Method of inverter linearization in electric machines through secondary modulation

The invention claimed is:

1. A method for controlling an electric motor, the method comprising: determining whether a magnitude of a desired voltage command falls between a first value and a second value utilizing a controller, the second value being greater than the first value; and if said magnitude of said desired voltage command falls between said first value and said second value, then pulse width modulating said desired voltage command between said first value and said second value over a determined sample period, thereby resulting in a modulated voltage command utilizing said controller; generating a first set of motor duty cycle commands based upon the modulated voltage command, utilizing said controller, said first set of motor duty cycle commands being received by an inverter operably coupled to said electric motor; generating a first set of modulated phase voltage signals based on the first set of motor duty cycle commands for controlling said electric motor, utilizing said inverter; and if said magnitude of said desired voltage command is greater than or equal to said second value, then generating a second set of motor duty cycle commands based upon said desired voltage command, utilizing said controller, said second set of motor duty cycle commands being received by said inverter; and generating a second set of modulated phase voltage signals based on said second set of motor duty cycle commands for controlling said electric motor, utilizing said inverter.

2. The method of claim 1, further comprising: computing a ratio between said desired voltage command and said second value; and multiplying said computed ratio by said time period, thereby determining an on time for said second value, said on time representing a portion of said time period when said desired voltage command is pulse width modulated to said second value.

3. The method of claim 2, further comprising: pulse width modulating said desired voltage command to said second value at a beginning of said time period; and following passage of said on time, pulse width modulating said desired voltage command to said first value for a remainder of said time period.

4. The method of claim 2, further comprising: obtaining a start time within said time period, when said desired voltage command is to be pulse width modulated to said second value; computing an end time from said start time and said on time, said end time representing a time when said desired voltage command is to be pulse width modulated to said first value; wherein said start time is randomly assigned to occur within said time period.

5. The method of claim 4, wherein if said end time is initially computed to be greater than said time period, then said end time is recomputed to be a difference between said initially computed end time and said time period.

6. A method for controlling an electric motor the method comprising: determining a desired voltage command signal from a torque command signal and a motor speed signal, utilizing a controller; determining a phase advancing angle from said torque command signal and said motor speed signal, utilizing said controller; determining whether a magnitude of said desired voltage command signal falls between a first value and a second value, said second value being greater than said first value, utilizing said controller; if said magnitude of said desired voltage command signal falls between said first value and said second value, then pulse width modulating said desired voltage command signal between said first value and said second value over a determined time period, thereby resulting in a pulse width modulated voltage command, utilizing said controller; generating a first set of motor duty cycle commands to be applied to an inverter coupled to said motor based upon a motor position signal, said phase advancing angle, and the pulse width modulated voltage command, utilizing said controller; generating a first set of modulated phase voltage signals based on said first set of motor duty cycle commands for controlling said electric motor, utilizing said inverter; if said magnitude of said desired voltage command is greater than or equal to said second value, then not pulse width modulating said desired voltage command signal, and generating a second set of motor duty cycle commands to be applied to said inverter coupled to said motor based upon a motor position signal, said phase advancing angle and said desired voltage command signal; and generating a second set of modulated phase voltage signals based on said second set of motor duty cycle command for controlling said electric motor, utilizing said inverter.

7. The method of claim 6, further comprising: computing a ratio between said desired voltage command signal and said second value; and multiplying said computed ratio by said time period, thereby determining an on time for said second value, said on time representing a portion of said time period when said voltage command signal is pulse width modulated to said second value.

8. The method of claim 7, further comprising: pulse width modulating said desired voltage command signal to said second value at a beginning of said time period; and following passage of said on time, pulse width modulating said desired voltage command signal to said first value for a remainder of said time period.

9. The method of claim 7, further comprising: obtaining a start time within said time period, when said desired voltage command signal is to be pulse width modulated to said second value; computing an end time from said start time and said on time, said end time representing a time when said desired voltage command signal is to be pulse width modulated to said first value; wherein said start time is randomly assigned to occur within said time period.

10. The method of claim 9, wherein if said end time is initially computed to be greater than said time period, then said end time is recomputed to be a difference between the initially computed end time and said time period.

11. An electric power steering system, comprising: a steering input device coupled to one or more steerable wheels; an assist actuator including an electric motor for providing an assist torque to said one or more steerable wheels, an inverter operably coupled to said electric motor; a controller operably coupled to said inverter, said controller configured to determine whether a magnitude of a desired voltage command falls between a first value and a second value, the second value being greater than the first value; and if said magnitude of said desired voltage command falls between said first value and said second value, then pulse width modulating said desired voltage command between said first value and said second value over a determined sample period, thereby resulting in a modulated voltage command; said controller further configured to generate a first set of motor duty cycle commands based upon the modulated voltage command, said first set of motor duty cycle commands being received by an inverter operably coupled to said electric motor; said inverter configured to generate a first set of modulated phase voltage signals based on the first set of motor duty cycle commands for controlling said electric motor; said controller further configured to generate a second set of motor duty cycle commands based upon said desired voltage command, if said magnitude of said desired voltage command is greater than or equal to said second value, said second set of motor duty cycle commands being received by said inverter; and said inverter further configured to generate a second set of modulated phase voltage signals based on said second set of motor duty cycle commands for controlling said electric motor.

