Braking control method according to friction of road surface

Abstract

A braking control method according to friction of road surface includes computing a real-time wheel speed according to a signal received from a wheel speed sensor; storing the real-time wheel speed as a wheel initial velocity when a braking event occurs; determining a relative-peak value according to the real-time wheel speed; estimating a vehicle deceleration according to the relative-peak value and the wheel initial velocity; computing an adjustment parameter according to the vehicle deceleration and a tire slip threshold, wherein the adjustment parameter reflects friction coefficient of road surface; and adjusting time length of an enhancement stage in an enhance-pressure control period of a stepped pressure-increasing phase according to the adjustment parameter; or adjusting time length of a reduction stage in a reduce-pressure control period of a stepped pressure-decreasing phase according to the adjustment parameter.

Claims

1. A braking control method according to friction of road surface, performed by a control module of an anti-lock braking system connected to a wheel speed sensor, wherein the control module receives a signal of a wheel speed from the wheel speed sensor and performs an intermittent brake mode including a stepped pressure-increasing phase or a stepped pressure-decreasing phase, and the braking control method is applied to the stepped pressure-increasing phase and the stepped pressure-decreasing phase; the braking control method comprising: computing a real-time wheel speed according to the signal received from the wheel speed sensor; storing the real-time wheel speed as a wheel initial velocity when a braking event occurs; determining a relative-peak value according to the real-time wheel speed; estimating a vehicle deceleration according to the relative-peak value and the wheel initial velocity; computing an adjustment parameter according to the vehicle deceleration and a tire slip threshold, wherein the adjustment parameter reflects a friction coefficient of road surface; adjusting a time length of an enhancement stage in an enhance-pressure control period of the stepped pressure-increasing phase according to the adjustment parameter; or adjusting a time length of a reduction stage in a reduce-pressure control period of the stepped pressure-decreasing phase according to the adjustment parameter; and controlling an increase or a decrease of a braking state according to the adjustment parameter.

2. The braking control method as claimed in claim 1, wherein the time length of the enhancement stage becomes longer when the friction coefficient of road surface becomes higher; and the time length of the enhancement stage becomes shorter when the friction coefficient of road surface becomes lower.

3. The braking control method as claimed in claim 2, wherein the time length of the reduction stage becomes shorter when the friction coefficient of road surface becomes higher; and the time length of the reduction stage becomes longer when the friction coefficient of road surface becomes lower.

4. The braking control method as claimed in claim 3, wherein the stepped pressure-increasing phase includes multiple said enhance-pressure control periods in sequence and of the same time length, and each one of the enhance-pressure control periods has the enhancement stage adjustable by the adjustment parameter and a retaining stage after the enhancement stage; and the stepped pressure-decreasing phase includes multiple said reduce-pressure control periods in sequence and of the same time length, and each one of the reduce-pressure control periods has the reduction stage adjustable by the adjustment parameter and a retaining stage after the reduction stage.

5. The braking control method as claimed in claim 4, wherein the adjustment parameter is represented as: $u=\frac{a}{1-\mathrm{ABSout}}$ wherein u is the adjustment parameter for reflecting the friction coefficient of road surface, a is the vehicle deceleration, and ABSout is the tire slip threshold higher than 0 and lower than 1.

6. The braking control method as claimed in claim 4, wherein the vehicle deceleration is represented as: $a_x=\frac{.Math.v_x-v_0.Math.}{t_x-t_0}$ wherein a.sub.x is the x.sup.th vehicle deceleration, v.sub.x is the x.sup.th relative-peak value, t.sub.x is the time when v.sub.x occurs, v.sub.0 is the wheel initial velocity, and t.sub.0 is the time when v.sub.0 occurs.

7. The braking control method as claimed in claim 4, wherein the wheel initial velocity is represented as: $v_{\mathrm{wheel}}=\frac{v_{\mathrm{rpm}}\times 2.Math.r}{60}\times \frac{60\times 60}{1000}\left(\frac{\mathrm{kilometer}}{\mathrm{hour}}\right)$ wherein v.sub.wheel is the real-time wheel speed, v.sub.rpm is a number of revolutions of the wheel per minute detected by the wheel speed sensor, and r is a radius of the wheel and a unit of the radius is meter.

