Abstract:
A modular electrical resistance sensor is positionable in gaps between brake linings for a drum brake. The modular sensor is worn with the brake linings resulting in steadily increasing electrical resistance of the modular sensor. A measurement circuit associated with the modular sensor is programmed to equate electrical resistance to the degree of wear when the sensor has assumed a steady state temperature at or near the ambient temperature. Otherwise, particularly during periods of use of the brakes, resistance and the degree of wear last calculated become arguments into a function for determining brake lining temperature.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The invention relates to brake lining wear sensing and brake lining transient temperature detection and more particularly to a modular electrical resistance sensor which may be used to implement wear and temperature sensing. 
   2. Description of the Problem 
   Effective brakes are essential to safe motorvehicle operation. Contemporary brake systems dissipate vehicle kinetic energy through brake friction pads as heat. These brake pads have a relatively short service life and require regular replacement. Heavy vehicles have historically exhibited problems with brake overheating, especially when the vehicles are descending along long grades. Overheating reduces stopping ability and accelerates brake pad wear. 
   Inspection of the brake system has traditionally involved disassembly of the wheel mechanism and visual examination of the pads. It has been recognized that it would be desirable to incorporate some kind of sensor into the brake pads that monitor wear of the pads without the need to disassemble the brake system. Were the same sensor used to monitor brake temperature the addition to vehicle complexity would be minimized. 
   Various brake lining wear detection systems and brake temperature measurement systems are known in the art. One such system for detecting wear provides a modular, progressive brake lining wear sensor. The sensor has a triangular, wedged shaped electrically resistive member disposed between a pair of conductive plates to define a triangular shaped sensor. The sensor is encapsulated within an erodable molding and connected to a sensing circuit by a pair of leads including a ground lead and a resistive lead. The ground lead and resistance lead emerge from the encapsulated sensor for connection to the sensing circuit. The sensor is disposed within the brake lining and is connected to the brake shoe. As the brake lining progressively wears, the triangular wedged shaped resistive member is also progressively worn away thus continuously changing the overall resistance of the sensor. The change in resistance provides for continuous indication of the state of wear of the brake lining. 
   Another sensor design provides both wear and temperature sensing. Here a plurality of parallel connected resistors are connected to a sensing circuit. A thermistor provides temperature sensing. The resistors and the separate thermistor are mounted, spaced from one another, on a printed circuit board and the entire unit encapsulated within a single molding. The thermistor is connected to a grounded lead as are each of the resistors. A ground lead, a resistance lead and a thermistor lead emerge from the encapsulated module for connection to the sensing circuit. The module is disposed between linings in a drum brake so that the module is worn away with the linings. With progressive wear the resistors (or at least the conductive loops in which individual resistors are connected) are progressively and sequentially worn away, increasing the resistance of the sensor in a series of discrete steps. Three resistors are used to indicate 4 discrete levels of brake lining wear. 
   SUMMARY OF THE INVENTION 
   According to the invention there is provided a brake lining temperature and wear measurement system in which a modular electrical resistance sensor is positioned in gaps between brake linings for a drum brake. The modular electrical resistance sensor comprises a thin film of copper or some other electrically conductive material disposed on a wearable substrate. Substrate and film are encapsulated in a thermally stable, wearable thermoplastic. The modular sensor is positioned with the brake linings to wear down with the lining. This results in steadily increasing electrical resistance of the modular sensor correlated with wear of the lining. A measurement circuit associated with the modular sensor is programmed to equate electrical resistance to the degree of wear and operates when the sensor has assumed a steady state temperature at or near ambient temperature. During periods when the brakes are in use, resistance and the degree of wear last calculated become arguments into a function for determining brake lining temperature. 
   Additional effects, features and advantages will be apparent in the written description that follows. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a perspective view of a drum brake assembly incorporating the brake wear and temperature sensor of the present invention. 
       FIG. 2  is a perspective view of a sensor module in accord with the present invention attached to a brake assembly. 
       FIG. 3  is a perspective view of a partially worn sensor module from  FIG. 2 . 
