Power-split hybrid drive train for electric bicycle

Abstract

An electric auxiliary driving system for a bicycle comprises a pedal shaft (1), a planetary gear mechanism (20), an auxiliary motor (M2) used for driving an output shaft (21) and a control motor (M1), and the transmission ratio between the pedal shaft (1) and the output shaft (21) is controlled through the planetary gear mechanism. A first one-way clutch (10) operably connected to the control motor (M1) and configured to prevent rotation of a rotatable member (4) of the planetary gear mechanism in a forward direction while allowing free rotation thereof in a reverse direction. A second one-way clutch (12) is configured to drivingly connect the auxiliary motor (M2) to the output shaft (21) when the auxiliary motor (M2) is turned on and to disengage the auxiliary motor (M2) from the output shaft (21) when the auxiliary motor is turned off but the output shaft (21) continues to rotate forward.

Description

Technical Field

[0001] The present invention relates to the field of electric bicycles (or "e-bikes" for short) having an electric motor to assist the rider's pedaling power. More particularly, the present invention relates to hybrid drivetrains for electric bicycles. Background Art

[0002] In recent years, power-split hybrid drivetrains for electric bicycles have been proposed. Some of these systems include a pedal shaft for operation by a rider, a planetary gear mechanism, an assist motor, and a control motor. The planetary gear mechanism is arranged to determine a transmission ratio between the pedal shaft and an output shaft for transmitting rotation to the rear wheel of the bicycle. The planetary gear mechanism is called a planetary "power-split" gear mechanism because it is arranged to transmit power from the pedals to the rear wheel of the bicycle through two paths: a mechanical path and an electrical path. Specifically, the planetary gear mechanism transmits power from the assist motor to the output shaft. In addition, the planetary gear mechanism adjusts the rotational speed of the pedal shaft based on the operation of the control motor.

[0003] WO2020 / 260772A1 discloses a power unit for a bicycle. The power unit comprises a pedal shaft, an output shaft arranged to transmit torque to a wheel, a main planetary gear set arranged to control the transmission ratio between the pedal shaft and the output shaft, an auxiliary motor connected to an auxiliary gear in the main planetary gear set, and a control motor connected to a control gear in the main planetary gear set. The control motor and the control gear constitute a control assembly of the power unit. The power unit comprises a one-way clutch, which is associated with the control assembly of the power unit and is arranged to transmit rotation only in a first rotational direction.

[0004] The first problem encountered in some planetary "power-split" transmission systems is that the torque generated by the pedals is always reacted by the motor which controls the gear ratio. When the bicycle is ridden at a low speed with a low gear ratio, the gear ratio controls the motor speed very low, so the motor efficiency is very low. Most of the electrical energy supplied to the motor is lost in the form of heat. This results in a shortened life of the motor and also inefficient use of the electrical energy stored in the battery.

[0005] Another problem with planetary "power-split" drivetrains is that at high speeds and high gear ratios, the ratio control motor spins quickly to maintain the desired pedal speed while still responding to torque from the pedals. As a result, the ratio control motor can deliver high levels of mechanical energy to the system, so no additional assistance from the traction motor is needed. The traction motor can be turned off, but still needs to spin in unison with the rear wheels. Therefore, the traction motor generates electrical energy in this situation, which can cause damage to the electronic system. Summary of the invention

[0006] Against the foregoing background, the present invention provides an electrically assisted drive system for a cycle having the features defined in claim 1. Preferred embodiments are defined in the dependent claims.

[0007] The present invention provides an electric auxiliary drive system for a bicycle, comprising a pedal shaft operated by a rider, a planetary gear mechanism, an auxiliary motor, a control motor, a first one-way clutch and a second one-way clutch. The planetary gear mechanism is configured to determine the transmission ratio between the pedal shaft and an output shaft to transmit rotation to a rear wheel of the bicycle. The auxiliary motor is drivingly connected to the output shaft. The control motor is drivingly connected to the planetary gear mechanism to control the transmission ratio between the pedal shaft and the output shaft. The first one-way clutch is operably connected between the proportional control motor and a rigid component fixedly mounted on a bicycle frame, and is configured to prevent a first rotatable member of the planetary gear mechanism from rotating in a forward direction, and to release and allow the first rotatable member of the planetary gear system to rotate freely in a second, opposite direction. The second one-way clutch is configured to be operably connected between the auxiliary motor and the output shaft, the second one-way clutch is configured to drive the output shaft in the forward direction when the auxiliary motor is turned on, thereby assisting the bicycle to move forward, and to separate the auxiliary motor from the output shaft when the auxiliary motor is turned off but the output shaft continues to rotate in the forward direction.

