Wheel driving apparatus and electric vehicle including the same

An electric bike includes a wheel driving apparatus. The wheel driving apparatus includes an electric wheel drive motor which drives a wheel, a gap changer which changes a gap length in the wheel drive motor, and a motor control unit which controls the wheel drive motor and the gap changer. The motor control unit calculates a target gap length for the gap changing motor in the gap changer based on an accelerator opening-degree signal, a rotation speed, a q-axis electric-current command value, a power source voltage, and a voltage utilization rate. Then, a feedback control is provided to the gap changer based on a difference value between the target gap length and the actual gap-length. A good vehicle characteristic is obtained without being affected by individual variability or operating environment of the electric motor through efficient drive of the electric motor from a low-speed range through a high-speed range.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wheel driving apparatus and an electric vehicle, and more specifically, to a wheel driving apparatus which drives a wheel with an electric motor, and to an electric vehicle including the wheel driving apparatus.

2. Description of the Related Art

Conventionally, electric vehicles such as electric scooters drive their wheels with an electric motor by controlling the motor's revolution (rotation speed) based on the amount of accelerator operation. When controlling the electric motor's rotation speed, a wide driving range from a high-torque low-rotation-speed to a low-torque high-rotation-speed is used so as to drive the electric motor efficiently.

As an electric motor to achieve this, there is known a variable air-gap permanent-magnet motor which is disclosed in JP-A 2005-168190, for example. The variable air-gap permanent-magnet motor includes a disc-shaped rotor provided with a permanent magnet, a stator having an armature coil and disposed to oppose the rotor, and a hydraulic mechanism for sliding thereby moving the stator inside the motor's casing. With this arrangement, by controlling the hydraulic mechanism, a gap length between the rotor and the stator is adjusted.

JP-A 2005-168190 discloses an arrangement in which a relationship between motor rotation speed and target gap length is stored in a reference map, for example, and when the motor is driven, the hydraulic mechanism is operated so as to achieve a target gap length which is appropriate to the actual motor rotation speed. The document also discloses an arrangement that a relationship between motor rotation speed and target gap length is stored in advance in a reference map, for example, in accordance with estimated motor shaft torque values, and when the motor is driven, a target gap length is obtained from the estimated shaft torque value and the motor rotation speed, and then the hydraulic mechanism is operated so as to achieve the obtained target gap length.

An output characteristic of electric motors differs from one motor to another depending on individual variability of the motor (for example, mechanical dimension tolerance of the air gap and magnetic flux density tolerance of the permanent magnets) as well as the motor's operating environment (for example, magnet temperature and power source voltage). For this reason, setting the target gap length in accordance with motor rotation speed as in the electric motor disclosed in JP-A 2005-168190 will result in an inability to achieve an appropriate gap length if the actual motor output characteristic is not identical with the expected characteristic (characteristic as designed).

Now, description will be made for the output characteristic of electric motors (a relationship between rotation speed and torque).

As shown inFIG. 26, electric motors have a characteristic that their maximum torque decreases linearly with an increase in the rotation speed (the amount of revolutions per unit time). Also, in order to protect motor driving circuits such as inverters, an upper limit current value is set for the electric current which is applied to the electric motor. Since the electric motor's torque is generally proportional to the applied current, the upper limit current value limits a maximum value of the applied current. For example, if the inverter's circuit element (FET) is set to have an upper limit current value of 100 A, a maximum torque will be a torque attained when this upper limit current value is applied. Therefore, actual drive control range for the electric motor is represented by a hatched range shown inFIG. 27.

When the gap length of the electric motor is changed, an output characteristic (rotation-speed vs. torque characteristic) of the electric motor changes.FIG. 28shows an example where the gap length is changed in three levels. InFIG. 28, a solid line shows Characteristic1which is a characteristic when the gap length is short (gap length G1), a broken line shows Characteristic3which is a characteristic when the gap length is long (gap length G3), and a dashed line shows Characteristic2which is a characteristic when the gap length is in the middle (gap length G2: G1<G2<G3). Under the state of Characteristic1, high torque is obtained but the motor cannot rotate at a high speed. Under the state of Characteristic3, the motor can rotate at a high speed but high torque is not obtainable. Characteristic2is an intermediate characteristic between the two. Therefore, by changing the gap length, it is possible to achieve a drive control characteristic of the electric motor as indicated by a solid line inFIG. 29. For example, by changing the gap length in the pattern of G1→G2→G3, it is possible to change the output characteristic in the pattern of Characteristic1→Characteristic2→Characteristic3. By using an electric motor which has such a capability of varying the gap length for a wheel drive motor of an electric vehicle, a vehicle characteristic capable of driving from a high-torque low-rotation-speed range through a low-torque high-rotation-speed range can be achieved.

In order to drive the electric motor efficiently, it is desirable that the gap length is switched at a switching timing where the electric motor is capable of performing at its maximum potential, i.e., on its output characteristic line. Here, reference will be made toFIG. 30to explain the gap length switching timing. In this example, the gap length is varied in three levels (G1, G2, G3: G1<G2<G3). At a time of starting the electric motor, a short gap length G1is selected in preference of torque over rotation speed. In this case, the state of the electric motor (torque, rotation speed) changes on Current Limiting Line L1in a direction indicated by arrows. The current Limiting Line L1shows torques obtained when the gap length is G1and the applied current is the upper limit current value. Also, Current Limiting Line L2shows torques obtained when the gap length is G2and the applied current is the upper limit current value, whereas Current Limiting Line L3shows torques which are obtained when the gap length is G3and the applied current is the upper limit current value.

If the gap length is changed from G1to G2when the electric motor's rotation speed has increased to a cross point between Current Limiting Line L1and Characteristic1line (rotation speed n1), it is possible to change the output characteristic at a point where the motor is performing at its maximum potential. At this point, the output torque decreases to a level limited by Current Limiting Line L2, and the rotation speed increases while maintaining this torque. Likewise, when the state of the electric motor (torque, rotation speed) has reached a cross point between Current Limiting Line L2and Characteristic2line (rotation speed n2), the gap length is switched from G2to G3. As described above, it is possible to make the electric motor perform at its maximum potential capacity by switching the gap length on the output characteristic line.

In a case where the switching of the gap length is based on motor rotation speed as in JP-A 2005-168190, there would be no problem in switching the gap length at those points where the rotation speed n1or n2has been detected if the output characteristic of electric motors is identical with the expected characteristic (characteristic as designed). However, if the actual output characteristic of the electric motor is different from the expected characteristic, the switching timing can be inappropriate and such problems may arise as the electric motor's potential is not utilized sufficiently or the drive feeling is not as good.

For example, take a case that the electric motor's torque tends to be weaker than the output characteristic shown inFIG. 30. Then, when the gap length is switched as shown inFIG. 31at each point where the rotation speed n1or n2is detected, switching of the gap length tends to be too early as compared to the increase in rotation speed. In this case, the electric motor's potential capability cannot be utilized in the areas circled by broken lines. In other words, the rotation speed could have been increased further while maintaining the maximum torque, yet the gap length was switched, making it impossible to obtain advantageous torques available from the electric motor.

Conversely, in a case where the electric motor's torque tends to be stronger than the expected characteristics, switching timing will be too late if the gap length is switched as shown inFIG. 32, at each point where the rotation speed n1or n2is detected. In this case, the torque will drop dramatically before the gap length is switched as shown in the area enclosed by a broken line. This torque drop will result in a decreased drive feeling. For example, when the vehicle is starting to run, there will be a feeling for a moment that the vehicle is losing acceleration.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodiments of the present invention provide a wheel driving apparatus and an electric vehicle that drives an electric motor efficiently from a low-speed range through a high-speed range without being affected by individual variability or operating environment of the electric motor, thereby offering a good vehicle characteristic.

According to a preferred embodiment of the present invention, there is provided a wheel driving apparatus for driving a wheel of an electric vehicle. The apparatus includes an electric motor arranged to drive the wheel, a gap changer arranged to change a gap length which is a length of an air gap in the electric motor, an accelerator information obtaining device arranged to obtain accelerator information regarding an amount of acceleration in the electric vehicle, a voltage calculation device arranged to calculate a control voltage command value based at least on the accelerator information obtained by the accelerator information obtaining device, a voltage utilization rate calculation device arranged to calculate a voltage utilization rate based on a maximum value of the control voltage command value and the control voltage command value calculated by the voltage calculation device, and a gap controller arranged to control the gap changer based on the voltage utilization rate calculated by the voltage utilization rate calculation device, so as to adjust the gap length.

In the electric motor, there is provided an air gap which functions as a magnetic resistance, i.e., a magnetic resistance space in the magnetic path. By changing the gap length, the magnetic resistance is changed and an output characteristic of the electric motor changes. In a preferred embodiment, the gap controller controls the gap changer based on the voltage utilization rate to change the gap length, and therefore to change the output characteristic of the electric motor.

As shown inFIG. 33, the points where the voltage utilization rate is 100% are on the output characteristic line (maximum torque line). The voltage utilization rate decreases as the electric motor's state of control becomes lower than the output characteristic line. This gives the electric motor a reserved capacity to output torque. In this case, the voltage utilization rate can be expressed as a value which can be obtained by first obtaining a torque value at the given state of control of the electric motor from the output characteristic chart inFIG. 33, and then dividing the torque value by a maximum torque (a torque on the characteristic line) which is available at the given rotation speed. For example, when the electric motor's state of control is at Point A inFIG. 33, the voltage utilization rate is 40%.

In an electric vehicle in which a wheel is driven by an electric motor, it is necessary, as the electric motor rotates at a higher speed, to shift to a low-torque high-rotation-speed characteristic (e.g., Characteristic3inFIG. 28). At this occasion, if the electric motor has a reserved torque outputting capacity, i.e., if the motor's state of control is below the characteristic line as indicated by Point A inFIG. 33, there is not an urgent need for switching to the low-torque high-rotation-speed characteristic, and it is possible to increase the rotation speed while maintaining the current torque. As the rotation speed is increased, the electric motor's state of control shifts from Point A to the right-hand direction, and eventually reaches a point where the voltage utilization rate is 100%.

Therefore, controlling the gap length based on the voltage utilization rate makes it possible to switch the output characteristic at an appropriate timing. In this case, ideally, the gap length should be changed at the point where the voltage utilization rate is 100%. However, in practice, a delayed response in the gap changer or the system must be taken into consideration and therefore it is reasonable to switch the gap length at a voltage utilization rate of about 90% to about 95%, for example.

In a preferred embodiment of the present invention, the output characteristic is changed by adjusting the gap length of the electric motor based on the voltage utilization rate, and therefore it is possible to change the output characteristic always at an appropriate timing without being affected by individual variability or operating environment of the electric motor. Thus, it is possible to sufficiently utilize the potential capability of the electric motor, and reduce a decrease in drive feeling when the vehicle is starting/accelerating. As a result, it is possible to drive the electric motor efficiently from a low-speed range through a high-speed range and to obtain a good vehicle characteristic.

Preferably, the gap controller controls the gap changer so as to increase the gap length if the voltage utilization rate is greater than a first threshold value, and to decrease the gap length if the voltage utilization rate is smaller than a second threshold value which is smaller than the first threshold value.

In other words, when the electric motor's control voltage-command value increases and the voltage utilization rate exceeds the first threshold value, then the gap controller increases the gap length. This changes the output characteristic of the electric motor, making available a high rotation speed. On the other hand, when the electric motor's control voltage-command value decreases and the voltage utilization rate becomes smaller than the second threshold value, then the gap controller decreases the gap length. This changes the output characteristic of the electric motor, and a high torque is obtained. As described, it is possible to control the output characteristic of the electric motor easily. Since no adjustment is made to the gap length when the voltage utilization rate is between the first threshold value and the second threshold value, it is possible to prevent such a problem of hunting in the control, and to achieve stable vehicle driving.

Further preferably, the wheel driving apparatus further includes an angular acceleration information obtaining device arranged to obtain angular acceleration information regarding the electric motor's angular acceleration, and a threshold value setting device arranged to decrease at least one of the first threshold value and the second threshold value with an increase in the angular acceleration information obtained by the angular acceleration information obtaining device.

When accelerating the electric vehicle, the electric motor's control voltage-command value is increased to increase the motor rotation speed. However, if there is a delay in the response from the gap changer to this motor control, there is a risk that it will become impossible to increase the motor rotation speed quickly. In other words, there is a risk that the shift in the motor characteristic from the high-torque low-rotation-speed characteristic to the low-torque high-rotation-speed characteristic caused by the gap length change will not appropriately follow the increase in the electric motor's rotation speed. On the other hand, when decelerating the electric vehicle, the electric motor's control voltage-command value is decreased to decrease the motor rotation speed. In this case again, a delayed response is not desirable.

