Abstract:
Device for determining wear in a carbon ceramic brake disk. The device includes a coil arrangement having at least one coil structured and arranged to generate a magnetic field in the brake disk and to detect an eddy current in the brake disk, and an arcuate measuring area.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority under 35 U.S.C. §119(a) of German Utility Model Application 20 2011 103 105.9 filed Jul. 12, 2011, the disclosure of which is expressly incorporated by reference herein in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the invention relates to a device for determining the wear of a carbon ceramic brake disk. The device includes at least one coil for generating a magnetic field in the brake disk and for detecting an eddy current in the brake disk. 
     2. Discussion of Background Information 
     A “carbon ceramic brake disk” is a brake disk comprising a carbon ceramic, wherein the carbon ceramic comprises a ceramic matrix as well as carbon fibers embedded in the matrix. 
     In such carbon ceramic brake disks an oxidation of the carbon fibers and therefore wear occurs due to high operating temperatures. This wear cannot be reliably recognized by mere optical inspection. An improved recognition of wear is achieved by inductive methods of measurement. The principle used in this type of measurements is based on eddy current damping; either using two coils (EP 1 387 166) operating continuously, or one or two coils in pulsed operation (DE 10 2008 051 802). The disclosures of EP 1 387 166 and DE 10 2008 051 802 are expressly incorporated by reference herein in their entireties. In these documents the excellent correlation between the inductive measurement and a gravimetric determination of the wear is disclosed. A conventionally traded profometer (www.proceq.com) is used as measuring device. Using these types of techniques, it is not necessary to disassemble the brake disk for measurement. It is also noted that the measuring values are independent on soiling and the presence of liquids. 
     The by far largest problem of such procedures lies in the fact that, due to the unavoidable inhomogeneity of the material, the measured values can differ strongly as a function of location (variations up to 100% are observed). A further variation is caused by the venting ducts extending within the disk. A conventional device can, upon a dislocation of a few millimeters, display a value that deviates by more than 10%. Since the drop of the measured value between a new and a worn disk is at approximately 40 to 50%, such a location dependence is greatly disadvantageous for a measurement. 
     DE 10 2008 051 802 describes a positioning technique by a gauge and mechanical positioning device, which, however, is found to non-viable. 
     Further it must be noted that reliable measuring values can only be recorded when the measuring device lies exactly against the disk, which leads to additional requirements regarding the shape of the device if the same is to be operated by hand at a location of service. 
     A further disturbing factor is due to the metallic elements, such as the caliper, axle shaft and fender, present around a built-in brake disk. 
     SUMMARY OF THE EMBODIMENTS 
     Therefore, embodiments of the invention are directed to a device of the type mentioned above that allows a more reliable measurement of the wear. 
     According to embodiments, the device for determining wear in a carbon ceramic brake disk includes a coil arrangement having at least one coil adapted and structured to generate a magnetic field in the brake disk and for detecting an eddy current in the brake disk. The coil arrangement has an arcuate measuring area. 
     In this context, the “measuring area” is the area in a measuring plane (i.e., in a plane parallel to the plane of the coils) that is reached by the field of the coil or coils during the measurement. In particular, the measuring area includes those locations in the measuring plane where the flux of the magnetic field generated by the coil(s) is at least 50% of a maximum value of the flux of the magnetic field in the measuring plane. 
     By the measuring area in accordance with the embodiments, a non-local measurement can be carried out over an extended region of the brake disk, which makes the measurement less sensitive to local inhomogeneities of the carbon ceramic. 
     Advantageously, the coil arrangement comprises at least three coils, in particular more than three coils, as well as a drive for generating, by means of the coils, a magnetic field in the measuring area. 
     Embodiments of the invention are directed to a device for determining wear in a carbon ceramic brake disk. The device includes a coil arrangement having at least one coil structured and arranged to generate a magnetic field in the brake disk and to detect an eddy current in the brake disk, and an arcuate measuring area. 
     According to embodiments, the arcuate measuring area may be positionable in a ring having a radial width smaller than 2 cm and an inner radius between 10 and 15 cm, and a length tangentially along the ring of at least 8 cm. 
     In accordance with other embodiments of the invention, the at least one coil can include at least three coils structured and arranged to generate a magnetic field in the measuring area. 
     Further, the at least one coil may include more than three coils structured and arranged to generate a magnetic field in the measuring area. The device can also include a common carrier plate on which the coils. The coils can be formed by conductive leads on the carrier plate. Moreover, each coil can extend around a center and the centers of the coils may be arranged on an arcuate curve. The centers of the coils can be arranged on a segment of a circle. Still further, each coil may extend around a center and the centers of the coils may be arranged on at least two arcuate, parallel curves. The centers of the coils can be arranged on at least two segments of concentric circles. The device may also include a common carrier plate on which the coils are arranged, and the common carrier plate one of forming a measuring surface or abutting on an inner side of a wall section forming the measuring surface. Further, each two neighboring coils on different curves can be poled anti-parallel to each other. 