[0008] Preferably, the planetary gear mechanism includes a sun gear fixed to rotate with the output shaft, a ring gear, a set of planetary gears located between the sun gear and the ring gear, and a planetary gear carrier fixed to rotate with the pedal shaft and supporting the planetary gears. According to one embodiment, the ring gear meshes with a first pinion mounted on a first shaft driven by the control motor, and the first one-way clutch is operably connected between the first shaft of the control motor and a rigid component fixedly mounted to the bicycle frame. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] In order to better understand the present invention, several preferred embodiments will now be described by way of example with reference to the accompanying drawings, in which:

[0010] Figure 1 is a schematic cross-sectional view of the main components of an electric bicycle drive system according to an embodiment of the present invention.

[0011] Figure 2 The torque distribution relationship through the planetary gear mechanism is schematically shown.

[0012] Figure 3 , Figure 4 and Figure 5 The power flow within the system during the three phases of acceleration of a bicycle is schematically depicted. DETAILED DESCRIPTION

[0013] See first Figure 1 An electric bicycle drive system comprises two electric motors M1, M2 and a planetary gear mechanism 20 having an output shaft 21. Fixed for rotation with the output shaft 21 is a sprocket 6 which drives the rear wheel (not shown) of the bicycle.

[0014] Reference numeral 1 represents a pedal shaft or crankshaft operated by a rider R. The pedal shaft 1 passes through this assembly and connects two conventional pedal cranks and a foot support assembly (not shown) mounted outside the drive unit. The pedal shaft 1 receives the torque and speed provided by the rider and transmits it to the planetary gear carrier 2.

[0015] The electric motor M1 is called the "control" motor or "ratio controlling" motor because it drives a gear of the planetary gear mechanism which controls the transmission ratio between the output shaft and the pedal shaft. The electric motor M2, referred to herein as the "auxiliary" motor (or "traction" motor), generates power which is transmitted to the output shaft 21.

[0016] The planetary gear mechanism is also referred to herein as a planetary "power-split" gear mechanism because it is arranged to transmit power from the pedals to the rear wheel of the bicycle via two paths, as described below: a mechanical path MR and an electrical path ER. Specifically, the auxiliary motor M2 transmits power to the output shaft. In addition, the planetary gear mechanism adjusts the rotational speed of the pedal crankshaft 1 due to the operation of the control motor M1.

[0017] The electric bicycle drive system is contained in a housing (not shown), preferably mounted centrally (at the "bottom bracket") to a frame of the bicycle. Typically, the housing provides mounting and reaction points for the rolling bearings that are rotatable pedal crankshafts 1. The housing may also contain an electronic controller (not shown) for the drive system.

[0018] The planetary gear mechanism 20 includes a planetary gear carrier 2 that supports a plurality of planetary gears 3 and applies the rider's torque and speed to the planetary gear system. The planetary gear carrier 2 is fixed for rotation with the pedal shaft 1. The pedal shaft 1 receives the torque and speed provided by the rider and transmits it to the planetary gear carrier 2.

[0019] The power split planetary gear mechanism includes a sun gear 5, which is driven to rotate by the auxiliary motor M2 and rotates with the sprocket 6 located on the right side of the system. The sun gear 5 rotates together with the sprocket 6 and has external teeth that mesh with the planetary gears 3.

[0020] A ring gear 4 has internal teeth meshing with the planetary gears 3 and a plurality of external teeth meshing with a first pinion 9 directly driven by the proportional control motor M1.

