Abstract:
A method for detecting particles in a fluid stream includes generating a measurement field that can be passed through by the fluid stream, acquiring and evaluating measurement values of the fluid stream passing through the measurement field, and detecting at least one particle by way of a distinctive sequence of measurement values. Each of the distinctive successions of measurement values is acquired and evaluated to determine if a particle or a gas bubble is passing through the measurement field.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method for detecting particles in a fluid stream, comprising generating a measurement field that can be passed through by the fluid stream, recording and evaluating of measurement values of the fluid stream passing through the measurement field, and detecting at least one particle, in each case by a characteristic sequence of measurement valves. 
     BACKGROUND OF THE INVENTION 
     Such a method is known, for example, from WO 2007/088015 A1. In the known method, a magnetic field covering at least sections of the fluid stream is generated by at least one field coil, and, via two sensor coils, each of which may be connected to an evaluation device. The presence of a particle in the fluid stream is detected from the signal induced in the respective sensor coil. If a particle flows through the section or the magnetic field covered by the field coil, the voltage induced in the respective sensor coil changes in such a way that a detection signal may be tapped and evaluated via the evaluation unit. In the known method, two sensor coils wound in opposite directions are used. As a result of those two sensor coils the sensitivity with respect to the particle to be detected is increased. For example, smaller particles having a size of 50 to 100 micrometers may also be detected. 
     By such methods, particles are detected in the fluid or fluid stream in a system for cooling and/or for lubricating components of a drive unit with the aid of a fluid, in particular for cooling and/or for lubricating a gear unit of a wind turbine. Maintenance of such systems in wind turbines is particularly costly due to the high mounting location of the gear unit. Detecting at an early stage particle loads caused by wear of mechanical components or by dirt entering from the outside that may particularly damage to the cooling system and/or lubricating system is therefore desirable. In addition to the particles, gas bubbles, in particular air bubbles, are sometimes also detected as particles, to be trapped in the fluid guided through the respective system. Because of the false positive identification of air bubbles as particles, early damage detection is adversely affected, since an actual rise in the particle count rate may only occur beyond the signal noise to be associated with the air bubble signals. Consequently, an excessively high particle load is sometimes indicated and potential damage is indicated prematurely. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an improved method for detecting particles in a fluid stream, which enables the actual particle load of the fluid in a fluid stream to be reliably determined. 
     This object is basically achieved by a method where the characteristic sequence of measurement values is recorded and evaluated in each case in terms of whether a particle or a gas bubble is passing through the measurement field. 
     Typically, air bubbles are detected as gas bubbles. The gas bubbles or air bubbles may appear due to foam in gear units, which are lubricated with the aid of the fluid in the fluid stream. The gear unit is, in particular, a wind turbine gear unit, which is difficult to access at its high mounting location. According to the invention, a gas bubble detection takes place to the extent that a gas bubble as such is detected and is differentiated from a particle. The relevant particle load, in other words, particle contamination, includes exclusively the actual particles present in the fluid or fluid stream. 
     The method according to the invention may be used to measure and monitor the load or contamination of a fluid stream with particles. However, measuring and monitoring the load of a fluid stream with gas bubbles can be done, for example, to determine an associated foam formation. The method according to the invention is not limited in use to fluids or fluid streams in a system for cooling and/or lubricating components of a drive unit, such as a gear unit. Additional applications in the manufacturing, in the transport and in the processing of fluids of any type are possible. 
     In one preferred variant of the method according to the invention, the measurement values are each recorded and evaluated for an observation period, which period is greater than the measurement duration for a sequence of measurement values characteristic of a particle and/or a gas bubble. This variant of the method is appropriate, in particular if the sequence of measurement values characteristic of a particle has a measurement duration that differs from that of a sequence of measurement values characteristic of a gas bubble. As soon as a particle or gas bubble is detected by the associated characteristic sequence of measurement values, it is verified whether the measured sequence is in fact characteristic of a particle or of a gas bubble by observing the further curve of the measurement values beyond the respective measurement duration. Preferably, the observation period is at least double, preferably 3.5 times the corresponding measurement duration. 
     The sequence of measurement values characteristic of a particle typically has a signal shape. Another sequence of measurement values characteristic of a gas bubble has another signal shape. Furthermore, the signal shape for a particle and the other signal shape for a gas bubble may at least partially coincide, in other words, follow, at least in sections, the same curve. In this preferred variant, the method according to the invention may be very efficient, i.e., may be carried out with minimal expenditure of time or measurement effort, since initially the particle characterized by the shorter signal, a particle or a bubble, is detected, and with an immediately following measurement, a determination is made whether in fact this particle or the particle characterized by a longer signal shape, a gas bubble or a particle, is present in the fluid stream. 
