Abstract:
An electrohydraulic brake system where brake pressure is controlled by the combined action of an apply valve and a dump valve by the implementation of a method of controlling the voltage applied to the apply and dump valve. The EHB pressure control system receives a desired wheel pressure command and, with a caliper pressure feedback signal, implements an algorithm to compute one voltage command for the apply valve and another for the dump valve, corresponding to a requested flow from the hydraulics. The voltage command drives current control electronics. The electronics in turn power the solenoids of the proportional apply and dump poppet valves to control flow in or out of the brakes and modulate wheel pressure as required. Use of the algorithm to control the electromagnetic poppet valves achieves the commanded pressure at a vehicle&#39;s brakes reliably and with good control in all states of flow through the valve. The algorithm is a function of the existing pressure within the system and whether there is bulk or leakage flow through the valves.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of International Application No. PCT/US02/23464 filed Jul. 24, 2002, the disclosure of which is incorporated herein by reference, and which claimed priority to U.S. patent application Ser. No. 09/919,445 filed Jul. 31, 2001 (issued as U.S. Pat. No. 6,634,722), the disclosure of which is incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates in general to braking systems, and in particular to an algorithm for electronically controlling poppet valves in an electrohydraulic brake system to control the pressure of brake fluid applied to vehicle wheel brakes.  
           [0003]    Traditional hydraulic brake systems include a brake pedal operated by the driver of a vehicle. The brake pedal operates a master cylinder, causing the master cylinder to send pressurized hydraulic brake fluid to the wheel brakes of a vehicle. This is sometimes referred to as foundation or base braking—the basic braking called for by the operator of a vehicle. Over the years, engineers have worked to improve the performance of the braking system of vehicles by augmenting or replacing the base braking function with another braking operation.  
           [0004]    Electrohydraulic brake (EHB) systems utilize electronically controlled valves, pumps, or a combination thereof to augment, replace, or control the base braking operation of a vehicle brake system. One of the first of many advanced braking functions that has been developed for vehicles is ABS (Antilock Braking System), which typically involves the operation of valves and pumps to selectively release and re-apply brakes during a braking operation. While typical base braking is commanded by the operator, ABS braking controls the vehicle brakes to recover from and limit skidding of a vehicle&#39;s wheels due to braking the wheels harder than permitted by the available coefficient of friction of the road surface. Since pumps and valves are electronically controlled to augment the base braking operation, a vehicle equipped with ABS may generally be said to have an EHB system. Another advanced braking function that may be accomplished by a properly configured EHB system is VSC (Vehicle Stability Control), which is a system for selectively actuating vehicle brakes to improve the stability of a vehicle during vehicle maneuvers. Other braking applications producing a pressure command input to the present invention include DRP (Dynamic Rear Proportioning—a system for controlling the front to rear proportioning of a vehicle braking command), TC (Traction Control—which typically involves selective application of brakes during vehicle acceleration to recover from and limit skidding of a vehicle&#39;s wheels due to accelerating the wheels faster than permitted by the available coefficient of friction of the road surface), ACC (Autonomous Cruise Control—a cruise control system that can actuate vehicle brakes to maintain proper vehicle spacing relative to a vehicle in front) and similar functions.  
           [0005]    Various forms of electrohydraulic braking systems have been proposed. For example U.S. Pat. No. 5,941,608, Campau, et al. discloses an electronic brake management system with manual fail-safe capabilities. A subset of electrohydraulic braking systems is an electronic brake management system (EBM). EHB systems can allow braking to be primarily controlled by the vehicle driver with a conventional master cylinder system. Additionally, an electronically controlled portion of the system operates the brakes under certain conditions, i.e. anti-lock, traction control, etc. Primary braking is controlled electronically in Electronic Brake Management systems. Normally, the vehicle driver or a safety system generates an electronic signal, which in turn operates the pumps and valves to achieve a braking pressure within the system. A pedal simulator creates the effect for the driver of applying direct braking pressure while also providing a back-up braking system in case of a failure of the primary system. In the back-up system, the pedal simulator acts as a master cylinder during the failure event and provides the hydraulic pressure that actuates the brakes.  
           [0006]    In Campau, et al., as well as in many EHB systems, there are a series of valves in the hydraulic circuit that operate to apply pressure to or dump pressure from the brake. In one embodiment of Campau, proportional control valves are provided for each vehicle brake. In a first energized position, an apply position, a proportional control valve directs pressurized hydraulic fluid supplied to the proportional control valve from a fluid conduit to an associated fluid separator unit. In a second energized position, the maintain position, the proportional control valve closes off the port thereof which is in communication with the associated fluid separator unit, thereby hydraulically locking the associated fluid separator piston of the fluid separator unit in a selected position. In a de-energized position, the release position, the spool of the proportional control valve is moved by a spring to a position where the proportional valve provides fluid communication between the associated fluid separator unit and a reservoir. This vents pressure from the associated fluid separator unit allowing the piston thereof to move back to an unactuated position under the urging of an associated spring, thereby reducing pressure at the associated wheel brake.  
           [0007]    The positions of the proportional control valves are controlled such that the pressure of fluid in the hydraulic circuit is controlled proportionally to the current of the energizing electrical signal. As a result, the exact position of a proportional control valve is proportional to the electrical control signal. Thus, the proportional control valves can be positioned at an infinite number of positions rather than just the three positions described above.  
           [0008]    Traditional valve control encompasses two positions: open or closed. In a closed position, ideally, there is no flow through the valve. In the open position, there is flow through the valve in proportion to the degree the valve is opened. Voltage across a controlling solenoid dictates the amount of valve opening. There are also two traditional types of valves that are used in hydraulic systems: spool valves and poppet valves. A major difference between poppet valves (used herein) and spool valves is that there is significant amount of leakage associated with poppet valves. How to deal with poppet valve leakage is therefore important to the performance of an EHB system. A common practice is to minimize the effect of leakage through careful mechanical design and software algorithm. However, valve leakage may also be used to increase system resolution, reduce valve clicking noise if a valve is supposed to be open, while being eliminated to minimize flow consumption if a valve is supposed to be closed. This creates a system where the beneficial aspects of valve leakage can be taken advantage of while the negative aspects thereof can be suppressed.  
           [0009]    The terms “bulk flow” and “leakage flow” are used to describe the primary types of flow through valves as used in this application. “Bulk flow”, as used herein, means the flow in a valve that occurs when the moving valve element (such as the valve poppet in poppet valves) is off of its seat. “Bulk flow” can also be described as flow through an open valve. “Leakage flow”, as used herein, means that flow that generally occurs during closed valve operation due to limitations in the manufacturing process and the design of the valve. Depending on conditions of operation as well as the type of valve and its manufactured characteristics, some flow can leak through it, even with the valve in a “closed” position. “Leakage flow” as used herein also means that flow which occurs when the moving element is not fully seated against its seat or is only intermittently in contact with the seat (e.g. “chattering” or “simmering”). Either leakage or bulk flow can be “laminar flow” or “turbulent flow”; however, it can be expected that bulk flow will be turbulent flow. Laminar flow is classically defined as a well ordered pattern of flow whereby fluid layers are assumed to slide over one another. Turbulent flow is irregular or unstable flow.  
