Brake force sensor

A brake system including a brake pad shaped and located to apply pressure to a brake rotor and an actuator shaped and located to apply pressure to the brake pad to cause the brake pad to apply pressure to the rotor. The system further includes a sensor material which varies in resistance when the actuator applies pressure to the brake pad, wherein the sensor material include an electrically insulating material with electrically conductive particles distributed therein. The system further includes a controller operatively coupled to the sensor material to detect a change in resistance of the sensor material.

TECHNICAL FIELD

The present invention is directed to a brake force sensor, and more particularly, to a brake force sensor having a resistance that varies with pressure.

BACKGROUND

Motorized vehicles are increasingly integrated with computers and computer control systems to provide quick and responsive control systems for the vehicle. Thus, it is desired to accurately track the status of various compartments of a vehicle for processing by the computer/computer system. In particular, as vehicles incorporate “drive by wire” and “brake by wire” systems it is desired to monitor the force applied by a brake (i.e. the force applied by the brake caliper or brake pad to the brake rotor). Although the position of the pedal of the brake system may be monitored to provide an estimate of brake force, it is of course more accurate to detect the brake force closer to the actual application of the brake. Accordingly, there is a need for a brake force sensor which can measure the brake force, as well as a brake force sensor which is reliable and robust.

SUMMARY

The present invention is a brake force sensor which can accurately measure brake force and which is reliable and robust. In particular, the brake force sensor of the present invention may utilize a material having an electrical resistance that varies with pressure, and the sensor material may be located in or immediately adjacent to the brakes to determine the brake force.

In one embodiment the invention is a brake system including a brake pad shaped and located to apply pressure to a brake rotor and an actuator shaped and located to apply pressure to the brake pad to cause the brake pad to apply pressure to the rotor. The system further includes a sensor material which varies in resistance when the actuator applies pressure to the brake pad, wherein the sensor material include an electrically insulating material with electrically conductive particles distributed therein. The system further includes a controller operatively coupled to the sensor material to detect a change in resistance of the sensor material.

In another embodiment the present invention is a motorized vehicle including a chassis, a plurality of wheels supporting the chassis, with each wheel including a brake rotor rotationally coupled thereto, and a brake pad shaped and located to selectively apply pressure to an associated rotor to thereby brake an associated wheel. The vehicle includes an actuator shaped and located to apply pressure to the brake pad to thereby cause the brake pad to apply pressure to the associated rotor and a sensor material which varies in resistance when the actuator applies pressure to the brake pad. The sensor material includes an electrically insulating material with electrically conductive particles distributed therein. The vehicle further includes a controller operatively coupled to the sensor material to detect a change in resistance of the sensor material.

Other objects and advantages of the present invention will be apparent from the following description and the accompanying drawings.

DETAILED DESCRIPTION

As shown inFIG. 1, a vehicle, generally designated10, may include a pair of front wheels12,14and a pair of rear wheels16,18. Each wheel12,14,16,18may include an associated brake or brake subsystem, each generally designated20. The vehicle10may include an electronic controller, processor, CPU, microprocessor, computer or the like (together termed a “controller”24herein) to monitor the status of the vehicle10, brakes20and its various subsystems. For example, the controller24may monitor the linear speed, linear acceleration, rotational speed, rotational acceleration, road conditions, atmospheric conditions, slip status of the wheels, the vehicle heading requested by the driver (i.e., position of the steering wheel), brake position or pressure, brake pressure requested by the driver, throttle position, throttle position requested by the driver, etc. The controller24may control various systems of the vehicle10, such as anti-lock brake systems, vehicle stability control systems, traction control systems, etc.

Each brake20may be, for example, a disc brake, a drum brake or other types of brakes. In the illustrated embodiment, each brake20is a disc brake and includes a brake pad26located on opposite sides of the associated wheel (or rotor), which may be operated to exert a pressure on the associated wheel or rotor to thereby slow rotation of the wheel. For example, as shown inFIG. 2, a brake subsystem20may include a pair of brake pads26located on opposite sides of a rotor30. The rotor30is typically rotationally coupled to one of the wheels12,14,16,18.

Each brake pad26is coupled to a backing plate28,29and the brake pads26and backing plates28,29may be generally located inside a caliper31. Each backing plate28,29may be considered to be part of the associated brake pad26. The caliper31may include a pressure cavity36located therein, and the brake subsystem20includes a piston46located between the pressure cavity36and one of the backing plates28. The caliper31includes an end flange48located adjacent to the backing plate29.

