Method for direct voltage saturation calculation and prevention of inverter voltage saturation

Abstract

A voltage saturation prevention algorithm used as at least part of a method of controlling an electric vehicle, wherein the electric vehicle comprises an electric motor, a controller, and an inverter. The controller receives a control signal with an instruction to operate the electric motor, then sends a switching signal corresponding to the control signal to the inverter, wherein the inverter provides a plurality of output signals for operation of the electric motor. The method includes determining the expected amplitude of the plurality of output signals based on the instruction to operate the electric motor, calculating the amount of modification of the plurality of output signals required to prevent the expected amplitude from reaching a saturation value, and modifying, based on the calculation, the instruction to operate the electric motor to prevent the expected amplitude from reaching the saturation value. The method is implemented in software, without any additional hardware.

Claims

1. A method of controlling an electric vehicle that comprises an electric motor and an inverter, the method comprising: receiving a control signal comprising an instruction to operate the electric motor, the instruction comprising a specific command current value; determining an expected amplitude of a plurality of output signals for the inverter to provide for operating the electric motor; determining, as a function of the specific command current value without using any measurement systems, a modulation index; calculating, based on the modulation index and a maximum allowable modulation index determined based on a current modulation strategy of the inverter, a saturated voltage modifier value indicating an amount of modification of the plurality of output signals required to prevent the expected amplitude from reaching a saturation value, the calculated saturated voltage modifier value limited to a maximum allowable value based on an application-specific design parameter; and modifying, based on the calculation, the specific command current value to operate the electric motor to prevent the expected amplitude from reaching the saturation value; wherein the specific command current comprises: a commanded quadratic current component value I.sub.q,cmd; and a commanded direct current component value I.sub.d,cmd; and modifying the specific command current value comprises: determining a modified commanded quadratic current component value, I.sub.q,cmd,modified, by calculating:
Iq, cmd, modified=Iq,cmd×Ecompensator; wherein E.sub.compensator is the saturated voltage modifier value; determining a modified direct current component value, I.sub.d,cmd,modified, by calculating: $\mathrm{Id},\mathrm{cmd},\mathrm{modified}=\mathrm{Id},\mathrm{cmd}-\frac{\mathrm{Iq},{\mathrm{cmd}}^2\times \mathrm{Ecompensator}}{.Math.\mathrm{Id}.Math.\max };\mathrm{and}$ wherein |Id|.sub.max is a maximum potential value of the direct current component for the electric motor.

2. The method of claim 1, wherein the method is implemented in software.

3. The method of claim 1, wherein the plurality of output signals is a plurality of voltage components.

4. The method of claim 1, wherein calculating the amount of modification of the plurality of output signals is based on current angle modification.

5. The method of claim 4, wherein a modification in the current angle results in a reduction of amplitude of the plurality of output signals.

6. The method of claim 1, wherein the method automatically corrects errors in a calibration table of the electric motor.

7. An electric vehicle comprising: an electric motor; an inverter providing a plurality of output signals for operation of the electric motor; and a controller in communication with the inverter and performing operations comprising: receiving a control signal comprising an instruction to operate the electric motor, the instruction comprising a specific command current value; determining an expected amplitude of a plurality of output signals for the inverter to provide for operating the electric motor; determining, as a function of the specific command current value-without using any measurement systems, a modulation index; calculating, based on the modulation index and a maximum allowable modulation index determined based on a current modulation strategy of the inverter, a saturated voltage modifier value indicating an amount of modification of the plurality of output signals required to prevent the expected amplitude from reaching a saturation value, the calculated saturated voltage modifier value limited to a maximum allowable value based on an application-specific design parameter; and modifying, based on the calculation, the specific command current value to operate the electric motor to prevent the expected amplitude from reaching the saturation value; wherein the specific command current comprises: a commanded quadratic current component value I.sub.q,cmd; and a commanded direct current component value I.sub.d,cmd; and modifying the specific command current value comprises: determining a modified commanded quadratic current component value, I.sub.q,cmd,modified, by calculating:
Iq,cmd,modified=Iq,cmd×Ecompensator; wherein E.sub.compensator is the saturated voltage modifier value; determining a modified direct current component value, I.sub.d,cmd,modified, by calculating: $\mathrm{Id},\mathrm{cmd},\mathrm{modified}=\mathrm{Id},\mathrm{cmd}-\frac{\mathrm{Iq},{\mathrm{cmd}}^2\times \mathrm{Ecompensator}}{.Math.\mathrm{Id}.Math.\max };$  and wherein |Id|.sub.max is a maximum potential value of the direct current component for the electric motor.

8. The electric vehicle of claim 7, wherein the method is implemented in software.

9. The electric vehicle of claim 7, wherein the plurality of output signals is a plurality of voltage components.

10. The electric vehicle of claim 7, wherein calculating the amount of modification of the plurality of output signals is based on current angle modification.