[0021] The planet gears 3 are free to rotate relative to the planet carrier 2 and therefore apply equal tangential forces to the gear-teeth of the ring gear 4 and the sun gear 5, regardless of the relative speed of these components. Thus, a fixed proportion of the torque from the rider is distributed to the ring gear 4, while the remainder of the torque from the rider is distributed to the sun gear 5.

[0022] The output shaft 21 may be a hollow tubular shaft through which the pedal shaft 1 passes.

[0023] The sprocket or belt sprocket 6 drives a chain or a toothed belt 7 for driving the rear wheel of the bicycle.

[0024] A traction gear 8 is fixed to rotate with the sun gear 5 and the sprocket 6 (or sprockets). The traction gear 8 has external teeth that mesh with a second pinion 11 that is directly driven by the auxiliary motor M2.

[0025] A first one-way clutch 10 releasably connects a shaft S1 of the proportional control motor M1 to a rigid component 22, which is fixed to the bicycle frame or formed integrally with the bicycle frame. Preferably, the rigid component 22 can be a housing of the drive unit.

[0026] The first one-way clutch 10 is arranged to block the rotation of the ring gear 4 in the forward direction (i.e., the forward direction is the direction of rotation of the pedals, chain and wheels when the bicycle moves forward), but allows the ring gear 4 to rotate freely in the reverse direction. According to one embodiment, the first one-way clutch 10 may include a one-way roller bearing pressed into a bore in the housing 22.

[0027] like Figure 1 In the exemplary embodiment shown, the first axis S1 of the control motor M1 is parallel to the rotation axis of the ring gear 4 .

[0028] According to another embodiment (not shown), the first shaft S1 of the control motor M1 can be positioned perpendicular to the direction of the rotation axis of the ring gear 4, whereby the ring gear 4 forms internal teeth meshing with the planetary gears 3 and side teeth meshing with the first pinion 9 according to a bevel gear arrangement.

[0029] The second pinion 11 is connected to the shaft S2 of the auxiliary motor M2 via a second one-way clutch 12. The second pinion 11 is meshed with the traction gear 8, and the auxiliary motor M2 can assist and drive the bicycle forward.

[0030] The second one-way clutch 12 connecting the auxiliary motor M2 to its pinion 11 is arranged to engage when the auxiliary motor M2 attempts to drive the traction gear 8 in the forward direction to assist in driving the bicycle forward. When the auxiliary motor M2 is turned off but the traction gear 8 continues to rotate forward, the second one-way clutch 12 allows the traction gear 8 to rotate freely.

[0031] The first and second one-way clutches 10, 12 may be in the form of a pawl and ratchet, for example, or a sprag clutch having rollers that ride up a ramp within a cage, or a belt or strip wrapped around a shaft.

[0032] Due to the above arrangement, the control motor M1 controls the ratio between the pedal speed and the bicycle speed by controlling the speed of the ring gear 4. The auxiliary motor M2 applies torque to the sprocket 6 by driving the traction gear 8 to assist the bicycle to move forward.

[0033] The electric bicycle drive system described above operates in the following manner. When the bicycle starts from rest, the rider applies torque to the system by pedaling. This torque is transmitted to the system via the pedal shaft 1, the planetary gear carrier 2 and the planetary gear 3. The planetary gear 3 then distributes the applied torque between the ring gear 4 and the sun gear 5.

[0034] The schematic diagram of the torque distribution between the ring gear 4 and the sun gear 5 is as follows: Figure 2 As shown, where:

[0035] Tc = torque applied to planetary gear carrier 2;

[0036] Zr = radius of planetary gear carrier 2;

[0037] Zs = radius of the planetary gear;

[0038] Fr = tangential force applied to the ring gear 4;

[0039] Fs = tangential force applied to the sun gear 5;

[0040] And where Fr = Fs = 1 / 2*Tc / Zr

[0041] Tr = torque applied to the ring gear: Tr = Fr*(Zr+Zs); and

[0042] Ts = torque applied to the sun gear: Ts = Fs*(Zr-Zs)

[0043] Initially, starting from a standstill ( Figure 3 ), the control motor M1 is turned off. The torque Tr is applied to the ring gear 4 in a forward direction, however the first one-way clutch 10 is arranged to prevent the forward rotation of the ring gear 4. Therefore, the torque Tr is reacted by the first one-way clutch 10, and the ring gear 4 remains stationary. Therefore, all the power provided by the rider R is transferred to the sun gear 5 and is transferred to the bicycle wheel via the sprocket or chain wheel 6 and the chain or belt 7. The transmission ratio between the pedal and the wheel of the bicycle is expressed as follows:

[0044]

[0045] Due to the action of the first one-way clutch 10, which allows the ring gear 4 to rotate in the reverse direction but not in the forward direction, there is a minimum overall transmission ratio when the ring gear 4 is stationary. The ratio of the planetary gear system and the ratio of the chain or belt can be arranged so that the ratio of the ring gear 4 when it is stationary is equal to a suitable ratio for starting the bicycle from rest or climbing a steep hill. As an indication, for a touring or commuter bicycle, this minimum ratio may have a value of about 1.

[0046] During a start from rest, the traction motor M2 may be powered by the battery 17 via the controller 16 to assist the rider in moving the bicycle forward. The traction motor M2 applies torque via the second one-way clutch 12 and the pinion 11 to move the traction gear 8 forward, thereby assisting in accelerating the bicycle forward. The second one-way clutch 12 is arranged to be locked when the traction motor M2 applies torque to the traction gear 8 in the forward direction.

[0047] When the bicycle starts to increase speed, in order to keep the speed of the pedals (i.e. the speed of the pedal shaft 1) at a speed that is comfortable for the rider, the overall gear ratio needs to be increased. This is done by giving the control motor M1 ( Figure 4 ) is energized to rotate the ring gear 4 in the opposite direction. This action unlocks the first one-way clutch 10, which is arranged to allow the ring gear 4 to rotate freely in the reverse direction. The speed of the control motor M1 is controlled to maintain the required ring gear speed (Wr), which is given by the following equation:

[0048] Wr=(Wc(Zr+Zs)–WsxZs) / Zr

[0049] in:

[0050] Wc = desired speed of pedal shaft 1;

[0051] Ws = rotation speed of sun gear 5;

[0052] Wr = the speed required by the ring gear 4;

[0053] Zr and Zs are the system radii that define the lever ratio within the planetary gear system, such as Figure 2 As shown in the graphic.

[0054] When the speed of the control motor M1 starts to increase to maintain a comfortable pedaling speed, it starts to provide mechanical power to the system:

[0055] Power M1 =Wr*Tr

[0056] Wherein, Tr is the torque applied to the ring gear 4 to react to the rider's pedaling torque (such as Figure 2 shown).

[0057] Following the law of conservation of energy, the mechanical power provided by control motor M1 supplements the mechanical power provided by traction motor M2 to help the rider move the bicycle forward.

[0058] As the bicycle speed increases further, the torque Tr applied by the rider tends to remain substantially constant, while the speed of the control motor M1 continues to increase to maintain a comfortable pedaling speed, thereby increasing the power of the control motor M1. The power of the control motor M1 can be set at certain points by programming the controller 16. M1 ) becomes sufficient to deliver the desired electric assist power to the bicycle, and no further assistance from the traction motor M2 is required. The traction motor M2 can then be turned off ( Figure 5 ) to save electrical energy. The bicycle continues to move forward under the power of the rider and the assistance of the proportional control motor M1, so the traction gear 8 continues to rotate in the forward direction. However, the traction motor M2 no longer applies a forward torque to the traction gear 8, so the second one-way clutch 12 is unlocked to allow the second pinion 11 to rotate freely relative to the shaft of the traction motor M2. Therefore, the electric motor M2 is allowed to stop and it does not transmit a deceleration torque to the traction gear 8 or thus to the wheels of the bicycle.

[0059] It can be observed that if the first one-way clutch 10 does not exist, when the speed ratio is lower ( Figure 3) Starting from standstill, the torque Tr will need to be reacted solely through the electric motor M1. Since current must pass through the motor's windings to generate the necessary reaction torque, the control motor M1 consumes electrical energy. However, since the control motor M1 is stationary, this motor does not generate any mechanical energy. Therefore, all the electrical energy will be converted into heat in the motor windings, which is both inefficient and detrimental to the durability of the motor.