     According to still other embodiments of the instant invention, the device may further include a voltage supply and a driver. The driver can be structured and arranged to connect the coils, in parallel to each other, to the voltage supply. The driver can be structured and arranged to disconnect the coils from the voltage supply and to sum inductive voltages generated over the coils after disconnecting the coils from the voltage supply. 
     In accordance with further embodiments of the invention, mutually neighboring coils can be poled anti-parallel to each other. 
     According to still other embodiments, the device may also include a housing and stops arranged on the housing being structured and arranged to radially abut against the brake disk. The device can also include a measuring surface structured and arranged to abut against a front side of the brake disk. The stops can include projections extending transversally to the measuring surface. The projections may include rollers. The device can also include a light source for generating a light field as a positioning aid. The light source and the stops can be mutually arranged to generate a light strip on a front face of the brake disk upon abutting the device against the brake disk. 
     According to still other embodiments, the device can also include a light source structured and arranged to generate a light field as a positioning aid. 
     In accordance with still yet other embodiments of the present invention, a diameter of the at least one coil can be between 10 and 15 mm. 
     Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein: 
         FIG. 1  shows a first perspective view of the device; 
         FIG. 2  shows a view of the device abutting against a brake disk; 
         FIG. 3  shows a representation of the arrangement of the measuring coils; 
         FIG. 4  is a schematic representation of the measuring area on the brake disk; 
         FIG. 5  shows a measured value as a function of the angle; 
         FIG. 6  is a partial block circuit diagram of the device; and 
         FIG. 7  shows a device with two rows of measuring coils. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the present invention only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show structural details of the present invention in more detail than is necessary for the fundamental understanding of the present invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the present invention may be embodied in practice. 
     The device illustrated in  FIGS. 1 and 2  include a housing  1  with a measuring surface  2  intended to abut against a front face  3  of brake disk  4  during the measurement. Further, stops  5 ,  6  are provided on housing  1  to extend transversally, in particular, perpendicularly, to measuring surface  2 . Stops  5 ,  6  are arranged to radially abut housing  1  against an outer edge  7  of brake disk  4 , i.e., they are used to radially align housing  1  with respect to brake disk  4 . Stops  5 ,  6  are designed as projections that extend over measuring surface  2 . Advantageously, exactly two such projections are employed in order to ensure a defined radial abutment on brake disk  4 . 
     The device further includes a display  8 , advantageously arranged on a side of housing  1  that is opposite to measuring surface  2  so that it can be seen by the user. When the device is abutting correctly against brake disk  4 , a side  9  of housing  1  is arranged to face the axle of brake disk  4 . Further, side  9  includes a light source  11 , which can include, e.g., a semiconductor laser having a light beam is extended in a direction perpendicular to front face  3  to form a planar light field  12 . Planar light field  12  can serve as a positioning help because light source  11  and stops  5 ,  6  should preferably be mutually aligned so that, when the device correctly abuts against brake disk  4 , a strip of light  13  is created on front face  3 . The user can use strip of light  13  for azimuthally aligning the device in a defined manner with respect to one or more marks  14  arranged on brake disk  4 . This solution requires no additional mechanical marker and is suited for all disk sizes. For better identification, light source  11  may be modulated in brightness. 
     The user can place housing  1  against brake disk  4  in the manner shown in  FIG. 2 , where the projections  5 ,  6  align the device radially and measuring surface  2  provides an axial alignment. To move the device to the correct azimuthal angle position, the user moves it along the circumference of brake disk  4 . To simplify this motion, projections  5 ,  6  can be formed by rollers, e.g., rotatable cylinders, which roll along outer edge  7  of brake disk  4 . 
     As has been mentioned, the measurement is carried out by one or more coils.  FIG. 3  shows an advantageous coil arrangement with several coils  15 . In this embodiment, coils  15  are arranged side by side in a row, such that their centers lie along an arcuate curve  16 , in particular a segment of a circle. The center of a circular coil is understood to be the axis that the coil is wound around. 
     Coils  15  form an arcuate measuring area  18 , as it is shown in dashed lines in  FIG. 4 . Measuring area  18  lies within a ring  19  between two concentric circle lines  20 ,  21 . Radial width D of the ring (i.e., the distance between circle lines  20 ,  21 ) is smaller than 2 cm. Length L of measuring area  18  measured tangentially along the ring is at least 8 cm. Inner radius R of ring  19  lies between 10 and 15 cm. With a measuring area  18  of this type, a substantial region of a conventional brake disk can be reached, without metallic parts of the attachment or edge regions of the brake disk falling within measuring area  18 . 