           [0010]    While the above-described system and other existing systems have effectively managed the operation of valves in an EHB system, there is a need for greater incremental control of the valves, as well as accounting for the various flow states through the valves. One limitation in controlling valves in a closed or near-closed position is that it is difficult to control the change in pressure applied to the brake. As a result, pressure to a brake would increase more than demanded by the vehicle user. There also is a need to account for the noise or clicking of valves when fluctuating between a near closed and closed position. The algorithm described below provides finer control of proportional valves in a braking system by controlling the voltage applied to operate the valve. Also described, as part of the invention, is the method of operating the valves to account for leakage and bulk flow as well as preventing flow while in a closed state.  
         SUMMARY OF THE INVENTION  
         [0011]    This invention relates to electrohydraulic braking (EHB) systems for vehicles and in particular to a method (algorithm) for controlling the pressure of brake fluid applied to the wheel brakes thereof. The algorithm interprets a braking pressure command input from any of a variety of braking functions, and controls the electromagnetic poppet valves to achieve the commanded pressure at a vehicle&#39;s brakes reliably and with good control in all states of flow (leakage or bulk) through the valves. Additionally, the pressure control system accepts inputs from advanced braking functions, such as ABS, VSC, TC, DRP, ACC, etc. More specifically, the purpose of the pressure control system is to provide adequate wheel pressure control with respect to dynamic response, tracking error, robustness and stability objectives, while taking into account issues such as vibration, noise and flow consumption. The pressure control system is intended for use with base braking functions, such as that described in U.S. Pat. No. 6,226,586, granted May 1, 2001, the disclosure of which is incorporated herein by reference.  
           [0012]    The EHB pressure control system receives a desired wheel pressure command, and with a caliper pressure feedback signal, computes one voltage command for the apply valve and another one for the dump valve, both corresponding to a requested flow from the hydraulics. The voltage command drives current control electronics. The electronics in turn selectively power the apply and dump poppet valves to control flow in or out of the brakes and modulate wheel pressure as required.  
           [0013]    Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a simplified schematic view of a vehicle braking system and a portion of the electronic controls therefore.  
         [0015]    [0015]FIG. 2 is a control space partition depicting voltage related to the solenoid, of an apply or dump valve, versus the flow demanded through the related valve.  
         [0016]    [0016]FIG. 3 is a switch logic function schematic of the applied pressure control algorithm.  
         [0017]    [0017]FIG. 4 is a switch logic function schematic of the applied pressure control algorithm showing the signal path for operating the apply valve in the bulk flow control region and holding the dump valve firmly closed.  
         [0018]    [0018]FIG. 5 is a switch logic function schematic of the applied pressure control algorithm showing the signal path for operating the apply valve in the leakage flow control region and holding the dump valve firmly closed.  
         [0019]    [0019]FIG. 6 is a switch logic function schematic of the applied pressure control algorithm showing the signal path for operating the apply valve and the dump valve in the pressure control region holding both valves firmly closed.  
         [0020]    [0020]FIG. 7 is a switch logic function schematic of the applied pressure control algorithm showing the signal path for operating the dump valve in the leakage flow control region and holding the apply valve firmly closed.  
         [0021]    [0021]FIG. 8 is a switch logic function schematic of the applied pressure control algorithm showing the signal path for operating the dump valve in the bulk flow control region and holding the apply valve firmly closed. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]    Referring now to the drawings, there is illustrated in FIG. 1 a portion  10  of the electronic circuitry processing braking signals that implements a pressure control algorithm according to the present invention, together with a greatly simplified schematic representation of a typical EHB system  12 . A more detailed description of an EHB system for which the algorithm of the present invention may suitably be used is described in U.S. Pat. No. 5,941,608 to Campau et al., the disclosure of which is hereby incorporated by reference. However, it should be understood that it is believed that the present invention may suitably be practiced in a variety of other EHB systems, including without limitation, the EHB system described in the SAE Technical Paper Series No. 950762, “Intelligent Braking for Current and Future Vehicles”, and No. 960991, “Electrohydraulic Brake System—The First Approach to Brake-By-Wire Technology”, the disclosures of which are also incorporated by reference. It should also be understood that although only one wheel brake  22  is shown in FIG. 1, that in actual vehicle use the system shown can be implemented to cooperate with a plurality of wheel brakes. This includes, but is not limited to, multiple pressure command signals, gain schedules, linear controllers, switch logic circuits, valve table values, apply and dump valves, pressure feedback sensors and pumps.  
         [0023]    The simplified EHB system  12  includes a fluid reservoir  14 . A pump  16  pumps hydraulic brake fluid from the reservoir  14 . The pump  16  (typically complemented by a high-pressure accumulator, not shown) supplies pressurized hydraulic brake fluid to an apply valve  18 , which is preferably implemented as a normally closed solenoid  19  operated poppet valve. When the apply valve  18  is opened, pressurized hydraulic brake fluid passes through the apply valve  18 , and flows through a conduit  20  to a vehicle wheel brake  22 . A fluid conduit  24  is connected to the conduit  20  between the wheel brake  22  and the apply valve  18 , and provides a return path for hydraulic brake fluid from the wheel brake  22  to the reservoir  14 . A normally open solenoid  27  operated dump valve  26  is disposed in the fluid conduit  24  to control the flow of hydraulic brake fluid through the fluid conduit  24 .  
         [0024]    The terms “bulk flow” and “leakage flow” have been referred to above (for example, in the Background and Summary of the Invention). “Bulk flow”, as used in this application, means the flow in a valve that occurs when the moving valve element (such as the valve poppet in poppet valves like the apply valve  18  and the dump valve  26 ) is off of its seat. “Bulk flow” can also be described as the flow through an open valve. “Leakage flow”, as used in this application, means that flow that generally occurs during closed valve operation due to limitations in the manufacturing process and the design of the valve. Depending on conditions of operation as well as the type of valve and its manufactured characteristics, some flow can leak through it, even with the valve in a “closed” position. “Leakage flow”, as used in this application, also means that flow which occurs when the moving valve element is not fully sealed against its seat or is only intermittently in contact with the seat (e.g. “chattering” or “simmering”). For example, in a poppet valve, such as the apply valve  18  or dump valve  26 , leakage flow may occur when the valve poppet is held so lightly against the valve seat that, due to manufacturing imperfections, the valve poppet is only partially contacting the associated seat. Leakage flow also occurs when the valve poppet is intermittently in contact with the seat. Either leakage or bulk flow can be “laminar flow” or “turbulent flow”; however it can be expected that bulk flow will be turbulent flow. Laminar flow is classically defined as a well ordered pattern of flow whereby fluid layers are assumed to slide over one another. Turbulent flow is irregular or unstable flow.  
         [0025]    Although poppet valves are preferred for use in this system, spool valves are also used in hydraulic systems. A major difference between poppet valves and spool valves is that there is a significant amount of leakage associated with poppet valves. How to deal with leakage is therefore important to the performance of the EHB system  12 . A common practice is to minimize the effect of leakage through careful mechanical design and software algorithm. However, part of the invention strategy is to utilize leakage to increase system resolution in a low flow demand situation. Additionally, the valves are controlled to reduce the clicking noise of a valve, caused by intermittent contact of the moving valve element with the seat, if the valve is supposed to be open. This control will also eliminate leakage, as much as possible, to minimize flow consumption if the valve is supposed to be closed. This strategy takes advantage of the “good” aspects of leakage as well as suppressing the “bad” aspects of leakage.  
