Abstract:
A sensor element includes an acoustic emission sensor for detecting acoustic emission. The sensor element has a second sensor for a second measured variable which is different from acoustic emission. Furthermore, a sensor element is provided, which includes an acoustic emission sensor for detecting acoustic emission and includes an interface for receiving an external sensor signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and hereby claims priority to International Application No. PCT/EP2012/056697 filed on Apr. 12, 2012, the contents of which are hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    The invention relates to a sensor element with an acoustic emission sensor for measuring acoustic emission. 
         [0003]    The condition monitoring of industrial installations is becoming increasingly important. The term ‘Acoustic Emission’ is used below. This term has established itself in the technical domain as a precise designation of a technology with which structure-borne sound is measured which does not occur in the case of reversible material changes, but only in the case of irreversible material changes. An evaluation of structure-borne sound in the ultrasound range (acoustic emission) is recognized as a tool for identifying material defects and material fatigue processes. In a range of applications, acoustic emission provides characteristic signals enabling an inference to be made regarding the process to be monitored, for example for bearing monitoring, tool monitoring or corrosion detection. The acoustic emission signal alone does not often provide evidence which is sufficiently clear. For example, heating processes similarly generate an acoustic emission due to thermal expansion. 
         [0004]    Sensors for measuring acoustic emission are typically manually produced piezo sensors with a broadband or resonant characteristic. Measuring systems are available for general laboratory applications or for special applications, such as tool monitoring on machine tools. These systems evaluate only the acoustic emission signal. The pure evaluation of the measured acoustic emission signals is susceptible to noise signals and misinterpretations. Following the transfer of the acoustic emission data from the acoustic emission sensor into a higher-order device, a correlation with other measured quantities can be carried out (for example by MATLAB on the PC). However, the devices required for this purpose are complex and costly, and are unsuitable for an integration into industrial environments. 
       SUMMARY 
       [0005]    One possible object is to provide a sensor element with an acoustic emission sensor with which the performance of measurement tasks is simplified. Furthermore, an potential object is to provide a monitoring system, in particular a corrosion monitoring system, a bearing monitoring system or a machine monitoring system with which the performance of measurement tasks is simplified. 
         [0006]    The inventors propose that the sensor element with an acoustic emission sensor for measuring acoustic emission comprises a second sensor for a second measured quantity which is different from acoustic emission. As a result, a processed (refined) sensor output can be provided with only one sensor component and cost for a further component, wiring cost and/or cost for a subsequent processing of the raw measured values can be at least partially saved. Furthermore, a precise positioning of the second sensor in relation to a position of the acoustic emission sensor is thus reliably ensured. 
         [0007]    The inventors also propose a sensor element with an acoustic emission sensor for measuring acoustic emission comprises an interface for picking up an external sensor signal. 
         [0008]    The external sensor signal can be provided, for example, from a rotational speed sensor or a different sensor which cannot be integrated into the sensor element due to the remoteness of the measurement location or for structural reasons. A rotational speed measurement is often advantageous for the evaluation of condition monitoring sensors, since the diagnosis quality can be significantly improved by the additional evidence from a supplementary sensor. Furthermore, a rotational speed measurement by synchronization with periodic disturbance quantities enables an improved suppression of these disturbance quantities. 
         [0009]    In terms of the monitoring system, the monitoring system comprises the proposed sensor element. 
         [0010]    Embodiments provide that the second sensor is a temperature sensor for measuring a temperature level and/or a temperature gradient, or that the second sensor is an oscillation sensor for measuring an oscillation characteristic, or that the second sensor is a magnetic field sensor for measuring a magnetic field strength and/or a magnetic field direction. The oscillation sensor can also be referred to as a vibration sensor. The selection of the sensors can be adapted according to the monitoring task. 
