Abstract:
A method and apparatus are provided for detecting the degradation of a permanent magnet in a motor of an electric or hybrid electric vehicle. A voltage monitor ( 102 ) is directly coupled to a traction motor ( 38 ) and/or generator motor ( 30 ) to detect a permanent magnet induced voltage within the motor at a predetermined speed and no load condition ( 300 ). A controller ( 100 ) compares the detected permanent magnet induced voltage with an expected reference voltage that represents an expected permanent magnet induced voltage at full magnetization and the predetermined speed ( 302 ). The controller produces an indication of magnetization based on the reference voltage, the detected permanent magnet induced voltage, and the predetermined speed. The indication of magnetization is stored for future reference ( 306 ). Also, a safety indicator is produced for a user of the vehicle if the indication of magnetization is below a safety threshold.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to a hybrid electric vehicle (HEV) or an electric vehicle, and specifically to permanent magnet degradation in motors/generators in hybrid electric and electric vehicles.  
           [0003]    2. Discussion of the Prior Art  
           [0004]    The need to reduce fossil fuel consumption and emissions in automobiles and other vehicles predominately powered by internal combustion engines (ICEs) is well known. Vehicles powered by electric motors attempt to address these needs. Another alternative known solution is to combine a smaller ICE with electric motors into one vehicle. Such vehicles combine the advantages of an ICE vehicle and an electric vehicle and are typically called hybrid electric vehicles (HEVs). See generally, U.S. Pat. No. 5,343,970 to Severinsky.  
           [0005]    The HEV is described in a variety of configurations. In one configuration, the electric motor drives one set of wheels and the ICE drives a different set. Other, more useful, configurations exist. For example, a series hybrid electric vehicle (SHEV) configuration is a vehicle with an engine (most typically an ICE) connected to an electric motor called a generator. The generator, in turn, provides electricity to a battery and another motor, called a traction motor. In the SHEV, the traction motor is the sole source of wheel torque. There is no mechanical connection between the engine and the drive wheels. A parallel hybrid electrical vehicle (PHEV) configuration has an engine (most typically an ICE) and an electric motor that work together in varying degrees to provide the necessary wheel torque to drive the vehicle. Additionally, in the PHEV configuration, the motor can be used as a generator to charge the battery from the power produced by the ICE.  
           [0006]    A parallel/series hybrid electric vehicle (PSHEV) has characteristics of both PHEV and SHEV configurations and is sometimes referred to as a “powersplit” configuration. In one of several types of PSHEV configurations, the ICE is mechanically coupled to two electric motors in a planetary gear-set transaxle. A first electric motor, the generator, is connected to a sun gear. The ICE is connected to a carrier. A second electric motor, a traction motor, is connected to a ring (output) gear via additional gearing in a transaxle. Engine torque can power the generator to charge the battery. The generator can also contribute to the necessary wheel (output shaft) torque if the system has a one-way clutch. The traction motor is used to contribute wheel torque and to recover braking energy to charge the battery. In this configuration, the generator can selectively provide a reaction torque that may be used to control engine speed. In fact, the engine, generator motor and traction motor can provide a continuous variable transmission (CVT) effect. Further, the HEV presents an opportunity to better control engine idle speed over conventional vehicles by using the generator to control engine speed.  
           [0007]    The generator motor and the traction motor include permanent magnets. These permanent magnets may demagnetize by accident and may degrade or demagnetize over time due to temperature, high current ripples, power ripples, vibration and aging. The demagnetization may degrade vehicle performance such as output power/torque and efficiency. Indeed, the demagnetization may reach a point where safety becomes an issue. More specifically, demagnetization may result in less torque being available to drive the wheels at a critical point, for example, to pass a vehicle. And, demagnetization may result in less energy being available for regenerative braking, which may adversely affect stopping distance/time.  
           [0008]    U.S. Pat. No. 5,650,706 issued to Yamada et al. (“Yamada”) is directed to a control device for a salient pole type permanent magnet motor. The object of that device is to prevent torque from lowering due to demagnetization of the magnet. A magnetic flux of the permanent magnet is calculated or inferred by determining an electromotive force of the permanent magnet in accordance with a voltage and current supplied to the permanent magnet motor, a rotational speed of the motor, and an inductance of the permanent magnet motor. This electromotive force is compared to a reference electromotive force representative of a fully magnetized permanent magnet. This process is complex and cumbersome. Direct detection of demagnetization is suggested in Yamada by using certain sensors, such as a Hall device or a magnetoresistance element. These direct detection methods suggested in Yamada are relatively expensive and impact serviceabilty due to location of a complex sensor in the motor housing. Also, demagnetization beyond a safety limit is not monitored and reported for safety-related actions.  
