Abstract:
The invention relates to a vibrating gyroscope comprising vibrating cylinder ( 1 ) that is magnetically or electrostatically excited. Regularly distributed masses ( 19 ) designed to lower the vibration frequency of said cylinder are arranged thereon. The inventive gyroscope is much more accurate than conventional gyroscopes and can be produced easily at low cost. The invention can be used to measure angular rotation or angular speed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention concerns a vibrating gyroscope for accurately measuring angular rotations. Compared with the techniques generally used, this gyroscope proves to be more effective, occupies less space, is simple to embody and is less expensive. 
     2. Description of the Invention 
     Vibrating gyroscopes are based on the effect of Coriolis forces due to a rotation imposed on moving masses. 
     Several embodiments have been previously proposed for embodying a vibrating element sensitive to angular speeds. 
     The method most frequently used consists of making an annular, hemispherical or cylindrical test body of revolution vibrate perpendicular to its axis of symmetry and of observing the movement of the vibration modes when it is subjected to rotation around said axis. 
     In the most general case of annular, hemispherical or cylindrical test bodies, the main difficulty derives from the compromise that has to be made between the resonance frequency which increases with the spatial requirement reduction and the time constant which determines the performance and which is improved when the resonance frequency is low. For example, it is virtually impossible to embody a cylindrical test body having a thin wall, a volume smaller than 2 cm3 and a resonance frequency lower than 6 kHz. Now it would be desirable to have small test bodies resonating only between 2 and 3 kHz so as to obtain much improved performances. 
     The second difficulty originates from the embodiment of the excitation and vibration measuring device, it being understood that the term ‘excitation’ denotes all the commands required for the proper functioning of these gyroscopes. 
     Solution put forward to date for creating, detecting and maintaining vibration are basically of the electromagnetic, electrostatic or piezo-electric types. 
     The electrostatic solutions have advantageous performances when they are used under vacuum so as to reduce losses. Because they require extremely small air gaps, they are difficult to implement inside or outside a hemispherical or cylindrical wall and are thus generally expensive. 
     The piezo-electric solutions use either a cylinder made fully of a piezo-electric material, or small piezo-electric elements mounted, most frequently by glueing, on a metal cylinder. The solutions have one major drawback when used in gyrometric applications for which they are basically adapted of being unable to adjust the axis of excitation with respect to the vibrating body which generally has one overriding direction for which performances are optimum. 
     For various reasons and in particular for reasons of cross talk, the means for detecting and exciting the vibrations of certain embodiments, are heterogeneous and are spaced as far a s possible from one another. 
     For example, the U.S. Pat. No. 4,793,195 describes a gyrometer with a vibrating cylinder provided with electrostatic detection and magnetically excited at a frequency half its vibration frequency so as to reduce these effects. 
     The French patent application 97/12129 describes a gyrometer with multiplexed magnetic detection and excitation which clearly resolves the difficulty of crosstalk between excitation and detection but whose performances are limited by Vie resonance frequency which remains high. 
     OBJECT OF THE INVENTION 
     The present invention brings about an improvement which, in a given spatial requirement, makes it possible to choose the resonance frequency and via its principle offers new possibilities for simply and cheaply embodying electromagnetic or electrostatic detection and excitation means. 
     So as to reach this result, the thin-walled test body of revolution comprises at its periphery evenly distributed masses separated by intervals which increase the moving mass when said test body is excited on vibration Openings can be fitted in the thin wall of the cylinder and not covered by the masses so as to adjust the stiffness of the end of these masses and thus the resonance frequency. This makes it possible to significantly reduce the resonance frequency of said test body and thus increase performances. 
     By acting on the shape of the openings, it is possible to favour certain types of movements of additional masses and thus embodying inexpensive flat electrostatic or magnetic detection/excitation units able to be placed at the right of a flat open extremity of the test body and thus extremely easy to adjust. 
