Abstract:
A force sensor for an electromechanical brake has a closed ring having a C-shaped profile that is open inwardly. Force is introduced along the inner circumference of the ring.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
   The present invention relates to a measuring device for an electro mechanical brake, and to an electro mechanical brake for a motor vehicle. 
   In the case of electro mechanical brakes in the field of motor vehicles, the braking force is produced by electric motor and transmitted mechanically to the brake shoes. In the case of disc brakes actuated by electric motor, the force sensor is arranged in the force flux and serves the purpose of accurately measuring the force exerted on the brake disc, in order to be able to drive the motor appropriately. 
   DE 101 51 561 A1 dispenses a force sensor for an electro mechanical brake that is designed as a ring and provided with three projections projecting in the axial direction. Support regions extending in the radial direction of the ring are formed centrally between the projections. The force is introduced via the axial projections, the reaction forces being introduced via the support regions. Pairs of strain gauges are arranged between axial projections and support regions on the ring element. The force sensor is deformed in an undulating fashion when the braking force is applied to it. The deformation is converted into the introduced braking force by the strain gauges and by an evaluation device. 
   SUMMARY OF THE INVENTION 
   It is the object of the invention to create a measuring device for an electro mechanical brake that supplies signals of adequate magnitude as far as possible in conjunction with simple manufacture. The object is achieved according to the invention by means of a measuring device having the features of claim  1 , and by means of an electro mechanical brake. Advantageous refinements are respectively the subject matter of the sub claims. 
   The measuring device according to the invention has an annular force sensor and an associated strain gauge. The force sensor has the shape of a closed circular ring. The ring has in cross section a C-shaped profile with two parallel limbs arranged spaced apart from one another. In contrast with the annular force sensor already known from the above prior art, the force sensor according to the invention is designed as a ring with a C-shaped cross section and not a rectangular one. Force is introduced into the limbs in the case of the force sensor according to the invention, as a result of which the ring is deformed and the spacing between the ends of the limbs is reduced. The strain gauge is arranged at least on one of the limbs and detects the strain produced by the bending of the limb. The strain of the limb can then be converted into the magnitude of the introduced force using methods known per se. The limbs preferably point into the center of the ring. 
   The use of a C profile for force measurement results in a substantially higher accuracy of the signals and enables very reliable measurement of the forces occurring in the case of an electromechanical brake. 
   In a preferred refinement, the transition from the base of the C profile to the limbs is rounded. A radius of curvature of approximately 1.5 mm is preferably used in this case. The ring has regions for introducing a force acting between an actuating element for a brake shoe and a caliper of the brake, the regions expediently being situated at the free end of the limbs on the outside. The regions for introducing the force preferably respectively run annularly along the free end of the limbs. 
   A strain gauge is arranged next to the force introduction region on the outside of the limb in order to measure the force introduced. In a preferred refinement, the strain gauge is arranged on the outside of a limb which points toward the caliper. 
   The strain gauge is preferably arranged on one of the limbs along the circumference. The signals of the strain gauges distributed over the circumference are averaged in order to evaluate them. 
   The strain gauge has measurement strain resistors that advantageously extend in the radial direction of the ring. The signals of the measurement strain resistors are led out by connecting individual resistors in parallel and/or series, or by means of resistance bridges. 
   In a particularly preferred refinement, silicon strain gauges are provided as strain gauge. Such silicon strain gauges are known, for example, from WO 01/08227. These are semiconductor strainometers that have a resistance substrate layer and a layer, supported by the latter, made from electrically conducting silicon. A particular advantage of the silicon strain gauges is that the latter exhibit a particularly small thickness. 
   The force sensor preferably consists of a precipitation hardenable steel when use is made of silicon strain gauges. Steel of type 17-4PH or Inconel 718 is preferably used here. The silicon strain gauges are preferably connected to the force sensor by means of lead borate glass solder. 
   Overall the use of a precipitation hardenable steel (PH steel) lends the force sensor a substantially greater strength. By comparison with the steels that are suitable for the known application of thick layers for the strain measuring elements, a PH steel has more than twice the strength and tensile yield strength. All PH steels contain nickel in order to permit precipitation hardening. The nickel content lowers the hardening temperature. During cooling, each steel changes volume upon exceeding the hardening temperature; if said change in volume is below the hardening temperature of a processed thick resistance layer, the layer peels off. Consequently, despite their strength and high tensile yield strength, PH steels cannot be used with processed thick resistance layers. 
   The silicon strain gauges as described in WO 01/08227 are bonded to the measuring ring by means of lead borate glass solder. 
   The object according to the invention is likewise achieved by means of an electromechanical brake having the measuring device described above, force being introduced into the ring via projections in the caliper. In the case of the associated second region for introducing force, as well, force is preferably introduced into the ring via projections on an actuating element for the brake shoes. The projections for introducing force into the measuring device preferably have a spherical bearing surface. As a result, a circular force introduction region is defined when force is introduced in an annularly running fashion. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The measuring device according to the invention and the electromechanical brake are explained below in more detail with the aid of an exemplary embodiment. 
     In the drawing: 
       FIG. 1  shows the schematic diagram of an electromechanical brake, 
       FIG. 2  shows a view of a detail from  FIG. 1 , 
       FIG. 3  shows a perspective view of the force sensor according to the invention, 
       FIG. 4  shows a perspective view of the sectioned force sensor, 
       FIG. 5  shows a deformation occurring in the force sensor, and 
       FIG. 6  shows schematic diagram of strain resistors in the force sensor, the resistors being arranged radially. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  is a diagrammatic view of an electromechanical brake for a brake disc  10 . Two brake shoes  12  and  14  are arranged at the outer rim of the brake disc  10  on opposite sides thereof. The brake shoe  12  is mounted on a caliper  16 . The brake shoe  14  is supported on a pressure piston  18 . The pressure piston  18  has an inner thread, and is widened in the region of its connection to the brake shoe  14  to form a circumferential flange  20 . The pressure piston  18  is supported displaceably in a sleeve  22 . The sleeve  22  is guided through an opening in the caliper  16 . At its end pointing toward the brake shoe  14 , the sleeve  22  is provided with an outwardly projecting flange  24 . The flange  24  has on its side pointing away from the brake shoe a projection  26  whose free end is spherically rounded. 
   At its end pointing away from the brake disc  10 , the sleeve  22  has a base  28  that is provided with a central bore  30 . Running in an axial direction in the pressure piston  18 , which is arranged in the sleeve  22 , is a spindle  32  whose shaft  34  projects from the central bore  30 . The end of the shaft  34  pointing away from the caliper is provided with a pinion  36 . The pinion  36  is rotated by a suitably designed gear (not illustrated) that is driven by a schematically illustrated motor  38 . The spindle  32  is supported by balls  40  in the pressure piston  18 . 
   An annular force sensor  42  is arranged between the sleeve  22  and caliper  16 . 
   During operation, the motor exerts a torque on the pinion  36 , as a result of which the spindle  32  exerts an axial force on the pressure piston  18 . The brake shoes  12  and  14  are thereby pressed against the brake disc  10  with the applied force. The reaction force of the pressure piston  18  is transmitted by the bearing of the spindle  32  in the sleeve  22  onto the flange  24  where this force acts on the force sensor  42 . The force sensor  42  experiences a reaction force of the caliper  16 . 
   As illustrated in  FIG. 2 , the caliper  16  also has a projection  44  whose end is of spherical design. As may be seen from  FIG. 2 , force is introduced into the force sensor  42  via the projections  26  and  44  in an annular fashion. 
     FIGS. 3 and 4  show the force sensor designed as a ring. On its inside, the ring has a circumferential groove  46  that defines two limbs  48  and  50 . As illustrated in  FIG. 4 , the depth of the groove  46  is preferably selected such that the base  52  of the ring has a greater thickness than the two limbs  48  and  50 . 
     FIG. 5  shows a diagram of the mode of operation of the force sensor  42 . The couple  54  and  56  presses the limbs  48  and  50  together such that their spacing is reduced from magnitude D to magnitude d. The bending of the limbs causes on their outer sides  58  and  60  a strain that is measured by a strain measuring system  62  (compare  FIG. 2 ). The magnitude of the introduced forces  54 ,  56  can be calculated from the measured strain.