In the case of an analog brake system, the vehicle10includes a brake pedal32which is mechanically, electrically, or otherwise operatively coupled to a master cylinder34that is filled with brake fluid (not shown). The pressure cavity36is coupled to the master cylinder34via an inlet brake line38and an outlet brake line42. The inlet brake line38includes an inlet valve or apply valve40located therein to control the flow of brake fluid from the master cylinder34to the pressure cavity36. The outlet brake line42includes an outlet valve or release valve44located therein to control the flow of brake fluid from the pressure cavity36to the master cylinder34. The apply valve40and release valves44may be two-position valves which allow the flow of fluid therethrough in one direction when open, and which block the flow of fluid therethrough when closed.

During a normal braking operation, a driver presses on the brake pedal32which causes pressure in the master cylinder34to rise. The increase in pressure in the master cylinder34is transmitted to the pressure cavity36(represented by arrow50of inlet brake line38). An increase in pressure in the pressure cavity36(represented by arrow52) urges the piston46to the left (as indicated by arrows54) against the associated backing plate28and thereby causes the associated brake pad26to be urged into contact with the rotor30. The frictional engagement of the brake pad26and rotor30causes the rotation of the rotor30and associated wheel to slow.

Simultaneously, pressure in the pressure cavity36causes the caliper31to move to the right, shown by arrow56, thereby causing the end flange48of the rotor30to engage the other backing plate29and urge the associated brake pad26into contact with the rotor30. In this manner, the backing plates28,29, piston46, caliper31and/or end flange48alone or in various combinations, may be considered an actuator shaped and located to press against a brake pad26to cause the brake pad26to apply pressure against the brake rotor30. When a driver releases the brake pedal32, pressure in the system is reduced which allows the brake pads26to move away from the rotor30.

Although the system shown inFIG. 2is a hydraulic brake system, it should be understood that the present invention may also be used in an electric (electromechanical or brake-by-wire) brake system, as well as a hybrid brake system. In this case an electric motor and gearing is utilized to apply pressure to the backing plates28,29and brake pads26in a well-known manner. In this case the electric motor, ball screw, backing plates28,29or other components, alone or in various combinations, may be considered an actuator.

In the system shown inFIG. 1, the controller24is connected to the brake pedal32, and/or master cylinder34and/or each of the brakes20via brake control lines62,64,66,68to control the application of braking pressure to each of the wheels12,14,16,18. Each brake control line62,64,66,68may be either a mechanical line (i.e. a hydraulic line filled with brake fluid in communication with the master cylinder34and/or pressure cavities52), or an electric/electronic control line (i.e. for controlling operation of the valves40,42or an electric motor), or both a mechanical line and an electric/electronic line such that the controller24is operatively coupled to the brakes20. Each brake control line62,64,66,68may also provide feedback from the brakes20to the controller24.

Each brake subsystem20may include a sensor70(FIG. 2) for detecting or estimating the pressure applied by the brake pads26to the rotor30. In one embodiment the sensor70may include a sensor material67which varies in electrical resistance with pressure. In particular, each brake pad26may include or be made of the sensor material67. However, the sensor material67may not necessarily be part of a brake pad26, and can be located in other positions, as will be described in greater detail below. In one embodiment, the brake pads26may be made entirely of the sensor material67. For example, because brake pads26are typically made of an electrically insulating material, the sensor material67may be formed by taking typical brake pads/brake pad material and adding electrically conductive particles throughout the brake pads/brake pad material (although a wide variety of materials, including existing brake pad material, could be suitable for use as the sensor material67depending upon the nature of such materials). In the configuration shown inFIG. 2, the brake pads26are made entirely of (or substantially entirely of), and therefore also serve as, the sensor material67.

The brake pads26/sensor material67may be a material which experiences a change in electrical resistance with pressure. In one embodiment, each brake pad26/sensor material67is a composite material which includes an electrically insulating material59that carries electrically conductive particles61therein (seeFIG. 7). The electrically insulating material59may be made of or include a wide variety of materials, including but not limited to fiberglass, asbestos, KEVLAR® synthetic fibrous material or other synthetic fibrous materials, organic materials, ceramics, etc. The electrically insulating material59acts as a matrix material, and should be elastically deformable.

The electrically conductive specks or particles61may be metal particles (such as brass or copper) or nearly any other electrically conductive particles. The electrically conductive particles61preferably have a variety of different sizes to ensure efficient packing of the electrically conductive particles61in the electrically insulating matrix59. Thus, although the size of the electrically conductive particles61may vary, the electrically conductive particles61may have a size of between about 500 Angstroms and about 50,000 Angstroms (FIG. 7not necessarily being to scale). The size and distribution of the conductive particles61will affect the electrical performance of the sensor material67. In one case the electrically conductive particles61may constitute between about 10 and about 30 percent of the sensor material67by volume, or between about 10 and about 40 percent by volume, or between about 10 and about 50 percent by volume.

