Patent Publication Number: US-9423463-B2

Title: Method and control unit for an electric motor or generator

Description:
The present invention relates to a control unit for an electric motor or generator and in particular a control unit for identifying a fault in an electric motor or generator. 
     With increased interested being placed in environmentally friendly vehicles there has been a corresponding increase in interest in the use of electric vehicles. 
     Electric vehicles typically use an electric motor to provide both drive for the vehicle and regenerative braking for stopping the vehicle. To effect regenerative braking, rotary motion of a drive wheel connected to an electric motor is converted into electric energy. 
     However, when using an electric motor in an automotive application it is desirable that the electric motor does not cause unintended acceleration or braking to occur. 
     Accordingly, it is desirable for an electric motor to have sensors for measuring/monitoring electric motor parameters, where a fault will typically be identified if an electric motor parameter exceeds a threshold value. 
     However, by determining whether a fault has occurred based directly on measured parameter values can result in one off spurious measurements being incorrectly identified as a fault. Further the types of faults identifiable in an electric motor will be limited by the number of parameters being measured. 
     It is desirable to improve this situation. 
     In accordance with an aspect of the present invention there is provided a method and a control unit for an electric motor or generator according to the accompanying claims. 
     The invention as claimed provides the advantage of reducing the risk of falsely identifying transient or noise conditions or spurious one off measurements within an electric motor as fault conditions. 
    
    
     
       The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates an exploded view of an electric motor as used in an embodiment of the present invention; 
         FIG. 2  illustrates an exploded view of the electric motor shown in  FIG. 1  from an alternative angle; 
         FIG. 3  illustrates an example arrangement of coil sets for an electric motor according to an embodiment of the present invention; 
         FIG. 4  illustrates a three phase stator current complex space vector; 
         FIG. 5  illustrates a closed loop control system for an electric motor according to an embodiment of the present invention; 
         FIG. 6  illustrates a three phase stator current reference frame with a d, q rotating reference frame. 
     
    
    
