Abstract:
A wheel bearing unit that measures the contact forces between the tire and the road under all driving situations including operation of the brake, and provides information for the vehicle operation control systems and for optimizing the braking process, the unit including first sensors arranged on the stationary part of the wheel bearing in order to measure the forces acting in the wheel bearing, and second sensors arranged on a support members for the disk-brake caliper for continuously determining the acting braking forces.

Description:
FIELD OF THE INVENTION 
     The invention concerns a wheel bearing, which enables conclusions to be drawn as to the forces which act between the road surface and the tires of a motor vehicle particularly by measuring the forces currently acting within the stationary ring of the wheel bearing. 
     BACKGROUND OF THE INVENTION 
     To improve the safety and traveling comfort of modern vehicles, the electronic control systems require increasingly more information about the current driving situation for the operation of the vehicle. In addition to the current data on the engine, the gearbox and the rotational speed of the individual wheels, the current forces and force directions (wheel contact forces) which act on the individual tires will also be employed in future for control of vehicle operation. There have been various attempts to record these forces: e.g. measuring forces in the tire or inserting measurement adaptors between the wheel rim and the acceptance flange (DE 196 27 385 or “Erfassung der mehr axialen Fahrbetriebsbelastungen mit dem Meβrad “Velos””, A. Rupp, W. Diefenbach, V. Grubisic, ATZ Automobiltechnische Zeitung 96 (1994)). These attempts at a solution all have the problem that the forces must be transmitted from a rotating part. For this purpose, additional complexity is necessary, such as the employment of telemetric data transmission or the use of sliding contacts. In this connection, one obvious possibility for recording these wheel contact forces is to use the wheel bearings, because these transmit to the vehicle framework all the forces which act on the tire. 
     EP 0 432 122 A2 shows a wheel bearing unit in which sensors are arranged at various locations on the stationary ring. The forces which act on the tire can be determined from the forces measured on the stationary ring of the wheel bearing during travel in a straight line or travel round a bend. Despite the large number of measurement locations, the problem with this measurement arrangement appears at the moment when the vehicle brake is actuated. Tests have shown that the wheel contact forces can no longer be exactly determined during the operation of the brake. 
     SUMMARY OF THE INVENTION 
     The invention has the object of indicating a measurement arrangement for a wheel bearing of a motor vehicle, from which measurement arrangement the wheel contact forces can be determined, even during braking. 
     DESCRIPTION OF THE INVENTION 
     This object is achieved by the invention. 
     The forces which act between the tire and the road surface are measured indirectly by means of the reaction forces in the wheel bearing. For this purpose, sensors (for example strain gauges or thin-film strain gauges) are arranged on the stationary ring of the wheel bearing and the reaction forces (the stress change) in the stationary ring of the wheel bearing are determined by these sensors. The wheel contact forces can be determined directly from these loads during travel without braking. During a braking process, however, the reaction forces in the wheel bearing are so strongly influenced by the forces introduced via the disk brake that the wheel contact forces can no longer be exactly determined. Only knowledge of the position and magnitude of the currently acting forces during the braking process makes it possible to draw conclusions about the forces and the current coefficient of friction between the road surface and the tire from the total forces which are recorded by the sensors on the stationary ring of the wheel bearing. 
     The spatial position of the center of gravity of the braking forces during the braking process can be continuously determined by the continuous measurement of the forces at the suspension points or in the support members of the flange-connected disk-brake caliper (center of gravity of the braking forces—calculated point of action of the vector of the braking forces on the brake disk or on the brake linings). The position of the center of gravity of the braking forces depends on the braking force introduced, the current coefficient of friction and the contact relationships between the brake lining and the disk brake. The current center of gravity of the braking forces can therefore be displaced both during a braking process and during the operating period. After reduction of the measured forces in the wheel bearing by the influence of the braking forces, it is possible to use the resulting wheel contact forces to carry out the braking process better than was possible with the prior art. 
     A further surprising advantage of the measurement arrangement according to the invention relative to EP 0 432 122 A2 and all the measurement wheels, is that for tangentially introduced wheel forces, the braking forces and the influences due to the road surface can be computationally differentiated by means of this measurement arrangement. If a vehicle travels over a soft subsurface (snow, gravel, soft ground, puddle), an accumulation of material forms in front of the tire and this must be pushed away. Pushing away in this manner causes a tangential force in the tire which has the same effect as the braking force. Due to the recording of the current braking forces, these horizontally acting forces can be unambiguously subdivided into braking forces and into forces which act from the road surface onto the tire. This advantage is all the more important because none of the measurement systems, which carry out measurements in the region of the wheel rim and of the tire, can differentiate between these forces. This possibility optimizes the braking process in the soft subsurface. 
     In a braking process of a driven wheel, furthermore, the forces which act in the brake disk can be differentiated from the driving forces (or driving torque), which are introduced via the drive train. This differentiation is necessary to permit optimum braking of these wheels when the clutch is not released (i.e. there is engine braking). This differentiation of the forces into driving forces from the drive train and braking forces is impossible with all the systems which carry out measurements in the wheel rim or in the tire. 
     The advantage of the arrangement of the sensors lies in the fact that all the sensors are arranged on a stationary component. This has particular advantages during the manufacture and fastening of the sensors. It avoids the problems of running cables to different components. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the forces and torques which fundamentally have to be measured, as known from the prior art for example, DE 196 27 385 A1. 
     FIG. 2 shows the force relationships which appear between the brake disk, the wheel bearing and the road surface during a braking process. 
     FIGS. 3 a ,  3   b  show the influence of the position of the disk-brake caliper on the forces acting in the wheel bearing during the braking process. 
     FIGS. 4 a ,  4   b  show the influence, on the wheel bearing, which can appear due to the displacement of the position of the braking center of gravity within the brake lining during a braking process. 
     FIGS. 5 a ,  5   b  show the force relationships which act on the wheel bearing during a braking process in the soft subsurface. Separation into braking influences and road surface influences. 
     FIG. 6 shows a wheel bearing with support members, which are integrated on the stationary ring, for accepting the brake-disk caliper and associated sensors. 
     FIGS. 7 a ,  7   b  show a wheel bearing with additional fastening appliances for accepting the support member of the brake-disk caliper on the stationary rim of the wheel bearing, and associated sensors. 
     FIG. 8 shows a wheel bearing with the brake-disk caliper support members fastened to the swivel bearing, and a possible arrangement of the sensors. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a tire with the forces Fx, Fy, Fz and the torques Mx, My, Mz. The object of the measurement system according to the invention is to record these forces and torques, which act between the tire  7  and the road. 
     FIG. 2 represents a wheel during the operation of the brake. So as not to overload the representation with force arrows, it is based on travel in a straight line. The known wheel contact forces Fyk during travel round a bend are not represented. The tire  7 , the brake disk  8 , the disk-brake caliper  4  (or brake lining) and the fastening of the brake-disk caliper  6  to the stationary outer ring  1   a  of the wheel bearing  1  are shown in this diagrammatic representation. The braking force FtanB acts on the brake disk, which is connected to the inner ring l b  of the wheel bearing  1  and retards the tire. The sensors  2  of the measurement arrangement are arranged on the stationary ring part  1   a . The sensors  3  determine the forces in the brake support members in order, by this means, to determine the current point of action  5  (center of gravity) of the braking force and the magnitude of the braking force FtanB. The direction of motion of the wheel is designated by v. The braking force FtanB and the force FxR 1  follow from the lever relationships between the tire and the current center of gravity of the braking force. The forces FtanB and FxR 1  are supported in the wheel bearing between the inner ring and the outer ring. These forces induce reaction forces there in the form of FtanL and FxL 1 . The weight forces FzR between the tire  7  and the road surface  10  cause the forces FzL in the wheel bearing  1 . 
     In consequence, there are always three force components (FxL 1 , FzL, FtanL) between the inner ring  1   b  and the stationary outer ring  1   a  of the wheel bearing  1  during travel in a straight line with the brake in operation. 
     My (FtanB) or My (FxR 1 ) is the torque which acts during the operation of the brake. 
     The forces acting in the wheel bearing  1  between the inner ring  1   b  and the outer ring  1   a , and which are measured by means of the sensors  2 , are added geometrically in FIG. 3 a  in order to determine the resulting bearing force FresL,. In order to determine the wheel contact forces (FxL 1 , FzL), the influence of the braking forces FtanL are calculated from the measured bearing forces FresL. The current coefficient of friction between the tire and the road surface is determined from the ratio between FxL 1  and FzL. In all other details, FIG. 3 a  corresponds to FIG.  2 . 
     FIG. 3 b  shows that the arrangement of the disk-brake caliper  4   a  has a fundamental effect on the resulting bearing forces FresL. FIGS. 3 a  and  3   b  are drawn in with the same wheel contact forces  9  designated in each case. This representation shows clearly how the position of the disk-brake caliper  4   a  and therefore of the braking center of gravity  5   a  influences the measurement result FresL. 
