Abstract:
Some embodiments are directed to a brake system for use with a vehicle. The brake system can include a sensor that is configured to sense at least one condition relating to interaction between at least one wheel of the vehicle and a surface upon which the vehicle travels. A controller can receive data from the sensor and be configured to instruct a brake modulator to cause a front brake assembly to modulate the speed of rotation of a front wheel via one of a normal mode and a pulsing mode based on the sensed data. The controller can also be configured to instruct the brake modulator to cause a rear brake assembly to modulate the speed of rotation of a rear wheel via the pulsing mode if the front brake assembly is engaged in reducing the speed of rotation of the front wheel via the pulsing mode.

Description:
BACKGROUND 
     The disclosed subject matter relates to vehicle braking apparatus, and methods of use and manufacture thereof. More particularly, the disclosed subject matter relates to methods and apparatus for enhancing braking efficiency of vehicles that include multiple rotating wheels. 
     Various types of vehicles are propelled for travel on land, such as along an improved or unimproved path of travel, via multiple wheels (including tires), such that the vehicle&#39;s motion is effected via rotation of the wheels. Some of the vehicles include a pair of front wheels disposed at laterally opposite sides of the vehicle and a pair of rear wheels. The pair of front wheels can be rotatably mounted to a respective hub (also referred to as a wheel carrier or a knuckle). The front wheel hubs can be connected to the vehicle by a respective independent suspension assembly, or by a common beam. In either arrangement, the pair of front wheels can be collectively referred to as a front axle. The pair of rear wheels can be associated with a corresponding structure; i.e., a respective wheel hub, and an independent suspension or a common beam, and can be referred to collectively as a rear axle. 
     SUMMARY 
     Numerous types of braking systems can be used to slow rotation of the wheels to thereby reduce vehicle speed. For example, a brake assembly can be provided at each wheel. Each brake assembly can include a rotating element that is rigidly connected to, and that rotates with, the wheel (or the rotatable portion of the wheel hub) and a stationary element that can be attached to elements of the vehicle frame so as not to be rotatable with the wheel, and can be disposed adjacent the rotating element. In a brake assembly configured as a disc brake assembly, the rotating element can be a disc, and the stationary element can include a caliper that houses at least one piston, and at least one brake pad movable in response to fluid pressure acting on the piston. In a brake assembly configured as a drum brake assembly, the rotating element can be configured as a drum, and the stationary element can include a pad on each of a pair of shoes that pivot into and out of engagement with the inner cylindrical surface of the drum. The shoes can be actuated by a mechanical linkage such as but not limited to a cable, or a piston subjected to fluid pressure. 
     In order to reduce the wheel&#39;s speed of rotation, the brake caliper or shoe can be manipulated to press the brake pad(s) against a face or surface of the brake disc or drum. This contact between the brake pad(s) and the brake disc or drum results in friction and reduces the rotational speed of the brake disc or drum, which correspondingly reduces the speed of rotation of the wheel by virtue of the rigid connection between the brake disc and the wheel. This manipulation of the caliper (to press the brake pad(s) against the brake disc) can be accomplished in various ways, such as via the selective supply of hydraulic fluid to the calipers through hydraulic lines or pneumatic fluid through pneumatic lines. 
     These and other types of braking systems are subject to various challenges. For example, it is beneficial to control the braking, and in particular the reduction in rotational speed of the wheels, so as to maintain the brake torque that the stationary element applies to the rotatable element without breaking the grip between the tire and the travel surface. If the tires losses grip with the travel surface and slides, the stopping distance can increase. 
     The grip between the tire and the travel surface is a function of the coefficient of friction (represented the Greek letter p, and also referred to as surface-mu) associated with the travel surface. For example, the surface-mu for asphalt has a relatively high value, and the surface-mu for an ice covered surface has a relatively low value. Maintaining each tire&#39;s grip with the travel surface can enhance control over the vehicle&#39;s path of travel (i.e., steering), and can enhance vehicle braking (i.e., reduces the stopping distance or length of travel of the vehicle from application of braking pressure until the vehicle becomes stationary). For example, locking the wheels (stopping rotation of the wheels) while the vehicle continues to travel can result in a loss of steering control and thus an inability to control the vehicle&#39;s direction of travel and also can negatively impact vehicle stopping distance. Furthermore, slipping of the wheels (disproportionate rotational velocity of the wheels compared to vehicle speed) can have similar detrimental effects on vehicle control, such as increased stopping distance. 
     Locking or slipping of the wheels can occur in a variety of circumstances. For example, the wheels may lock or slip when the vehicle is traveling at a very high speed, and a relatively large amount of pressure is applied to the brake disc by the brake pad(s). However, application of a lower amount of pressure to the brake disc may also cause the wheels to lock or slip if the surface upon which the vehicle travels is slippery, such as due to the presence of snow, ice, rain, etc. In other words, locking or slipping of the wheels is more likely under relatively low surface-mu conditions. 
