ELECTRO-HYDRAULIC BRAKE SYSTEM

Abstract: An electro-hydraulic brake system (30) comprising: a master cylinder (32); a pump (34) the output being communicated to a pressure regulating valve (50), a motor (36) for rotating the pump; an isolation valve (42) for selectively connecting one of the master cylinder and pump to a brake cylinder(s) (44) and a signal generator (40) to activate the pump. The pressure regulating valve connected between the master cylinder and pump to regulate pump output pressure at a determinable level in proportion to the pressure generated by the master cylinder and for providing a path to drain the brake cylinder(s) to a reservoir during intervals of decreasing master cylinder pressure. The system may also include an excess flow valve(s) (90) to prevent the reservoir from being drained by the pump in the event of a malfunction in the system. Various embodiments of the regulating valve are described which add additional failure mode protection.

Description: ELECTRO-HYDRAULIC BRAKE SYSTEM BACKGROUND AND SUMMARY OF THE INVENTION The present invention relates to an electro- hydraulic braking system in which the primary braking force is supplied to brake cylinders by a pump moved by an electric motor. A simplified version of an electro-hydraulic braking system is shown in FIGURE 1. The system comprises a pump 12 powered by a motor 14 in response to control signals generated by an electronic control unit (ECU) 16. The pump directly pressurizes a brake cylinder or cylinders generally shown as 18. The speed of the motor 14/pump 12 is controlled so as to modulate brake system pressure in accordance with a commanded brake pressure signal. A by-pass orifice 20 is provided across the pump 12 which enhances the stability of motor/pump speed and allows for the rapid release of brake cylinder pressure when required. FIGURE 2 illustrates an improvement to the simplified system shown in FIGURE 1 in which a master cylinder 22 has been added to provide brake system redundancy in the event that the pump 12 or electronics fail. In this system the motor control signal is generated by comparing master cylinder pressure as generated by a pressure sensor 24, to a signal indicative of the pressure in the brake cylinder 18 as determined by pressure sensor 26. This technique could also be used in the system of FIGURE 1. An isolation valve 28 is used to communicate either the master cylinder or the output of the pump to one or more brake cylinders 18. The isolation valve may be a pressure piloted isolation valve responsive to pump output pressure. The system illustrated utilizes, in a general sense, a flow control device 20 across the pump. This- flow control device may be implemented in many different ways, including the fixed orifice shown in FIGURE 1 or the solenoid valve shown in FIGURE 2. While either of the systems shown in FIGURES 1 and 2 work well, certain time delays will exist until the pump output pressure builds satisfactorily. To reduce any time delay these systems require an extremely fast responding motor and also require motor speed control electronics which are useful in regulating the speed of the pump such that during in the steady state the output pressure of the pump is equal to the commanded or master cylinder pressure. It is an object of the present invention to provide a more rapid responding, less expensive electro-hydraulic system than that illustrated in the above figures. According, the invention comprises: an electro-hydraulic brake system comprising: a master cylinder; a pump the output communicated to a pressure regulating valve means; a motor for rotating the pump at a determinable speed; isolation valve means selectively connecting one of the master cylinder and pump to a brake cylinder or group of brake cylinders and first means for generating a signal indicative of operator initiated brake activity to cause activation of the pump. The valve means, is connected between the master cylinder and pump and regulates the output pressure of the pump at a determinable level in proportion to the pressure generated by the master cylinder and for providing a path to drain the brake cylinder(s) to a reservoir during intervals of decreasing master cylinder pressure. The system may also include an excess flow valve(s) which will prevent the reservoir from being drained by the pump in the event of a malfunction such as a defective or leaky brake cylinder or a hole (leak) in a hydraulic line. Various embodiments of the valve means are described which add additional failure mode protection to the system. Many other objects and purposes of the invention will be clear from the following detailed description of the drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: FIGURES 1 and 2 illustrates simplified versions of an electro-hydraulic braking system. FIGURE 3 illustrates an improved electro- hydraulic system incorporating features