ELECTRIC ASSIST DEVICE FOR A BICYCLE

Abstract

An electric assist device for a bicycle comprising a shaft coupled in rotation with a pair of cranks and being able to be rotated in a positive direction by an electric motor, a connection mechanism being placed between the motor and the shaft, which connection mechanism has at least three distinct states. The states includes State 1, termed assist state, in which the motor transmits a torque to the shaft via a drive part which rotates at the same speed as the shaft, State 2, termed freewheel state, in which the rotation of the shaft in the positive direction is greater than that of the drive part, and State 3, termed disconnection state, in which a rotation in the positive direction or in the negative direction of the shaft cannot cause the motor to rotate.

Claims

1. An electric assist device for a bicycle comprising a shaft secured in rotation with a pair of cranks and that can be driven in rotation in a positive direction by an electric motor, wherein a connection mechanism is placed between the motor and the shaft, the connection mechanism having at least three distinct states: state 1, called assist state, in which the motor transmits a torque to the shaft via a driving piece which turns at the same speed as the shaft, state 2, called free-wheel state, in which the rotation of the shaft in the positive direction is greater than that of the driving piece, and state 3, called disconnection state, in which a rotation in the positive direction or in the negative direction of the shaft cannot drive the rotation of the motor.

2. The device as claimed in claim 1, wherein the cranks are fixed to the two ends of the shaft.

3. The device as claimed in claim 1, wherein a reducing gear is placed between the motor and the shaft so as to reduce the speed of rotation of the motor.

4. The device as claimed in claim 3, wherein the reducing gear comprises a gear train.

5. The device as claimed in claim 4, wherein the reducing gear is a reducing gear of epicyclic type.

6. The device as claimed in claim 1, wherein the connection mechanism comprises blocking pieces that are displaced between an “engaged” position and a “disengaged” position and means allowing the blocking pieces to be kept in disengaged position.

7. The device as claimed in claim 6, wherein the blocking pieces are pivoting ratchets whose distal ends can engage with teeth integral to the shaft when the ratchets are in engaged position.

8. The device as claimed in claim 7, wherein the means allowing the ratchets to be kept in disengaged position comprise a ratchet synchronization ring and a friction ring secured to one another, said friction ring coming into contact with a fixed part of the assist device so as to generate a friction torque during its rotation.

9. The device as claimed claim 8, wherein, upon a rotation of the friction ring in the positive direction, the latter generates a torque which tends to engage the ratchets and, conversely, upon a rotation in the negative direction, the friction ring generates an opposite torque which tends to disengage the ratchets.

10. The device as claimed in claim 8, wherein the friction ring is an elastically deformable ring bearing with prestress on the fixed part of the device on at least two zones.

11. The device as claimed in claim 1, wherein the connection mechanism comprises a freewheel and a clutch mechanism of centrifugal clutch type which passes from the closed position to the open position when the speed of rotation of the motor is less than a given speed V1.

12. The device as claimed in claim 11, wherein the clutch mechanism comprises a plurality of sectors, each equipped with friction pads that can come into contact with a friction cylinder and in that, in open position, the friction pads are not in contact with the friction cylinder, whereas, in closed position, the sectors, subjected to the centrifugal force, exert an effort on the friction cylinder such that sectors and friction cylinder are secured in rotation and that the motor can transmit a non-zero torque to the driving shaft.

13. The device as claimed in claim 11, wherein the clutch mechanism is placed between the motor and the reducing gear.

14. An electric assist method for a bicycle equipped with a device as claimed in claim 1, and comprising a microcontroller which controls the rotation of the motor in a first direction, called positive direction, when assistance is desired by the user, said positive direction being such that it generates the forward motion of the bicycle; wherein, when the speed of rotation of the cranks is zero or when the latter turn in the direction opposite to the forward motion of the bicycle, the microcontroller controls the rotation of the motor in the direction opposite the first direction, for a short time period.

15. The electric assist method for a bicycle equipped with a device as claimed in claim 11, wherein the reducing gear has a reduction ratio R, wherein, when the speed of rotation of the motor is less than V1, the clutch mechanism is in open position, said speed V1 being such that V1/R<25 rpm.

