Patent Publication Number: US-7225062-B2

Title: Device for indicating the vibration level of a vehicle

Description:
The present invention relates to a device for indicating the vibration level of a vehicle. 
   BACKGROUND OF THE INVENTION 
   The field of the invention is that of analyzing vibration, and more particularly vibration in an appliance that is provided with an engine driving a rotary propulsion member. A typical example of an application of the invention is a helicopter, a vehicle which presents a high level of vibration that is due essentially to the main rotor serving to support the helicopter and to enable it to move. 
   It is important to characterize the vibration present in such a vehicle since vibration is a sign of stresses that can lead to a fatigue phenomenon and that therefore have a direct influence on safety. Vibration can also degrade the performance of various items of equipment installed on the vehicle. In addition, vibration in a helicopter cabin is instrumental in determining passenger and pilot comfort. It goes without saying that pilot comfort is also an essential feature for safety. 
   Thus, a system is known for continuously recording vibration within a helicopter, which system is known as a health usage monitoring system (HUMS). The system records a warning in a removable monitoring cartridge if the level of vibration exceeds a predetermined threshold at a frequency Ω, 2Ω, or BΩ, where Ω represents the speed of rotation of the rotor and B the number of blades mounted on the rotor. The warning can be detected only in a ground station equipped to play back the information recorded in the cartridge. The pilot does not have access to this information while on a mission. In addition, no means is provided to give an overall assessment of the vibration level that is not restricted to an identified frequency, but rather reflects the overall behavior of the vehicle. 
   OBJECTS AND SUMMARY OF THE INVENTION 
   An object of the present invention is thus to provide a device which indicates the vibration level of a vehicle, preferably in real time, and over a broad range of frequencies, by means of a single data item. 
   According to the invention, the device indicates at least a first vibration level derived from a first acceleration signal, and it comprises: 
   first analysis means for resolving the first acceleration signal into a plurality of axial analysis signals each occupying an identified frequency band; 
   a first memory in which a plurality of axial weighting coefficients are stored, each corresponding to one of said identified frequency bands; and 
   a first control circuit for summing the p th  powers of the axial analysis signal weighted by the axial weighting coefficients, thereby producing the first vibration level. 
   In addition, the device includes a first comparator for producing a first alert signal when the first vibration level exceeds a first predetermined threshold. 
   In addition, the device includes a first concentrator member arranged so that the plurality of axial analysis signals come from a plurality of measurement signals, each coming from a distinct sensor. 
   In a first option, the first concentrator member sums the measurement signals. 
   In a second option, the first concentrator member retains the greatest of said measurement signals. 
   Preferably, the device receives a second acceleration signal and comprises: 
   second analysis means for resolving the second acceleration signal into a plurality of radial analysis signals, each contained in an identified frequency band; 
   a second memory having a plurality of radial weighting coefficients recorded therein, each corresponding to one of said identified frequency bands; and 
   a second control circuit CC for summing the p th  powers of the radial analysis signals weighted by the radial weighting coefficients in order to produce a second vibration level. 
   In addition, the device includes a second comparator for producing a second alert signal when the second vibration level exceeds a second predetermined threshold. 
   In addition, the device further includes a second concentrator member arranged so that the plurality of radial analysis signals come from a plurality of evaluation signals, each coming from a distinct sensor. 
   In a first embodiment, the second concentrator member sums said evaluation signals. 
   In a second embodiment, the second concentrator member retains the greatest of said evaluation signals. 
   Advantageously, the device includes a combiner member for producing a warning signal when a combination of the two alert signals exceeds a third predetermined threshold. 
   By way of example, the combiner member sums the q th  powers of the alert signal, the value of q preferably being equal to 1. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention appears below in greater detail in the following description of embodiments given by way of illustration and with reference to the accompanying figures, in which: 
       FIG. 1  shows an analysis module of the invention; 
       FIG. 2  is a graphical representation of the axial and radial coefficients; and 
       FIG. 3  is a block diagram of a device of the invention. 
   