Thus the “mixture” or composite material which forms the sensor material67may, as a whole, have the property that the material67changes in resistance as a function of pressure. Further, the sensor material67need not necessarily be made of the composite material including an electrically insulating matrix59and electrically conductive materials61described above. Instead, the sensor material67may also be or include materials whose smallest unit has piezoresistive qualities, such as semiconductors, crystals or the like. Further, materials which are primarily conductive material, which are loaded with particles or parts of insulating materials, may also be used as the brake pads26/sensor material67.

Accordingly, when the brake pad26/sensor material67experiences pressure (as applied by the associated actuator28,29,48,31,46or motors, etc.), the brake pad26/sensor material67may experience a change in its electrical resistance. In particular, when the brake pad26/sensor material67is sufficiently loaded with electrically conductive particles61and pressure is applied to the brake pad26/sensor material67, the electrically insulating material59of the brake pad26/sensor material67elastically deforms and the electrically conductive particles61are pressed into contact with each other (or contact or surface area between the electrically conductive particles61increases). For example,FIG. 7illustrates two particles61′ pressed into contact with each other.

When exposed to sufficient pressure and under a voltage (either an AC or a DC voltage), percolation currents between the conductive particles61are created to increase the conductivity and decrease resistance of the brake pad26/sensor material67. The electrically insulating material59of the sensor material67is preferably generally elastic in character so that when the applied force is removed the electrically insulating material59(and the sensor material67as a whole) returns to its original position or shape and the resistance of the brake pads26/sensor material67returns to its original level.

The brake pads26may have a wide variety of resistances, and in one embodiment have a resistance of between about 50 kOhms and about 100 MOhms when the brake pads26are not exposed to any mechanical pressure. The brake pads26may have a variety of thickness, for example, between about 0.01 inches and about 1 inch, or between about 0.03 inches and about 0.5 inches. A typical braking force applied by an actuator to the brake pads26may be up to about 5,000 Newtons. When exposed to a braking force of about 5,000 Newtons, the brake pads26may experience a change in resistance of at least about 5 kOhms, or at least about 10 kOhms, or at least about 50 kOhms, but could be more or less depending upon the choice of materials. Thus, the brake pads26/sensor material67may experience a change in resistance of at least 0.1 percent, or at least about 1%, or at least about 2%, or at least about 10%, or at least about 20% when exposed to a braking force of about 5,000 Newtons.

The vehicle10or controller24may include various methods for tracking the change in resistance of the brake pads26/sensor material67. For example, as shown inFIG. 2, each brake20may include a set of leads72,74,76,78, wherein at least two leads are electrically coupled to each brake pads26/sensor material67.FIGS. 3,4and5illustrate a connection to a brake pad26associated with backing plate28. As can be seen, the backing plate28may include an opening80formed therethrough to expose the brake pads26/sensor material67underneath. A lead72is then passed through the opening80and into contact with the top surface of the brake pads26/sensor material67.

The lead72is preferably coupled to the brake pads26/sensor material67at the center of the opening80. Another lead74is coupled to the outer surface of the backing plate28. The free ends of the leads72,74are then connected to a current or voltage source, and to the controller24or a voltmeter or the like which can track the change in resistance across the leads72,74. If desired, the controller24, voltmeter, etc. may be coupled to the leads72,74by a wireless connection. The sensing system described above is a DC measurement system. However, it should be understood that the current passed through the leads72,74could be an AC current and that is processed by an AC resistance sensing system.

As shown inFIG. 5, the path82of current flowing from lead72to lead74includes a part84in which the current flows across the brake pad26, and a part86which flows across the backing plate28. Because the backing plate28may provide very low (essentially zero) resistance, any change in resistance of the brake pads26/sensor material67will affect the resistance of the part84of the current flow across the brake pads26/sensor material67and can be measured or sensed by the controller24. Thus part84of the total current flow82across the brake20provides a variable resistance which can be tracked.

Rather than having one lead72electrically coupled to the brake pads26/sensor material67and another lead74electrically coupled to the backing plate28, both leads may be electrically coupled to the brake pad26. For example, as shown inFIG. 3, instead of or in addition to hole80, the backing plate may include an auxiliary hole80′ formed therein, and an auxiliary lead74′ is passed through the hole80′ to make electrical contact with the brake pad26. In this manner, a current path flowing from the tip of lead72to the tip of lead74′ is in theory more sensitive to changes in resistance of the brake pads26/sensor material67, which should provide a more sensitive brake force sensor70by essentially doubling the length of the current path84.

The brake force (i.e. change in resistance of the brake pads26/sensor material67) may be monitored at only a single brake pad26of a wheel/rotor30, or in each brake pad26separately, or both brake pads26collectively. When it is desired to monitor the braking force at each brake pad26separately (i.e., as shown inFIG. 2), the brake pads26should be electrically isolated from each other by, for example, insulating bushings (not shown). The change in resistance at both brake pads26can be collectively monitored by connecting a wire to the one of the brake pads26/backing plates28and another wire to the other one of the brake pads26/backing plates29, and then running a current from one brake pad26to the other, for example, via the rotor30which is highly conductive and has essentially no electrical resistance. Changes in temperature may of course affect the measured resistance in the brakes20, and an interface circuit or other temperature-compensating structure/devices may be used to accommodate the temperature effects in a well-known manner.

Further, the brake force sensor70may also operate as a worn brake sensor. In particular, the controller24may be programmed such that the controller24detects a worn brake lining when the resistance of the brake pads26/sensor material67becomes sufficiently low (i.e., when no braking force is applied to brake pads26/sensor material67). For example, when the brake pads26become sufficiently thin, an electrical current may be carried across the associated rotor30. In other words, when the brake pads26become very thin (i.e. on the order of the size of the conductive particles61or less) then the rotor30may effectively provide a short circuit and the current will no longer travel along the brake pad26and current path84would be eliminated. Thus, the controller24may be programmed to detect a low resistance and send a signal that the brake pad26is worn and should be replaced.

Furthermore, the particles61may be arranged a varying manner in the brake pad26so as to allow the controller24to detect a worn brake pad26, even before the brake pad26become significantly worn. For example, the size and/or density of the particles may be arranged in a gradient across the thickness of the brake pad26. In particular, in one example particles61having a larger average size may be located on the side of the brake pad26facing the backing plate28, and particles61having a smaller average size may be located the side of the brake pad26facing the rotor30. In this manner, wearing of the brake pads20will result in a gradual change in the average resistance provided by the brake pad26, and this gradual change can be monitored to provide an estimate of brake pad wear and/or remaining brake pad life.

FIGS. 1–5illustrate an embodiment of the invention wherein the sensor material of the brake force sensor70is located in or formed as part of the brake pad26. However, the sensor material need not necessarily be located in or formed as part of the brake pad26, and can be placed at a variety of other locations. The sensor material may be located in any other location which experiences a force proportional to or related to that applied by an actuator to the brake pads26. The sensor material67may be positioned to experience a change in pressure when the piston46/caliper31presses against the associated backing plate28,29to cause the associated brake pad26to apply pressure against the brake rotor30.

For example, as shown inFIG. 6, a brake20′ may include a piece of sensor material86that is located between the actuator (i.e. piston46) and the associated backing plate28. In this case, the sensor material86may transmit at least part of the pressure from the piston46to the backing plate28and pad26. In addition, or in the alternative, another piece of sensor material88may be located between the end flange48and backing plate28. Various leads90may then be coupled to these sensor materials86,88and coupled to the controller24such that the controller24can monitor the resistance of the sensor materials86,88and therefore the applied brake force. If desired, another piece of sensor material88can be located on the opposite side of the brake20′ so that the other piece of sensor material88is located between backing plate28and brake pad26.

In this arrangement (i.e., when the sensor material86,88is not part of the brake pad26) wear or replacement of the brake pad26does not affect the sensor/sensor material86,88. Furthermore, if the sensor material86is located inside the piston46(as shown inFIG. 6) the sensor material86may be protected from ambient conditions.

The present invention provides a relatively simple, accurate and robust sensor/system for measuring the applied brake force. The present invention provides advantages over strain gages (i.e. piezoresistive (semiconductor), carbon resistive, bonded metallic wire and foil resistant strain gauges) because such strain gauges must be securely bonded to a braking component. Reliably bonding the strain gauge to a braking component is difficult due to the materials, high temperatures, and high temperature cycles of the braking components. Furthermore, conventional strain gauges experience thermal offsets, RF-pick-up, magnet pick-up and plastic deformation. The present invention can be easily fit into existing brake systems with little to no modification.

In addition, the sensor or sensor material can be implemented in a wide variety of systems and uses beyond the use in brakes and brake systems described herein. For example, whenever applied stress or pressure is desired to be sensed (for example when a first pressure component applies pressure to a second pressure component) the sensor material of the present invention may be utilized. Thus, a washer made of the sensor material described herein may be placed between the head of a bolt and an anchor component to which the bolt is being threadedly attached. Wires or leads can then be attached to the washer, and its electrical properties measured or monitored to determine the force applied by the bolt. Of course, this is merely a single example of a wide variety of settings and structures in which the sensor material may be utilized.

Having described the invention in detail and by reference to the preferred embodiments, it will be apparent that modifications and variations thereof are possible without departing from the scope of the invention.