     The embodiment of the invention described is a control unit for an electric motor for use in a wheel of a vehicle. However, the control unit can be located remotely to the electric motor. The motor is of the type having a set of coils being part of the stator for attachment to a vehicle, radially surrounded by a rotor carrying a set of magnets for attachment to a wheel. For the avoidance of doubt, the various aspects of the invention are equally applicable to an electric generator having the same arrangement. As such, the definition of electric motor is intended to include electric generator. In addition, some of the aspects of the invention are applicable to an arrangement having the rotor centrally mounted within radially surrounding coils. As would be appreciated by a person skilled in the art, the present invention is applicable for use with other types of electric motors. 
     For the purposes of the present embodiment, as illustrated in  FIG. 1 , the in-wheel electric motor  40  includes a stator  252  comprising a rear portion  230  forming a first part of the housing of the assembly, and a heat sink and drive arrangement  231  comprising multiple coils and electronics to drive the coils. The coil drive arrangement  231  is fixed to the rear portion  230  to form the stator  252  which may then be fixed to a vehicle and does not rotate relative to the vehicle during use. The coils themselves are formed on tooth laminations to form coil windings, which together with the drive arrangement  231  and rear portion  230  form the stator  252 . 
     A rotor  240  comprises a front portion  220  and a cylindrical portion  221  forming a cover, which substantially surrounds the stator  252 . The rotor includes a plurality of permanent magnets  242  arranged around the inside of the cylindrical portion  221 . For the purposes of the present embodiment 32 magnet pairs are mounted on the inside of the cylindrical portion  221 . However, any number of magnet pairs may be used. 
     The magnets are in close proximity to the coils on the assembly  231  so that magnetic fields generated by the coils in the assembly  231  interact with the magnets  242  arranged around the inside of the cylindrical portion  221  of the rotor  240  to cause the rotor  240  to rotate. As the permanent magnets  242  are utilized to generate a drive torque for driving the electric motor, the permanent magnets are typically called drive magnets. 
     The rotor  240  is attached to the stator  252  by a bearing block  223 . The bearing block  223  can be a standard bearing block as would be used in a vehicle to which this motor assembly is to be fitted. The bearing block comprises two parts, a first part fixed to the stator and a second part fixed to the rotor. The bearing block is fixed to a central portion  233  of the wall  230  of the stator  252  and also to a central portion  225  of the housing wall  220  of the rotor  240 . The rotor  240  is thus rotationally fixed to the vehicle with which it is to be used via the bearing block  223  at the central portion  225  of the rotor  240 . This has an advantage in that a wheel rim and tyre can then be fixed to the rotor  240  at the central portion  225  using the normal wheel bolts to fix the wheel rim to the central portion of the rotor and consequently firmly onto the rotatable side of the bearing block  223 . The wheel bolts may be fitted through the central portion  225  of the rotor through into the bearing block itself. With both the rotor  240  and the wheel being mounted to the bearing block  223  there is a one to one correspondence between the angle of rotation of the rotor and the wheel. 
       FIG. 2  shows an exploded view of the same assembly as  FIG. 1  from the opposite side showing the stator  252  comprising the rear stator wall  230  and coil and electronics assembly  231 . The rotor  240  comprises the outer rotor wall  220  and circumferential wall  221  within which magnets  242  are circumferentially arranged. As previously described, the stator  252  is connected to the rotor  240  via the bearing block at the central portions of the rotor and stator walls. 
     Additionally shown in  FIG. 1  are control devices  80  carrying control electronics, otherwise known as motor drive controllers or inverters. 
     A V shaped seal  350  is provided between the circumferential wall  221  of the rotor and the outer edge of the stator housing  230 . 
     The rotor also includes a set of magnets  227  for position sensing, otherwise known as commutation magnets, which in conjunction with sensors mounted on the stator allows for a rotor flux angle to be estimated. The rotor flux angle defines the positional relationship of the drive magnets to the coil windings. Alternatively, in place of a set of separate magnets the rotor may include a ring of magnetic material that has multiple poles that act as a set of separate magnets. 
     To allow the commutation magnets to be used to calculate a rotor flux angle, preferably each drive magnet has an associated commutation magnet, where the rotor flux angle is derived from the flux angle associated with the set of commutation magnets by calibrating the measured commutation magnet flux angle. To simplify the correlation between the commutation magnet flux angle and the rotor flux angle, preferably the set of commutation magnets has the same number of magnet or magnet pole pairs as the set of drive magnet pairs, where the commutation magnets and associated drive magnets are approximately radially aligned with each other. Accordingly, for the purposes of the present embodiment the set of commutation magnets has 32 magnet pairs, where each magnet pair is approximately radially aligned with a respective drive magnet pair. 
     A sensor, which in this embodiment is a Hall sensor, is mounted on the stator. The sensor is positioned so that as the rotor rotates each of the commutation magnets that form the commutation magnet ring respectively rotates past the sensor. 
     As the rotor rotates relative to the stator the commutation magnets correspondingly rotate past the sensor with the Hall sensor outputting an AC voltage signal, where the sensor outputs a complete voltage cycle of 360 electrical degrees for each magnet pair that passes the respective sensors. 
     To aid in the determination of the direction of the rotor, the sensor may also have an associated second sensor placed 90 electrical degrees apart. 
     As illustrated in  FIG. 3 , the motor  40  in this embodiment includes 8 coil sets  60  with each coil set  60  having three coil sub-sets  61 ,  62 ,  63  that are coupled to a respective control device  80 , where each control device  80  and respective coil sub-sets form a three phase logical or sub electric motor that can be controlled independently of the other sub motors. The control devices  80  drive their respective sub motor with a three phase voltage supply, thereby allowing the respective coil sub-sets to generate a rotating magnetic field. Although the present embodiment describes each coil set  60  as having three coil sub-sets  61 ,  62 ,  63 , the present invention is not limited by this and it would be appreciated that each coil set  60  could have two or more coil sub-sets. Equally, although the present embodiment describes an electric motor having eight coil sets  60  (i.e. eight sub motors) the motor could have one or more coil sets with an associated control device. 
     Each control device includes a three phase bridge inverter which, as is well known to a person skilled in the art, contains six switches. The three phase bridge inverter is coupled to the three subset coils of a coil set  60  to form a three phase electric motor configuration. Accordingly, as stated above, the motor includes eight three phase sub-motors, where each three phase sub-motor includes a control device  80  coupled to the three sub-set coils of a coil set  60 . 
     Each three phase bridge inverter is arranged to provide PWM voltage control across the respective coil sub-sets  61 ,  62 ,  63  to provide a required torque for the respective sub-motors. 
     For a given coil set the three phase bridge switches of a control device  64  are arranged to apply a single voltage phase across each of the coil sub-sets  61 ,  62 ,  63 . 
     Although the in-wheel electric motor described in the present embodiment includes a plurality of logical sub-motors, as person skilled in the art would appreciate the electric motor may be of a conventional design without the use of logical sub-motors. 
     In this embodiment, each control device  80  is substantially wedge-shaped. This shape allows multiple control devices  80  to be located adjacent each other within the motor, forming a fan-like arrangement. 
     The control device  80  switches can include semiconductor devices such as MOSFETs or IGBTs. In the present example, the switches comprise IGBTs. However, any suitable known switching circuit can be employed for controlling the current. One well known example of such a switching circuit is the three phase bridge circuit having six switches configured to drive a three phase electric motor. The six switches are configured as three parallel sets of two switches, where each pair of switches is placed in series and from a leg of the three phase bridge circuit. 
     The plurality of switches are arranged to apply an alternating voltage across the respective coil sub-sets. 
     As described above, the plurality of switches are configured to form an n-phase bridge circuit. Accordingly, as is well known to a person skilled in the art, the number of switches will depend upon the number of voltage phases to be applied to the respective sub motors. Although the current design shows each sub motor having a three phase construction, the sub motors can be constructed to have two or more phases. 
     The wires (e.g. copper wires) of the coil sub-sets can be connected directly to the switching devices as appropriate. 
     The control device  80  includes a number of electrical components for controlling the operation of the switches mounted on the control device  80 . Examples of electrical components mounted on the control device  80  include control logic for controlling the operation of the switches for providing PWM voltage control and interface components, such as a CAN interface chip, for allowing the control device  80  to communicate with devices external to the control device  80 , such as other control devices  80  or a motor controller. Typically the control device  80  will communicate over the interface to receive torque demand requests and to transmit status information. Typically, the sensor for determining the rotor flux angle will be mounted on a control device  80 , which in turn will be mounted to the stator. Optionally, for additional redundancy a sensor may be mounted to each control device  80 . 
     The sinusoidal voltage waveforms generated in the electric motor by the control devices  80  under the control of a motor controller (not shown) are created using Field Orientation Control, where the resultant rotor flux and stator currents are represented by respective vectors which are separated by 120 degrees as illustrated in  FIG. 4  by the three axes a, b, c. 
     As illustrated in  FIG. 4 , currents ia, ib, ic represent the instantaneous current in the respective stator coils in the A, B, and C axis of a three phase current reference frame, where the stator current vector is defined by,
         where α=e (i*2*Π/3) .       

