Method and apparatus for reducing recurrent fluctuations in motor torque

Claims

What is claimed is:

1. A method for reducing recurrent ripple in electric motor output torque and comprising the steps of:

generating at least one synchronized reference signal at a frequency of at least one frequency component of said torque ripple;

combining said synchronized reference signal into an electric current drive to said motor, which drive determines motor shaft rotation speed; and

adjusting phase and magnitude of said at least one synchronized reference signal, while said shaft of said motor is rotating, to reduce said at least one frequency component, said adjusting step comprising the steps of:

recurrently analyzing the frequency spectrum of said speed of said motor shaft to indicate the extent of presence of said at least one frequency component;

successively approximating phase and amplitude adjustment values required in said generated at least one signal to reduce the corresponding selected frequency component in said spectrum to at least a predetermined level; and

modifying said at least one synchronized reference signal, while said shaft of said motor is rotating, by said phase and amplitude adjustment values.

2. A method for reducing recurrent ripple in electric motor output torque and comprising the steps of:

generating at least one synchronized reference signal at a frequency of at least one frequency component of said torque ripple;

adjusting phase and magnitude of said at least one synchronized reference signal, while a shaft of said motor is rotating, to reduce said at least one frequency component; and

combining said synchronized reference signal into an electric current drive to said motor, said combining step comprising:

storing digitally encoded samples of first and second sets of phase related sine wave alternating current signals, said first set being in a predetermined phase relationship with said at least one synchronized reference signal and said second set being in 120 electrical degrees leading phase relationship with said first set;

recurrently addressing said sets of samples at a rate determined by the speed of rotation of said motor to read them out;

converting the outputs of said addressing step to analog signal form as first and second analog signals, respectively;

adjusting amplitudes of the outputs of said converting step in accordance with said at least one synchronized reference signal;

combining amplitude adjusted outputs of said amplitude adjusting step to form a third analog signal at 240 electrical degrees leading phase relationship with respect to said first analog signal; and

applying said first, second, and third analog signals to said three-phase drive current input connections of said motor, respectively.

3. A method for reducing recurrent ripple in electric motor output torque, said motor being connected in a velocity servo loop including an encoder driven by said motor for maintaining motor velocity approximately equal to a predetermined reference velocity, said method comprising the steps of:

generating at least one synchronized reference signal at a frequency of at least one frequency component of said torque ripple, said generating step including the steps of:

storing for each of said at least one frequency components a set of digitally encoded successive sample values representing a sine wave at the frequency of a corresponding one of said at least one frequency components; and

reading said sample values out at a rate which is a function of an output of said encoder thereby forming a digital sample value signal train representing each said sine wave;

combining said synchronized reference signal into an electric current drive to said motor; and

adjusting phase and magnitude of said at least one synchronized reference signal, while a shaft of said motor is rotating, to reduce said at least one frequency component, said adjusting step comprising the steps of:

providing for each of said digital signal trains a sine wave phase adjustment value as an initial address pointer for reading said sample values of said digitally encoded sample value set for that one component;

receiving from said encoder a motor shaft position indexing signal and at least a first and a second quadrature related pulse train signal waves at a single frequency which is much higher than the recurrence rate of said indexing signal;

reading said pointer;

testing for the presence of said indexing signal, and

if it is not present, testing for the presence of said first pulse train signal,

if said first pulse train signal is not present, repeating said testing step for said indexing signal; and

if said first pulse train signal is present, testing the direction of motor rotation as indicated by said first and second pulse train signals, and incrementing or decrementing said pointer depending upon the result of said direction testing step;

outputting said stored value from the storing step address indicated by the new value of said pointer; and

repeating said reading, testing, and outputting steps.

4. A method for reducing recurrent ripple in electric motor output torque, said motor being connected in a velocity servo loop including an encoder driven by said motor for maintaining motor velocity approximately equal to a predetermined reference velocity, said method comprising the steps of:

generating at least one synchronized reference signal at a frequency of at least one frequency component of said torque ripple, said generating step including the steps of:

storing for each of said at least one frequency components a set of digitally encoded successive sample values representing a sine wave at the frequency of a corresponding one of said at least one frequency components; and

reading said sample values out at a rate which is a function of an output of said encoder thereby forming a digital sample value signal train representing each said sine wave;

combining said synchronized reference signal into an electric current drive to said motor; and

adjusting phase and magnitude of said at least one synchronized reference signal, while a shaft of said motor is revolving, to reduce said at least one frequency component;

said at least one synchronized reference signal generating step produces a plurality of said synchronized reference signals and a corresponding plurality of of said digital signal trains, each at a frequency of a different frequency component of said torque ripple, said at least one synchronized reference signal generating step further compising the steps of:

converting each of said digital signal trains into a corresponding alternating current analog signal; and

summing said analog signals to produce a sum signal; and

said combining step combines said sum signal into said electric current drive.