8. The braking control method as claimed in claim 4, wherein the control module determines the relative-peak value according to a slope change of curve of the real-time wheel speed from a positive slope to a negative slope.

9. The braking control method as claimed in claim 3, wherein the adjustment parameter is represented as: $u=\frac{a}{1-\mathrm{ABSout}}$ wherein u is the adjustment parameter for reflecting the friction coefficient of road surface, a is the vehicle deceleration, and ABSout is the tire slip threshold higher than 0 and lower than 1.

10. The braking control method as claimed in claim 2, wherein the adjustment parameter is represented as: $u=\frac{a}{1-\mathrm{ABSout}}$ wherein u is the adjustment parameter for reflecting the friction coefficient of road surface, a is the vehicle deceleration, and ABSout is the tire slip threshold higher than 0 and lower than 1.

11. The braking control method as claimed in claim 1, wherein the time length of the reduction stage becomes shorter when the friction coefficient of road surface becomes higher; and the time length of the reduction stage becomes longer when the friction coefficient of road surface becomes lower.

12. The braking control method as claimed in claim 11, wherein the stepped pressure-increasing phase includes multiple said enhance-pressure control periods in sequence and of the same time length, and each one of the enhance-pressure control periods has the enhancement stage adjustable by the adjustment parameter and a retaining stage after the enhancement stage; the stepped pressure-decreasing phase includes multiple said reduce-pressure control periods in sequence and of the same time length, and each one of the reduce-pressure control periods has the reduction stage adjustable by the adjustment parameter and a retaining stage after the reduction stage.

13. The braking control method as claimed in claim 12, wherein the adjustment parameter is represented as: $u=\frac{a}{1-\mathrm{ABSout}}$ wherein u is the adjustment parameter for reflecting the friction coefficient of road surface, a is the vehicle deceleration, and ABSout is the tire slip threshold higher than 0 and lower than 1.

14. The braking control method as claimed in claim 12, wherein the vehicle deceleration is represented as: $a_x=\frac{.Math.v_x-v_0.Math.}{t_x-t_0}$ wherein a.sub.x is the x.sup.th vehicle deceleration, v.sub.x is the x.sup.th relative-peak value, t.sub.x is the time when v.sub.x occurs, v.sub.0 is the wheel initial velocity, and t.sub.0 is the time when v.sub.0 occurs.

15. The braking control method as claimed in claim 12, wherein the wheel initial velocity is represented as: $v_{\mathrm{wheel}}=\frac{v_{\mathrm{rpm}}\times 2.Math.r}{60}\times \frac{60\times 60}{1000}\left(\frac{\mathrm{kilometer}}{\mathrm{hour}}\right)$ wherein v.sub.wheel is the real-time wheel speed, v.sub.rpm is a number of revolutions of the wheel per minute detected by the wheel speed sensor, and r is a radius of the wheel and a unit of the radius is meter.

16. The braking control method as claimed in claim 12, wherein the control module determines the relative-peak value according to a slope change of curve of the real-time wheel speed from a positive slope to a negative slope.

17. The braking control method as claimed in claim 11, wherein the adjustment parameter is represented as: $u=\frac{a}{1-\mathrm{ABSout}}$ wherein u is the adjustment parameter for reflecting the friction coefficient of road surface, a is the vehicle deceleration, and ABSout is the tire slip threshold higher than 0 and lower than 1.

18. The braking control method as claimed in claim 1, wherein the adjustment parameter is represented as: $u=\frac{a}{1-\mathrm{ABSout}}$ wherein u is the adjustment parameter for reflecting the friction coefficient of road surface, a is the vehicle deceleration, and ABSout is the tire slip threshold higher than 0 and lower than 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a block diagram of an anti-lock braking system of the present invention;

(2) FIG. 2 is a flow chart of an embodiment of the present invention;

(3) FIG. 3A is a waveform diagram of a real-time wheel speed of the present invention;

(4) FIG. 3B is a waveform diagram of the adjustment parameter of the present invention;

(5) FIG. 4A is a waveform diagram of braking pressure in the stepped pressure-increasing phase of the present invention;

(6) FIG. 4B is a time diagram of the enhance-pressure control period of the present invention;

(7) FIG. 5A is a time diagram of the enhance-pressure control period of the present invention;

(8) FIG. 5B is a time diagram of the enhance-pressure control period of the present invention;

