Abstract:
A vibration detector. A machine, such as a gas turbine engine, contains a rotor bearing and a gear. A nearby sensor produces a train of pulses as teeth of the gear travel past. The frequency of the pulse train indicates rotational speed of the rotor. In addition, vibration of the rotor causes the gear to orbit about another center. The orbiting causes amplitude modulation, frequency modulation, or both, in the pulse train. Detection of the modulation indicates the presence of vibration. Thus, a single pulse train, produced by a single sensor, is used to indicate both speed, and the presence of vibration.

Description:
FIELD OF THE INVENTION 
     The invention relates to vibration sensing. 
     BACKGROUND OF THE INVENTION 
     Gas turbine engines are commonly equipped with one, or more, accelerometers to detect vibration. Because the accelerometers sometimes malfunction, back-up accelerometers are often provided. The accelerometers add weight to the engine. They also increase costs of manufacturing, design, and maintenance. Further, some accelerometers are fragile, and easily damaged. 
     SUMMARY OF THE INVENTION 
     The invention mitigates some, or all, of the disadvantages just identified. One form of the invention detects vibration by analyzing an existing pulse train which is produced by an existing sensor, and presently used for speed measurement. Under the invention, the existing pulse train is used to indicate both speed and vibration. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a gear  4  and a prior-art reluctance sensor. 
     FIGS. 2,  3 , and  4  illustrate three d ifferent positions of tooth  16 , and the corresponding reluctance seen by sensor  8 . 
     FIG. 5 is a plot of reluctance seen by sensor  8  in FIG. 4, plotted against angular position. 
     FIGS. 6 and 7 illustrate passage of a single tooth  16  past sensor  8 . 
     FIG. 8 illustrates a single pulse, produced by electronic circuit  12  in FIGS. 6 and 7, when tooth  16  passes the sensor  8 . 
     FIG. 9 illustrates a pulse train  72 , produced by electronic circuit  12  in FIG. 6, when teeth repeatedly pass the sensor  8 . 
     FIG. 10 illustrates how vibration of disc  86 , which represents gear  4  in FIG. 1, can be represented as orbiting of shaft  88  about axis  98 . 
     FIG. 11 shows several different rotational positions of the apparatus of FIG.  10 . 
     FIG. 12 illustrates plots  100  and  124  of FIG. 11, and is used to show a velocity change. 
     FIG. 13 illustrates amplitude modulation of the pulse train  72  of FIG.  9 . 
     FIG. 14 illustrates how reference block  165 , indicating a tooth  16  in FIG. 1, follows a non-symmetrical path when orbiting of disc  86  occurs, thereby causing the amplitude modulation of FIG.  13 . 
     FIG. 15 illustrates frequency modulation of the pulse train  72  of FIG.  9 . 
     FIG. 16 is a flow chart illustrating procedures implemented by one form of the invention. 
     FIG. 17 illustrates one form of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention utilizes a pulse train produced by a sensor. Numerous types of sensor can be used. For simplicity, this discussion will be framed in terms of a generic reluctance sensor. 
     FIG. 1 illustrates a prior-art system, including a toothed gear  4 , a reluctance sensor  8 , and an electronic circuit  12 . The electronic circuit  12  detects passage of each tooth  16  past the reluctance sensor  8 . The electronic circuit  12  produces a pulse (not shown) in response to each tooth  16 , on output  17 . 
     “Reluctance” refers to magnetic reluctance. In general, magnetic reluctance depends on (1) the amount, and (2) magnetic permeability, of material located within dashed region  20 . For example, tooth  16  in FIG. 2 is positioned above a reference line  24 . The reluctance seen by sensor  8  is indicated by point  28  in the plot  38 . 