5. The recurrent ripple reducing method of claim 4, in which said adjusting step comprises:

recurrently analyzing the frequency spectrum of said output torque to produce a representation of each of said different frequency components of said torque ripple; and

recurrently adjusting the amplitude and phase of said analog signal version of said each of digital signal trains to reduce the magnitude of said representation of its frequency components in said spectrum.

6. Apparatus for reducing recurrent speed variations in a velocity servo by reducing ripple in the torque output of an electric motor included in said servo and comprising:

means, responsive to motor output speed and shaft position, for generating a synchronized reference signal at a frequency of at least one frequency component in said ripple;

means for combining said synchronized reference signal, in a ripple reducing sense, a drive current for said motor, which drive current affects the motor shaft rotation speed; and

means for adjusting phase and amplitude of said synchronized signal to reduce said at least one frequency component, said amplitude and phase adjusting means including digital signal processor means comprising:

means for recurrently analyzing the frequency spectrum of said torque output; and

means, operating in a successive approximation mode, and responsive to recurrent spectrum analysis of said torque output spectrum, for determining phase and amplitude adjustment values required to reduce said at least one frequency component.

7. Apparatus for reducing recurrent speed variations in a velocity servo by reducing ripple in the torque output of an electric motor included in said servo and comprising:

means, responsive to motor output speed and shaft position, for generating a synchronized reference signal at a frequency of at least one frequency component in said recurrent ripple;

means for combining said synchronized reference signal, in a ripple reducing sense, into a drive current for said motor, said combining means comprising means for connecting said motor in a velocity servo loop including an encoder, driven by said motor, and an amplification means, said loop being responsive to an output of said encoder for approximately maintaining motor speed at a predetermined reference speed; and

said amplification means has an open loop bandwidth approximately an order of magnitude larger than the bandwidth of said velocity servo loop and comprises:

first amplifier means, responsive to said encoder output and a predetermined reference voltage, for producing an uncorrected error signal, said first amplifier means having its alternating current gain set to fix the overall bandwidth of said velocity servo loop and said generating means;

means responsive to said synchronized reference signal and to said uncorrected error signal for producing a ripple corrected error signal; and

second amplifier means responsive to said ripple corrected error signal for producing said drive current, said second amplifier means having an open loop bandwidth aproximately an order of magnitude larger than the bandwidth of said velocity servo loop; and

means for adjusting phase and amplitude of said synchronized reference signal to reduce said at least one frequency component.

8. Apparatus for reducing recurrent speed variations in a velocity servo by reducing ripple in the torque output of an electric motor included in said servo, said velocity servo including a speed and phase encoder, driven by said motor, and a power amplifier, responsive to an output of said encoder for coarsely maintaining motor speed at a predetermined reference speed;

a plurality of means, responsive to motor output speed and shaft position, for generating a plurality of synchronized reference signals, each at a frequency of a different frequency component in said recurrent ripple;

each of said generating means comprises:

means, responsive to an output of said encoder, for producing a digital signal representation of said ssynchronized reference signals, and

means for converting said digital signal representation to analog signal form;

means for adjusting phase and amplitude of said synchronized reference signals;

said adjusting means comprising means for adjusting the phase of said digital signal representation and means for adjusting the amplitude of said analog signal form to reduce said different frequency component;

means are provided for summing synchronized reference signal outputs of said plurality of synchronized reference signal generating means; and

means for combining an output of said summing means, in a ripple reducing sense, into a drive current for said motor.

