Methods and systems for evaluating permanent magnet motors

A method for evaluating a permanent magnet motor, which includes a rotor with a plurality of magnets mounted thereon, and a stator with a plurality of windings in proximity to the rotor and coupled to an inverter, includes spinning the motor such that a voltage is induced in the windings of the stator and the inverter; measuring the voltage on the inverter; calculating the voltage constant from the motor from the measured voltage; comparing the voltage constant to accepted voltage constants; and identifying the motor as not acceptable if the voltage constant is outside of a range of the accepted voltage constants.

TECHNICAL FIELD

The present invention generally relates permanent magnet motors, and more particularly relates to methods for evaluating permanent magnet motors.

BACKGROUND

Hybrid vehicle systems typically utilize one or more electric, permanent magnet motors as part of a transmission system that provides a propulsion source to compliment the engine. The accuracy of the manufacture, service, and operation of these motors to produce specified torque profiles is important to the consistent and efficient operation of the vehicle.

Various issues can arise in the manufacture, assembly, transport, service, and use of the motors that can affect the performance. For example, the magnet flux strength of the motors can be diminished due to factors such as the particular characteristics of the magnets, including field strength and effects of heat and vibration. Similarly, if the stators of the motors are not properly wound with the specified number of turns per coil, performance can be affected. Another factor that contributes to the performance of the motors is the material utilized to fabricate the stators of the motors. Typically, steel or a similar material forms part of the magnetic circuit through with the magnet flux of the motor flows. As an example, the magnetic permeability of the steel can vary with the types of material used to manufacture the motors, and this variation can impact the performance of the motor.

One mechanism of evaluating a motor involves the calculation and evaluation of a voltage constant, which is a function of the number of windings of the stator, the permeability of the flux path, and the field strength of the rotor magnets. Conventional methods calculate the voltage constant of the motor based on voltage measurements from the motor itself. However, these methods are typically unavailable when the motor is installed in the transmission.

Accordingly, it is desirable to provide methods for evaluating permanent magnet motors in transmissions in a variety of situations, such as during operation, after manufacturing or during service of the vehicle. In addition, it is desirable to provide such methods in an economical and convenient manner. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.

BRIEF SUMMARY

In accordance with an exemplary embodiment, a method is provided for evaluating a permanent magnet motor, which includes a rotor with a plurality of magnets mounted thereon, and a stator with a plurality of windings in proximity to the rotor and coupled to an inverter. The method includes spinning the motor such that a voltage is induced in the windings of the stator and the inverter; measuring the voltage on the inverter; calculating the voltage constant from the motor from the measured voltage; comparing the voltage constant to a set of accepted voltage constants; and identifying the motor as not acceptable if the voltage constant is outside of a range of the set of accepted voltage constants.

In accordance with another exemplary embodiment, a method is provided for evaluating permanent magnet motors in a transmission during operation. The motors includes first and second motors, each including a rotor with a plurality of magnets mounted thereon, and a stator with a plurality of windings in proximity to the rotor and coupled to first and second inverters. The method includes measuring a first voltage for the first motor on the first inverter; calculating a first voltage constant of the first motor from the first voltage; comparing the first voltage constant to a set of accepted voltage constants; and identifying the first motor as not acceptable if the first voltage constant is outside of a range of the accepted voltage constants.

In accordance with yet another exemplary embodiment, an automotive system includes an internal combustion engine; and a two-mode, compound-split, electromechanical transmission coupled to the internal combustion engine. The transmission includes an input member to receive power from the internal combustion engine; an output member to deliver power from the transmission; a first motor and a second motor that are coaxially aligned and coupled to the output and input members; a power inverter coupled to the first and second motors; a measurement device coupled to the power inverter for measuring a first voltage from the first motor; and a processor coupled to the measurement device. The processor is configured to receive the first voltage measured by the measurement device; calculate a first voltage constant from the first voltage; compare the first voltage constant to an accepted voltage constant; and identify the first motor as not acceptable if the first voltage constant varies by more than a predetermined amount from the accepted voltage constant.

