The present invention relates to a method for determining the torque of an electric motor and a drivetrain for a motor vehicle.
Electric motors are used for various technical applications. For example, in electric or hybrid vehicles, electric motors are used for driving a motor vehicle. Continuous and precise detection of the torque generated by the electric motor is necessary for the use of electric motors in motor vehicles, in order to be able to avoid inadvertent accelerations or decelerations of the motor vehicle. For this purpose, the torque is determined from the measured phase currents of the electromagnets of the electric motor and the rotation angle of the rotor of the electric motor using a machine model. The machine model contains operation-dependent parameters, e.g., the temperature of the stator and the rotor or the rotor flux, which must be complexly adapted. Errors or inaccuracies in the calculation path of the machine model result in incorrectly ascertained torques, which may have safety-relevant consequences for the motor vehicle. For this reason, there are methods for performing a plausibility check of the torque calculated using the machine model from measured values of the voltage and the current, the torque therefore being disadvantageously dependent on the precision and functionality of the sensors for these measured values.
A method for operating a variable-speed electric motor, in particular a brushless servo motor, is discussed in DE 41 22 391 A1. By regulating the motor variables current, position angle, and speed, the speed is determined from the position angle measured with the aid of a position sensor by a filter.
A method according to the present invention for determining the torque of an electric motor on a motor vehicle, a torque being generated by a rotor of the electric motor and the torque being transmitted from a drivetrain to at least one drive wheel, including the following steps: measuring the rotation angle of a rotor of the electric motor, determining the torque generated by the rotor, the at least one drive wheel driven by the rotor being fixed, the stationary rotor not generating any torque, and a first rotation angle of the stationary rotor being measured, a torque subsequently being applied by the rotor to the drivetrain, so that the rotor carries out a rotational movement due to the limited stiffness of the drivetrain and subsequently, in an equilibrium state between the torque generated by the rotor and a counter torque of the drivetrain, a second rotation angle of the rotor being measured, the torque generated by the rotor and/or the total stiffness of the drivetrain being ascertained from the measured values of the first and second rotation angles of the rotor. The torque of the electric motor may therefore advantageously be determined without measured values for the current or the voltage of the electric motor being necessary, and therefore no errors occur due to a lack of precision or functionality of the sensors for the measured values.
In one variant, the torque generated by the rotor and/or the total stiffness of the drivetrain is/are ascertained using a physical model.
In another specific embodiment, the physical model is a dual-mass oscillator having a first mass with a rotational moment of inertia of the rotor of the electric motor, a second mass having a second equivalent rotational moment of inertia, and a torsion bar spring as the drivetrain between the first mass and the second mass.
The drivetrain advantageously includes a transmission and/or a drive shaft and/or a differential and/or a converter, by which the torque is transmitted from the electric motor to the at least one drive wheel.
In another embodiment, the rotational moment of inertia of the first mass and the equivalent rotational moment of inertia of the second mass are determined for the physical model.
In a supplementary specific embodiment, the at least one drive wheel, in particular all drive wheels, do not carry out any rotational movement between the points in time of the measurement of the first rotation angle and the second rotation angle of the rotor.
The total stiffness of the torsion bar spring is advantageously set equal to the total stiffness of the drivetrain between the electric motor and the at least one parking brake.
In another variant, the equivalent rotational moment of inertia of the second mass also includes, in addition to the rotational moment of inertia of the at least one drive wheel, the fictional rotational moment of inertia of the motor vehicle, the fictional rotational moment of inertia of the motor vehicle being calculated from the translational moment of inertia of the motor vehicle.
In particular, when the at least one parking brake is disengaged, the drivetrain transmits the torque of the electric motor from the electric motor to the at least one drive wheel and, when the at least one parking brake is engaged, the drivetrain transmits the torque of the electric motor from the electric motor to the at least one parking brake.
The second angle of the rotor may be measured in the equilibrium state when the rotor is essentially stationary. An essentially stationary rotor means that the angular velocity of the rotor is less than one revolution per minute.
In another specific embodiment, the torque generated by the rotor and/or the total stiffness of the drivetrain is/are ascertained using an equation, the equation may read Me1=(c/u2)*(φR2−φR1) and Me1 being the torque of electric motor (1) in this case, c being the total stiffness of the drivetrain, u being the transmission ratio of the transmission, φR2 being the second rotation angle of the rotor, and φR1 being the first rotation angle of the rotor.
A drive unit according to the present invention, in particular an electric or hybrid drive unit, for a motor vehicle, including: an electric motor having a sensor for detecting the rotation angle of a rotor of the electric motor, which may be a control unit, which may be an internal combustion engine, a drivetrain, at least one drive wheel, a parking brake for the at least one drive wheel, a method described in the present application being executable therein.