           [0009]    Therefore, a need exists for an improved method for monitoring permanent magnet degradation and determining whether a permanent magnet has degraded beyond a safe limit.  
         SUMMARY OF THE INVENTION  
         [0010]    Accordingly, an object of the present invention is to provide a monitor for permanent magnet degradation for an electric or a hybrid electric vehicle (HEV).  
           [0011]    Another object of the present invention is to provide a safe and direct method for determining the magnetic flux of a permanent magnet in a motor.  
           [0012]    Yet another object of the present invention is to determine a state of magnetism of a permanent magnet to adjust a torque of a generator motor to control the speed of an internal combustion engine.  
           [0013]    Other objects of the present invention will become more apparent to persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying figures.  
           [0014]    In accordance with one aspect of the present invention, a device is provided for monitoring the state of magnetization of the permanent magnet in a motor. The device includes a voltage monitor that detects a permanent magnet induced voltage within the motor at a predetermined speed and no load condition. The voltage monitor is coupled to a processor that receives the permanent magnet induced voltage and compares the permanent magnet induced voltage to a reference voltage that reflects the permanent magnet induced voltage for the motor with a fully magnetized permanent magnet. The processor determines an indication of magnetism of the permanent magnet as a function of the detected permanent magnet induced voltage, the reference voltage, and the predetermined speed. The indication of magnetism is stored for subsequent use as a safety indicator, to calibrate motor torque, and to indicate that the motor can not provide the demanded torque or regenerative braking. And, in the case of a generator motor, the indication of magnetism indicates whether the generator motor is capable of demanded control of the engine speed. The motor is preferably a traction motor or generator motor for an electric or hybrid electric vehicle.  
           [0015]    In accordance with another aspect of the present invention, a method is provided for determining magnet degradation in a permanent magnet of a motor. First a permanent magnet (PM) induced voltage of a motor is detected. Preferably, the permanent magnet induced voltage is detected by inducing a voltage in coils wrapped around the stator teeth of a motor. The voltage is induced at a predetermined speed by the rotation of a rotor that includes the permanent magnets. The detected permanent magnet induced voltage is compared to a reference voltage that reflects full magnetism of the permanent magnets at the predetermined speed. An indication of magnetism of the permanent magnets is produced as a function of the detected permanent magnet induced voltage, the reference voltage and the predetermined speed. The indication of magnetism is stored for subsequent use as a safety indicator, to calibrate the motor torque, and to indicate that the motor can not provide the demanded torque or regenerative braking.  
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0016]    The foregoing objects, advantages, and features, as well as other objects and advantages, will become apparent with reference to the description and figures below, in which like numerals represent like elements and in which:  
         [0017]    [0017]FIG. 1 is a block diagram illustrating a hybrid electric vehicle (HEV) configuration in accordance with a preferred embodiment of the present invention.  
         [0018]    [0018]FIG. 2 is a block diagram of a transaxle management unit in accordance with a preferred embodiment of the present invention.  
         [0019]    [0019]FIG. 3 is a cross sectional view of a motor in accordance with a preferred embodiment of the present invention.  
         [0020]    [0020]FIG. 4 is a flow diagram illustrating a method of detecting permanent magnetism degradation in accordance with a preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    The present invention relates to electric vehicles and, more particularly, hybrid electric vehicles (HEVs). FIG. 1 illustrates a parallel/series hybrid electric vehicle (powersplit) configuration in accordance with the present invention.  
         [0022]    In the HEV of FIG. 1, a planetary gear set  20  mechanically couples a carrier gear  22  to an engine  24  via a one way clutch  26 . The planetary gear set  20  also mechanically couples a sun gear  28  to a generator motor  30  and a ring (output) gear  32 . The generator motor  30  also mechanically links to a generator brake  34  and is electrically linked to a battery  36 . A traction motor  38  is mechanically coupled to the ring gear  32  of the planetary gear set  20  via a second gear set  40  and is electrically linked to the battery  36 . The ring gear  32  of the planetary gear set  20  and the traction motor  38  are mechanically coupled to drive wheels  42  via an output shaft  44 .  
