position indicator for receiving information therefrom concerning the position of the rotor and for providing a control signal for each axis having values selected from a stored table of values in accordance with the calculated rotational speed and the instantaneous angular position of the rotor, the values being selected to correct the amplitude of the current vector command to compensate for non-sinusoidal variation of the magnetic field in the air gap as a function of the angular position of the rotor; and

(c) control circuit means responsive to a velocity error signal indicative of the difference between the calculated rotational speed and the commanded rotational speed and the control signal provided by the computing means and for providing a current vector command signal to the excitation current generating means which is adjusted in magnitude to provide a substantially sinusoidal tongue distribution for each phase.

2. A motor control system for driving at a constant speed a multiple phase brushless DC motor, including at least two stator windings, a permanent magnet rotor and a position indicator for indicating the angular position of the rotor, the system comprising:

(a) excitation current generating circuit means connected for providing excitation current to the stator windings in response to a current command vector signal;

(b) computing means coupled to the position indicator for receiving information therefrom concerning the position of the rotor and for providing a control signal for each axis having values selected from a stored table of values in accordance with the calculated rotational speed and the instantaneous angular position of the rotor, the values being selected to advance the phase of control signal in a non-linear manner, according to the formula φ

=tan^-1 (ω

T_E), where T_E is the time constant of the closed loop current and ω

is the electrical angular velocity; and

(c) control circuit means responsive to a velocity error signal indicative of the difference between the calculated rotational speed and the commanded rotational speed and also responsive to the control signal provided by the computing means and for providing a current vector command signal to the excitation current generating means which is advanced in phase to compensate for variations of phase angle between the excitation current in the stator windings and the magnetic field as a function of motor speed.

3. A motor control system for driving at a constant speed a multiple phase brushless DC motor, including at least two stator windings, a permanent magnet rotor and a position indicator for indicating the angular position of the rotor, wherein the DC motor has a trapezoidal fluid distribution and wherein the stator excitation current for a particular phase is multiplied by the factor and K(θ

) is defined by -h(θ

)/sθ

where

space="preserve" listing-type="equation">k(θ

)=Σ

.sub.n b.sub.n Sin (θ

)

space="preserve" listing-type="equation">s(θ

)=h(θ

)+b.sub.1 Sin (θ

)

where n=2k+1 and k=1, 2, 3 and b_n is the magnitude of the nth harmonic, the system comprising;

(a) excitation current generating circuit means connected for providing excitation current to the stator windings in response to a current command vector signal;

(b) computing means coupled to the position indicator for receiving information therefrom concerning the position of the rotor and for providing a control signal for each axis having values selected from a stored table of values in accordance with the calculated rotational speed and the instantaneous angular position of the rotor, the values being selected to advance the phase of control signal in a non-linear manner; and

(c) control circuit means responsive to a velocity error signal indicative of the difference between the calculated rotational speed and to the commanded rotational speed and the control signal provided by the computing means and for providing a current vector command signal to the excitation current generating means which is advanced in phase to compensate for variations of phase angle between the excitation current in the stator windings and the magnetic field as a function of motor speed.

4. A motor control system for driving at a constant speed a multiple phase brushless DC motor, including at least two stator windings, a permanent magnet rotor and a position indicator for indicating the angular position of the rotor, the system comprising:

(a) excitation current generating circuit means connected for providing excitation current to the stator windings in response to a current command vector signal;

(b) computing means coupled to the position indicator for receiving information therefrom concerning the position of the rotor and for providing a control signal for each axis having values selected from a stored table of values in accordance with the calculated rotational speed and the instantaneous angular position of the rotor, the values being selected to advance the phase of control signal in a non-linear manner and to correct the amplitude of the current vector command to compensate for non-sinusoidal variation of the magnetic field in the air gap as a function of the angular position of the rotor; and

(c) control circuit means responsive to a velocity error signal indicative of the difference between the calculated rotational speed and the commanded rotational speed and the control signal provided by the computing means and for providing a current vector command signal to the excitation current generating means which is advanced in phase to compensate for variations of phase angle between the excitation current in the stator windings and the magnetic field as a function of motor speed and is adjusted in magnitude to provide a substantially sinusoidal torque distribution for each phase.

5. The invention of claim 4 wherein the control circuit means multiplies the control signal from the computing means by the velocity error signal to provide the current command vector signal to the excitation current generating circuit means.

6. In combination with a brushless motor, an inverter drive circuit comprising:

(a) command generating circuit means for generating a sinusoidal command voltage having a frequency proportional to the desired rotational velocity of the brushless motor over an operating range of velocities;

(b) signal generating circuitry for producing a substantially triangular voltage signal having a frequency substantially higher than the anticipated frequency of the sinusoidal command voltage at the upper end of the operating range of velocities;

(c) pulse width modulating means coupled to receive the command voltage from the command generating circuit means and the alternating voltage signal generated by the further generating circuitry and for producing a modified command voltage having first and second levels, the modified command voltage having the first level when the command voltage and the alternating voltage are both positive and having the second level when the command voltage and the alternating voltage signals are both negative;

(d) commutor means coupled to the output of the pulse width modulating means to receive the modified command voltage therefrom and for driving the brushless motor with a motor drive current having a sinusoidal fundamental component at a frequency corresponding to the frequency of the command voltage and a superimposed alternating signal having a frequency corresponding to the frequency of the alternating voltage signal and a magnitude substantially less than the magnitude of the fundamental component for driving the brushless motor smoothly;

(e) shaft position sensing means for generating a signal indicative of the angular position of the shaft of the brushless motor;

(f) first control means operatively coupled to the shaft position sensing means for receiving the output from the shaft position sensing means and for delivering a phase modified output signal and for calculting a corrective lead angle for the sinusoidal command voltage which is non-linearly dependent upon the rotational velocity of the motor shaft and the electrical characteristics of the motor; and

(g) signal processing means for receiving the output signal from the first control means and the voltage command signal and for generating a phase corrected output voltage having a correction factor selected to produce a motor drive current having minimal discontinuities over a wide range of operating velocities.

7. The method of optimizing the excitation of a brushless DC motor comprising the steps of:

(a) sensing the absolute position of the motor shaft angle and estimating the shaft angle velocity during a particular time increment;

(b) using the estimated velocity and the motor'"'"'s electrical time constant, looking in a storage table for an optimum advancing angle;

(c) determining the optimum advancing angle by the relationship φ

=tan^-1 (ω

T_E), where φ

is the optimum advancing angle, ω

is the estimated motor velocity and T_E is the motor electrical time constant;

(d) adding the optimized advancing angle to the commutation sinusoidal angle; and

(e) providing the final angle to the commutation memory for driving the motor utilizing a pulse width modulator inverter.