11. The electric vehicle of claim 10, wherein a modification in the current angle results in a reduction of amplitude of the plurality of output signals.

12. The electric vehicle of claim 7, wherein the method automatically corrects errors in a calibration table of the electric motor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The features, objects, and advantages of the disclosed embodiments will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein:

(2) FIG. 1 is a perspective view depicting an exemplary embodiment of an electric vehicle that may include an electric motor drive system.

(3) FIG. 2 is a flow diagram depicting the structure of an exemplary electric motor drive system in electric vehicles.

(4) FIG. 3A is a side view depicting an exemplary embodiment of an electric motor for an electric vehicle. FIG. 3B is a side view depicting an exemplary current vector coordinate system for the electric motor of FIG. 3A.

(5) FIG. 4 is a signal block diagram depicting an exemplary embodiment of a voltage saturation prevention algorithm.

DETAILED DESCRIPTION

(6) One aspect of the disclosure is directed to a voltage saturation prevention algorithm used as at least part of a method of controlling an electric vehicle.

(7) References throughout this document to “one embodiment,” “certain embodiments,” “an embodiment,” or similar term mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of such phrases in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner on one or more embodiments without limitation. For example, two or more of the innovative algorithms described herein may be combined in a single implementation, but the application is not limited to the specific exemplary combinations of voltage saturation prevention algorithms that are described herein.

(8) As used herein, the terms “a” or “an” shall mean one or more than one. The term “plurality” shall mean two or more than two. The term “another” is defined as a second or more. The terms “including” and/or “having” are open ended (e.g., comprising). The term “or” as used herein is to be interpreted as inclusive or meaning any one or any combination. Therefore, “A, B or C” means “any of the following: A; B; C; A and B; A and C; B and C; A, B and C”. An exception to this definition will occur only when a combination of elements, functions, steps or acts are in some way inherently mutually exclusive.

(9) A detailed description of various embodiments is provided; however, it is to be understood that the disclosed embodiments are merely exemplary and may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the disclosed embodiments.

(10) FIG. 1 is a perspective view depicting an exemplary embodiment of an electric vehicle 100 that may include an electric motor drive system 200. The electric vehicle 100 shown in FIG. 1 is exemplary. The electric motor drive system 200 may be installed in any vehicle with use for an electric motor drive system, including but not limited to hybrid vehicles.

(11) FIG. 2 is a flow diagram depicting the structure of an exemplary electric motor drive system 200 in electric vehicles. The algorithm depicted in FIG. 4 may be installed as part of the inverter control board 210, which in one embodiment is designed to control the inverter 220 at its commanded value with a good dynamic and steady-state response. In one embodiment, the inverter 220 feeds power to the electric motor 230, which in turn applies torque to a plurality of vehicle wheels 240.

(12) In one embodiment, the inverter control board 210 receives a control signal with an instruction to operate the electric motor 230. In one embodiment, the control signal comprises a torque command. In one embodiment, the torque command may be an instruction for the electric motor 230 to operate with a specific torque value in order to achieve a desired velocity for the electric vehicle 100. The torque command may be determined based on input from systems including but not limited to an acceleration pedal of the electric vehicle 100, a brake pedal of the electric vehicle 100, and a cruise control system of the electric vehicle 100. In one embodiment, the inverter control board may receive further feedback regarding the operating conditions of the electric motor drive system 200 or other systems by means of an analog input or a resolver input.

(13) In one embodiment, the inverter control board 210 performs a series of calculations using the received instruction in order to produce a switching signal for the inverter 220. The switching signal may be designed to use space vector pulse width modulation (SVPWM) to operate the legs of the inverter 220 to produce a plurality of output signals corresponding to the received instruction to operate the electric motor 230. In one embodiment, the plurality of output signals is a set of current components. In one embodiment, the plurality of output signals includes a first output signal and a second output signal, wherein the first output signal is a current component that is related to the direct axis as a flux component, and wherein the second output signal is a current component that is related to the quadrature axis as a torque component. In one embodiment, the plurality of output signals may be mapped to a dq frame of reference.

(14) FIG. 3A is a side view depicting an exemplary embodiment of an electric motor 230 for an electric vehicle 100. In one embodiment, the electric motor 230 includes a rotor 231 and a stator with a plurality of stator coils 232. In one embodiment, the rotor 231 may include a permanent magnet. In one embodiment, the stator coils 232 may receive power from the inverter 220 to produce a magnetic field by means of SVPWM. In one embodiment, the electric motor drive system 200 rotates the magnetic field of the stator in order to induce rotation in the rotor 231 and propel the electric vehicle 100.