         [0011]    A 3D Hall-effect sensor, for example, can be used to measure the magnetic field strength and/or the magnetic field direction. The measurement of a magnetic fingerprint which is characteristic of a machine condition, is thus possible. Different evaluation strategies are conceivable: evaluation of an intrinsic magnetic field of the machine (for example on a motor) and/or a rotational speed determination from a magnetic field change of a rotating magnetic field of an electric motor or an electric generator. It is also possible to evaluate a modulation of a magnetic field (“DC magnetic field”), the direction of which remains constant, in order to determine a rotor position of a linear motor by evaluation of a shunt change on end stops or on the arresting of the rotor. If a 3D magnetic field sensor is used, the alignment of the sensor in relation to the magnetic field is uncritical, since the magnetic field vector can be evaluated. 
         [0012]    Advantageous further developments of the sensor element comprise a third sensor for measuring a temperature level, a vibration characteristic and/or a magnetic field strength and/or a magnetic field direction. 
         [0013]    Also under the first aspect, the sensor element can comprise an interface to pick up an external sensor signal. Advantages resulting therefrom have already been explained. 
         [0014]    It is preferred if the sensor element comprises an evaluation device to generate a consolidated and/or condensed sensor signal by evaluation of a sensor signal of the acoustic emission sensor, taking into account the second measured quantity and/or the external sensor signal. The sensor can comprise one or more algorithms for signal fusion of the measured quantities. An algorithm of this type may comprise, for example, a simple threshold value monitoring or a correlation calculation between two measured quantities. The algorithms can be available as diagnosis blocks which can be separately or jointly activated and/or deactivated. 
         [0015]    It is particularly preferred if a program code is loadable into the evaluation device and/or if a program code is executable in the evaluation device. Application-specific evaluation algorithms can thereby be loaded into the sensor element separately or combined with one another and optionally executed there. It can be provided that the program code can be loaded into the sensor element via a different interface or via the same interface as the program code. 
         [0016]    It is similarly advantageous if the evaluation device is prepared in order to carry out a correlation between signals which are measurable by the first and the second sensor and/or by the first and the third sensor and/or by the first and the fourth sensor and/or by a pair of the second to the fourth sensors. The reliability of a condition characteristic value selected by the sensor element can thereby be increased. 
         [0017]    Embodiments provide that the evaluation device is prepared in order to carry out a correlation between the external sensor signal and a sensor signal of the first and/or the second and/or the third and/or the fourth sensor. The reliability of a condition characteristic value selected by the sensor element can thereby also be increased. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
           [0019]      FIG. 1  shows a schematic block diagram of a sensor element, and 
           [0020]      FIG. 2  shows, not to scale, a variation with time in a plausibility characteristic value depending on different, similarly shown, temporarily variable measured quantities. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0021]    Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
         [0022]    The monitoring system  60  shown in  FIG. 1  for monitoring a monitoring object  18  comprises a higher-order monitoring device  26  and a sensor element  10  connected thereto. The sensor element  10  comprises a plurality of sensors  11 ,  12 ,  13 ,  14  for physically different measured quantities, a data acquisition circuit  20 , an evaluation device  22  for acquired measured values  51 ,  52 ,  53 ,  54 ,  55  and an interface  24  for connecting the higher-order monitoring device  26 . 
         [0023]    The first sensor  11  is an acoustic emission sensor for generating electrical signals depending on a strength and/or direction of measured acoustic emission. The second sensor  12  is a temperature sensor for generating electrical signals depending on a measured temperature level and/or a strength and/or direction of a temperature gradient. The third sensor  13  is a vibration sensor for generating electrical signals depending on a strength, frequency and/or direction of measured vibrations. The fourth sensor  14  is a magnetic field sensor for generating electrical signals depending on a strength and/or direction of a measured magnetic field. 
         [0024]    Optionally, the sensor element  10  also comprises an interface  28  for feeding signals  55  from one or more external sensors  15 . Independently therefrom, signals  55  can also be fed from an external sensor  16  via the interface  24  which is provided for the connection of the sensor element  10  to the higher-order monitoring device  26 . One embodiment appropriate for some applications provides that the interface  24 ,  28  for the external sensor  15 ,  16  is prepared in order to feed a rotational speed signal  55  from a rotational speed sensor  15 ,  16  and/or a bearing current signal  55  from a bearing current sensor  15 ,  16 . 