     As a second example, tooth  16  in FIG. 3 is positioned at the reference  24 . The reluctance is indicated by point  32 . As a third example, tooth  16  in FIG. 4 is positioned below the reference  24 . The reluctance is indicated by point  36 . 
     A generalized plot  39  of reluctance versus position is given in FIG.  5 . It is symmetrical about the reference  24 , and has a minimum point  40 , corresponding to point  32  in FIG.  3 . 
     The electronic circuit  12  does not necessarily measure reluctance itself, but often measures a parameter related to the reluctance. For example, FIG. 6 shows a hatched tooth  16 . The hatched tooth  16  passes the sensor  8 , and moves to the position shown in FIG.  7 . In response to this movement, the electronic circuit  12  produces a signal  41  resembling that in FIG.  8 . Because of the measurement technique utilized, signal  41  indicates more the slope of the reluctance plot  39  in FIG. 5, rather than the reluctance itself. 
     For simplicity, the plot of FIG. 8 does not exactly indicate the slopes of every point of the plot  39  in FIG. 5, but only general features. In FIG. 8, region  48 , which is negative, corresponds to region  52  in FIG. 5, which has a negative slope. Region  56  in FIG. 8, which is positive, corresponds to region  60  in FIG. 5, which has a positive slope. Point  64  in FIG. 8, which has a value of zero, corresponds to point  40  in FIG. 5, which has zero slope. 
     As successive teeth  16  pass the reluctance sensor  8  in FIG. 7, a train  72  of pulses  74  is generated, as in FIG.  9 . If a reluctance sensor is used which measures actual reluctance, as opposed to the slope, then the train of pulses (not shown) will contain a sequence of the plots  39  of FIG.  5 . 
     In the ideal case, the pulses  74  within the train  72  in FIG. 9 will be identical in shape, and the time intervals  76  between adjacent pulses will be identical. The ideal case requires the toothed gear  4  in FIG. 7 to be perfectly symmetrical, perfectly homogeneous in magnetic permeability, and rotating at a constant speed about a fixed center  82  in FIG.  6 . 
     However, if vibration occurs, the ideal case will no longer exist. The toothed gear  4  will not only rotate about its center  82 , but center  82  will orbit about another center. FIG. 10 illustrates the situation. Disc  86  represents the toothed gear  4  in FIG.  6 . Disc  86  in FIG. 10 is supported by shaft  88 , and rotates about axis  90 . Center  82  is shown. 
     In addition, to illustrate the orbiting, shaft  88  is supported by a second disc  94 . Second disc  94  rotates about second axis  98 . FIG. 11 illustrates a sequence of positions which the components of FIG. 10 will occupy during their combined rotation and orbiting. 
     In FIG. 11, plot  100  indicates the relative arrangement of the components at an initial, reference time. Reluctance sensor  8  is shown, as is shaft  88 . An arm  107  is superimposed, to illustrate the fact that disc  94  acts as a crank arm in supporting shaft  88 . Arm  107  rotates about center  98 . A second arm  105  is shown, to illustrate the fact that disc  86  acts as a crank arm in supporting reference square  106 , which represents a tooth  16  of FIG.  1 . In FIG. 11, arm  105  rotates about shaft  88 . 
     It is assumed, for simplicity, that both discs  86  and  94 , and thus both crank arms  105  and  107 , rotate at the same angular speed. 
     FIG. 11 shows seven plots. Table 1, below, indicates the amount of rotation occurring in each plot. A single amount of rotation is indicated in Table 1 for each plot, because, as stated above, both crank arms  105  and  107  rotate at the same angular speed, although about different centers. 
     Since they rotate at the same speed, at any given time, their angular displacements from the initial position of plot  100  will be identical. That is, at any given time, both cranks  105  and  107  will experience the same total rotation, but about different centers. 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 PLOT IN 
                 TOTAL AMOUNT OF 
               