     Field Oriented Control is based on projections that transform a two co-ordinate time invariant system, where a stator current or voltage component is aligned with a quadrature axis q and a magnetic flux component is aligned with a direct axis d, into a three phase time and speed dependent system and vice versa. 
     Using a closed loop control system, an example of one being illustrated in  FIG. 5 , a required torque τ, which is represented by an input i q * value, and a required magnetic flux λ, which is represented by an input i d * value, are generated in the electric motor. 
     It should be noted, however, that under normal circumstances a permanent magnet synchronous electric motor will typically have i d  set to zero. 
     The closed loop control system  100  illustrated in  FIG. 5  includes a Proportional Integral PI controller located between the respective motor input id* and iq* current demands, which correspond to driver torque demand, and the coil windings of the electric motor. 
     The closed loop control system  100  includes current sensors for measuring current flow in the coil windings of the electric motor. To determine the corresponding id and iq current values a Clarke Transform and a Park Transform are used. 
     A Clarke Transform uses three phase currents ia, ib, and ic that flow in the respective stator coils, which drive the electric motor, to calculate currents in a two phase orthogonal stator axis system iα and iβ. A Park transformation is then performed by a Park Transform to transform the two fixed co-ordinate stator axes iα and iβ to the two co-ordinate time invariant system id and iq, which defines a d, q rotating reference frame.  FIG. 6  illustrates the relationship of the stator current in the d,q rotating reference frame with respect to the two phase orthogonal stator axes iα and iβ and the a, b and c stationary reference frame. 
     Under normal drive conditions the rotor phase angle θ r , otherwise known as rotor flux angle of the drive magnets, which is defined by the rotor magnetic flux vector ψ R , and the stator electrical phase angle θ e  should ideally be aligned with the q-axis, thereby maintaining synchronization between the rotor phase angle θ r  and the stator electrical phase angle θ e . 
     To allow the Park Transform  102  to derive a time invariant transformation the rotor phase angle θ r  is provided to the Park Transform, where the rotor phase angle θ r  is determined using the rotor commutation magnets and position sensor mounted on the control device  80 . 
     The measured id and iq values are fedback to the respective input id* and iq* current values, where an error value based on the difference between the measured id and iq currents and the driver torque demand (i.e. the id* and iq* current demand) is calculated. 
     The respective error values for id and iq are fed to the respective PI controller. 
     Based on the error values provided to the respective PI controller, the PI controllers determine the respective vd and vq values (i.e. the PI controller&#39;s manipulated variables) for driving the electric motor. The PWM voltage used to generate the ia, ib, and ic currents that flow in respective coil windings are derived from the vd and vq values via the use of an inverse Park Transform, as is well known to a person skilled in the art. 
     Although the present embodiment utilizes a PI controller feedback loop, any control loop feedback mechanism may be utilized. 
     A process for identifying fault conditions associated with an electric motor according to a first embodiment will now be described. 
     Using a model of an electric motor, which according to the present embodiment is a permanent magnet synchronous electric motor, an estimate of the motor id and iq current is calculated using the input vd and vq voltages generated by the respective PI controllers. 
     For example, a permanent magnet motor can be modeled using voltage equations: 
                       v   d     =         R   s     ⁢     i   d       -       ω   e     ⁢     L   sq     ⁢     i   q       +       L   sd     ⁢       ⅆ     i   d         ⅆ   t             ⁢     
     ⁢       v   q     =         R   s     ⁢     i   q       +       ω   e     ⁢     L   sd     ⁢     i   d       +       ω   e     ⁢     ψ   m       +       L   sq     ⁢       ⅆ     i   q         ⅆ   t                     (     Equ   .           ⁢   1     )               
where R s  is the resistance of the respective stator coil windings, L sd  and L sq  are the components of inductance of the stator coil windings, ω e  is the electrical frequency and ψ m  is the flux resulting from the permanent magnets.
 
