CONTROL APPARATUS FOR HYDRAULIC CONTINUOUSLY VARIABLE SPEED TRANSMISSION

Abstract: In a continuously variable speed transmission having a hydraulic pump (P), a hydraulic motor (M), a hydraulic closed circuit hydraulically connecting the pump with the motor, a control apparatus comprises a slip sensor (61,63) for detecting slip of the driving wheels and hydraulic control means (20) for lowering hydraulic pressure in the hydraulic closed cirucit in accordance with magnitude of the slip of the wheels, in such a way that the driving force of the driving wheels is reduced less than a slip limit force.

Description: The invention relates to a hydraulic continuously variable speed transmission composed of a hydraulic pump and a hydraulic motor, and more particularly to a control apparatus for such variable speed transmission mounted on a vehicle. There have been proposed various types of hydraulic continuously variable speed transmissions (see, for example, Japanese Patent Laid-Open Publication No.56(1981)-95722); wherein the transmission comprises a constant displacement hydraulic pump driven by an engine and a variable displacement hydraulic motor driven by hydraulic pressure supplied through a hydraulic circuit from the hydraulic pumps to transmit the rotation to the wheels. The transmissions are adapted to continuously vary the transmitted engine speed to the wheels by controlling displacement of the hydraulic motors. In such hydraulic continuously variable speed transmissions, the driving torque acting on the wheels depends on the hydraulic pressure generated in the hydraulic circuit and on the displacement of the hydraulic motor (i.e. the reduction ratio). This hydraulic pressure in the hydraulic circuit is limited only by a relief valve within a range below a given maximum pressure (relief hydraulic pressure). If the pressure is below this relief hydraulic pressure, the hydraulic pressure in the hydraulic circuit is determined in accordance with the driving load of the wheels. Therefore, when for example, driving on a road having a low frictional coefficient, if the torque on a wheel exceeds the driving load (the gripping force of wheels) that is determined by the frictional coefficient between the road and the wheel, the wheel receiving that driving torque will slip and the driving (gripping) force thereof will be reduced, and as, a result, the vehicle will not be able to acquire necessary driving force or acceleration. This problem occurs not only in driving the wheels by the engine but also in decelerating the vehicle with engine braking. An object of the invention is to provide a control apparatus capable of preventing the slip of the wheels and resultant reduction of the driving forces thereof in a vehicle equipped with a continuously variable speed transmission. Another object of the invention is to provide a control apparatus capable of suppressing the driving forces on the wheels within a slip limit, the apparatus preferably being capable of preventing the slip of the wheels both when the engine power is transmitted to the wheels and when decelerating a vehicle with engine braking. EP-A-226844 discloses a control apparatus for a continuously variable speed transmission for a vehicle having a hydraulic pump connected to an engine, a hydraulic motor connected to driving wheels, and a hydraulic closed circuit hydraulically connecting said hydraulic pump with said hydraulic motor, at least one of said hydraulic pump and said hydraulic motor being of a variable displacement type, the control apparatus comprising a slip sensor for detecting slip of the driving wheels driven by said hydraulic motor. The arrangement disclosed in this EP-A 226844 has a control device associated with each hydraulic motor driving each wheel. This control device is in addition to regulating devices which in normal circumstances control the motors via a closed hydraulic system. When slip is detected, the control device associated with the motor which is applying too much torque is operated. Each control device has a valve which is opened when the control device is operated so that the increased control pressure from the control lead section is able to act on the regulating device to thereby override it resulting in a decrease of the swept volume of the swash plate of the motor concerned. Thus, the control device does not act on the closed hydraulic circuit, but overrides the regulating device to decrease the swept volume of the motor rather than decreasing the pressure applied by the closed hydraulic circuits to the motor. The present invention is characterised in that the control apparatus further comprises hydraulic control means for lowering hydraulic pressure in said hydraulic closed circuit by exhausting hydraulic fluid from said hydraulic closed circuit in accordance with the magnitude of the slip detected by said slip sensor, in such a way that the driving force applied to said driving wheels is reduced so as to be less than a slip limit force. The engine speed is transmitted to the wheels through a continuously variable speed transmission comprising a hydraulic pump and hydraulic motor. The control apparatus functions in such a way that when the driving force exceeds the driving load determined by the frictional force (tire's gripping force) between the wheels and the road surface causing slip of the wheels, the slip is sensed by the slip sensor, according to which the hydraulic pressure in the hydraulic circuit is lowered to reduce the driving force of the wheels, suppressing the slip. Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, wherein: FIG. 1 is a schematic diagram of the hydraulic circuit of a continuously variable speed transmission in accordance with the invention, FIGS. 2 and 3 are graphs showing the relationship between the duty ratio of the solenoid valve of the above control apparatus and the relief hydraulic pressure P H , and the relationship between the slip ratio and the duty ratio, respectively, FIG. 4 is a flow chart of the control of the said control apparatus, FIG. 5 is a cross sectional view of a clutch valve and a servo unit used in the hydraulic pressure control for a second embodiment according to the invention. Fig. 1 shows a hydraulic circuit diagram of a continuously variable speed transmission provided with a control apparatus, according to the invention. The continuously variable speed transmission has a constant displacement hydraulic pump P driven by the engine E through the input shaft 1 and a variable displacement hydraulic motor M connected to the output shaft 2 driving the wheels W₁. The hydraulic pump and motor P and M constitute a closed hydraulic circuit along with two hydraulic lines: the first hydraulic line La by which the delivery port of the pump P communicated with the suction port of the motor M, and the second hydraulic line Lb by which the suction port of the pump P communicates with the delivery port of the motor M. A charge pump 10 driven by the engine E is connected to the closed circuit through a charge hydraulic line Lh having a check valve 11 and through a third hydraulic line Lc having a pair of check valves 3 and 3. Hydraulic oil pumped up by the charge pump 10 from an oil sump 15 with its pressure regulated by a charge pressure relief valve 12 is supplied to a lower pressure side of the aforesaid hydraulic lines La and Lb through the check valves 3 and 3. The internal pressure of the first line La is high when the engine E drives the wheels W₁ through the pump P and the motor M (the internal pressure of the second line Lb is low at this time.) On the other hand, the internal pressure of the second line Lb is high when the engine E is driven by the wheels W₁ creating engine braking phenomenon ( the internal pressure of the first line La is low at this time). The engine braking phenomenon occurs at a deceleration state of the vehicle for example. And, a fourth hydraulic line Ld having a shuttle valve 4 is connected to the aforesaid closed circuit. To the shuttle valve 4 is connected fifth and sixth hydraulic lines which respectively have a high pressure relief valve 6 and a low pressure relief valve 7 and are inserted into the oil sump 15. The shuttle valve 4 is a 2-port 3-position selector valve, which operates in response to a hydraulic pressure difference of the first and second hydraulic lines to connect the oneof the first or second hydraulic lines La having higher pressure to the fifth hydraulic line Le as well as to connect the one having lower pressure to the sixth hydraulic line Lf. Therefore, the relief hydraulic pressure of a higher pressure-side line is regulated by the high pressure relief valve 6, and the relief hydraulic pressure of the other lower pressure-side line is regulated by the low pressure relief valve 7. Between the first and second hydraulic lines La and Lb is provided a seventh hydraulic line Lg to short circuit both lines. The seventh hydraulic line Lg is provided with a clutch valve 5, a variable throttle valve to control the opening degree or hydraulic resistance of the line. Therefore, the throttle control of the clutch valve 5 can effect clutch control to control the transmission of driving force from the hydraulic pump P to the hydraulic motor M. The engine E drives the hydraulic pump P, and the hydraulic pressure generated by the pump P drives the hydraulic motor M for rotation which is transmitted to the wheels W₁ through the output shaft 2 to drive the wheels W₁. The hydraulic motor M can be, for example, a swash plate type axial piston motor which permits the speed reduction ratio of the continuously variable speed transmission T to be steplessly changed by controlling the tilt angle of the swash plate. The fifth hydraulic line Le of the hydraulic closed circuit into which a high-pressure fluid is introduced, is connected via a hydraulic line 51 with a pressure control valve 20 which is used as a means for controlling the hydraulic pressure. The control valve 20 is composed of a ball 21, a spool 22, and a housing in which the ball 21 and the spool 22 are inserted. The housing has a first cylindrical hole 20a diameter D₁ forming a first hydraulic chamber 25a, a second cylindrical hole 20b of a diameter D₂ for receiving therein the central large-diametrical portion 20b of spool 22, and a third cylindrical hole 20c of a diameter D₃ for receiving therein the right small-diametrical portion 22c of spool 22. The ball 21 has a larger diameter than the first cylindrical hole 20a, and is arranged to the right of this first cylindrical hole 20a in