To tackle this issue, the first threshold value is decreased with an increase in the electric motor's angular acceleration. This arrangement makes it possible to increase the gap length earlier in the course of accelerating the electric vehicle. Therefore, it becomes possible to increase the motor rotation speed quickly and to reduce delay in the response from the gap changer. On the other hand, if the second threshold value is decreased with an increase in the electric motor's angular acceleration, i.e., if the second threshold value is increased with a decrease in the electric motor's angular acceleration, then it becomes possible to decrease the gap length earlier in the course of decelerating the electric vehicle. Therefore, it becomes possible to decrease the motor rotation speed quickly and to reduce delay in the response from the gap changer. It should be noted here that the electric motor's angular acceleration is the electric motor's rotational acceleration, and is obtained by differentiating the motor's angular speed or rotation speed by time. The value takes a positive value when the electric motor is accelerating, while it takes a negative value when the electric motor is decelerating.

Further, preferably, the gap controller controls the gap changer based on the voltage utilization rate if the accelerator information has a value greater than a predetermined value, but does not control the gap changer based on the voltage utilization rate if the accelerator information has a value not greater than the predetermined value. For example, when the accelerator is operated to be moved in a closing direction to decelerate the electric vehicle, the electric motor's control voltage-command value decreases and the electric motor's voltage utilization rate decreases with the operation. In such a situation, the electric motor has a reserved capacity to output more torque than the demanded torque. However, if gap length adjustment is made based on the voltage utilization rate, the gap length is shortened, and results in increased iron loss. Thus, by not performing the gap length adjustment based on the voltage utilization rate if the accelerator information is not greater than the predetermined value, it becomes possible to reduce influence from iron loss.

Preferably, the wheel driving apparatus further includes a rotation speed information obtaining device arranged to obtain rotation speed information regarding a rotation speed of the electric motor. With this arrangement, the gap controller provides a first control mode of controlling the gap changer based on the voltage utilization rate if the accelerator information has a value greater than a predetermined value, and a second control mode of controlling the gap changer based on the rotation speed information obtained by the rotation speed information obtaining device if the accelerator information has a value not greater than the predetermined value. It is preferable that the gap controller controls the gap changer so as to increase the gap length with an increase in the rotation speed information in the second control mode. In this case, the electric motor's induced voltage does not exceed a DC power source voltage value, and this makes it possible to control the electric motor at a high-speed rotation.

Further preferably, the voltage calculation device includes an electric-current command value calculation device arranged to calculate a q-axis electric-current command value in a d-q axis coordinate system for the electric motor based on the accelerator information. With this arrangement, the gap controller controls the gap changer so as to increase the gap length with a decrease in the q-axis electric-current command value in the second control mode. When the amount of accelerator operation is small and the accelerator information is small, i.e., when the q-axis electric-current command value is small, there is no need for changing the output characteristic of the electric motor toward the high-torque low-rotation-speed side. Therefore, the gap length adjustment should be addressed for reduced iron loss in the electric motor. It is possible to reduce iron loss by increasing the gap length thereby reducing the amount of magnetic flux across the coils in the electric motor.

Further, preferably, the electric vehicle further includes a DC power source arranged to supply electric power to the electric motor. With this arrangement, the wheel driving apparatus further includes a voltage information obtaining device arranged to obtain voltage information regarding a voltage in the DC power source, and the gap controller controls the gap changer so as to increase the gap length with a decrease in the voltage information in the second control mode. In this case, the electric motor's induced voltage does not exceed the DC power source voltage, and this makes it possible to maintain the control of the electric motor.

Preferably, the wheel driving apparatus further includes a motor stoppage detector arranged to detect a stoppage of rotation of the electric motor. With this arrangement, the gap controller controls the gap changer so as to minimize the gap length regardless of the voltage utilization rate if the motor stoppage detector detects a stoppage of rotation of the electric motor. Therefore, a high torque is obtained when the electric vehicle is started next time, offering good acceleration.

Further preferably, the electric vehicle further includes a main switch, and the gap controller controls the gap changer so as to maximize the gap length regardless of the voltage utilization rate if the main switch is turned off. This arrangement makes it possible to reduce cogging torque which is generated by rotation of the electric motor in a case where the electric vehicle is pushed by hand for transportation.

Further, preferably, the wheel driving apparatus further includes a gap length detector arranged to detect the gap length. With this arrangement, the gap controller provides a feedback control to the gap changer based on a difference between a target gap length obtained by calculation based on the voltage utilization rate and the gap length detected by the gap length detector. In this case, an actual gap-length is fed back to the target gap length which was calculated based on the voltage utilization rate, to calculate an amount of control for the gap changer, and based on the calculation result, the gap changer is controlled. This improves the gap changer's response, making it possible to further improve the output characteristic of the wheel driving electric motor.

Preferably, the electric motor is provided by an axial air gap motor which includes a rotor having a rotation shaft, and a stator which faces the rotor at a distance in a direction in which the rotation shaft extends, and the gap changer changes the gap length between the rotor and the stator. By changing a relative positional relationship between the rotor and the stator in the axial direction of the rotation shaft as described above, the gap length can be easily changed.

Further preferably, the electric motor is an axial air gap motor which includes a rotor having a rotation shaft, and a stator facing the rotor at a distance in a direction in which the rotation shaft extends. In addition, the stator includes a first stator and a second stator opposed to each other at a distance in the direction in which the rotation shaft extends and variable in terms of relative position in a rotating direction of the rotor. With this arrangement, the gap changer varies the relative position of the first stator and the second stator in the rotating direction thereby changing the gap length between the first stator and the second stator. In this case, the gap length change can be performed easily even if there is no space provided for relative movement in the axial direction of the rotation shaft.

The above-described wheel driving apparatus can be used suitably in electric vehicles.

It should be noted here that the accelerator information is information relevant to the amount of acceleration, and has a value which varies as the amount of acceleration varies.

The angular acceleration information is information relevant to the electric motor's angular acceleration, and has a value which varies as the angular acceleration varies.

The rotation speed information is information relevant to the electric motor's rotation speed, has a value which varies as the rotation speed varies. It should be noted here that the electric motor's rotation speed is an amount of rotation per unit time which can be expressed in terms of rpm, for example, or other suitable units of measure.

The voltage information is information relevant to a voltage of DC power source, and has a value which varies as the voltage varies.

Other features, elements, steps, characteristics, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments with reference to the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring toFIG. 1, an electric bike1according to the first preferred embodiment of the present invention preferably includes a wheel2, a DC power source3, an accelerator4, a main switch5, and a wheel driving apparatus10. The DC power source3includes a battery, for example, and supplies electric power for driving the electric bike1. The main switch5is a switch which starts and stops a control system for the entire electric bike1.

The wheel driving apparatus10includes a wheel driving electric motor (hereinafter called “wheel drive motor”)100which provides a rotating torque to the wheel2, a gap changer200which changes the gap length GAP, i.e., a length of an air gap in the wheel drive motor100, and a motor control unit300which controls the wheel drive motor100and the gap changer200.

FIG. 2andFIG. 3are schematic sectional views which show a configuration of the wheel drive motor100and the gap changer200.

The wheel drive motor100is preferably a three-phase permanent magnet axial air gap electric motor. The wheel drive motor100includes a stator110and a rotor120. The stator110is disposed to face the rotor120at a predetermined distance provided in a direction (axial direction) in which a rotation shaft123of the rotor120extends.

The stator110includes an annular shaped stator yoke111made of magnetic steel plate, for example; a plurality (for example, eighteen, in the present preferred embodiment) of teeth112on the stator yoke111; and a plurality of coils113wound around the teeth112. The stator yoke111has a main surface which has a plurality (for example, eighteen, in the present preferred embodiment) of fitting holes111aprovided equidistantly in a circumferential direction. In each of the fitting holes111a, one of the teeth112is fitted, with its end into the fitting hole111a, so that each of the teeth112is magnetically connected with the stator yoke111.

With each of the teeth112fixed to the stator yoke111, and with each of the teeth112having the coil113wound around, the entire stator110is molded into a plastic or resin (hereinafter referred to as resin), and has an annular shape. In other words, the stator110is integrated into an annular shape by an annular shaped resin body114.

Each of the teeth112is defined, for example, by a plurality of magnetic steel plates laminated one onto another. In each of the teeth112, the surface which faces the rotor120is not covered by the resin, is perpendicular or substantially perpendicular to the rotation shaft123, and has a flat surface.

Referring toFIG. 4, the annular shaped resin body114has a center opening115and an outer circumferential portion116, each embedded with a plurality of metal collars117. The collars117and the stator yoke111have insertion holes (not illustrated) and by inserting set screws into these holes, the stator110is fixed to a cylindrical mount6which is part of the vehicle body.

Referring toFIG. 5, the rotor120includes a rotor yoke121made of iron, for example, and a magnet122fixed to the rotor yoke121. The rotor yoke121has an outer plate portion121ain the form of an annular shaped disc to which the magnet122is fixed, a tapered portion121bextending from an inner circumferential end of the outer plate portion121awhile decreasing in its diameter, an inner plate portion121cwhich is an annular shaped disc extending inward from an inner circumferential end of the tapered portion121bin parallel or substantially parallel to the outer plate portion121a, and a cylindrical portion121dextending axially from an inner circumferential end of the inner plate portion121c.

The motor rotation shaft123is inserted into the cylindrical portion121d. The cylindrical portion121dhas an inner circumferential surface provided with axial female splines (not illustrated), whereas the motor rotation shaft123has an outer circumferential surface provided with axial male splines (not illustrated). The female splines and the male splines are mated with each other slidably in the axial direction. Therefore, the motor rotation shaft123is non-rotatable but axially slidable with respect to the cylindrical portion121d. The motor rotation shaft123has an end (an upper end as seen inFIG. 2) which is connected with a wheel shaft of the wheel2via an unillustrated speed reduction mechanism.

As shown inFIG. 5, the magnet122is provided, for example, as a single member such as a ring-shaped bond magnet magnetized to have a plurality of N poles and S poles arranged alternately with respect to each other in a circumferential direction. The magnet122is fixed to the outer plate portion121aof the rotor yoke121so as to face the teeth112of the stator110. In the present preferred embodiment, the magnet122preferably includes six pairs of mutually adjacent pole regions (in each pair of regions, one is magnetized to an N pole and the other is magnetized to an S pole) on the surface facing the teeth112. It should be noted here that, instead of the magnet122as a ring-shaped single plate, a plurality of individually magnetized, generally square or fan-shaped magnet pieces may be fixed to the outer plate portion121aso that an S pole and an N pole are arranged alternately with respect to each other.

Returning toFIG. 2, the gap changer200includes a feed screw mechanism210, a gap changing electric motor (hereinafter, referred to as “gap changing motor”)220, and a gap length detector230. The feed screw mechanism210includes a threaded cylindrical body211which surrounds the cylindrical portion121dof the rotor120and has an outer circumferential surface provided with a male thread211a, and a cylindrical nut212which has an inner circumferential surface provided with a female thread212ato thread on the male thread211aof the threaded cylindrical body211. The outer circumferential surface of the threaded cylindrical body211has an end provided with a gear213.

A bearing214is provided between the inner circumferential surface of the threaded cylindrical body211and the motor rotation shaft123, making the threaded cylindrical body211and the motor rotation shaft123coaxially rotatable to each other. Also, a bearing215is provided between the inner circumferential surface of the nut212and the outer circumferential surface of the cylindrical portion121dof the rotor120, to support the cylindrical portion121dof the rotor120coaxially and rotatably with respect to the nut212.

An anti-rotation groove212bis provided axially and for a predetermined length in the outer circumferential surface of the nut212. A projection6aprovided on the mount6is inserted slidably into the anti-rotation groove212b. Therefore, the nut212is movable only in the axial direction and is non-rotatable with respect to the mount6. Also, the anti-rotation groove212bhas two ends which function as mechanical stoppers to limit the axial position of the nut212.

The gap changing motor220is fixed to unillustrated portion of the vehicle body, and has an output shaft221provided with a gear222fixed thereto. The gear222engages with the gear213of the threaded cylindrical body211. Therefore, when electric power is supplied to the gap changing motor220and the output shaft221rotates, the threaded cylindrical body211rotates. In this movement, since the nut212is non-rotatable with respect to the mount6, the rotating movement of the threaded cylindrical body211is converted to an axial movement of the nut212. Therefore, the rotor120which is rotatably supported by the nut212is moved in the axial direction. For this reason, it is possible to adjust the gap length GAP, which is the length of an air gap between the stator110and the rotor120(a space between the teeth112and the magnet122opposed in parallel or substantially parallel to each other), by controlling the gap changing motor220.FIG. 2shows a state where the gap length GAP is at its minimum.FIG. 3shows a state where the gap length GAP is at its maximum, with the nut212raised with respect to the mount6.