12. A computer storage medium readable by a computer and storing instructions for execution by said computer for facilitating a method comprising: determining whether a magnitude of a desired voltage command falls between a first value and a second value, said second value being greater than said first value; and if said magnitude of said desired voltage command falls between said first value and said second value, then pulse width modulating said desired voltage command between said first value and said second value over a determined time period, thereby resulting in a modulated voltage command; and generating a first set of motor duty cycle commands based upon said modulated voltage command, said first set of motor duty cycle commands being received by an inverter operably coupled to an electric motor for controlling said electric motor; if said magnitude of said desired voltage command is greater than or equal to said second value, then generating a second set of motor duty cycle commands based upon said desired voltage command, said second set of motor duty cycle command being received by said inverter for controlling said electric motor.

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BACKGROUND

The present disclosure relates generally to automobile steering systems and, more particularly, to a method of inverter linearization in electric machines such as electric power steering (EPS) motors through secondary modulation of a motor voltage command thereto.

Electric power steering (EPS) has been recently implemented in motor vehicles to improve fuel economy and has even started to replace hydraulic power steering in certain vehicles. One way to accomplish improved fuel economy is through the reduction or elimination of losses inherent in traditional steering systems. To this end, electric power steering requires power only on demand. Commonly, in such EPS systems, an electronic controller is also configured to require significantly less power under a small or no steering input condition. This dramatic decrease from conventional steering assist is the basis of the power and fuel savings.

A polyphase permanent magnet (PM) brushless motor is typically used in EPS systems as the actuator for providing a mechanical assist to the vehicle's steering mechanism. Such a motor is generally excited with a sinusoidal field to provide lower torque ripple, noise, and vibration as compared to those motors excited with a trapezoidal field. Theoretically, if a motor controller produces polyphase sinusoidal currents with the same frequency and phase as that of the sinusoidal back electromotive force (EMF), the torque output of the motor will be a constant, and zero torque ripple will be achieved. However, due to practical limitations of motor design and controller implementation, there are always deviations from pure sinusoidal back EMF and current waveforms. Such deviations usually result in parasitic torque ripple components at various frequencies and magnitudes. Various methods of torque control can influence the magnitude and characteristics of this torque ripple.

In EPS drive systems based on a voltage mode controlled sinusoidal PM drive, a full bridge power inverter is employed to apply a pulse width modulated (PWM) voltage across the motor phases. Unfortunately, these inverters (particularly those used for sinusoidal brushless motors) suffer, however, from several linearity issues. More specifically, an inverter used in conjunction with a sinusoidal motor with phase grounding can generate considerable torque ripple at relatively low voltage commands. In addition, when the voltage command is low, the torque output versus voltage commands characteristics are fairly non linear as the result of dead time and switching time associated with the power transistors of the inverter. Accordingly, it is desirable to reduce the torque ripple and non-linearity associated with low voltage commands in EPS systems, thereby enhancing the performance of the EPS system.

SUMMARY

The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a method for implementing control signal linearization for an electric motor. In an exemplary embodiment, the method includes receiving a desired voltage command to be applied to the motor and determining whether the magnitude of the desired voltage command falls between a first value and a second value. If the magnitude of the desired voltage command falls between the first value and the second value, then modulating the desired voltage command between the first and the second value over a determined sample period, thereby resulting in the application of a modulated voltage command to the motor.

Preferably, the first value is zero and the second value is a determined threshold voltage value. If the desired voltage command is greater than or equal to the second value, then the desired voltage command is not modulated between the first and said second values before being applied to the motor. In addition, a ratio is computed between the desired voltage command and the second value. Then, the computed ratio is multiplied by the sample period, thereby determining an on time for the second value. The on time represents a portion of the sample period during which the desired voltage command is modulated at the second value.

In one embodiment, the desired voltage command is modulated to the second value at the beginning of the sample period. Following the passing of the on time, the desired voltage command is modulated to the first value for the remainder of the sample period. Alternatively, a start time is obtained within the sample period, at which time the desired voltage command is to be modulated to the second value. Then, an end time is computed from the start time and the on time, the end time representing the time at which the desired voltage command is to be modulated to the first value. The start time may be randomly assigned to occur within said sample period. If the end time is initially computed to be greater than the sample period, then the end time is recomputed to be the difference between the initially computed end time and the sample period.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures:

FIG. 1 is a schematic diagram of a motor vehicle provided with an electric power steering system, suitable for practicing an embodiment of the present disclosure;

FIG. 2 is a block diagram of an exemplary PM motor control system that may be used for controlling the torque of a sinusoidally excited PM electric machine, such as the EPS motor in FIG. 1;

FIG. 3 is a modified block diagram of the motor control system shown in FIG. 2, in accordance with an embodiment of the invention, illustrating the addition of a secondary modulation block for implementing a secondary time-based modulation of the voltage command signal; and

FIG. 4 is a flow diagram illustrating a first embodiment of a method for implementing the secondary modulation shown in FIG. 3;

FIG. 5 is a flow diagram illustrating an alternative embodiment of the method for implementing the secondary modulation shown in FIG. 4; and

FIGS. 6(a) and 6(b) are signal waveform illustrating an example of the PWM technique of the embodiment in FIG. 5.

