Abstract:
A method of preventing a vacuum from developing in a low-pressure area of a vehicle anti-lock brake system comprises providing periodically opening normally closed electromagnet valves when brakes are not applied, the valves connecting wheel brakes to a tandem master cylinder by low pressure lines passing through the low-pressure area. As a result, pressures in a tandem master cylinder of the system and the low-pressure area level off and thus a vacuum trapped in the low-pressure area is eliminated.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to anti-lock brake systems (ABS) and/or traction control systems (ASR, for anti-slip regulation), more particularly to evacuating and filling an ABS unit, and still more particularly to preventing the system from developing air pockets in a low-pressure accumulator area affecting braking performance. 
     2. Description of the Related Art 
     ABS (and ASR) is a part of the standard equipment in medium-class and upper-class vehicles. These systems are becoming increasingly included in lower-category and low-cost cars. 
     ABS and their variations typically comprise a hydraulic control unit (HCU) that houses hydraulic components of the system. The HCU is connected to the vehicle brake system between its master cylinder and calipers of wheel brakes. 
     A conventional hydraulic brake system, a one-wheel circuit  10  of which is shown in FIG. 1, comprises a tandem master cylinder (TMC) common for all wheels (shown is an inlet  12  from TMC), a HCU  14 , and a wheel caliper (shown in FIG. 1 is an outlet  16  to the wheel caliper). Depicted within dotted lines is a low-pressure portion  18  of the HCU  14 . The portion  18  typically contains a low-pressure accumulator  20 , an input portion  22  of a return pump  24 , and associated lines  26  and  28 . A motor  30  drives the return pump  24 . Depicted in FIG. 1 are also a return pump outlet valve  32 , a noise damper  34 , and two electromagnet valves: a pressure buildup normally open (NO) valve  36  and a pressure reduction normally closed (NC) valve  38 . The pressure buildup electromagnet NO valve  36  is disposed in a brake line  40  connecting the inlet  12  from TMC to the outlet  16 . A check valve  42  is connected in parallel to the NO valve  36 . The pressure reduction electromagnet NC valve  38  is placed in a return line  44  that eventually, through the return pump  24  reunites with a main brake line  46 . 
     Prior to filling the system including the circuit  10  with brake fluid, a vacuum is applied at the TMC to remove air from the system. When the vacuum is applied to the HCU, a portion of the unit, namely the low-pressure circuit, is isolated and does not evacuate its air. Any air bubbles still present in the hydraulic medium may bring about such an elasticity of fluid columns formed by the medium that the brake pressures required in wheel brake cylinders are not brought about to the requisite level. 
     ABS/ASR brake systems have several NC electromagnet valves; the valves of return pumps used in these brake systems are normally closed as well. The problem therefore exists that pockets of air may be present downstream of the NC electromagnet valves, and also downstream of the return pump valves. The air in these pockets may possibly be removed only with difficulty using liquid. 
     It has been proposed that at least the NC valves be triggered electrically upon evacuation and prior to filling of the system, and thus opened. To do so, special plug connections must be used to supply current to these electromagnet valves. The connections of this kind are expensive and involve operating costs when used, let alone that they are also complicated to manufacture. For many applications, therefore, it is the customer&#39;s desire not to energize the NC valves during the evacuation/filling process because of those issues involved. 
     To avoid the need to electrically energize the valves at the assembly plant, a one-time bleed valve with reset capability for an anti-lock hydraulic control unit has been put forward to address the problem of evacuating the low-pressure circuit. The bleed valve comprises a piston that can be sealingly installed into a bore made in the body of the hydraulic control unit. The bore is sized to create a seal when the piston is pushed into the bore in response to a brake pressure. On one of its ends, the bore is in communication with the primary hydraulic pressure circuit. On another end, it is connected with the low-pressure line. Due to such a structure of the bleed valve, an air communication is established between the primary hydraulic pressure circuit and the low-pressure line when the piston sits loosely in the bore. By virtue of that communication, air can be evacuated from the low-pressure brake circuit when it is exposed to a vacuum applied to the primary hydraulic pressure circuit. 
     However, all the advantages of such a solution notwithstanding, the placing of the bleed valve into the hydraulic system still may not fully prevent a vacuum from developing over time in the low pressure accumulating area after several normal brake pedal depressions. Specifically, the vacuum can be created upon brake pedal release. The accumulated vacuum may lead to air gaining access into the system and thus to a “soft pedal”. Also, when the HCU subsequently goes into an ABS mode, the trapped air can be pumped into the brake circuit causing reduced brake performance. 
     Accordingly, a need as yet exists in the art to provide a reliable means for evacuating the low-pressure circuit. 
     SUMMARY OF THE INVENTION 
     Therefore, a primary object of the present invention is to provide a hydraulic brake system that would retain all the advantages of the systems in the art, not being subject to their drawbacks discussed in the above. 
     Another object of the present invention is to provide a method of preventing a vehicle brake system from developing a vacuum in its low-pressure area. 
     In accordance with the principles of the present invention confined to a vehicle anti-lock brake system of the type including a tandem master cylinder, wheel brakes, and a hydraulic control unit with NO valves connecting the tandem master cylinder with the wheel brakes through primary hydraulic pressure lines, and NC valves that connect the wheel brakes to the tandem master cylinder by low-pressure lines passing through a low-pressure area, the method provides for opening the NC valves periodically when brakes are not applied. 
     Specifically, the periodical opening of the NC valves is performed each time the ignition is shut off. 