The rotor yoke121, the magnet122, the teeth112and the stator yoke111in the wheel drive motor100define a magnetic path, and the air gap is arranged within the magnetic path. Since the air gap functions as a magnetic resistance provided in the magnetic path, changing the gap length GAP changes the amount of magnetic flux which flows through the air gap, thereby changing the output characteristic (rotation speed-torque characteristic) of the wheel drive motor100. Namely, as shown in an example inFIG. 28, the shorter the gap length GAP, the more toward the high-torque low-rotation-speed characteristic (Characteristic1); and the longer the gap length GAP becomes, the more toward the low-torque high-rotation-speed characteristic (Characteristic3). Therefore, by controlling the gap length GAP, it becomes possible to use a wide operation range from a high-torque low-rotation-speed to a low-torque high-rotation-speed.

The gap length detector230, which detects an axial position (in the axial direction of the motor rotation shaft123) of the rotor120, in order to detect the gap length GAP, includes an encoder231which detects a rotation speed of the threaded cylindrical body211, and an origin-position sensor232which detects an origin position of the nut212. The encoder231includes a plurality of magnets231adisposed equidistantly in a circular manner on a ring-shaped stepped portion of the threaded cylindrical body211where the gear213is provided, and a magnetic sensor231bprovided at a position facing the magnets231a. The magnetic sensor231bincludes two hole effect ICs. These hole effect ICs are provided at a predetermined circumferential space from each other on a substrate233fixed to a portion of the vehicle body, and output pulse signals which have a 90-degree phase difference when the threaded cylindrical body211rotates. Therefore, by detecting these pulse signals, it is possible to detect an angle of rotation of the threaded cylindrical body211as well as the direction of rotation. The origin-position sensor232includes a magnet232aprovided at an end of the nut212, and a magnetic sensor232bprovided in the substrate233. When the nut212moves to the position which minimizes the gap length, the magnetic sensor232bcomes close enough to the magnet232aand outputs an origin position detection signal. The magnetic sensor232bis provided by a hole effect IC.

As shown inFIG. 6, the wheel drive motor100is also provided with an encoder130which outputs a pulse signal in accordance with the motor's number of revolutions. The encoder130includes a plurality (three, in the present preferred embodiment) of hole effect ICs. The stator110is provided with a plurality (eighteen, in the present preferred embodiment) of teeth112as described, and the hole effect ICs are provided within three gaps (each between two mutually adjacent teeth) in four consecutive teeth112. As the rotor120rotates and an N pole or an S pole of the magnet122passes across the hole effect ICs, the hole effect IC's output level is reversed. In other words, the three hole effect ICs switch their output signal level between LOW level and HIGH level each time the three phase electricity in the wheel drive motor100, i.e., Phase U, Phase V and Phase W, make their advances by an electric angle of 180 degrees, thereby outputting pulse signals while the wheel drive motor100is rotating. From these pulse signals outputted by the encoder130, it is possible to detect an electric angle, rotation speed and rotation angle, etc. of the wheel drive motor100. Further, by differentiating the rotation speed (the amount of revolutions per unit time) of the wheel drive motor100by time, it is also possible to detect rotational acceleration (angular acceleration).

Next, description will be made of a control system which controls the wheel drive motor100and the gap changing motor220, with reference toFIG. 6.

The wheel drive motor100and the gap changing motor220are controlled by the motor control unit300. The motor control unit300includes an electronic controller310provided primarily by a microcomputer which has a CPU, a ROM, a RAM, etc.; an inverter circuit350as a motor driving circuit which supplies electric power to the wheel drive motor100in accordance with commands from the electronic controller310; an H bridge circuit360as a motor driving circuit which supplies electric power to the gap changing motor220in accordance with commands from the electronic controller310; and a voltage monitor circuit370which monitors a power source voltage Vb of the DC power source3. It should be noted here thatFIG. 6is a functional block diagram which shows components of the electronic controller310categorized by function. Processes are performed when control programs for the microcomputer to be described below are executed by the electronic controller310.

The electronic controller310is connected with the main switch5; an accelerator operation-amount sensor301; the encoder130; electric current sensors302which measure an electric current of the three phases (Phase U, Phase V, Phase W) flowing through the wheel drive motor100; the gap length detector230of the gap changer200; and the voltage monitor circuit370, via an unillustrated input interface.

The accelerator operation-amount sensor301outputs an accelerator opening-degree signal in accordance with the amount of operation made to the accelerator4by a human operator, as accelerator information, to the electronic controller310. In the present preferred embodiment, the accelerator operation-amount sensor301includes a variable resistor which varies electric resistance value in accordance with the operation made to the accelerator4, and outputs a voltage across the variable resistor as the accelerator opening-degree signal.

The electronic controller310includes an electric-current command value calculator311. The electric-current command value calculator311calculates an accelerator opening-degree AO based on the accelerator opening-degree signal from the accelerator operation-amount sensor301. The accelerator opening-degree AO is a ratio of an actual amount of accelerator operation to a maximum value of the amount of accelerator operation, i.e., a ratio of the accelerator opening-degree signal which is obtained by the accelerator operation-amount sensor301to a maximum possible value of the accelerator opening-degree signal.

With the above-described arrangement, the electric-current command value calculator311calculates an electric-current command value (target electric-current value) for driving the wheel drive motor100, based on the accelerator opening-degree AO and the rotation speed n of the wheel drive motor100from the rotation speed calculator312. The rotation speed calculator312calculates the rotation speed n of the wheel drive motor100based on the pulse signal from the encoder130.

The electronic controller310supplies the wheel drive motor100, eventually, with sin wave electric power composed of three phases (Phase U, Phase V, Phase W) which have a 120-degree electric-angle phase difference from one phase to another. Also, the electronic controller310performs a power supply control in a two-phase d-q axis coordinate system. In other words, a feedback control is provided using the d-q axis coordinate system in which the d axis represents the direction of magnetization of the magnet122and the q axis represents a direction which is perpendicular to the d axis. Based on this system, the feedback control is performed to a q-axis current which is an electric component that generates torque and a d-axis current which is an electric component that reduces induced voltage.

The electric-current command value calculator311calculates an electric-current command value in the d-q axis coordinate system. A q-axis electric-current command value Iq* is calculated by multiplying an upper limit current value Iqmax by an accelerator opening-degree AO (Iqmax×AO/100). A d-axis electric-current command value Id* is obtained from a calculation using at least one of the accelerator opening-degree AO, the rotation speed n of the wheel drive motor100and the q-axis electric-current command value Iq*, as parameter information.

For example, the upper limit current value Idmax is multiplied by the accelerator opening-degree AO to obtain the d-axis electric-current command value Id* (Idmax×AO/100). Another example is to use the rotation speed n of the wheel drive motor100as the parameter information. In this case, optimum values of the Id* corresponding to various values of the rotation speed n are obtained in advance through experiments, for example, and are stored in the form of map data in the ROM, and the d-axis electric-current command value Id* is obtained from the map data, for a given rotation speed n of the wheel drive motor100. As still another example, optimum values of the Id* corresponding to parameter information of the q-axis electric-current command value Iq* may be obtained in advance through experiments, for example, and be stored in the form of map data in the ROM, so that the d-axis electric-current command value Id* is obtained from the map data, for a given q-axis electric-current command value Iq*. Also, optimum values of the Id* corresponding to parameter information of the accelerator opening-degree AO and the q-axis electric-current command value Iq* may be obtained in advance through experiments, for example, and be stored in the form of three-dimensional map data in the ROM, so that the d-axis electric-current command value Id* is obtained from the three-dimensional map data, for a given accelerator opening-degree AO and q-axis electric-current command value Iq*. It should be noted here that the d-axis electric-current command value Id* may be set to zero (Id*=0) constantly, regardless of the above-described accelerator opening-degree AO, the rotation speed n of the wheel drive motor100or the q-axis electric-current command value Iq* information.

The upper limit current value Iqmax is set in accordance with the rotation speed n of the wheel drive motor100, by using a reference map shown inFIG. 7. As shown in the reference map, the upper limit current value Iqmax is set to a constant value when the rotation speed n of the wheel drive motor100is not greater than a predetermined rotation speed n1, but when the rotation speed n of the wheel drive motor100exceeds the predetermined rotation speed n1, the value Iqmax is decreased following the increase in the rotation speed n. Therefore, the electric-current command value calculator311calculates the q-axis electric-current command value Iq* based on the accelerator opening-degree AO obtained by the accelerator operation-amount sensor301and the rotation speed n of the wheel drive motor100obtained by the rotation speed calculator312.

It should be noted here that the upper limit current value Iqmax may be set to a constant value not only when the rotation speed n of the wheel drive motor100is not greater than a predetermined rotation speed n1but also when the rotation speed n is greater than the predetermined rotation speed n1, instead of using the reference map characteristic shown inFIG. 7.

The upper limit current value Idmax is obtained as a value which satisfies the following mathematical expression (1) for example:
Imax>√{square root over (Iqmax2+Idmax2)}  (1)

Imax is a predetermined, maximum allowable current for the switching devices in the inverter circuit350.

The electric-current command value calculator311outputs the q-axis electric-current command value Iq* to a q-axis difference calculator313, and the d-axis electric-current command value Id* to a d-axis difference calculator314respectively. The q-axis difference calculator313calculates a difference value (Iq*−Iq) in the d-q axis coordinate system, i.e., a difference between the actual q-axis current value Iq which has passed through the wheel drive motor100and the q-axis electric-current command value Iq*, and outputs the obtained difference value to a q-axis proportional item integral calculator315. Also, the d-axis difference calculator314calculates a difference value (Id*−Id) in the d-q axis coordinate system, i.e., a difference between the actual d-axis current value Id which has passed through the wheel drive motor100and the d-axis electric-current command value Id*, and outputs the calculated difference value to the d-axis proportional item integral calculator316.

In this case, calculation of the actual q-axis current value Iq and the actual d-axis current value Id is performed through conversion of the actual three-phase current values (Iu, Iv, Iw) which are measured by the electric current sensor302into values on the d-q axis coordinate system by a three-phase/two-phase coordination system converter317. It should be noted here that in order to convert the three-phase coordination system into the d-q axis coordinate system, the three-phase/two-phase coordination system converter317uses an electric angle θ as electric angle information, which is a value calculated by an electric angle calculator318. The electric angle calculator318calculates the electric angle θ of the wheel drive motor100based on the pulse signal outputted from the encoder130which detects rotation of the wheel drive motor100.

The q-axis proportional item integral calculator315performs proportional item integral calculation based on the difference value (Iq*−Iq), and gives a control amount which will make the actual q-axis current value Iq follow the q-axis electric-current command value Iq*. Likewise, the d-axis proportional item integral calculator316performs proportional item integral calculation based on the difference value (Id*−Id), and gives a control amount which will make the actual d-axis current value Id follow the d-axis electric-current command value Id*. The control amounts thus calculated are outputted to a voltage-command value calculator319, and converted into a q-axis voltage-command value Vq* and a d-axis voltage-command value Vd* in the voltage-command value calculator319.

The q-axis voltage-command value Vq* and the d-axis voltage-command value Vd* are outputted to a two-phase/three-phase coordination system converter320. Using the electric angle θ calculated by the electric angle calculator318, the two-phase/three-phase coordination system converter320converts the q-axis voltage-command value Vq* and the d-axis voltage-command value Vd* into three-phase voltage-command values Vu*, Vv*, Vw*, and outputs duty control signals (PWM control signals) in accordance with these voltage-command values Vu*, Vv*, Vw* to the inverter circuit350.

The inverter circuit350, which is placed between a positive terminal and a ground terminal of the DC power source3, is a three-phase bridge circuit including six switching devices provided by, for example, MOSFETs. Each switching device has its gate supplied with the duty control signal. The inverter circuit350turns on and off each of the switching devices according to the duty ratio given by the duty control signals, and supplies the wheel drive motor100with driving electric power at a voltage in accordance with the voltage-command values Vu*, Vv*, Vw*.

As described, in the power application control of the wheel drive motor100, the electric-current command value (Iq*, Id*) is set in accordance with the amount of accelerator operation, i.e., the accelerator opening-degree AO in the d-q axis coordinate system, and to this value, the actual current (Iq, Id) which has actually passed through the wheel drive motor100is fed back for calculation of the voltage-command value (Vq*, Vd*), and then, duty control of the inverter circuit350is performed using the voltage-command values (Vq*, Vd*).

Next, a control system of the gap changing motor220in the electronic controller310will be described. It should be noted here that control processes for the gap changing motor220will be described below in detail with reference to flow charts. Therefore, only a brief description will be given here as to the functional configuration in the electronic controller310.

