Abstract:
A slipring with at least one slip ring module having at least one sliding track, and at least one wear indication track. A wear indication circuit is connected to the at least one wear indication track for monitoring electrical properties of the wear indication track and signaling an abnormal slipring condition. A wear indication track may be exposed to higher load, higher rotation speed, higher brush pressure force, or other lifetime-reducing influence as compared to a normal sliding track.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of pending International Application No. PCT/EP2015/073764 filed on Oct. 14, 2015, which designates the United States and claims priority from the European Application No. 14188832.1, filed on Oct. 14, 2014. The disclosure of each of the above-identified patent documents is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to slip-rings for transmission of electrical signals between rotating parts. Specifically, it relates to wear monitoring in context of transmission of electrical signals between the rotating parts and a slip-ring device with improved wear monitoring. 
         [0004]    2. Description of Relevant Art 
         [0005]    Electrical slipring devices are used to transfer electrical power and/or signals between a rotating part and a stationary part of a given system. Such devices are used in different applications, such as wind energy plants or computer tomography scanners, for example. There are also several military and aerospace applications, in which sliprings find their use. 
         [0006]    It is common to all of these applications, that a long lifetime and a low contact resistance as well as a low contact noise are required from the slipring. Furthermore, in specific applications such as those involving a CT scanner, comparatively high speeds caused by a rotation of up to four revolutions per second in a circumference of about 5 meters require specific attention. The same applies for specific environmental requirements that appear in, for example, aerospace applications. 
         [0007]    Sliprings are generally constructed around a first part having sliding tracks and a second part having brushes for sliding on or gliding along the sliding tracks. Due to the mechanical friction there appears mechanical wear, which causes the slipring to degrade over time. 
         [0008]    U.S. Pat. No. 4,831,302 A discloses a wear indicator that determines the length of a carbon brush and therefore indicates the wear of the brush. In most sliprings the sliding tracks have significantly longer lifetimes than the brushes, but these sliding tracks are also susceptible to wear. Currently, there is no means providing an indication of the wear of sliding tracks. 
       SUMMARY 
       [0009]    The embodiments of the present invention are directed to providing a slipring having a reliable wear indicator, which is able to indicate wear of a slip-ring brush and/or of a sliding track. 
         [0010]    In a first embodiment a slipring module or a slipring assembly is provided that comprises a plurality of sliding tracks, at least one of which is configured as a wear indication (or wear indicator) track. The at least one wear indicator track preferably is a sliding track that is not used for any other signal transmission and is used for wear indication only. A sliding brush, operably corresponding to the wear indication sliding track, is a wear indication sliding brush. Together, the wear indication sliding track and wear indication sliding track form a wear indication track-brush pair. Preferably, there is a wear indication (evaluation circuit that evaluates the status and/or quality and/or functionality of the wear indicator track. The circuit may be configured to measure at least one of a voltage drop, a noise, a bit error rate, a temperature, a contact resistance and contact interrupts caused by a worn sliding track to generate a warning signal. In a very simple embodiment, there would be a detection of wear if the transmission is interrupted due to wear. Preferably, there are two wear indicator tracks, and the wear indicator sliding tracks or the brushes are electrically connected together to form a loop for a test current of the evaluation circuit. Most preferably, at least one of the wear indication tracks is exclusively connected to the wear indication circuit. 
         [0011]    Alternatively, the wear indication track may be used for transmission of a signal that is of low importance for the system into which the slipring assembly is integrated, but which can easily be detected such that a failure of the wear indication track can be recognized by identifying a failure of such low-importance signal. Preferably, any component of the wear indication track-brush pair has shorter lifetime, as compared to that of the remaining sliding tracks and sliding brushes of the slipring. Most preferably, a sliding track and/or a corresponding sliding brush from a wear indication track-brush pair is made pre-worn, in that has a degree of mechanical wear that is higher than a degree of wear of any remaining sliding tracks and/or sliding brushes. 
