Abstract:
This invention describes the addition of mechanical reference members (MRM) to MEMS gyroscopes and accelerometers in order to enable the measurement of their scale factor and bias characteristics. The measurements can be made prior to or during operation of the instruments. This approach is attractive since MEMS devices are subject to drift of these characteristics with time, with the environment and with application conditions. The mechanical reference members are used to provide a rotation rate reference for the gyroscopes and an acceleration reference for the accelerometers.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims priority of provisional application No. 60/284,348, filed on Apr. 17, 2001. 
     
    
     
       BACKGROUND OF INVENTION  
         [0002]    Typically, gyroscopes and accelerometers are tested on stable test tables/stations that can provide precise inputs to determine the scale factor and bias characteristics of the instrument. This is an accurate method for factory/baseline instrument testing. The factory can test a large quantity of devices simultaneously to spread the high cost of testing. However, when MEMS devices are in the field, since they are low cost, they cannot economically be re-tested due to the high cost of factory testing. A built-in test approach is needed to monitor the scale factor and bias characteristics as they drift with time, temperature and due to application conditions.  
           [0003]    Present approaches measure the instrument characteristics fully at the factory over ranges of temperatures and vibrations and other testable environments. The data is then used to compensate the instruments during operation, while also measuring the environment. The disadvantage of this approach for MEMS devices is that an insufficient amount of testing may have been done during testing for all the conditions that are likely to occur. In addition, MEMS devices are likely to be unstable and drift with time as stresses relieve themselves and as materials creep, and so forth. Time dependent drifts of the characteristics may be unpredictable.  
         SUMMARY OF INVENTION  
         [0004]    This invention relates to the addition of integral mechanical reference members (MRM) to MEMS gyroscopes and accelerometers in order to enable the measurement of their scale factor and bias characteristics. The measurements can be made prior to and/or during operation of the instruments. This approach is attractive since MEMS devices are subject to drift of these characteristics with time, with the environment and with application conditions. The mechanical reference member provides a rotation rate reference for the gyroscopes, and an acceleration reference for the accelerometers. The built-in/internal capability to measure the characteristics as needed would greatly reduce the up front testing and thus the instrument cost.  
           [0005]    It is an object of this invention to add a mechanical reference member (MRM) to a MEMS gyroscope that allows the gyroscope to be rotated about its input axis at a known rotation rate (rotation angle with time). The mechanical member will be referred to herein as the Rotation Mechanical Reference Member (RMRM). By inputting different rotation rates, the gyroscope characteristics (scale factor and bias) can be determined.  
           [0006]    It is a further object of this invention to rotate the RMRM according to a variety of input waveforms that describe rotation angle with time. One example is a ramp waveform that describes a constantly varying angle with time to a maximum angle; the waveform is then reset to zero angle and repeated. The input rate in this case is constant and can serve as a reference rotation rate value. By varying the ramp period, other reference rotation rates can be applied.  
           [0007]    It is a further object of this invention to rotate the RMRM with a sinusoidal waveform that varies the rotation rate with time. In this case the rotation rate is varied from zero rotation rate to a maximum rotation rate that generates an output that varies sinusoidally from zero output to a maximum output. By varying the amplitude of the input rate oscillation, the linearity can be determined in addition to the gyroscope scale factor and bias.  
           [0008]    It is a further object of this invention to add a mechanical reference member to a MEMS accelerometer that allows the accelerometer to be accelerated along its input axis. The mechanical reference member will be referred to herein as the Acceleration Mechanical Reference Member (AMRM). By inputting different acceleration levels, the accelerometer characteristics (scale factor and bias) can be determined.  
           [0009]    It is a further object of this invention to accelerate the AMRM according to a variety of input waveforms that describe acceleration with time. A sinusoidal variation of acceleration with time produces a sinusoidal output with time. By varying the amplitude of the input acceleration oscillation, the scale factor and bias of the accelerometer can be determined.  
           [0010]    It is a further object of this invention to add an integral member to the MEMS gyroscope that changes the input axis direction in order to remove the bias from the gyroscope data.  
           [0011]    It is a further object of this invention to add an integral member to the MEMS accelerometer that changes the input axis direction in order to remove the bias from the accelerometer data.  
