Patent Description:
Permanent magnet synchronous electric motors operate via the production of a rotating magnetic field, which is typically formed via current flowing through coil windings mounted on a stator. The coil windings typically form a group of phase windings distributed around the stator that are coupled together. For a three phase electric motor or generator three sets of phase windings are connected together in either a star or a delta configuration.

An illustration of a six phase electric motor or generator having six sets of phase windings connected in a star configuration is shown in <FIG>, where one end of each of the windings are connected at a common point, known as a star point <NUM>.

During operation of the electric motor or generator a different voltage phase is applied or generated across each group of phase windings. Accordingly, for an n-phase electric motor or generator an n-phase voltage is applied across the respective phase windings of the electric motor or generator.

The efficiency of a permanent magnet synchronous electric motor or generator is optimised by ensuring that the electrical phase, that is to say electrical phase angle, that is applied to the phase windings is synchronised with the magnetic field generated by permanent magnets, which are typically mounted on the rotor, such that the electrical phase angle is at a fixed phase offset to the magnetic field for a given torque/speed. In other words, the electrical phase angle is synchronised with the rotor magnetic flux angle.

To allow the electrical phase of the respective phase windings to be synchronised with the magnetic field, that is to say the rotor magnetic flux angle, generated by the permanent magnets mounted on the rotor, a ring of magnets or ring of multiple magnetic poles in the form of a commutation magnet ring is mounted on the rotor that has a matching configuration to the permanent magnets mounted on the rotor, which act as drive magnets, where the commutation magnets are used to estimate the rotor magnetic flux angle of the drive magnets.

Typically mounted on the stator is a Hall sensor that is arranged to measure the field strength from the commutation magnet ring. To allow the direction of the rotor to be determined and for improved accuracy a second Hall sensor is typically placed <NUM> electrical degrees away from the first Hall sensor.

As the rotor rotates relative to the stator the Hall sensor outputs an AC voltage signal that allows the rotor magnetic flux angle to be estimated. The rotor magnetic flux angle is determined from the signal output from the Hall sensor via trigonometric calculation. For a two Hall sensor configuration one Hall sensor represents the sine of the rotor flux angle and the second Hall sensor signal represents the cosine of the rotor flux angle. The output AC voltage signals have a frequency proportional to the speed of the rotor. By way of illustration, <FIG> represents the output from two Hall sensors mounted on the stator that have been separated by an electrical phase angle of approximately <NUM> degrees. As illustrated, two sinusoidal signals are output, where one output signal is shifted by approximately <NUM> degrees with respect to the other output signal.

However, due to manufacturing tolerances the magnets within the commutation magnet ring will typically be offset by varying amounts with respect to the permanent magnets mounted on the rotor. Although a general offset between the magnets in the commutation magnet ring and the corresponding permanent magnets mounted on the rotor can be calibrated out, a varying offset can result in an error between the measured and the actual rotor flux angle of the rotor. An error between the measured and the actual rotor flux angle of the rotor can result in the electrical phase of the respective phase windings not being fully synchronised with the magnetic field generated by permanent magnets mounted on the rotor.

Typically the varying error between the measured and actual phase angle will be sinusoidal over a full mechanical revolution of the rotor relative to the stator, as illustrated in <FIG>. The error in rotor flux angle can result in loss of torque, torque ripple, acoustic noise and a decrease in the efficiency of the electric motor.

<CIT> describes an electric motor having an energising magnet that is magnetised in such a manner that two output signals are generated by two rotation position detectors per revolution, where the output signals are shifted in phase with respect to one another and have an asymmetric voltage response, and a control signal which is a function of the rotation speed of the rotor is formed from these output signals by means of an evaluation circuit.

<CIT> describes a motor control circuit that includes two sensor chips, where each sensor chip only controls part of a winding set.

<CIT> describes an electric motor having a plurality of sub-motors.

In accordance with an aspect of the present invention there is provided a method, an electric motor or generator according to the accompanying claims.

The present invention is directed to providing an electric motor or generator system for use in a wheel of a vehicle according to independent claim <NUM> and a method of operating an electric motor or generator system for use in a wheel of a vehicle according to independent claim <NUM>. Preferred embodiments are set out in the dependent claims <NUM> to <NUM>.

The present invention will now be described with reference to the accompanying drawings, in which:.

The embodiment of the invention discloses an electric motor or generator system for use in a wheel of a vehicle. 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. Various aspects of the invention are equally applicable to an electric generator having the same arrangement. In addition, some of the aspects of the invention are applicable to an arrangement having the rotor centrally mounted within radially surrounding coils.

As illustrated in <FIG>, the in-wheel electric motor <NUM> includes a stator <NUM> comprising a rear portion <NUM> forming a first part of the housing of the assembly, and a heat sink and drive arrangement <NUM> comprising multiple coils and electronics to drive the coils. The coil drive arrangement <NUM> is fixed to the rear portion <NUM> to form the stator <NUM> 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 <NUM> and rear portion <NUM> form the stator <NUM>.

A rotor <NUM> comprises a front portion <NUM> and a cylindrical portion <NUM> forming a cover, which surrounds the stator <NUM> according to the invention. The rotor includes a plurality of permanent magnets <NUM> which are arranged around the inside of the cylindrical portion <NUM>. For the purposes of the present embodiment <NUM> magnet pairs are mounted on the inside of the cylindrical portion <NUM>. However, any number of magnet pairs may be used.

The magnets are in close proximity to the coils on the assembly <NUM> so that magnetic fields generated by the coils in the assembly <NUM> cooperate with the magnets <NUM> arranged around the inside of the cylindrical portion <NUM> of the rotor <NUM> to cause the rotor <NUM> to rotate. As the permanent magnets <NUM> are utilized to generate a drive torque for driving the electric motor, the permanent magnets are typically called drive magnets.

The rotor <NUM> is attached to the stator <NUM> by a bearing block <NUM>. The bearing block <NUM> 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 <NUM> of the wall <NUM> of the stator <NUM> and also to a central portion <NUM> of the housing wall <NUM> of the rotor <NUM>. The rotor <NUM> is thus rotationally fixed to the vehicle with which it is to be used via the bearing block <NUM> at the central portion <NUM> of the rotor <NUM>. This has an advantage in that a wheel rim and tyre can then be fixed to the rotor <NUM> at the central portion <NUM> 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 <NUM>. The wheel bolts may be fitted through the central portion <NUM> of the rotor through into the bearing block itself. With both the rotor <NUM> and the wheel being mounted to the bearing block <NUM> there is a one to one correspondence between the angle of rotation of the rotor and the wheel.

<FIG> shows an exploded view of the same assembly as <FIG> from the opposite side showing the stator <NUM> comprising the rear stator wall <NUM> and coil and electronics assembly <NUM>. The rotor <NUM> comprises the outer rotor wall <NUM> and circumferential wall <NUM> within which magnets <NUM> are circumferentially arranged. As previously described, the stator <NUM> is connected to the rotor <NUM> via the bearing block at the central portions of the rotor and stator walls.

Additionally shown in <FIG> are control devices <NUM> carrying control electronics, otherwise known as motor drive controllers or inverters.

A V shaped seal <NUM> is provided between the circumferential wall <NUM> of the rotor and the outer edge of the stator housing <NUM>.

According to the invention, the rotor also includes a set of magnets <NUM> for position sensing, otherwise known as commutation magnets, which in conjunction with position 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, as an example not belonging to the invention, 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, the exemplary set of commutation magnets has <NUM> magnet pairs, where each magnet pair is approximately radially aligned with a respective drive magnet pair.

According to the invention, two sensors, which in this embodiment are Hall sensors, are mounted on the stator in diametrically opposite positions, that is to say <NUM> mechanical degrees apart. The sensors are positioned so that as the rotor rotates each of the commutation magnets that form the commutation magnet ring respectively rotates past the respective sensors.

As the rotor rotates relative to the stator the commutation magnets correspondingly rotate past the respective sensors with each Hall sensor outputting an AC voltage signal, where each sensor outputs a complete voltage cycle of <NUM> electrical degrees for each magnet pair that passes the respective sensors.

<FIG> illustrates a representation of a commutation magnet ring, having <NUM> magnet pairs, with two sensors positioned approximately <NUM> mechanical degrees apart with respect to the commutation magnet ring.

According to the invention, to aid in the determination of the direction of the rotor, each position sensor also has an associated second position sensor placed <NUM> electrical degrees apart.

As illustrated in <FIG>, the motor <NUM> in this embodiment includes <NUM> coil sets <NUM> with each coil set <NUM> having three coil sub-sets <NUM>, <NUM>, <NUM> that are coupled to a respective control device <NUM>, where each control device <NUM> 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 <NUM> 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 <NUM> as having three coil sub-sets <NUM>, <NUM>, <NUM>, the present invention is not limited by this and it would be appreciated that each coil set <NUM> could have two or more coil sub-sets. Equally, although the present embodiment describes an electric motor having eight coil sets <NUM> (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 <NUM> to form a three phase electric motor configuration. Accordingly, as stated above, the exemplary motor includes eight three phase sub-motors, where each three phase sub-motor includes a control device <NUM> coupled to the three sub-set coils of a coil set <NUM>.

Each three phase bridge inverter is arranged to provide PWM voltage control across the respective coil sub-sets <NUM>, <NUM>, <NUM> to provide a required torque for the respective sub-motors.

For a given coil set the three phase bridge switches of a control device <NUM> are arranged to apply a single voltage phase across each of the coil sub-sets <NUM>, <NUM>, <NUM>.

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 <NUM> is substantially wedge-shaped. This shape allows multiple control devices <NUM> to be located adjacent each other within the motor, forming a fan-like arrangement.

The control device <NUM> 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 <NUM> includes a number of electrical components for controlling the operation of the switches mounted on the control device <NUM>. Examples of electrical components mounted on the control device <NUM> 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 <NUM> to communicate with devices external to the control device <NUM>, such as other control devices <NUM> or a master controller. Typically the control device <NUM> will communicate over the interface to receive torque demand requests and to transmit status information. Typically, the at least two sensors for determining the rotor flux angle will be mounted on separate control devices <NUM>, which in turn are mounted to the stator. Optionally, for additional redundancy a sensor may be mounted to each control device <NUM>.

The sinusoidal voltage waveforms generated in the electric motor by the control devices <NUM> 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 <NUM> degrees as illustrated in <FIG> by the three axis A, B, C.

As illustrated in <FIG>, 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 is = ia + αib + α<NUM>ic, where α = e(i*<NUM>*π/<NUM>).

Field Oriented Control is based on projections that transform a three phase time and speed dependent system into 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.

Using a closed loop control system, an example of one being illustrated in <FIG>, a required torque τ, which is represented by an input iq value, and a required magnetic flux λ, which is represented by an input id value, are compared with actual values measured from the electric motor.

It should be noted, however, that under normal circumstances a permanent magnet synchronous electric motor will typically have id set to zero.

The closed loop control system <NUM> illustrated in <FIG> performs a comparison of a required torque value with measured values and performs the associated voltage and current control required to drive the electric motor using Park and Clarke Transforms.

A Clarke Transform <NUM> uses measured values of at least two of the three phase voltage va, vb, and vc that are used to drive the electric motor to calculate voltages in a two phase orthogonal stator axis vα and vβ. A Park transformation is then performed by a Park Transform <NUM> to transform the two fixed co-ordinate stator axis vα and vβ to a two co-ordinate time invariant system vd and vq, which defines a d, q rotating reference frame. <FIG> illustrates the relationship of the stator voltage in the d,q rotating reference frame with respect to the two phase orthogonal stator axis vα and vβ and the a, b and c stationary reference frame.

Under normal drive conditions the rotor phase angle θr, otherwise known as rotor flux angle of the drive magnets, which is defined by the rotor magnetic flux vector ΨR, and the stator electrical phase angle θe should ideally be aligned with the q-axis, thereby maintaining synchronization between the rotor phase angle θr and the stator electrical phase angle θe.

To allow the Park Transform <NUM> to derive a time invariant transformation the rotor phase angle θr is provided to the Park Transform, where the rotor phase angle θr is determined using the rotor commutation magnets and position sensors mounted on the control devices <NUM>.

As described above, the rotor phase angle θr of the rotor is determined using two position sensors, which in this embodiment are Hall sensors, which are mounted on separated control devices. For the purposes of the present embodiment, to improve accuracy each position sensor forms part of a separate position sensor assembly, with each position sensor assembly having a second position sensor, where both position sensor assembly position sensors are mounted <NUM> electrical degrees apart. Accordingly, for the purposes of the present embodiment, at least two position sensor assemblies are mounted on separate control devices.

As described above, although only two position sensors, or for the purposes of the present embodiment sensor assemblies, are required to be mounted in substantially diametrically opposite positions on the stator, each control device <NUM> may have a sensor or sensor assembly. By having a position sensor mounted on a number of the control devices this has the advantage of providing redundancy should a fault occur with one set of sensors.

Although the present embodiment incorporates active Hall sensors, according to the invention also inductive sensors, that include a magnet and coil mounted adjacent to a toothed ring, can be used. As described above, as the rotor rotates relative to the stator, the position sensors output an AC voltage signal, where the outputted AC voltage signals have a frequency proportional to the speed of the rotor.

The phase of the voltage signal output by the respective position sensors corresponds to the phase of the commutation magnet ring flux, that is to say flux angle. To determine the rotor phase angle θr to allow its use in the Park Transform, the commutation magnet ring flux, which is defined by the voltage signals output by the position sensors, needs to be calibrated to correct for variations in relative position of each of the commutation magnets and their associated drive magnet.