       FIG. 4  is a front view of a sensor mounted on a circuit board. 
       FIG. 5  is a side cross sectional view of the sensor module. 
       FIG. 6  is a graph illustrating resistance of the sensor against temperature at various degrees of wear of the sensor. 
       FIG. 7  is a flow chart of a program executed by the measurement circuit for estimating wear and temperature of the brake linings. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is preferably employed in a drum brake assembly  10  as shown in  FIG. 1 . However, the modular sensor  20  may be adopted for employment in other types of brake assemblies. Referring now to  FIG. 1 , a brake drum  13  has an inner brake surface  14  for frictionally engaging the brake lining  11 . An actuator such as an S-cam arrangement displaces the brake shoes  15  outwardly towards the inner brake surface  14  bringing brake lining  11  into contact with the inner brake surface of the drum  13 . Brake linings  11  are mounted to the brake shoe  15  to frictionally engage the brake drum  13  and thus provide braking force. The generic brake drum arrangement  10  and actuation mechanism is completely conventional and is well known in the art. 
   Modular sensor  20  is preferably mounted between a pair of brake lining surfaces  11  in a gap  18  with a distal end substantially flush with the outer or friction surface of the brake lining  11 . The specific connection of modular sensor  20  to brake shoe  15  is not shown. The specific manner of connection is not critical to the present invention. Any suitable connection that maintains radial alignment of the modular sensor  20  during braking may be employed. 
     FIG. 2  is a perspective view of the preferred embodiment of the present invention prior to any significant wear. Modular sensor  20  is positioned on brake shoe  15  between the brake lining portions  11 . A clip  22 , having one end partially embedded in the sensor encapsulation, may be used to secure modular sensor  20  to brake shoe  15 . Resistance lead  26  and ground lead  24  extend from the encapsulation material of sensor module  20 . 
     FIG. 3  is a perspective view of the modular sensor  20  of  FIG. 2  in a partially worn state. Modular sensor  20 , including its encapsulating material, the sensor material and sensor backing are all made of erodable material resulting in the sensor wearing to conform substantially to the profile of the brake lining  11  at all times. As more of modular sensor  20  is worn, the resistance of the sensor increases as explained below. Resistance measurements may be equated with either brake lining  11  temperature and brake lining wear, although the measurements cannot be simultaneously equated to both variables. A determination of wear must precede brake lining temperature estimation as explained below. Generally speaking, the progressive increase in resistance indicates the progression of brake lining wear, as determined under conditions of a steady state, and known, brake lining temperature. Resistance lead  26  may be seen connected to a measurement circuit  28 , which may be implemented in a number of different ways. Measurement circuit  28  may incorporate an analog to digital converter, a data sending unit, cabling, and a programmable microcomputer attached to receive data over the cabling (not shown). A motor vehicle ambient temperature sensor  29  may be advantageously employed if present to provide ambient temperature readings to the measurement circuit  28 . Such elements are believed well within those skilled in the art. 
   Referring to  FIGS. 4 and 5  a sensor  31  suitable for encapsulation to form sensor module  20  is illustrated. Sensor  31  comprises a backing such as a circuit board  30  from one of the major surfaces of which has been etched a copper or metal coated pad  32 . Metal coated pad  32  includes rails  34  and  38  and a thin film sheet  36  located between the rails and connected to the rails along two opposed edges. Thin film sheet  36  should be sufficiently Thin relative to the rails  34 ,  38  to exhibit substantially greater resistivity than the rails. As circuit board  30  is worn down from the top in the direction indicated by the arrow “A”, and thin film sheet  36  is worn down along a free edge of the sheet between rails  34  and  38 , the area of thin film sheet  36  decreases. A consequent increase in the resistance of sheet  36  between rails  34  and  38  results. Electrical connection to rails  34  and  38  may be made by connection to pads  42  and  44 , which are shown op a side of circuit board  30  bordering the etched major face. Alternatively, the leads may be taken off from pads left on the front, etched major face. Both front and back major faces of circuit board  30  are coated with a heat resistant, erodable thermoplastic resin, or similar electrically insulative, heat resistant material. 
