Control device of a start-assist system

Abstract: A start-assist system provided in a vehicle having two rear axles, one of which is a driven axle (200) includes an air actuator (505) attached to the driven axle; a level sensor (650) detecting loads applied to the axles; an air supply source (810) supplying compressed air to the actuator; a first valve (812) disposed in an air conduit for supplying air to the air actuator; a second valve (840) for releasing air from the air actuator; and control means (850) controlling the first valve and the second valve. The control means opens the first valve to introduce air to the air actuator when a signal from the level sensor indicates that the vehicle stops and is unladen, and opens the second valve to release air from the air actuator when the signal from the level sensor indicates that the vehicle stops and is laden.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a method of controlling a start-assist system installed in a vehicle of a type having two rear axles. Description of the Prior Art Fig. 10 is a schematic explanatory view of a vehicle having two rear axles. A big size vehicle, labelled 1, has a front axle 21 mounted in a front location of a frame 10 to support front wheels 21a. The vehicle 1 has two rear axles 31 and 32 which support wheels 31a and wheels 32a. These two rear axles 31, 21 are supported by the frame 10 via a suspension 40, and the frame 10 supports cargo 50 thereon. The whole weight WG of the vehicle 1 including the cargo 50 is divided to a load WF applied to the front wheels 21a and a load WR applied to the rear wheels 31a and 32a, and the ground G bears both these loads WF and WR. Since the vehicle 1 has two axles 31 and 32 in a rear half, the load WR applied to the entirety of the rear half of the vehicle 1 is divided to a load WRF supported by the rear-front axle 31 and a load WRR supported by the rear-rear axle 32. By limiting the maximum possible load applied to a single axle in this manner, vehicles for larger loads can be constructed. Among various vehicles having two rear axles, so-called 6x2 vehicles have been developed, in which the power transmission system is simplified by using the rear-front axle 31 as a drive axle for receiving the drive force from the engine and using rear-rear axle 32 as an idler axle which does not receive the drive force. In these 6x2 vehicles, the drive wheels 31a must generate a sufficient propulsive force. In this connection, the load WRF applied to the wheels 31a must be large enough to generate a sufficient propulsive force between the wheel 31a and the ground G. When the vehicle is laden, the load WRF applied to the drive axle 31 is large enough to generate a sufficient propulsive force at the wheels 31a. However, when the vehicle 1 is unladen, the load WRF to the drive axle 31 may be too small, and the wheels 31a may fail to produce a sufficient propulsive force againstthe ground G. In particular, when the vehicle is going to start moving on a ground having a small friction coefficient, it often fails to start successfully due to slippage of the wheels 31a. FR-A-2606711 discloses a device with speed and level sensors, which is automatically deactivated when a load limit on the drive axle is exceeded. Such system comprises six valves controlling five air springs and allows the driver to adjust the air pressure within the two air springs of the drive axle (which are linked together) and the two air springs of the idler axle. By increasing the load on the drive axle through, for example, operation of a manual switch, this system may also serve as a start-assist device. The present Applicant previously proposed a start-assist system for use in a vehicle configured to drive one of two rear axles, as described in Japanese Patent Application JP-A-7 237 425. The invention of the above application uses an air actuator for adjusting the load to the drive axle and a level sensor for detecting a change in load to axles and for controlling behaviors of the air actuator, in order to increase the driving force exerted when the vehicle starts to move. The use of this system certainly increases the traction produced by the drive wheels upon a starting run of the vehicle and improves the starting performance of the vehicle. The above start-assist system also improves the running efficiency of a vehicle on ascending slopes and the braking reliability on slopes. The above start-assist system works well when the vehicle is unladen. However, if the start-assist system is activated when the vehicle is laden, where a load large enough to produce a traction is applied to two rear axles, then an additional load is applied to the drive axle. Such a large load reduces the life of the axle and precipitates abrasion of drive wheels. OBJECTS AND SUMMARY OF THE INVENTION It is therefore an object of the invention to provide a method of controlling a start-assist system overcoming the above-indicated disadvantages. Accordingly there is provided a method of controlling a start-assist system equipped in a vehicle having two rear axles one of which is a drive axle and the other is an idler axle, and including an air actuator attached to the drive axle to increase the load to drive wheels mounted on the drive axle, an air supply source for supplying compressed air to the air actuator; a first valve disposed in an air conduit for supplying air to the air actuator; a second valve for releasing air from the air actuator; and a control unit for controlling the first valve and the second valve. The control unit opens the first valve to introduce air to the air actuator when a signal indicates that the vehicle is stopped and unladen, and opens the second valve to release air from the air actuator when the signal indicates that the vehicle is stopped and laden. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an explanatory view of a control device implementing the method of controlling a start-assist system according to a first embodiment of the invention; Fig. 2 is a flow chart of the method according to the first embodiment; Fig. 3 is a flow chart of the process of control by a modified version of the control method according to the first embodiment; Fig. 4 is a plan view of a vehicle equipped with a start-assist system; Fig. 5 is a side elevation of the vehicle of Fig. 3; Fig. 6 is a front elevation of a drive axle Fig. 7 is an explanatory view of a load detector; Fig. 8 is an explanatory view of a level sensor; Fig. 9 is an explanatory view of a process of judgement by the level sensor that the vehicle stops; Fig. 10 is an explanatory view showing a general aspect of a vehicle having two rear axles; Fig. 11 is an explanatory view of a control device implementing the method of controlling a start-assist system according to a second embodiment of the invention; and Fig. 12 is a flow chart of the method according to the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 4 is a fragmentary plan view of a vehicle equipped with a start-assist system to which the method disclosed by the invention is applied. Fig. 5 shows a construction of a trunnion suspension equipped with the start-assist system according to the method disclosed by the invention. Fig. 6 is a fragmentary front elevation of a drive axle and its peripheral elements. Fig. 7 is an explanatory view of a load detector used in the invention. With reference to Figs. 4 to 6, the start-assist system is equipped in the vehicle having the trunnion suspension to increase the drive force required when the vehicle starts to run. The vehicle has a frame 10 disposed at right and left sides of the vehicle body to support a drive axle 200 and an idler axle 300. An engine disposed at a front portion of the vehicle, a propeller axle 60 coupled to a gear box transmit power to a differential gear mechanism contained in a differential gear case 70 which is integral with the drive axle 200. The trunnion suspension has trunnion brackets 110 fixed on the lower surface of the frame 10. The trunnion bracket 110 rotatably supports a trunnion axle which in turn supports a central portion of a leaf spring 130. The leaf spring 130 is a laminated plate member made by stacking several to ten leaf springs. The drive axle 200 is a unit including a drive axle rotatably contained in a cylindrical housing 210 and supporting drive wheels 250 at opposite ends. In general, two drive wheels 250 are two at each end of the drive axle and four in total. Provided on the housing 210 of the drive axle 200 is a sliding sheet 220 which receives one end of the leaf spring 130. The leaf spring exerts a resilient force for urging the drive axle 200 onto the ground. Spring pressers 230 are provided at opposite ends of the sliding sheet 220. A first-torque rod is attached to the lower surface of the drive axle housing 210 via a bracket. The first torque rod is coupled to a second torque rod through a coupling member extending through the trunnion bracket 110. The idler axle 300 does not directly receive the driving force from the engine but rotatably supports idler wheels 350. In general, the idler wheels 350 are two at each end of the idler axle 300 and four in total; however, they may be only one at each end. A central portion of the drive axle housing 210 projects upward and downward for containing the differential gear case 70. A bracket 260 is mounted on the top of the upper projecting portion to which a third torque rod 150 is coupled. With the above-explained construction of the trunnion suspension, any appropriate ratio of loads distributed to both axles can be selected by appropriately determining the distance between the center of the trunnion axle and the center of the drive axle 200 and the distance between the center of the trunnion axle. The drive axle 200 and the idler axle 300 can move along circular orbits of radii R1 and R2, and the distance between both axles varies between D and D1. A start-assist system to which the invention is applied is mounted on the drive axle 200. A start-assist system, labelled 500, includes an air actuator 505 and its mounting structure. The air actuator 505 comprises a diaphragm 510, piston member 520 disposed under the diaphragm 510, and upper plate 530 disposed on the diaphragm 510. A