         [0026]    A pressure control algorithm contains specific functions that have been designed so that a complete closed-loop control system performs as required by receiving an indication of the actual braking pressure, P b ,  30  at the wheel brake  22  as a pressure feedback signal  32 . The pressure control algorithm can also be implemented utilizing an estimated pressure at the wheel brake  22  as the pressure feedback signal  32 . For example, the feedback signal  32  may be derived from a pressure measured at a location remote from the wheel brake  22  (for example, in a central hydraulic control unit) as modified by an electronic model of the piping between the remote location and the wheel brake  22 . The electronic model accounts for transient pressure effects on the fluid supplied to the wheel brake  22  from the remote location.  
         [0027]    The functional components present in the current EHB pressure control algorithm include a primary pressure control law  44 , described below, which is designed to calculate the demanded flow, Q dem    36  into or out of the wheel brake  22  to achieve a desired braking effect. This is based on the demanded pressure, P cmd    50 , supply pressure, P s    28 , and brake pressure, P b    30  and various gain factors  38 ,  40 ,  112 . The gain factors are based on gain schedules  42 , described below, which are used in the pressure control law  44  to account for non-linearities in the braking system  12 . Deadband reduction is also used to minimize the response delay. Finally, switch logic  34  is implemented to determine whether the demanded flow, Q dem ,  36  at the brake is positive or negative. As a result, the switch logic  34  determines whether voltage will be applied to the apply valve  18  or dump valve  26 , and whether bulk flow or leakage flow needs to be used.  
         [0028]    The first functional component present in the algorithm is the primary control function and is designed to achieve the steady state and dynamic performance requirements in an EHB system  12 . To measure the performance of the EHB system  12 , a pressure control law  44  is implemented to determine the required inputs and outputs of the system. The inputs to the pressure control law function  44  are filtered demanded brake pressure, P cmd    50 , which is the pressure that the algorithm is trying to establish at the brake; measured (or estimated) wheel brake pressure, P b    30 , and measured supply pressure at the pump outlet, P s    28 .  
         [0029]    The outputs of the pressure control law function  44  are demanded flow, Q dem    36 , and scheduled gain multipliers 1/K  38  and K h    40 . The flow demand Q dem    36  is the commanded flow rate for fluid to flow to or from the wheel brake  22 . The scheduled gain multipliers 1/K  38  and K h    40  are used to modify the demanded flow, Q dem    36  in bulk and leakage flow regions, respectively.  
       Pressure Control Law/Gain Schedules/Deadband Reduction  
       [0030]    Gain schedules  42  are related to the gain factors  38 ,  40  described above. Gain factors  38 ,  40  are used to account for non-linearities within the hydraulic system  12 . The values that compensate for the non-linearities are established by experimentation and vary for an individual valve, brake caliper or other element in the system or for the system itself. A gain schedule  42  is a table of values for the element based upon certain operating conditions. Depending on the operating conditions at the time a flow is demanded, corresponding gain values  38 ,  40  from the gain schedule  42  are factored into the pressure control law  44 .  
         [0031]    The demanded flow, Q dem    36 , is computed as a linear function of the pressure demand rate and the error between the demanded pressure, P cmd    50 , and the estimated wheel pressure, P b    30 , and represents the flow into or out of the wheel brake  22  (caliper). Two types of gain schedules  42  are used in the pressure control law  44 , which allow the use of a linear control signal despite the presence of non-linearities in the system. The primary gain factor, K h    40 , which represents the system hydraulic non-linearity, is intended to compensate for the change in flow gain as a consequence of pressure difference across the valve and the non-linear brake load characteristics. Specifically, K h    40  is the hydraulic gain factor that accounts for the non-linearity of the pressure versus volume characteristic of the brake caliper  22  and the non-linearity in the throttling of a valve.  
         [0032]    The gain factor 1/K  38  considers the overall hardware non-linearity, including the hydraulic non-linearity which is accounted for by K h    40  as well as additional system non-linearities such as non-linear armature displacement in response to a changing magnetic force applied through the solenoids  19 ,  27 . Preferably, however, only gain factor K h    40  is used in the leakage flow regions  102 ,  110  since the change in displacement of the armature in those flow regions is insignificant. Finally, a deadband reduction scheme is used to reduce delays in valve response to an applied voltage when the valve is in its normal position (closed for an apply valve  18 , open for a dump valve  26 ).  
         [0033]    Another force on the armature comes from the valve seat when the valve (apply valve  18  or dump valve  26 ) is completely closed and acts against the magnetic force. While this force helps reduce the leakage if the valves are supposed to be closed, its presence creates a deadband to the system if the valves are supposed to be opened. If the voltage boundary where the valves are closed and the seat force is zero, then the deadband can be reduced using that value.  
         [0034]    The look-up table factors (voltage), which are Voltage Boundary Table 1   46  and Voltage Boundary Table 2   48 , are implemented in the control law  44  to adjust the voltage applied across the valve (apply valve  18  or dump  26 ) based on pre-existing forces on the valve. The look-up tables  46 ,  48  give the voltages necessary to take the valves from a de-energized state to a just-closed position given the existing pressure differential. The just-closed position is where the valves are closed and the seat force is zero. For the apply valve  18 , the pressure differential is established by considering the supply pressure  28  minus the wheel brake pressure  30  (P s -P b ). For the dump valve  26 , the pressure differential is the same as the wheel brake pressure  30  (P b ) because the pressure in the fluid reservoir  14  is zero. The de-energized state for the apply valve  18  is normally closed and the de-energized state for the dump valve  26  is normally open. Voltage Boundary Table 1   46  supplies the voltage values to be used when a voltage is applied to the apply valve  18  and Voltage Boundary Table 2   48  supplies the values respective to the dump valve  26 . The table values  46 ,  48  account for pre-load force from a spring connected to the valve armature and the pressure differential force across the valve  18 ,  26 . The table values  46 ,  48  will be determined based on experimentation and the natural characteristics of a particular valve.  
       Switch Logic/Bulk and Leakage Flow Control Modes  
       [0035]    The EHB system  12  uses two proportional poppet valves, apply valve  18  and dump valve  26 , as the means of actuation of the brake caliper  22  in the apply and release directions, respectively. The apply valve  18  is used solely for pressure increases and the dump valve  26  solely for pressure decreases. Therefore, switch logic  34  is needed to generate two voltage signals, V apply    52  and V dump    54 , from one control command. It is also important to distinguish between bulk flow control and leakage flow control because of the distinct characteristics each possesses, as described above.  
         [0036]    The inputs to the switch logic function are, as above, flow demand, Q dem    36 ; measured/estimated wheel brake pressure, P b    30 ; measured supply pressure, P s    28 ; scheduled gain multipliers 1/K  38  and K h    40 , and the Voltage Boundary Table 1   46  and Table 2   48 . The outputs of the switch logic function are applied voltages on both the apply valve  18  and dump valve  26 , V apply    52  and V dump    54 . The switch logic function  34  generates actual voltages, V apply    52  and V dump    54 , which are applied to the apply-side solenoid  19  and the dump-side solenoid  27 , respectively.  
         [0037]    [0037]FIG. 2 depicts the control space  100  for operation of the EHB system  12 , which has been partitioned into five regions based on switch logic that generates voltages V apply    52  and V dump    54 . These regions of control of the EHB system  12  are apply valve bulk flow control  102 , apply valve leakage flow control  104 , pressure control  106 , dump valve leakage flow control  108  and dump valve bulk flow control  110 .  