   Elements that are shown in more than one figure are given the same references in all of them. 
   MORE DETAILED DESCRIPTION 
   With reference to  FIG. 1 , a first analysis module MOD 1  comprises first, second, and third sensors S 1 , S 2 , and S 3 , disposed in the cabin of a helicopter to measure axial vibration, i.e. vibration directed along the axis of the rotor. Each of the three sensors delivers a respective first, second, or third measurement signal C 1 , C 2 , or C 3  which represents the axial acceleration to which the sensor is subject. The sensors are preferably accelerometers or strain gauges. 
   A first concentrator member MM 1  receives the three measurement signals C 1 , C 2 , and C 3  and combines them to form a first acceleration signal γ 1 . In a first option, it sums the measurement signals. 
   In a second option, it reproduces the greatest measurement signal as the first acceleration signal. 
   It should be observed that the invention also applies when only one sensor is provided, in which case it delivers the first acceleration signal γ 1  directly and the first concentrator member MM 1  is omitted. 
   First analysis means FT 1  transposes the first acceleration signal γ 1  from the time domain into the frequency domain; in other words, the means FT 1  performs the discrete Fourrier transform on the signal. Thus, in a first table T 1 , it produces a plurality of axial analysis signals γ 1   1 , γ 1   2 , . . . , γ 1   i , . . . , γ 1   n , giving the value for acceleration in successive narrow frequency bands, where the union of said bands defines the frequency domain of the analysis. 
   A first memory RM 1  contains axial weighting coefficients KA 1 , KA 2 , . . . , KA i , . . . , KA n  which correspond to the various frequency bands identified by the first analysis means FT 1 . These coefficients represent the relative importance of a frequency band in the general level of vibration of the cabin. 
   It will be understood that some kinds of vibration are much more harmful than others. Thus, if it is desired to monitor the balance of the helicopter rotor, frequencies that are multiples of the speed of rotation are particularly symptomatic. In contrast, if it is desired to pay attention more particularly to comfort, reference may advantageously be made to the standard ISO 2631 which, for a given duration of exposure, defines the maximum levels of vibration that are acceptable as a function of frequency. 
   With reference to  FIG. 2 , simple means for defining the axial weighting coefficient KA consist, for a given frequency (or frequency band) in subtracting from an arbitrary constant the value corresponding to the maximum level along the vertical axis or z axis as specified in that standard. Thus, in logarithmic coordinates, the representation of KA as a function of frequency approximates to a straight line of rising slope in the range 1 hertz (Hz) to 4 Hz, followed by a level portion in the range 4 Hz to 8 Hz, and terminated by a straight line of falling slope in the range 8 Hz to 100 Hz. 
   A first control circuit CC produces a first vibration level VIB 1  by summing the p th  powers of the axial analysis signals γ 1   i  weighted by the axial weighting coefficients KA i : 
   
     
       
         
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                 KA 
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   Advantageously, p is equal to 2. 
   A first comparator COMP 1  produces a first alert signal AL 1  if the first vibration level VIB 1  exceeds a first predetermined threshold TH 1 . 
   Preferably, the device has a second analysis module MOD 2  for producing a second vibration level VIB 2  and a second warning alert AL 2 . The second module is not shown in detail in the drawings, since it is analogous (in particular identical) to the first analysis module. 
   The second analysis module MOD 2  has fourth, fifth, and sixth sensors disposed in the cabin to measure vibration that is radial, i.e. perpendicular to the axis of the rotor. These three sensors deliver respective first, second, and third evaluation signals C 1 , C 2 , and C 3  representative of the radial acceleration to which the respective sensors are subject. 
   A second concentrator member receives the three evaluation signals for the purpose of combining them (in application of a function) to produce a second acceleration signal; this member preferably either sums the evaluation signals or else retains the greatest one of them. 
   Second analysis means transposes the second acceleration signal from the time domain into the frequency domain. This thus produces a second table comprising a plurality of radial analysis signals. 
   A second memory contains radial weighting coefficients KR 1 , KR 2 , . . . , KR i , . . . , KR n  which corresponds to different frequency bands identified by the second analysis means. 
   With reference to  FIG. 2 , a simple means for defining the radial weighting coefficients KR consists, for a given frequency (or frequency band), in subtracting from an arbitrary constant the value that corresponds to the maximum level along the x or y axis, i.e. in a horizontal plane. Thus, in logarithmic coordinates, the representation of KR as a function of frequency approximates to a straight line of slope that falls from 1 Hz to 100 Hz. 
   A second control circuit produces a second vibration level by summing the p th  powers of the radial analysis signals weighted by the radial weighting coefficients. Preferably, this circuit sums the second powers (p=2) of the weighted signals. 
   A second comparator produces a second alert signal AL 2  if the second vibration level exceeds a second predetermined threshold. 
   With reference to  FIG. 3 , the device of the invention thus comprises the first and second analysis modules MOD 1  and MOD 2  together with a combiner member AO. This member produces a warning signal W when any combination of the alert signals AL 1 , AL 2  exceeds a third predetermined threshold. Preferably, this combination is the sum of the q th  powers of the two alert signals, where a simple solution consists in giving q the value 1. 
   Naturally, the present invention is subject to numerous variations concerning its implementation. Although several embodiments are described, it will readily be understood that it is not possible to identify exhaustively all possible embodiments. Naturally, it is possible to envisage replacing any of the means described by equivalent means without going beyond the ambit of the present invention.