lower portion of the diaphragm 510 is attached to a bracket on the side of the drive axle. An upper portion of the diaphragm 510 is attached to a bracket on the part of the frame. Used as the vehicle frame 10 is a channel material having a channel-shaped cross section, for example. An attachment member 570 of the frame-side bracket is fixed to the inner side of the frame 10. A bracket 560 opening downward is integrally mounted to the attachment member 570, and securely contains an upper plate 530 of the air actuator 505 inside. An air inlet 540 of the air actuator 505 projects upward passing through the bracket 560. The air inlet 540 is connected to a compressed air source through an air supply pipe which will be explained later. Since big-sized vehicles, in general, are equipped with an air compressor and a high-pressure tank for assisting clutching and braking actions, the source of the compressed air can be utilized for this purpose. Under the condition with no compressed air introduced to the air actuator 505, the drive axle 200 and the idler axle 300 receive loads of a ratio equal to that in an ordinary trunnion suspension. When compressed air is introduced into the air actuator 505, the diaphragm 510 expands and biases the drive axle 200 toward the ground. This biasing force increases the load applied to the drive wheels 250 and hence increases the propulsive force produced between the drive wheels 250 and the ground. Therefore, if the start-assist system is activated when the vehicle starts to run where a large propulsive force is required, the vehicle can start smoothly and reliably. The air actuator used as an element of the start-assist system is liable to break down because the diaphragm, in particular, is made of a flexible material. The air actuator is therefore protected by locating it at the inner side of the vehicle frame and by covering it with the bracket. The trunnion suspension permits the housing to rotate about the center line of the vehicle axle when the vehicle axle moves vertically. This rotation causes deviations at the upper support portion and other support portions of the air actuator; however, adverse affection of these deviations can be minimized by locating the air actuator on the drive axle housing 210. In Fig. 7, an axle load detector, labelled 600, comprises a leaf spring 610 and a level sensor 650 for detecting a change in position of the center of the leaf spring 610. The two-rear-axle vehicle has a first rear axle 200 and a second rear axle 300. The first rear axle nearer to the front axle comprises the housing 210 containing an axle, and a differential gear case 70, and behaves as a drive axle. The second rear axle 300 has a housing 310 and behaves as an idler axle which does not receive the driving force. It is of course possible to configure the second rear axle 300 to behave as a drive axle. A front end portion of the leaf spring 610 is supported on a rear portion of the differential gear case 70 of the drive axle 200 via a bracket 620. A rear end portion of the leaf spring 610 is supported on a front portion of the idler axle housing 310 via a bracket 630. A bush 622 is provided between the bracket 620 and the leaf spring 610 to absorb relative twist or other stress between the bracket and the leaf spring. Also the idler axle bracket 630 similarly supports a leaf spring via a bush. The leaf spring 610 supports the lower end of a vertically extending rod 640 at its central portion via mount members 642, 643. The upper end of the rod 640 is coupled to a horizontally extending arm 645 which rotates a axle 652 of the level sensor 650. The level sensor 650 is fixed to the chassis of the vehicle to electrically detect the amount of rotation of the axle 652. When any change occurs in loads applied to the drive axle 200 and the idler axle 300, the suspension deforms by changing the vertical distance between the position of the rod support portion LC of the leaf spring and the position of the level sensor 650. Therefore, by electrically detecting a change in this distance, loads applied to the drive axle 200 and the idler 300 can be detected. Among various types of suspension for two-rear-axle vehicles, there is one in which the inter-axle distance D varies with changes in heights of the axles 200 and 300 from the ground. If the axle load detector is used to a vehicle having a suspension of this type, the sensitivity of detecting loads to axles can be improved. That is, in a suspension where the inter-axle distance D increases with load with respect to that under the unladen status, the distance between the front-end attachment LF and the rear-end attachment LR of the leaf spring 610 increases, and the rod support portion LC rises bringing the rod 640 upward. Since the rod 640 rotates the axle 652 of the sensor 650 by the amount of the displacement of the leaf spring 610 plus the vertical displacements of the axles 200 and 300 from the ground, the angle of rotation of the axle 652 becomes large, and the sensitivity is improved. The sensor 650 varies in its output voltage depending on the position of the arm 645. Thus, as shown in