         [0023]    The planetary gear set  20  splits the engine output energy into a series path from the engine  24  to the generator motor  30  and a parallel path from the engine  24  to the drive wheels  42 . Engine speed can be controlled by varying the split to the series path while maintaining the mechanical connection through the parallel path. The traction motor  38  augments the engine power to the drive wheels  42  on the parallel path through the second gear set  40 . The traction motor  38  also provides the opportunity to use energy directly from the series path, essentially running off power created by the generator motor  30 . This reduces losses associated with converting energy into and out of chemical energy in the battery  36  and allows all engine energy, minus conversion losses, to reach the drive wheels  42 .  
         [0024]    A vehicle system controller (VSC)  46  controls many components in this HEV configuration by connecting to each component&#39;s controller. An engine control unit (ECU)  48  connects to the engine  24  via a hardwire interface. The ECU  48  and VSC  46  can be housed in the same unit, but are preferably separate controllers. The VSC  46  communicates with the ECU  48 , as well as a battery control unit (BCU)  50  and a transaxle management unit (TMU)  52  through a communication network, such as a controller area network (CAN)  54 . The BCU  50  connects to the battery  36  via a hardwire interface. The TMU  52  controls the generator motor  30  and traction motor  38  via a hardwire interface. More specifically, TMU  52  includes a controller that executes a stored program to determine the torque of generator motor  30  and traction motor  38 . Also, in accordance with the present invention, TMU  52  detects and stores an indication of the magnetization of permanent magnets in generator motor  30  and traction motor  38 . In particular, a voltage sensor incorporated in generator motor  30  and a voltage sensor in traction motor  38  determine a voltage that is proportional to the magnetization of permanent magnets in generator motor  30  and traction motor  38 , as described below.  
         [0025]    [0025]FIG. 2 is a block diagram of a portion of transaxle management unit  52  shown interfaced to generator motor  30  and traction motor  38  in accordance with a preferred embodiment of the present invention. TMU  52  preferably includes a controller  100 , a voltage monitor  102 , a voltage monitor  104 , an inverter  106  and an inverter  108 . Inverter  106  is coupled to traction motor  38  to provide a three-phase AC current to traction motor  38 . The three-phase AC current is derived from a DC current from battery  36 . Similarly, inverter  108  is coupled to generator motor  30  to provide a three-phase AC current to generator motor  30 . The three-phase AC current is also derived from a DC current from battery  36 . Inverter  106  an inverter  108  are coupled to controller  100  such that controller  100  provides input signals to inverters  106 , 108  to determine a current provided to generator motor  30  and traction motor  38 , respectively. In accordance with the present invention, a voltage monitor  102  is coupled to traction motor  38  to determine a permanent magnet induced voltage of traction motor  38 . Similarly, a voltage monitor  104  is coupled to generator motor  30  to determine permanent magnet induced voltage in generator motor  30 . The permanent magnet induced voltages from traction motor  38  and generator motor  30  are used by controller  100  to determine a state of the permanent magnets contained within traction motor  38  and generator motor  30 .  
         [0026]    Controller  100  preferably includes a processor  110  and a memory  112 . Processor  110  comprises one or more microprocessors, micro-controllers, or the like. Controller  100  preferably executes a stored program to determine, store and transmit an indication of the state of magnetism of the permanent magnets contained within generator motor  30  and traction motor  38 . Most preferably, memory  112  includes a non-volatile memory component that stores an indication of the state of magnetism of the permanent magnets in generator motor  30  and traction motor  30 .  
         [0027]    Voltage monitors  102 , 104  preferably include a voltage sensor  114  and a voltmeter  116 . Voltage sensor  114  is directly coupled to its respective motor to determine a permanent magnet induced voltage at a predetermined speed of the motor  38 . The voltmeter  116  provides the voltage from voltage sensor  114  to controller  100  for use in determining the state of magnetism of the permanent magnets in generator motor  30  and traction motor  38 . Preferably, the voltmeter is housed external to the motor. Most preferably, the voltmeter is hardware available on the vehicle that is reused for the magnetization monitoring function, which function is only required periodically.  