(9) FIG. 6A is a waveform diagram of braking pressure in the stepped pressure-decreasing phase of the present invention;

(10) FIG. 6B is a time diagram of the reduce-pressure control period of the present invention;

(11) FIG. 7A is a time diagram of the reduce-pressure control period of the present invention;

(12) FIG. 7B is a time diagram of the reduce-pressure control period of the present invention;

(13) FIG. 8 is a block diagram of an electronic stability control system (ESC);

(14) FIG. 9 is a block diagram of an anti-lock braking system (ABS);

(15) FIG. 10A is a waveform diagram of vehicle velocity and wheel speed after a braking event occurs;

(16) FIG. 10B is a waveform diagram of wheel acceleration after a braking event occurs;

(17) FIG. 10C is a waveform diagram of braking pressure after a braking event occurs;

(18) FIG. 11A is a waveform diagram of vehicle velocity and wheel speed after a braking event occurs; and

(19) FIG. 11B is a waveform diagram of braking pressure after a braking event occurs.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

(20) With reference to FIG. 1, an anti-lock braking system (ABS) essentially comprises a control module 10 and a wheel speed sensor 11 electrically connected to the control module 10. The control module 10 is electrically connected to a braking system 20 of a vehicle (such as a car, a truck, a scooter, and so on) for signal transmission. The control module 10 receives a signal v.sub.rpm of wheel speed from the wheel speed sensor 11 wherein the signal v.sub.rpm of the wheel speed is a number of revolutions of the wheel per minute detected by the wheel speed sensor 11. The control module 10 will compute a real-time wheel speed v.sub.wheel according to the signal v.sub.rpm. The real-time wheel speed v.sub.wheel will be represented as:

(21) $v_{\mathrm{wheel}}=\frac{v_{\mathrm{rpm}}\times 2\pi r}{60}\times \frac{60\times 60}{1000}\left(\frac{\mathrm{kilometer}}{\mathrm{hour}}\right)$

(22) In the above equation, r is a radius of the wheel and a unit of the radius is meter (m). The unit of the real-time wheel speed v.sub.wheel is kilometers-per-hour.

(23) In general, after the vehicle is started, the control module 10 will record the real-time wheel speed v.sub.wheel. With reference to FIG. 2, at first, the control module 10 determines whether a braking event occurs or not (STEP S01). When the barking event occurs, the control module 10 further determines whether a vehicle state meets a threshold condition or not (STEP S02). When the vehicle state meets the threshold condition, the control module 10 actively intervenes the operation of the vehicle (STEP S03). For example, the vehicle state will include the real-time wheel speed v.sub.wheel. When the real-time wheel speed v.sub.wheel is decreasing while the variation of the real-time wheel speed v in a unit of time is equal to or higher than a threshold as the threshold condition, which means the revolution speed of the wheel rapidly slows down, the wheels will be rapidly locked up. On the contrary, in the STEP S02, when the vehicle state does not meet the threshold condition, which means the decrease of the wheel speed is acceptable and the vehicle is still controllable by the driver, the control module 10 would not intervene the operation of the vehicle and the vehicle will be normally braked. Besides, in the STEP S02, when the vehicle state does not meet the threshold condition, the real-time wheel speed v.sub.wheel is deemed as a vehicle estimation speed (STEP S04) and then the control module 10 returns to the STEP S01.

(24) After the control module 10 actively intervenes the operation of the vehicle, the control module 10 will perform an intermittent brake mode. The intermittent brake mode includes a pressure-decreasing phase, a pressure-retaining phase, and a pressure-increasing phase, wherein the pressure-decreasing phase, the pressure-retaining phase, and the pressure-increasing phase occur in sequence. The pressure-decreasing phase will be a stepped pressure-decreasing phase. The pressure-increasing phase will be a stepped pressure-increasing phase. As a result, the intermittent brake mode will include the stepped pressure-decreasing phase or the stepped pressure-increasing phase or both.

(25) It is to be noted that the intermittent brake mode, conditions to enter the stepped pressure-increasing phase and the stepped pressure-decreasing phase, and pressure increasing speed in the stepped pressure-increasing phase and pressure decreasing speed in the stepped pressure-decreasing phase, performed by ABS are conventional arts, and thus would not be described in detail herein.