of the present invention. FIGURE 4 illustrates an alternate embodiment of a pressure regulating valve. FIGURE 5 illustrates another brake system. FIGURE 6 illustrates another embodiment of a valve. DETAILED DESCRIPTION OF THE DRAWINGS FIGURE 3 schematically illustrates an electro-hydraulic brake system 30. The system includes a master cylinder 32 and a pump 34 such as a positive-displacement pump rotated by a motor 36. The motor 36 is activated through a relay 38 by a control signal generated by an ECU 40. As shown the output signal from a brake light switch 80 (or similar command) is used to activate the relay 38. While an ECU 40 is shown, this could be eliminated. The ECU 40 may in its simplest form include electronic filters and a buffer or amplifier circuit for powering the relay 38. The output of the pump 34 and the output of the master cylinder 32 are communicated to an isolation valve generally shown as 42 similar in function to the isolation valve 28 of FIGURE 2. If the pump does not produce adequate pressure, isolation does not occur and conventional manual braking is still available via the master cylinder. In this sense, the failure of the pump is similar to the failure of a vacuum booster found in conventional power brake systems. The system 30 further includes a pressure regulator which is also referred to as valve 50. As will be .seen the valve 50 regulates pump pressure i.e., the pressure supplied to the brake cylinder(s) at a determinable ratio of master cylinder pressure. FIGURE 3 shows one embodiment of the valve 50 which comprises a first piston 52 exposed to master cylinder pressure, movable within a piston passage 56. The piston includes a pin or closure element 54. One end 60 of the piston passage 56 is communicated to the pump 34 as well as to the brake cylinder or cylinders 44. The piston passage 56 includes a by-pass passage 58 communicated to a reservoir 62. A first orifice 70 is provided in the piston passage 56 and defines a flow area A__ which is equal to or less than the area A 2 of the first piston 52 that is exposed to master cylinder pressure. As can be seen, the first orifice 70 is located between the by-pass passage 58 and the end 60 of the piston passage 56. The system 30 additionally includes a means for generating a signal indicative of operator initiated brake activity. As illustrated in FIGURE 3, and as mentioned above, an indication that the operator has stepped on the brake pedal 82 is sensed by the closure of a brake light switch generally shown as 80. The brake light switch is communicated to the ECU 40 which in turn activates relay 38 to cause the motor 36 to rotate at a relatively constant speed defined by the pump pressure needed to create a force balance across the first piston 52. Alternatively, an indication of braking activity can be generated by utilizing a force sensor 84 to measure brake pedal force or a pressure sensor 86 to measure developed master cylinder pressure. One advantage of using the brake light switch 80 as opposed to a force transducer 84 or pressure sensor 86 is that the brake light switch 80 will generate a signal slightly before a force sensor or pressure sensor will generate its corresponding signal. Using the brake light switch as a measure of brake activity reduces time delays permitting a more rapid energization of the motor 36/pump 34. In operation, when the. operator depresses the brake pedal 82 pressure in the master cylinder will increase. The output pressure of the master cylinder is communicated to the brake cylinder or cylinders 44 through the isolation valve. The nonactivated state of the isolation valve is shown in FIGURE 3. When the brake pedal 82 is depressed, the brake light switch 80 output signal is communicated to the motor relay 38. During the initial moments of operation of the system, the master cylinder pressure will exceed the pressure generated by the pump 34. During this time, the master cylinder pressure acts upon the first piston 52 causing the element 54 to fully close the orifice 70. With the orifice 70 closed the full output flow capacity of the pump 34 is directed to flow to the isolation valve 42. The isolation valve 42 should typically be designed to change its state at a relatively low output pump pressure such as 30-50 psi. When the isolation valve changes state, it isolates the master cylinder 32 from the brake cylinder 44 and directly communicates the output of the pump to the brake cylinder 44. As the output pressure of the pump builds, this pressure acts upon the lower end of the element 54, moving it from the first orifice 70 in opposition to the forces generated on the first piston 52.by master cylinder pressure. It is desirable to regulate the output pressure of pump 34 to a determinable pressure and as a function of master cylinder pressure. In its equilibrium condition, the valve 50 will regulate pump pressure P to be equal to: P = A 