Fig. 8, CPU 850 determines that the vehicle is laden when the arm 645 is elevated by the rod 640 beyond a given position, and determines that the vehicle is unladen when the rod 640 remains below the given position. Fig. 9 shows a typical aspect of activation of the level sensor. The abscissa shows time, and the ordinate shows positions of the arm of the level sensor. Since the output voltage of the sensor varies with the position of the arm as stated above, the ordinate may be regarded as changes in output voltage of the sensor. In the period of time from a to b which has passed with no change in the position of the arm, CPU determines that the vehicle has stopped. In addition, since the position of the arm is in the unladen range, CPU determines that the vehicle lies unladen. In the period of time from b to c where the position of the arm changes in the given time, CPU determines that the vehicle currently runs. When CPU determines so, it does not determine whether the vehicle is laden or unladen. When the vehicle stops at the point of time c and is loaded with cargo at the point of time d, the position of the arm of the level sensor enters in the laden region, and when a given time passes at the position, CPU determines that the vehicle has stopped and has been laden. After the point of time e, CPU determines from changes in position of the arm that the vehicle currently runs, and does not determine whether it is laden or unladen. The given period of time for determining whether the vehicle has stopped may be, for example, about 5 to 10 seconds. Fig. 1 is an explanatory view showing a general aspect of a control device implementing the method of controlling a start-assist system according to an embodiment of the invention. The control device 800 of a start-assist system comprises an air tank 810 containing air which is introduced into the air actuator 505 of the start-assist system 500 through a first control valve A812, a check valve 814 and a conduit 820, and increases the load to the drive axle 200. CPU 850, used as a control unit, is activated by an actuator switch 860 and receives a signal from the level sensor 650 and a speed sensor 856 to control the first valve (A) 812 and a second valve (B) 840. The first valve 812 controls the supply of air to the conduit 820, and the second valve 840 releases air from the conduit 820. Fig. 2 is a flow chart showing the process of control by the control device 800 according to the above embodiment. The process starts with step S10. In step S11, the speed of the vehicle is detected. If the vehicle stops, the process goes to step S12, and detects whether the output signal of the level sensor 650 indicates the unladen condition. If the output signal of the level sensor 650 shows the unladen condition, then the flow proceeds to step S13 and opens the first valve 812 to introduce air into the air actuator 505. In step S14, when it is detected that a given time has passed after the valve 812 is opened, the valve is closed in step S15. Thus, the air actuator expands large enough to make the start-assist system operative, and a flow of the process ends. When it is detected in step S11 that the vehicle stops, then the flow proceeds to step S12. If the output of the level sensor indicates the laden condition in step S12, the flow goes to S20 where the second valve 840 is opened to release air from the air actuator 505. In step S21, when a predetermined time passes after the valve 840 is opened, the valve is closed in step S22. Thus, the start-assist system is deactivated. Fig. 3 is a flow chart showing the process of control by a modified version of the control device 800 according to the same embodiment. This process of control is substantially the same as the control process shown in the flow chart of Fig. 2 except that the flow goes to step S20 when the speed of the vehicle reaches 20 km/h in step S16 after the first valve 812 is closed, and opens the second valve 840 to introduce air into the air actuator 505. According to the above-described embodiment, in a vehicle having two rear axles one of which behaves as a drive axle, and equipped with the start-assist system which includes the air actuator attached to the drive axle and activated upon starting the vehicle to increase the load, the system is automatically activated when detecting that the vehicle is unladen and stops, whereas it is deactivated when detecting that the vehicle is laden and stops or when the speed of the vehicle exceeds a predetermined value. Therefore, it is sufficient for the driver to operate the actuator switch, and the system is automatically deactivated when it is not required. Thus the durability of the system is improved. Fig. 11 shows an explanatory view showing a general aspect of a control device implementing the method of controlling a start-assist system according to a second embodiment of the invention. This embodiment is substantially the same as the former embodiment except that CPU 850 does not include a speed sensor 856. Fig. 12 is a flow chart showing the process of control by the control device 800 according to the second embodiment. The process starts with step S10. In step S11, it is confirmed with a signal from the level sensor 650 whether the predetermined time has passed while the arm of the level sensor 650 keeps the current position. If the time already passes at the position, it is determined that the vehicle has stopped, and the flow proceeds to step S12 to detect whether the output signal from the level sensor 650 indicates the unladen condition. If the output of the level sensor shows the unladen condition, then the process goes to step S13 and opens the first valve 812 to introduce air into the air actuator 505. In step S14, when it is detected that a given time has passed after the valve 812 opened, the valve is closed in step S15. Thus, the air actuator expands large enough to make the start-assist system operative, and a flow of the process ends. In step S11, it is confirmed whether the predetermined time has passed since the arm of the level sensor takes the current position. If the time passes at the position, it is determined that the vehicle has stoppped, and the flow proceeds to step S12. If the output of the level sensor indicates the laden condition in step S12, the process goes to S20 where the second valve 840 is opened to release air from the air actuator 505. In step S21, when a predetermined time passes after the valve 840 is opened, the valve is closed in step S22. Thus, the start-assist system is deactivated. According to the second embodiment described above, in a vehicle having two rear axles one of which behaves as a drive axle, and equipped with the start-assist system which includes the air actuator attached to the drive axle and activated upon starting the vehicle to increase the load, the system is automatically activated when detecting that the vehicle is unladen and stops, whereas it is deactivated when detecting that the vehicle is laden and stops. Therefore, it is sufficient for the driver to operate the actuator switch, and the system is automatically inactivated when it is not required. Thus the durability of the system is improved. Moreover, the second embodiment is advantageous also in requiring no additional sensor to detect that the vehicle stops. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly, such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs. FIG. 1 810 Air Tank 812 Valve A 840 Valve B 856 Speed Sensor 650 Level Sensor 505 Air Actuator FIG. 2 S11 Speed = 0 km/h? S12 Does Signal from Level Sensor indicates unladen state? S13 Open Valve A S14 Time has passed since Valve A opened? S15 Close Valve A S20 Open Valve B S21 Time has passed since Valve B opened? S22 Close Valve B FIG. 3 (See Fig. 2) S16 Speed < 20 km/h? S17 Speed = 0 km/h? FIG. 8 Unladen Laden FIG. 9 Position of Arm of Level Sensor Laden Unladen Time Stopped & unladen Run Stopped & unladen Stopped & laden Run FIG. 11 (See Fig. 1) FIG. 12 S11 Time has passed with Arm kept there? S12 Signal from Level Sensor indicates "unladen"? S13 Open Valve A S14 Time has passed since Valve A opened? S15 Close Valve A S20 Open Valve B S21 Time has passed since Valve B opened? S22 Close Valve B

Claims: A method of controlling a start-assist system equipped in a vehicle having two rear axles one of which is a drive axle (200) and the other is an idler axle (300), and including an air actuator (505) attached to the drive axle (200) to increase the load to drive wheels mounted on the drive axle (200), an air supply source (810) for supplying compressed air to said air actuator (505), a first valve (812) disposed in an air conduit for supplying air to said air actuator (505), a second valve (840) for releasing air from said air actuator (505), and a control unit (850) for controlling said first valve (812) and said second valve (840), characterised in that: said control unit (850) opens said first valve (812) to introduce air to said air actuator (505) when a signal indicates that the vehicle is stopped and unladen, and said control unit opens said second valve (840) to release air from said air actuator (505) when the signal indicates that the vehicle is stopped and laden. The method according to claim 1, characterised in that the signal which indicates whether the vehicle is stopped or not is obtained by a speed sensor (856), and the signal which indicates the vehicle is laden or not is obtained by a level sensor (650) for detecting the level of the rear axles. The method according to claim 1, characterised in that the signal which indicates whether the vehicle is stopped or not is obtained by a level sensor (650) for detecting the level of the rear axles and the signal which indicates the vehicle is laden or not is obtained by the same level sensor (650).

Inventors: HIGUCHI MINORI, HIRAHARA HIDETO, HOSOYA HARUYUKI, KANO MASAAKI, HIGUCHI, MINORI, HIRAHARA, HIDETO, HOSOYA, HARUYUKI, KANO, MASAAKI, Higuchi, Minori, c/o Isuzu Motors Lim., Hirahara, Hideto, c/o Isuzu Motors Lim., Hosoya, Haruyuki, c/o Isuzu Motors Lim., Kano, Masaaki, c/o Isuzu Motors Lim.
Applicants: ISUZU MOTORS LTD, ISUZU MOTORS LIMITED
Application year: 1996
Main classification: B60G17
IPC codes: B60G17, B60G21, B62D61