               
                 ROTATION 
                 FIG. 11 
                 (Degrees) 
               
               
                   
               
             
             
               
                   
                 100 
                 zero 
               
               
                   
                 104 
                 30 
               
               
                   
                 108 
                 60 
               
               
                   
                 112 
                 90 
               
               
                   
                 116 
                 120 
               
               
                   
                 120 
                 150 
               
               
                   
                 124 
                 180 
               
               
                   
               
             
          
         
       
     
     Two significant features of the combined rotations of FIG. 11 are significant. One is that the distance between disc  86  and the reluctance sensor  8  changes, as the rotations occur. As a specific example, distance  128  in plot  124  is greater than distance  132  in plot  108 . The change in the distance will change the reluctance signal produced by sensor  8 . 
     A second feature is that the velocity with which disc  86  passes the reluctance sensor  8  changes as the combined rotation and orbiting occurs. FIG. 12 illustrates this change, and contains copies of plots  100  and  124  from FIG.  11 . All rotation is counter-clockwise. To simplify the explanation, only the component of velocity in the vertical direction in FIG. 12 will be considered. “Up” and “down” are labeled in the center-of the Figure. 
     In plot  100 , vector  140  represents the velocity of shaft  88  in the vertical direction. Since shaft  88  is the axle about which disc  86  rotates, vector  140  also represents the translational velocity of the entire disc  86 , in the upward direction. Since reference box  106  is attached to disc  86 , vector  140  also represents one velocity component of box  106  in the upward direction. 
     In addition, vector  144  represents the additional velocity of box  106 , due to the rotation of disc  86  about shaft  88 . The net velocity of box  106  in the upward direction is the vector sum of vectors  140  and  144 . The net velocity is relatively high, compared with that of plot  124 , which will now be considered. 
     In plot  124 , shaft  88  is now moving downward, because shaft  88  rotates counterclockwise about shaft  98 . Vector  148  indicates its downward component of velocity. Since disc  86  is attached to shaft  88 , vector  148  also indicates the downward translational velocity of the entire disc  86 . Thus, box  106  has a component of velocity in the downward direction, indicated by arrow  148 , due to the downward translation of disc  86 . 
     In addition, box  106  has a component of velocity in the upward direction, because of the rotation of disc  86  about shaft  88 . Vector  152  indicates that component. The net velocity of box  106  in the vertical direction is the vector sum of vectors  148  and  152 . The net velocity is relatively low, compared with that of plot  100 , because vectors  148  and  152  for plot  124  oppose each other. 
     Therefore, vibration of disc  86  in FIG. 10 can take the form of orbiting of shaft, or center,  88  about axis  98 . Disc  86  represents the toothed gear  4  of FIG.  6 . The orbiting causes two events to occur in the parameter measured by the sensor  8  and electronic circuit  12  in FIG.  6 . 
     One event is that the orbiting causes a change in the reluctance signal, because the orbiting causes the reluctance seen by sensor  8  to change. The second event is that the orbiting changes the tangential speed at which the circumference of disc  86  in FIG. 11 passes the sensor  8 . Since the teeth  16  in FIG. 6 are located at that circumference, their speed will change as orbiting occurs. 
     FIGS. 13 and 15 illustrate how these two events affect the pulse train  72  in FIG.  9 . FIG. 13 shows a type of amplitude modulation: the amplitudeat point  160  is larger than that at point  164 . The amplitude change is caused by the movement of disc  86  in FIG. 11 toward, and away from, sensor  8 . The orbiting about center  98  causes the movement. Amplitude is measured from zero to a point such as  160 . 
     Most of the pulses shown in FIG. 13 are not symmetrical about the zero amplitude axis  163 . The reasons for this are complex, and depend partly on the technique used to produce the pulse train shown in the Figure. However, one factor affecting the lack of symmetry is shown in FIG.  14 . 
     In FIG. 14, plots  170 ,  174 ,  175 , and  179  show four successive positions of reference block  165 . These four positions are superimposed together in plot  183 , and labeled with their corresponding plot numbers. 
     Plot  183  indicates that the path of the block  165  is not symmetrical about axis  24 . This lack of symmetry is partly responsible for the lack of symmetry in FIG. 13, about the zero-amplitude axis  163 . For example, in a very general sense, point  160  in FIG. 13 may correspond to the position of block  165  in plot  170  in FIG. 14, where reluctance is somewhat high. Point  161  in FIG. 13 may correspond to the position of block  165  in plot  179  in FIG. 14, where reluctance is somewhat low. Plot  183  in FIG. 14 illustrates the two positions in a single plot, more clearly showing the difference in reluctance. 
     FIG. 15 shows a type of frequency modulation: the frequency is higher at time  184  than at time  188 . The higher frequency, that is, a smaller time interval between adjacent pulses, would occur in, for example, plot  100  in FIG.  11 . In plot  100 , tangential speed is relatively larger, as explained in connection with FIG.  12 . 
     A smaller frequency, that is, a larger time interval between adjacent pulses, would occur in plot  124  of FIG.  11 . In plot  124 , tangential speed is relatively smaller. 