opposition thereto. To the right of the ball 21, the left end 22a of the spool 22 is opposed to. Accordingly, as the ball 21 is abutted against the right surface of the first cylindrical hole 20a, the right end of the first hydraulic chamber 25a is shut by the ball 21. Between the right end surface of the central large-diametrical portion 22b of the spool 22 inserted in the second cylindrical hole 20b and the housing is formed a second hydraulic chamber 25b. Between the right end surface of the right small-diametrical portion 22c of spool 22 inserted in the third cylindrical hole 20c and the housing is formed a third hydraulic chamber 25c. The first hydraulic chamber 25a and the third hydraulic chamber 25c communicate with a hydraulic line 51 (hydraulic line 52) connected with the first hydraulic line La, while the second hydraulic chamber 25b is connected with a solenoid valve 151 via a hydraulic line 54. Here it should be understood that the solenoid valve 151 is used for controlling, in response to the duty ratio signal sent from a controller 100, the communication between the charge hydraulic line 53 branching from a charge hydraulic line Lh which is pressure-controlled by the charge pressure relief valve 12, and a hydraulic line 54 connected with said second hydraulic chamber 25. The valve 151 is a normal-open type valve which is opened when the duty ratio is 0% but fully closed when the duty ratio is 100%. Consequently, upon the ball 21 acts the rightward force of Fa ( = Pa * Aa, where Aa(cross sectional area of first hydraulic chamber) = π/4*D₁²) due to the hydraulic pressure Pa given from the first hydraulic line La into the first hydraulic chamber 25a through the hydraulic line 51, and upon the spool 22 acts the leftward force Fb ( = Pb *Ab, where Ab (cross sectional area of the second hydraulic chamber) = π/4*(D₂² - D₃²)) due to the hydraulic pressure Pb supplied Into the second chamber 25b through the hydraulic line 54 and controlled by the solenoid valve 151, and upon the spool also acts a leftward force Fc ( = Pa*Ac, where Ac(cross sectional area of the third hydraulic chamber) =π/4*D₃²) . As understood from the foregoing description, the hydraulic pressure acting on the ball 21 and the spool 22 balances when Fa = Fb + FC , but the ball 21 is pushed to the right when Fa > Fb + Fc making the hydraulic line 51 communicate with a sixth hydraulic line Lf which is pressure-control led by a low-pressure relief valve 7. Hence when Fa > Fb + Fc , the fluid in the first hydraulic line La flows into the sixth hydraulic line Lf through the line 51, pressure-control valve 20 and line 55 to make the pressure lower. We now discuss equilibrium equation Fa = Fb + Fc below. This equation when expressed in terms of the cross sectional areas of each hydraulic chamber and the pressure in it, reads Pa * Aa = Pb * Ab + Pa * Ac which is, upon rearrangement, equivalent to Pa * (Aa - Ac) = Pb * Ab (1) From this equation we see that by controlling the pressure Pb in the second hydraulic chamber 25b, the magnitude P H of the pressure Pa required for attaining the above equilibrium can be set in the first and third hydraulic chambers 25a and 25c, respectively, through the hydraulic line 51. When the pressure Pa in the first hydraulic chamber 25a becomes higher then P H , the ball 21 is pushed to the right to connect the hydraulic line 51 with line 55. As a result of this, the pressure in the line 51, i.e. pressure Pa in the first hydraulic line La, will never exceed the preset pressure P H , which is the relief hydraulic pressure for the first hydraulic line La. It is noted that the hydraulic pressure Pb in the second hydraulic chamber 25b can be controlled by the duty ratio control of the solenoid valve 151, and hence that by giving the duty ratio signals generated in the controller 100 to the solenoid valve 151 through the line 100a, the maximum hydraulic pressure (relief hydraulic pressure) P H can be controlled. Details of the operations in controlling the relief hydraulic pressure in the first hydraulic line La will be now described below. The relief hydraulic pressure P H is set to be maximum when the duty ratio is 0% in response to the duty ratio signal from the controller 100, as shown in FIG. 2, and minimum when the duty ratio is 100%. This duty ratio control is effected to suppress the slip of the wheels when the driving force of the wheels exceeds the driving load (the gripping force of the tires) as determined by the frictional forces between the wheels and the road surface. To do this, slip is detected if any (S1) and slip rate thereof is determined (S2). As shown in FIG. 1 the rotational speed of driving wheels driven by the hydraulic motor M is detected by a first speed sensor 61, and the detection signal from which is transmitted through a line 62 to the controller 100. The rotational speed of free-rotation wheels W₂ is detected by a second speed sensor 63, whose detection signal is also transmitted to the controller through a line 64. When the driving wheel W₁ slips, there will be produced a difference in rotational speed between the wheels W₁ and W₂, from which the slip rate is calculated in the controller 100. Based on the slip rate found in this way, a duty ratio is set (S3) in correspondence to a magnitude of the