The electronic controller310includes a target gap length calculator321; a voltage utilization rate calculator322; a difference calculator323; a proportional item integral calculator324; a duty ratio calculator325; and an actual gap-length calculator326. The target gap length calculator321is supplied with the accelerator opening-degree signal obtained by the accelerator operation-amount sensor301; the rotation speed n of the wheel drive motor100calculated by the rotation speed calculator312; the q-axis electric-current command value Iq* calculated by the electric-current command value calculator311; the power source voltage Vb of the DC power source3detected by the voltage monitor circuit370; the voltage utilization rate Vrate calculated by the voltage utilization rate calculator322; and the actual gap-length GAP calculated by the actual gap-length calculator326. Based on these input data, the target gap length calculator321calculates a target gap length GAP* which is a target value for the gap length in the gap changing motor220, and outputs the value to the difference calculator323.

The difference calculator323calculates a difference value (GAP*−GAP) which is a difference between the target gap length GAP* and the actual gap-length GAP calculated by the actual gap-length calculator326, and outputs the value to the proportional item integral calculator324. Based on the difference value (GAP*−GAP), the proportional item integral calculator324performs proportional item integral calculation to give a control amount which will make the actual gap-length GAP follow the target gap length GAP*. The duty ratio calculator325outputs a duty ratio signal in accordance with the calculation result to an H bridge circuit360.

The H bridge circuit360, which is placed between the positive terminal and the ground terminal of the DC power source3, is a bridge circuit including four switching devices provided by, for example, MOSFETs. Each switching device has its gate supplied with the duty control signal. The H bridge circuit360turns on and off each of the switching devices according to the duty ratio given by the duty control signals, and supplies the gap changing motor220with driving electric power at a voltage in accordance with the duty ratio. Also, the H bridge circuit360switches the direction of rotation of the gap changing motor220from normal rotation to reverse rotation or vise versa in accordance with the direction of the applied electricity.

The actual gap-length calculator326calculates the actual gap-length GAP, which is an actual gap length, based on the pulse signal and the origin position detection signal from the gap length detector230(the encoder231and the origin-position sensor232) of the gap changer200, and outputs the calculation result to the target gap length calculator321.

The voltage utilization rate calculator322calculates a voltage utilization rate Vrate based on the q-axis voltage-command value Vq* and the d-axis voltage-command value Vd* calculated by the voltage-command value calculator319, and outputs the calculation results to the target gap length calculator321.

Here, description will be made of the voltage utilization rate. In the present preferred embodiment, a limit to the amount of power application to the wheel drive motor100is calculated on the basis of the d-q axis coordinate system and therefore, the voltage control command value is given as the q-axis voltage-command value Vq* and the d-axis voltage-command value Vd* on the d-q axis coordinate system. In this case, a combined vector of the q-axis voltage-command value Vq* and the d-axis voltage-command value Vd* represents the voltage-command value Vc. Therefore, the voltage-command value Vc is expressed by the following mathematical expression (2), i.e., as a square root of a sum of the q-axis voltage-command value Vq* raised to the second power (Vq*2) and the d-axis voltage-command value Vd* raised to the second power (Vd*2):
Vc=√{square root over (Vq*2+Vd*2)}  (2)

This voltage-command value Vc is used as an amplitude value of the control voltage.

As shown in the following mathematical expression (3), the voltage utilization rate Vrate is a ratio of the actually calculated voltage-command value Vc to a maximum control voltage command value (hereinafter called “maximum control voltage command value”) Vcmax for driving the wheel drive motor100:

Generally, when performing a PWM control to the inverter circuit350, a duty ratio is set based on a carrier wave W1and a modulation wave W2as shown inFIG. 8, from a magnitude correlation between the two waves. In other words, the duty ratio is set on the basis of a time ratio between a time for which the modulation wave W2is higher than the carrier wave W1and a time for which the modulation wave W2is lower than the carrier wave W1. In the present example, a maximum wave height value of the carrier wave W1is identical with a maximum wave height value of the modulation wave W2and thus, when the modulation wave W2attains the maximum wave height value of the carrier wave W1, the switching devices in the inverter circuit350is controlled substantially at 100% duty ratio. Therefore, the maximum control voltage command value Vcmax is set to the maximum wave height value of the modulation wave W2. It should be noted here that although the present example uses a case where the maximum wave height value of the carrier wave W1is identical with the maximum wave height value of the modulation wave W2, these may not necessarily be identical with each other. Also, the modulation wave W2shown inFIG. 8may have at least one of its cycle period and amplitude be increased or decreased with respect to the values shown inFIG. 8.

In the present preferred embodiment, the accelerator operation-amount sensor301and the electric-current command value calculator311function as an accelerator information obtaining device. The electric-current command value calculator311, the q-axis difference calculator313, the d-axis difference calculator314, the q-axis proportional item integral calculator315, the d-axis proportional item integral calculator316and the voltage-command value calculator319function as a voltage calculation device. The voltage utilization rate calculator322functions as a voltage utilization rate calculation device. The target gap length calculator321, the difference calculator323, the proportional item integral calculator324, the duty ratio calculator325and the H bridge circuit360function as a gap controller. The target gap length calculator321functions as an angular acceleration information obtaining device, the threshold value setting device and motor stoppage detector. The encoder130and the rotation speed calculator312function as a rotation speed information obtaining device. The electric-current command value calculator311represents an electric-current command value calculation device. The voltage monitor circuit370represents a voltage information obtaining device. The gap length detector230and the actual gap-length calculator326function as a gap length detector.

Next, description will be made for a wheel driving control process performed by the electronic controller310, with reference to a flowchart inFIG. 9.FIG. 9shows a wheel drive main control routine. The wheel drive main control routine is stored in the ROM in the electronic controller310as a control program. The wheel drive main control routine includes not only a driving control process of the wheel drive motor100but also a control process of the gap length by the gap changer200.

The main control routine starts when the main switch5provided in the electric bike1is turned on. When the main control routine starts, the electronic controller310first initializes the overall system of the motor control unit300in Step S10. For example, the microcomputer is initialized and initial diagnosis is performed to check if there is any abnormality in the system. Subsequently, the electronic controller310initializes the gap changing motor220in Step S12. This process performed in Step S12will now be described with reference to a flowchart inFIG. 10.FIG. 10shows a subroutine as the gap changing motor initialization process.

In the gap changing motor initialization routine, the electronic controller310first reads a signal from the origin-position sensor232of the gap length detector230in Step S121. Subsequently, in Step S122, a determination is made whether or not the signal read from the origin-position sensor232is an origin position detection signal. The origin-position sensor232outputs an origin position detection signal when the rotor120is at a position where the gap length GAP takes its minimum value. Therefore, if the rotor120is at the position to give the minimum gap length, the determination made by the electronic controller310in Step S122is “Yes” and the process moves to Step S129. On the other hand, if the rotor120is not at the point to give the minimum gap length, the determination in Step S122is “No”, and the process moves to Step S123.

First, description will be made for a process starting from Step S123. In Step S123, the electronic controller310determines that the rotating direction of the gap changing motor220should be a direction which will decrease the air gap. Next, in Step S124, a driving power of the gap changing motor220is determined. The driving power is set to a predetermined initialization value.

Subsequently, in Step S125, the electronic controller310outputs a drive signal (duty control signal) in accordance with the above-determined rotating direction and the drive power, to the H bridge circuit360, and drives the gap changing motor220. Next, in Step S126, a signal from the origin-position sensor232of the gap length detector230is read, and in Step S127a determination is made whether or not an origin position detection signal has been outputted from the origin-position sensor232. The electronic controller310repeats the process in Steps S125through S127until an origin position detection signal has been detected. Then, upon detection of the origin position detection signal (S127: Yes), a power application stop signal is outputted to the H bridge circuit360in Step S128to stop the operation of the gap changing motor220.

When the process in Step S128is finished, or if Step S122has determined that the rotor120is already at the position to give the minimum gap length, Step S129is performed to reset an accumulated count of the number of pulse signals outputted from the encoder231of the gap length detector230, to zero. Therefore, after the process in Step S129has been performed, it is possible to detect the axial position of the rotor120, i.e., the gap length, by counting the pulse signals (performing addition or subtraction depending on the rotating direction) outputted from the encoder231. After finishing the process in Step S129, the process leaves this subroutine, and proceeds to perform a process in Step S14of the main control routine.

Now, description will go back to the main control routine shown inFIG. 9.

In Step S14, the electronic controller310reads an accelerator opening-degree signal from the accelerator operation-amount sensor301. Subsequently, in Step S16, the electronic controller310calculates an accelerator opening-degree AO based on the accelerator opening-degree signal. The accelerator opening-degree AO is calculated by dividing the accelerator opening-degree signal from the accelerator operation-amount sensor301by a maximum value of the accelerator opening-degree signal. In Step S18, the electronic controller310calculates a q-axis electric-current command value Iq* and a d-axis electric-current command value Id* for driving the wheel drive motor100. In other words, the electronic controller310calculates a q-axis electric-current command value Iq* by multiplying an upper limit current value Iqmax, which is a value set in accordance with the rotation speed n of the wheel drive motor100by an accelerator opening-degree AO. Also, the electronic controller310calculates a d-axis electric-current command value Id*, using at least one of the accelerator opening-degree AO, the rotation speed n of the wheel drive motor100and the q-axis electric-current command value Iq* as parameter information. The above-described process in Steps S14through S18is a process performed by the electric-current command value calculator311.

In Step S20, the electronic controller310detects actual current values (Iu, Iv, Iw) of the current flowing through the wheel drive motor100by using the electric current sensor302. Next, in Step S22, calculation is made to obtain an electric angle θ of the wheel drive motor100based on pulse signals outputted from the encoder130which detects rotation of the wheel drive motor100. The process in Step S22is a process performed by the electric angle calculator318.

In Step S24, the electronic controller310calculates an actual q-axis current value Iq and an actual d-axis current value Id which are values obtained by converting the three-phase actual current values (Iu, Iv, Iw) into values on the d-q axis coordinate system. The process in Step S24is a process performed by the three-phase/two-phase coordination system converter317.

In Step S26, the electronic controller310performs a calculation process of the voltage utilization rate Vrate. The process in Step S26will be described with reference to a flowchart shown inFIG. 11.FIG. 11shows a subroutine as the voltage utilization rate calculation process.

In the voltage utilization rate calculation routine, first, Step S261is performed to calculate a proportional item Pvd of the d-axis voltage-command value. The proportional item Pvd of the d-axis voltage-command value is obtained by the following mathematical expression (4), by first obtaining a difference value (Id*−Id) between the d-axis electric-current command value Id* and the actual d-axis current value Id, and then multiplying the obtained value by a coefficient Kpd:
Pvd=Kpd×(Id*−Id)  (4)

Next, in Step S262, calculation is made to obtain an integral item Ivd of the d-axis voltage-command value. The integral item Ivd of the d-axis voltage-command value is obtained by the following mathematical expression (5), by first obtaining a difference value (Id*−Id) between the d-axis electric-current command value Id* and the actual d-axis current value Id, then multiplying the obtained value by a coefficient Kid, and then adding a previous integral item value Ivdold to the obtained value:
Ivd=Kid×(Id*−Id)+Ivdold(5)

The voltage utilization rate calculation routine is a part of the main control routine and is repeated periodically in a predetermined interval. Therefore, the previous value Ivdold is a value of the integral item Ivd calculated in the previous cycle.

Next, in Step S263, the d-axis voltage-command value Vd* is calculated. The d-axis voltage-command value Vd* is obtained by adding the proportional item Pvd to the integral item Ivd (Vd*=Pvd+Ivd).

Then, in Step S264, the process determines whether or not an absolute value |vd*| of the d-axis voltage-command value Vd* is greater than a predetermined upper limit value Vdmax. If the value is greater than the upper limit value Vdmax (S264: Yes), then Step S265is performed to put an upper-limit value limitation on the d-axis voltage-command value Vd* so that its absolute value is equal to the upper limit value Vdmax. On the other hand, if the absolute value |Vd*| of the d-axis voltage-command value Vd* is not greater than the upper limit value Vdmax (S264: No), Step S265is skipped.

Next, in Step S266, the electronic controller310calculates a proportional item Pvq of the q-axis voltage-command value. The proportional item Pvq of the q-axis voltage-command value is obtained by the following mathematical expression (6), by first obtaining a difference value (Iq*−Iq) between the q-axis electric-current command value Iq* and the actual q-axis current value Iq, and then multiplying the obtained value by a coefficient Kpq:
Pvq=Kpq×(Iq*−Iq)  (6)

Next, in Step S267, the integral item Ivq of the q-axis voltage-command value is calculated. The integral item Ivq of the q-axis voltage-command value is obtained by the following mathematical expression (7), by first obtaining a difference value (Iq*−Iq) between the q-axis electric-current command value Iq* and the actual q-axis current value Iq, then multiplying the obtained value by a coefficient Kiq, and then adding the obtained value to a previous value Ivqold of the integral item:
Ivq=Kiq×(Iq*−Iq)+Ivqold(7)

Then, in Step S268, the q-axis voltage-command value Vq* is calculated. The q-axis voltage-command value Vq* is obtained by adding the proportional item Pvq and the integral item Ivq (Vq*=Pvq+Ivq).