     Antilock brake systems (ABS) can be used to enhance brake performance, such as under the circumstances discussed above. Some ABS control the brake caliper so that the brake pad(s) apply intermittent pressure to the brake disc to prevent (or reduce the likelihood of) the wheels locking or slipping while the vehicle continues to move along the surface. This intermittent pressure can be achieved by pulsing the hydraulic fluid pressure supplied to the brake calipers. ABS can be initiated under a variety of conditions, such as based on sensed conditions relevant to the interaction between the wheels and the surface on which the vehicle travels, which as discussed above is at least partially dependent upon the magnitude of the pressure applied to the brake wheel by the brake pad(s), the surface-mu value of the surface on which the vehicle travels, etc. 
     Another strategy to enhance braking performance involves controlling the front brake calipers so that the associated brake pad(s) apply a higher pressure to the front brake disc, than is applied by the brake pad(s) associated with the rear brake disc. This strategy can accommodate the weight shift onto the front axle, and off of the rear axle, when the vehicle brakes. 
     This strategy of applying more pressure to the front brake disc than to the rear brake disc can be combined with ABS, so that ABS is engaged for the front braking assemblies by virtue of the higher pressure applied by the front brake pad(s) to the front brake calipers. However, only applying ABS (i.e., pulsing pressure to the brake disc) to the front brake assemblies may not yield the desired brake performance, and thus it may be beneficial to also apply ABS to the rear brake assemblies. More particularly, it may be beneficial to modulate the magnitude of hydraulic fluid pressure applied to the rear brake calipers to initiate ABS for the rear brake assemblies when the front brake assemblies are determined to be operating via ABS. In other words, it may be beneficial to automatically cause the rear brake assembly to operate under ABS when the ABS is engaged for the front brake assemblies. 
     Alternate embodiments are intended to include a braking system that can rely on electric actuators and electrical communication lines for actuation of the stationary member. For example, an electric motor can be mounted directly on a brake caliper to thereby move a piston to actuate the caliper. In such a configuration, motors can actuate the rear brake calipers directly and without the use of hydraulic fluid, to initiate ABS for the rear brake assemblies when the front brake assemblies are determined to be operating via ABS. 
     Some embodiments are therefore directed to a brake system for use with a vehicle that includes front and rear wheels that are configured for rotation on a surface upon which the vehicle travels. The brake system can include multiple brake assemblies including a front brake assembly provided at the front wheel, and a rear brake assembly provided at the rear wheel. A brake modulator can be configured to cause each of the brake assemblies to engage and thereby reduce speed of rotation of the associated wheel, via one of a normal mode in which the brake assembly substantially continuously reduces speed of rotation, and a pulsing mode in which the brake assembly sporadically modulate speed of rotation. A sensor can be configured to sense at least one condition relating to interaction between at least one of the wheels and the surface upon which the vehicle travels. A controller can receive data from the sensor and be configured to instruct the brake modulator to cause the front brake assembly to modulate the speed of rotation of the front wheel via one of the normal mode and the pulsing mode based on the sensed data. The controller can also be configured to instruct the brake modulator to cause the rear brake assembly to modulate a braking force applied to the rear wheel until the rear wheel begins slipping, and then to instruct the rear brake assembly to modulate the speed of rotation of the rear wheel via the pulsing mode if the front brake assembly is engaged in reducing the speed of rotation of the front wheel via the pulsing mode. 
     Some other embodiments are directed to a control assembly for a brake system for a vehicle that includes front and rear wheels that are configured for rotation on a surface upon which the vehicle travels. The brake system can include multiple brake assemblies including a front brake assembly provided at the front wheel, and a rear brake assembly provided at the rear wheel; and a brake modulator that is configured to cause each of the brake assemblies to engage and disengage and thereby reduce speed of rotation of the associated wheel, via one of a normal mode in which the brake assembly substantially continuously reduces speed of rotation, and a pulsing mode in which the brake assembly sporadically modulates speed of rotation. The control assembly can include a sensor that is configured to sense at least one condition relating to interaction between at least one of the wheels and the surface upon which the vehicle travels. The control assembly can also include a controller that receives data from the sensor and that is configured to instruct the brake modulator to cause the front brake assembly to modulate the speed of rotation of the front wheel via one of the normal mode and the pulsing mode based on the sensed data. The controller can also be configured to instruct the brake modulator to cause the rear brake assembly to modulate a braking force applied to the rear wheel until the rear wheel begins slipping, and then to instruct the rear brake assembly to modulate the speed of rotation of the rear wheel via the pulsing mode if the front brake assembly is engaged in reducing the speed of rotation of the front wheel via the pulsing mode. 