     Any suitable means for determining the values of R s , L sd  , L sq , ω e  and ψ m  may be used, for example by measurement. 
     To provide expressions for the rate of change of current the voltage equations are rearranged as illustrated in Equ. 2. 
     
       
         
           
             
               
                 
                   
                     
                       
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     To obtain an estimates for i d  and i q , Equ. 2 is integrated. 
     The estimated id and iq values may be used to provide an estimate for the torque generated by the motor using a torque equation, for example: 
                     T   e     =       3   2     ⁢       N   P     ⁡     (         ψ   m     ⁢     i   q       +       (       L   sd     -     L   sq       )     ⁢     i   d     ⁢     i   q         )                 (     Equ   .           ⁢   3     )               
where T e  is the electrical torque and N P  is the number of motor pole pairs. As can be seen from Equ. 3, the torque generated by the electric motor is proportional to iq when id equals zero or L sd  equals L sq .
 
     The estimated id and iq current values derived from equations 2 are compared with the measured id and iq current values. 
     If the difference between the estimated id and/or iq values and the measured id and/or iq values are greater than a first predetermined value a counter value is increased. Any suitable means may be used to identify the first predetermined value, for example by experimentation or based on specific faults that are induced in the electric motor. However, if the first predetermined value is set too low there will be a greater likelihood that transient conditions will be falsely identified as a fault. In contrast, if the first predetermined value is set too high failure conditions in the electric motor may not be identified. 
     If the difference between the estimated id and/or iq values and the measured id and/or iq values are less than the first predetermined value the counter value is decreased. 
     Although the present embodiment compares measured id and/or iq values with estimated id and iq values, a comparison may be made with other motor parameters, for example estimated and measured values of torque or power. 
     If the counter value exceeds a second predetermined value this indicates that there is a fault with the electric motor and a fault condition is triggered. 
     The frequency at which the comparison of the estimated and measured values is made will typically be based upon the amount of time before which a fault condition becomes critical. For example, if a fault condition needs to be identified within 500 ms and the comparison is performed every 100 ms, a count value of 5 would indicate that a fault has occurred. If the comparison is performed very 50 ms, a count value of 10 would indicate that a fault has occurred. 
     Any suitable fault notification may be utilised, for example a fault indicator may be set to notify the driver of the vehicle that the vehicle should be taken to a service/diagnostic centre and/or the performance of the electric motor may be limited, for example the motor torque may be reduced or a reduced maximum torque set. 
     Using a fault counter that is incremented and decremented based on the difference between estimated and measured current values has the advantage of minimising the risk of transient or noisy conditions being identified as a fault condition, where an out of tolerance condition is only identified as a fault if the condition continues for a predetermined time period or is identified a predetermined number of times. 
     In an alternative embodiment, for an electric motor having a plurality of sub-motors, as described above, the input vq and vd voltages for the respective sub-motors may be compared. 
     If the difference between the input vq and vd voltages between two sub-motors is greater than a first predetermined value a counter value is increased. As with the first embodiment, the first predetermined value can be determined by any suitable means. Preferably, to aid the location of a fault condition to a specific sub-motor the input vq and vd voltages for three sub-motors are compared. 
     If the difference between the respective vq and vd values are less than the first predetermined value the counter value is decreased. 
     Although, this embodiment compares respective sub-motor vq and vd values, a comparison may be made with other motor parameters, for example current. 
     If the counter value exceeds a second predetermined value this indicates that there is a fault with one or more of the sub-motors and a fault condition is triggered. 
     Any suitable fault condition may be utilised, for example a fault indicator may be set to notify the driver of the vehicle that the vehicle should be taken to a service/diagnostic centre and/or the performance of the electric motor may be limited, for example the motor torque may be reduced or a reduced maximum torque set. 
     Both of the embodiments for identifying a fault in an electric motor described above may be implemented in an electric motor together or separately.