         [0038]    [0038]FIG. 3 indicates how the switch logic  34  develops the voltage (V apply    52 ) that is applied to the apply valve solenoid  19 . FIG. 3 also illustrates how the switch logic  34  develops the voltage (V dump    54 ) supplied to the dump valve solenoid  27 . Three modes of control of flow through each of the valves  18  and  26  are described in further detail below. Each region of flow control  102  through  110  has a unique combination of modes of control of the apply valve  18  and dump valve  26 . Each flow control region also has associated with it a respective V apply    52  and V dump    54  (V apply1    220 , V apply2    222 , V apply3    224 , V dump1    226 , V dump2    228 , or V dump3    230 ). The applications of each apply and dump voltage  220  through  230  is described below.  
         [0039]    A pseudo-code can be developed that defines the functionality of the switch logic function  34  for a normally closed apply valve  18  and a normally open dump valve  26 . If a point on the axis representing the flow demand  36  (FIG. 2) is positive and is greater than the gain factor K h    40  multiplied by sigma  112  (Condition 1   248 ), then the EHB system  12  will operate in the apply valve bulk flow region  102  with the apply valve  18  being opened to the desired position. Specifically, the open apply valve  18  in a bulk flow region  102  is operated based on the valve&#39;s natural characteristics (including the overall hardware non-linearity), and the hydraulic non-linearity. The function then calculates a respective voltage  52 ,  54  to be applied to the apply valve  18  and dump valve  26  to achieve apply bulk flow and firmly close the dump valve  26 . Stated in equation form, the voltages for the apply valve  18  and dump valve  26  are: 
           V   apply   =V   apply1 =Table 1 +( Q   dem −sigma* K   h )*(1 /K ) 
           V   dump   =V   dump3 =Table 2 +beta 
         [0040]    Beta  114  is a pre-determined voltage quantity designed to cause the dump valve  26  to be firmly closed when a voltage in the amount of beta  114  is applied to it. If the demanded flow  36  is negative and less than the gain factor K h    40  multiplied by—sigma  112  (Condition 4   250 ), then the valves operate in dump bulk flow region  110  with the dump valve  26  being opened to the desired position. Represented in equation form, it is the opposite of the previous function. 
           V   apply   =V   apply3 =Table 1 −beta 
           V   dump   =V   dump1 =Table 2 +( Q   dem +sigma* K   h )*(1 /K ) 
         [0041]    The conditions under which the bulk flow control modes described above will be implemented can be also stated in another manner. If the absolute value of the demanded flow  36  is greater than the absolute value of sigma  112  multiplied by K h    40 , then the apply valve  18  and dump valve  26  operate in either the apply valve bulk flow region  102  or the dump valve bulk flow region  110 . A first valve will be opened to allow bulk flow through it while a second valve is held firmly closed. If the dump valve  26  is the second valve, the apply valve  18  and the dump valve  26  operate in the apply valve bulk flow region  102 . If flow is demanded to the brake  22 , then the apply valve  18  is the first valve. If the apply valve  18  is the second valve then the apply valve  18  and dump valve  26  are operated in the dump valve bulk flow region  110 . If flow is demanded away from the brake  22 , then the first valve is the dump valve  26 . This condition, determined by Condition 1   248  and Condition 4   250 , can be set forth in equation form as well: 
         |Q dem |&gt;|sigma*K h | 
         [0042]    If flow is demanded to the brake  22 , but the demanded flow is less than sigma  112  multiplied by K h    40 , but greater than switch (evaluated by Condition 2   252  and Condition 1   248 ), then the apply valve is operated in apply valve leakage flow region  104 , while the dump valve is held firmly closed. The opposite remains true here as well. If flow is demanded away from the brake  22  and the value is greater than negative sigma  112  times K h    40 , but less than negative switch  116  (Condition 4   250  and Condition 3   254 ), then the dump valve  26  is operated in dump valve leakage flow region  108  and the apply valve  18  is held firmly closed. Both are represented in equation form as follows. When operating in the apply valve leakage flow control region  104 , the voltage applied can be determined by the following equation:  
         V   apply     =       V   apply2     =       Table                 1     +       (       Q   dem     -     sigma   *     K   h         )     *     betal     sigma   *     K   h                                     
  V   dump   =V   dump3 =Table 2 +beta 
         [0043]    When operating in the dump valve leakage flow control region  108 , the voltage can be determined by the following equation: 
           V   apply   =V   apply3 =Table 1 −beta 
         [0044]    [0044]         V   dump     =       V   dump2     =       Table                 2     +       (       Q   dem     +     sigma   *     K   h         )     *     betal     sigma   *     K   h                                       
         [0045]    The actuating conditions can be alternatively stated as follows. If the absolute value of the demanded flow  36  is less than the absolute value of sigma  112  multiplied by K h    40 , then a first valve will be controlled in the leakage flow region while a second valve will be held firmly closed. If flow is demanded to the brake  22 , then the apply valve  18  will be operated in the leakage flow region  104  and the dump valve  26  will be firmly held closed. If flow is demanded away from the brake  22 , then the dump valve  26  will be operated in leakage control region  108  and the apply valve  18  will be held firmly closed. Similarly, the following equation represents the above condition under which, if satisfied, a leakage control mode will be implemented. 
         |switch|&lt;|Q dem |&lt;|sigma*K h | 
         [0046]    If no flow is demanded to or from the brake  22  (determined by Condition 3   254 ) whether or not a braking signal exists, then both the apply  18  and dump valves  26  will be firmly held closed and operate in the pressure control region  106 . In equation form, The pressure control region is described below. 
           V   apply   =V   apply3 =Table 1 −beta 
           V   dump   =V   dump3 =Table 2 +beta 
         [0047]    The condition under which pressure control  106  will operate can also be expressed in a similar form as the conditions above. Satisfaction of the following condition will result in a pressure control mode being applied to the valves. 
         |Q dem |&lt;|switch| 
         [0048]    Table 1   46  and Table 2   48  represent “closing boundaries” for the apply valve  18  and the dump valve  26 , respectively. The closing boundaries are the voltage values at which the apply  18  and dump valves  26  are just-closed and there is no extra force on the valve seat. Beta  114 , Beta 1    115 , sigma  112  and switch  116  are described in greater detail below.  
         [0049]    [0049]FIG. 2 is a graph illustrating the above pseudo-code in that FIG. 2 graphically depicts the voltage  126  related to the solenoids  19 ,  27  of the apply valve  18  and the dump valve  26  versus the flow demanded  36  through the associated valve. It is important to note that FIG. 2 is for a given wheel brake pressure, P b    30 . Depending on the given pressure, a subsequent pressure demand signal  36  can request an increase or a decrease from that pressure. The horizontal axis depicts the flow demanded  36  by the user of the braking system, presumably the driver of the vehicle. A positive demanded flow, Q dem    36 , indicates that flow is demanded towards the brake  22  and a negative flow demand indicates flow is demanded away from the brake  22 . The vertical axis depicts voltage  126  related to the respective solenoids  19 ,  27  to open or firmly hold closed the apply  18  or dump valves  26 .  
         [0050]    The apply valve voltage curve, U a    120 , depicts the voltage  126  versus demanded flow  36  for the apply valve  18  and the dump valve voltage curve, U d    122 , represents the voltage  126  versus demanded flow  36  for the dump valve  26 . Both curves represent voltages relative to the “closing boundaries” in the following way: 
           V   apply =Table 1 + U   a   
           V   dump =Table 2 − U   d   
         [0051]    The Table 1   46  value is a voltage that will move the valve armature to a just-closed position. Because the apply valve  18  is normally closed, U a  needs to be added to Table 1   46 . The three regions of valve operation—bulk flow  102 , leakage flow  104  and pressure control  106 —on the flow demand versus voltage curve of FIG. 2 is based on “closing boundary” denoted by Table 1   46 . The Table 2   48  value is a voltage that will move the valve armature to a just closed position as well. Because the dump valve  26  is normally open, U d  needs to be subtracted from Table 2   48 . The three regions of operation—bulk flow  110 , leakage flow  108  and pressure control  106 —on the flow versus demand curve depicted in FIG. 2, are based on “closing boundary” denoted by Table 2   48 .  