     Still other embodiments are directed to a method of operating a brake system for a vehicle that includes front and rear wheels that are configured for rotation on a surface upon which the vehicle travels. The brake system can include multiple brake assemblies including a front brake assembly provided at the front wheel, and a rear brake assembly provided at the rear wheel; and a brake modulator that is configured to cause each of the brake assemblies to disengage and thereby modulate speed of rotation of the associated wheel, via one of a normal mode in which the brake assembly substantially continuously reduces speed of rotation, and a pulsing mode in which the brake assembly sporadically modulates speed of rotation. The method can include: sensing at least one condition relating to interaction between at least one of the wheels and the surface upon which the vehicle travels; instructing the brake modulator to cause the front brake assembly to modulate the speed of rotation of the front wheel via one of the normal mode and the pulsing mode based on the sensed at least one condition; and instructing the brake modulator to cause the rear brake assembly to modulate a braking force applied to the rear wheel until the rear wheel begins slipping, and then to instruct the rear brake assembly to reduce the speed of rotation of the rear wheel via the pulsing mode if the front brake assembly is engaged in reducing the speed of rotation of the front wheel via the pulsing mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosed subject matter of the present application will now be described in more detail with reference to exemplary embodiments of the apparatus and method, given by way of example, and with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic view of a braking system for a vehicle in accordance with the disclosed subject matter. 
         FIG. 2  is a flowchart depicting an algorithm in accordance with the disclosed subject matter. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     A few inventive aspects of the disclosed embodiments are explained in detail below with reference to the various figures. Exemplary embodiments are described to illustrate the disclosed subject matter, not to limit its scope, which is defined by the claims. Those of ordinary skill in the art will recognize a number of equivalent variations of the various features provided in the description that follows. 
     I. Overview 
       FIG. 1  is a schematic view of a braking system  12  for a vehicle  10  in accordance with the disclosed subject matter. The vehicle  10  shown in  FIG. 1  can be configured for use on paved roadways, and can be referred to as a passenger vehicle. However, the braking system  12  can be used with any vehicle that is configured to travel along any one or combination of improved, unimproved, and/or unmarked roadways and paths constituted by gravel, dirt, sand, etc. For example, embodiments are intended to include or otherwise cover any other type of automobile, including passenger car, truck, ATV, etc. 
     As described below and shown in the exemplary embodiment of  FIG. 1 , the braking system  12  includes disc brakes. However, alternate embodiments of the braking system  12  can include any other type of braking system, such as drum brakes, regenerative brakes, etc. The exemplary embodiment of the braking system  12  described below can rely on a hydraulic fluid for actuation of the stationary member into engagement with the rotatable member. However, exemplary embodiments are intended to include a braking system  12  that can rely on pneumatic fluid, or electric actuators and electrical communication lines (also referred to as a brake-by-wire system) for actuation of the stationary member. 
     II. Braking System 
     The exemplary vehicle  10  of  FIG. 1  can include a pair of front wheels  16 L,R referred to collectively as a front axle  14 , and a pair of rear wheels  20 L,R referred to as a rear axle  18 . The front wheels  16 L,R and the rear wheels  20 L,R can each include hubs, rims, and tires. Front brake assemblies  22 L,R and rear brake assemblies  24 L,R of the braking system  12  are respectively provided adjacent the hubs of the front wheels  16 L,R and the rear wheels  20 L,R. The front brake assemblies  22 L,R can include a rotating member that rotates with the respective front wheels  16 L,R, and a stationary member that selectively engages the rotating member to slow rotation of the rotating member. 
     In the exemplary embodiment of  FIG. 1 , the front brake assemblies  22 L,R include front brake discs  26 L,R, front brake calipers  28 L,R, and front brake pads  30 L,R. Similarly, in the  FIG. 1  embodiment, the rear brake assemblies  24 L,R include rear brake discs  32 L,R, rear brake calipers  34 L,R, and rear brake pads  36 L,R. However, exemplary embodiments are also intended to include and otherwise cover alternate brake assemblies including rotating and fixed members, such as drum brake assemblies. In  FIG. 1 , each of the brake calipers  28 L,R,  34 L,R is adjacent a contact portion of the respective brake discs  26 L,R,  32 L,R, and is configured to cause the respective brake pads  30 L,R,  36 L,R to clamp the contact portion. 
     The brake discs  26 L,R,  32 L,R may be solid, cross-drilled, slotted, or waved, and the brake calipers  28 L,R,  34 L,R may have an adequate number of pistons to achieve desired braking effects, such as one, two, four, six, etc. The calipers  28 L,R,  34 L,R can be fixed calipers or floating calipers. Additionally, the brake discs  26 L,R,  32 L,R may be made of cast-iron, steel, or carbonceramic composite, while the brake calipers  28 L,R,  34 L,R may be made of cast-aluminum or cast-iron. However, embodiments are intended to cover forming these components from any beneficial material. 
     As described below, the braking system  12  of the exemplary vehicle  10  also includes front and rear hydraulic lines  38 ,  39  connecting the brake calipers  28 L,R,  34 L,R to a brake modulator  40  positioned within the vehicle  10 . The brake modulator  40  can include a master cylinder and a brake booster to convert user input via a brake pedal  42  (or other manually actuable device) to hydraulic pressure, as described below. The brake modulator  40  is in turn connected to the brake pedal  42 , and upon input from a user (such as a driver of the vehicle  10 ), pressing the brake pedal  42  causes the brake modulator  40  to pressurize the hydraulic lines  38 ,  39  with hydraulic fluid, thereby causing the brake calipers  28 L,R,  34 L,R to clamp the contact portions of the respective brake discs  26 L,R,  32 L,R. 