The sensor is a resolver or a digital sensor in particular.
An exemplary embodiment of the present invention is described in greater detail hereafter with reference to the appended drawings.
A drive unit 9, which is configured as a hybrid drive unit 10, for a motor vehicle 11 is shown in
In this physical model of dual-mass oscillator 5, the two differential equations 1a and 1b apply:
J
R*φ″R=Mem−c/u*(φR/u−φF)−d/u*(φ′R/u−φ′F) (1a)
J
F*φ″F=c/u*(φR/u−φF)−d/u*(φ′R/u−φ′F)−ML (1b)
In this case, φR is the rotation angle of first mass 6 or rotor 2, φ′R=dφR/dt is the angular velocity of first mass 6, and φ″R=dφ′R/dt is the angular acceleration of first mass 6. Similarly, φF is the rotation angle of second mass 7, φ′F=dφF/dt is the angular velocity of second mass 7, and φ″F=dφ′F/dt is the angular acceleration of second mass 7. Torsion bar spring 8 has total stiffness c and total damping d, which correspond to total stiffness c and total damping d of drivetrain 12 between electric motor 1 and the two parking brakes 25.
Motor vehicle 11 is stationary, so that drive wheels 20 do not carry out a rotational movement and drive wheels 20 are fixed using the two parking brakes 25. Electric motor 1 does not generate a torque Me1, rotor 2 is stationary and a first rotation angle φR1 of rotor 2 is detected. After the fixing of the two drive wheels 20, a torque Me1 is applied using electric motor 1 to drive shaft 14 and this torque Me1 causes a torsion of drivetrain 12 or torsion bar spring 8 because of limited total stiffness c, since the total stiffness is not infinite. In particular drive shaft 14 and half shafts 19 are twisted. Drivetrain 12 or torsion bar spring 8 applies a counter torque ML or a load torque ML to rotor 2 until, in an equilibrium state, angular velocity φ′R=dφR/dt of first mass 6 or rotor 2 is equal to zero. A second rotation angle φR2 of rotor 2 is detected in this equilibrium state.
Therefore, angular acceleration φ″R=dφ′R/dt of first mass 6 or rotor 2 is also equal to zero and therefore in differential equation (1a), JR*φ″R=0. Furthermore, in addition to angular velocity φ′R=dφR/dt of first mass 6 or rotor 2, the angular velocity of second mass 7 is also φ′F=dφF/dt=0, so that also in differential equation (1a), d/u*(φ′R/u−φ′F)=0. The following equation (2) therefore arises in the equilibrium state for Mem:
M
em
=c/u*(φR/u−φF) (2)
In this equation, φF=0, because drive wheels 20 are fixed and therefore have not established a rotation angle. φR is the difference of second and first detected rotation angles φR, i.e., rotation angle φR of rotor 2 or first mass 6 in the case of fixed drive wheels 20 until the equilibrium state.
Therefore, the equation (3) arises:
M
em=(c/u)2*(φR2−φR1) (3)
The transmission ratio is a known variable and total stiffness c of drivetrain 12 is a mechanical parameter, which either may be calculated from the design data and/or may be determined using measurements.
Equations (2) and (3) therefore allow the calculation of torque Mem of electric motor 1 independently of measured values for the voltage and the current on electric motor 2 and parameters of a machine model for electric motor 1.
Torque Mem of electric motor 1, which is calculated using equations (2) and (3), may be used, e.g., for a calibration of the parameters of a machine model, for a calibration of the current sensor system of electric motor 1, or for a determination of total stiffness c of drivetrain 12, if torque Mem is known.
Considered as a whole, significant advantages are connected with the method according to the present invention and drive unit 9 according to the present invention. Torque Mem, generated by electric motor 1 may be determined in a simple way, without complex and error-prone machine models of electric motor 1 being required. Only a brief stoppage of motor vehicle 11 is necessary; therefore, it may be carried out with motor vehicle 11 on a test stand, for example, or also during a brief stoppage of motor vehicle 11 at a traffic light during normal driving operation, if parking brakes 25 are briefly operated by the controller of motor vehicle 11 during the stoppage and, during the operation of the parking brakes, for a period of time of less than one second, e.g., in the range of 300 ms to 500 ms, torque Mem is generated by electric motor 1, the difference between second and first detected rotation angles φR may therefore be measured. The method may be used for all types of electric motors 1, e.g., GM, ASM, PSM, or GRM.
Number | Date | Country | Kind |
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102010030365.8 | Jun 2010 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2011/057138 | 5/4/2011 | WO | 00 | 12/11/2012 |