Abstract:
This invention relates to a tri-axial accelerometer with a magnetic sensing element, and has little or no magnetic shielding. Magnetic fields which would otherwise adversely affect the output of the accelerometer are measured and removed from the calculated accelerations in each dimension.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to a tri-axial accelerometer, and in particular to an improvement upon granted patent GB 2 213 272, the disclosure of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     In patent document GB 2 213 272 there is disclosed a tri-axial accelerometer configured as a force balance device with a magnetic proof mass constrained to a null or zero position by an orthogonal set of electrical force coils. An advantage of the device described in that disclosure is that the use of a moving magnet enables the force coils to be body mounted and eliminates the need for electrical flexure connections.  
         [0003]     A disadvantage of a magnetic proof mass is that the magnet will experience a torque proportional to any magnetic field (such as the Earth&#39;s magnetic field for example) perpendicular to the longitudinal (Z) axis.  
         [0004]      FIG. 1  of GB 2 213 272 is reproduced herein (also as  FIG. 1 ) for ease of reference. The magnet  6  and carrier  50  are constrained at one end by flexure  9 . Forces acting upon the magnet, either as a result of accelerations or magnetic fields, are balanced by a radial force in the flexure and an opposing force from electrical currents flowing through the force coils  30 , 32  (in the X direction) and corresponding force coils for the Y direction which are not seen in  FIG. 1  (but which are numbered  29  and  31  in GB 2 213 272). Thus, there is a magnetic sensitivity on the X axis from a radial magnetic field in the X direction and a magnetic sensitivity on the Y axis from a radial magnetic field in the Y direction. The Z axis is not sensitive to a magnetic field in the Z direction because such a field generates no force (torque) upon the magnet  6 .  
         [0005]     In order to seek to eliminate the effect of a magnetic field, GB 2 213 272 discloses magnetic shielding  5  which reduces magnetic fields acting in the X and Y directions to near zero.  
         [0006]     However, magnetic shielding such as that disclosed has a first disadvantage in requiring great concentricity and mechanical stability of the shield  5  in order to minimise magnetically induced radial forces. The magnetic shielding also has a second disadvantage in increasing the overall size of the tri-axial accelerometer; removing the magnetic shielding allows a reduction in the overall size of the accelerometer without changing any of the other components, and allows its use in applications where an accelerometer such as that of GB 2 213 272 would not be possible.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention seeks to provide a tri-axial accelerometer similar to that of GB 2 213 272, but without magnetic shielding. Alternatively stated, magnetic fields acting upon the accelerometer are catered for in a different way than the magnetic shielding of GB 2 213 272. Preferred embodiments of the present invention have no magnetic shielding at all, but in other embodiments it is possible that some of the componentry surrounding the magnet provides some magnetic shielding, albeit not intentionally.  
         [0008]     According to the present invention, there is provided a tri-axial accelerometer comprising a housing, a permanent magnet disposed within said housing, a flexural mounting element attached to said housing and mounting said magnet so that said magnet is displaceable with respect to three mutually perpendicular measurement axes in response to an applied force, and sensing means for sensing displacement of said magnet and for providing a respective output signal proportional to the component of the applied force along each of the three measurement axes, characterised in that means are provided to measure the magnetic field along two of the three mutually perpendicular axes, the measurement of the magnetic field being incorporated into the output signal.  
         [0009]     The invention therefore uses separate means to measure the magnetic field (acting in the X and Y directions), and subtracts or removes the effect of that field from the output of the accelerometer.  
         [0010]     Desirably, the means to measure the magnetic field in each of the two axes is a respective fluxgate. In this respect, it is known that in many applications of an accelerometer, a fluxgate will already be present to measure a component of the Earth&#39;s magnetic field, and the present invention can take advantage of the presence of such an instrument. In particular, a widespread application of a tri-axial accelerometer is in the control of a steerable drill bit used to drill for oil and gas, and the steering component would typically not only carry an accelerometer to determine the local direction of the Earth&#39;s gravitational field, but would also incorporate fluxgates to determine the local direction of the Earth&#39;s magnetic field.  