Subsequently, in Step S269, a determination is made whether an absolute value |vq*| of the q-axis voltage-command value Vq* is greater than a predetermined upper limit value Vqmax. If the value is greater than the upper limit value Vqmax (S269: Yes), Step S270is performed to put an upper-limit value limitation on the q-axis voltage-command value Vq* so that its absolute value |Vq*| is equal to the upper limit value Vqmax. On the other hand, if the absolute value |vq*| of the q-axis voltage-command value Vq* is not greater than the upper limit value Vdmax (S269: No), Step S270is skipped.

The process in Steps S261through S270are a process performed by the above-described d-axis difference calculator314, the d-axis proportional item integral calculator316, the q-axis difference calculator313, the q-axis proportional item integral calculator315and the voltage-command value calculator319.

In Step S271, the electronic controller310calculates a voltage utilization rate Vrate. The voltage utilization rate Vrate is calculated by the above-described mathematical expression (3) based on the d-axis voltage-command value Vd* calculated in Step S265, the q-axis voltage-command value Vq* calculated in Step S270, and the maximum voltage-command value Vcmax. After the voltage utilization rate Vrate is calculated, the process leaves this subroutine and proceeds to the process in Step S28of the main control routine inFIG. 9.

In Step S28of the main control routine, the electronic controller310converts the q-axis voltage-command value Vq* and the d-axis voltage-command value Vd* into three-phase voltage-command values Vu*, Vv*, Vw*. This process is a process performed by the two-phase/three-phase coordination system converter320. Subsequently, in Step S30, duty control signals (PWM control signals) which are signals in accordance with the voltage-command values Vu*, Vv*, Vw* are outputted to the inverter circuit350. Therefore, the wheel drive motor100is supplied with electric power in accordance with the voltage-command values Vu*, Vv*, Vw*.

In Step S32, the electronic controller310performs a gap length control process. Description will now cover the gap length control process in Step S32, with reference to a flowchart shown inFIG. 12.FIG. 12shows a subroutine as the gap length control process.

In the gap length control routine, the electronic controller310first determines in Step S321whether or not the wheel drive motor100is rotating. The determination is made based on whether or not the rotation speed n from the rotation speed calculator312has a value of zero. If the wheel drive motor100is not rotating (S321: No), the target gap length GAP* is set to the minimum gap length in Step S322. This is to set the output characteristic to a high-torque low-rotation-speed characteristic so that a high torque will be obtained when the wheel drive motor100is started. In this case, the setting for the output characteristic is like Characteristic1inFIG. 28, for example. The process in Step S321is a process performed by the target gap length calculator321.

On the other hand, if the wheel drive motor100is rotating (S321: Yes), Step S323checks whether or not the accelerator opening-degree AO is greater than a predetermined opening-degree A1(for example, 0.7). If the accelerator opening-degree AO is greater than the opening-degree A1, the process moves to a gap length setting process (which represents the first control mode in the present preferred embodiment) which is based on the voltage utilization rate Vrate and starts from Step S324. On the other hand, if the accelerator opening-degree AO is not greater than the predetermined opening-degree A1, the process proceeds to a gap length setting process (which represents the second control mode in the present preferred embodiment) which is not based on the voltage utilization rate Vrate and starts from Step S328.

First, description will be made for the case where the accelerator opening-degree AO is greater than the predetermined opening-degree A1.

In Step S324, the electronic controller310calculates an angular acceleration of the wheel drive motor100, and sets threshold values R1, R2based on the angular acceleration. The angular acceleration is obtained from the rotation speed n (the amount of rotation per unit time) of the wheel drive motor100which is obtained from a pulse signal string outputted by the encoder130provided in the wheel drive motor100through differentiation of the value by time. The threshold values R1, R2are utilized to adjust the gap length in accordance with the voltage utilization rate Vrate as will be explained below in Step S325. The threshold values R1, R2are set by using a reference map shown inFIG. 13, for example. As shown in the reference map, the threshold values R1, R2are decreased to smaller values as the angular acceleration increases.

When the output characteristic of the wheel drive motor100is changed, there would be no problem if it is possible to increase/decrease the gap length right at the moment as the wheel drive motor100makes acceleration/deceleration. However, in reality, it takes time for the gap changing motor220to adjust the gap length. The threshold values R1, R2are provided for this reason, to enable to start the gap changing motor220earlier when the angular acceleration of the wheel drive motor100is large.

Subsequently, Step S325determines the size of the voltage utilization rate Vrate. In other words, a determination is made whether or not the voltage utilization rate Vrate is greater than the threshold value R1; smaller than the threshold value R2; or not smaller than the threshold value R2and not greater than the threshold value R1. The voltage utilization rate Vrate utilized here is the value calculated in Step S271. If the voltage utilization rate Vrate is smaller than the threshold value R2(Vrate<R2), Step S326sets the target gap length GAP* to a value which is obtained by subtracting a unit gap length GAP0from the current target gap length GAPc* (GAP*=GAPc*−GAP0). In other words, the target gap length GAP* is shortened.

If the voltage utilization rate Vrate is greater than the threshold value R1(Vrate>R1), Step S327sets the target gap length GAP* to a value which is obtained by adding the unit gap length GAP0to the current target gap length GAPc* (GAP*=GAPc*+GAP0). In other words, the target gap length GAP* is made longer.

If the voltage utilization rate Vrate is not smaller than the threshold value R2and not greater than the threshold value R1(R2≦Vrate≦R1), no setting change is made to the target gap length GAP*, and Step S331is performed to read pulse signal counting information which is outputted from the encoder231of the gap changer200. The pulse signal outputted from the encoder231is counted by an unillustrated counting routine, and a value of the count is read in Step S331. The value of the count is a value obtained by adding or subtracting the number of pulse signals depending on the rotating direction of the gap changing motor220.

Subsequently, in Step S332, the pulse signal count value is accumulated, and based on the accumulated value, the amount of rotation (total amount of rotation) of the gap changing motor220from the origin position is calculated. The amount of rotation of the gap changing motor220corresponds to the actual gap-length GAP. The process in Steps S331through S332corresponds to the process performed by the actual gap-length calculator326inFIG. 6. Next, in Step S333, a stop signal to stop the gap changing motor220is outputted to the H bridge circuit360, and then the process leaves this subroutine for a while.

If Step S323determines that the accelerator opening-degree AO is not greater than the predetermined opening-degree A1(AO≦A1), the gap length setting based on the voltage utilization rate Vrate is not performed, but Steps S328through S330are performed to set the target gap length GAP*.

Here, description will first cover reasons why the gap length setting based on the voltage utilization rate Vrate is not performed if the accelerator opening-degree AO is not greater than the predetermined opening-degree A1. For example, take a case where the electric bike1is decelerated on the run. The driver moves the accelerator4to the closing direction and therefore, the accelerator opening-degree AO and the control voltage (voltage-command value Vc) of the wheel drive motor100decrease, and thus the voltage utilization rate Vrate decreases. In this moment, the wheel drive motor100has an excess in its torque output capacity as compared to the demanded torque. Setting the target gap length GAP* based on the voltage utilization rate Vrate under such a situation will shorten the gap length (see Step S326), resulting in an unfavorable large iron loss. The gap length also affects copper loss in the wheel drive motor100. For these reasons, in this preferred embodiment, the gap length setting based on the voltage utilization rate Vrate is not performed when the accelerator opening-degree AO is not greater than the predetermined opening-degree A1, but the target gap length GAP* is set as follows so that iron loss and copper loss in the wheel drive motor100will be reduced.

First, in Step S328, the electronic controller310obtains the power source voltage Vb from the voltage monitor circuit370. Next, in Step S329, a target gap length setting map appropriate for the obtained power source voltage Vb is selected.FIG. 14shows an example of such a target gap length setting map. The target gap length setting map allows setting of the target gap length GAP* from two parameters, i.e., the motor rotation speed n and the motor electric-current command value (q-axis electric-current command value Iq*). As the target gap length setting map, a plurality of maps are prepared each for a different characteristic suitable for a given power source voltage Vb. Therefore, in Step S329, one map is selected from a plurality of target gap length setting maps, in accordance with the obtained power source voltage Vb. Subsequently, in Step S330, the target gap length GAP* is set by using the selected target gap length setting map. The process in Steps S321through S330corresponds to the process performed by the target gap length calculator321inFIG. 6which has been described above.

Description will now cover the target gap length setting map shown inFIG. 14. In this map, the horizontal axis represents the rotation speed n of the wheel drive motor100whereas the vertical axis represents the motor electric-current command value (q-axis electric-current command value Iq*). The map is for setting the target gap length GAP* in order to provide optimum control of iron loss and copper loss in the wheel drive motor100. In this map, a region on the right side, enclosed by a broken line, is a controllable region Af where motor impedance is taken into account whereas a region on the left side, enclosed by a broken line, is an optimum control region Ac where iron loss and copper loss will be optimized. In the controllable region Af, Lines Lf1through Lf12are drawn to indicate borders of the target gap length GAP*. In the optimum control region Ac, Lines Lc1through Lc8are drawn to indicate borders of the target gap length GAP*.

Description will now cover how the target gap length setting map is used in setting the target gap length GAP*. A region enclosed by Line Lf1and Line Lc1(a region on the left of Line Lf1and above Line Lc1) is a 1-mm region where the target gap length GAP* is set to 1 mm. A region enclosed by Line Lf2and Line Lc2(a region on the left of Line Lf2and above Line Lc2) excluding the 1-mm region is a 2-mm region where the target gap length GAP* is set to 2 mm. Likewise, a region enclosed by Line Lf3and Line Lc3excluding the 1-mm and the 2-mm regions is a 3-mm region where the target gap length GAP* is set to 3 mm. In other words, a region enclosed by Line Lfn (n=1 through 12) and Line Lcn (n=1 through 8) excluding the 1-mm through (n−1) mm regions is an n-mm region where the target gap length GAP* is set to n mm. For a region where the motor rotation speed n is greater than in the 8-mm region, or for a region where the motor electric-current command value is smaller than in the 8-mm region, the target gap length GAP* is set to a value greater than 8 mm and having a border given by one of Lines Lf8through Lf12in the controllable region Af.

Following the target gap length setting map, the target gap length GAP* increases with increase in the rotation speed n of the wheel drive motor100. Also, the target gap length GAP* increases with decrease in the motor current (q-axis electric-current command value Iq*).

The target gap length setting map is developed in advance, through experiments, for example, by finding optimum gap length setting values which reduce iron loss and copper loss. The optimum gap length in the controllable region Af varies with the power source voltage Vb of the DC power source3which serves as a power source for the inverter circuit350. In the present preferred embodiment, therefore, a plurality of target gap length setting maps each indicating optimum gap length values for a given power source voltage Vb are stored in the ROM. Under such an arrangement, a target gap length setting map appropriate for a given power source voltage Vb is selected in Step S329. In this case, one map is selected from a plurality of target gap length setting maps, and then the target gap length GAP* is obtained based on the selected map. There may be a case where two consecutive maps are selected from a plurality of target gap length setting maps, and the target gap length GAP* is obtained through interpolation of the two maps.

Setting is made as shown inFIG. 15for example, so that Lines Lf1through Lf12are shifted on the higher side of the rotation speed n of the wheel drive motor100as the power source voltage Vb becomes higher. In other words, if the same principle is stated from the opposite perspective, setting is made so that lines Lf1through Lf12are shifted on the lower side of the rotation speed n of the wheel drive motor100as the power source voltage Vb becomes lower. Therefore, the target gap length GAP* setting region shifts to the right or left on the characteristic chart depending on the power source voltage Vb. With this arrangement, a correction is made by shortening the target gap length GAP* as the power source voltage Vb increases, even if the rotation speed n of the wheel drive motor100and the motor current (q-axis electric-current command value Iq*) are the same. Likewise, a correction will also be made in the reverse way to make the target gap length GAP* longer when the power source voltage Vb decreases.

Note that inFIG. 15, the gradient of Lines Lf1through Lf12in the controllable region Af is decreased with an increase in the power source voltage Vb. However, the gradient of Lines Lf1through Lf12may be increased with an increase in the power source voltage Vb as shown inFIG. 16depending on the motor characteristic. Also, it is not necessary to make a shift or change the gradient in all of the Lines-Lf1through Lf12. Changes may be made to the characteristic only partially.

Description will now return to the gap length control routine inFIG. 12.