         [0028]    [0028]FIG. 3 is a sectional view of generator motor  30  including a preferred voltage sensor in accordance with the present invention. A similar arrangement is preferred for traction motor  38 . Generator motor  30  includes a rotor  200  and a stator  202 . Permanent magnets  208  are mounted within rotor  200 . The motor windings  204  (as exemplary shown between two stator teeth) are wrapped around the teeth  205  in slots  206  in stator  202  in the traditional manner. In accordance with the invention, a sensor coil  210  is wrapped around the teeth  205  in slots  206  in stator  202 . As shown in FIG. 3, sensor coil  210  is preferably located adjacent rotor  200  at an edge of the teeth  205  closest to a gap between stator  202  and rotor  200 . Preferably, sensor coil  210  comprises a wire having a very high gauge and a few turns. The sensor coil  210  is coupled to voltmeter  116  and serves as a voltage sensor  114 . Sensor coil  210  is used to determine a permanent magnet induced voltage in generator  30 . More specifically, when no current is running through motor windings  204 , a voltage is induced in sensor coil  210  due to the rotation of rotor  200  and a magnetic field generated by permanent magnets  208 . This voltage is sensed by voltmeter  116  and is transmitted to controller  100 .  
         [0029]    [0029]FIG. 4 is a flow diagram illustrating a method for determining and indicating permanent magnet degradation in a motor in accordance with the present invention. The method is described below with reference to the preferred embodiments described above in FIGS.  1 - 3 .  
         [0030]    First, the permanent magnet induced voltage of the motor is determined ( 300 ). In the preferred embodiment, this is accomplished by inducing a voltage in sensor coil  210  during a period of time when no current is flowing in the motor windings, i.e., there is no load. Preferably, voltmeter  116  quantifies the voltage induced in the sensor coil  210 . The no load condition occurs when there is zero current in the stator windings of the motor. For example, the no load condition occurs when the vehicle is at idle, for example, stopped at a stop light, and also, when the vehicle is at cruising speed and there is no current in the motor windings. Another exemplary no load condition occurs when the generator motor is not supplying any torque to the wheels or receiving torque from the engine to charge the batteries. The PM induced voltage is preferably induced by the rotation of rotor  200 , including permanent magnets  208 . This causes a magnetic field that induces the voltage in the sensor coil. Most preferably, rotor  200  is rotated at a predetermined speed and the inverter contacts that supply current to the motor are opened during permanent magnet induced voltage measurement. The TMU  52 , and more specifically, controller  100  determines when to measure the permanent magnet induced voltage in light of the state of the vehicle, which state is preferably obtained via controller area network  54  or any other suitable means.  
         [0031]    The permanent magnet induced voltage is proportional to the magnetic field (flux) and the speed of rotation of the rotor. Hence, the strength of the permanent magnet is readily obtained where the speed and permanent magnet induced voltage are known.  
         [0032]    After the permanent magnet induced voltage is detected, the permanent magnet induced voltage is compared to a reference voltage that reflects a permanent magnet induced voltage at no demagnetization and the same predetermined speed at which the permanent magnet induced voltage is detected ( 302 ). That is, the reference voltage is the value expected for the permanent magnet induced voltage if the permanent magnet is fully magnetized. Preferably, the reference voltage is stored in TMU  52 . Any difference between the reference voltage and the detected permanent magnet induced voltage is used to determine an indication of the amount of degradation of the permanent magnet. This indication is preferably stored in a non-volatile memory for further reference ( 304 ). Also, the indication of magnetic strength is compared to a safety threshold to determine if the permanent magnet has reached a point of degradation where safety precautions should be taken ( 306 ). Most preferably, if the magnetic strength is below a predetermined safety threshold, an indication is made to a user of the vehicle, for example, through an audible or visual indication that is transmitted via controller area network  54  ( 308 ).  
         [0033]    The stored indication of actual magnetic strength ( 304 ) is used in any suitable manner, and in addition to being used for a safety threshold check as described above, may also be used in calibrating the TMU  52  to more accurately drive the inverter to force the motor to provide the torque required. And, the indication of magnetization of the permanent magnet is used to define the maximum regenerative braking and available wheel torque for stopping distance limits and commanded torque, respectively.  
         [0034]    As discussed above, the present invention provides a simple and effective method of determining the state of magnetism of a permanent magnet in a motor of a vehicle. Advantageously, the state of magnetism is compared with a safety threshold and an indication of safety problems is made available to a user of the vehicle. Also, the state of magnetism is used to calibrate a torque from the motor and to indicate that the motor can not provide the demanded torque or regenerative braking.  
         [0035]    The above-described embodiments of the invention are provided purely for purposes of example. Many other variations, modifications, and applications of the invention may be made.