(15) FIG. 3B is a side view depicting an exemplary current vector coordinate system for the electric motor 230 of FIG. 3A. In one embodiment, the electric motor 230 has a rotation angle value θ. Rotation angle θ may represent a current angle of the motor, wherein the current angle is the angle between the current vector i.sub.dq and the d-axis. In one embodiment, the current angle is defined as

(16) $\theta =a\tan \left(\frac{i_q}{i_d}\right)$

(17) FIG. 4 is a signal block diagram depicting an exemplary embodiment of a voltage saturation prevention algorithm 300. According to one embodiment, the voltage saturation prevention algorithm 300 modifies the commanded current components comprising the output signal only when the voltage amplitude is saturated and exceeds a maximum allowable value.

(18) In one embodiment, the modulation index m is defined as

(19) $m=\frac{2\sqrt{V_d^2+V_q^2}}{V_{DC}}$
and modulation index differential m.sub.diff may be expressed with the following equation:
m.sub.diff=m−m.sub.max
where m.sub.max is the maximum allowable modulation index value before the electric motor drive system 200 is considered to be experiencing voltage saturation. m.sub.max may vary according to a number of factors, including but not limited to the hardware architecture of the inverter and the PWM strategy.

(20) In one embodiment, saturated voltage modifier E.sub.compensator is the result of feeding m.sub.diff through a compensator. The compensator may be a proportional (P) compensator, proportional-integral (PI) compensator, or any other compensator type chosen by the designer. In one embodiment, the compensator uses windup or non-windup limiters to limit E.sub.compensator to a minimum and maximum allowable value. In one embodiment, the minimum allowable value of E.sub.compensator is 0, representing a condition of no voltage saturation and normal electric motor operation. In one embodiment, the maximum allowable value of E.sub.compensator is E.sub.max (wherein E.sub.max is an application-specific design parameter), representing a condition of maximum voltage saturation adjustment.

(21) In one embodiment, the modified quadrature current component I.sub.q,cmd,modified may be expressed with the following equation:
I.sub.q,cmd,modified=I.sub.q,cmd−I.sub.q,cmd×E.sub.compensator
wherein I.sub.q,cmd is the original current component before factoring in voltage saturation. The above equation may be reduced to the following equation:
I.sub.q,cmd,modified=I.sub.q,cmd(1E.sub.compensator)
In one embodiment, E.sub.compensator is equal to zero and I.sub.q,cmd,modified is equal to I.sub.q,cmd. In this condition, the voltage saturation prevention algorithm 300 has determined that the electric motor drive system 200 is not experiencing voltage saturation and no current modification is necessary. In one embodiment, E.sub.compensator is not equal to zero and I.sub.q,cmd,modified has a magnitude equal to a fraction of the magnitude of I.sub.q,cmd. In this condition, the voltage saturation prevention algorithm 300 has determined that the electric motor drive system 200 is experiencing voltage saturation and that a modification of current value is necessary to prevent that condition.

(22) In one embodiment, the modified direct current component I.sub.d,cmd,modified may be expressed with the following equation:

(23) $I_{d,\mathrm{cmd},\mathrm{modified}}=I_{d,\mathrm{cmd}}-I_{q,\mathrm{cmd}}\times \left(I_{q,\mathrm{cmd}}{E}_{\mathrm{compensator}}\right)\times \frac{1}{{.Math.I_d.Math.}_{\max }}$
wherein I.sub.d,cmd is the original direct current command before factoring in voltage saturation and |I.sub.d|.sub.max is the maximum potential value of the direct current component that the electric motor drive system 200 may have. The above equation may be reduced to the following equation:

(24) $I_{d,\mathrm{cmd},\mathrm{modified}}=I_{d,\mathrm{cmd}}-\frac{I_{q,\mathrm{cmd}}^2{E}_{\mathrm{compensator}}}{{.Math.I_d.Math.}_{\max }}$
In one embodiment, E.sub.compensator is equal to zero and I.sub.d,cmd,modified is equal to I.sub.d,cmd. In this condition, the voltage saturation prevention algorithm 300 has determined that the electric motor drive system 200 is not experiencing voltage saturation and no current modification is necessary. In one embodiment, E.sub.compensator is not equal to zero and I.sub.d,cmd,modified has a magnitude greater than the magnitude of I.sub.d,cmd. In this condition, the voltage saturation prevention algorithm 300 has determined that the electric motor drive system 200 is experiencing voltage saturation and that a modification of current value is necessary to prevent that condition.

(25) In one embodiment, m is determined by the inverter control board 210 as a function of the current component values I.sub.d,cmd and I.sub.q,cmd. In one embodiment, the ability to determine m without using measurement systems allows the voltage saturation prevention algorithm 300 to operate without adding any additional hardware to the electric motor drive system 200.

(26) While this disclosure makes reference to exemplary embodiments, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the claimed embodiments.