After the process from Steps S321through S330and the target gap length GAP* has been set, next, the electronic controller310proceeds to Step S334and reads pulse signal counting information outputted from the encoder231of the gap changer200. Subsequently, in Step S335, the pulse signal count value is accumulated, and based on the accumulated value, the amount of rotation (total amount of rotation) of the gap changing motor220from the origin position is calculated. The amount of rotation of the gap changing motor220corresponds to the actual gap-length GAP. The process in Steps S334through S335is the same as the process in Steps S331and S332described above.

Subsequently, in Step S336, the electronic controller310calculates a drive power for the gap changing motor220, i.e., a duty ratio of the duty control signals to be outputted to the H bridge circuit360. In this case, a proportional item integral calculation is performed based on a difference value (GAP*−GAP), i.e., a difference between the target gap length GAP* calculated in Steps S321through S330and the actual gap length GAP calculated in Step S335, and then the duty ratio is set in accordance with the calculation result.

Subsequently, in Step S337, a determination is made whether or not the rotor120of the wheel drive motor100has reached a position which reaches the target gap, i.e., whether or not the actual gap-length GAP calculated in Step S335has become equal to the target gap length GAP*.

The process in Steps S336and S337corresponds to the process performed by the difference calculator323, the proportional item integral calculator324and the duty ratio calculator325inFIG. 6.

If the actual gap-length GAP is equal to the target gap length GAP* (S337: Yes), Step S333is performed to output a stop signal to the H bridge circuit360for stopping the gap changing motor220, and then the process leaves this subroutine for a while.

On the other hand, if the actual gap-length GAP is not equal to the target gap length GAP* (S337: No), Step S338determines whether or not the specified rotating direction of the gap changing motor220is the direction to decrease the air gap. In other words, the process checks whether or not the target gap length GAP* is smaller than the actual gap-length GAP. If the specified rotating direction of the gap changing motor220is the direction to decrease the air gap (S338: Yes), Step S339is performed to output a duty control signal to the H bridge circuit360in a power application direction for decreased air gap, to drive the gap changing motor220. On the other hand, if the specified rotating direction of the gap changing motor220is the direction to increase the air gap (S338: No), Step S340is performed to output a duty control signal to the H bridge circuit360in a power application direction for increased air gap to drive the gap changing motor220. The duty control signal outputted in Steps S339and S340is the duty control signal of the duty ratio which was calculated in Step S336above.

After outputting the drive signal or stop signal to the H bridge circuit360, the electronic controller310leaves the gap length control routine.

According to the gap length control routine, the gap length is adjusted based on the voltage utilization rate Vrate if the accelerator opening-degree AO is greater than the predetermined opening-degree A1. In other words, if the voltage utilization rate Vrate is greater than the threshold value R1, the gap length is made longer (S327), but if the voltage utilization rate Vrate is smaller than the threshold value R2, the gap length is shortened (S326), whereas if the voltage utilization rate Vrate is not smaller than the threshold value R2and not greater than the threshold value R1, the gap length is not changed (S333). Therefore, it is possible to switch the motor output characteristic among a plurality of settings such as Characteristic1through Characteristic3as shown inFIG. 28. It should be noted here thatFIG. 28shows only three settings for the sake of easier understanding. However, in the present preferred embodiment, switching is made in a greater number of settings. In this arrangement, the gap length (motor output characteristic) is switched in accordance with the voltage utilization rate Vrate and therefore, and an appropriate switching timing is assured.

If the accelerator opening-degree AO is not greater than the predetermined opening-degree A1, the gap length adjustment based on the voltage utilization rate Vrate is not performed, and the gap length is adjusted on the basis of the target gap length setting map. Therefore, it is possible to reduce iron loss and copper loss in the wheel drive motor100.

Now, an example of the gap length adjustment which uses a target gap length setting map will be described for a case Of stopping a running electric bike1.

As shown inFIG. 17, assume that the state of control (rotation speed n of the wheel drive motor100, q-axis electric-current command value Iq*) of the wheel drive motor100is at Point A, and then, assume that the accelerator opening-degree AO is reduced to zero. In this instance, the q-axis electric-current command value Iq* decreases to zero, but the wheel drive motor100does not stop instantly. Therefore, the target gap length GAP* stays at 9 mm (Point B) for a while. The wheel2continues to turn for some time and then stops. Therefore, the relationship between the target gap length GAP* and the rotation speed n of the wheel drive motor100shifts from Point B to Point C as the rotation speed n decreases. In this way, the target gap length GAP* increases from the initial 6 mm (Point A) to 9 mm (Point B) for a while, and then decreases to the smallest setting of 1 mm (Point C). Therefore, during the period when the rotation speed n of the wheel drive motor100is high, a long target gap length GAP* is set to reduce iron loss and thereafter, the target gap length GAP* is made shorter along with decrease in the rotation speed n of the wheel drive motor100.

Another example will be explained with reference toFIG. 18.

Assume that the state of control (rotation speed n of the wheel drive motor100, q-axis electric-current command value Iq*) of the wheel drive motor100is at Point A. Then, assume that only the rotation speed n of the wheel drive motor100increases while the q-axis electric-current command value Iq* is constant. The state of control shifts to Point B. Therefore, the target gap length GAP* which is based on the target gap length setting map is increased from 1 mm (Point A) to 6 mm (Point B). This reduces iron loss. From this state, if the human driver wants a strong torque and increases the accelerator opening-degree AO, then the q-axis electric-current command value Iq* increases. In this instance, the state of control of the wheel drive motor100shifts from Point B to Point C. Therefore, the target gap length GAP* which is set by using the target gap length setting map is decreased from 6 mm (Point B) to 1 mm (Point C). The decreased gap length allows the same torque at a lower current value to be obtained. This makes it possible to increase the torque of wheel drive motor100without increasing too much in copper loss.

Copper loss is an electric power loss component caused by the electric resistance in the coil. With i being the value of current flowing through the coil, and r being the electric resistance value of the coil, copper loss is expressed as i2×r. On the other hand, in the motor characteristic, even if the value of current applied to the coil is the same, the torque becomes greater as the gap length becomes shorter, i.e., the torque becomes smaller as the gap length becomes longer. Therefore, if the gap length GAP is made small with respect to the demanded torque, only a small current value i is necessary to apply to the coil113, and this makes it possible to reduce copper loss. For this reason, it is possible to reduce copper loss in the example inFIG. 18, by decreasing the target gap length GAP* from 6 mm (Point B) to 1 mm (Point C).

As described above, in the present preferred embodiment, the target gap length setting map is based on results of experiments conducted with comprehensive evaluation of a balance between the driver's desire (torque) and influence from iron loss and copper loss.

When the gap length control routine shown inFIG. 12is completed, the electronic controller310advances the process to Step S34of the main control routine shown inFIG. 9. In Step S34, a determination is made whether or not the main switch5has been turned off. If it has not been turned off (S34: No), the process in Steps S14through S34are repeated. Then, when the human driver turns off the main switch5, a determination given in Step S34turns to “Yes”, whereupon the wheel drive main control routine comes to an end.

The present preferred embodiment provides the following advantages described below.

A motor output characteristic is varied by adjusting the gap length GAP in the wheel drive motor100thereby increasing/decreasing the magnetic flux which flows between the magnet122of the rotor120and the teeth112of the stator110. Therefore, it is possible to drive the wheel drive motor100efficiently from a low-speed range through a high-speed range and to obtain a good vehicle characteristic. Also, it is possible to change the gap length GAP easily by changing a relative positional relationship between the rotor120and the stator110in the axial direction of the rotation shaft123.

Since the gap length GAP is adjusted based on the voltage utilization rate Vrate, it is possible to change the output characteristic always at an appropriate timing without being affected by individual variability or operating environment of the wheel drive motor100. Therefore, it is possible to sufficiently utilize the potential capability of the wheel drive motor100. Also, the present invention makes it possible to eliminate such a problem as excessive decrease in output torque caused by a delay in changing the output characteristic when the electric vehicle1is accelerating, and to reduce a deteriorated drive feeling.

In the gap length control, the gap length GAP is increased to achieve a characteristic capable of obtaining high rotation speed if the voltage utilization rate Vrate has exceeded the threshold value R1; the gap length GAP is decreased to achieve a characteristic capable of obtaining high torque if the voltage utilization rate Vrate has become smaller than the threshold values R2; and no adjustment is made to the gap length GAP, i.e., no change is made to the output characteristic if the voltage utilization rate Vrate is not smaller than the threshold values R2and not greater than the threshold values R1(Steps S325through327, and S333inFIG. 12). As described above, it is possible to control the output characteristic of the wheel drive motor100easily. Also, since no adjustment is made to the gap length GAP when the voltage utilization rate Vrate is not smaller than the threshold value R2and not greater than the threshold value R1, it is possible to prevent such a problem of hunting in the control, achieving stable vehicle drive.

In the gap length control, the threshold values R1, R2are decreased with an increase in the angular acceleration of the wheel drive motor100(seeFIG. 13). This makes it possible to prevent or minimize a delayed response of the gap changer200.

For example, when accelerating the electric bike1, the control voltage to the wheel drive motor100is increased to increase the motor rotation speed n. However, if there is a delay in the response from the gap changer200to this motor control, switching of the output characteristic is delayed, causing a risk that it will become impossible to increase the motor rotation speed n quickly. Thus, the threshold values R1, R2are deceased in such a case as this, to allow the voltage utilization rate Vrate to surpass the threshold value R1at an earlier timing so as to increase the gap length GAP with the acceleration of the wheel drive motor100.

On the other hand, when decelerating the electric bike1, the control voltage to the wheel drive motor100is decreased to decrease the motor rotation speed n. In this case again, however, if there is a delay in the response from the gap changer200, it becomes impossible to obtain an optimum motor output characteristic. In this case, the angular acceleration of the wheel drive motor100takes a negative value, so the threshold values R1, R2are increased to allow the voltage utilization rate Vrate to become smaller than the threshold value R2at an earlier timing so as to decrease the gap length GAP with the acceleration of the wheel drive motor100.

When the wheel drive motor100is not rotating, i.e., when the electric bike1is stopped, the gap length GAP is set to a minimum value to provide a high-torque low-rotation-speed characteristic. Therefore, a high torque is obtained when the electric bike1is started the next time, offering good acceleration (Step S322inFIG. 12).

During a period when the accelerator opening-degree AO is not greater than the predetermined opening-degree A1(0.7 for example), a gap length setting based on the voltage utilization rate Vrate is not performed. Instead, a setting of the target gap length GAP* is performed using a target gap length setting map, based on the rotation speed n of the wheel drive motor100and on the motor current (q-axis electric-current command value Iq*). Therefore, it is possible to achieve good reduction in iron loss and copper loss in the wheel drive motor100(Steps S323, and S328through S330inFIG. 12). For example, the target gap length GAP* is increased with an increase in the rotation speed n of the wheel drive motor100. This eliminates a case where the induced voltage in the wheel drive motor100exceeds the power source voltage Vb of the DC power source3. This makes it possible to control the wheel drive motor100in a high-rotating-speed range. Also, the target gap length GAP* is increased with a decrease in the motor current (q-axis electric-current command value Iq*). This makes it possible to reduce iron loss.

Since the target gap length GAP* is made longer with decrease in the power source voltage Vb, there is no case where the induced voltage in the wheel drive motor100exceeds the power source voltage Vb of the DC power source3. This makes it possible to maintain control of the wheel drive motor100.

In the gap length control, the detected actual gap-length GAP is fed back, and drive control of the gap changing motor220is performed by using proportional item integral calculation (PI) based on the difference (gap*−gap) between the actual gap-length GAP and the target gap length GAP*. Therefore, the response is greatly improved (Step S334inFIG. 12).

Next, description will be made for an electric bike1aaccording to a second preferred embodiment of the present invention.

Referring toFIG. 1, the electric bike1adiffers from the electric bike1according to the first preferred embodiment in that the wheel driving apparatus10in the electric bike1is replaced by a wheel driving apparatus10a. Further, referring also toFIG. 2, the wheel driving apparatus10adoes not have the gap length detector230used in the gap changer200of the wheel driving apparatus10, and uses a motor control unit400in place of the motor control unit300. Otherwise, the wheel driving apparatus10apreferably is essentially the same as in the electric bike1, so by utilizing the same reference symbols as in the electric bike1, repetitive description will be skipped.

FIG. 19is a system configuration diagram of the motor control unit400.

The motor control unit400includes an electronic controller410provided primarily by a microcomputer; an inverter circuit350as a motor driving circuit which supplies electric power to the wheel drive motor100in accordance with commands from the electronic controller410; an H bridge circuit360as a motor driving circuit which supplies electric power to the gap changing motor220in accordance with commands from the electronic controller410; and a voltage monitor circuit370.

The electronic controller410provides drive control to the wheel drive motor100and to the gap changing motor220independently. A control system for the wheel drive motor100is the same as in the first preferred embodiment, so no more description will be given.