16. The electric assist method for a bicycle equipped with a device as claimed in claim 12, wherein the reducing gear has a reduction ratio R, wherein, when the speed of rotation of the motor is less than V1, the clutch mechanism is in open position, said speed V1 being such that V1/R<25 rpm.

17. The electric assist method for a bicycle equipped with a device as claimed in claim 13, wherein the reducing gear has a reduction ratio R, wherein, when the speed of rotation of the motor is less than V1, the clutch mechanism is in open position, said speed V1 being such that V1/R<25 rpm.

Description

[0044] The invention will be better understood in light of the following description. The drawing attached thereto comprises:

[0045] FIG. 1 is an overview of a bicycle incorporating an electric assist system according to an example of the invention.

[0046] FIG. 2 is a view of an enlargement of a part of FIG. 1, centered on the housing of the crankset.

[0047] FIG. 3 is a perspective view of the device according to a first embodiment of the invention.

[0048] FIG. 4 is an exploded and partial perspective view of the device of FIG. 3.

[0049] FIG. 5 is a longitudinal cross-sectional view of the device of FIG. 3.

[0050] FIGS. 6 and 7 are, respectively, views in transverse cross section D-D and B-B of the device of FIG. 3 when the clutch mechanism is in closed position.

[0051] FIGS. 8 and 9 are, respectively, views in transverse cross section D-D and B-B of the device of FIG. 3 when the clutch mechanism is in open position.

[0052] FIG. 10 is a view in transverse cross section on the plane E-E.

[0053] FIG. 11 is a view in longitudinal cross section of a device according to a second embodiment of the invention.

[0054] FIG. 12 is an exploded and partial perspective view of the device of FIG. 11.

[0055] FIG. 13 is a detail view of FIG. 12.

[0056] FIG. 14 is a view in transverse cross section F-F, when the clutch mechanism is in closed position.

[0057] FIG. 15 is a view in transverse cross section F-F, when the clutch mechanism is in open position.

[0058] FIG. 1 illustrates an electric assist bicycle 1 incorporating an assist system according to the present invention. As is known, this bicycle comprises a frame to which are fixed two wheels. The rear wheel 15 is the drive wheel and is driven by the crankset, consisting of a pair of cranks 16 and two pedals 17. The right crank comprises a star 161 whose ends serve as fixing for a toothed chain ring 18 of the transmission chain 19.

[0059] FIG. 2 shows the crankset 2 housing which forms an integral part of the frame and links the various tubes thereto. The assist motor 3 is housed inside an element forming the frame of the bicycle. It can for example be housed inside the seat tube 12, or in the oblique tube 13, or in one of the bases 14 linking the crankset to the hub of the rear wheel 5. Alternatively, the motor can be added onto the frame, for example by being fixed to one of these tubes 12, 13, 14 or by being fixed to the housing 2 of the crankset.

[0060] The motor 3 is an electric motor. It is preferably powered by a battery added onto the frame, or housed inside the frame, or even worn by the user. The motor 3 drives a motor member 31 in rotation.

[0061] Preferably, the motor member 31 is driven in rotation about the same axis of rotation as the output of the motor. This axis of rotation is at right angles to that about which the driving shaft 4 of the crankset turns. The axis of rotation of the crankset 4 and the axis of rotation of the motor 3 correspond, respectively, to the axes X and Z of the orthogonal reference frame illustrated in FIG. 2. Thus, the axis of rotation of the motor 3 is at right angles to that of the shaft 4 of the crankset.

[0062] A casing 21 is housed inside the housing 2 and this casing is closed by a cover 22. The shaft 4 of the crankset passes right through the casing 21. FIG. 3 shows the casing 21 before it is inserted into the crankset housing. At each of its ends, the shaft 4 comprises means 41 allowing for the fixing of the cranks 17. In the case described here, these means consist of splines.

[0063] The casing 21 has a through opening allowing the passage of the shaft 4 of the crankset. In order to allow precise guidance and a good rigidity of the shaft 4 inside the housing 2, two rolling members 43, 44 are provided, typically ball bearings. These ball bearings are visible in FIG. 5.

[0064] The housing 2 and the casing 21 also have a top passage 23 allowing the motor member 31 to enter into the housing 2.