     Therefore, two changes occur as the disc  86  in FIG. 10 rotates and orbits. One involves the changes in distance between disc  86  in FIG.  11  and the sensor  8 . These changes cause changes in reluctance. The changes in reluctance cause amplitude modulation of the pulse train, as shown in FIG.  13 . 
     The second change involves the changes in tangential speed of the disc  86 . The changes in speed cause frequency modulation, as shown in FIG.  15 . 
     FIG. 16 is a flow chart of logic used to detect the amplitude and frequency modulations shown in FIGS. 13 and 15. Block  190  indicates that the pulse train, such as that of FIG. 9, is received. The pulse train may, or may not, contain the amplitude modulation or frequency modulation shown in FIGS. 13 and 15. 
     Block  192  in FIG. 16 indicates that rotational speed of the toothed gear  4  in FIG. 1 is-computed. For example, assume that the spacing between gear teeth  16  is ten degrees. If 15 pulses are counted in 0.01 seconds, then rotational speed is computed as (15×10) degrees/0.01 second. This quotient corresponds to 15,000 degrees per second, or roughly 41 revolutions per second. 
     Block  194  indicates that amplitude modulation is detected. Such detection is well known, and numerous different techniques can be used. As a simple example, the amplitude of each pulse  74  in FIG. 9 can be stored in a stack memory. The stack memory may contain 1,000 memory locations. When the stack becomes filled, the earliest amplitudes stored in it become lost. 
     As a specific example, amplitudes  1  through  1 , 000  may be stored in the stack, in sequence. At this time, the stack has become filled. When amplitude  1 , 001  is added, amplitude  1  becomes lost. When amplitude  1 , 002  is added, amplitude  2  becomes lost, and so on. 
     A detection routine looks for deviations in the amplitudes stored in the stack. As a simple example, the detection routine may scan the stack, and find both the largest amplitude and the smallest amplitude. If the difference between them exceeds a threshold, then unacceptable vibration is inferred. 
     Block  196  in FIG. 16 indicates that frequency modulation is detected. Such detection is well known, and numerous different techniques can be used. As a simple example, a second stack may be used, containing the time intervals between each adjacent pair of the 1,000 amplitudes stored in the first stack. A detection routine may scan the second stack, looking for the largest and the smallest interval. If the difference between them exceeds a threshold, then unacceptable vibration is inferred. 
     Block  198  indicates that a warning is issued if unacceptable vibration is found. For example, a warning signal can be transmitted to the cockpit of an aircraft, if either amplitude modulation or frequency modulation exceeds a limit. 
     Alternately, numerical values indicating the amount of frequency modulation, and amount of amplitude modulation, can be displayed to an operator, such as a pilot. In communications work, modulation of a carrier is commonly expressed as a percentage, such as fifty-percent modulation. This convention can be used by the invention. 
     Other, more complex, approaches can be undertaken in detection of the modulations. For example, one goal may be to detect excessive deviation, in frequency and amplitude, of a measured pulse train from an ideal pulse train. To identify the deviation, a Fast Fourier Transform, FFT, of the pulse train is taken. 
     If the pulse train is an ideal pulse train, containing identical pulses, identically spaced, it will have a given distribution of Fourier terms. Further, if the pulses are true sine waves, a single Fourier term will exist. 
     Modulation of the pulse train, either in amplitude or frequency, will alter the terms of the Fourier series. If the alteration exceeds a threshold, then unacceptable vibration will be inferred. As a simple example, if the base frequency term, plus the three lowest three harmonics, change by ten percent each, then unacceptable vibration will be inferred. More generally, if any of the first N harmonics change by X percent each, then unacceptable vibration will be inferred. 
     FIG. 17 illustrates one form of the invention. A turbofan aircraft engine  203  is shown, containing a high pressure compressor  200 , a high pressure turbine  204 , a fan  208 , and a low pressure turbine  212 . Toothed gear  4  is shown, and is used to measure speed of fan  208 . Toothed gear  4  need not actually function as a gear, but can be used as a toothed wheel solely to produce pulses. 
     Block  216  represents the reluctance sensor and associated electronics, which produces the pulse train  72  of FIG.  9 . 
     The computation indicated by the flow chart of FIG. 16 is undertaken by apparatus represented byblock  220  in FIG.  17 . Alternately, the computation of block  220  can be performed by the digital engine control  224 . 
     The engine control  224  is known in the art. It measures various operating parameters, such as component speeds, airflows, and pressures. Based on those parameters, it schedules, or controls, other parameters, such as fuel-air ratio, blade cooling, and stator vane angle. The control  224  contains a microprocessor (not shown) which can perform the computations described in connection with FIG.  16 . 
     The discussion herein has been framed in terms of a reluctance sensor. However, a reluctance sensor is not required. Other sensors can produce the pulse train of FIG. 9, in response to the passage of teeth on a wheel. The sensor used should produce pulses of different sizes, when distance to the teeth changes. The sensor should also produce pulses, in response to passage of teeth  16  in FIG. 1, so that the pulse frequency changes, when the speed of passage of the teeth changes. Some examples of sensors are Hall Effect sensors, optical proximity sensors, and microwave proximity sensors. 
     Numerous substitutions and modifications can be undertaken without departing from the true spirit and scope of the invention. What is desired to be secured by Letters Patent is the invention as defined in the following claims.