     SUMMARY OF THE INVENTION 
     Thus, the invention concerns a vibrating gyroscope of the type comprising: 
     a thin and vibrating element and approximately generated by rotation, 
     excitation means for generating vibrations at least one point of the vibrating element so as to make appear on said vibrating element vibration modes able to be modified under the effect of an angular speed of rotation, and 
     means for detecting said vibrations and arranged so as to be able to detect said vibration modes, 
     characterised in that the vibrating element approximately generated by rotation at receives at least three and preferably eight masses forming vibrating masses and preferably constituted by excessive thicknesses of the vibrating element itself. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     There follows hereafter non-restrictive embodiments of the invention given solely by way of example with reference to the accompanying drawings on which: 
     FIG. 1 is a skeleton diagram showing the functioning of a vibrating gyroscope, 
     FIG. 2 is a cutaway side view of the vibrating gyroscope of the invention, 
     FIG. 3 is an axial cutaway view showing the vibrating gyroscope of FIG. 2 along the direction A, 
     FIG. 4 is a cutaway side view of the vibrating gyroscope of FIG. 2 in a variant with excitation and electrostatic detection, 
     FIG. 5 shows two views of a variant of the test body of the vibrating gyroscope of FIG. 2, 
     FIG. 6 is a cutaway side view of the variant of the test body of FIG. 5, 
     FIG. 7 shows two views of a preferred variant of the test body of the vibrating gyroscope of FIG. 2, 
     FIG. 8 is a cutaway side view of the variant of the test body of FIG. 7, 
     FIG. 9 is a cutaway side view of a variant of the gyroscope of FIG. 2 using the test body of FIG. 7 and a flat electromagnetic excitation/detection unit, 
     FIG. 10 is an axial cutaway view of the variant of the gyroscope of FIG. 9 along the direction A, 
     FIG. 11 is a cutaway side view of a variant of the gyroscope of FIG.  9  and using a flat electrostatic excitation/detection unit, and 
     FIG. 12 is a skeleton diagram of the electric and electronic circuits of the vibrating gyroscope of the invention in the multiplexing version with excitation and detection functions adapted to the electrostatic detection and excitation gyroscopes of FIGS.  4  and  11 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As previously mentioned and shown on FIG. 1, a vibrating gyroscope comprises a test body  1 , having an axis of symmetry  6 , cylindrical for example (FIG. 1 a ) but which can be hemispherical or have any other shape of revolution, and which is excited on vibration (FIG. 1 b ) along two initial directions  2  and  3  perpendicular to each other and to the axis  6  of the test body  1  so that four nodes  4  and four vibration antinodes  5  appear, the movements of the portions situated on the vibration antinodes being in phase opposition for the two initial excitation directions  2  and  3 . 
     When the test body  1  is subjected to an angular speed rotation Ω around an axis parallel to the axis of symmetry  6 , the vibration nodes do not rotate with the test body. They no longer remain fixed in space but they rotate with respect to the inertial space at an angular speed ω=K.Ω which depends on the geometry and angular speed of the test body. The theoretical ratio K between the angular speed of the test body and that of the vibration nodes also depends on the vibration mode. For example, it is possible to make the test body vibrate with six vibration nodes and six vibration antinodes, but the corresponding configuration is less favourable for the gyroscopic measurement. 
     Thus, the vibration nodes  4  are not linked to the test body  1 , but move with respect to the latter with an angular speed, also proportional to the angular speed of the test body itself. 
     FIG. 2 shows a cutaway view of a preferred embodiment of the vibrating gyroscope of the invention. 
     This comprises: 
     a test body or vibrating cylinder  1 , 
     a support  7 , 
     an external cylindrical box  8 , 
     a magnetic detector exciter or stator  9 , 
     an electronic circuit  10 , 
     fixing, cabling and closing means. 
     The test body is embodied in the form of an approximately cylindrical. vibrating element or vibrating cylinder  1  having an axis of symmetry  6 , a wall  11  open at one of its extremities  12  and closed at its other extremity  13  by a wall forming a bottom  14 . The wall  11  of the vibrating cylinder  1  is thin and regular on one portion of its length  15  close to the bottom  14 . Said bottom comprises an external portion  16  having approximately the same thickness as that of the wall  11  of the cylinder and at the centre a thicker portion  17 . 