(26) In the embodiment of the present invention, the control module 10 actively intervenes the operation of the vehicle to adjust a time length of an enhancement stage in an enhance-pressure control period of the stepped pressure-increasing phase according to friction of road surface, or adjust a time length of a reduction stage in a reduce-pressure control period of the stepped pressure-decreasing phase according to the adjustment parameter. The detailed contents of the present invention are described in the following paragraphs.

(27) 1. Wheel Initial Velocity

(28) As mentioned above, after the vehicle is started, the control module 10 records the real-time wheel speed v.sub.wheel. In the embodiment of the present invention, the control module 10 stores the real-time wheel speed v.sub.wheel as a wheel initial velocity when a braking event, such as when the brake pedal of the vehicle is pressed down, occurs. With reference to FIG. 3A, the control module 10 will determine the braking event at t.sub.0, and the real-time wheel speed v.sub.wheel at t.sub.0 is stored as the wheel initial velocity v.sub.0. In other words, t.sub.0 is the time that v.sub.0 occurs.

(29) 2. Relative-Peak Value

(30) During the intermittent brake mode, the real-time wheel speed v.sub.wheel varies with time. The control module 10 will determine a relative-peak value according to the real-time wheel speed v.sub.wheel. For example, with reference to FIG. 3A, when the slope of the curve of the real-time wheel speed v.sub.wheel is changed from a positive slope to a negative slope at t.sub.1, the control module 10 determines the real-time wheel speed v.sub.wheel at t.sub.1 as the relative-peak value. Hence, the control module 10 will determine the relative-peak value according to a slope change of the curve of the real-time wheel speed v.sub.wheel from a positive slope to a negative slope. As time goes on, the control module 10 will determine several relative-peak values v.sub.2, v.sub.3 . . . during the process of the intermittent brake mode.

(31) 3. Estimation of a Vehicle Deceleration

(32) In the embodiment of the present invention, the vehicle deceleration is estimated according to the relative-peak value and the wheel initial velocity v.sub.0 and will be represented as:

(33) $a_x=\frac{\left|v_x-v_0\right|}{t_x-t_0}$

(34) In the above equation, a.sub.x is the x.sup.th vehicle deceleration, v.sub.x is the x.sup.th relative-peak value, t.sub.x is the time when v.sub.x occurs, v.sub.0 is the wheel initial velocity, and t.sub.0 is the time when v.sub.0 occurs. With reference to FIG. 3A, when the control module 10 determines the first relative-peak value v.sub.1 at t.sub.1, the control module 10 will estimate a first vehicle deceleration a.sub.1 according to the first relative-peak value v.sub.1 and the wheel initial velocity v.sub.0. The first vehicle deceleration a.sub.1 will be represented as:

(35) $a_1=\frac{.Math.v_1-v_0.Math.}{t_1-t_0}$

(36) Furthermore, based on the first vehicle deceleration a.sub.1, the control module 10 estimates a first vehicle velocity v.sub.vehicle,1 that will be represented as:
v.sub.vehicle,1=v.sub.0−a.sub.1×t
In the above equation, t is an elapsed time after the braking event occurs.

(37) As time goes on, when the control module 10 determines the second relative-peak value v.sub.2 at t.sub.2, the control module 10 then estimates a second vehicle deceleration a.sub.2 according to the second relative-peak value v.sub.2 and the wheel initial velocity v.sub.0. The second vehicle deceleration a.sub.2 will be represented as:

(38) $a_2=\frac{.Math.v_2-v_0.Math.}{t_2-t_0}$

(39) Furthermore, based on the second vehicle deceleration a.sub.2, the control module 10 estimates a second vehicle velocity v.sub.vehicle,2 that will be represented as:
v.sub.vehicle,2=v.sub.0−a.sub.2×t

(40) In the above equation, t is an elapsed time after the braking event occurs.

(41) As a result, after the braking event occurs, as time goes on, the control module 10 will estimate several vehicle decelerations according to the relative-peak values and the wheel initial velocity (STEP S05). Besides, the vehicle decelerations and the wheel initial velocity will be used to estimate the vehicle velocity v.sub.vehicle. The estimated vehicle velocity v.sub.vehicle will be represented as:
v.sub.vehicle=v.sub.0−a×t

(42) In the above equation, t is an elapsed time after the braking event occurs.