         [0052]    Sigma  112  is a constant used as a leakage gain factor, described above. Sigma  112  calibrates the control law  44  to the leakage characteristics of the installed type of valve to match the slope of the flow versus the change in current curve. For example, depending on the valve used, if leakage is high, then a high sigma  112  value is used to compensate, whereas if there is low leakage flow, a low sigma  112  value is used. If there is no leakage, then sigma  112  will be zero. On the apply valve voltage curve  120  and dump valve voltage curve  122 , the slopes of the curves in their respective bulk control regions  102 ,  110  are gain factors 1/K  38  and −1/K  38 .  
         [0053]    The demanded flow  36 , and its positive or negative value, is chosen as the selector of the switch logic  34  so that a uniform non-control band can be achieved with respect to pressure error. Use of a so-called non-control band is intended to eliminate valve chattering caused by system noise.  
         [0054]    When the demanded flow  36  falls in some neighborhood of zero, the valve can chatter which is deemed to result entirely from noise in the system resulting in both valves varying from a closed to slightly unclosed position rapidly. This is where a first voltage quantity, beta  114 , becomes relevant. The beta  114  value is the magnitude of the minimum additional voltage required to firmly close the valve, depending on individual valve characteristics, to eliminate leakage. This first voltage quantity, beta  114 , is determined experimentally based on individual valve properties. The value of beta  114  can also be determined as a function of the pressure differential across a valve. For an apply valve  18 , the pressure differential is between the supply pressure, P s    28 , and the brake pressure, P b    30 . For a dump valve  26 , the pressure differential is simply P b    30 . A second voltage quantity, beta 1    115 , is less than or is equal to the first voltage quantity, beta  114 , and is used to set the slope of the apply valve voltage curve, U a    120 , and the dump valve voltage curve, U d    122 , in their respective leakage flow regions  104 ,  108 . Beta 1    115  is the magnitude of the additional voltage applied to a just-closed valve to reduce leakage flow to a level from which leakage is utilized to modulate brake pressure. Beta 1    115  can also be determined as a function of the pressure differential across a valve. For an apply valve  18 , the pressure differential is between the supply pressure, P s    28 , and the brake pressure, P b    30 . For a dump valve  26 , the pressure differential is simply P b    30 . As can also be seen in the U a    120  and U d    122  equations, above, is that when the valves  18 ,  26  are held firmly closed, beta  114  will be subtracted from Table 1   46  and added to Table 2   48 , such that beta  114  moves the valve armature into a firmly closed position from a just-closed position.  
         [0055]    The set value of beta  114  and beta 1    115  can be the same or different. However, from a physical standpoint, by adding a second voltage quantity, beta 1    115 , (see FIG. 3) there can be a lesser value for holding a valve shut when it may be desired to have some usable leakage flow while holding the valve shut to help the driver moderate pressures. For example setting the second voltage quantity, beta 1    115 , to 0.5V (which allows some leakage to occur) and the first voltage quantity, beta  114 , to 1.5V (where the valve is held firmly shut with essentially no leakage) allows for this control scheme. The voltage values used in this example are not meant to limit the voltage values that can be used for beta  114  and beta 1    115  in terms of magnitude and/or proportion, and are used for example only. This example, with beta 1    115  at 0.5V and beta  114  at 1.5V, is also shown in FIGS. 3-8. However the voltage values are only shown in the drawings as exemplary of values that can be used and should not be construed to limit the values at which beta 1    115  and beta  114  can be fixed. Additionally, voltage quantities beta 1    115  and beta  114  can have different values for an apply valve and a dump valve, and can also be a function of pressure differential across a valve.  
         [0056]    Switch positions  116  on the apply and dump voltage curves  120 ,  122  depict threshold points set to avoid a response to noise in the EHB system  10 . Switch  116  is the magnitude of the demanded flow signal  36  below which the first voltage quantity, beta  114 , is applied to the valve. Application of the first voltage quantity, beta  114 , to the valve will firmly close the valve so that there will be no response to minor variations in the signal. As the differential pressure increases (for a normally open valve), the voltage at which a position on the dump voltage curve, U d    122 , equals its Table 2  value  48  increases. As the pressure differential decreases, the voltage at which a position of the dump valve voltage curve  122  equals its Table 2  value  48  decreases. The converse is true for a normally closed valve. As described above, the Table values  46 ,  48  are look-up values implemented in the control law  44  to adjust the voltage applied across the valve based on pre-existing forces on the valve.  
         [0057]    [0057]FIG. 3 illustrates the switch logic function and depicts the switch logic function schematic of the applied pressure control algorithm  34  for each of the flow control regions  102 ,  104 ,  106 ,  108 ,  110 . FIGS. 4-8 show the switch logic function for each individual control state. However, what is shown is only the ultimate signal flow after the signal has been processed through each comparator  256 ,  258 ,  260 ,  262 . In actual application of the algorithm, signals will be evaluated at every stage of the schematic as shown in FIG. 3. Only the applicable signals will pass through each comparator, addition or multiplication box and output a resultant voltage. Measured supply pressure, P s    28 , and measured (or estimated) wheel pressure, P b    30 , are used to determine the pressure differential, delta P a    203  across the apply valve  18 . The resultant pressure differential is input to valve closing boundary Table 1   46  for the apply valve  18 . The wheel pressure, P b    30  is input to Table 2   48 , for the dump valve  26 . The demanded flow, Q dem    36  and gain factors sigma  112 , K h    40  and voltage quantity, beta 1    115  are also factored together, multiplied by scheduled gain factor 1/K  38  and combined with the proper Table value  46 ,  48 . This signal then continues to one of the valve position voltage schemes, V apply1    220 , V apply2    222 , V apply3    224 , V dump1    226 , V dump2    228  or V dump3    230  corresponding with the proper flow control mode. The routing of the signals to the proper schematic location corresponds to processing the voltage position on the apply  18  or dump valve  26  voltage curves. V apply1    220  or V dump1    226  correspond to the upper region of the curves (indicating Bulk Flow—apply  102 ,—dump  110 ), V apply2    222  or V dump2    228  corresponds to the mid-region (indicating Leakage Flow—apply  104 ,—dump  108 ) and V apply3    224  or V dump3    230  corresponds to the base region (Pressure Control  106 ). The signals then are modified according to the algorithm respective to whether Leakage Flow, Bulk Flow or Pressure Control is the goal. Finally, the signal moves through the appropriate Switch ( 274 ,  276 ,  278 ,  280 ) to determine the output voltage.  