     The brake modulator  40  can be any appropriate device, system or component that can alter the fluid pressure in any combination of the brake assemblies  22 L,R,  24 L,R independently of the vehicle operator&#39;s input via the brake pedal  42 . Exemplary embodiments are intended to include a brake modulator  40  that can adjust brake pressure simultaneously and equally to all of the brake assemblies  22 L,R,  24 L,R. Exemplary embodiments are also intended to include a brake modulator that can provide a unique brake pressure to each of the brake assemblies  22 L,R,  24 L,R. Further details of an exemplary brake modulator in accordance with the disclosed subject matter will be provided below. 
     When the exemplary vehicle  10  is in motion and the wheels  16 L,R,  20 L,R are rotating, the brake discs  26 L,R,  32 L,R are also rotating due to being fixed with the wheels  16 L,R,  20 L,R. Clamping the contact portions of the brake discs  26 L,R,  32 L,R creates friction between the brake pads  30 L,R,  36 L,R and the brake discs  26 L,R,  32 L,R, slowing rotation of both the brake discs  26 L,R,  32 L,R and the wheels  16 L,R,  20 L,R simultaneously. As rotation of the wheels  16 L,R,  20 L,R slow, the vehicle  10  slows down accordingly and may ultimately come to a complete stop. 
     As discussed above, ABS involves monitoring wheel speed to determine whether wheels are slipping under braking force. Therefore, the exemplary embodiment includes front wheel sensors  44 L,R and rear wheel sensors  46 L,R positioned adjacent the front wheels  16 L,R and the rear wheels  20 L,R, respectively. The wheel sensors  44 L,R,  46 L,R measure a rotational velocity of the respective wheels  16 L,R,  20 L,R and transmit those measurements to an ABS control module  48  via sensor lines  54 . 
     The ABS control module  48  includes both a control processor  50  and a control memory  52  that function together to provide ABS control over the front axle  14  and the rear axle  18  of the vehicle  10 . The ABS control module  48  may also be configured to provide ABS control over both of the wheels  16 L,R of the front axle  14 , or over either of the wheels  16 L,R individually. Similarly, ABS control may be provided by the ABS control module  48  over both of the wheels  20 L,R of the rear axle  18 , or over either of the wheels  20 L,R individually. Both of the above-described circumstances may be analyzed to determine whether an axle is under ABS control. As described above, the ABS control module  48  receives rotational velocity measurements of the wheels  16 L,R,  20 L,R, and then determines whether any of the wheels  16 L,R,  20 L,R have begun slipping (i.e., rotational velocity of the wheel is less than what a rotational velocity of a free-rolling wheel at an identical vehicle speed would be) due to a brake caliper slowing rotation of a brake disc proportionately more than speed of the vehicle is decreasing, thereby causing a wheel connected to the brake disc to slide along a travel surface at a rotational velocity less than free-rolling. As will be discussed below, in embodiments featuring regenerative braking, a regeneration motor may be implemented instead of a brake caliper to slow rotation of the respective wheel. In the present embodiment, the ABS control module  48  can permit an amount of slipping in the wheels  16 L,R,  20 L,R before the wheels  16 L,R,  20 L,R are considered to “begin slipping,” thus having a target slip rate which the wheels  16 L,R,  20 L,R must reach to qualify as slipping. The target slip rate can help the vehicle  10 , specifically the wheels  16 L,R,  20 L,R, maintain grip/traction with the surface on which the vehicle  10  is traveling while retaining stability and the ability to steer. As mentioned above, a slip rate is a ratio of a rotational velocity of a wheel (typically under braking force) compared to a rotational velocity of a free-rolling wheel (absent braking resistance, or regenerative braking through implementation of a regeneration motor as referenced above) at an identical vehicle speed. To illustrate this concept, a slip rate of zero percent equates to a wheel that is rolling freely, while a slip rate of 100% equates to a wheel that is fully locked and not rotating at all. For example, a target slip rate in some embodiments may therefore be approximately 10 percent, meaning that the wheels  16 L,R,  20 L,R will not be considered to have begun slipping until their rotational velocity is 10 percent less than what a rotational velocity of a free-rolling wheel would be at an identical vehicle speed. Embodiments may have target slip rates greater than or less than 10 percent as the previous example is for illustrative purposes. Slipping of the wheels  16 L,R,  20 L,R is detected when the wheel sensors  44 L,R,  46 L,R measure disproportionately decreased rotational velocity of the respective wheels  16 L,R,  20 L,R, indicating that the wheels  16 L,R,  20 L,R are slipping. After detecting slipping of the wheels  16 L,R,  20 L,R, the control processor  50  of the ABS control module  48  can enter ABS control over appropriate brake assemblies  22 L,R,  24 L,R via the brake modulator  40 , which is described below. 
