Abstract:
There is disclosed a dual output compressive mode accelerometer having first and second output channels, comprising:
       a supporting base;   first and second transducers mounted on the supporting base adjacent to one another, each transducer comprising a piezoelectric element and a seismic mass, the piezoelectric element positioned between the supporting base and the seismic mass; and   a rigid mechanical coupling between the first and second transducers, the rigid mechanical coupling coupled to both of the first and second transducers above the supporting base.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a dual accelerometer which comprises two (or more) transducers, each providing an output. 
       BACKGROUND OF THE INVENTION 
       [0002]    Accelerometers are used for measuring vibrations in machines, such as gas turbines or jet engines. In aircraft engines there is a requirement for redundancy in almost all components, including monitoring equipment. It is therefore known to provide dual accelerometers, so that should one accelerometer fail, the other can be switched in to provide the necessary is information to the engine control system. 
         [0003]    One form of an accelerometer used in gas turbine engines comprises a piezoelectric crystal stack held on a pre-tensioned rod between a supporting base and a seismic mass. The sensor works by providing electrical signals indicative of the compression of the piezoelectric stack between the seismic mass and the supporting base. 
         [0004]    A dual output accelerometer of this type is disclosed in U.S. Pat. No. 4,586,377. U.S. Pat. No. 4,586,377 describes a system in which two piezoelectric crystal stacks are located on a single pre-tensioned bolt, to provide two output channels. This system suffers from the problem that if one crystal stack breaks or crumbles, both signals are greatly reduced or lost. Also, if the pre-tensioned central rod breaks, both signals are lost. 
         [0005]    Another problem with this type of accelerometer system is that when in an environment in which there are significant transverse vibrations, the output signals can be significantly distorted to the point where they are no longer useful. Furthermore, the pre-tensioned rod may fail. This is the case where the two accelerometers are placed on a single rod one above the other, or if they are placed side by side. If two accelerometers are placed side by side, when one accelerometer fails, it is quite likely that the other will also fail, as they are both subject to substantially the same conditions. 
         [0006]    It is an object of the invention to provide a dual channel accelerometer that is more reliable under transverse vibrations. It is a particular object to provide an accelerometer that is more reliable under the conditions typically found in an aircraft gas turbine engine. 
       SUMMARY OF THE INVENTION 
       [0007]    The invention is defined in the appended independent claims, to which reference should be made. Preferred features are set out in the dependent claims. 
         [0008]    In a first aspect, the invention provides a dual output compressive mode accelerometer having first and second output channels, comprising:
       a supporting base;   first and second transducers mounted on the supporting base adjacent to one another, each transducer comprising a piezoelectric element and a seismic mass, the piezoelectric element positioned between the supporting base and the seismic mass; and   a rigid mechanical coupling between the first and second transducers, the rigid mechanical coupling coupled to both of the first and second transducers above the supporting base.       
 
         [0012]    By coupling the two transducers together above the supporting base e.g. at the top or middle of the accelerometer stacks, the resonant frequency of the accelerometer assembly in the transverse mode is significantly increased and can be increased to a value outside the normal operating frequency of a gas turbine engine. “Transverse” in this context means transverse to a transducer axis, where the axis passes through the supporting base, the piezoelectric element and the corresponding seismic mass. The transducer axis corresponds to the direction of sensitivity of the accelerometer. 
         [0013]    The first and second output channels are connected to the first and second piezoelectric elements respectively. 
         [0014]    Each of the two transducers may share the same seismic mass, with the shared seismic mass forming the rigid mechanical coupling between the first and second transducers. 
         [0015]    Alternatively, the first and second transducers may each have a separate seismic mass and the rigid mechanical coupling may be a bracing element coupled to both seismic masses or to another part of each transducer. For example, the bracing element may be coupled to a pre-tensioned rod or bolt running through each transducer in an axial direction. 
         [0016]    The accelerometer may further comprise a casing fixed to the supporting base. The bracing element may be coupled to the casing. The bracing element may be coupled to the casing at a number of points on the casing so that movement of the transducers is substantially prevented in all transverse directions. This allows two output channels to be maintained when the sensor is subject to high transverse loads or vibrations. Alternatively, or in addition, the bracing element may be rigidly fixed to the supporting base, for example by one or more rigid pins. 
         [0017]    The bracing element may be formed from a solid piece of material or made up of multiple layers of thinner material. 
         [0018]    In embodiments in which the first and second transducers share a single seismic mass, a bracing element may be provided in addition and may be coupled to a casing. 