The electronic controller410includes a gap-change command section421, a voltage utilization rate calculator322and a duty-signal output section425, in order to provide the drive control to the gap changing motor220. The gap-change command section421is supplied with the rotation speed n of the wheel drive motor100calculated by the rotation speed calculator312; and the voltage utilization rate Vrate calculated by the voltage utilization rate calculator322. Based on these, the gap-change command section421obtains a power-application/power-stoppage command signal and a rotating direction command signal of the gap changing motor220, and outputs these to the duty-signal output section425. The process performed by the gap-change command section421will be described below in detail with reference to a flowchart.

The duty-signal output section425outputs a signal of a constant duty-ratio to the H bridge circuit360based on the power-application/power-stoppage command signal and the rotating direction command signal inputted from the gap-change command section421. The gap changing motor220is driven in accordance with the outputted signal.

In the present preferred embodiment, the gap-change command section421, the duty-signal output section425and the H bridge circuit360function as a gap controller. The gap-change command section421functions as an angular acceleration information obtaining device, a threshold value setting device and a motor stoppage detector. Other configuration relationships are preferably the same as in the first preferred embodiment.

Next, a wheel driving control process performed by the electronic controller410will be described.

A wheel drive main control routine in the second preferred embodiment shown inFIG. 9differs from that of the first preferred embodiment in an initialization process of the gap changing motor220in Step S12and in a gap length control process in Step S32. Therefore, description will be made only for these differences.FIG. 20shows a subroutine as the initialization process of the gap changing motor220in Step S12.FIG. 21shows a subroutine as the gap length control process in Step S32.

First, the initialization process of the gap changing motor220will be described with reference toFIG. 20.

When the gap changing motor initialization routine is started, the electronic controller410first resets a time count (time count=0) of a time counter in Step S1201. The time counter is a software timer. Next, in Step S1202, the rotating direction of the gap changing motor220is determined to be a direction for decreasing the gap. Subsequently, in Step S1203, the driving power to drive the gap changing motor220is determined. The driving power is set to a predetermined initialization value, but the value may be the same as a driving power which is used in the gap length control to be described below.

Subsequently, in Step S1204, the electronic controller410outputs a drive signal (duty control signal) in accordance with the rotating direction and the driving power as determined above, to the H bridge circuit360, and drives the gap changing motor220. Next, in Step S1205, the time counter makes a count up, and in Step S1206, a determination is made whether or not the elapsed time counted by the time counter has reached a set time. Driving of the gap changing motor220is continued until the elapsed time has reached the set time. It should be noted here that even if the rotor120has returned the origin position before the elapsed time has reached the set time, the rotor120is held at the origin position by a mechanical stopper (two end portions of the anti-rotation groove212bin the nut212) of the gap changer200. When the set time is reached (S1206: Yes), a power application stop signal is outputted to the H bridge circuit360in Step S1207, and the gap changing motor220is stopped. It is preferable that the set time is an amount of time necessary for the rotor120in the vehicle driving motor100to return from a position which gives the maximum gap length to a position which gives the minimum gap length (origin position).

With this arrangement, the rotor120in the vehicle driving motor100is reliably returned to the origin position where the minimum gap length is attained. When the rotor120in the vehicle driving motor100has been returned to the origin position, the gap changing motor initialization routine comes to an end, and the process moves to Step S14in the main control routine.

Next, the gap length control process in Step S32of the main control routine will be described with reference toFIG. 21.

When the gap length control routine starts, the electronic controller410first determines whether or not the wheel drive motor100is rotating in Step S3201. The determination is made based on whether or not the rotation speed n from the rotation speed calculator312has a value of zero. Here, description will start with a case where the wheel drive motor100is rotating (S3201: Yes).

If the wheel drive motor100is rotating, a flag F is set to “1” in Step S3202. At the time when the control routine is started, the flag F is in “0” state. Subsequently, in Step S3203, the threshold values R1, R2are set based on the angular acceleration of the wheel drive motor100. This process is the same as the process in Step S324according to the previous preferred embodiment. Next, in Step S3204, a determination is made as to the size of the voltage utilization rate Vrate. In other words, a determination is made whether or not the voltage utilization rate Vrate is greater than the threshold value R1; smaller than the threshold value R2; or not smaller than the threshold value R2and not greater than the threshold value R1. A value used as the voltage utilization rate Vrate is the value calculated in the above-described Step S271.

If the voltage utilization rate Vrate is smaller than the threshold value R2(Vrate<R2), the electronic controller410performs Step S3205, and sets the rotating direction of the gap changing motor220to the direction for decreasing the gap length in the wheel drive motor100. If the voltage utilization rate Vrate is greater than the threshold value R1(Vrate>R1), the electronic controller410performs Step S3206, and sets the rotating direction of the gap changing motor220to the direction for increasing the gap length in the wheel drive motor100. If the voltage utilization rate Vrate is not smaller than the threshold value R2and not greater than the threshold value R1(R2≦Vrate≦R1), Step S3207is performed to clear a count value of a one-way continuous power application time counter to zero. Then, Step S3208is performed to output a duty control signal to the H bridge circuit360for stopping the gap changing motor220. Therefore, the gap changing motor220is stopped if the voltage utilization rate Vrate is not smaller than the threshold value R2and not greater than the threshold value R1. It should be noted here that the one-way continuous power application time counter is a software timer which measures the amount of time for which the gap changing motor220is continuously supplied with power in the same rotating direction. The count value in the timer is cleared to zero each time the rotating direction of the gap changing motor220is switched as well as each time a stop command signal is outputted.

After the rotating direction of the gap changing motor220is set in Step S3205or in Step S3206, Step S3209determines whether or not the rotating direction is the same as the previous rotating direction. This gap length control routine is part of the main control routine and is repeated in a predetermined short cycle. Therefore, in Step S3209, a comparison is made between the rotating direction in the previous cycle and the rotating direction which was set in the current cycle to see if the two directions are the same. If the rotating direction of the gap changing motor220is reverse from the previous rotating direction, Step S3210is performed to clear the count value in the one-way continuous power application time counter. If the rotating direction of the gap changing motor220is the same as the previous rotating direction, Step S3210is skipped.

Subsequently, in Step S3211, the electronic controller410determines whether or not the continuous power application time measured by the one-way continuous power application time counter has exceeded the set time. If the continuous power application time has not exceeded the set time (S3211: No), Step S3212is performed to count up the one-way continuous power application time counter, and then in Step S3213, a drive signal (duty control signal) in accordance with the rotating direction is outputted to the H bridge circuit360. As a result, the gap changing motor220rotates in the set rotating direction to change the gap length in the wheel drive motor100.

On the other hand, if Step S3211determines that the continuous power application time has exceeded the set time, Step S3207is performed to clear the count value in the one-way continuous power application time counter, and thereafter, Step S3208is performed to output a duty control signal for stopping the gap changing motor220to the H bridge circuit360.

Since the present preferred embodiment does not include the gap length detector230, it is not possible to accurately detect the axial position of the rotor120in the wheel drive motor100, i.e., the gap length. Instead, the one-way continuous power application time counter is utilized to limit time. This makes it possible to prevent such a problem that the rotor120has come to the position which gives the maximum or minimum gap length yet power application to the gap changing motor220continues. In this case, it is preferable that the set time, which is an amount of time that limits the continuous power application time, is an amount of time necessary for the rotor120in the vehicle driving motor100to move from the position which gives the maximum gap length to the position which gives the minimum gap length (origin position). This makes sure that the gap length GAP is reliably adjusted within the range from the minimum gap length to the maximum gap length, and makes sure to prevent the problem of excessive power application to the gap changing motor220.

After the duty control signal is outputted to the H bridge circuit360in Step S3208or Step S3213, the electronic controller410leaves the gap length control routine for a while and proceeds to Step S34in the main control routine.

Description will now cover a case where Step S3201has determined that the wheel drive motor100is not rotating. In this case, Step S3214first checks whether or not the flag F is set to “1”. If the flag F is set to “1” (S3214: Yes), Step S3215checks whether or not the continuous power application time measured by the one-way continuous power application time counter has exceeded the set time.

If the continuous power application time has not exceeded the set time (S3215: No), Step S3216is performed to set the rotating direction of the gap changing motor220to the direction for decreasing the gap length in the wheel drive motor100, and then the process proceeds to Step S3212. Therefore, the gap length GAP in the wheel drive motor100is decreased by the gap changing motor220. This is to set the output characteristic of the wheel drive motor100to a high-torque low-rotation-speed characteristic so that a high torque will be obtained when starting the wheel drive motor100.

The above-described tasks of applying power to the gap changing motor220and checking the power application continuation time is repeated while the electric bike1is stopped. When the power application continuation time has reached the set time, the rotor120in the wheel drive motor100is at the position which gives the minimum gap length. Thus, when it is determined that the power application continuation time has exceeded the set time (S3215: Yes), the electronic controller410performs Step S3217to set the flag F to “0”, performs the above-described clearing process of the one-way continuous power application time counter (S3207) and the stopping process of the gap changing motor220(S3208), and then leaves the gap length control routine. Hence, thereafter, checking on the flag F in Step S3214will result in “No” as long as the vehicle continues to be in the stopped state, and thus, power application to the gap changing motor220is prevented.

According to this preferred embodiment, the gap length GAP is controlled based on the voltage utilization rate Vrate as in the first preferred embodiment. Therefore, it is possible to change the motor output characteristic always at an appropriate timing without being affected by individual variability or operating environment of the wheel drive motor100. As a result, it is possible to efficiently utilize the potential capability of the wheel drive motor100from a low-speed range through a high-speed range, and to provide a good vehicle characteristic. Also, the present preferred embodiment can control the gap length GAP without the gap length detector230, allowing cost reduction, size reduction and weight reduction of the electric bike1accordingly. The gap length control is simple and does not require a high processing capability of the microcomputer.

Also, like in the first preferred embodiment, the threshold values R1, R2are set in accordance with the angular acceleration of the wheel drive motor100. This makes it possible to reduce response delay of the gap changer200. Further, when the electric bike1is stopped, the gap length GAP is set to its minimum to provide a high-torque low-rotation-speed characteristic. Therefore, a high torque is obtained when the electric bike1is started the next time, which provides good acceleration.

The present invention is not limited to the above-described preferred embodiments, and can be varied in many ways.

In each of the above-described preferred embodiments, the wheel drive main control routine (seeFIG. 9) is brought to the end when the main switch5is turned off. However, for example, the arrangement may be that when turning off of the main switch5is detected (S34: Yes), a duty control signal of a predetermined duty ratio is outputted to the H bridge circuit360so that drive control of the gap changing motor220is performed until the rotor120of the wheel drive motor100comes to the position which gives the maximum gap length. In other words, there may be a step after Step S34in the wheel drive main control routine for adjusting the gap length GAP of the wheel drive motor100to its maximum value regardless of the voltage utilization rate when the main switch5is turned off. In this case, the wheel drive main control routine is brought to the end after this step is performed. Such an arrangement reduces cogging torque generated by rotation of the wheel drive motor100in a case where the electric bike1must be pushed by hand for transportation.

In the first preferred embodiment, a gap length setting process based on the voltage utilization rate Vrate (which represents the first control mode) is performed if the accelerator opening-degree AO is greater than the predetermined opening-degree A1, but on the other hand, if the accelerator opening-degree AO is not greater than the predetermined opening-degree A1, the voltage utilization rate Vrate is not used as a base but a gap length setting process is performed which takes into account influences from iron loss and copper loss (which represents the second control mode) (FIG. 12: S323through S330). However, the arrangement may not perform such a switching of gap length control mode based on the accelerator opening-degree AO. For example, a gap length setting process based on the voltage utilization rate Vrate may be always performed regardless of the accelerator opening-degree AO.

In the first and the second preferred embodiments, setting of the threshold values R1, R2of the voltage utilization rate Vrate are based on the angular acceleration of the wheel drive motor100. However, the threshold values R1, R2may be fixed to constant values regardless of the angular acceleration. Also, only one of the threshold values R1, R2may be set based on the angular acceleration of the wheel drive motor100.

Further, in the first and the second preferred embodiments, the gap length GAP is set to a minimum when the wheel drive motor100is not rotating (FIG. 12: S321, S322;FIG. 21: S3201, S3216). However, such a process may not be performed.

The gap length GAP may be obtained by any appropriate device. For example, the gap length detector230(the encoder231and the origin-position sensor232) may be replaced by an arrangement where a magnet is provided on one of the threaded cylindrical body211and the substrate233and a magnetic sensor is provided on the other for detecting magnetism from the magnet, so that the gap length GAP can be obtained on the basis of magnetic change detected by the magnetic sensor. Another arrangement may be that a magnetic sensor is provided on the nut212, and a magnet is provided on the substrate233so that the gap length GAP can be obtained on the basis of magnetic change detected by the magnetic sensor.