(43) In the embodiment of the present invention, the vehicle decelerations and the estimated vehicle velocity are updated with the variation of the real-time wheel speed v.sub.wheel.

(44) In order to estimate the vehicle velocity at the time after to and before t.sub.1, with reference to FIG. 3A, in the embodiment of the present invention, a preset vehicle deceleration a.sub.preset is used. The control module 10 estimates a reference vehicle velocity v.sub.ref after to and before t.sub.1 according to the preset vehicle deceleration a.sub.preset (STEP S03A). The reference vehicle velocity v.sub.ref will be represented as:
v.sub.ref=v.sub.0−a.sub.preset×t

(45) In the above equation, t is an elapsed time after the braking event occurs and before t.sub.1. Afterwards, the control module 10 compares the reference vehicle velocity v.sub.ref with the real-time wheel speed v.sub.wheel. When the reference vehicle velocity v.sub.ref is higher than the real-time wheel speed v.sub.wheel, the reference vehicle velocity v.sub.ref is deemed as an estimated vehicle velocity. On the contrary, when the reference vehicle velocity v.sub.ref is lower than the real-time wheel speed v the real-time wheel speed v is deemed as the estimated vehicle velocity (STEP S03B). The preset vehicle deceleration a.sub.preset will be higher than 0 and lower than 1 g, wherein g is equal to 9.8 (meter/second).

(46) 4. Adjustment Parameter for Reflecting the Friction Coefficient of Road Surface

(47) In the embodiment of the present invention, after the control module 10 intervenes the operation of the vehicle, the control module 10 computes an adjustment parameter according to a present vehicle deceleration obtained in the STEP S05 and a tire slip threshold (STEP S06). The adjustment parameter will be represented as:

(48) $u=\frac{a}{1-\mathrm{ABSout}}$

(49) In the above equation, u is the adjustment parameter for reflecting the friction coefficient of road surface, a is the vehicle deceleration, and ABSout is the tire slip threshold.

(50) The tire slip threshold ABSout is a constant preset in the control module 10. The value of the tire slip threshold ABSout is higher than 0 and lower than 1, i.e., 0<ABSout<1. When the anti-lock braking system determines that an actual tire slip is equal to or higher than the tire slip threshold ABSout, the anti-lock braking system will control the braking system 20 to stop a pressure-decreasing mode and start a pressure-increasing mode, and that would be an inherent function of the conventional anti-lock braking system. However, the inherent function will affect the vehicle deceleration. As a result, the estimated vehicle deceleration will be lower than an actual vehicle deceleration. In order to overcome the inconsistency, (1-ABSout) in the present invention is a correction factor for the adjustment parameter to meet the actual condition. In the arts of vehicles, a tire slip equation will be represented as:
tire slip (%)=|vehicle velocity−wheel speed|/vehicle velocity×100(%)

(51) In comparison of dry road surface and wet road surface, the dry road surface has a higher friction coefficient than the wet road surface. The performance of the wheels rotating on the dry road surface would be better than that on the wet road surface. Hence, when the vehicle is braked, the vehicle deceleration of the wheels corresponding to the dry road surface will be lower than that corresponding to the wet road surface. In addition, the adjustment parameter is computed according to the vehicle deceleration. Hence, the adjustment parameter will reflect the friction coefficient of road surface. In other words, lower adjustment parameter corresponds to lower vehicle deceleration and lower friction coefficient of road surface, and higher adjustment parameter corresponds to higher vehicle deceleration and higher friction coefficient of road surface.

(52) With reference to FIG. 3B, as time goes on, several adjustment parameters will be respectively computed according to different vehicle decelerations. For example, a first adjustment parameter u.sub.1 will be computed according to the first vehicle deceleration a.sub.1 and represented as:

(53) $u_1=\frac{a_1}{1-\mathrm{ABSout}}$

(54) A second adjustment parameter u.sub.2 will be computed according to the second vehicle deceleration a.sub.2 and represented as:

(55) $u_2=\frac{a_2}{1-\mathrm{ABSout}}$

(56) Calculation of the following adjustment parameters will be deduced from the above descriptions.

(57) 5. Control the Braking System Based on the Adjustment Parameters

(58) With reference to FIGS. 4A and 4B, when the process performed by the control module 10 enters any one of the stepped pressure-increasing phases P.sub.increase, a present adjustment parameter has been computed. The stepped pressure-increasing phase P.sub.increase includes one or more than one enhance-pressure control period T.sub.increase. Each enhance-pressure control period T.sub.increase includes an enhancement stage in a time length T1 and a retaining stage in a time length T2. The retaining stage is sequentially after the enhancement stage. The control module 10 adjusts the time lengths of the enhancement stage and the retaining stage according to the present adjustment parameter in the enhance-pressure control period T.sub.increase. Hence, the control module 10 controls the increase of braking state according to the adjustment parameters (STEP S07). When the adjustment parameter reflects a higher friction coefficient of road surface, the time length T1 of the enhancement stage in the enhance-pressure control period T.sub.increase is adjusted by the control module 10 to be longer. On the contrary, when the adjustment parameter reflects a lower friction coefficient of road surface, the time length T1 of the enhancement stage in the enhance-pressure control period T.sub.increase is adjusted by the control module 10 to be shorter.

(59) For example, in comparison of FIGS. 5A and 5B, when the adjustment parameter reflects a higher friction coefficient of road surface, the time length T1 of the enhancement stage will be extended as shown in FIG. 5A. Because the time length of each enhance-pressure control period T.sub.increase is the same, the time length T2 of the retaining stage becomes shorter accordingly. On the contrary, as shown in FIG. 5B, when the adjustment parameter reflects a lower friction coefficient of road surface, the time length T1 of the enhancement stage will be shortened. Because the time length of the enhance-pressure control period T.sub.increase is the same, the time length T2 of the retaining stage becomes longer accordingly. In FIGS. 5A and 5B, as an example, the time length of the enhance-pressure control period T.sub.increase will be 20 milliseconds (ms), wherein T1 will be 15 ms and T2 will be 5 ms in FIG. 5A, and T1 will be 5 ms and T2 will be 15 ms in FIG. 5B.

(60) With reference to FIGS. 6A and 6B, when the process performed by the control module 10 enters any one of the stepped pressure-decreasing phases P.sub.decrease, a present adjustment parameter has been computed. The stepped pressure-decreasing phase P.sub.decrease includes one or more than one reduce-pressure control period T.sub.decrease. Each reduce-pressure control period T.sub.decrease includes a reduction stage in a time length T3 and a retaining stage in a time length T4. The retaining stage is sequentially after the reduction stage. The control module 10 adjusts the time lengths of the reduction stage and the retaining stage according to the present adjustment parameter in the reduce-pressure control period T.sub.decrease. Hence, the control module 10 controls the decrease of braking state according to the adjustment parameters (STEP S07). When the adjustment parameter reflects a higher friction coefficient of road surface, the time length T3 of the reduction stage in the reduce-pressure control period T.sub.decrease is adjusted by the control module 10 to be shorter. On the contrary, when the adjustment parameter reflects a lower friction coefficient of road surface, the time length T3 of the reduction stage in the reduce-pressure control period T.sub.decrease is adjusted by the control module 10 to be longer.

(61) For example, in comparison of FIGS. 7A and 7B, when the adjustment parameter reflects a higher friction coefficient of road surface, the time length T4 of the retaining stage will be extended and the time length T3 of the reduction stage is accordingly shortened as shown in FIG. 7A. On the contrary, as shown in FIG. 7B, when the adjustment parameter reflects a lower friction coefficient of road surface, the time length T3 of the reduction stage will be extended and the time length T4 of the retaining stage is accordingly shortened. In FIGS. 7A and 7B, as an example, the time length of the reduce-pressure control period T.sub.decrease will be 20 milliseconds (ms), wherein T3 in FIG. 7A will be 5 ms and T3 in FIG. 7B will be 15 ms.

(62) In conclusion, the braking control method of the present invention brakes the vehicle according to the adjustment parameters. Because the adjustment parameters reflect the friction coefficient of road surface, the present invention will adaptively adjust the time length of the enhancement stage in the enhance-pressure control periods T.sub.increase or adaptively adjust the time length of the reduction stage on the reduce-pressure control periods T.sub.decrease in different conditions of road surfaces. Hence, the wheels of the vehicle avoid being rapidly locked up and retain rotation in a certain speed to maintain the friction against the road surface.

(63) Even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes will be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.