To calibrate the commutation magnet flux angle for use as the rotor flux angle, an electrical angle offset for both sensors is defined with respect to the rotor magnet pairs, where the offset defines the difference between the measured electrical phase angle of a commutation magnet and the electrical phase angle of its associated drive magnet.

Each position sensor as commutation sensor has a calibrated phase angle offset value, which represents the average difference between the commutation magnet flux angle and the rotor flux angle. However, since the commutation magnet pole pairs will typically not be equally spaced over a mechanical revolution, unless corrected there will be an additional offset error between the calculated commutation flux angle and the rotor flux angle that is broadly sinusoidal in shape even though the average error is zero. Table <NUM> illustrates the sinusoidal nature of the offset error between the calculated commutation flux angle and the rotor flux angle resulting from unequal spacing between the commutation magnets and their respective drive magnet, where for each of the <NUM> magnet pairs a calibration offset is determined at both sensors. The final column of Table <NUM> contains an average of the determined offset values for each magnet pair at both sensors.

The error values shown in Table <NUM> are in electrical angles and shown the electrical angle error relative to the rotor magnet flux assuming that the motor has been calibrated so that the average error is zero, that is to say an average offset value has already been applied to these values.

<FIG> illustrates a graphical representation of the calibration data according to the invention, where the x axis represents the respective magnet pairs and the y axis represents the flux angle offset value. As illustrated in <FIG>, the measured offset value at each sensor varies in a sinusoidal manner across the commutation magnet pairs with the measured values for both sensors being approximately <NUM> mechanical degrees out of phase. The out of phase sinusoidal variation between the two sensors is indicative of a varying offset error.

However, because the second position sensor is <NUM> mechanical degrees from the first position sensor, the error, which is sinusoidal in nature, between the commutation flux angle and the rotor flux angle is opposite in sign to that seen by the first sensor.

However, as the present embodiment only allows a single calibration offset to be stored for each position sensor, that is to say only a single offset value per position sensor can be used for all magnets, the separate offset error values per magnet pair listed in Table <NUM> cannot be used in the present embodiment to correct for the varying offset error.

However, because the two offset errors associated with the respective position sensors that vary on top of the average offset error are opposite in sign and have approximately the same magnitude, to compensate for the varying offset error the respective control devices are arranged to use the average of the two flux angles measured by the two sensors that are positioned in substantially diametrically opposite positions on the stator. As the two varying offset error values are sinusoidal in nature and opposite in sign, by averaging the two measured flux angles according to the invention this largely cancels out the varying phase angle offset error.

The final column in Table <NUM> provides an average of the varying offset error values for two position sensors positioned in substantially diametrically opposite positions on the stator, which corresponds to the averaging of the phase angles measured by the two substantially diametrically opposite sensors where the main offset value for the general offset between the commutation magnets and drive magnets has already been corrected for.

As a result of averaging the varying offset errors for each sensor it can be seen that there is no offset error greater than <NUM> electrical degrees despite there being an error of more than <NUM> degrees in the signals from each sensor.

The rotor flux angle derived using the average rotor flux angle is used as the rotor flux angle θr.

Using the calibration data, which is preferably stored within the electric motor or generator system, by comparing the determined varying offset error, derived from the measured phase angle, with the calibration data it is possible to determine the rotor position relative to the stator. Once the relative rotor position has been determined, should one of the position sensors fail it will be possible to apply the appropriate varying offset error correction based on rotor position.

In an alternative embodiment, the varying offset error for more than two position sensors may be used.

Claim 1:
An electric motor or generator system for use in a wheel of a vehicle comprising a rotor (<NUM>) having a first set of permanent magnets (<NUM>); a stator (<NUM>) having a first position sensor, as a Hall sensor or an inductive sensor, mounted on the stator (<NUM>) and a second sensor, as a Hall sensor or an inductive sensor, mounted in a diametrically opposite position on the stator relative to the first position sensor, wherein the rotor (<NUM>) surrounds the stator (<NUM>), wherein the first position sensor is arranged to output a first signal indicative of a first rotor flux angle associated with the first set of permanent magnets (<NUM>) as the rotor (<NUM>) rotates relative to the stator (<NUM>); and the second position sensor is arranged to output a second signal indicative of a second rotor flux angle associated with the first set of permanent magnets (<NUM>) as the rotor (<NUM>)rotates relative to the stator (<NUM>); characterised by means arranged to determine a corrected rotor flux angle by averaging the first rotor flux angle indicated by the first position sensor and the second rotor flux angle indicated by the second position sensor, for cancelling out at the first and second position sensors a varying sinusoidal phase angle offset error between a calculated commutation flux angle resulting from the first set of permanent magnets (<NUM>) and the rotor flux angle resulting from drive magnets (<NUM>) of the rotor (<NUM>).