         [0025]    With reference to  FIG. 2 , it will now be explained, using the example of a bearing diagnosis, how a plausibility characteristic value  46 , which is used as a measure of an applicability and/or validity of a measured acoustic emission activity  41 , can be generated by the sensor element  10  from measured values  51 ,  52 ,  53 ,  54 ,  55  of a plurality of physically different measured quantities  41 ,  42 ,  45 . In the example, it is assumed that the bearing  18  is operated in a normal operating phase  33  with a more or less constant normal operating rotational speed  450 . At the beginning of the commissioning of the bearing  18 , a run-up phase  31  initially takes place in which the rotational speed  42  is increased to the normal operating rotational speed  450 . The run-up phase  31  is followed by a warm-up phase  32  in which, although the normal operating rotational speed  450  has already been reached, the bearing  18  is only gradually heated to a normal operating temperature  420 . The commissioning phase therefore comprises a run-up phase  31  and a warm-up phase  32  which partially overlap one another in time. No bearing diagnosis is carried out during the commissioning phase  31 ,  32 . In the normal operating phase  33  after the commissioning phase  31 ,  32 , the rotational speed  42  is more or less constant. Temperature changes in the commissioning phase  31 ,  32  are therefore not caused by rotational speed changes. Bearing diagnoses which produce plausible results can be carried out during the quasi-stationary condition of the normal operating phase  33 . In the example, a substantial increase in the acoustic emission  41  and a slight to substantial increase in the temperature  42  are observed at the end  34  of the normal operating phase  33 . Increasing bearing wear can be inferred from the simultaneous occurrence of the substantial increase in the acoustic emission  41  in conjunction with the tangible temperature increase. This can be used in the sensor element  10  to generate a warning signal (with a corresponding condition characteristic value) in a timely manner in order to initiate maintenance measures. The sensor element  10  is flexibly parameterizable in order to implement an adaptation of the evaluation method according to specific applications or monitoring objects  18  (such as, for example, pumps, bearings, gears, fans, compressor monitoring). The data  52 ,  53 ,  54 ,  55  to be fused with the acoustic emission signal  51 , the respective fusion method and also evaluation rules and/or evaluation weightings are defined in each case for this purpose. Different application-specific methods of this type are described in detail below. 
         [0026]    Example of cavitation detection in pumps: A fusion of acoustic emission detection and temperature detection is appropriate, since cavitation is strongly temperature-dependent. A synchronization with the pump rotational speed  45  is required for the localization of the cavitation source. To do this, an external rotational speed input  28 , a network signal (e.g. of a PTP telegram) or an evaluation of a magnetic field sensor  14  of the sensor element  10  can be provided (PTP=Precision Time Protocol). The signal  53  of the vibration sensor  13  of the sensor element  10  represents an indicator of the severity of damage. If this additional signal  53  has a high intensity, a plausibility  46  of the acoustic emission signal  51  increases, justifying the initiation of a deactivation of the pump  18 . This plausibility  46  (as a probability) can be used as additional information to a condition characteristic value of the pump  18 . 
         [0027]    Example of bearing diagnosis: Acoustic emission occurs in the high frequency range in bearings  18  during a run-up phase  31  due to a thermal expansion of machine components  18 . Considered alone, this appears to reveal severe bearing damage. However, there is in fact no real damage signal, but rather material relaxation with expansion due to heating. An appropriate acoustic emission evaluation in order to assess the question of whether any bearing damage is present is possible only in the thermally stable condition. The detection and monitoring of the warm-up process by an additional temperature sensor  12  is appropriate in order to avoid too fast a run-up in the cold condition. An excessive heating results in a reduction in the bearing gap (bearing clearance) and in a ‘seizure’ of the bearing  18 . A viscosity of the lubricant and the type of friction can be inferred through fusion of temperature measurement and acoustic emission measurement. 