         [0019]    Preferably each transducer assembly further includes a pre-tensioned bolt, to which the piezoelectric elements and the seismic mass or masses are coupled. The rigid mechanical coupling may be connected to each bolt. 
         [0020]    In a second aspect of the invention, there is provided a dual output compressive mode accelerometer having first and second output channels, comprising a supporting base, first and second piezoelectric elements mounted on the supporting base adjacent to one another and a seismic mass coupled to both of the piezoelectric elements such that the piezoelectric elements are positioned between the seismic mass and the supporting base. 
         [0021]    The first and second output channels are preferably connected to the first and second piezoelectric elements respectively. 
         [0022]    The output from each transducer output channel can be tuned to ensure identical output from each channel when subjected to the same input vibration or loading conditions. 
         [0023]    It is of course possible to have more than two output channels and more than two piezoelectric elements. The inventions can be extended to as many output channels as required, with the transducers each coupled to one other transducer by a rigid coupling. 
         [0024]    In accordance with a further aspect of the invention, there is provided a gas turbine engine comprising a dual output accelerometer in accordance with either of the preceding aspects of the invention, and an engine control system connectable to both output channels of the dual output accelerometer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    Embodiments of the invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which: 
           [0026]      FIG. 1   a  is a schematic cross sectional view of a first embodiment of the invention; 
           [0027]      FIG. 1   b  is a schematic plan view of the embodiment of  FIG. 1   a;    
           [0028]      FIG. 2   a  is a schematic cross sectional view of a second embodiment of the invention; 
           [0029]      FIG. 2   b  is a schematic plan view of the embodiment of  FIG. 2   a;    
           [0030]      FIG. 3   a  is a schematic cross sectional view of a third embodiment of the invention; 
           [0031]      FIG. 3   b  is a schematic plan view of the embodiment of  FIG. 3   a;    
           [0032]      FIG. 4   a  is a schematic cross sectional view of a fourth embodiment of the invention; 
           [0033]      FIG. 4   b  is a schematic plan view of the embodiment of  FIG. 4   a;    
           [0034]      FIG. 5   a  is a schematic cross sectional view of a fifth embodiment of the invention; 
           [0035]      FIG. 5   b  is a schematic plan view of the embodiment of  FIG. 5   a;    
           [0036]      FIG. 6   a  is a schematic cross sectional view of a sixth embodiment of the invention; 
           [0037]      FIG. 6   b  is a schematic plan view of the embodiment of  FIG. 6   a;    
           [0038]      FIG. 7   a  is a schematic cross sectional view of a seventh embodiment of the invention; and 
           [0039]      FIG. 7   b  is a schematic plan view of the embodiment of  FIG. 7   a.    
       
    
    
     DETAILED DESCRIPTION 
       [0040]      FIG. 1   a  illustrates the basic elements of a first embodiment of a dual output accelerometer in accordance with the invention. The accelerometer comprises two piezoelectric crystal stacks  10 ,  11  arranged side by side on a solid supporting base  12 . The piezoelectric crystal stacks  10 ,  11  are mounted on tensioning bolts  15 ,  16  fixed to the support surface  12 . Mounted on the tensioning bolts above the piezoelectric elements is a single seismic mass  17 . Nuts  18 ,  19  are provided at the top of the tensioning bolts to securely fix the seismic mass above the piezoelectric crystal stacks. An outer casing  13  is provided to protect the accelerometer assembly. 
         [0041]      FIG. 1   b  is a plan view of the embodiment shown in  FIG. 1   a.    
         [0042]    Instead of pre-tensioned bolts  15 ,  16 , the components of the transducers may be welded, glued or fastened to one another in any other suitable way. The casing  13  is welded to the supporting base  12 , but may also be fixed in another suitable way. 
         [0043]    Under the action of vibrations in the direction of the sensitive axis (indicated by arrow  14 ) the seismic mass generates compression and decompression forces on the piezoelectric elements  10 ,  11 . The piezoelectric elements then produce an electrical signal representative of the compression and decompression which can be transmitted to output channels. 
         [0044]    Under the action of vibrations in directions perpendicular to the sensitive axis, which are referred to herein as transverse directions, the piezoelectric elements  10 ,  11  experience bending stresses. The bending stresses lead to a distortion of the signals output to the output channels, particularly when the frequency of vibration coincides with the transverse resonant frequency of the accelerometer assembly. 
         [0045]    In the embodiment of  FIGS. 1   a  and  1   b , the seismic mass  17  forms part of both transducer assemblies and at the same time ties the two assemblies together to provide structural rigidity. This provides a much higher transverse resonant frequency than would be the case if there were two separate seismic masses, one coupled to each pre-tensioned bolt, but not coupled to one another. By coupling the transducers in this way, the transverse resonant frequency of the accelerometer can be made much higher than the typical frequency experienced in, for example, a gas turbine engine on an aircraft. 
         [0046]      FIG. 2   a  illustrates an alternative embodiment. In the embodiment of  FIG. 2   a  each transducer has a separate seismic mass  20 ,  21  but a bracing member  22  is provided to connect the two adjacent transducer stacks. The bracing element  22  is coupled to the seismic masses  20 ,  21  at the top end, above the tensioning nuts  18 ,  19 . The bracing element  22  is also fixed to the outer casing  13 . 
         [0047]    The bracing member  22  provides for an increase in the transverse resonant frequency of the accelerometer assembly in the same manner as the shared seismic mass of the embodiment shown in  FIGS. 1   a  and  1   b . The bracing member in this example is formed of spring steel but may be formed on other suitably rigid materials, such as stainless steel or Inconel® alloys. The bracing member may be formed from a single component or may be formed from a plurality of components layered together or otherwise joined. The bracing may also be corrugated to provide stiffness in the transverse direction and flexibility in the axial direction 
         [0048]      FIG. 2   b  is a plan view of the embodiment shown in  FIG. 2   a . As can been seen from  FIG. 2   b  the bracing element  22  is fixed to the casing  13  on each side so as substantially to prevent transverse motion of the seismic masses  20 ,  21  in all transverse directions. This further reduces the distorting effect of transverse vibration on the output signals. 
         [0049]    The bracing member  22  is fixed to the housing by welded or brazed joints and to the seismic masses  20 ,  21  by welded joints. 
         [0050]      FIG. 3   a  illustrates a third embodiment of the present invention. The third embodiment is the same as the second embodiment of the invention, except that the bracing element  32  is provided at around the midpoint of the seismic masses  20 ,  21  and is of a slightly different shape.  FIG. 3   b  is a plan view of the embodiment shown in  FIG. 3   a  and it can be seen from  FIG. 3   b  that the bracing element  32  is coupled to the outer casing  13  on all four sides, to prevent transverse movement in all directions. The joints of the bracing element  32  with the casing  13  are more extensive than in the second embodiment. The bracing element may be fixed to the seismic masses by any suitable means, in this example welding. 
         [0051]      FIG. 4   a  is a fourth embodiment of the present invention. The fourth embodiment is the same as the second embodiment of the invention, except that the bracing element  42  is not connected to the casing  13 , as can be seen from  FIG. 4   b , and is of a different shape. 
         [0052]      FIG. 5   a  illustrates a fifth embodiment of the present invention. The fifth embodiment is the same as the third embodiment of the invention, except that the bracing element  52  is not connected to the casing  13 , as can be seen from  FIG. 5   b , and is of a different shape. 
         [0053]      FIG. 6   a  illustrates a sixth embodiment of the invention. The sixth embodiment is the same as the fourth embodiment in most respects. However, in the embodiment of  FIG. 6   a , the bracing element  62  is fixed to a top surface of the casing  13  by a pair of welded pins  64 ,  66 .  FIG. 6   b  is a plan view of the sixth embodiment illustrating possible positions for the pins. 
         [0054]      FIG. 7   a  illustrates a seventh embodiment of the invention. The seventh embodiment is the same as the sixth embodiment except that the bracing element  72  is fixed to base  12  by a pair of welded pins  74 ,  76 , rather than to the casing  13 .  FIG. 7   b  is a plan view of the seventh embodiment illustrating possible positions for the pins. 
         [0055]    The same or similar materials may be used in all of the described embodiments for the various components. The supporting base  12  may be formed from an alloy such as Inconel® alloy 600 or Inconel® alloy 625 and the casing may be formed or stainless steel. But the materials chosen will depend on the operating environment for the accelerometer and their compatibility. Suitable materials for the tensioning bolts  15 ,  16  are Titanium and Inconel® alloy 718. The piezoelectric elements may be formed of a natural material such as quartz or Tourmaline, a man-made quartz such as Langatate, or a man-made polycrystalline material such as Bismuth Titanate. The seismic masses may be formed or Tungsten or any other suitable high density material. The bracing member may be formed of spring steel or other suitably rigid materials, such as stainless steel or Inconel® alloys. 
         [0056]    It is also possible to use a single seismic mass as shown in  FIGS. 1   a  and  1   b  but fixing the seismic mass to the housing to provide off-axis stabilisation. This can be done by directly coupling the seismic mass to the housing or by using a bracing element as shown in  FIGS. 2   a  and  2   b  for example.