9. Apparatus for reducing recurrent speed variations in a velocity servo by reducing ripple in the torque output of an electric motor included in said servo, said velocity servo including a speed and phase encoder, driven by said motor, and a power amplifier, responsive to an output of said encoder for coarsely maintaining motor speed at a predetermined reference speed; said apparatus comprising:

three-phase drive current input connections for said motor, and means, including said power amplifier and responsive to motor speed, for developing a three-phase alternating current drive current for said motor;

means, responsive to motor output speed and shaft position, for generating a synchronized reference signal at a frequency of at least one frequency component in said recurrent ripple, said generating means comprising means, responsive to an output of said encoder, fopr producing a digital signal representation of said synchronized reference signal, and means for converting said digital signal representation to analog signal form;

means for combining said synchronized reference signal, in a ripple reducing sense, into drive current for said motor, said combining means comprising means for providing a reference speed signal, means for summing said synchronized reference signal and said reference speed signal to produce a speed error signal, and means, in said developing means and responsive to said speed error signal, for modifying said three-phase alternating current drive current, said modifying means comprising:

an up/down counter, responsive to said motor speed for recurrently counting through a predetermined counting range;

means for separately storing digitally encoded sample values defining a first, essentially sinusoidal, alternating current wave and a second, essentially sinusodial, alternating current wave, said second wave leading said first wave in phase by 120 electrical degrees;

means for applying bit-parallel outputs of said counter to address sequentially different locations of said storing means to read out said sample values of said first and second waves;

means for separately converting said readout sample values of said first and second waves to corresponding first and second analog signal waves;

means, resposive to said first and second analog signal waves, for producing a third analog signal alternating current wave which leads said second wave by an additional 120 electrical degreees; and

means for separately amplifying each of said first, second, and third analog signal waves before application to said three-phase input connections of said motor; and

means for adjusting phase and amplitude of said synchronized reference signal to reduce said at least one frequency component, said adjusting means comprising means for adjusting the phase of said digital signal representation and means for adjusting the amplitude of said analog signal form to reduce said at least one frequency component.

10. A signal function generator for generating an alternating current signal wave for controlling speed of an electric motor and comprising:

means for receiving signals indicating motor shaft position and speed variation for said motor;

means, responsive to said signals, for generating a digital signal sample train having a frequency equal to a torque ripple frequency component in the output of said motor;

means for adjusting phase of said of said sample train in a direction to reduce said ripple frequency;

means for converting said phase adjusted sample train to a corresponding analog signal wave at said frequency;

means for adjusting the amplitude of said analog signal wave in a direction to reduce said ripple;

at least one additional set of generating means, phase adjusting means, converting means, and amplitude adjusting means, said generating means of said at least one additional set being responsive to the same signals from said receiving means but generating sample trains having different frequencies equal to different torque ripple frequency components of said motor; and

means for summing all phase and amplitude adjusted analog signal waves from said converting means and said amplitude adjusting means to form saide alternating current signal wave for motor speed control.

FIELD OF THE INVENTION

The present invention relates to speed control refinement, by control of torque output from an electric motor, and it relates in particular to compensating for recurrent fluctuations (i.e., ripple) in output torque.

BACKGROUND OF THE INVENTION

Some electric motors are operated at low speeds at which there is an essentially periodic fluctuation, or ripple, in the output torque. Such ripple, one form of which is called "cogging," is primarily due to factors such as unbalanced and/or coarse resolution driving phases or limited numbers of poles on the rotor and/or stator. These kinds of factors give rise to periodic variations in the magnetic field strength in the air gap between cooperating rotor and stator poles as the motor turns and hence to the ripple effect. Torque ripple can also occur as a result of, e.g., irregularities in motor bearings which impose corresponding irregularities in the loading of the motor. Torque ripple is necessarily accompanied by corresponding motor speed variations which also are essentially periodic and at frequencies higher than the motor speed in revolutions per minute (RPM). If the motor is driving, e.g., a coating drum onto which a fluid is being sprayed, the speed fluctuations result in a deposited film which has an undesirable nonuniform thickness.

Electric motor output torque ripple is one of the factors that concerns electric motor system designers. Several aspects of the problem are described in "Drive Scheme Holds Key To DC Motor Performance" by M. B. McCormick, EDN, Jan. 19, 1989. It is said that a motor with sinusoidal commutation has nearly zero torque ripple over a complete revolution of the motor. However, in some applications significant troublesome ripple still remains.