         [0011]     The measurement of the magnetic field can be removed from the output signal of the accelerometer either by summing a suitably calibrated and scaled correction factor into the accelerometer output, or by removing the magnetic field measurement during calculation of the acceleration.  
     
    
     BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0012]     The invention will now be described in more detail, by way of example, with reference to the accompanying drawings, in which:  
         [0013]      FIG. 1  reproduces  FIG. 1  of GB 2 213 272 for ease of reference;  
         [0014]      FIG. 2  shows an electrical circuit representative of a fluxgate for measuring a magnetic field; and  
         [0015]      FIG. 3  shows an electrical circuit similar to that of  FIG. 5  of GB 2 213 272 for measuring acceleration in a chosen direction, into which the output of the fluxgate is incorporated. 
     
    
     DETAILED DESCRIPTION  
       [0016]      FIG. 1  reproduces a prior art arrangement, specifically  FIG. 1  of granted GB patent 2 213 272. A full description of that document is not necessary in this application, and reference is made to the original document for a full description.  
         [0017]     An element of  FIG. 1  which is particularly relevant to the present invention is the magnetic shielding provided by casing  5 , which casing is electrically insulated from the housing  2 , and is made from a magnetic alloy such as radiometal.  
         [0018]     In the present invention on the other hand, the casing  5  is omitted, and in preferred embodiments the remaining components of  FIG. 1  are utilised without modification. Accordingly, in embodiments of the present invention, the magnet  6  experiences not only forces induced by acceleration (which it is desired to measure), but also torque induced by magnetic field in the X and Y directions (which it is desired to eliminate or ignore).  
         [0019]     Specifically, a magnetic field in the X direction, i.e. left/right in the plane of the paper as viewed in  FIG. 1 , will induce a rotational torque upon the magnet in the plane of the paper (i.e. the X-Z plane), in either the clockwise or counter-clockwise direction depending upon the field direction. Similarly, a magnetic field in the Y direction, i.e. into/out of the plane of the paper as viewed in  FIG. 1 , will induce a rotational torque upon the magnet in the plane perpendicular to the paper and from top to bottom of the paper (i.e. the Y-Z plane). On the other hand, a magnetic field in the Z direction, i.e. top/bottom of the paper as viewed will induce no torque upon the magnet  6  as the magnet  6  is arranged with its poles aligned with the Z direction.  
         [0020]     A magnetic field in the X or Y directions will not induce movement of the magnet  6 , but will induce torque, and because the magnet is held at one of its ends by the plate  9 , that torque will result in movement of the support member  7 , and in particular movement of the annular plate  13  relative to the circuit board  15 . As is stated in GB 2 231 272, and in particular with reference to  FIG. 3  of that document, the circuit board  15  carries another plate which is separated into four quadrants or plate portions. Two of the plate portions are aligned with the X-axis and two are aligned with the Y-axis, (the plate portions are numbered  18 - 21  in  FIG. 3  of GB 2 213 272).  
         [0021]     Accordingly, movement of the plate  13  in the X and Y directions relative to the plate portions carried by the circuit board  15  results in a change of the capacitance between the plate  13  and the respective plate portions which will be measurable.  
         [0022]     The object of the present invention is to permit the movement of the plate  13  which is induced by a magnetic field to be distinguished from the movement of the plate  13  induced by accelerations, so that the output signal which is indicative of the acceleration is not distorted or corrupted by a magnetic field.  
         [0023]     In order to eliminate the magnetic field from the output signal it is first necessary to measure the magnetic field, and in this embodiment this is achieved by a pair of fluxgates  100 , one aligned with the X axis of the accelerometer  1 , the other aligned with the Y axis of the accelerometer  1 .  