     In the exemplary embodiment, the ABS control module  48  can be linked with (or combined with) a vehicle stability control module that can control stability of the vehicle by modulating hydraulic pressure to a brake assembly to slow rotation of any given wheel, as described above. Modulation by the ABS control module  48  can include increasing or decreasing adjustment of the brake assemblies  22 L,R,  24 L,R by an appropriate amount to achieve balanced operation of the respective wheels  16 L,R,  20 L,R. Specifically, modulating the hydraulic pressure may necessitate increasing or reducing pressure in the hydraulic lines  38 , 39  to actuate or release actuation of the brake assemblies  22 L,R,  24 L,R. The ABS control module  48  may also be linked to a brake actuator such as in instances of regenerative braking including regeneration motors configured to slow rotation of a wheel, as well as a brake booster. 
     The exemplary brake modulator  40  shown in  FIG. 1  can include a valve assembly  56  connected to a pressurized fluid source  60 , and a pressure sensor  58 . Pressurized fluid flows between the fluid source  60  and the hydraulic lines  38 ,  39  pursuant to a disposition of the valve assembly  56 , which is controlled by the control processor  50  of the ABS control module  48  via signals transmitted through the control line  62 . The control line  62  connects the valve assembly  56 , the fluid source  60 , and the pressure sensor  58  to the ABS control module  48 . The pressure sensor  58  measures and transmits the hydraulic pressure in the hydraulic lines  38 ,  39  to the ABS control module  48 , thereby allowing the control processor  50  to determine an appropriate disposition of the valve assembly  56  to facilitate hydraulic fluid flow to achieve a desired hydraulic pressure. For example, the control processor  50  can dispose the valve assembly  56  to supplement hydraulic pressure in the hydraulic lines  38 ,  39  as a result of input from the driver in the form of actuating the brake pedal  42 . Alternatively, the control processor  50  can dispose the valve assembly  56  to reduce hydraulic pressure as a result of input from the driver in the form of releasing the brake pedal  42 . 
     III. Method of Operation 
       FIG. 2  is a flowchart depicting an algorithm in accordance with the disclosed subject matter. The ABS control module  48  of the braking system  12  can employ the algorithm described below during instances of driving when the vehicle  10  is in a state of motion, either traveling forward or backward along a surface of travel, hereinafter referred to as a travel surface. 
     When the vehicle  10  is being driven and is in motion, the control processor  50  of the ABS control module  48  may begin an initial step of the algorithm by initiating a start step S 100 . Once the control processor  50  has initiated the start step S 100 , the algorithm proceeds to a brake inquiry step S 102  to determine whether the driver has actuated the braking system  12  by actuating the brake pedal  42 . The brake pedal  42  can be actuated manually by the driver, or automatically actuated by an autonomous system. As described above, pressing the brake pedal  42  modulates pressure in the hydraulic lines  38 ,  39  and causes the brake calipers  28 L,R,  34 L,R to clamp the brake discs  26 L,R,  32 L,R accordingly. The control processor  50  can in turn detect the pressure modulation in the hydraulic lines  38 ,  39  via the control line  62  connected to the pressure sensor  58  of the brake modulator  40 . If the control processor  50  does detect modulated pressure in the hydraulic lines  38 ,  39  resulting from the driver actuating the brake pedal  42 , the algorithm proceeds to a first ABS inquiry step S 104  described below. However, if the control processor  50  does not detect a pressure modulation in the hydraulic lines  38 ,  39 , then the driver is determined to have not actuated the brake pedal  42 , and the control processor  50  proceeds to an end step S 124  and exits the algorithm without performing additional steps. 
     The first ABS inquiry step S 104  of the algorithm determines whether a single axle of the vehicle  10 , such as the front axle  14 , is under ABS control. In other words, at this step the control processor  50  determines whether ABS control has been entered over either the front axle  14  or the rear axle  18  due to the wheels  16 L,R,  20 L,R slipping on the travel surface under braking. The control processor  50  may further determine whether ABS control has been entered over each individual wheel  16 L,R,  20 L,R of either the front axle  14  or the rear axle  18 . As described above, the wheels  16 L,R,  20 L,R and connected brake discs  26 L,R,  32 L,R can disproportionately slow rotating due to clamping force of the calipers  28 L,R,  34 L,R overcoming frictional grip of the wheels  16 L,R,  20 L,R (through the tires) to the travel surface. If either the front wheels  16 L,R or the rear wheels  20 L,R begin slipping, rotational velocity measurements of the corresponding wheel sensors  44 L,R,  46 L,R being transmitted to the ABS control module  48  reflect the slipping. The ABS control module  48  then actuates ABS control over the slipping wheels. 