2 / ! x P mc where P mc is master cylinder pressure. If A 2 is greater than x a boosted pump pressure is generated analogous to the output of a conventional power brake. As can be appreciated, the pressure regulating action of the valve 50 works on a force balance principle. The master cylinder pressure acts on the working area of its associated piston (piston 52) and produces a force which is balanced by the pump 34 acting on an associated element (pin or element 54) . The pin 54/piston 52 will move until sufficient flow passes through the orifice 70 to produce the required pressure difference. When using a conventional service brake system the operator modulates the force applied to the brake pedal 82 to vary the pressure applied to the brake cylinder 44. This action also happens in the present invention. The pressure generated by the pump will follow master cylinder pressure which is proportional to brake pedal force. While FIGURE 3 illustrates a separate master cylinder and valve 50, it should be appreciated that the valve 50 can be incorporated within a modified master cylinder. Failure of a portion of the hydraulic system downstream of the isolation valve 42 such as a broken line or leaking brake cylinder could result in not only a loss of braking to the affected wheel but also allow the pump to completely drain the reservoir so that the pump cannot pressurize other brake cylinders. This undesirable effect can be circumvented by placing an excess flow valve 90 between the pump 34 and isolation valve 44. It should be appreciated that FIGURE 3 shows a single brake channel. FIGURE 5 shows a plurality of excess flow valves and a two channel brake system from which the benefit of these valves is more readily apparent. Reference is made to FIGURES 4 and 5. FIGURE 4 illustrates an alternate valve 50'. FIGURE 5 illustrates an exemplary braking system 30' for the control of four brake cylinders 44. The valve 50', also shown schematically in FIGURE 5, is functionally analogous in operation to valve 50. An added feature is that this valve 50' is adapted to communicate to a master cylinder having primary and secondary master cylinder chambers 32a and 32b, respectively providing failure redundancy in operation. The valve 50' includes a housing 100 defining a plurality of ports 102, 104, 106 and 108, adapted to respectively communicate to the primary master cylinder chamber 32a, pump 34, reservoir 62 and the secondary master cylinder chamber 32b. The valve 50' includes the first piston 52 (see FIGURE 4) and closure element 54 and a valve seat 110 defining the orifice 70. Port 104 illustrates the use of an inverted SAE fitting which may also be used in any of the various ports of the valve 50' . The piston 52 is slidably received within a bore 114 which also supports a second piston 116. The valve 50' includes an-additional two ports 120a and 120b which communicate to the isolation valves 42a and 42b shown in FIGURE 5. The exemplary system of FIGURE 5 shows a cross-split brake configuration in which the primary master cylinder 32a is communicated to the left front and to the right rear brake cylinders 44 through a proportioning valve 122a. Secondary master cylinder pressure is used to control the right front and left rear brake cylinders through a second proportioning valve 122b. Returning to FIGURE 4, the passage 114, between the connection points of the primary and secondary master cylinder, is also communicated to the exhaust port through a passage 126. This passage 126 provides a region of atmospheric pressure about piston 116. The purpose of this vent or passage 126 is to insure that if any of the regulator seals such as 128 fail, this failure will be detectable. As an example, if one of the seals 128 fail the secondary master cylinder chamber pressure will decrease as brake fluid will flow to the reservoir. This low pressure will be detected by a low pressure switch in the secondary master cylinder chamber. The pressure switch will typically activate a light on the dashboard informing the driver of the failure. If such a failure of a component in the valve were not detectable then a subsequent failure of, for example a seal in the primary master cylinder would result in the loss of complete braking control. Another feature of the valve 50' is that it is operable in the event of a failure of one or the other master cylinder chambers or in the hydraulic lines connecting these chambers to the valve 50'. The operation of the valve 50' is as follows. As is typically the case, the pressure generated in the primary master cylinder will be approximately 20 to 50 psi (1.38-3.45 bar) greater than the pressure generated in the secondary master cylinder. With the system connected as shown in FIGURE 5, the primary master cylinder pressure is received into chamber 130 of FIGURE 4. This pressure force urges piston 52 downwardly and piston 116 upwardly against stop 