DESCRIPTION OF AN EXEMPLARY EMBODIMENT

Exemplary embodiments described herein provide a method for evaluating permanent magnet motors within a two-mode, hybrid, compound-split, electromechanical transmission. Evaluation methods are particularly provided after manufacturing or during service of the transmission by calculating a voltage constant based on voltage measurements taken at the inverter coupled to the motors and comparing the voltage constant to predetermined values. If the calculated voltage constant is within an acceptable threshold range from the predetermined values, then it can be determined that the motors were properly manufactured, installed, and/or maintained.

In the description below, the structural and functional components of the transmission and motors are first described, including an explanation of the relationship between the voltages generated by the motor within the transmission and the voltages measured at the inverters. The conditions and methods for evaluating the motors will then be provided in further detail.

In accordance with an exemplary embodiment, a two-mode, hybrid, compound-split, electromechanical transmission10is depicted inFIG. 1. The hybrid transmission10has an input member12, such as a shaft, that may be directly driven by an engine14. A transient torque damper (not shown) may be incorporated between the engine14and the input member12of the transmission10.

The engine14may be a fossil fuel engine, such as a diesel engine. In the exemplary embodiment, the engine14, after start-up, and during the majority of its input, operates at a range of speeds from approximately 600 to approximately 6000 RPM. Although the particular speed and horsepower output of the engine14can vary, for the purpose of effecting a clear understanding of the hybrid transmission10, an available output of about 300 horsepower from engine14will be assumed for the description of an exemplary installation.

The transmission10includes three planetary gear sets24,26and28. The first planetary gear set24is connected to the input member12and has a ring (or “outer”) gear member30that circumscribes a sun (or “inner”) gear member32. Any number of planet gear members34are rotatably mounted on a carrier36such that each planet gear member34can rotate and mesh with both the outer gear member30and the sun gear member32.

The second planetary gear set26also has a ring gear member38circumscribing a sun gear member40. A number of planet gear members42are rotatably mounted on a carrier44such that each planet gear42engages both the ring gear member38and the sun gear member40.

The third planetary gear set28also has a ring gear member46circumscribing a sun gear member48. A number of planet gear members50are rotatably mounted on a carrier52such that each planet gear50engages both the ring gear member46and the sun gear member48.

The first and second planetary gear sets24and26are compounded in that the sun gear member32of the first planetary gear set24is conjoined, as through a hub plate gear (or first interconnecting member)54, to the ring gear member38of the second planetary gear set26. The conjoined sun gear member32of the first planetary gear set24and the ring gear member38of the second planetary gear set26are continuously coupled to a first motor56. As used herein, the term “motor” can include a generator. The first motor56is described in further detail below.

The first and second planetary gear sets24and26are further compounded in that the carrier36of the first planetary gear set24is conjoined, as through a shaft60, to the carrier44of the second planetary gear set26. As such, carriers36and44of the first and second planetary gear sets24and26, respectively, are conjoined. The shaft60selectively connects to the carrier52of the third planetary gear set28through a clutch (or “second clutch” CL2)62, which assists in the selection of the operational modes of the hybrid transmission10. As used herein the term “clutch” refers to any device capable of transmitting rotation that can be engaged and disengaged, such as for example, a friction clutch, a multi-plate wet clutch, a magnetorheological (MR) fluid clutch, or a motor-generator clutch.

The carrier52of the third planetary gear set28is coupled directly to the transmission output member64. When the hybrid transmission10is used in a land vehicle, the output member64may be connected to the vehicular axles (not shown) that may, in turn, terminate in the drive members (also not shown). The drive members may be either front or rear wheels of the vehicle on which they are employed, or they may be the drive gear of a track vehicle.