         [0024]     A suitable electrical circuit for a fluxgate  100  is shown in  FIG. 2 . This comprises a waveform generator  102  which produces two square wave outputs, a first drive signal along line  104  and a second demodulating signal along line  106 . The frequency of the demodulating output is twice that of the drive output, and these outputs are out of phase as shown in  FIG. 2 .  
         [0025]     The line  104  is connected to the input of an amplifier  110 , the output of which is connected in series to a first input coil  112  and a second input coil  114 . The input coils  112  and  114  lie adjacent to respective ferromagnets  116 ,  120 . The ferromagnets  116 ,  120  extend beyond the input coils  112 ,  114  and also lie adjacent respective output coils  122 , 124 .  
         [0026]     The output coils  122 ,  124  are arranged in series between an earth point  126  and the input of a pre-amplifier  130 .  
         [0027]     The output of the pre-amplifier is connected to a phase sensitive demodulator  132  which is also connected to line  106  so that the phase sensitive demodulator receives the demodulating waveform signal. The output of the phase sensitive demodulator  132  is connected to an output amplifier  134 , and a feedback resistor  136  is connected between the output of the output amplifier  134  and the input of the pre-amplifier  130 .  
         [0028]     The ferromagnets  116 ,  120  are arranged to be parallel with each other, and also parallel to the axis of each of the respective coils  112 ,  114 ,  122 ,  124 , that axis defining the sensing axis S of the fluxgate  100 .  
         [0029]     The coil  112  is designed to match the coil  114 , the coil  122  is designed to match the coil  124 , and similarly the ferromagnet  116  matches the ferromagnet  120 . Accordingly, in the absence of an external magnetic field, the magnetic field induced into the ferromagnet  116  by the coil  112  exactly matches the opposing magnetic field induced into the ferromagnet  120  by the coil  114 , and the current induced in the coil  122  by the ferromagnet  116  exactly matches the opposing current induced in the coil  124  by the ferromagnet  120 , during each cycle of the drive signal.  
         [0030]     It is arranged that the ferromagnets  116 ,  120  reach saturation during each cycle of the drive signal, and in the absence of a magnetic field the ferromagnets  116 ,  120  will reach saturation at the same time in each cycle.  
         [0031]     In these circumstances, the current induced in the coil  122  directly opposes the current induced in coil  124  and no current flows to the preamplifier  130 , so that the output of the output amplifier  134  is also zero.  
         [0032]     In the presence of a magnetic field acting in the direction of the sensing axis S, however, the magnetic field will reinforce the magnetic field induced in one of the ferromagnets  116 ,  120 , and oppose the magnetic field induced in the other of the ferromagnets (depending upon the direction of magnetisation of the ferromagnets during that part of the cycle of the drive signal). This causes the system to become unbalanced with one of the ferromagnets  116 ,  120  reaching saturation before the other, and consequently the current induced into the coils  122 ,  124  no longer balances for all of each half-cycle of the drive signal. A resultant current therefore flows into the pre-amplifier during a part of each half-cycle of the drive signal.  
         [0033]     The resultant current is demodulated by the phase sensitive demodulator  132  and amplified by the output amplifier  134 , so that a signal V HX  is outputted along the line  140 , which signal V HX  is proportional to the magnetic field H X  parallel to the sensing axis S of the fluxgate  100  (the fluxgate  100  of  FIG. 2  having its sensing axis S aligned with the X axis of the accelerometer).  
         [0034]      FIG. 3  shows a force balance circuit substantially identical to that shown in  FIG. 5  of GB 2 213 272. A complete explanation of the circuit is provided in that earlier document and will not be repeated in its entirety here. However, in general terms the circuit  200  acts to provide current through force balance resistors  30 ,  32  (see  FIG. 1 ) so as to oppose movements of the magnet  6  and keep the magnet  6  in its null or zero position.  