[0060] It can also be observed that if the second one-way clutch 12 does not exist, when accelerating at high speed ( Figure 5 ), the control motor M2 will continue to rotate and it will apply a deceleration torque to the traction gear 8 and thus transmit to the wheels of the bicycle. In addition, voltage will be generated in the motor winding, which may damage the electronic control system of the motor.

[0061] The advantages and benefits of the present drive system will be understood below.

[0062] Since the heat generation of the proportional control motor M1 is reduced, the efficiency and durability of the drive system are improved.

[0063] Due to the locking properties of the first one-way clutch 10, the bicycle can be ridden (in the lowest gear ratio) when no power is supplied to either motor, for example when the battery is dead. This is an inherent feature of the system's behavior - no special locking device or operating mode of the system needs to be selected.

[0064] Since the traction motor M2 can be mechanically disconnected from the system when not needed, system efficiency is improved at higher vehicle speeds.

[0065] Since the traction motor M2 cannot generate braking torque, it cannot generate any voltage in its windings that could damage the electronic system.

[0066] There is no need for either motor to regenerate any electrical energy. This results in simplified motor control and electronics, as well as a simplified battery management system that does not require boosting the motor voltage to charge the battery.

Claims

1. An electric auxiliary drive system for a bicycle, comprising: a pedal shaft (1) operated by a rider; a planetary gear mechanism (20) configured to determine a transmission ratio between the pedal shaft and an output shaft (21) to transmit rotation to a rear wheel of the bicycle; an auxiliary motor (M2) for driving an output shaft (21); a control motor (M1) drivingly connected to the planetary gear mechanism and controlling the transmission ratio between the pedal shaft (1) and the output shaft (21) through the planetary gear mechanism; a first one-way clutch (10) operably connected between the control motor (M1) and a rigid component (22) fixedly mounted on a bicycle frame, wherein the first one-way clutch (10) is configured to prevent a first rotatable member (4) of the planetary gear mechanism from rotating in a forward direction, and to release and allow the first rotatable member of the planetary gear mechanism to freely rotate in a reverse direction; as well as A second one-way clutch (12) is operably connected between the auxiliary motor and the output shaft, wherein the second one-way clutch is configured to drivingly connect the auxiliary motor (M2) to the output shaft (21) when the auxiliary motor (M2) is turned on to drive the output shaft in the forward direction to assist the bicycle to move forward, and to decouple the auxiliary motor (M2) from the output shaft (21) when the auxiliary motor is turned off but the output shaft (21) continues to rotate in the forward direction.

2. The drive system of claim 1, wherein the planetary gear mechanism (20) comprises: a sun gear (5) fixed to rotate with the output shaft; a ring gear (4); a set of planetary gears (3) located between the sun gear (5) and the ring gear (4); and a planetary gear carrier (2) fixed to rotate with the pedal shaft (1) and to support the planetary gears (3); wherein the ring gear (4) meshes with a first pinion (9) mounted on a first shaft (S1) driven by the control motor (M1), and The first one-way clutch (10) is operatively connected between the first shaft (S1) of the control motor (M1) and the rigid component (22) fixedly mounted to the bicycle frame.

3. The drive system according to claim 2, wherein the ring gear (4) forms internal teeth meshing with the planetary gears (3) and external teeth meshing with the first pinion (9).

4. The drive system of claim 2, wherein the ring gear (4) forms internal teeth meshing with the planetary gears (3) and side teeth meshing with the first pinion (9) according to a bevel gear arrangement.

5. The drive system according to any one of the preceding claims, further comprising: a second pinion (11) releasably and drivingly connected to a shaft (S2) of the auxiliary motor (M2) via the second one-way clutch (12); and A traction gear (8) is fixed to rotate with the output shaft (21) and meshes with the second pinion (11).

6. A drive system as claimed in any one of the preceding claims, wherein the output shaft (21) comprises a central tubular portion extending axially and coaxially around the length of the pedal shaft (1).

7. An electric bicycle comprising a drive system as claimed in any one of the preceding claims.

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