The ring gear member46of the third planetary gear set28selectively couples to ground, represented by the transmission housing68, through a clutch (or “a first clutch” CL1)70. The first clutch70also assists in the selection of the operational modes of the hybrid transmission10, as will be described in further detail below. The sun gear48is continuously coupled to a second motor72. All the planetary gear sets24,26and28as well as the two motors56and72are shown coaxially oriented, as about the axially disposed shaft60. Both motors56and72are shown in this embodiment as being of an annular configuration that permits them to circumscribe the three planetary gear sets24,26and28such that the planetary gear sets24,26and28are disposed radially inwardly of the motor56and72. This configuration assures that the overall envelope, i.e., the circumferential dimension, of the transmission10is minimized.

A clutch (or “third clutch” CL3)73selectively couples the sun gear40with ground (i.e., with transmission housing68). A clutch (or “fourth clutch” CL4)75is operative as a lock-up clutch, locking planetary gear sets24,26, motor56, and the input12to rotate as a group, by selectively coupling the sun gear40with the carrier44. The sun gear40is also coupled to the sun gear48. Although one exemplary transmission arrangement is depicted inFIG. 2, the systems and methods disclosed herein can be provided for any gearing and clutch configuration.

The transmission10operates as a two-mode, compound-split, electromechanical, vehicular transmission. “Modes” of operation refer to circumstances in which the transmission functions are controlled by one clutch (e.g., clutch62or clutch70), and the controlled speed and torque of the motor56and72, one example of which is described in U.S. Pat. No. 5,009,301 which issued on Apr. 23, 1991 to General Motors Corporation. In one exemplary embodiment, a first mode is selected when the first clutch70is actuated in order to “ground” the ring gear member46of the third planetary gear set28. A second mode is selected when the first clutch70is released and the second clutch62is simultaneously actuated to connect the shaft60to the carrier52of the third planetary gear set128.

Additionally, certain “ranges” of operation may be achieved by applying an additional clutch (e.g., clutch62,73or75). When the additional clutch is applied (i.e., when two clutching mechanisms are applied), a fixed input to output speed ratio (i.e., a fixed gear ratio) is achieved. The rotations of the motors56and72will then be dependent on internal rotation of the mechanism as defined by the clutching and proportional to the input speed. In one embodiment, the first range falls within the first mode of operation when the first and fourth clutches70and75are engaged, and the second range falls within the first mode of operation when the first and second clutches62and70are engaged. A third fixed ratio range is available during the second mode of operation when the second and fourth clutches62and75are engaged, and a fourth fixed ratio range is available during the second mode of operation when the second and third clutches62and73are engaged.

The transmission10selectively receives power from the engine14. The transmission10also receives power from an electric storage device74. The electric storage device74may be one or more batteries or other types of storage devices. The electric storage device74communicates with an electrical control unit (ECU)76by transfer conductors78A and78B. The ECU76communicates with the first motor56by transfer conductors78C and78D, and the ECU76similarly communicates with the second motor72by transfer conductors78E and78F.

The ECU76obtains information from both the first and second motors56and72, respectively, the engine14and the electric storage device74. In response to an operator's action, or “operator demand” (e.g., from a drive range selector, an accelerator pedal, and/or a brake pedal), the ECU76determines what is required and then manipulates the selectively operated components of the hybrid transmission10appropriately to respond to the operator demand.

FIG. 2illustrates a cross sectional view of the motor56utilized in the transmission10described above. The motor56is described to provide a greater understanding of the exemplary motor evaluation methods described below. The motor depicted inFIG. 2is labeled as the first motor56, although it could also represent the second motor72.

The motor56can be a two pole, three phase, brushless permanent magnet machine, although the description below can be applicable to any number of poles. The motor56includes a shaft202for providing input to the motor56and receiving output from the motor56. A rotor204is coupled to the shaft202and includes a rotor core206with permanent magnets208mounted thereon. A stator210is separated from the rotor204by an air gap212and includes a stator core214with armature windings216positioned thereon.

As the rotor204rotates with respect to the windings214on the stator210, a voltage is induced in the windings216as specified by Faraday's Law, which is expressed as Equations (1) below.

eind=-ⅆλⅆt(1)
where λ is the total flux linking the stator winding and eindis the voltage induced on the coil.