   Referring to  FIG. 6 , graphs of resistance of the metallic thin film  36  against temperature, at various stages of wear (from the top of the sensor module  20  in the direction A), are shown. As is well known, the resistance of copper and most other metals increases linearly with temperature at temperatures typically encountered in motor vehicle operations. The graph illustrates curves  602 ,  604 ,  606 ,  608  and  610  for a sensor which is: wholly intact (0% wear); one fifth eroded (20% wear); two fifths eroded (40% wear); three fifths eroded (60% wear); and four fifths eroded (80% wear). As is readily seen, each resistance curve, for a constant degree of wear, is linear. However, resistance increases exponentially with destruction of the thin film  32  at any temperature and will be understood to increase without bound as the film is destroyed. The operator may choose at any given time to determine one of either the degree of destruction of the film or the temperature of the brakes. Determining brake temperature requires that brake pad wear is already determined. If the operator knows the temperature of the brake linings, wear of the linings can be estimated. If the degree of wear of the linings is known, then temperature of the linings may be estimated. Where a vehicle has stood for a period exceeding a minimum period of time, and the brakes have not been used, it may be assumed that the brakes take on the ambient temperature. This temperature may be measured by an sensor  29  on board the vehicle, such as an engine air intake temperature sensor, or the wear calculation can assume a value, e.g. 25 degrees Celsius, or the ambient temperature may be entered by the vehicle operator. Upon measuring the resistance of the sensor module  20  the measurement circuit  28  can determine which wear curve the point of intersection between resistance and temperature on the graph falls closest to. The selected curve is then be saved as the current wear value. When the vehicle is started and a driver begins to use the brakes, resistance in the film continues to be measured, but the result is mapped to the curve serving as the current estimate of wear to recover brake lining temperature. This may be implemented as a look up table. Thus the curves  602 ,  604 ,  606 ,  608  and  610  are predetermined and may be stored as values in a look up table on a programmable computer. 
   Referring to  FIG. 7  implementation of the invention is linked to vehicle operation to determine likely periods when the brakes have assumed a steady state temperature close to the ambient temperature. Upon vehicle start step  702  is executed to read the ambient temperature and the length of time that the vehicle has stood. Vehicle start may be any event marking a the end of a period where the vehicle has stood still, with the engine either idling or shut off. Step  704  marks determination as to whether the brake temperature is likely to be at a steady state near the ambient temperature. If the result of the test is in the affirmative, step  706  is executed to measure resistance of the brake lining resistance modules. Once the resistance has been determined step  708  is executed to determine wear of the sensor as a function of temperature and resistance. As noted above, these results may be precalculated and stored as a look up table graphically illustrated in  FIG. 6 . Step  710  represents selection of a curve (store wear). Wear of course may exceed a limit in which case a brake lining wear warning may be issued (step  711 ). 
   Once a new wear level has been determined, or following the NO branch from decision step  704 , vehicle start is confirmed at step  712 . Confirmation of vehicle start may be taken as an instance of operational application of the brakes. As long as the brakes are not applied the program may continue to loop back to step  702  for a wear measurement. Once the brakes are applied the YES branch is followed from step  712  to step  714 , representing another measurement of the resistance of the brake lining resistance sensors. Since the brakes have been used they cannot be assumed to be at ambient temperature any longer, and the measurements are instead used as an argument into the wear curve selected at step  710 . Temperature is returned at step  716  and may be displayed to the driver at step  718 . The returned temperature is compared to a critical limit temperature at step  720 . If the temperature does not exceed limit(s) the program loops back to step  712 . If the brake lining temperature exceeds the critical temperature, step  722  is executed to issue a warning and the program loops back to step  712 . 
   The invention provides a low cost mechanism utilizing a single sensor type located in the area of the brakes. The sensor realizes both wear and temperature monitoring for brake linings by utilizing on board computing capacity to monitor the context of the measurements. 
   While the invention is shown in only one of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit and scope of the invention.