         [0012]    Therefore, it is preferred for a component of the wear indication track-brush pair to have a shorter lifetime than that of the remaining tracks and/or sliding brushes. The wear indication track-brush pair may have passed a pre-wear procedure (which may be a run-in procedure, and preferably a run-in procedure under conditions that accelerate wear of a given sliding track and/or sliding brush, such as high temperature, high speed, or similar conditions). 
         [0013]    In a related embodiment, the wear indicator track-brush pair is pre-used or pre-worn or is at least made such that it shows the signs of wear earlier than the other tracks. Therefore the design lifetime of the wear indicator track-brush pair is shorter than the design lifetime of any other track-brush pair. 
         [0014]    In an alternative embodiment, a component of the wear indication track-brush pair may be manufactured by a process causing a shorter lifetime of such component compared to any of the remaining tracks and/or brushes. Such a process may, for example, be a galvanic plating with a thinner layer than that of a plating layer of a component of any other track-brush pair, such that the galvanic plating is worn earlier than the thicker platings of the remaining tracks. 
         [0015]    In another embodiment, there is a higher stress level imposed on the wear indicator track-brush pair, which causes a component of the wear indicator track-brush pair to show the signs of wear earlier than any other track and/or brush. This may, for example, be done by applying a higher brush pressure while pressing the brush against a given sliding track. This may be done by using a stronger spring for a carbon brush or by using a stiffer wire for the wire brush. It may also be done by applying less grease or oil to the track. In a further embodiment, it may be done by applying a higher current or at least a higher current density to the brush, which may, for example, be accomplished by using a thinner brush wire. 
         [0016]    In another embodiment, there may be provided an additional gear configured to rotate the wear indicator track at a higher speed than the other tracks. 
         [0017]    In yet a further embodiment, there may be a sliding brush, preferably a wire brush, which wears quicker than other brushes. This may in addition provide a reliable brush wear indication. 
         [0018]    Another embodiment provides a plurality of wear indicator tracks and/or brushes that may be designed differently from one another. 
         [0019]    In a further embodiment, there may be present a shield configured to protect the other signal tracks from wear of the wear indicator track. 
         [0020]    In a further embodiment, a sensor may be provided. The sensor may be a temperature sensor, which for example may detect over-temperature or which even may detect the temperature profile of the slipring and calculate lifetime expectancy independent of temperature. For example, extremely high or low temperature may shorten a lifetime, whereas the use of the slipring at moderate temperature levels may lengthen the lifetime. There may be an optical sensor that, for example, may be configured to detect electrical arcs at the slipring. There may be a shock and/or vibration sensor for detecting mechanical vibrations, which may be an indication of a worn slip-ring module. It may also detect external vibration, which further would reduce the lifetime of the slip-ring assembly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    In the following, the idea and implementations of the invention will be described by way of example, without limitation of the general inventive concept, and with reference to the drawings. 
           [0022]      FIG. 1  shows a side view of a first embodiment. 
           [0023]      FIG. 2  shows a sectional view of the first embodiment. 
           [0024]      FIG. 3  shows a circuit diagram of a first embodiment. 
           [0025]      FIG. 4  shows a simplified circuit diagram. 
           [0026]      FIG. 5  shows a further embodiment using a sensor. 
           [0027]      FIG. 6  shows a modified embodiment. 
           [0028]      FIG. 7  shows a simplified block diagram of the wear indication circuit. 
       
    
    
       [0029]    While the implementations of the invention are susceptible to various modifications and alternative forms, specific embodiments are illustrated in the drawings and are described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular disclosed form, but to the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
       DETAILED DESCRIPTION 
       [0030]    In  FIG. 1 , a side view of a preferred embodiment is shown. A slipring assembly  100  comprises a slipring module  110  and a slipring brush block  120 . The slipring module  110  may rotate about the rotation axis  15  and comprises an isolating body  10  having a plurality of sliding tracks. Here, four sliding tracks  11 ,  12 ,  13 , and  14  are shown, although there may be any other number of sliding tracks present. The sliding tracks are embedded in and/or held by the isolating body  10 . Preferably, the sliding tracks are isolated from each other. There may also be configurations, where at least some of the sliding tracks are connected together electrically. Such configurations may be useful for transferring higher currents or signals with lower levels of noise. Here, a preferred embodiment of sliding tracks having V-shaped grooves is shown. These V-grooves provide the advantage that they can guide wires sliding on them and keep the wires precisely on predetermined tracks. Any other type of sliding track may be used instead, such as, for example, a track having multiple grooves or a track without grooves arid having a planar surface. 