Assuming the magnets208are evenly distributed around the rotor204, the flux linkage can be written as Equation (2).
λ=Kvsin(ωt)  (2)
where Kvis the voltage constant of the motor and ω is the rotational frequency of the motor.

The voltage constant Kvis a function of various parameters of the motor design including: the number of winding216turns of the stator210; the magnet field strength of the rotor204; and the permeability of the flux path in the motor56. Substituting Equation (2) into Equation (1) results in Equation (3).

Equation (3) demonstrates that the magnitude of the voltage included on a phase of a winding216of the stator210is proportional to the voltage constant Kvand the rotational speed of the motor56.

Equation (4) illustrates that the voltage constant Kvis a function of the number of turns of the windings216, the permeability of the flux path, and the strength of the magnets208, as shown below.
Kv=ƒ(N,μ,Φ)  (4)
where N is the number of turns in the stator windings, μ is the permeability of the flux path, and Φ is the field strength of the rotor magnets.

Thus, Equation (3) for the induced voltage can be written as Equation (5), as shown below.
eind=−f(N,μ,Φ)ω cos(ωt)  (5)

As shown in Equation (5), the magnitude of the induced voltage is the product of the angular velocity of the rotor204and a function of the number of turns of the windings216, the permeability of the steel of the stator210, and the strength of the magnets208. If any of these parameters do not match the design criteria, the induced voltage on the motor56for a given speed will not match the values calculated based on the design values.

Referring toFIG. 3, the motor56includes, or can be coupled to, an inverter300to facilitate power flow to and from the motor300. In one exemplary embodiment, a measurement device302measures voltages at the inverter300. As described below, under certain conditions, the voltages measured at the inverter300correspond to the voltages induced on the windings216of the stator210of the motor56. As such, the voltages measured at the inverter300can used in an evaluation of the motor56.

The inverter300is a three phase circuit coupled to the motor (depicted as the first motor56, although it could also be the second motor72). The inverter300includes three pairs of series switches302,304, and306coupled to the battery74and the motor56. The first pair of switches302is coupled to the first phase314of the motor56at a first terminal308. The second pair of switches304is coupled to the second phase316of the motor56at the second terminal310. The third pair of switches306is coupled to the third phase318of the motor56at the third terminal312.

During operation, the inverter300creates a three phase voltage on terminals308,310, and312by changing the states of the three pairs of switches302,304, and306. As an example, the voltage at the inverter300(assuming current flowing out of inverter as positive) between two phases (A and B) is illustrated by Equation (6).

Typically, the inverter300actuates the switches302,304, and306to control the current that flows in and out of each terminal308,310, and312. If, as stated above, the inverter300is commanded to control zero current in the phase terminals using, for example, closed loop current regulators, when Ia=Ib=dIa/dt=dIb/dt=0, and the inverter voltage equation reduces to Equation (7).
Vab=Ea−Eb(7)

Since the currents are assumed zero, the induced voltage from Equation (3), can be equated with the terminal voltage given in Equation (7) to result in Equation (7).
Vab=−Kvω cos(ωt)  (8)

The relationship between quantities measured in the physical reference frame and a mathematical dq frame is illustrated in Equation (9).

[fdfq]=[1-12-12032-32]⁡[fafbfc](9)
where f can replaced with appropriate physical quantity, e.g., voltage, current, or flux.

Based on the transformation above in Equation (9), current regulators that control the switches302,304, and306can be expressed in the form of Equations (10).
Vd=PIregulator(Id commanded−Id measured)
Vq=PIregulator(Iq commanded−Iq measured)  (10)

The current regulators work on the measured current, as compared to the commanded current. As noted above, the commanded currents are designated as zero by the inverter300. Therefore, all of the non-zero terms on the right side of Equation (10) are measurable and the voltage that is needed to keep the currents zero is calculated as the current regulators drive the measured current to the desired current. The magnitude of the developed voltage can be calculated as shown in Equation (11).
Vref=√{square root over (Vd2+Vq2)}  (11)

The voltage should be measured below base speed, i.e., when the inverter output voltage is maximum. In other words, the base speed is the point at which the induced voltage on the motor56or72matches the supply voltage available to the inverter300. Comparing Vref of Equation (11) to Equation (8), and knowing the motor speed from measurement, the voltage constant Kv of the either of the motors56and72can be calculated and compared against a set of accepted values to thus evaluate the motor56or72.