132. As can be appreciated, the dynamics of valve 50', in this condition, are essentially identical to those of valve 50. In this operating condition, secondary master cylinder pressure does not play an operative role in regulating the output pressure of the pump. The valve 50' will continue to operate even in the face of a failure of the hydraulic system upstream of port 108 i.e. the secondary master cylinder chamber. In the event of a failure of the hydraulic system connected to the primary master cylinder chamber, no fluid pressure would be generated within chamber 130, however, secondary master cylinder pressure is communicated to chamber 134 which urges piston 116 downwardly and which in turn will cause piston 52 to close the orifice 70. As before, this action permits full pump output to be communicated to the brake cylinders 44 during pump start-up. In this failure mode, the valve 50' will regulate pump output pressure to be proportional to the pressure in the secondary master cylinder. Reference is made to FIGURE 6 which illustrates an alternate embodiment of a pressure regulating valve 50". This valve can be substituted into FIGURE 5. The valve 50" includes a housing 104 into which are received three pistons 206, 208 and 212. A pin 214 is secured to one of the pistons 206 or 208. In FIGURE 6 the pin 214 is secured to piston 208 and is slidably received within a bore 216 of piston 206. Pistons 206 and 208 are slidably received within a central bore 220. Piston 212 is received within another bore 222. Various dynamic seals such as 224 and 226 are provided to prevent leakage through the various bores 220 and 222. As illustrated seal 224 can be a GLYD ring while seal 226 is shown as an O-ring. Piston 212 supports a valve closure element 230 which is spherically shaped and adapted to seat upon a valve seat 232 defining an orifice 234. The valve 50" includes a plurality of ports 104, 106, 102 and 108 respectively connected to the pump 34, the reservoir 62, the primary master cylinder chamber 32a, and the secondary master cylinder chamber 32b. The valve 50" also includes a plurality of vents schematically shown as 242 and 246. As can be appreciated exterior vents are not necessary. Alternately, the bores 220 and 222 can be vented to atmosphere through internal passages (not shown) communicated to the reservoir port which is typically at atmospheric pressure. The piston 208 has a first working surface 260 of area A 3 which is exposed to primary master cylinder pressure. The piston 212 has a second working surface 262 of area A α which is also exposed to atmosphere through the vent 246. The operation of the system 200 is as follows. Fluid from the primary chamber 32a will fill the chamber 250 between pistons 206 and 208 and act on the first working surface 260 of piston 208 causing the piston 208 to press down on the working surface 262 of piston 212 causing element 230 to fully close orifice 234. This action enables the full output of the pump 34 to be communicated to the brake cylinder 44. As the pump pressure builds, it acts upon the piston 212 to move away from the valve seat 232 in opposition to the forces exerted on piston 208. As the piston 212 moves out from the orifice 234, the output of the pump 34 will be regulated to a determinable pressure which is a function of master cylinder pressure and more specifically a function of the primary master cylinder pressure. It can be shown that this determinable pressure equals: P = [(A 3 /(A 2 -A X )] x P pmc wherein P pmc is primary master cylinder pressure, A 3 is the area across piston 208, A 2 the area of the piston 212 and A 1 is the area of the orifice 234. As can be appreciated, the valve 50" shown in FIGURE 6 will permit the pressure generated by the pump to be greater than the pressure generated by the primary master cylinder pressure. If it is desired that the pump pressure be more closely related to master cylinder pressure, that is, that the relationship between pump pressure and primary master cylinder pressure approach unity, then the cross-sectional areas of the various pistons will be made equal. As can be seen valve 50" includes failure mode protection similar to that employed in valve 50' and will continue to operate even if primary or secondary master cylinder pressure is not communicated to the various ports. In the event of the failure of the primary master cylinder chamber, the output pressure of the pump 34 will be regulated as a function of the secondary master cylinder. The relationship is as follows: where P εmc is secondary master cylinder pressure and A 4 is the area of the piston 206 exposed to secondary master cylinder pressure. Many changes and modifications in the above described embodiment of the invention can, of course, be carried out without departing from the scope thereof. Accordingly, that scope is intended to be limited only by the scope of the appended claims.