         [0058]    A signal dependent upon Condition 1   248 , Condition 2   252 , Condition 3   254  or Condition 4   250  is also determined from the combination of sigma  112 , K h    40 , beta 1    115  and Q dem    36 . Satisfaction of Condition 1  through Condition 4  ( 248 ,  250 ,  252 ,  254 ), described above in the pseudo-code, determines whether to actuate one of the following control models: Bulk Flow Control for the apply  18  or dump  26  valves, Leakage Flow Control for the apply  18  or dump  26  valve, or Pressure Control. Only four Conditions are listed above, but five flow control models exist because the bulk flow control mode for the dump valve  26  is operated as the default condition if the other conditions are not satisfied. This is due to the dump valve  26  being normally held open and the apply valve  18  normally being held closed. Whether a Condition is satisfied determines whether switching functions, Switch 1   274 , Switch 2   276 , Switch 3   278  or Switch 4   280 , are in a positive or negative mode. If the signal is positive, then the primary signal is selected and moves through the switch. If the signal is negative, then the secondary signal moves through the switch.  
         [0059]    The signals from V apply1    220 , V apply2    222 , V apply3    224 , V dump1    226 , V dump2    228  or V dump3    230  proceed next through the proper switch depending on the satisfaction of Condition 1  through Condition 4  ( 248 ,  250 ,  252 ,  254 ). These Conditions are designed to serve as signals to determine whether there is a positive or negative signal going through the switches ( 274 ,  276 ,  278 ,  280 ). FIGS. 4-8 schematically represent the signal flow through the system for each of the flow control modes, however it should be understood that the signal will be processed during every braking operation through each Condition box and each Switch. Only the proper signal will pass to result in an output voltage. Ultimately, the signal is output, based on the above values, and actuates the apply  18  and dump  26  valves in the proportion demanded by the braking system to deliver the demanded braking pressure to the wheel brake  22 . It should be understood that this operation also applies to vehicles having multiple wheels and to the apply  18  and dump valves  26  for each wheel brake  22  in the system.  
         [0060]    The switch process, as explained above, allows a signal to pass through each Switch during every braking operation. However, each Switch allows a signal to pass based on satisfaction of a Condition. The Conditions used herein compare the relative values of the demanded flow value, Q dem    36 , to gain factors, sigma  112  times K h    40 , and switch  116  as described above in the pseudo-code. This process is described starting with Switch 1   274 . A primary signal, described with FIG. 4, is processed from multiplexer  264  comprising the outputs of adder boxes  208  and  216 , resulting in V apply1    220  and V dump3    230 . A signal from comparator  256 , described below, also enters Switch 1   274 . If Condition 1   248  is satisfied, indicating that the demanded flow, Q dem    36  is greater than sigma times K h  and Q dem  is greater than zero, the signal is positive representing that the apply valve  18  will be operated in the bulk flow control region. In such a case, the signal from multiplexer  264  passes through the Switch 1   274 , is de-multiplexed  282 , and a V apply    52  and V dump    54  voltage is output. If Condition 1   248  is not satisfied, indicating that the demanded flow, Q dem    36 , is any other value, the signal is negative indicating that a control mode other than a bulk flow control mode for the apply valve  18  is to be activated. In that case, a secondary signal is read from the output of Switch 2   276  and that signal passes through Switch 1   274 , is de-multiplexed  282  and outputs V apply    52  and V dump    54 .  
         [0061]    The output of Switch 2   276  is similarly determined. A primary signal enters Switch 2   276  from multiplexer  266  with the signal output from adder boxes  212  and  216  representing V apply2    222  and V dump3    230 . A signal from comparator  260 , determining if the demanded flow  36  in relation to Condition 2   252 , described below, is met. That signal also enters Switch 2   276 . If Condition 2   252  is satisfied, then the signal proceeds to Switch 1   274  and is assessed to determine if Condition 1   248  is also satisfied. Satisfaction of Condition 1   248  and Condition 2   252 , for purposes of applying the leakage flow control model, is to determine whether Q dem    36  is between switch  116  and sigma  112  times K h    40 . If so, a positive value Of Q dem    36  represents that the apply valve  18  will be operated in the leakage flow control region. The signal from multiplexer  266  then passes through Switch 2   276 , Switch 1   274  and is de-multiplexed through de-multiplexer  282  to produce the voltages V apply    52  and V dump    54 . If Condition 2   252  is satisfied but Condition 1   248  is not satisfied, then the control system will operate in the bulk flow control region as described above. If both Condition 2   252  and Condition 1   248  are not satisfied, indicating that Q dem    36  is less than or equal to switch  116 , then the signal at Switch 2   276  is negative. When the signal at Switch 2   276  is negative, a flow control mode for the apply valve  18  is a flow mode other than leakage flow or bulk flow. In such a case, a secondary signal is read from the output of Switch 3   278  and that signal passes through Switch 2   276  and Switch 1   274 , is de-multiplexed  282  and outputs V apply    52  and V dump    54 .  
         [0062]    The output of Switch 3   278  is also determined in a similar manner. A primary signal enters Switch 3   278  from multiplexer  268  with the signal output from adder box  210  and  216  representing V apply3    224  and V dump3    230 . A signal from Condition 3   254 , described below, also enters Switch 3   278 . If Condition 3   254  is satisfied, indicating that Q dem    36  is between negative switch  116  and positive switch  116 , then the signal is positive representing that the valves operate in a pressure control mode is to be activated. In such a case, the signal from multiplexer  268  passes through Switch 3   278 , Switch 2   276 , Switch 1   274  and is de-multiplexed  282  and V apply    52  and V dump    54  are output. If Condition 3   254  is not satisfied, indicating that Q dem    36  is any other value less than negative switch, a secondary signal is read from the output of Switch 4   280  and that signal passes through Switch 3   278 , Switch 2   276  and Switch 1   274 , is de-multiplexed  282  and outputs V apply    52  and V dump    54 . If Q dem    36  is greater than positive switch  116 , the signal is processed through Switch 2   276  and Switch 1   274  as described above.  
         [0063]    The output of Switch 4   280  is similarly determined. A primary signal enters Switch 4   280  from multiplexer  270  with the signal output from adder box  210  and  218  representing V apply3    224  and V dump2    228 . A signal from Condition 4   250 , described below, also enters Switch 4   280 . If Condition 4   250  is satisfied, indicating that Q dem    36  is greater than negative sigma  112  times K h    40 , then a positive value representing the dump valve  26  will operate in the leakage flow region, then the signal from multiplexer  270  passes through Switch 4   280  to Switch 3   278 . At Switch 3   278 , the signal is evaluated against Condition 3   254  to determine if Q dem    36  is less than negative switch  116 . If both Condition 3   254  and Condition 4   250  are met such that the demanded flow  36  is between negative switch  116  and negative sigma  112  times K h    40 , then the signal proceeds through Switch 2   276 , Switch 1   274 , is de-multiplexed  282  and a V apply    52  and V dump    54  voltage is output in leakage flow control mode applied to the dump valve  26 . If Condition 4   250  and Condition 3   254  are not satisfied, a secondary signal is read from the output of Switch 4   280  to determine whether the demanded flow is less than negative sigma  112 , which is the default position with the dump valve open and apply valve held closed. That signal then passes through Switch 3   278 , Switch 2   276  and Switch 1   274 , is de-multiplexed  282  and outputs V apply    52  and V dump    54  in a bulk flow control mode applied to the dump valve  26 . If Condition 4   250  is satisfied, but Condition 3   254  is not, then as the signal goes through Switch 3   278 , Switch 2   276  and Switch 1   274  there is a comparison to determine if the preceding conditions are satisfied, as described above.  