     The wall  11  of the vibrating cylinder bears on one portion of its height  18  close to its open extremity  12  at least three and preferably eight excess thicknesses or masses  19  evenly distributed and whose shape can be any. In one preferred embodiment of the invention, the height of these excess thicknesses  19  is parallel to the axis of symmetry  6  of the test body and approximately equal to half the total height of said test body. Their section as shown on FIG. 3 perpendicular to the axis of symmetry  6  is externally bordered by an arc of a circle  20  centred on said axis of symmetry  6 . It is bordered on the sides by two blanks  21  and  22  parallel to said axis of symmetry and orientated so that one blank  21  with an excess thickness  23  is parallel to one immediately adjacent blank  22  with an excess thickness  24 . This arrangement facilitates machining of said excess thicknesses by means of milling. 
     The bottom  14  is fixed at its centre onto the support  7  by an internal foot  25 . 
     The support of revolution  7  comprises a first portion  26  whose diameter is such that it is able to receive the external box  8 , and a second portion comprising two successive decreasing diameters  27  and  28 , the second diameter being used to act firstly as a support for the magnetic stator  9  on which coils  30  are placed, and secondly as a support for the vibrating cylinder  1 . 
     It is dimensioned so that the stator  9  is placed centred in the open extremity  12  of the vibrating cylinder  1  thus providing an air gap  29  having a thickness reduced as far as possible. 
     The magnetic exciter is embodied in the form of an eight-branched star  31  and thus comprises eight poles  32  on which the coils  30  are placed. 
     As described above, the gyroscope of the invention functions as follows by first of all making the hypothesis that the losses are nil and that the vibrations once established conserve their energy. The vibrations are initially created on two pairs of masses  23 ,  33  and  34 ,  35  for example placed on two perpendicular axes  3  and  2 , the other four masses  36  to  39  not vibrating. In the absence of rotation, the vibrating state does not change. In the presence of a rotation Ω around the axis  6 , the effect of the Coriolis forces results in a transfer of energy of the masses which initially vibrated towards the latter which did not vibrate so that the total energy is retained. If A is the initial peak amplitude of the vibrations of the two pairs of masses  23 ,  33  and  34 ,  35 , the peak amplitude of these vibrations at the end of a time t is written: 
     
       
           A=A .cos[2(1 −K ),  fΩ.dte].   
       
     
     Similarly, the peak amplitude of the vibrations of the four other masses  36 ,  37  and  38 ,  39  is written: 
     
       
           B=A .sin[2(1 −K ),  fΩ.dte].   
       
     
     Because the losses by rubbing in the material are not nil, the vibrations tend to dampen and need to be maintained so as to ensure functioning of the gyroscope. 
     To this effect, by using the well known principles of electronic circuits, the amplitudes of the vibrations of each of the 4 pairs of masses are measured and are used to draw up holding and correction voltages which are sent to the windings. Advantageously, the principles for multiplexing excitation and detection described in the French patent application no 97/12129 shall be used 
     For a gyrometer usage, again using the well-known principles of counter-reaction and preferably the multiplexing technique, the four masses  36 ,  37 ,  38  and  39  are kept immobile by sending to the corresponding windings a counter-reaction voltage opposing the effects of the Coriolis forces. This counter-reaction voltage is then representative of the angular speed Ω. The vibration amplitude of the masses  23 ,  33 ,  34  and  35  is kept constant. 
     Thus as shown on FIG. 4, the electromagnetic detection/excitation unit  9  can be replaced by an electrostatic detection/excitation unit  40 . To this effect, the stator  9  of FIG. 2 is replaced by a ring  41  made of a non-conducting material with at least two and preferably eight or more electrodes  42  being placed on the periphery of said ring. The outer diameter of this ring is such that the electrodes are found opposite the internal face of the cylinder  1  with an air gap  29  reduced as much as possible. 
     So as to improve the performances of the vibrating gyroscope, it may be necessary to reduce the stiffness provided by the thin portion of the vibrating cylinder. As shown on FIG. 5, one first variant of the invention consists of embodying in the thin wall  11  of the vibrating cylinder openings  43 , said openings being evenly distributed and centred approximately between the masses  19 . Seen from the side, FIG. 5 a  shows a vibrating cylinder pierced with eight relatively fine long openings  43  whose largest dimension is approximately parallel to the axis  6 . These openings preferably go down as far as the thin portion  16  of the bottom  14  of the vibrating cylinder. At their other extremity, they may be closer or further away from the upper portion of the thin wall  11  of the cylinder remaining between the masses, the thin wall portion remaining between said openings  43  and the extremity  12  of the cylinder constituting the elastic bridges  79  between the masses  19 . Moreover in the example shown, the height and position of the additional masses are such that the latter go past the height of the cylinder itself, thus forming notches on the side of the open extremity  12  of said cylinder  1 . 
     FIG. 6, which is a cutaway view of the vibrating cylinder described above, shows, by exaggerating it with respect to reality, the movement of two of the masses  19  under the effect of the vibrations. Because of the shapes retained and the position of the openings, it appears that the masses  19  on vibrating carry out a rotation movement approximately centred at a point  44  corresponding to the joining point between the extremity  13  of the cylinder  1  and the flat thin wall  16 . If one considers the movement from an upper corner  45  of the mass  19 , this drawing shows that under the effect of this rotation that the upper portion of the masses and in particular the point  45  is moved firstly by a translation movement perpendicular to the axis  6 , and secondly by a translation movement  50  parallel to said axis  6  but with a much more reduced amplitude. 
     Still with the aim of improving the performances by adjusting as best as possible the rigidities and the masses, it is possible to extend the openings  43  made in the wall  11  of the vibrating cylinder  1  onto the bottom  14  in the direction of the foot  25 . FIG. 7 is divided into FIG. 7 a  and  7   b , the first being a side view of the test body and the other a top view of said body. The openings  43  are extended by grooves  46 , preferably radial, on the bottom  14 . These grooves  46  are preferably narrowed towards the centre and the masses  19  are therefore connected to the centre by a cylindrical wall portion  47  and by a flat sector  48  perpendicular to said cylindrical wall portion  47 , said flat sector comprising a narrowing  49  close to the centre. 
     In addition, the openings  43  are extended on the wall  11  between the masses  19  in the direction of the extremity  12  of the vibrating cylinder  1  so that the remaining portion of said wall  11  between, said masses  19  is approximately reduced and constitutes an elastic bridge  79  between these masses. 
     Because of the narrowing  49  and the elongation towards the extremity  12  of the openings  43 , the most flexible portion of the link between the masses and the centre is located exactly at the location of this narrowing  49 . Thus, as shown on FIG. 8, the hinge point  44  of the movement of the masses  19 , which is located approximately at this most flexible location, is thus much closer to the axis  6  than that of the preceding variant of FIG.  6 . FIG. 8 also shows the movement of the masses  19  in the configuration of FIG.  7 . It appears that the translation movement  50  parallel to the axis  6  from the corner  45  is much larger and it can also be used to excite and detect the vibrations with an excitation/detection system having a flat interface with the vibrating element, said interface being constituted by air gaps  29 , as shown on FIG.  9 . In this configuration, the shape of the elastic bridges  79  is determined so as to harmonise the various rigidities and avoid creating parasitic resonance frequencies too close to the nominal frequency of the test body. 
     FIG. 9 thus shows a cutaway view of a first example of the gyroscope using this translation movement parallel to the axis  6  with an electromagnetic excitation/detection system having along with the vibrating element an interface of revolution  83  centred on the axis  6  and preferably flat. The masses  19  having a section perpendicular to the large axis  6  can be embodied with one extremity or face  51  situated on the side of the open portion  12  of the vibrating cylinder, said section being flat and thus able to be used with a simple electromagnet, the faces  51  of each of the masses  19  being approximately coplanar. 
     The detection/excitation unit then includes at least two, but preferably eight electromagnets  52  secured to the support  7  and whose magnetic cores  53  each have one flat extremity  54 . Said flat extremities  54  are coplanar between one another and each extremity is placed opposite a flat extremity  51  from one of the masses  19  with an air gap  29  reduced as much as possible, the air gaps  29  forming the interface  83 . 
     It is to be noted that the interface  83  could be slightly conical or even spherical or more generally have a not fully flat shape without departing from the context of the invention. 
     So as to be more effective by reducing losses, each of the electromagnets may include, as shown on FIG. 10, two short cores  53  having axes approximately parallel to the axis  6 , each core preferably being placed at an equal distance from said axis  6 , said cores being interconnected, preferably two by two, by a magnetic reinforcement  55  secured to the support  7  opposite the flat face  51  of the rings  19 . A winding  56  is placed around each of these cores  53 . So as to further reduce the magnetic losses, a plate  57  made of a low loss magnetic material and preferably having the same surface as the section of the masses  19 , is secured to each flat face  51  of said masses and via the air gap  29  close a magnetic circuit constituted by a pair of cores  53  and their reinforcement  55 . The outer face of the magnetic plate then constitutes the flat face  51 . 
     In this configuration as in the configuration of FIG. 4, the gyroscope may advantageously use the multiplexing technique described in the French patent application no 97/12129. It can also be used as a gyroscope or a gyrometer. 
     The vibrating cylinder of FIG. 7 is also clearly suitable in the use of an electrostatic flat interface detection/excitation system, as shown on FIG. 11 showing also a gyroscope equipped with this system. This system comprises a non-conducting crown  58  secured to the support  7  opposite the flat faces  51  of the masses  19 . Secured to this crown are at least and preferably eight electrodes  59 , each electrode being placed opposite one of the faces  51  of said masses  19 . 
     The vibrating cylinder is positioned on the support so that the air gap  29  between the electrodes and the faces  51  constituting the interface  83  is as reduced as possible. 
     This latter variant preferably uses one multiplexed electronics unit whose principle is shown on the diagram of FIG.  12  and which avoids any problem of crosstalk between the excitation signals and the detection signals. 
     In this solution, the electrodes  59  are in turn used to excite and then detect the vibrations of each of the masses, knowing that it is also possible to use one portion of the electrodes for detection and use the others for excitation. 
     The electrodes are connected by pairs, the electrodes of a given pair being placed symmetrically with respect to the axis  6 . 
     The faces  51  of the electrically interconnected masses  19  form a counter-electrode  77  fed by a circuit  78 . 
     Each of the pairs of electrodes is connected to a circuit changer  89 ,  80 ,  81 ,  82  and controlled by a sequencer  60 . When the circuit changers are in the position B, the system functions in detection mode. When they are in the position C, they function in the excitation mode. 
     In the detection mode, the signal derived from the pairs of electrodes firstly respectively  61 ,  65  and  63 ,  67  and secondly  62 ,  61  and  64 ,  68  are sent by means of two differential amplifiers  69  and  70  respectively to a calculation circuit  71  which works out on four outlets  73 ,  74  and  75 ,  76  respectively four excitation voltages two by two in antiphase sent to the pairs of electrodes by means of the circuit changers when the latter move into the excitation position C. 
     The calculation circuit  71  works out the excitation frequency so that the latter corresponds to the resonance frequency of the masses of the vibrating cylinder. 
     The circuit  71  works out outgoing information  72  which represents the, rotation fΩ.dt of the gyroscope. 
     The sequencer  60  is synchronised by the excitation frequency with the aid of a signal derived from the calculation circuit  71 . 
     The calculation circuit also makes the corrections required to the errors brought about by the residual resonance deviations existing between the two vibration modes situated 45° from each another. 
     In a gyrometer use mode, the calculation circuit controls the vibration of the masses situated opposite the electrodes  62 ,  66  and  64 ,  68  to be nil, both in phase and in quadrature and thus compensate the resonance deviations. 
     The operating frequency of the sequencer  60  is a sub-multiple of the actual frequency of the vibrating cylinder  1 . The cyclic ratio of switching between the excitation time and the detection time may be 1/1. It can also advantageously be 1/2, 1/3, 1/4 or even lower, this depending on the excess voltage of said vibrating cylinder. The switchings of the excitation function to the detection function are preferably carried out at the time when the voltage on the electrodes  61  to  68  moves to zero. The switchings of the detection, function to the excitation function are preferably carried out at the time the voltage control sine wave in said electrodes moves to zero. 
     So as to obtain better distribution of the various resonance modes of the vibrating element, it is possible to replace all or part of the cylindrical wall  11  and the bottom  14  of the vibrating cylinder by a curved surface and apply to this new element all the arrangements and improvements described above, this new vibrating element then having for example a hemispherical, ellipsoid, parabolic shape etc., without departing from the context of the invention. 
     Finally, the use of additional masses can also be applied to a ring-shaped vibrating element without departing from the context of the invention.