In accordance with an exemplary embodiment, the motors56and72can be evaluated during operation. Knowledge of the voltage constants of the first and second motors56and72during vehicle operation is desirable for several reasons. First, the fidelity of the control of the first and second motors56and72can be improved by comprehending changes in the voltage constant over the life of the first and second motors56and72. For example, magnet field strength changes can impact the operation of the first and second motors56and72. The control fidelity is improved by updating the switching commands to the first and second motors56and72based on the knowledge of the changes in the first and second motors56and72. Additionally, knowledge of the voltage constant of the first and second motors56and72can be used to trigger diagnostics and to provide warning to the automobile owners if one of the first and second motors56and72is experience performance issues.

When the vehicle is in operation, the first and second motors56and72will rotate with various speed relationships in dependence on the configuration of the clutches70,62,73, and75in the transmission10. The speed relationships and configurations can be utilized to determine the voltage constant of the first and second motors56and72during operation.

For example, in the first fixed gear, both the first and second motors56and72and the input speed at the input member12are all maintained at the same rotational speed by the gearing and clutch configuration. Additionally, the engine14is directly coupled to the transmission output64through the gear ratio of the transmission10. This allows the transmission10to provide propulsion torque to the vehicle, and to spin the first and second motors56and72solely with the torque from the engine14. Thus, the first and second motors56and72will spin at the same speed as the engine14, but will not be required to generate torque. When these conditions exist, the ECU76can enter an operational test mode and utilize the inverter voltage of the first and second motors56and72to determine the voltage constants.

Accordingly, referring additionally toFIG. 4, the first step402of a method400for evaluating the motors56and72during operation is to spin the motors56and72. In a step404, the method evaluates whether or not the motors56and72are in a fixed gear operating point. If not, transmission10is placed into a fixed gear operating point, as indicated by step410. The first and second motors56and72can be tested any time the transmission10is in a fixed gear state since the motor torques are not required to satisfy the vehicle torque requests. In fixed gears, the operating point selection algorithms naturally tend to select points that require low motor torques based on the efficiency of these operating points. At low torque values, there is very little power loss. If the gear ratio is such that the engine14is also using fuel at these points, the hybrid optimization algorithms selected operating points where the motor torques are low or zero to eliminate any electrical loss. Therefore, during the course of normal driving, many opportunities are present to conduct the test of the first and second motors56and72. However, particularly advantageous circumstances can be utilized for the operational test of the first and second motors56and72.

The operating point selection logic in the ECU76can be modified to favor fixed gear operating points that require no motor torque, particularly if a significant amount of time has passed since the system was in this type of operating point. This allows the ECU76regular opportunities to conduct the operational test.

In one exemplary embodiment, the ECU76can place the transmission in an operating condition in which the operational test could be conducted. In this embodiment, the ECU76can apply the more detailed information it has relative to the last time a successful test was completed and the operation of the first and second motors56and72to determine when a test is needed. This enables the ECU76to periodically test the motors56and72at more favorable points. Generally, these points would correspond to fixed gear, low motor speed operating points.

When these conditions are present, in a fourth step420, the voltages at the inverters of the first and second motors56and72can be measured by the measurement device302(FIG. 3). The measurement device302can be, for example, a sensor or other suitable device for measuring the voltage at the inverter. After measurement, in a fifth step430and a sixth step440, the voltage constants of the first and second motors56and72can be calculated and compared to the design specification value to evaluate the motors56and72. If the voltage constants derived from the voltage measurements of the first and second motors56,72are within an acceptable threshold of the predetermined values, then the motors56,72are deemed to be properly manufactured, installed, and/or maintained. Conversely, if the voltage constants are outside of the threshold, it may indicate an issue with the motors56,72. In this scenario, a service message indicating the results may be provided, as indicated in step450.