         [0035]     The force balance circuit  200  of  FIG. 3  is for the X direction, and it will be understood that an identical circuit is provided for the Y direction. The inputs to the force balance circuit V XA  and V XB  comprise voltage signals indicative of a change in capacitance between the plate  13  and the respective plate portions of the circuit board  15  which are aligned with the X direction, and so are dependent upon movements of the plate  13  in the X direction (i.e. the plate portions numbered  19  and  21  in GB 2 213 272). The circuit  200  can provide a current through the force balance resistors  30  and  32  proportional to the movement of the plate  13  and so as to oppose the movement of the plate  13 , as described in GB 2 213 272.  
         [0036]     In embodiments of the present invention there is no magnetic shielding, so that the signals V XA  and V XB  are dependent not only upon movements induced in the plate  13  by acceleration, but also movement induced in the plate  13  by torque resulting from a magnetic field in the X direction. The circuit of  FIG. 3  upstream of the force balance resistors  30 ,  32  cannot distinguish between those movements, and will oppose both types of movement equally, resulting in the magnet  6  being moved to a null or zero position in which the plate  13  is centrally located with respect to the plate portions upon the circuit board  15 .  
         [0037]     In this regard, it will be noted that the null or zero position of the magnet  6  in the presence of a magnetic field may not be the same as the null or zero position absent a magnetic field, because of deviations in the support  7  caused by the torque upon the magnet  6 . The very high amplifications used, however, mean that actual movement of the magnet  6  is very small indeed, for example of the order of microns.  
         [0038]     In addition, actual movement of the magnet  6  in the presence of a magnetic field is of no consequence to operation of the device, since the plate  13  will still be centrally located with respect to the plate portions and movement induced by acceleration will still be responded to and opposed.  
         [0039]     What is important, however, is that the measured magnetic field be compensated for in the output signal V X  of the circuit  200 , or otherwise be removed from the measured acceleration. In the present embodiment, the magnetic field is removed electrically, i.e. the output signal V HX  from the fluxgate  100  is added to the voltage output of the circuit  200  (it can readily be arranged that the output signal V HX  is made negative if the output from the amplifier  49  is positive, and vice versa.  
         [0040]     In order to determine the correct scale factor for the signal V HX , a resistor  202  is used between the output from the output amplifier  134  of the circuit  100  and the connection  204  where the signal V HX  is communicated to the circuit  200 .  
         [0041]     The size of the resistor  202  is determined by calibration. For example, the accelerometer is oriented with its Z axis vertical. A known magnetic field is then applied horizontally and the accelerometer rotated. In such circumstances there are no accelerations in the X and Y directions, so that the outputs V X  and V Y  should both be zero. The output from the amplifier  49 , and the output V HX  from the circuit  100  (and similarly the output V HY ) will not be zero, however, because of the applied magnetic field, and the values of these outputs will change cyclically with rotation of the accelerometer. With a particular value for the resistor  202 , however, for each of the circuits  200  (i.e. for the X and Y directions respectively) the signals will cancel out so that the output signal V X , and similarly the output signal V Y , remain zero as the accelerometer is rotated.  
         [0042]     Alternatively, the measured magnetic field can be removed during the calculation of the acceleration. Thus, it will be understood that the output signal V X  is not a measure of the acceleration but is proportional to the acceleration. Suitable calibration and calculation is required to determine the actual acceleration for the measured output signal V X . When the accelerometer is subsequently in use, the signal V HX , which it itself suitably calibrated, can be incorporated into the calculation to remove the effect of the magnetic field from the calculated acceleration (and similarly for the output in the Y direction).  
         [0043]     It will be understood that a fluxgate is not required for the Z axis since the accelerometer is not sensitive to magnetic fields in that direction, i.e. the Z axis is aligned with the axis joining the poles of the magnet and so no torque is generated by a magnetic field in that direction. The accelerometer can, however, measure accelerations in the Z direction as is described in GB 2,213,272.