           [0012]    This invention features an improved micro electromechanical systems (MEMS) gyroscope with an internal rotational reference, the gyroscope comprising a rotor member driven to oscillate about a rotor axis and coupled to an output member that is adapted to oscillate about an output axis that is orthogonal to the rotor axis, and means for resolving the oscillation about the output axis of the output member, the output member oscillation at least in part being induced by gyroscope rotation about a gyroscope input axis that is orthogonal to both the rotor axis and the output axis, the improvement comprising: an integral mechanical reference member flexurally coupled to the gyroscope and adapted to oscillate the gyroscope about the input axis; means for moving the mechanical reference member about the input axis; and means, responsive to the means for resolving, for determining the output member oscillation caused by the mechanical reference member oscillation, as a measure at least one of the scale factor and bias offset of the gyroscope.  
           [0013]    The improvement may further comprise means for changing the orientation of the gyroscope input axis by 180 degrees to factor out bias offset from the input rate determination. The mechanical reference member movement about the input axis is preferably repetitive. Preferred motions are sinusoidal, a sawtooth, and a ramp.  
           [0014]    The mechanical reference member may be coupled to the output member. In situations in which the gyroscope has an outer member, the mechanical reference member may be coupled to the outer member.  
           [0015]    This invention also features an improved micro electromechanical systems (MEMS) gyroscope with an internal rotational reference, the gyroscope comprising a rotor member driven to oscillate about a rotor axis and coupled to an output member that is adapted to oscillate about an output axis that is orthogonal to the rotor axis, and means for resolving the oscillation about the output axis of the output member, the output member oscillation at least in part being induced by gyroscope rotation about a gyroscope input axis that is orthogonal to both the rotor axis and the output axis, the improvement comprising: an integral means for causing motion of the output member about the output axis; and means, responsive to the means for resolving, for determining the output member motion caused by the integral means, as a measure at least one of the scale factor and bias offset of the gyroscope.  
           [0016]    Also featured is an improved micro electromechanical systems (MEMS) gyroscope with an internal rotational reference, the gyroscope comprising a rotor member driven to oscillate about a rotor axis and coupled to an output member that is adapted to oscillate about an output axis that is orthogonal to the rotor axis, and means for resolving the oscillation about the output axis of the output member, the output member oscillation at least in part being induced by gyroscope rotation about a gyroscope input axis that is orthogonal to both the rotor axis and the output axis, the improvement comprising: integral means for changing the orientation of the gyroscope relative to the input axis by 180 degrees, to provide for factoring out of the gyroscope bias.  
           [0017]    The invention is also applicable to an improved micro electromechanical systems (MEMS) accelerometer with an internal acceleration reference, the accelerometer comprising a proof mass adapted to move along an input axis in response to acceleration, and means for resolving movement of the proof mass along the input axis, the improvement comprising: an integral mechanical reference member flexurally coupled to the accelerometer and adapted to move back and forth along the input axis; means for moving the mechanical reference member along the input axis, the movement causing movement of the proof mass along the input axis; and means, responsive to the means for resolving, for determining at least one of the scale factor and bias offset of the accelerometer.  
           [0018]    In this aspect, the improvement may further comprise means for changing the orientation of the input axis by 180 degrees to factor out bias offset from the input acceleration determination. The mechanical reference member movement along the input axis is preferably repetitive. Preferred motions are sinusoidal, a sawtooth and a ramp.  
           [0019]    The mechanical reference member may be coupled to the output member. In cases in which the accelerometer has an outer member, the mechanical reference member may be coupled to the outer member.  
           [0020]    This invention also features an improved micro electromechanical systems (MEMS) accelerometer with an internal acceleration reference, the accelerometer comprising a proof mass adapted to move along an input axis in response to acceleration, and means for resolving movement of the proof mass along the input axis, the improvement comprising: integral means for causing motion of the proof mass along the input axis; and means, responsive to the means for resolving, for determining at least one of the scale factor and bias offset of the accelerometer.  
           [0021]    Also featured is an improved micro electromechanical systems (MEMS) accelerometer with an internal acceleration reference, the accelerometer comprising a proof mass adapted to move along an input axis in response to acceleration, and means for resolving movement of the proof mass along the input axis, the improvement comprising: integral means for changing the orientation of the accelerometer relative to the input axis by 180 degrees, to provide for factoring out of the accelerometer bias.  