         [0028]    Example of bearing currents on engine bearings: Bearing currents similarly express themselves through acoustic emission  41 . The acoustic emission  41  typically correlates with an engine vibration, since the discharge in the bearing  18  always occurs at particularly high vibration amplitudes (at which a bearing clearance constricts to a minimum). A magnetic field sensor  14  also can similarly supply signals during bearing current events. A classification of the type of the bearing currents is possible with the sensor element  10 :
       Acoustic emission  41  and temperature increase are an indication of ohmic bearing current or bearing current due to spark erosion.   Bearing current flashovers with spark erosion usually occur with low-frequency vibrations of the installation. The lubricant film thickness is modulated, and acoustic emission  41  and magnetic field pulses occur during bearing current events. The resulting damage (groove formation in the outer ring and later polygonization of the inner ring) can be detected with a low-frequency vibration sensor  13 .       
 
         [0031]    The progress of bearing current damage and of the condition of the monitoring object  18  can be tracked by joint evaluation of acoustic emission data  51 , temperature data  52  and vibration data  53  (possibly magnetic field data  54  and rotational speed data  55  also, the latter e.g. by magnetic field measurement) in the evaluation device  22 . Alternatively or additionally to the rotational speed data  55 , data from an external bearing current monitoring  15 ,  16  can also be used in the joint evaluation as an external data signal  55 . 
         [0032]    The sensor element  10  preferably comprises a digital interface  24 . It is advantageous if the interface  24  supports an interface standard for a wired or for a wireless data connection (for example an Ethernet standard such as Fast Ethernet Physical, a CAN standard, a WLAN standard and/or Bluetooth). It is also appropriate if an adaptation can be carried out according to the specific application via the digital interface  24  along with the communication with the condition monitoring infrastructure  26 . Signals with or without a timestamp can be transmitted via the digital interface  24 . A transmission of the signals with a timestamp enables a synchronization with other system elements. As a further possible additional benefit, a localization of signal sources can be carried out independently thereof by timestamping and a plurality of sensors (for example on a pump head) via an amplitude or transit time method. 
         [0033]    It can be provided that characteristic values are transmitted or are internally stored in normal operation. The storage can be effected in a ring buffer. A further development can be provided that a histogram is produced with consolidation of the oldest values. 
         [0034]    A detailed analysis can be provided if damage events occur. A ‘snapshot’ of the measurement data  51 ,  52 ,  53 ,  54 ,  55  captured at high resolution can be transmitted for this purpose. A data compression can be used here. 
         [0035]    The sensor element  10  can differ from known sensor elements in one or more of the following features:
       A fusion of the sensor system for acoustic emission with additional quantities is supported in a sensor component  10  (in an integrated sensor component), wherein the additional quantities are, for example, a vibration, a temperature  42  and/or a magnetic field.   The sensor system  10  has integrated adaptable algorithms for fusion of the measured quantities and for acquiring additional information (for example rotational speed information  45  from a magnetic field change).   A probability  46  of the occurrence of consolidated condition characteristic values is determined by a plausibility monitoring of monitored condition data  51 ,  52 ,  53 ,  54 ,  55  and one of a plurality of possible condition characteristic values is selected as the result and is made available via the interface  24  of the higher-order monitoring device  26  as sensor output of the sensor element  10 .       
 
         [0039]    The sensor element  10  can offer one or more of the following advantages compared with known sensor elements:
       A simple adaptation of the sensor element  10  (of the integrated measurement system) to different measurement tasks is possible.   An integrated magnetic field sensor  14  enables rotational speed detection from the magnetic field, with no communication with the converter being required for this purpose.   The sensor element  10  is retrofittable at low cost, and its installation cost is low.   A plausibility check of acoustic emission signals  51  is possible through fusion with further measured quantities. The sensor element  10  is resilient to a misinterpretation of acoustic emission signals  51 .   The data volume is reduced due to the local data fusion of different physical quantities  41 ,  42 ,  45  in the sensor element  10  (in the integrated sensor element).   The wiring requirement is reduced, as a result of which the reliability of the monitoring system  60  is also improved.   The system costs for integration and multiple use of subsystems (communication interface, microprocessor, etc.) are reduced.   The adaptability of the sensor element  10  reduces type and part diversity and enables high quantities.       
 
         [0048]    The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention covered by the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in Superguide v. DIRECTV, 69 USPQ2d 1865 (Fed. Cir. 2004).