     For example, the signal shape for a particle may have at least one extremum, preferably two extrema, and/or follow a periodic curve, preferably a sinusoidal curve. The other signal shape for a gas bubble may also have at least three extrema, preferably four extrema and/or follow a periodic curve, preferably a sinusoidal curve. Each extremum is expediently identified and evaluated by the exceeding of and the subsequent dropping below an upper limit value or by the dropping below and the subsequent exceeding of a lower limit value of the recorded measurement values. In the case of signal shapes for the particle and the gas bubble having a different number of extremum values or extrema, with the number of consecutively measured extrema, whether a particle or a gas bubble is present in the fluid stream can be determined. 
     To avoid a falsification of the measurement result caused by a noise of the measurement values, such noise is taken into consideration in a preferred variant of the invention in such a way that only measurement values outside a predefined noise band between a lower noise value and an upper noise value are taken into consideration. 
     Additional advantages and features of the invention are apparent from the figures and the following description of the drawings. The aforementioned features and those cited below may, according to the invention, be implemented alone or in arbitrary combinations in conjunction with the method according to the invention. 
     Other objects, advantages and salient features of the present invention will become apparent from the following detailed description, which, taken in conjunction with the drawings, discloses a preferred embodiment of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the drawings that form a part of this disclosure and that are purely schematic and are not to scale: 
         FIGS. 1 a  and 1 b    are a flow chart of a method according to an exemplary embodiment of the invention; and 
         FIGS. 2 a -2 d    show curves of measurement values recorded and evaluated for a defined observation period, taking into account a noise band, lower and upper limit values, a sequence of measurement values characteristic of a particle and/or another sequence of measurement values characteristic of a gas bubble. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1 a    shows the steps of the method according to the invention from the first start B 1  of the registering of a characteristic sequence of measurement values to the second start B 2  of the identification or verification of whether a particle or a gas bubble has been detected. In the first step S 1 , it is verified whether eight measurement values or data points lie within a noise band. The noise band is defined by an upper noise value, noise level+, and a lower noise value, noise level−. In the exemplary embodiment shown, a particle is characterized by a signal shape having two consecutive extrema E 1 , E 2 , and a gas bubble is characterized by another signal shape having four consecutive extrema E 1 -E 4 .  FIGS. 2 a  through 2 c    each show the signal shape for a particle.  FIG. 2 d    shows the other signal shape for a gas bubble. A pause between two consecutive signal shapes characteristic of a particle or a gas bubble, in other words, a pause between two signals, is identified via at least eight measurement values or data points within the noise band N. As long as at least eight data points do not lie within the noise band N, it is possible, for example, that a preceding sequence of measurement values, or a signal sequence characteristic of a particle or gas bubble, for example, is not yet completed. A characteristic signal shape, in other words, a signal, includes far more than eight measurement values or data points P 1 -P 8 . 
     In the second step S 2 , it is queried whether the data points or measurement values exceed an upper trigger level t+ or drop below a trigger level t−. The upper trigger level t+ and the lower trigger level t− represent inasmuch an upper limit level and a lower limit level. In the curve of the measurement values, or the measurement signals, shown in  FIG. 2 a   , the measurement values lying between the data points P 2  and P 3  and the measurement values lying between the data points P 6  and P 7  meet this condition. 
     In the third step S 3 , it is evaluated, whether a maximum having measurement values above the upper trigger level t+ or a minimum having measurement values below the lower trigger level t− is present.  FIG. 2 a    shows a measurement signal having a maximum value M 1  as the first extremum E 1  between the data points P 2  and P 3 , as well as a minimum value M 2  as the second extremum E 2  between the data points P 6  and P 7 . 
     In the fourth step S 4 . 1  for a minimum and S 4 . 2  for a maximum, the length λ/2 of the first half-wave is determined. In the exemplary embodiment shown, the signal shapes characteristic of a particle or a gas bubble are formed, following a sinusoidal curve, in such a way that the wave length λ of a sine wave characteristic of a particle having two extrema E 1 , E 2  corresponds to the measurement duration of this characteristic sequence of measurement values. The beginning of the first half-wave is defined in conjunction with the exceeding of or the dropping below the respectively associated trigger level t+, t−, and the end of the first half-wave by the dropping below or exceeding of the respective, opposite trigger level t−, t+. As soon as the respective opposite trigger level t+, t− is fallen short of or exceeded, in other words, broken through, the condition for the signal trigger is met and the entire wave length λ of the sine wave or sine curve is calculated from the length λ/2 of the first half-wave. 