slip rate, the ratio being greater for a greater slip ratio, and a signal indicative of the duty ratio is output to the solenoid valve 151 (S4). This lowers the hydraulic pressure in the first hydraulic line La, lowering the driving force of the driving wheels W₁ to suppress the slip thereof. Namely, the slip of the driving wheels is suppressed because the hydraulic pressure in the first hydraulic line La is reduced by the action of a pressure control valve 20 to lower the driving force less than the slip limit thereof. Although the above description refers to the slip control operation for suppressing the slip of the driving wheel W₁ (driven by the engine) by controlling the hydraulic pressure in the first hydraulic line La by means of the hydraulic control valve 20, similar control will be carried out when the driving wheel W₁ slips caused by the high pressure in the second hydraulic line Lb due to engine braking during deceleration. In the above example, the hydraulic pressure control valve 20 is used as a means of controlling the hydraulic pressure. However, the invention is not limited by the above example: similar control may also be provided by a clutch valve 5 disposed in a seventh hydraulic line Lg short circuiting the first and second hydraulic lines La and Lb. Such a clutch valve as this will be described below by way of second example. The constitution and operation of the clutch valve 5 are as follows. The clutch servo unit 80, as shown in Fig. 5, comprises a cylinder member 81, a piston member 82 inserted in the cylinder member 81 slidably to the right and left on Fig.5, a cover member 85 fixed to cover the cylinder chamber into which the piston member 82 is inserted, and a spring 87 pushing the piston member 82 to the left on Fig. 5, A piston 82a on the piston member 82 divides aforesaid cylinder chamber 81 into a left cylinder chamber 83 and a right cylinder chamber 84 to both of which hydraulic lines 112 and 110 are respectively connected via respective ports 86a and 86b. The hydraulic oil in the hydraulic line 110 is transferred from the charge pump 10 (whose delivery pressure is regulated by the charge pressure relief valve 12) through a hydraulic line 101, while the hydraulic oil in the hydraulic line 112 is transferred from the hydraulic line 101 through a hydraulic line 111, when the hydraulic oil is diverged into the line 111 and passes through a orifice 111a in the line 111, hydraulic oil pressure is controlled by two duty-ratio-controlled solenoid valves 155 and 156. Here, the solenoid valve 156 is provided to control the flow rate of the hydraulic oil flowing from the line 111 ( having the orifice 111a therein) to the line 112 based on the duty ratio signals, while the solenoid valve 155 is disposed between a hydraulic line 113 diverging from the line 112 and a hydraulic line 114 communicates with the drain through an orifice 114a, to control the flow of the hydraulic oil from the line 113 to the drain based on the duty signals. Therefore, to the right cylinder chamber 84 via the line 110 is applied the charge pressure regulated by the charge pressure relief valve 12, while to the left cylinder chamber 83 is applied a pressure from the line 112 lower than the charge pressure, by the action of the aforesaid two solenoid valves 155 and 156. In this connection, the force applied on the piston member 82 from the right side (that is, a force due to the hydraulic pressure P1 in the right cylinder chamber 84 plus the force of the spring 87) balances with the force applied on the piston member 82 from the left side (that is, a force due to the hydraulic pressure P2 in the left cylinder chamber 83), even when P2 is lower than P1, because the area of the right cylinder chamber 84 subject to oil pressure is designed to be much smaller than that of the left cylinder chamber 83. Therefore, if the solenoid valves 155 and 156 control the hydraulic pressure (in the left cylinder chamber 83) supplied from the line 112 so as to be larger than the pressure P1, the piston member 82 will be moved to the right, while when the solenoid valves 155 and 156 control the hydraulic pressure in the left cylinder chamber 83 supplied from the line 112 so as to be smaller than the pressure P1, the piston member 82 will be moved to the left. The movement of the piston member 82 to the left or right is transmitted to the clutch valve 5 through a link 88. The clutch valve 5 consists of a stationary member 5a having a first valve port 5b therein, and a rotary member 5c having a second valve port 5d rotatably inserted in the stationary member 5a. An arm 5c engaging with the rotary member 5c is connected to the aforesaid link 88, thus allowing rotation of the rotary member 5c in accordance with the movement of the aforesaid piston member 82. When the rotary member 5c is rotated, the communication between the first and second valve ports 5b and 5d varies from "fully open" to "fully closed". When the piston member 82 is moved to the leftmost as shown in Fig.5, the communication in the clutch valve 5 is "fully open", while as the piston member 82 moves to the right, the communication varies gradually to "fully closed". Because the first valve port 5b communicates with the first line La and the second valve port 5d communicates with