         [0031]    The slipring brush block comprises a brush carrier  20 , which may be a printed circuit board or any other isolating material. It may also comprise a conducting material such as a metal, with isolated portions for holding the brushes. The brush block  20  holds a plurality of sliding brushes. In this embodiment, four wire brushes are shown, but there may be present any other number of brushes and any other kind of brushes. For example, the multi-fiber brushes or carbon brushes can be used. The brushes are spaced such that they fit to corresponding sliding tracks of the slipring module. There must not necessarily be one brush per sliding track, but there may also be a plurality of brushes contacting a given sliding track to increase current capability and/or reduce noise and/or contact resistance. 
         [0032]    In this embodiment, there are a first sliding brush  21 . (having a first section  21   a  and a second section  21   b,  as shown in a side view of  FIG. 2 ) that contacts first sliding track  11 ; a second sliding brush  22  that contacts a second sliding track  12 , a third sliding brush  23  that contacts a third sliding track  13 , and a fourth sliding brush  24  that contacts fourth sliding track  14 . Sliding brushes  22 ,  23 ,  24  are structured by analogy with the brush  21 . 
         [0033]    Preferably, the first sliding track  11  together with the first sliding brush  21  are used for wear indication. They may be used together with the second sliding track  12  and the second sliding brush  22 , as will be shown later. Of course any other sliding tracks together with their corresponding sliding brushes may be used for wear indication. 
         [0034]    In  FIG. 2 , a sectional view of the first embodiment is shown in a plane cut through lines A-A of  FIG. 1 . It is preferred that the slipring module have a free bore, for example for carrying cables. A connector  16  is shown, which may be a soldering point or soldering pin or a connector, which contacts the first sliding track  11 . A connecting cable may be soldered to this connector. Preferably, the other sliding tracks also have connectors to contact the sliding tracks from the inner side of the isolating body. 
         [0035]    In  FIG. 3 , a circuit diagram of a first embodiment is shown. The slipring assembly comprises a main signal path through the third sliding brush  23  together with the third sliding track  13 , and the fourth sliding brush  24  together with the fourth sliding track  14 , which is accessible through the first brush connection  41 , the second brush connection  42 , and the first ring connection  43  and the second ring connection  44 . The assembly further includes a wear indication circuit  50  that has a first test port  51  and a second test port  52  connected to the first sliding brush  21  and the second sliding brush  22 , which are in contact with the first sliding track  11  and the second sliding track  12 . Both sliding tracks are connected to each other, thereby allowing a current flow between the first test port  51  and the second test port  52 . The test results are output via the signal port  53 . The third sliding brush  23  together with the third sliding track  13 , and the fourth sliding brush  24  together with the fourth sliding track  14  are used for normal signal and/or power transmission over the slipring assembly. As the normal signal paths and the sliding tracks and brushes used for wear detection are completely separated, the overall design is comparatively simple. No care must be taken about electrical connections, unwanted currents and noise. 
         [0036]    In  FIG. 4 , a simplified circuit diagram is shown. In this embodiment, only three sliding tracks together with brushes are used. Here, the second test port  52  of the wear indication circuit  50  is connected to the first signal path comprising third sliding brush  23  and the third sliding track  13 . Here, at least a common sliding track and a sliding brush are shared with the main signal path. This configuration reduces the number of required tracks. In this example sliding brush  22  and sliding ring  12  are no more required. 