     For the subsequent determination of the measured signal shape beginning at the second start B 2  and, corresponding to the detected particle, of a particle or a gas bubble, the wave length λ, preferably 3.5 times that of the wave length λ, is stored in an analysis buffer. The observation period to be subsequently evaluated begins with the first exceeding or falling below of the associated trigger level t+, t−, in  FIG. 2 a    at data point P 2 , and ends after 3.5 times the wavelength λ at the aforementioned time or data point P 2 . The representations of  FIGS. 2 b -2 d    differ from the representation of  FIG. 2 a    in that in the former, in each case, no noise band is delineated. 
       FIG. 1 b    shows the steps according to the method of the present invention of determining the signal shape from the second start B 2  to two alternative results, result R 1  for a particle, or result R 2  for a gas bubble. In the fifth step S 5 , an upper limit value T+ and a lower limit value T− are calculated from the amplitudes of the measured sine waves, in other words, from the extremum values M 1 , M 2  of both extrema E 1 , E 2 . In the exemplary embodiment shown, each limit value T+, T− is, in each case, ⅛ of the associated extreme value M 1 , M 2 . Partial ratios deviating from the former may be selected. If, comparable to the representation in  FIG. 2 d   , two or more maxima E 2 , E 4  and/or two or more minima E 1 , E 3  are present, the global maximum and the global minimum, in  FIG. 2 d   , the second extremum E 2  formed as the maximum and the third extremum E 2  formed as the minimum, are used for determining the upper limit value T+ and the lower limit value T−, respectively. 
     Furthermore, the length of the observation period is set at 3.5 times the wavelength A, wherein multiples deviating therefrom, for example, two-fold, are conceivable. The numbering of the data points or measurement values and the numbering of the extrema are each reset to 0. The now evaluated and analyzed observation period begins with the first exceeding of or dropping below the associated limit value T+, T−, and ends after 3.5 times the wavelength λ as of the aforementioned data point or measurement value. In addition to the wavelength λ, at least one data point or measurement value, expediently, the respective extreme value M 1 -M 4 , is stored in a cache or analysis buffer for each extremum E 1 -E 4 . 
     The data points or extreme values M 1 -M 4  stored in the cache or analysis buffer are verified in the sixth step S 6  with respect to the number of extrema E 1 -E 4  or extreme points present. Based on the first extreme value M 1 , which lies above the upper limit value T+ or below the lower limit value T−, the number 1 of the extrema E 1 -E 4  when exceeding or dropping below the respective, opposite limit value T−, T+, is incremented in each case, i.e., increased by 1. 
     Once the measurement values pass through the observation period and are analyzed in the process, in the seventh step S 7 , whether a maximum of two or more extreme points have been counted is determined. Two extreme points E 1 , E 2  corresponding to the representations in  FIGS. 2 b  and 2 c    indicate, in accordance with the signal type or the characteristic signal shape, a particle as result 1 R 1 . Four extrema E 1 -E 4  indicate, in accordance with the representation of  FIG. 2 d   , the other signal shape characteristic of a gas bubble, and lead to result R 2 . 
     The representations of  FIGS. 2 b  and 2 d    differ in that in  FIG. 2 b    a global minimum E 2  follows a global maximum E 1 , and in  FIG. 2 c    a global maximum E 2  follows a global minimum E 1 .  FIG. 2 d    shows a succession of a local minimum E 1 , a global maximum E 2 , a global minimum E 3  and a local maximum E 4 . Crucial for the detection of a gas bubble is the number of more than two extreme points E 1 -E 4 . Thus, all extrema E 1 -E 4  may have the same amplitude, in other words, identical extreme values M 1 -M 4  in terms of amount. Furthermore, the signal measured in the observation period may, in departing from a sinusoidal curve, be a square wave signal, a triangle signal or the like. 
     The local extrema, the local minimum E 1  as first extremum and the local maximum M 4  as fourth extremum shown in  FIG. 2 d    represent a type of back and forth oscillation for a succession of two global extrema, the global maximum E 2  and the global minimum E 3 , which are characteristic of a particle. In the situation in  FIG. 2 d   , the length λ/2 of the half-wave is determined and, accordingly, the wavelength λ is calculated, and the length of the observation period is defined by the beginning of the minimum E 1  as first extremum, as well as the subsequent exceeding of the upper limit value T+. 
     While one embodiment has been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined in the claims.