[0065] A reducing gear 5 makes it possible to transmit the efforts from the motor member 31 to the shaft 4 of the crankset. The reducing gear 5 is of epicyclic type with dual planet gear, in order to allow for a strong reduction ratio within a reduced volume.

[0066] The motor member 31 is a pinion which meshes with a crown ring 51 that is movable in rotation about an axis identical to that X of the shaft 4.

[0067] Preferably, the motor member 31 is a bevel pinion and the movable crown ring 51 bears beveled toothing complementing that of the motor member 31.

[0068] The crown ring 51 is mounted to rotate freely about the shaft 4 of the crankset. For that, rolling members can be provided, such as ball bearings 511, 512, disposed between the shaft 4 and the crown ring 51.

[0069] The movable crown ring 51 is secured in rotation to a planet carrier 52. Preferably, the movable crown ring 51 and the planet carrier 52 form one and the same piece. Alternatively, provision can be made for the crown ring and planet carrier to be distinct pieces secured to one another.

[0070] The planet carrier 52 has recesses configured to accommodate planet gears 53. There are preferably at least three of these planet gears 53 and they are configured to move in one and the same plane at right angles to the axis X of the shaft 4.

[0071] In this nonlimiting example, each planet gear is secured to a shaft mounted to rotate in a housing borne by the planet carrier 52. To this end, rolling bearings are provided, typically ball bearings, mounted between the shaft of the planet gear and the housing in order to minimize the friction losses of this reducing gear.

[0072] Each planet gear 53 comprises two pinions 531, 532. The two pinions 531, 532 of each planet gear 53 are placed parallel to one another and are secured to one another in their movement.

[0073] A first pinion 531 meshes with a fixed crown ring 54 secured to the casing 21 and therefore secured to the frame of the bicycle. This fixed crown ring 54 bears internal toothing. A second pinion 532 meshes with toothing borne by a second crown ring 55. The second crown ring 55 is secured with a driving piece 56 which constitutes the output of the reducing gear 5. The speed of rotation of the electric motor which can be several thousands of revolutions per minute is reduced until the output of the reducing gear is less than 100 rpm. For example, in the designed configuration, the reduction ratio of the bevel torque is 5 followed by the epicyclic reducing gear which has a ratio of 21, i.e. a total ratio of 105 between the speed of the motor and the output speed of the reducing gear.

[0074] The driving piece 56 is linked to the shaft 4 via the connection mechanism according to the invention. The latter comprises a declutchable freewheel 6, which comprises two ratchets 61 and a synchronizing ring 62. The ratchets 61 are mounted to pivot inside a housing 561 formed in the driving piece 56. They are kept there by virtue of a cage 57 fixed to the driving piece 56 by four screws. The ratchets 61 pivot between an “engaged” position represented in FIG. 7, in which their distal end is engaged with one of the teeth 42 formed on the driving shaft 4, and a “disengaged” position represented in FIG. 9.

[0075] A synchronizing ring 62 synchronizes the swiveling movement of the two ratchets 61. It is mounted to pivot about the axis X by virtue of a flange 562 which guides it. The synchronizing ring 62 has a small amplitude of rotation with respect to the driving ring 56. In fact, the synchronizing ring is fixed to the ratchets at a distance of a few millimeters from their pivoting point.

[0076] The two ratchets 61 are 180° opposed making it possible to balance the efforts transmitted by theoretically transmitting a pure torque with no radial component, and by substantially halving the effort transmitted by each of the ratchets. The ratchets preferably have an end with a beak engaging with teeth 42 formed on the shaft 4. Thus, as soon as the end of the ratchet comes into contact with the shaft 4, the ratchet will then continue its travel to engage automatically at the bottom of the tooth despite the frictions opposing its engagement, allowing them to remain perfectly stable once engaged, the synchronizing ring 62 mounted to pivot on the output of the reducing gear makes it possible to synchronize the rotation thereof in order for them to engage correctly simultaneously. FIGS. 7 and 9 show, in cross section B-B, the two ratchets 61, and the synchronizing ring 62, in engaged position, respectively, in disengaged position.