In the first preferred embodiment, the amount of drive of the gap changing motor220is determined by a proportional item integral of the difference (GAP*−GAP) between the target gap length GAP* and the actual gap-length GAP. However, the amount of drive may be fixed, and only the rotating direction of the gap changing motor220may be determined. In this case, it is possible to reduce the process burden on the electronic controller310(the microcomputer).

In the first and the second preferred embodiments, the voltage utilization rate calculator322calculates the voltage utilization rate, using a control voltage command value provided by an output from the voltage-command value calculator319. However, an output from the two-phase/three-phase coordination system converter320may be used.

In the first and the second preferred embodiments, the gap changing motor220is used to change the gap length GAP. However, other actuators such as a solenoid may be used.

Many variations are possible also in the device arranged to change the gap length in the wheel drive motor100. For example, in the first and the second preferred embodiments, the rotor120is moved axially in order to change the gap length GAP between the rotor120and the stator110. However, a different air gap may be provided in the magnetic path, so that the gap length of this particular gap is changed. An example will be described hereinafter.

FIG. 22is a schematic view showing a wheel drive motor500and a gap changer600as a variation. The wheel drive motor500is preferably a three-phase axial-gap electric motor. The wheel drive motor500includes a stator510and a rotor520. The stator510is disposed to face the rotor520at a predetermined distance provided in a direction (axial direction) in which a rotation shaft523of the rotor520extends.

The stator510includes a first stator511which faces the rotor520at an axial predetermined distance, and a second stator512which does not face the rotor520but faces the first stator511at an axial predetermined distance.

The first stator511includes a plurality (eighteen, in the present example) of first teeth513which are disposed equidistantly on the same circle, and a plurality of coils514each wound around one of the first teeth513. These first teeth513and the coils514are molded into a resin, integrated into an annular shape, and are fixed to an unillustrated mount which is part of the vehicle body.

Each of the first teeth513is provided by a plurality of magnetic steel plates laminated one onto another, with its axial ends flat and not covered by the resin. In each of the first teeth513, an end513a(seeFIG. 23,FIG. 24) which faces the rotor520has a long circumferential dimension, and thus has a T-shaped side view. The coil514is wound around a portion of the first teeth513excluding the end513a. Each of the first teeth513includes an end513bwhich has chamfered circumferential corners facing the second stator512.

The second stator512includes an iron stator yoke515and a plurality (the same number as the first teeth513) of second teeth516which are fixed to the stator yoke515. The stator yoke515is formed as an annular shaped disc, and includes a plate portion515ato which the second teeth516are fixed; and a cylindrical portion515bwhich extends from an inner circumferential end of the plate portion515ain an axial direction of the rotor520. The plate portion515ahas a plurality of fitting holes515cspaced equidistantly in a circumferential direction. In each of the fitting holes515c, one of the second teeth516is fitted, with its end fitted into the hole, so that second teeth516are magnetically connected with the stator yoke515.

Each of the second teeth516faces one of the first teeth513in the first stator511when the stator yoke515is at its origin position of rotation. Each of the second teeth516projects beyond the plate portion515aby a predetermined distance, and has a tip end516awhich has its circumferential corners chamfered.

The rotor520includes, for example, an iron rotor yoke521, and a plurality (twelve, in the present example) of magnets522which are fixed to the rotor yoke521. The rotor yoke521includes an outer plate portion521ain the form of an annular shaped disc and has the magnets522fixed thereon; a tapered portion521bextending from an inner circumferential end of the outer plate portion521awhile decreasing in its diameter; an inner plate portion521cwhich is an annular shaped disc extending inwardly from an inner circumferential end of the tapered portion521bin parallel or substantially parallel to the outer plate portion521a; and a cylindrical portion521dextending axially from an inner circumferential end of the inner plate portion521c.

A motor rotation shaft523is inserted into the cylindrical portion521d. The motor rotation shaft523is fixed to the cylindrical portion521dso as not to be rotatably or axially movably with respect thereto. Therefore, the motor rotation shaft523rotates integrally with the rotor520. The motor rotation shaft523has an end (the upper end as seen inFIG. 22) which is connected with a wheel shaft of the wheel2via an unillustrated speed reduction mechanism. The magnets522are disposed on a main surface of the outer plate portion521a, which is a surface opposed to the first stator511, equidistantly and in alternating polarity on the same circle.

The cylindrical portion521dof the rotor520is rotatably supported by a cylindrical mount533via bearings531and532. The cylindrical mount533is fixed to the vehicle body. The cylindrical mount533has an outer circumference to which the above-described cylindrical portion515bof the second stator512is mounted. The cylindrical portion515bhas an inner circumferential surface which makes slidable contact with an outer circumferential surface of the cylindrical mount533, and thus is rotatable with respect to the cylindrical mount533, but is not axially movable since the movement is prevented by an unillustrated stopper. Therefore, the second stator512is relatively rotatable with respect to the first stator511in the rotating direction of the motor rotation shaft523, but is not relatively movable in the axial direction.

The gap changer600changes a relative position of the second stator512to the first stator511in the rotating direction, and includes a gap length changing motor601, and a gear portion602which transmits rotation of the gap length changing motor601to the second stator512. The gap length changing motor601is fixed to part of the vehicle body. The gear portion602includes a gear515dwhich is provided on the outer circumferential surface of the cylindrical portion515bin the second stator512, and a gear604which is fixed to an output shaft603of the gap length changing motor601. The gear604engages with the gear515dto transmit a rotating force of the gap length changing motor601to the second stator512. It is preferable to provide reduction gears appropriately between the gear515dand the gear604if the gap length changing motor601has a low resolution.

FIG. 23andFIG. 24are development schematics which show a positional relationship between the first teeth513and the second teeth516when the wheel drive motor500is viewed from the outer circumferential side.FIGS. 25A and 25Bare schematic diagrams which gives a two-dimensional interpretation of a state where the second teeth516are opposed to the first teeth513(FIG. 25A) and a state where they are not opposed to each other (FIG. 25B).

In the wheel drive motor500, when the first teeth513and the second teeth516are opposed to each other, a magnetic path (broken lines in the figure) as shown inFIG. 23, is formed by the rotor520, the first stator511, and the second stator512. In this case, a space between the first teeth513and the magnets522, and a space between the first teeth513and the second teeth516are air gaps which function as magnetic resistances in the magnetic path. A gap length (the length of the air gap) between the first teeth513and the magnets522will be called GAP1, and a gap length between the first teeth513and the second teeth516will be called GAP2.

When the second stator512is rotated to move the second teeth516to a position where they are not opposed to the first teeth513, a magnetic path (broken lines inFIG. 24) is formed as shown inFIG. 24, by the rotor520and the first stator511. In this case, a space between the first teeth513and the magnets522, and a space between mutually adjacent two first teeth513are air gaps which function as magnetic resistances in the magnetic path. A gap length of the air gap between mutually adjacent two first teeth513will be called GAP3.

When setting the output characteristic of the wheel drive motor500to a high-torque low-rotation-speed characteristic, the second stator512is held at the origin position as shown inFIG. 23, i.e., a position where the first teeth513and the second teeth516are opposed to each other. In this case, the gap length GAP1between the first teeth513and the magnets522, as well as the gap length GAP2between the first teeth513and the second teeth516are substantially the same distance in practical application, and are extremely small. Therefore, these air gaps have a small magnetic resistance.

On the other hand, the gap length GAP3between mutually adjacent first teeth513is greater than the gap length GAP2between the first teeth513and the second teeth516. Therefore, the magnetic resistance in the air gap between mutually adjacent first teeth513is greater than the magnetic resistance in the air gap between the first teeth513and the second teeth516. It should be noted here that the gap length GAP2and the gap length GAP3has a relationship expressed as 2×GAP2<GAP3.

Thus, a magnetic flux which is generated between an N-pole magnet522and an adjacent S-pole magnet522as shown inFIG. 23flows through the air gap (the gap length GAP1) between the N-pole magnet522and the first teeth513→the first teeth513→the air gap (the gap length GAP2) between the first teeth513and the second teeth516→the second teeth516→the stator yoke515→the adjacent second teeth516→the air gap (the gap length GAP2) between the adjacent second teeth516and the adjacent first teeth513→the adjacent first teeth513→the air gap (the gap length GAP1) between the adjacent first teeth513and the S-pole magnet522→the S-pole magnet522→the rotor yoke521, with very little of the flux penetrating the air gap (the gap length GAP3) between the first teeth513.

When setting the output characteristic of the wheel drive motor500to a low-torque high-rotation-speed characteristic, the second teeth516are moved as shown inFIG. 24, to the middle of two adjacent first teeth513so that the first teeth513and the second teeth516are not opposed to each other. In this case, the gap length between the first teeth513and the second teeth516increases from GAP2to GAP4. For this reason, the gap length GAP3between two adjacent first teeth513is now smaller than the gap length GAP4between the first teeth513and the second teeth516. Therefore, the magnetic resistance between adjacent first teeth513is now smaller than the magnetic resistance between the first teeth513and the second teeth516.

Thus, a magnetic flux which is generated between an N-pole magnet522and an adjacent S-pole magnet522as shown inFIG. 24flows through the air gap (the gap length GAP1) between the N-pole magnet522and the first teeth513→an end513aof the first teeth513→the air gap (the gap length GAP3) between the end513aof the first teeth513and an end513aof the adjacent first teeth513→the end513aof the adjacent first teeth513→the air gap (the gap length GAP1) between the adjacent first teeth513and the S-pole magnet522→the S-pole magnet522→the rotor yoke521. In this case, therefore, there is virtually no magnetic flow through portions of the first teeth513where the coils514are wound around.

As described, according to the variation again, it is possible to change the motor output characteristic by changing the gap length of a gap in the magnetic path of the wheel drive motor500. In the present example, description was made for two positions of the second teeth516, i.e., of the second stator512. However, the motor output characteristic can be varied to multiple levels by adjusting the relative position of the second stator512to the first stator511in the rotating direction through drive control performed to the gap changing motor601. In this case, it is preferable as in the preferred embodiment described above, that the motor control unit300or400is used to control the gap adjusting motor601based on the voltage utilization rate.

For example, through a drive control performed to the gap changing motor601, the relative angle of rotation of the second teeth516to the first teeth513, i.e., the relative angle of the second teeth516to the first stator511, may be controlled in a range from the origin position up to a maximum of ten degrees, with an increment of one degree. In this case, an angle detection sensor (not illustrated) may be provided to detect the rotation angle of the second stator512, so as to obtain a difference between the actual detected rotation angle and a target rotation angle which is set in accordance with the voltage utilization rate, and to provide feedback control to the gap changing motor601in accordance with the obtained difference.

It should be noted here that when changing the relative position of the second teeth stator512to the first stator511in the rotating direction, the air gap between the two stators changes as follows. While the second teeth516are opposed to the first teeth513, the gap length continues to be GAP2and the change occurs only in the cross-sectional area. Thereafter, when the second teeth516comes to a position where they are not opposed to the first teeth513, the gap length becomes GAP4and varies in accordance with the rotation angle. Therefore, it is preferable that a range of rotation of the second stator512within which desired motor output characteristics are available should be found for use in the gap length control.

It is also possible to employ a sensor-less system as in the second preferred embodiment in the control of the relative rotation angle of the second stator512to the first stator511. In this case, it is preferable to provide a mechanical stopper (not illustrated) which limits an end position of rotation of the second stator512, and a timer which limits one-way continuous power application time to the gap changing motor601.

According to this variation, the gap length is changed through adjustment of the relative position in the rotating direction between the first stator511and the second stator512and therefore, the gap length change can be performed easily without providing an axial space for relative movement as is needed in the first and the second preferred embodiments.

It should be noted here that the magnets provided on the main surface, i.e., a surface of the outer plate portion521afacing the first stator511, may be provided by, for example, a single member such as a ring-shaped bond magnet magnetized to have a plurality of N poles and S poles to occur alternately with each other in a circumferential direction.

In the above-described preferred embodiment, an accelerator opening-degree AO is used as accelerator information, and the accelerator opening-degree AO is compared to a predetermined opening-degree A1in Step S323inFIG. 12. However, accelerator information may be provided by the amount of accelerator operation, i.e., an accelerator opening-degree signal. In this case, the accelerator operation-amount sensor301represents accelerator information obtaining device. Then, the accelerator opening-degree signal is compared to a predetermined value in Step S323inFIG. 12.

Angular acceleration information is not limited to the angular acceleration per se of the wheel drive motor100. Rotation speed information is not limited to rotation speed per se of the wheel drive motor100. Voltage information is not limited to a power source voltage Vb in the DC power source3.

The present invention is suitably applied not only to electric bikes but also any other electric vehicles.