     Instead of a single row of coils  15 , it is also possible to use at least two rows of coils  15 . This is illustrated in  FIG. 7 , where the two rows of coils  15  are arranged on two parallel, arcuate curves  16 ′,  16 ″, in particular, on two segments of concentric circles. Advantageously, each two neighboring coils  15  arranged on different curves are poled anti-parallel, as it is shown by the signs + and − in  FIG. 7 . In this manner, the field of one coil is deflected into the respective neighboring coil, so that it extends through a substantial volume of brake disk  4  without extending very deeply into brake disk  4 . This allows preventing the field from exiting through the opposite side of brake disk  4 , where it might be affected by metal parts. 
     In principle, also in the embodiment of  FIGS. 3 and 4 , each two neighboring coils can be poled anti-parallel. However, it has also been found that the use of anti-parallel poled coils in an arrangement with two rows of coils according to  FIG. 7  is particularly advantageous. 
     The term “poled anti-parallel” as used in the embodiments is to be understood to mean that the fields generated by the two coils are anti-parallel to each other. This can be achieved, e.g., by winding the two coils in opposite winding directions and by sending currents of equal phases through them, or by winding the two coils in the same winding direction and by sending oppositely phased currents through them. 
     The coils  15  shown in  FIG. 3  may be advantageously arranged on a common carrier plate  25 , which simplifies their mounting and mutual alignment. Advantageously, they are designed as concentric conducting leads on carrier  25 , implemented as a multi-layer printed circuit. 
     In the embodiment shown in  FIG. 3 , carrier plate  25  lies against a wall section  26  of housing  1  that forms measuring surface  2 . Advantageously, carrier plate  25  is laminated to wall section  26 . Alternatively, carrier plate  25  can form the outer wall of housing  1  and therefore measuring surface  2  itself. Both these embodiments allow positioning of coils  15  close to and in very well defined spatial relation relative to the surface of brake disk  4 . In particular, the distance between coils  15  and the sample does not vary when the force pressing the one against the other changes. This is important because a variation of the distance by only a few tenths of a millimeter can lead to very large signal variations. 
     Coils  15  have a diameter that corresponds approximately to the half thickness of the sample such that their field extends sufficiently deep into the brake disk  4  without a substantial part of the field exiting from the opposite side of brake disk  4 . In order to fulfill these requirements for typical brake disks, an advantageous diameter of coils  15  is in a range between 10-15 mm. If the coils are non-rotationally symmetric, this is the diameter tangential to the brake disk if the measuring device is applied in its measuring position against brake disk  4 . 
     A review of  FIG. 5  shows that the selected geometry satisfies the requirements. The wiggly line shows the position dependence of the signal when measuring with a single coil whose diameter corresponds approximately to the disk thickness. A part of the modulation is caused by the venting channels—overlaid and non-periodic are variations due to the natural inhomogeneity of the composite. The smoothed curve is created when sampling with the device described here. The vertical axis shows the measuring value, in linear units, the horizontal axis the angle or azimuthal position of the device along the outer edge of the brake disk  4 . 
       FIG. 6  shows a possible embodiment of the coil circuit. Accordingly, a driver  30  is provided, which controls the operation of coils  15  and generates a magnetic field in measuring area  18  via coils  15 . An electronic switch  31  is attributed to or associated with each coil, i.e., coils  15  can, by closing switches  31 , be connected, parallel to each other and to the supply voltage from a voltage source  32 . This parallel configuration allows using a voltage source  32  with low voltage and without voltage converter, such as a simple battery. When switches  31  are interrupted, coils  15  are disconnected from the voltage supply and an inductive voltage is generated over each coil due to the eddy currents in brake disk  4 . These inductive voltages are added computatively or electrically by driver  30 . In this manner, a comparatively strong signal is generated even if only a low supply voltage is used. 
     Advantageously, as shown in  FIG. 1 , the device has an interface  39  for exchanging data with external equipment, e.g., in order to generate a protocol of the measurements that have been carried out. Further, one or more buttons  40  can be arranged on the device for storing and/or marking a current measuring value. 
     In principle, it is also possible to equip the device with a single coil only, which has an arcuate cross section. However, as such a coil has a high inductance, it needs more power and is slower in operation. In addition, its field reaches deeply, which gives rise to a risk that components arranged behind the brake disk may be included in the measurement. For these reason, it is advantageous to use several coils, and in particular more than three coils. 
     It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to an exemplary embodiment, it is understood that the words which have been used herein are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular means, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims.