Abstract:
In a measuring device for the measurement of forces in a vehicle undercarriage, more particularly of the brake torque on a vehicle undercarriage, e.g. an aircraft landing gear, a sensor is introduced into a hollow connecting element that is transversally loaded by said forces, which sensor produces a measuring signal in function of a deformation of said connecting element. Distance measuring elements which detect the distance of the inner wall of said connecting element from said sensor are used as measuring elements.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a device for measuring a force in a vehicle undercarriage, more particularly the brake torque, said force being transmitted to said vehicle undercarriage by a bar-shaped member and said bar-shaped member being loaded transversally by said force. The invention further relates to a sensor for such a device. 
   PRIOR ART 
   The brakes of aircraft consist of stacks of mutually interleaved brake disks that are pressed against each other by hydraulic or electric actuators. One of the stacks is connected to the respective wheel. The other stack is connected to the stationary part of the landing gear for receiving the brake torque. In order to transmit the brake torque, i.e. the torque that appears when the brakes are activated, to the landing gear, the latter stationary stack is non-rotatably locked to the landing gear in a suitable manner. Generally, this is achieved by a fastening device that is arranged on the stationary stack eccentrically with respect to the axis of the wheel, in the simplest case a bore. A bolt serves for connecting the stationary stack to the landing gear directly or via a torque arm. This bolt is highly stressed by the torque in the transversal direction and is consequently made of a high-strength material. However, since its diameter is generally relatively large, it is made hollow in order to reduce its weight. 
   For various reasons it is desirable to measure the momentary braking action. To this end, U.S. Pat. No. 4,474,060 suggests designing the bushing that is normally arranged between the mentioned bolt and the respective receiving opening as a torque sensor. However, the disadvantage of this solution is that it involves a modification of the elements which serve for force transmission, thereby causing considerable expenditure for the certification of this solution. The certification is relatively time-consuming and costly and may furthermore be required, in the extreme case, for each aircraft type separately. 
   Similar problems in the measurement of the brake torque may also be encountered in other types of vehicles whose braking systems are similar to those of aircraft. Furthermore, in the undercarriages of aircraft and other vehicle types, other forces whose measurement is desirable or important may appear, e.g. due to bumps, suspension, damping elements, vehicle weight, etc. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to provide a device for measuring forces in a vehicle undercarriage, more particularly the brake torque, that can be mounted without any substantial interventions in the transmission path of the brake torque. 
   This is accomplished by a device wherein at least one sensor is arranged in the interior of said bar-shaped member and measures the deformation of said bar-shaped member that is due to said transversal load. The following claims indicate preferred embodiments and sensors for use in the device. 
   Accordingly, the device comprises a sensor located in a connecting element that is generally bar-shaped and is transversally loaded and concomitantly deformed by the force or forces that is/are to be measured, e.g. by the brake torque. More particularly, the sensor is designed to detect the distance between the sensor and the inner walls of the cavity in the connecting element in which the sensor is located. Preferentially, capacitive or inductive distance measuring elements are used for this purpose. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be further explained by means of an exemplary embodiment and with reference to figures. 
       FIG. 1  schematic illustration of an aircraft landing gear (bogie); 
       FIG. 2  longitudinal section of a connecting element comprising a sensor of the invention; 
       FIG. 3  cross-section according to III-III in  FIG. 2 , connecting element in the unstressed condition; 
       FIG. 4  as  FIG. 3  but connecting element loaded by brake torque; 
       FIG. 5  block diagram; and 
       FIG. 6  block diagram of a variant of the circuit of  FIG. 5 . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1  shows the basic structure of an aircraft landing gear  1 . On a landing gear strut  2 , a bogie train  4  is mounted by a pivot  3 . The bogie train supports wheels  5 . Wheels  5  are provided with brakes  6  that are actuatable by (e.g. hydraulic) actuators  7 . The stationary disk stack of brake  6  has a lever  8  with a bore  9 . A torque arm  12  is fastened to bore  9  as well as to an attachment point  10  and transmits the brake torque from brake  7  to landing gear  1  during brake application. 
   The depicted basic construction of an aircraft landing gear corresponds to the state of the art for larger aircraft. Alternatively, instead of using torque arm  12 , it is also common, especially in smaller aircraft, to transmit the torque from the brake directly to the landing gear, e.g. by a direct bolt connection. 
     FIG. 2  shows a longitudinal section of the connection between brake torque arm  12  and the stationary part  14  of brake  6 , the above-mentioned lever  8  being considered as belonging to stationary part  14 . 
   Bolt  16  extends through bore  9  in lever  8  as well as through a bore  15  at the end of brake torque arm  12 . Bolt  16  is made of a high-strength material and is largely hollow to reduce its weight. However, during brake application, it is still noticeably deformed. For example, a deformation of 4/10 mm has been observed in a bolt having an internal diameter of 50 mm. 
   Bolt  16 , which is hollow, contains sensor  20 . At its end on the right in the figure, enclosure  22  is provided with projections or has such an overall diameter that it is in close contact with inner wall  26  of bolt  16 . Bolt  16  as well as end  24  of sensor  20  are here traversed by a bore through which a pin  28  is pushed. Pin  28  is held in a bore  30  in an orientation ring  32  that is attached to lever  8 , i.e. to the stationary part  14  of brake  6 . The purpose of this device is to lock the sensor in a predetermined, fixed orientation relative to the brake torque (arrow  34 ). 
   On the outside of portion  36  of sensor  20  on the left in  FIG. 2 , O-rings  38  are attached. The latter serve the purpose of maintaining this part of sensor  20  approximately centrally and of absorbing the deformations of bolt  16  when it is loaded by brake torque  34 , enclosure  22  of sensor  20  being substantially rigid. Portion  36  of sensor  20  comprises an inductive distance measuring element  40  and an associated supply and evaluation circuit on a circuit board  42 . The sensor enclosure is sealed by a plate  44  on which electric connector  46  is located through which the electrical connections (not shown) are established. 
   As appears more clearly in  FIGS. 3 and 4 , inductive measuring element  40  is essentially composed of two perpendicularly arranged coil assemblies  50  and  51  located on a cruciform core  52 . Core  52  has a high magnetic permeability. More specifically, it is composed of a stack of a magnetically soft material in order to avoid eddy currents that might appear during AC excitation of coil assemblies  50 ,  51 . 
   Arms  54  of core  52  along with the outer ends of coils  50 ,  51  are maintained in corresponding bores respectively recesses of enclosure  22  such that the ends of arms  54  represent a part of the enclosure surface of sensor  20 . In this manner, a magnetic field emitted from core  52  through arms  54  may leave respectively enter into the sensor unrestrictedly. In order not to disturb the propagation of such a magnetic field, enclosure  22  of sensor  20  is made, at least in the area near inductive distance measuring element  40 , of a material having a low magnetic permeability. 
   Inductive distance measuring element  40  serves for measuring radial distances between bolt  16  and sensor  20 , as illustrated in  FIGS. 3 and 4 . Due to the deformation of bolt  16  into an oval (see  FIG. 4 ), the distances in the direction of force  34  (distances  75 ,  76 ) decrease and those perpendicularly to force  34  (distances  77 ,  78 ) increase. Since this is independent from the direction in which the force is acting along arrow  34 , the measurement also fulfills the frequently demanded requirement of measuring the absolute value of force  34 . 
   Although a simple coil assembly with a bar-shaped core would be sufficient for the measurement, the cruciform arrangement of two coil assemblies is provided in order to be able to separate the effect of brake torque  34  from other influences and furthermore to allow a simpler derivation of the brake torque from the measuring signals of inductive distance measuring element  40 . Moreover, errors on account of an imprecisely centered position of measuring element  40  within bolt  16  are eliminated. 
   A prerequisite for using an inductive distance measuring element is that bolt  16  is also made of a material having a high magnetic permeability, which is commonly the case today. The usual high-strength materials for these components exhibit sufficient magnetic properties in this respect. 
   For the measurement, the coil pairs  50 ,  51  are separately supplied with an alternating current, and the alternating voltage across the coils is measured. By a synchronous demodulation of these voltages by a voltage having the same frequency but which is offset by 90°, the imaginary part of the voltage is obtained, i.e. the part that is due to inductance. Therefrom, using the evaluation described in more detail below, it is possible to generate a measuring signal that is proportional to the brake torque. 
   The circuitry around inductive distance measuring element  40  is schematically illustrated in  FIG. 5 . An oscillator  58  generates a voltage U osc  having a frequency ω and whose amplitude is predetermined by an externally preset voltage U REF . By adjusting U REF , a possible temperature dependence of inductive distance measuring element  40  can be compensated. This will not be further discussed hereinafter, but it is conceivable to arrange a temperature probe in sensor  20  and to adjust U REF  in function of its signal. 
   U osc  is converted by two current-voltage converters  60 ,  62  into currents I A  and I B  that are supplied to coils A  50  and B  51 . The voltages across A and B are supplied to synchronous demodulators  64 ,  66  to which the output signal U osc  of oscillator  58 , shifted 90° by an integrator  68 , is supplied as the second signal. After low-pass filtering in respective low-pass filters  70 ,  71 , output signals U A  and U B  are obtained which correspond to the pure inductance of coil assemblies  50 ,  52 , respectively, i.e. without their ohmic components. Low-pass filters  70 ,  71  serve for eliminating the carrier frequency. The two voltages U A  and U B  are supplied to an analog or digital processing unit  73  which divides the difference of the input signals by the sum of the input signals, thereby yielding output signal U OUT . As will be demonstrated, this voltage is proportional to force F acting upon bolt  16 . 
   For the purposes of the following derivation it will be assumed that coil assemblies A and B are each the result of serial connections of ideal inductances L A  respectively L B  and of ohmic components R A  respectively R B . The ohmic component includes iron losses, the ohmic resistance of conductors, etc. As far as alternating voltages and currents are concerned, the currents and voltages indicated below shall normally be considered as vectorial values. 
   The voltage induced in coil assembly A (that corresponds to coil pair  50 ) by current I A  is:
 
 U   A   =U   L     A     +U   R     A     Eq. 1
 
and:
 
U L     A   =L A I A ω  Eq. 2
 
where:
     U L     A    alternating voltage component due to the pure inductance,   U R     A    component due to the parasitic ohmic components.   

   The pure inductance L A  of coil assembly A is equal to: 
                   L   A     =         n   A   2     ⁢     Λ   A       =         n   A   2     ⁢     μ   0     ⁢           ⁢       A     p   A         d   A         =       K   A       d   A                   Eq   .           ⁢   3               
where:
     n A  number of windings of A   μ 0  magnetic permeability   A p  pole cross-section of A   d A  air gap in the magnetic circle of A, i.e. the sum of distances  75  and  76  ( FIG. 4 )   K A  constant: K A =n A   2 μ 0 A p     A   /d A      
   The variation of air gap d A , equivalent to the sum of distances  75  and  76 , is approximately proportional to brake torque F:
 
 d   A   =d   0   +CF   Eq. 4
 
where:
     C mechanical constant, dependent upon bolt  16 .   d 0  air gap d A  in no-load condition (F=0)   

   From equations (2), (3), and (4) it follows that: 
                   U     L   A       =       I   A     ⁢           ⁢   ω   ⁢           ⁢     K   A     ⁢     1       d   0     +   CF                 Eq   .           ⁢   5               
and by an analogous derivation for coil assembly B:
 
   
     
       
         
           
             
               
                 
                   U 
                   
                     L 
                     B 
                   
                 
                 = 
                 
                   
                     I 
                     B 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   ω 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     K 
                     B 
                   
                   ⁢ 
                   
                     1 
                     
                       
                         d 
                         0 
                       
                       - 
                       CF 
                     
                   
                 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 6 
               
             
           
         
       
     
   
   Furthermore, with an identical, symmetrical design of coil pairs  50 ,  51 , the following applies:
 
I A ωK A =I B ωK B   Eq. 7
 
   When Eq. (7) is entered into Eq. (5) and (6), one obtains for U OUT : 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         U 
                         OUT 
                       
                       = 
                       
                         
                           
                             U 
                             
                               L 
                               A 
                             
                           
                           - 
                           
                             U 
                             
                               L 
                               B 
                             
                           
                         
                         
                           
                             U 
                             
                               L 
                               A 
                             
                           
                           + 
                           
                             U 
                             
                               L 
                               B 
                             
                           
                         
                       
                     
                   
                 
                 
                   
                     
                       = 
                       
                         
                           K 
                           ⁡ 
                           
                             [ 
                             
                               
                                 1 
                                 
                                   
                                     d 
                                     0 
                                   
                                   + 
                                   CF 
                                 
                               
                               - 
                               
                                 1 
                                 
                                   
                                     d 
                                     0 
                                   
                                   - 
                                   CF 
                                 
                               
                             
                             ] 
                           
                         
                         
                           K 
                           ⁡ 
                           
                             [ 
                             
                               
                                 1 
                                 
                                   
                                     d 
                                     0 
                                   
                                   + 
                                   CF 
                                 
                               
                               + 
                               
                                 1 
                                 
                                   
                                     d 
                                     0 
                                   
                                   - 
                                   CF 
                                 
                               
                             
                             ] 
                           
                         
                       
                     
                   
                 
                 
                   
                     
                       = 
                       
                         CF 
                         
                           d 
                           0 
                         
                       
                     
                   
                 
                 
                   
                     
                       = 
                       
                         
                           K 
                           2 
                         
                         ⁢ 
                         F 
                       
                     
                   
                 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 8 
               
             
           
         
       
     
   
   Thence, U OUT  is proportional to brake torque F. 
   The division by (U L     A   +U L     B   ) in Eq. 8 is difficult to perform analogically and also relatively demanding digitally.  FIG. 6  shows a variant where this division is avoided by keeping (U L     A   +U L     B   ) constant. 
   The circuit of  FIG. 6  largely corresponds to that of  FIG. 5 , especially with regard to the components designated by concordant reference numerals. 
   In contrast to  FIG. 5 , the voltages across coils A  50  and B  52  are supplied to an adder  77 . The resulting sum U A +U B  is supplied to a third synchronous demodulator  79  whose output delivers the sum U L     A   +U L     B    after adequate smoothing by a low-pass filter  80 . This signal is supplied to a PI controller  81  as the actual value while U REF  is the command value. Optionally, the regulating behavior can be further improved by the addition of a differential component (PID controller). 
   PI or PID controller  81  controls the amplitude of oscillator  58 . 
   Thus the factor 
           1       U     L   A       +     U     L   B               
is constant, and the result is:
   U   OUT *( U   L     A     +U   L     B   )= U   L     A     −U   L     B   =( U   L     A     +U   L     B   ) K   2   F=K   3   F   Eq. 9 
where K 3 =constant.
 
   Thus, the output signal of adder  83  preceded by inverter  85 , i.e. the difference U L     A   −U L     B   , is directly indicative of the force F, and the demanding division is avoided. In particular, this variant can also be implemented by analog means. 
   A particular advantage of the described sensor is that it is insertable into existing connecting bolts  16  without the need of altering the mechanical construction in a way that would require a recertification. Moreover, the sensor can be mounted respectively inspected or replaced on location, i.e. during regular aircraft maintenance. 
   From the preceding description of an exemplary embodiment, numerous modifications are accessible to those skilled in the art without leaving the scope of the invention that is solely defined by the claims. Conceivable are the following, inter alia:
         Using other distance measuring elements than inductive ones, e.g. capacitive ones or measuring elements based on eddy currents; in the case of capacitive elements, the indicated evaluation circuits would have to be supplied with alternating current and the blind current would be measured as the equivalent of the imaginary component of the signal of inductive measuring elements.   Arranging the measuring element in a completely sealed enclosure of the sensor. In this case, the measuring element, e.g. inductive distance sensor  40 , can be fastened to a support inside sensor enclosure  24 .   Building up the distance sensor of two separate sensors yet preferably at a small distance along bolt  16 , i.e. each near the junction of the two parts  12 ,  14  that are connected by the bolt and where the strongest deformation of the connecting element (bolt  16 ) by the arising forces is to be expected;   Using a different core material for the inductive measuring element, e.g. one that is based on ferrites.