           [0022]    Further featured in the invention is an improved micro electromechanical systems (MEMS) inertial instrument with an internal reference, the instrument comprising a first structure adapted to move relative to an input axis in response to motion, and means for resolving movement of the first structure relative to the input axis, the improvement comprising: an integral mechanical reference member flexurally coupled to the instrument and adapted to move relative to the input axis; means for moving the mechanical reference member relative to the input axis, the movement causing movement of the first structure relative to the input axis; means for resolving motion of the first structure relative to the input axis; and means, responsive to the means for resolving, for determining the scale factor and bias offset of the instrument. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0023]    Other objects, features and advantages will occur to those skilled in the art from the following descriptions of the preferred embodiments, and the accompanying drawings, in which:  
         [0024]    [0024]FIG. 1 shows an Output versus Input characteristic with which gyroscope scale factor and bias are defined for this invention.  
         [0025]    [0025]FIG. 2 shows an Output versus Input characteristic of this invention with superimposed oscillatory input and resulting oscillatory output.  
         [0026]    [0026]FIG. 3 shows an Output versus Input characteristic of this invention with superimposed oscillatory input added to a DC input.  
         [0027]    [0027]FIG. 4 a  illustrates how the flip of the input axis of this invention can be used to separate bias from the actual output according to this invention.  
         [0028]    [0028]FIG. 4 b  illustrates, in a close-up view, how the bias is separated from the actual output.  
         [0029]    [0029]FIG. 5 is a conceptual rendition of a MEMS gyroscope of this invention with a Rotation Mechanical Reference Member.  
         [0030]    [0030]FIG. 6 is a conceptual rendition of a MEMS gyroscope of this invention with a Rotation Mechanical Reference Member and Flip Member that enables flip of the input axis by 180 degrees.  
         [0031]    [0031]FIG. 7 is a conceptual rendition of a MEMS accelerometer of this invention with an Acceleration Mechanical Reference Member.  
         [0032]    [0032]FIG. 8 is a conceptual rendition of a MEMS accelerometer of this invention with an Acceleration Mechanical Reference Member and Flip Member that enables flip of the input axis by 180 degrees.  
         [0033]    [0033]FIG. 9 is a conceptual rendition of a MEMS gyroscope of this invention with a Rotation Member that rotates the gyroscope input axis into Zero Degrees Alignment and 180 Degrees Alignment.  
         [0034]    [0034]FIG. 10 is a conceptual rendition of a second MEMS gyroscope of this invention with a Rotation Mechanical Reference Member.  
         [0035]    [0035]FIG. 11 is a conceptual rendition of a second MEMS gyroscope of this invention with a Rotation Mechanical Reference Member and a Flip Member.  
         [0036]    [0036]FIG. 12 is a conceptual rendition of a MEMS Tuned Flexure Accelerometer of this invention with an Acceleration Mechanical Reference Member.  
         [0037]    [0037]FIG. 13 is a conceptual rendition of a MEMS Tuned Flexure Accelerometer of this invention with an Acceleration Mechanical Reference Member and a Flip Member.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0038]    Mechanical Reference Member Mechanization  
         [0039]    This invention may be accomplished with mechanical means added to gyroscopes and accelerometers to enable internal testing of at least scale factor and bias characteristics. The mechanical means introduces a known rotation rate to the gyroscope and a known acceleration to the accelerometer. The mechanical means preferably comprises a gimbal that rotates the gyroscope about the input axis, and a gimbal that allows the accelerometer to be translated along its input axis. The rotation and acceleration inputs are most likely repetitive (e.g. oscillatory), with an amplitude and frequency to be determined by the needs of the gyroscope and accelerometer; scale factor and bias are the primary characteristics, while non-linearity is also important. The amplitude of the oscillatory input needs to be known precisely or held constant relative to a voltage reference.  
         [0040]    MEMS size and MEMS integration allows these mechanical members to be built into (i.e., integrated into) the instruments.  
         [0041]    Modes of Operation  
         [0042]    The mechanical reference inputs can be operated during an initializing phase prior to use, operated continuously before and during operation, or operated as needed.  
         [0043]    Scale Factor and Bias Characteristics  
         [0044]    Scale factor and bias characteristics are common to the gyroscope and to the accelerometer. They are obtained by applying different inputs to the instruments and measuring the outputs. When the data is plotted, the scale factor is the slope of the linear region of the curve, and the bias is the offset, as shown in FIG. 1.  
         [0045]    One method under the invention of measuring the scale factor and bias is to use a MRM to apply to the instrument a sinusoidal input, as shown in FIG. 2. The output is also sinusoidal. By taking the ratio of output amplitude to input amplitude, the scale factor is obtained. By processing the output signal to obtain the DC level, the bias is determined. The bias, however, can only be determined when zero input rate is applied.  