     As discussed above, front brake bias can cause the front wheels  16 L,R to slip and enter ABS control before the rear wheels  20 L,R slip and enter ABS control. Under these circumstances, both the front axle  14  and the rear axle  18  may not be under ABS control simultaneously for a variety of reasons. For example, in the exemplary embodiment and as described above, pressing the brake pedal  42  applies a greater braking force of the braking system  12  to the front axle  14  than the rear axle  18 . Therefore, the greater braking force applied to the front axle  14  may cause the front axle  14  to slip and actuate ABS control while a lesser braking force applied to the rear axle  14  is not sufficient to cause slipping of the rear axle  14  and actuate ABS control. In this manner, a single axle of the vehicle  10  (such as the front axle  14 ) may be under ABS control while another axle (such as the rear axle  18 ) is not under ABS control. Alternatively, the front wheels  16 L,R may be traveling over a travel surface with a surface-mu lower than that of a travel surface over which the rear wheels  20 L,R are traveling. Therefore, a braking force may cause the front wheels to enter ABS control due to the more slippery surface, while ABS control is not entered over the rear wheels. If a single axle is under ABS control, then the control processor  50  proceeds to a first surface verification step S 106  of the algorithm described below. However, if ABS control has not been actuated over either the front axle  14  or the rear axle  18 , then the control processor  50  proceeds to an end step S 124  and exits the algorithm without performing additional steps. 
     As described above, each potential travel surface has a surface-mu value representative of the slipperiness of the travel surface, measured as a coefficient of friction, μ. For instance, a snow-covered road may have a higher surface-mu value than that of an ice-covered road, a rain-covered road may yet have a higher surface-mu value than that of a snow-covered road, and a dry asphalt surface may yet have a higher surface-mu value than that of a rain-covered road. The following step of the algorithm determines whether or not actuating ABS control over the wheels  16 L,R,  20 L,R of the vehicle  10  will be beneficial based on conditions of the travel surface. 
     Once the control processor  50  has determined that a single axle is under ABS control, the algorithm then enters the first surface verification step S 106  to determine whether a surface-mu value of the travel surface is permissible by being within a surface-mu value. The surface-mu range includes all coefficients of friction representative of surfaces of travel on which the algorithm is intended to operate. In other words, the first surface verification step S 106  analyzes whether or not properties of the travel surface are appropriate for actuation of ABS control, given the hydraulic pressure resulting from the driver input on the brake pedal  42 , which is hereinafter referred to as system brake pressure and will be further described below. For example, the vehicle  10  may be traveling on an ice-covered surface or a snow-covered surface, and the surface-mu value of one surface may be impermissible for operation of the algorithm, while the other surface represents a permissible surface-mu value for advancing to the next step of the algorithm. If the surface-mu value is permissible, then the algorithm proceeds to the first pressure range step S 108  described below. However, if the surface-mu value is not permissible, then the control processor  50  proceeds to an end step S 124 , and exits the algorithm without performing additional steps. 
     After the surface-mu value determination of the first surface verification step S 106 , the control processor  50  then determines whether a system brake pressure is an acceptable value at the first pressure range step S 108 . In other words, the algorithm checks the system brake pressure resulting from the driver input via the brake pedal  42  to interpret the driver&#39;s intent regarding a path of travel of the vehicle  10 . In embodiments featuring an autonomous system for automatically actuating the brake pedal  42 , the algorithm checks the system brake pressure resulting from the autonomous system input via the brake pedal  42  to interpret the autonomous system&#39;s intent regarding a path of travel of the vehicle  10 . 
     For illustrative purposes, the following describes an embodiment in which the brake pedal  42  can be actuated manually by the driver. The first pressure range step S 108  analyzes whether the driver briefly tapped and then released the brake pedal  42  indicative of intent for a minor adjustment to the path of the vehicle  10 , or if the driver applied an emergency braking force to the brake pedal  42  indicative of intent to quickly reduce speed of the vehicle  10  and potentially come to a complete stop, dramatically altering the path of the vehicle  10 . The algorithm can define an acceptable value of the system brake pressure as that which indicates that the driver intends to quickly reduce speed of the vehicle  10  and potentially come to a complete stop, dramatically altering the path of the vehicle  10 . The aforementioned acceptable value of the system brake pressure as defined by the algorithm represents a tunable range, and can be adjusted to encompass any amount of system brake pressure such as under light, moderate, or heavy braking. At this step, if the algorithm determines that the system brake pressure is an acceptable value, the algorithm proceeds to the next step described below. However, if the system brake pressure is not an acceptable value, then the control processor  50  proceeds to an end step S 124  and exits the algorithm without performing additional steps. 
     Next, the control processor  50  will enter a modulate pressure step S 110 . In the modulate pressure step S 110 , the algorithm autonomously modulates the hydraulic pressure to enter ABS control over the other axle that was not put under ABS control from the aforementioned system brake pressure due to driver input. As described above, for the control processor  50  to reach this step in the algorithm, it must have been determined that a single axle is under ABS control. The current step therefore achieves ABS control on both axles  14 ,  18  by modulating system brake pressure to enter ABS on the other axle. Specifically, the algorithm results in the control processor  50  causing the brake modulator  40  to modulate hydraulic pressure in the hydraulic lines  38 ,  39  connected to the other axle until the other axle begins slipping and ABS control is entered. For example, hydraulic pressure in the hydraulic lines  39  is modulated to enter ABS control over the rear axle  18 . 