Similarly, in "A Brushless Motor Evaluation" by B. Bessenyei and T. Niertit, Motion, July/August 1988, pages 3, 4, 6-8, and 10, solutions such as balancing the motor currents and adding a high-inertia load are considered. However, significant ripple still remains in some applications. [Note that, due to a typographical error, the figure numbers for FIGS. 5 and 6 in this paper were inadvertently interchanged.]

Flywheels have long been used to smooth torque ripple in the output of various kinds of motors, including electrical motors; but flywheels are bulky, and inconvenient to use for motors operated at speeds below say 100 RPM. Efforts also have been made to combat the torque ripple problem by specific motor structure design techniques, and one example is shown in the U.S. Pat. No. 4,947,066 to B. A. Ghibu et al.

There are many motor control servo systems that use position signals derived from a motor output to control motor speed. In these, any given speed change and correction may take place over the course of multiple revolutions of the motor armature. Two examples are shown in the U.S. Pat. Nos. 4,885,793 to J. Tabuchi and 4,259,698 to S. Takada. Such motor speed control servo systems are typically found in systems in which the motor speed is substantially greater than the recurrence rate of any speed changes of interest. Those systems usually have a relatively slow response which is unable to deal with an application in which there are recurrent speed variations during each revolution of the motor's armature as is the case where torque ripple is a problem.

A number of attempts have been made to reduce the impact of torque ripple by employing electronic circuit means. In U.S. Pat. No. 4,868,477 to F. J. Anderson et al., static drive current measurements are made at each of many angular positions of the motor shaft; and for each torque rating at which the motor is to be operated the current values are stored in read only memories (ROMs) for the respective motor drive current phases. Those values are then read out to provide drive current when the motor is being operated under normal rotational load. This is an awkward and time consuming process which provides compensation in the form of a composite of all torque effects which appear under static conditions. The U.S. Pat. No. 4,943,760 to J. V. Byrne et al. is another of the type that uses static tests to determine a compensating waveform for each phase of motor drive current.

U.S. Pat. No. 3,919,609 to H. Klautschek et al. shows the comparison of actual output torque measurements to a reference torque and use of the difference to correct drive current for reducing ripple. This too involves measurement of gross torque, and its real time style of operation puts a significant premium on dealing adequately with issues of circuit and hardware inertia and time constants. Another torque comparison and current control system is shown in U.S. Pat. No. 4,240,020 to T. Okuyama et al.

U.S. Pat. No. 4,890,048 to L. W. Hunter shows compensation for torque ripple due to motor drive current supply effects by employing an accumulator to dampen hunting caused by the input signal.

SUMMARY OF THE INVENTION

Impact of the torque ripple problem is reduced in accordance with the present invention by the method steps of generating at least one synchronized reference signal at a frequency of at least one frequency component of the torque ripple; combining the synchronized reference signal into an electric current drive to the motor; and adjusting phase and magnitude of the at least one synchronized reference signal, while a shaft of the motor is revolving, to reduce the at least one frequency component.

In one embodiment, apparatus for reducing recurrent speed variations in a velocity servo by reducing ripple in the torque output of an electric motor included in the servo comprises means, responsive to motor output speed and shaft position, for generating a synchronized reference signal at a frequency of at least one frequency component in the recurrent ripple; means for combining the synchronized reference signal, in a ripple reducing sense, into a drive current for the motor; and means for adjusting phase and/or amplitude of the synchronized reference signal to reduce the at least one frequency component.

An embodiment of the synchronized reference signal generating means includes at least one synchronous function generator for generating an alternating current signal wave for controlling speed of an electric motor and comprising means for receiving signals indicating motor shaft position and speed variation; means, responsive to the signals, for generating a digital signal sample train representing an analog signal frequency equal to a torque ripple frequency component in the output of the motor; means for adjusting phase of the sample train in a direction to reduce the ripple frequency; means for converting the phase adjusted sample train to the analog signal; and means for adjusting the amplitude of the analog signal in a direction to reduce the ripple.

In one embodiment, the modified drive current is applied to the motor through amplification means having an open loop bandwidth about an order of magnitude larger than the bandwidth of the servo loop comprising the motor and the speed control circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional insight on various features and advantages of the invention can be obtained from a consideration of the following Detailed Description in connection with the appended claims and the attached drawings in which:

FIG. 1 is a block diagram of a motor speed control system employing the invention for manual