         [0046]    The motion of the mechanical reference members determines the reference rotation rate and acceleration inputs applied.  
         [0047]    Scale Factor and Bias Characteristics During Operation  
         [0048]    During operation, the bias cannot be distinguished from the actual output. FIG. 3 shows the modification required to FIG. 2 to illustrate this case. The input is comprised of the actual input to be measured plus the reference oscillation used to measure the characteristics. The input oscillation is shown displaced in the horizontal direction. The output contains the oscillation that can be used to calculate the scale factor as was done for the zero input case of FIG. 2. The DC component of the output is the sum of the actual output plus the bias. The problem is that the bias can change during operation and therefore is assumed to be actual output. An additional procedure is thus needed to separate the bias from the actual output.  
         [0049]    Separation of the Actual Output from Bias During Operation  
         [0050]    A different procedure is needed to separate the actual output from the bias during operation of the instrument. The procedure is to change the input axis orientation by 180 degrees. This has the effect of changing the sign of the output. Assuming that the procedure does not introduce an additional bias contribution, the bias should not change. FIG. 4 a  illustrates the characteristic for the initial input axis alignment, and for the 180 degrees re-alignment. Two output curves are shown with the same slopes but of opposite sign and that pass through the same bias. FIG. 4 b  is used to observe a close-up of the output for the case of a given input. R 0  is the output for the Zero Degrees Input Axis Alignment. R 180  is the output for the 180 Degrees Input Axis Alignment. The actual data is then obtained by (R 0 −R 180 )/2.  
         [0051]    A second approach is to change the input over a small angle rather than through 180 degrees. The change in either case can be accomplished by any useful means, for example by flipping the instrument, or turning the instrument through the desired angular change (typically 180 degrees).  
         [0052]    Accuracy of Mechanical Reference Member Motion  
         [0053]    The accuracy of the mechanical reference member motion can be controlled relative to a voltage standard. An alternative approach is to include fiducials in the structures that are placed at known intervals (like marks on rulers). The mechanical reference member is controlled by measuring its motion relative to the fixed fiducials. Fiducials can include capacitive fingers, pits in the materials, reflective surfaces or magnetic writing, etc.  
         [0054]    The invention relates to the addition of mechanical members to MEMS gyroscopes and accelerometers that enable two techniques for measuring the scale factor, bias and linearity characteristics of gyroscopes and accelerometer. The members are referred to as the Mechanical Reference Member and the Flip Member. The Rotation Mechanical Reference Member rotates the gyroscope about its input axis. The Acceleration Mechanical Reference Member translates the accelerometer along the input axis. The Flip Member rotates the input axis for both instruments from the zero degree alignment to the 180 degree alignment. The purpose is to change the sign of the output data.  
         [0055]    MEMS Gyroscope with the Mechanical Reference Member  
         [0056]    One MEMS gyroscope is described in U.S. Pat. No. 5,712,426, incorporated herein by reference. A MEMS gyroscope with integral reference member is shown in FIG. 5. Gyroscope  10  comprises the Rotor Member  2  connected with flexures  4  to Output Member  6  that is connected with flexures  8  to the Rotation Mechanical Reference Member (RMRM)  9 . RMRM  9  is connected with flexures  12  to case  14 .  
         [0057]    During operation, Rotor Member  2  is oscillated sinusoidally at a frequency and amplitude about Rotor Axis  16 . When case  14  is rotated about Input Axis  18 , Output Member  6  responds with an oscillation about the Output Axis  19  with the same frequency, and with an amplitude that is proportional to the rotation rate input. The output then is the amplitude of the Output Member oscillation. For this configuration, the Rotor Member is oscillated out of-the-plane of the device, and the Output Member oscillates in the plane. Other configurations are possible, as explained more filly below. The input axis, output axis and rotor axis are mutually orthogonal.  
         [0058]    To add a reference rotation rate about the Input Axis, the Rotation Mechanical Reference Member is actuated to rotate about the Input Axis. The Rotation Rate that is input depends on the application. A sinusoidal rotation rate input can be used to determine the scale factor and bias-at-rest. A constant Rotation Rate can be obtained with a sawtooth or ramp waveform. Different waveform periods can be used to vary the rotation rate input and obtain the gyroscope data from which the scale factor and bias are calculated.  