DETAILED DESCRIPTION

Disclosed herein is a method of inverter linearization in electric motors (such as those used in EPS systems) through secondary modulation of a motor voltage command thereto. Briefly stated, the method overcomes the aforementioned problems by interpolating between two known "good" operating points through a secondary, time-based domain modulation. A first good operating point is established as a defined threshold voltage command value, wherein the motor is considered to exhibit acceptable torque ripple and linearity at values above the threshold. At values below the threshold, the torque ripple and linearity is considered unacceptable. The second good operating point is defined at the "zero" command, at which point the inverter also produces very low torque ripple. Thus, for command values greater than zero, but less then the threshold value, a time based modulation is implemented wherein the command is modulated to the threshold value for a period of time proportional to the relationship between the actual desired value and the threshold value.

Referring initially to FIG. 1, there is shown a representative environment for practicing an embodiment of the present disclosure, in which a motor vehicle 10 is provided with an electric power steering system 12. The electric power steering system 12 may include a conventional rack and pinion steering mechanism 14 having a toothed rack 15 and a pinion gear (not shown) under a gear housing 16. As the steering wheel 18 is turned, an upper steering shaft 20 turns a lower shaft 22 through a universal joint 24. The lower steering shaft 22 turns the pinion gear. The rotation of the pinion gear moves the pinion rack 15, which then moves tie rods 28 (only one shown). In turn, tie rods 28 move steering knuckles 30 (only one shown) to turn wheels 32.

An electric power assist is provided through a controller 34 and a power assist actuator comprising a motor 36. The controller 34 receives electric power from a vehicle electric power source 38 through a connection 40. The controller 34 also receives a signal 41 representative of the vehicle velocity, as well as steering pinion gear angle signal 44 from a rotational position sensor 42. As the steering wheel 18 is turned, a torque sensor 46 senses the torque applied to steering wheel 18 by the vehicle operator and provides an operator torque signal 48 to the controller 34. In addition, as the rotor of motor 36 turns, rotor position signals 50 are generated within the motor 36 and are also provided to the controller 34. In response to vehicle velocity, operator torque, steering pinion gear angle and rotor position signals received, the controller 34 derives desired motor phase voltages. The motor phase voltages are provided to motor 36 through a bus 52, thereby providing torque assist to steering shaft 20 through worm 54 and worm gear 56. As is described in greater detail later, the controller 34 is configured to develop the necessary voltage(s) to be applied to the motor 36 such that the desired torque is generated. Accordingly, a storage medium 58 may be used to contain instructions for executing a computer-implemented process within controller 34, through the transmission of data signal(s) 60 therebetween.

FIG. 2 is a block diagram of an exemplary PM motor control system 100 that may be used for controlling the torque of a sinusoidally excited PM electric machine, such as the EPS motor 36 of FIG. 1. The control system 100 includes (but is not limited to) motor 36, a motor rotor position/velocity sensor assembly 102, the controller 34, a power circuit or inverter 106 and power supply 108. The velocity sensing portion of sensor assembly 102 may be embodied by a circuit or algorithm, for example. Again, the controller 34 is configured and connected to develop the necessary voltage(s) out of inverter 106 such that, when applied to the motor 36, the desired torque is generated. Because these voltages are related to the position and speed of the motor 36, the position and speed of the motor rotor are determined by the system 100. The sensor assembly 102 is connected to the motor 36 to detect the angular position, .theta., of the rotor. The sensor assembly 102 may sense the rotary position based on optical detection, magnetic field variations, or other methodologies. Exemplary position sensors include potentiometers, resolvers, synchros, encoders, and the like. The sensor assembly 102 outputs a position signal 110 indicating the angular position of the rotor.

Again, the motor speed (denoted .omega.) may be measured, calculated, or otherwise derived from a combination thereof. For example, the motor speed .omega. is calculated as the change of the motor position .theta. as measured by the sensor assembly 102 over a prescribed time interval. Alternatively, the motor speed .omega. may be determined as the derivative of the motor position .theta. from the equation .omega.=.DELTA..theta./.DELTA.t, where .DELTA.t is the sampling time and .DELTA..theta. is the change in position during the sampling interval. In FIG. 2, the sensor assembly 102 determines the speed of the rotor and outputs a corresponding speed signal 112.

The position signal 110, speed signal 112, and a torque command signal 114 are each applied to the controller 34. The torque command signal 114 is representative of the desired motor torque value for the assist motor 36. The controller 34 then determines a voltage command amplitude V.sub.ref (shown as signal 116) and its phase advance angle .delta. needed to develop the desired torque according to the torque command signal 114, the position signal 110 and the speed signal 112, as well as other fixed motor parameter values. Although not shown in FIG. 2, the controller 34 could also include a linearization function block that provides a linearization offset function for V.sub.ref 116 in order to minimize torque ripple.

For a three-phase motor, three sinusoidal reference signals that are synchronized with the moto