     After the control processor  50  autonomously modulates the hydraulic pressure at the modulate pressure step S 110  of the algorithm to enter ABS on the other axle, the control processor  50  will enter a second surface verification step S 112  of a pressure modulation section of the algorithm that, as described below, is repeated until the control processor  50  verifies that the driver input (via the brake pedal  42 ) does alter the intended braking pressure by the threshold amount. For example, as described below, if the front axle  14  is under ABS control and the rear axle  18  is not, the control processor  50  will continually modulate hydraulic pressure to the rear brake assemblies  24 L,R of the rear axle  18  to actuate ABS control. This next step determines whether the surface-mu value is still permissible. In other words, the second surface verification step S 112  analyzes whether or not properties of the travel surface have changed, and if any change to the surface-mu is still within the permissible surface-mu range. For example, the vehicle  10  may transition from an ice-covered surface to a dry asphalt surface, changing the surface-mu value during progression of the algorithm by the control processor  50 . 
     If the surface-mu value does not change or changes but is still permissible, then the processor  50  proceeds to a second pressure range step S 114  of the pressure modulation section of the algorithm described below. However, if the surface-mu value changes to an impermissible surface-mu value, then the processor  50  exits the pressure modulation section and proceeds to a pressure reduction step S 122  to reduce brake pressure back to the intended system brake pressure, which is the hydraulic pressure that was originally requested by the driver via the brake pedal  42 . In other words, the driver input (through the brake pedal  42 ) controls the brake pressure applied again to brake assemblies  22 L,R,  24 L,R at both the front axle  14  and the rear axle  18 , and the processor  50  then proceeds to the end step S 124  of the algorithm. For example, the driver may only have pressed the brake pedal  42  to a sufficient degree to enter ABS control over the front axle  14  and not the rear axle  18 . Therefore, reducing hydraulic pressure back to the driver requested system brake pressure may serve to release the rear axle  18  from ABS control while maintaining ABS control over the front axle  14 . 
     As described above, the second pressure range step S 114  of the pressure modulation section occurs if the surface-mu value remains permissible. The algorithm again checks to determine whether the system brake pressure is still an acceptable value. In other words, the algorithm checks the system brake pressure resulting from the driver input to interpret the driver&#39;s intent regarding a path of travel of the vehicle  10 , and whether or not the driver&#39;s intent has changed since initialization of the algorithm. For example, this step analyzes whether the driver has partially or completely released the brake pedal  42  since initially pressing the brake pedal  42 , indicating a change in the intent of the driver regarding the desired path for the vehicle  10 . As described above, the algorithm can define an acceptable value of the system brake pressure as that which indicates that the driver intends to quickly reduce speed of the vehicle  10  and potentially come to a complete stop, dramatically altering the path of the vehicle  10 . Therefore, if the driver has not partially or completely released the brake pedal  42  and instead maintains the driver input via the brake pedal  42 , then the algorithm will determine that the system brake pressure is still an acceptable value, and the control processor  50  proceeds to a double axle ABS step S 116  of the algorithm described below. However, if the driver has partially or completely released the brake pedal  42 , indicating an intent not to dramatically altering the path of the vehicle  10  by quickly reducing the speed of the vehicle  10  or coming to a complete stop, then the control processor  50  exits the pressure modulation section of the algorithm and proceeds to the pressure reduction step S 122  to reduce brake pressure back to that which was originally requested by the driver input into the brake system  12  via the brake pedal  42 . 
     If the system brake pressure is still an acceptable value, then the control processor  50  enters double axle ABS step S 116  of the pressure modulation section of the algorithm. At this step, the control processor  50  determines whether both axles are under ABS control. Due to changing conditions, such as surface-mu value and system brake pressure, the initial autonomous modulation in system brake pressure may not have been sufficient to maintain ABS control over both axles. This step of the modulate pressure section verifies whether the initial modulation in system brake pressure was sufficient to enter ABS control over both axles, and if it was indeed sufficient and both axles are under ABS control, then the algorithm then exits the pressure modulation section and proceeds to the next step described below. However, if both axles are not under ABS control because, for example, the initial modulation in system brake pressure was insufficient to do so, then the control processor  50  exits the pressure modulation section of the algorithm and proceeds to the pressure reduction step S 122  to reduce brake pressure back to that which was originally requested by the driver input into the brake system  12  via the brake pedal  42 . 
     After exiting the pressure modulation section, the control processor  50  proceeds to a pressure maintenance step S 118  and maintains autonomous supplemental pressure based on the driver input. In other words, the algorithm maintains the modulated system brake pressure to keep both axles under ABS control if the driver input on the braking system  12  via the brake pedal  42  is maintained. 