the second line Lb constituting the hydraulic closed-circuit, the variation in the communication between the aforesaid first and second valve ports 5b and 5d can change the opening degree of the seventh line Lg of the short circuit line of the first and second lines La and Lb, thus effecting clutch control. In other words, based on signal from the controller 100, duty-ratio control of aforesaid solenoid valves 155 and 156 can perform a successful clutch control. The primary purpose of this clutch control is to provide smooth start/stopping and thus the control is conducted mainly at the time of starting and stopping. However, as stated above, it also functions as a slip protection means for the driving wheel W₁. Namely, as the wheel W₁ is detected slipping, the solenoid valves 155 and 156 are subjected to the duty ratio control in response to the signal from the controller 100 in accordance with the slip rate, resulting in opening the clutch valve 5 by an amount necessary for the control. This permits the working fluid, pressurized in the first hydraulic line La due to the driving of the driving wheel W₁ to flow into the less pressurized second hydraulic line Lb via the clutch valve 5 to lower the pressure in the first hydraulic line La. The driving force of the driving wheel W₁, and hence also the slip thereof, is reduced. In other words, the clutch valve 5 capable of such control serves not only as a clutch means by also as a hydraulic pressure control apparatus that lowers the hydraulic pressure in the driving hydraulic line when the driving wheel has slipped. With the above control when the frictional coefficient of the road surface is small and the driving force of a wheel is greater than the driving load determined by the frictional force, causing slip between them, the slip will be detected by the slip sensor, and the hydraulic pressure in the hydraulic circuit will be lowered in accordance with the detected slip rate to reduce the driving force of the wheel less than the slip limit thereof. Accordingly, further slip of the wheel is prevented keeping the gripping force of the driving wheels as high as possible.

Claims: A control apparatus for a continuously variable speed transmission for a vehicle having a hydraulic pump (P) connected to an engine (E), a hydraulic motor (M) connected to driving wheels (W₁), and a hydraulic closed circuit (La,Lb) hydraulically connecting said hydraulic pump (P) with said hydraulic motor (M), at least one of said hydraulic pump (P) and said hydraulic motor (M) being of a variable displacement type, the control apparatus comprising a slip sensor (61,63,100) for detecting slip (S1) of the driving wheels (W₁) driven by said hydraulic motor (M), characterised in that the control apparatus further comprises hydraulic control means (20) for lowering hydraulic pressure in said hydraulic closed circuit (La,Lb) by exhausting hydraulic fluid from said hydraulic closed circuit (La,Lb) in accordance with the magnitude of the slip (S1) detected by said slip sensor (61,63,100), in such a way that the driving force applied to said driving wheels (W₁) is reduced so as to be less than a slip limit force. A control apparatus as defined in claim 1, wherein said slip sensor comprises a front wheel speed sensor (63), a rear wheel speed sensor (61) and means (100) to calculate a slip rate based on a difference in wheel speed between front wheels and rear wheels. A control apparatus as defined in claim 1 or 2, wherein said hydraulic control means (20) controls the hydraulic pressure in a hydraulic line (La) of said hydraulic closed circuit through which hydraulic fluid from said hydraulic pump (P) is sent to said hydraulic motor (M) when said hydraulic motor (M) is driven by the hydraulic pressure supplied from said hydraulic pump (P) with said wheels being driven by said hydraulic motor (M). A control apparatus as defined in claim 1, 2 or 3, wherein said hydraulic control means (20) controls the hydraulic pressure in a hydraulic line (Lb) of said hydraulic closed circuit through which hydraulic fluid from said hydraulic motor (M) is sent to said hydraulic pump (P) when said hydraulic motor (M) is driven by driving force of said wheels (W₁) with said hydraulic pump (P) being driven by hydraulic pressure supplied from said hydraulic motor (M). A control apparatus as defined in any preceding claim, wherein said hydraulic control means includes a pressure control valve (20) which controls the hydraulic pressure in a hydraulic line (La) of said hydraulic closed circuit by controlling leakage of hydraulic fluid from said hydraulic line (La). A control apparatus as defined in any preceding claim wherein said hydraulic control means includes a clutch valve (5) to adjust an opening of a bypass line (Lg) which connects a hydraulic line (La) of said hydraulic closed circuit through which hydraulic fluid from said hydraulic pump (P) is sent to said hydraulic motor (M) with a hydraulic line (Lb) of said hydraulic closed circuit through which hydraulic fluid from said hydraulic motor (M) is sent to said hydraulic pump (P).

Inventors: MAKI, KAZUYA, YAMAGUCHI, KOUJI
Applicants: HONDA GIKEN KOGYO KABUSHIKI KAISHA
Application year: 1988
Main classification: F16H61
IPC codes: F16H61, B60K28, B60K17, F15B11, F16H59