         [0064]    Shown in FIG. 4 is the switch logic function schematic  34 ′ of the applied pressure control algorithm processing a signal to operate the apply valve  18  in a bulk flow control region while holding the dump valve  26  firmly closed. Initially, the vehicle operator requests a demanded flow, Q dem    36 , by applying or releasing a brake pedal (not shown). A signal detecting this demand travels to comparator  256  and Condition 1   248  is applied, determining the relative values of demanded flow, Q dem    36  to sigma  112  multiplied by K h    40 . The signal leaving comparator  256  is multiplied by 1/K  38  and is added under condition V apply1    220  through adder box  208  to a signal representing the current pressure in the system. The current system pressure is determined by calculating the supply pressure, P s    28  minus the brake pressure, P b    30 , defining a pressure differential, delta P a    203 . The signal next proceeds to box  204  where it is factored with the Table 1   46  value for the apply valve  18  signal. That signal also proceeds to adder box  208  and is added to the flow demand  36  signal described above. The brake pressure signal also travels a separate route, factored with Table 2   48  for the dump valve  26  and to adder box  216 , representing V dump3    230 . That signal is also added to beta  114 . Both pressure signals are multiplexed  264  and proceed to Switch 1   274  and are evaluated as described above. The output of Switch 1   274  is de-multiplexed and outputs V apply    52  and V dump    54 . To reiterate, V apply1    220  represents the apply valve in a bulk flow region, V dump3    230  represents the dump valve in a closed position with the additional beta value  114  (voltage factor) present to ensure that the valve  26  is held firmly closed.  
         [0065]    [0065]FIG. 5 shows the switch logic function schematic  34 ″ of the applied pressure control algorithm processing a signal to operate the apply valve  18  in a leakage flow control region while holding the dump valve  26  firmly closed. Initially, the operator of the vehicle demands a flow by applying or releasing the brake pedal. This signal travels in two directions. First, it is evaluated relative to Condition 2   252  at comparator  260 . It is there determined whether the demanded flow, Q dem    36  is greater or less than the switch  116  value. The signal then proceeds to Switch 2   276  and is processed as described above. The demanded flow, Q dem    36 , signal also travels to comparator  256 . There, the gain factor signal is compared thereto. Gain factor K h    40  is multiplied by sigma  112  and sends a signal to comparator  256  and is compared to the demanded flow, Q dem    36 . That signal then moves to Switch 1   274  as described above. The gain factor signals (sigma*K h ) is divided by beta 1    115  and then proceeds to multiplication box  242  where it is factored with the output of adder box  256 . The combined signal then moves to adder box  212  representing V apply2    222 . There, as above, it is added to the pressure differential signal, delta P a    203 . The pressure differential signal, delta P a    203 , is again factored through Table 1   46 , but proceeds to adder box  212 . The signal from the brake pressure proceeds as above to adder box  216 , and is added to beta 1    115 , as the dump valve  26  remains firmly closed. The signal from  212  and  216  are multiplexed  266  and proceed to Switch 2   276 , where they are processed and proceed as described above. The output signal from Switch 1   274  then is de-multiplexed  282  and is the voltage (V apply    52  and V dump    54 ) applied to the apply  18  and dump  26  valves, controlling the apply valve  18  in a leakage flow control region while holding the dump valve  26  firmly closed.  
         [0066]    As seen in FIG. 6, the switch logic function schematic  34 ′″ of the applied pressure control algorithm processing a signal to hold the apply  18  and dump  26  valves firmly closed. Initially, the operator of the vehicle initiates a signal demanding no flow by not applying or releasing the brake pedal. This signal travels to comparator  262  under Condition 3   254  and is checked relative to switch  116 . The signal then proceeds to Switch 3   278  where it is evaluated as described above. As above, the pressure differential, delta P a    203 , is determined and in this case will result in there being no pressure differential. Therefore, a first signal proceeds to adder box  210  representing V apply3    224  and is added to beta  114 . The beta value  114  is used to hold the valves firmly closed. The signal then proceeds to Switch 3   278  where it is processed with a signal from adder box  216 . The brake pressure, P b    30  is processed through Table 2   48  and is added to beta  114  in adder box  216  representing V dump3    230  indicating the valve is held firmly closed. The signals are multiplexed  268 , and proceed to Switch 3   278  and are evaluated and proceed as described above. The output of Switch 1   274  ultimately is de-multiplexed  282  and outputs V apply    52  and V dump    54 , both being voltages to hold the valves firmly closed.  
         [0067]    Illustrated in FIG. 7 is the switch logic function schematic  34   iv  of the applied pressure control algorithm processing a signal to operate the dump valve  26  in a leakage flow control region while holding the apply valve  18  firmly closed. Initially, a flow is demanded by the operator of the vehicle by applying or releasing the brake pedal. This signal travels to comparator  258  representing Condition 4   250 . There the flow demand  36  signal is compared to a gain factor signal. The gain factor signal is a combination of sigma  112  and K h    40  after it is processed through multiplication box  236 . The demanded flow is added to sigma  112  times K h    40  and is split in two direction (Q dem +(sigma*K h )). One such signal proceeds to Switch 4   280 . The flow demand  36  is also compared under Condition 3   254  relative to switch  116  and proceeds to Switch 3   278 . The other proceeds to multiplication box  244 . The sigma  112  times K h    40  signal is also processed with beta 1    115  through division box  240 . The combined signal then proceeds to multiplication box  244 . The output of  244  [(Q dem +sigma*K h )(beta 1 /(sigma*K h ))] proceeds to adder box  218  representing V dump2    228  which is the dump valve leakage flow region. The other factor entering  218  is the Table 2   48  processed brake pressure signal, P b    30 . The sum of those two signals proceeds to Switch 4   280  in combination with the signal from  210 . The apply valve  18  is held firmly closed, as dictated by V apply3    224 . Adder box  210  outputs the sum of the supply pressure, P s    28  through its Table 1   46  processing and beta  114 . The signals from  210  and  218  are multiplexed  270  and proceed to Switch 4   280  where they are processed as described above with the signal from  258 . From there the signal goes through Switch 3   278 , Switch 2   276  and Switch 1   274  where it is de-multiplexed  282  and finally output as V apply    52  and V dump    54 , controlling the dump valve  26  in a leakage flow control region and applying a voltage to hold the apply valve  18  firmly closed.  
         [0068]    As shown in FIG. 8 is the switch logic function schematic  34   v  of the applied pressure control algorithm processing a signal to operate the dump valve  26  in a bulk flow mode while holding the apply valve  18  firmly closed. Initially, the operator of the vehicle demands a flow by applying or releasing the brake pedal. The signal proceeds through comparator  258  representing Condition 4   250  with a gain factor signal of sigma  112  multiplied by K h    40  through  236 . The output of  258  is split and travels to Switch 4   280  as well as multiplication box  234 . At  234 , it is combined with gain factor 1/K  38 . The output of  234  is added to the brake pressure signal, P b    30 , processed through Table 2   48 , at adder box  214 . V dump1    226 , at adder box  214 , represents the dump valve bulk flow region  110 . The signal is multiplexed  272  then proceeds to Switch 4   280 . A pressure differential signal, delta P a    203 , processed through Table 1   46 , is further processed through  210  with beta  114 . The output of  210  also is multiplexed  272  and proceeds to Switch 4   280 , where it is processed with signals from  214  and  258 . The output of Switch 1   274  is de-multiplexed  282  and represent V apply    52  and V dump    54 , where V apply    52  is a voltage to hold the apply valve  18  firmly closed.  
         [0069]    Various signals are shown in FIGS. 3-8 as being multiplexed and de-multiplexed. It should be understood that the depiction of the signal in such a manner is done for purposes of simplicity in the figures only. The actual signals need not be multiplexed or de-multiplexed while being processed.  