     The control processor  50  then enters driver pressure reduction step S 120  and determines whether the driver intended braking pressure reaches a threshold by partially or completely releasing the brake pedal  42 . In other words, if the driver releases the brake pedal  42 , the algorithm interprets the driver input as intending to reduce the system brake pressure. The threshold of the driver intended braking pressure may be appropriate to reflect the driver&#39;s intention to no longer dramatically alter the path of the vehicle  10  by quickly reducing the speed of the vehicle  10  or come to a complete stop. For example, the driver may no longer wish to apply the brake system  12  to slow the vehicle  10  as the vehicle  10  has been sufficiently slowed. Therefore, if the driver input (via the brake pedal  42 ) does alter the intended braking pressure by the threshold amount, then the control processor  50  proceeds to the pressure reduction step S 122  before proceeding to the end step S 124  and exiting the algorithm, as described below. However, if the driver is not altering the intended braking pressure by the threshold amount because the driver is either maintaining input via the brake pedal  42  or is altering input via the brake pedal  42  by less than the threshold amount, then the control processor  50  then restarts the pressure modulation section, and again proceeds to check the surface-mu value and the system brake pressure before returning to the double axle ABS step S 116  to determine whether or not both axles are under ABS control, after which the control processor  50  proceeds to the pressure maintenance step S 118  and maintains autonomous supplemental pressure based on the driver input, followed by a return to the driver pressure reduction step S 120  to determine whether the driver intended braking pressure is reduced yet. This pressure modulation section is repeated until the control processor  50  verifies that the driver input (via the brake pedal  42 ) does alter the intended braking pressure by the threshold amount, at which point the control processor  50  proceeds to the pressure reduction step S 122  before proceeding to the end step S 124  and exiting the algorithm. 
     Finally, if the driver does alter intended braking pressure by the threshold amount via the driver input on the brake pedal  42 , then the algorithm reduces system brake pressure to the driver intended system brake pressure, and the control processor  50  proceeds to the end step S 124  and exits the algorithm. As described above, at this stage, the driver input through the brake pedal  42  once again controls the brake pressure applied to both the front axle  14  and the rear axle  18 . 
     By entering ABS control on both axles instead of a single axle, the vehicle  10  is able to decelerate more quickly. Particularly, the vehicle  10  is capable of slowing and stopping more effectively in relation to the driver input on relatively slippery surfaces such as snow, ice, rainwater, etc. More effective stopping and slowing yields more control over the vehicle  10  for the driver in instances of changing the path of the vehicle  10 . Under certain conditions, the driver may be unaware of how slippery a travel surface is, and therefore may not cause an appropriate hydraulic pressure in the hydraulic lines  38 ,  39  via the brake pedal  42  to effectively slow the vehicle  10 . The algorithm assists the driver in effectively slowing the vehicle  10  by supplementing insufficient hydraulic pressure input from the brake pedal  42 . 
     The above-described steps may also be performed in any appropriate order to achieve the described functionality of the brake system, and the brake system is thus not intended to be limited in functionality to the order of steps shown and described in the exemplary embodiment and shown in  FIG. 2 . For example, steps S 102 , S 104 , S 106 , and S 108  may be performed in any appropriate order to achieve the braking modulation outlined above. 
     IV. Alternative Embodiments 
     While certain embodiments of the invention are described above, and  FIGS. 1-2  disclose the best mode for practicing the various inventive aspects, it should be understood that the invention can be embodied and configured in many different ways without departing from the spirit and scope of the invention. 
     For example, in the disclosed embodiments, the disclosed algorithm is applied to a disc brake system of a vehicle. However, the disclosed algorithm may alternatively be used with any type of braking system, such as drum brake systems, regenerative braking systems, and/or other types of braking systems. 
     As disclosed above, embodiments are intended to be used with any type of vehicle. The power source of the vehicle can be an internal combustion engine, an electric motor, or a hybrid of an internal combustion engine and an electric motor. The power source configured as an internal combustion engine or a hybrid power source can have the engine output axis oriented in the longitudinal direction or in the traverse direction of the vehicle. The engine can be mounted forward of the front axles, rearward of the rear axles, or intermediate the front and rear axles. Particularly, the algorithm may be used in conjunction with a regenerative braking system of a vehicle having an electric motor. 
     In the disclosed embodiment, the algorithm is used with a passenger vehicle having two pairs of wheels referred to as a front axle and a rear axle. However, the algorithm may additionally be used with a heavy duty vehicle having multiple axles (pairs of wheels) such as three, four, five, or any number of appropriate pairs of wheels. 
     The algorithm may also be used with braking systems including brake modulators having any appropriate number of valves such as one, two, three, etc. and any appropriate number of hydraulic lines connecting the valves to brake assemblies. Furthermore, the brake system may include additional components, such as an accumulator, master cylinder, brake booster, etc., in configurations other than those discussed. The braking system and connecting lines may also be pneumatic or electromagnetic instead of hydraulic. 
     Embodiments are also intended to include or otherwise cover methods of using and methods of manufacturing any or all of the elements disclosed above. The methods of manufacturing include or otherwise cover processors and computer programs implemented by processors used to design various elements of the vehicle braking apparatus disclosed above. 
     While the subject matter has been described in detail with reference to exemplary embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. All related art references discussed in the above Background section are hereby incorporated by reference in their entirety.