     Upon periodical opening the NC valves, pressures in the tandem master cylinder and in the low-pressure area are equalized and that eliminates a vacuum trapped in the low-pressure area. 
     The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of preferred embodiments taken in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic diagram of a hydraulic brake system illustrating the area of application of the present invention. 
     FIG. 2 is a schematic diagram of an improved hydraulic brake system that the present invention can be applied to. 
     FIG. 3 illustrates a one-time bleed valve used with the system of FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 2 where a schematic diagram of a hydraulic brake system  100  that can host the present invention is shown, the system  100  comprises all the principle components of the conventional system  10  depicted in FIG.  1 . Specifically, it has a TMC supplying brake fluid to all wheels, the TMC being represented in FIG. 2 by an inlet  102 ; a HCU  104 ; and a wheel caliper of a wheel brake defined in FIG. 2 by an outlet  106  thereto. 
     A low-pressure portion  108  of the HCU  104  is shown within a dotted contour. It contains a low-pressure accumulator  110 , an input portion  112  of a return pump  114 , and associated low-pressure lines  116  and  118 . A motor  120  drives the return pump  114 . Illustrated in FIG. 2 are also a return pump outlet valve  122 , a noise damper  124 , and two electromagnet valves: a pressure buildup NO valve  126  and a pressure reduction NC valve  128 . The pressure buildup electromagnet NO valve  126  is disposed in a brake line  130  connecting the inlet  102  from TMC to the outlet  106 . A check valve  132  is also connected in parallel to the NO valve  126 . The pressure reduction electromagnet NC valve  128  is placed in a return line  134  that eventually, through the return pump  114  reunites with a primary hydraulic pressure circuit  136 . 
     In addition, the system  100  comprises a one-time bleed valve  138  of the type discussed in the above and connected between the low-pressure line  116  and the main brake line  136 . Shown in FIG. 3, the bleed valve  138  is structurally assembled in a bore  140  made in a body  142  of the HCU  104 . The bore  140  is arranged in the body  142  in such a way that on one of its ends it is connected to the line  130  of the TMC primary hydraulic pressure circuit  136 , and on another of its ends it is connected to the low-pressure line  116 . To obtain an easy access from the bore  140  to the line  130 , it was found convenient to make the bore  140  at a recessed part  144  of the body  142  that accommodates the NO valve  126 , and more specifically, immediately below the body  146  of the valve  126 . The valve  138  comprises a piston  148  with an annual groove  150  made on a head portion  152  of the piston  148 , and an O-ring  154  assembled in the groove  150 . The diameter of the bore  140  is sized to create a seal when the piston  148 /O-ring  154  combination is pushed downward into the bore  140 . The bore  140  is provided with a lead-in chamfer  156  to facilitate the piston/O-ring combination to be pushed into the bore. 
     Provided as an auxiliary feature of the bleed valve  148  is an access hole  158  to reset the valve  138  after a leak test. The feature will be discussed below in more detail. The access hole  158  can be sealed with a ball  160 . 
     The piston  148  is initially assembled loosely into the bore  140  so that it rests in the lead-in chamfer  156 . The body  146  of the NO valve  126  limits from above the piston&#39;s travel to allow only a predetermined amount of stroke when operating. 
     At the rest condition, the piston  148  is loosely set into the bore  140  so that it rests on the lead-in chamfer  156 . If an air-leak test is conducted on the HCU  104  prior to shipment of the system  100  to an OEM, high pressure is applied to the main circuit, that is to primary hydraulic pressure circuit  136 , the low-pressure circuit remaining at the atmospheric pressure. The pressure difference will force the piston  148  into the bore  140 , thus sealing the path between the lines  130  and  116 . After the test is completed, a rod (not shown) is inserted in the access hole  158  to push the piston back out of the bore  140  resetting it for the evacuation process. The feature of accessing the piston  148  through the hole  158  is not needed if the air-leak pretest is not required. 
     After the HCU  104  is installed into a vehicle, the evacuation process applies a vacuum to the main circuit  136  (and hence to the line  130 ). The valve  138  is forced open due to the atmospheric pressure in the low-pressure line  116 , thus allowing the air to be evacuated. When brake fluid is inserted into the system at low pressure, the valve  138  will close again. 
     When brake pressure is applied at the proof pressure load during a test on a vehicle assembly line, a large pressure drop will ensure that the piston  148  will move completely into the bore  140 . During normal brake operation, the piston  148  seals the line  130  from the low-pressure line  116 . When a transient vacuum is applied due to brake pedal release, friction of the O-ring  154  holds the valve  138  in place. Also, an undercut (not shown) may be used in the bore  140  to further prevent the valve  138  from moving backward after it has been seated. 
     A software controlling an electronic control unit (ECU, not shown in the drawings) of the vehicle provides for pulsing each of NC valves  128  not long after the engine is off. Preferably, the delay does not exceed several minutes. Additionally, during continuous drive, the software may pulse the NC valves periodically at the moments the brakes are not applied that may be detected, for example, via a brake light. As a result of opening the NC valves  128 , pressures in primary brake line  130  and low-pressure area  108  level off, and the vacuum developed in the low-pressure area is thus eliminated. 
     While the foregoing description relates to a preferred exemplary embodiment of the present invention, it is to be understood that this embodiment is given by example only and not in a limiting sense. Those skilled in the art may make various modifications and additions to the preferred embodiment chosen to illustrate the invention without departing from the spirit and scope of the present contribution to the art. Accordingly, it is to be realized that the patent protection sought and to be afforded hereby shall be deemed to extend to the subject matter claimed and all equivalence thereof fairly within the scope of the invention.