         [0037]    In  FIG. 5 , a further embodiment having a sensor is shown, using a sensor  69  which may be connected by a sensor port  54  to the wear indication circuit  50 . Here, only one connecting line is shown, which may comprise a plurality of electrical wires, as may be required by the sensor. The sensor may be a temperature sensor, which for example may be configured to detect over-temperature or even the temperature profile of the slipring and may allow the wear indication circuit to calculate lifetime expectancy independent from temperature. For example, extremely high or low temperature may shorten the lifetime, whereas using the slipring at moderate temperature levels may lengthen it. There may be an optical sensor that, for example, may be configured to detect electric arcs at the slipring. There may be a shock and/or vibration sensor for detecting mechanical vibrations, which may be an indication of a worn slipring module. It may also detect external vibration, which further would reduce the lifetime of the slipring assembly. 
         [0038]    In  FIG. 6 , a modified embodiment is illustrated. Here, the wear indication circuit  50  is connected to the sliding tracks  11  and  12 , whereas the short circuit is at the brushes  21  and  22  that are connected together. A similar modification may be applied to any embodiments, because a slipring may be operated in any direction. 
         [0039]    In  FIG. 7 , a simplified block diagram of the wear indication circuit  50  is shown. A test signal source  61 , which preferably is a DC current or voltage source (which also may be an AC current or voltage source), is connected via the first test port  51  and the second test port  52  to at least one sliding track and/or at least one sliding brush as shown above. The test signal source  61  may be controlled by an evaluation circuit  68 . This evaluation circuit  68  may set up a specific current or voltage profile. For example, during short periods, a comparatively high current may be delivered to the slipring for measuring the high-current performance. A series resistor  62  may be provided for measuring the current flowing through the sliding brush and sliding track, although current measurement may be done by other means like a hall sensor detecting the magnetic field of the current or a current transformer for measuring an AC current. The voltage at the series resistor  62  may be amplified by current measurement amplifier  63  and delivered to evaluation circuit  68 . A voltage measurement amplifier  64  may be provided for measuring the voltage between the at least one sliding brush and the sliding track connected to the first test port  51  and second test port  52 . Under normal operating conditions, the resistance of the slipring connection between the sliding brushes and sliding tracks may be comparatively low, so the voltage drop should be comparatively low. With increasing wear of the slip-ring, the voltage drop will increase. There may further be an AC voltage measurement amplifier  65 , which may be coupled via a capacitor  66  for measurement of AC or RF signals. Such signals may arise from contact noise, which may also increase with wear. Furthermore, a sensor amplifier  67  may be provided for delivering a signal in relation to the output of a sensor  69 , connected to sensor port  54 , to the evaluation circuit  68 . There may be a signal port  53  connected to the evaluation circuit  68 , by which the evaluation circuit  68  may signal an abnormal condition, a slip-ring OK signal, or even a complex numerical output, like the estimated total lifetime, the remaining lifetime, the total number of revolutions, or the estimated number of remaining revolutions. It is preferred if the evaluation circuit is a microcontroller, and it is further preferred if the signal port  53  is a port of a bus system. Such a bus system may be a CAN bus or any other industrial control bus, or Ethernet or any wireless communication interface. 
         [0040]    It will be appreciated by those skilled in the art that this invention provides sliprings for transfer of electrical signals and power. Modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           10  isolating body 
           11  first sliding track 
           12  second sliding track 
           13  third sliding track 
           14  fourth sliding track 
           15  rotation axis 
           16  connector 
           20  brush carrier 
           21 ,  21   a,    21   b  first sliding brush 
           22  second sliding brush 
           23  third sliding brush 
           24  fourth sliding brush 
           41  first brush connection 
           42  second brush connection 
           43  first ring connection 
           44  second ring connection 
           50  wear indication circuit 
           51  first test port 
           52  second test port 
           53  signal port 
           54  sensor port 
           61  test signal source 
           62  series resistor 
           63  current measurement amplifier 
           64  voltage measurement amplifier 
           65  AC voltage measurement amplifier 
           66  capacitor 
           67  sensor amplifier 
           68  evaluation circuit 
           69  sensor 
           100  slip-ring assembly 
           110  slip-ring module 
           120  slip-ring brush block