Claims: IN THE CLAIMS 1. An electro-hydraulic brake system (30) comprising: a master cylinder (32) ; a pump (34) the output being communicated to a valve means (50) ; a motor C36) for rotating the pump (34) at a speed related to the pressure developed by the pump; isolation valve means (42) for selectively connecting one of the master cylinder (32) and pump (34) to at least one brake cylinder (44) ; first means (80,84,86,38,40) for generating a signal indicative of operator initiated brake activity to cause activation of the pump (34) ; the valve means (50,50',50") , connected between the master cylinder (32) and pump (34) for regulating the output pressure generated by the pump (34) at a determinable pressure in proportion to the pressure generated by the master cylinder (32) and for providing a path, to drain the at least one brake cylinder (44) to a reservoir (62) during intervals of decreasing master cylinder pressure. 2. The system as defined in Claim 1 wherein the first means (80,84,86) generates a pump activation signal in advance of a substantial increase in master cylinder pressure. 3. The system as defined in Claim 1 wherein the first means includes a brake light switch (80) activated when an operator of the vehicle depresses a brake pedal connected to the master cylinder. 4. The system as defined in Claim 1 wherein the valve means (50) comprises: a first piston (52;212), having an area A 2 , exposed to master cylinder pressure, and a closure element (54,-230) movable in a piston passage (56;114;220) , for controlling flow through a first orifice (70,-234) having a flow area A : , the piston passage (56) communicated to the pump (34) and to the reservoir (62), one side of the first orifice (70) communicated to the reservoir, wherein when the pump pressure is less than the determinable pressure, the master cylinder pressure operates to cause the first piston to close the first orifice to enable the full output of the pump (34) to be communicated to the at least one brake cylinder (44) , and as the pump pressure builds such pressure acts on the first piston (54) to move same from the first orifice (70) to increase flow thereacross to regulate the output pressure of the pump to the determinable pressure. 5. The system (30) as defined in Claim 4 wherein the determinable pressure P equals wherein P rac is master cylinder pressure. 6. The system as defined in Claim 4 wherein the master cylinder includes a primary chamber (32a) and a secondary chamber (32b) , and wherein the valve means (50) includes second means (116;206;208) , for receiving fluid generally at the pressure levels generated in the primary and secondary chambers (32a, 32b) and for permitting the valve means to regulate pump pressure in the event that only one of primary master cylinder pressure or secondary master cylinder pressure is communicated to the valve means. 7. The system as defined in Claim 4 wherein the area A x is equal to or less than the area A 2 . 8. The system as defined in Claim 4 including excess flow valve means (90), situated downstream of the pump, for terminating pump flow in the event of a loss of pressurization downstream of the pump due to a malfunction. 9. The system as defined in Claim 4 wherein the master cylinder (32) and the valve means (50) are of integral construction. 10. The system as defined in Claim 6 wherein the second means includes a second piston (116) movable into contact with the first piston in response to secondary master cylinder pressure thereby controlling the position of the first piston relative to the first orifice. 11. The system as defined in Claim 6 wherein pump is communicated to the first piston at a location upstream of the first orifice (234) and a downstream side of the first orifice is communicated to atmospheric pressure and wherein the second means comprises: a second piston (208) , having a first working surface (262) of area A 3 , and an opposite contacting surface movable to contact with the first piston (212) having an area of A 2 in response to primary master cylinder pressure applied to the first working surface to move same relative to the first orifice, the contacting surface also being exposed to atmospheric pressure, and a third piston (206) , for moving the second piston in response to secondary master cylinder pressure during intervals when the level of secondary master cylinder pressure exceeds the level of primary master cylinder pressure. 12. The system as defined in Claim 13 wherein the determinable pressure equals P = [A 3 /(A 2 -A 1 )] x P pmc wherein P mc is pressure generated in the primary master cylinder chamber, and A α is the area of the orifice.

Inventors: PATIENT DANIEL JOSEPH, PHIPPS JACK RALPH, PATIENT, DANIEL, JOSEPH, PHIPPS, JACK, RALPH
Applicants: ALLIED SIGNAL INC, ALLIED-SIGNAL, INC.
Application year: 1992
Main classification: B60T13
IPC codes: B60T13, B60T15