[0077] The connection mechanism 8 also comprises means which allow the ratchets 61 to be controlled, and notably allow them to be kept in disengaged position. These means comprise a friction ring 72 and two pins 73. FIGS. 4, 5 and 10 show the geometry and the operation of the friction ring 72. The friction ring is an elastically deformable ring which comes to bear prestressed on a fixed part of the device on at least two zones. In the embodiment represented, the fixed part is a cylindrical barrel 221, protruding from the internal face of the cover 22. This ring comprises two diametrically opposed buttons 721 formed on the internal face of the ring. These buttons are provided to slip over the outer surface of the barrel 221 deriving from the cover 22. The friction ring 72 is secured in rotation to the synchronizing ring 62 via two pins 73 that are inserted into two diametrically opposite, radially oblong holes. To ensure a certain grip of the friction ring on the barrel 221, the internal diameter between the two buttons is, in the free state, slightly less by approximately 0.5 mm than the outer diameter of the barrel 221. The fitting of the friction ring will generate a certain out-of-roundness of the latter, the geometry of this friction ring must be dimensioned to generate two diametrically opposite radial efforts when it becomes out-of-round by 0.5 mm without generating high stresses on the latter in order for it to be able to withstand this level of stress without creep throughout the life of the product. It will be possible to choose a material of PBT or POM type to produce the ring because this friction piece must have a very good resistance to wear, and a very good elasticity, as well as a good behavior at the operating temperature to guarantee a very good stability of its clamping prestress throughout the lifetime of use of the product.

[0078] The diametral clamping of this friction ring 72 onto the barrel 221 will induce a stable and controlled friction torque opposing the rotation of the latter with respect to the barrel 221 which is itself linked to the housing of the frame 1.

[0079] As an example, very good operation is obtained by dimensioning the friction ring to obtain two diametrically opposite radial efforts at the friction buttons of approximately 5N under a diametral clamping of 0.5 mm with a stress level of less than 10 MPa on the latter, thus avoiding any risk of relaxation or creep over time, these radial efforts generated on the barrel of 35 mm diameter make it possible to generate a friction torque of approximately 25 mN.Math.m in both directions of rotation, more than sufficient to engage and disengage the ratchets, while generating a torque that is low enough not to induce any significant power loss upon the slippage of the latter, since this friction torque of 25 mN.Math.m generates a loss only of 0.2 watt at 80 revolutions per minute, which is negligible compared to the assist power and which does not make it possible to heat up and wear the zone of the buttons in contact with the barrel 221. The operation of the clutch mechanism will now be described in detail.

[0080] When the output of the reducing gear, that is to say the driving ring 56, is driven in the positive direction (normal direction of pedaling identified with a + sign in the figures), the synchronizing ring 62 will be driven in rotation in the same direction and it will drive with it the friction ring 72, the friction of the two buttons 721 on the barrel of the cover 22 will generate a friction torque which will be transmitted to the synchronizing ring 62. Since the latter has a certain amplitude of relative rotation with respect to the driving ring 56, the friction torque will cause it to be forced in the direction opposite the movement and therefore make it turn backward relative to the output of the reducing gear. This relative rotation tends to cause the ratchets to pivot in their direction of engagement provoking the connection in rotation of the motor with the driving shaft 4. The freewheel is then engaging. This state of engagement of the freewheel is represented in FIGS. 6 and 7. That corresponds to the state 1 of the connection mechanism, called assist state, or even to the assisted pedaling mode (phase 2) when the cyclist pedals less quickly than that of the assistance provided by the motor.

[0081] In the case where the cyclist increases his or her pedaling rate to exceed the output speed of the reducing gear (phase 3), the teeth 42 of the shaft 4 will lift the ratchets 61 on each passage of teeth and no torque will be transmitted between the driving ring and the shaft 4. The freewheel is then free to slip. This is the conventional operation of a freewheel. That corresponds to the state 2, called freewheel state, of the connection mechanism. It can be noted that, when the ratchet is lifted by a tooth 42 of the shaft 4, the synchronizing ring 62 and the friction ring 72 will thus assume a slight advance relative to the driving ring but the friction torque of the friction ring provoked by the friction of the buttons 721 on the cover 22 will remain substantially constant, provoking the immediate reengagement of the ratchets as soon as the tooth 42 is passed, so, in this freewheel phase, the friction ring acts in a way analogous to the return springs of the conventional ratchet freewheels.

[0082] Now, if the cyclist decides to backpedal (phase 5), the shaft 4 which turns in the negative direction will drive, via the ratchets, the driving ring 56 and the friction ring 72. The friction of the buttons 721 with the cover 221 generates a friction torque which opposes the rotation and makes the synchronizing ring turn relative to the driving ring until the two ratchets are lifted, making the motor coupling impossible. The connection mechanism is then in state 3, called disconnection state, as represented in FIGS. 8 and 9.

[0083] Indeed, this declutching of the ratchets is linked to the direction of relative rotation between the crown ring of the reducing gear and the cover of the crankset housing (fixed), so it will be possible to control the engagement of the ratchets by controlling the direction of rotation of the motor such that it is sufficient to order a small reverse rotation of the motor to make the driving ring 56 pivot by at least a few degrees backward, that is to say in the negative direction, to declutch the freewheel.

[0084] In practice, if the cyclist is moving forward with motor assist (phase 2), the ratchets are then engaged. From this state, if the cyclist decides to suddenly reverse (phase 5), or stop pedaling to be able to stop (phase 4), since the ratchets have engaging beaks, it can be possible for the ratchets not to be able to easily disengage despite the friction torque applied to the synchronizing ring, and for that it is desirable for a controlling microcontroller (not represented) to be able to detect this event either by a sensor of pedaling torque (not represented) which suddenly becomes zero or negative, or by a zero speed of rotation of the crankset detected by an angular sensor (not represented), or by a strong angular deceleration or even by a reversal of the direction of rotation of the motor or of the crankset and then order a reverse rotation of the motor with a greater reverse acceleration than that of the cyclist for a fraction of revolution which will disengage the ratchets and allow them to lift, this reverse motion will be able to be stopped almost immediately and the freewheel will then remain in this stable disengaged state without inducing the slightest friction or the slightest additional noise for the cyclist.

[0085] Finally, when the cyclist decides to switch from assistance or when the battery is drained (phase 1), the microcontroller will order a reverse rotation of the motor in order to place the freewheel in the disengaged state. Thus, the presence of the assist device induces no additional friction or noise.

[0086] In a variant embodiment of the first embodiment of the invention, a spring placed between the driving ring 56 and the synchronizing ring 62 keeps the ratchets in the disengaged position, for an absence of rotation or a negative rotation of the friction ring. The return torque of the spring must be less than the friction torque of the friction ring in order for a rotation of the latter in the positive direction to correctly provoke the engagement of the ratchets. Thus, if the cyclist backpedals, he or she will disengage the ratchets which will then, by virtue of the spring, be in a stable disengaged position (state 3). In this variant, the control of the motor in reverse is not necessary.

[0087] FIGS. 11 to 15 describe a second embodiment of the invention in which the connection mechanism 8 comprises a freewheel 6 and a clutch mechanism 7 that are distinct. Furthermore, the freewheel and clutch are not directly adjacent in the motor-wheel power transmission pathway. In fact, as will be seen later, a reducing gear 5 is inserted between the clutch and the freewheel. This embodiment differs also in that the assist motor 3 is a concentric motor with driving shaft 4. It is clearly understood that the type and the disposition of the motor are not limiting in the context of the invention and that the first embodiment described above could very well be equipped with a concentric motor with driving shaft.

[0088] FIG. 11 shows, in longitudinal cross section, the complete block before the latter is inserted into the crankset housing of the bicycle as is shown in FIG. 2. All of the elements are housed inside a casing 21 closed by a cover 22. The motor 3 drives the motor member 31 in rotation. The latter comprises three pins 311 protruding from its front-end surface which allow the reducing gear 5 to be driven through a clutch mechanism 7, the operation of which will be explained hereinbelow.

[0089] The reducing gear 5 in this embodiment is substantially identical to that described with the first embodiment. It will not be described again in detail. This is a reducing gear that has a reduction ratio R which is equal to 41 in this second embodiment. The input of the reducing gear is through the planet carrier 52 and the output of the reducing gear 5 consists of a driving ring 56 which drives the shaft 4 through a simple freewheel 6. This is a simple ratchet freewheel that is substantially identical to that described for the first embodiment of the invention, but in which the ratchets are engaged in traditional manner, each by a small spring in their position of engagement.

[0090] The clutch mechanism 7 comprises a clutch ring 77 and three sectors 74 housed inside said ring and distributed over the entire circumference. The clutch ring 77 is secured to the planet carrier 52. Each sector is mounted to pivot on a pin 311 and is stressed by a return spring 75 which links it to another pin. A friction pad 76 covers, at least partially, the periphery of each of the sectors 74. The springs 75 are chosen such that, when the clutch 7 is not rotating, the return force of the springs 75 stresses the sectors 74 toward the shaft 4 as can be seen in FIG. 14.

[0091] When the motor is not rotating, or rotating at a low speed, there is no contact, or no sufficient friction, between the friction pads 76 and the friction cylinder 771 of the clutch ring 77, and the clutch is open. The motor 3 is then no longer secured in rotation, with the reducing gear 5, and consequently the driving shaft, and this is so in both directions of rotation.

[0092] When the motor starts up at a sufficiently high speed, it drives the sectors, which, subjected to the centrifugal force, pivot about the pins 311. The centrifugal force which is exerted on the sectors then exceeds the return force of the springs and the friction pads are pressed against the friction cylinder 771 of the clutch ring. The speed of the motor is then V1, which is the wheel spin start speed. The centrifugal efforts exerted on the sectors increase with the square of the speed, the efforts generated on the friction pad will very quickly allow the clutch to stop the slipping of the latter with respect to the friction cylinder. The speed of the motor is then V2, that is to say the end of wheel spin speed at the maximum torque of the motor. The configuration described in FIG. 15 then applies. This configuration corresponds to the closed position of the clutch mechanism in which the motor is completely secured to the reducing gear 5 and the driving shaft 4.

[0093] The operation of a bicycle equipped with an assist device according to the second embodiment of the invention is detailed hereinbelow.

[0094] When the cyclist does not want assistance or when the battery is drained (phase 1), the centrifugal clutch 7 is open and the motor 3 is separated from the driving shaft 4. The motor-wheel transmission pathway is broken. That corresponds to the state 3 of the connection mechanism 8. Consequently, the motor cannot in any case be damaged by a rotation of the pedals, whatever the speed at which the latter are turned.

[0095] When the cyclist wants to benefit from the assistance and when the motor is running, the rotation of the motor at a speed greater than a given speed V2, for example 500 revolutions per minute, sets the clutch in closed position. The motor 3 contributes to the driving of the shaft 4 with the pedals (phase 2). That corresponds to the state 1 of the connection mechanism. If the cyclist pedals at a faster rate than that which the motor 3 can supply to the shaft 4, then the freewheel 6 is disconnected. In this configuration, the motor no longer contributes to the driving of the shaft (phase 3), and the connection mechanism is in the state 2.

[0096] When the cyclist stops pedaling (phase 4), a pedaling rate sensor informs the microcontroller of this and the microcontroller stops the motor. As soon as the rotation of the latter passes below the value V1, the clutch 7 switches to open configuration.

[0097] If, during use with assistance, the cyclist reverses with the pedals (phase 5), the transition from the speed of rotation of the crankset through a zero speed (and consequently less than V1) has the effect, as for phase 4, of opening the clutch and disconnecting the motor. The connection mechanism switches to the state 3. In these latter two cases, the state 3 of the connection mechanism is an unstable transient state because, as soon as the motor is once again driven at a speed V1, the connection mechanism switches back to the state 1.

[0098] In practice, the speed V1 is chosen such that, when the motor turns at the speed V1, the shaft 4, the speed of which is V1/R, is between 1 and 25 rpm. These speeds do not correspond to usual pedaling rates, which lie between 30 and 110 rpm. In the example represented here, the ratio of the reducing gear R being equal to 41, the speed V1 is between 41 and 1025 rpm.

[0099] The designer must determine the mechanical characteristics of the sectors and of the springs in order, when the motor turns at the speed V2, for 100% of the maximum torque of the motor to be transmitted, that is to say that there is no longer any slip. Preferably, the speed V2 corresponds to a speed of the shaft 4, V2/R, of approximately 40 rpm. In the example represented, V2 is approximately 1640 rpm. When the motor turns at a speed greater than V2, there is an assurance that there will not be any wheel spin, that is to say slip, and therefore overheating and loss of energy.