     FIGS. 4 a  and  4   b  represent the problem that the braking center of gravity  5   b ,  5   c  within the brake linings  4  can be displaced during the operation of the brake. Different values for the resultant force FresL in the wheel bearing I result, depending on the center of gravity  5   b ,  5   c  of the braking force FtanB. In order to arrive at the currently acting wheel contact forces (FxL 1 , FzL) during the operation of the brake, the current effects of the braking force FtanL (measured by means of the sensors  3  on the support member  6 ) must be continuously calculated from the resultant bearing force FresL. 
     FIGS. 5 a  and  5   b  show a surprising advantage relative to the known measurement wheels and measurement methods in the tire. FIG. 5 a  represents the problems which arise when a tire moves in a soft subsurface  11 . This soft subsurface leads to a displacement of the wheel contact forces. The force FxR 2  must be produced within this soft subsurface  11  in order to displace it. This force FxR 2  has a corresponding force in the wheel bearing FxL 2 . In this figure, the force relationships are shown without brake intervention. The evaluation of the measured braking forces (FtanB=0) shows that the horizontal force acts on the tire from the outside and is not caused by braking forces. In this figure, the support members  6  and the sensors  3  are not shown for reasons of clarity. 
     FIG. 5 b  represents the situation from FIG. 5 a  with braking now taking place on a soft subsurface  11 . The forces acting horizontally on the tire  7  are represented in this case by the two force vectors FxR 2  and FxR 1 . FxR 2  corresponds to the force which acts on the tire due to the soft subsurface  11 . FxR 1  corresponds to the force which occurs in the horizontal direction due to the operation of the brake. The remaining force arrows correspond to the representations in the figures previously mentioned. Because of the exact knowledge of the braking force FtanB and the force FxR 1 , which is proportional via the lever arm to the braking force, the resulting bearing force FresL can be reduced by calculation by the forces FtanL and FxR 1 . All the magnitudes of the currently acting wheel contact force are therefore known. This separation of the forces which act tangentially on the tire  7  is not possible by means of the known systems of measurement wheels or adaptors because these systems are not aware of the braking forces. Even in extreme driving situations, therefore, the exact force relationships between the tire and the wheel surface can be determined because of the arrangement, according to the invention, of the sensors  2 ,  3  on the stationary ring  1   a  of the wheel bearing  1  and on the support members  6  of the disk-brake caliper  4 . 
     FIG. 6 shows a wheel bearing  1  in which two support members  6   a , which accept the disk-brake caliper, are integrated on the housing  1   d  of the outer ring  1   a . The disk-brake caliper, the brake lining and the brake disk are not shown in this representation. The flange  13  for accepting the wheel rim or the brake disk is connected to the inner ring  1   b . The sensors  3  for measuring the braking force are arranged in the two arms of the support member  6   a . The sensors  2   a  for measuring. the forces are attached to the housing  1   d  of the outer ring  1   a.    
     FIG. 7 a  and FIG. 7 b  show a wheel bearing unit with disk-brake caliper fitted. FIG. 7 a  shows a wheel bearing unit with rotating flange part  13  and stationary outer ring  1   a . The sensors  2   b  are arranged on the outer ring  1   a . The sensors  2   b  are arranged in two planes in this representation so as to permit measurement of the tipping forces when the vehicle is traveling round a bend. The fastening holes  14  for acceptance in the swivel bearing (the swivel bearing is shown in FIG. 8) and the separate holes  15  for accepting the disk-brake caliper support member  6  are shown on the flange  1   e  of the stationary outer ring  1   a . The sensors  3   a , by means of which the braking force and the braking center of gravity are measured, are arranged in the flange region  1   e  at the holes  15 . Advantages arise in handling and assembly due to the spatial proximity of the sensors  2   b ,  3   a  because all the sensors  2   b ,  3   a  are arranged on a stationary part. 
     FIG. 7 b  shows FIG. 7 a  with the disk-brake caliper  18  fastened over the support members  6 . In this representation, the sensors  2   b ,  3   a  are not visible for perspective reasons. The disk-brake caliper  18  is fastened by means of the holes  15  through the disk-brake caliper support member  6 . The wheel bearing unit is connected to the swivel bearing by means of the holes  14 . The arrangement of the sensors  3   b  in the support members  6  of the disk-brake caliper  18  represents a further variant for determining the braking force and center of gravity. 
     FIG. 8 shows the swivel bearing  16 . The wheel bearing is seated in the hole  17  of the swivel bearing  16  and is fastened by means of the holes  19 . The wheel bearing is not shown in this representation. In this representation, the disk-brake caliper  18  with its support members is fastened to the swivel bearing  16 . The sensors  3   c  for determining the braking force and center of gravity are represented. 
     Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.