         [0070]    The preferred embodiment can be described as the operation of a control algorithm in an EHB system. It should be understood that this method can be used in any electronically controlled braking system. This includes a conventional system where the base-braking function is controlled in a conventional manner by a user actuating a brake pedal thereby operating a master cylinder to operate base braking under normal, no-slip braking conditions. The conventional system would also implement a conventional ABS system to control braking only under wheel-slip conditions or where electronic braking operates during specified special situations (i.e. traction control, vehicle stability control, etc.).  
         [0071]    In an EHB system where pressure in a brake is controlled by the combined action of a first valve and a second valve, controlling the voltage applied to the first valve and the second valve includes controlling the first valve using a bulk flow mode when |Q dem |&gt;|sigma*K h | and holding the second valve firmly closed. Also, the system controlling the first valve uses a leakage control mode when |switch|&lt;|Q dem |&lt;|sigma*K h |, and holds the second valve firmly closed. Additionally, the control system controlling the first valve and the second valve uses a pressure control mode when |Q dem |&lt;|switch|, holding both valves firmly closed. The system controlling the second valve uses a leakage control mode when |switch|&lt;|Q dem |&lt;|sigma*K h |, and holds the first valve firmly closed. Finally, the system controlling the second valve uses a bulk flow control mode when |Q dem |&gt;|sigma*K h |, and holds the first valve firmly closed.  
         [0072]    It should be further understood the invention can be used to control pressure in other vehicular and non-vehicular hydraulic systems using at least one poppet valve for controlling the application of pressurized hydraulic fluid to a hydraulic load or one poppet valve for controlling the relief of pressurized hydraulic fluid from a hydraulic load to a lower pressure portion of the hydraulic system.  
         [0073]    It should be understood that the electronic circuitry  10  processing signals to implement the pressure control algorithm according to the present invention would normally have a data carrier for storing the steps of the algorithm and possibly various process values. This data carrier maybe implemented in any suitable fashion. For example the data carrier may be (without limitation) a solid state memory chip such as a read only memory (ROM) device, a random access memory (RAM) device, a magnetic media device such as a computer disk or tape, or an optical memory device such a CD-ROM or DVD disc.  
         [0074]    The principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or exceeding the scope of the claims.  
       List of Components by Reference Numbers  
       [0075]    [0075] 10  Portion of electronic circuitry  
         [0076]    [0076] 12  Simplified schematic representation of a typical EHB system  
         [0077]    [0077] 14  Fluid reservoir  
         [0078]    [0078] 16  Pump  
         [0079]    [0079] 18  Apply valve  
         [0080]    [0080] 19  Solenoid  
         [0081]    [0081] 20  Conduit  
         [0082]    [0082] 22  Wheel brake  
         [0083]    [0083] 24  Fluid conduit  
         [0084]    [0084] 26  Dump valve  
         [0085]    [0085] 27  Solenoid  
         [0086]    [0086] 28  Supply pressure, P s    
         [0087]    [0087] 30  Braking pressure, P b    
         [0088]    [0088] 32  Pressure feedback signal  
         [0089]    [0089] 34  Switch logic function schematic  
         [0090]    [0090] 34 ′ Apply valve bulk flow control switch logic function schematic  
         [0091]    [0091] 34 ″ Apply valve leakage flow control switch logic function schematic  
         [0092]    [0092] 34 ′″ Pressure control switch logic function schematic  
         [0093]    [0093] 34   iv  Dump valve leakage flow control switch logic function schematic  
         [0094]    [0094] 34   v  Dump valve bulk flow control switch logic function schematic  
         [0095]    [0095] 36  Demanded flow, Q dem    
         [0096]    [0096] 38  Gain factor, 1/k  
         [0097]    [0097] 40  Gain factor, K h    
         [0098]    [0098] 42  Gain schedule  
         [0099]    [0099] 44  Primary pressure control law  
         [0100]    [0100] 46  Voltage Boundary Table 1   
         [0101]    [0101] 48  Voltage Boundary Table 2   
         [0102]    [0102] 50  Demanded pressure, P cmd    
         [0103]    [0103] 52  Voltage signal, V apply    
         [0104]    [0104] 54  Voltage signal, V dump    
         [0105]    [0105] 100  Control space for operation of the EHB system  
         [0106]    [0106] 102  Apply valve bulk flow control  
         [0107]    [0107] 104  Apply valve leakage flow control  
         [0108]    [0108] 106  Pressure control  
         [0109]    [0109] 108  Dump valve leakage flow control  
         [0110]    [0110] 110  Dump valve bulk flow control  
         [0111]    [0111] 112  Sigma  
         [0112]    [0112] 114  Beta  
         [0113]    [0113] 115  Beta 1    
         [0114]    [0114] 116  Negative switch  
         [0115]    [0115] 120  Apply valve voltage curve, U a    
         [0116]    [0116] 122  Dump valve voltage curve, U d    
         [0117]    [0117] 124  Control space x-axis for demanded flow, Q dem    
         [0118]    [0118] 126  Control space y-axis for voltage  
         [0119]    [0119] 130  Switch 1  point  
         [0120]    [0120] 132  Switch 2  point  
         [0121]    [0121] 134  Switch 3  point  
         [0122]    [0122] 136  Switch 4  point  
         [0123]    [0123] 202  Adder box  
         [0124]    [0124] 203  Pressure differential, delta P a    
         [0125]    [0125] 204  Box  
         [0126]    [0126] 206  Box  
         [0127]    [0127] 208  Adder box  
         [0128]    [0128] 210  Adder box  
         [0129]    [0129] 212  Adder box  
         [0130]    [0130] 214  Adder box  
         [0131]    [0131] 216  Adder box  
         [0132]    [0132] 218  Adder box  
         [0133]    [0133] 220  Valve position voltage scheme, V apply1    
         [0134]    [0134] 222  Valve position voltage scheme, V apply2    
         [0135]    [0135] 224  Valve position voltage scheme, V apply3    
         [0136]    [0136] 226  Valve position voltage scheme, V dump1    
         [0137]    [0137] 228  Valve position voltage scheme, V dump2    
         [0138]    [0138] 230  Valve position voltage scheme, V dump3    
         [0139]    [0139] 232  Multiplication box  
         [0140]    [0140] 234  Multiplication box  
         [0141]    [0141] 236  Multiplication box  
         [0142]    [0142] 238  Multiplexer  
         [0143]    [0143] 240  Division box  
         [0144]    [0144] 242  Multiplication box  
         [0145]    [0145] 244  Multiplication box  
         [0146]    [0146] 246  Multiplication factor  
         [0147]    [0147] 248  Condition 1   
         [0148]    [0148] 250  Condition 4   
         [0149]    [0149] 252  Condition 2   
         [0150]    [0150] 254  Condition 3   
         [0151]    [0151] 256  Comparator  
         [0152]    [0152] 258  Comparator  
         [0153]    [0153] 260  Comparator  
         [0154]    [0154] 262  Comparator  
         [0155]    [0155] 264  Multiplexer  
         [0156]    [0156] 266  Multiplexer  
         [0157]    [0157] 268  Multiplexer  
         [0158]    [0158] 270  Multiplexer  
         [0159]    [0159] 272  Multiplexer  
         [0160]    [0160] 274  Switch 1   
         [0161]    [0161] 276  Switch 2   
         [0162]    [0162] 278  Switch 3   
         [0163]    [0163] 280  Switch 4   
         [0164]    [0164] 282  De-multiplexer