         [0059]    MEMS Gyroscope with the RMRM and Flip Member  
         [0060]    [0060]FIG. 6 shows the gyroscope of FIG. 5 with the Rotation Mechanical Reference Member (RMRM)  9 . RMRM  9  is connected to Flip Member  22  rather than to case  25 . Flip Member  22  is connected by flip mechanisms  24  to case  25 . When Flip Member  22  is activated to flip from the zero angle to the 180 degree angle orientation, the full gyroscope and RMRM are flipped. The pointer indicates the alignment between A (0 degree orientation) and A′ (180 degree orientation). A MEMS gyroscope can be designed with just the Flip Member and not the RMRM.  
         [0061]    MEMS Accelerometer with the Acceleration Mechanical Reference Member  
         [0062]    One MEMS accelerometer  30  is described in FIG. 7. It comprises Output Member (Mass)  26  connected with flexures  28  to Acceleration Mechanical Reference Member (AMRM)  32  that is connected with flexures  34  to case  36 .  
         [0063]    During operation, mass  26  responds to acceleration along Input Axis  35  to translate along the same axis and in the opposite direction. The sensed displacement of the mass relative to the AMRM is the output. To add reference acceleration to the mass, the AMRM is accelerated. Different accelerations can be used to determine the scale factor and bias. Alternatively, sinusoidal acceleration input can be used to determine the scale factor and bias.  
         [0064]    MEMS Accelerometer with AMRM and Flip Member  
         [0065]    [0065]FIG. 8 shows the accelerometer  40  of FIG. 7 with AMRM  32  connected with flexures  38  to Flip Member  42  rather than to case  46 . Flip Member  42  is connected by flip mechanisms  44  to case  46 . When Flip Member  42  is activated to flip from the zero angle to the 180 degree angle orientation, the full accelerometer and AMRM are flipped. The pointer indicates the alignment between the B and B′ positions. A MEMS accelerometer can be designed having just the Flip Member and not the AMRM.  
         [0066]    Alternative to the Flip Member  
         [0067]    The function required to separate bias from actual output is to orient the instrument axis first along one axis and then in the opposite direction. The method described above to achieve this is to flip the gyroscope or accelerometer about the Flip Axis with the Flip Member as shown in FIGS. 6 and 8 for the gyroscope and accelerometer, respectively. An alternative method is to rotate the instrument in the plane using a Rotating Member as illustrated for the example of the gyroscope in FIG. 9. It applies to the accelerometer as well.  
         [0068]    For this gyroscope  50 , Rotor Member  51  is connected with flexures  52  to Output Member  53 , that is connected with flexures  55  to Rotation Member  56 . The RMRM is not shown in this example. Rotation Member  56  is connected with a rotating mechanism  58  to case  59 . The pointer indicates alignment between the C and C′ position.  
         [0069]    Alternative to Flip for the Isolation of Bias from Actual Rate  
         [0070]    The essential flip between the negative- and positive-sloped characteristic can also be obtained without mechanically turning or flipping the gyroscope from the zero to the 180 degree orientations. Effectively this can also be done by changing the phase of the rotor oscillation by 180 degrees.  
         [0071]    Application of Inventions to Non-MEMS Gyroscopes and Accelerometers  
         [0072]    The invention is primarily applied to MEMS devices because the MEMS technology provides for integration of the invention into the instruments. The invention is also appropriate for MEMS devices because these are low cost devices that cannot be tested in the conventional way due to cost considerations. However, other miniature technologies may emerge for which the Mechanical Reference Members and Flip Member are practical, and the invention would apply to these also. In particular, nano gyroscopes and accelerometers are an example of a miniature technology in which the MRMS/Flip Members can be integrated.  
         [0073]    This invention also applies to conventional instruments that are larger and more costly.  
         [0074]    All MEMS Gyroscopes and Accelerometers  
         [0075]    In the above description one gyroscope and one accelerometer were described with the added Mechanical Reference Member and Flip Member. However, the invention applies to all configurations and designs of gyroscopes and accelerometers.  
         [0076]    Second Gyroscope  
         [0077]    A second gyroscope  60  is illustrated in FIG. 10. Inner member  61  is attached by flexures  62  to Outer Member  63 . Outer Member  63  is attached with flexures  64  to Rotation Mechanical Reference Member (RMRM)  65 , that is attached with flip mechanisms  66  to case  67 .  
         [0078]    Operationally, the Outer Member oscillates about the Rotor Axis  68  thereby oscillating the Inner Member about the Rotor Axis. When subjected to rotation rate about the Input Axis  69 , the Inner Member oscillates about Output Axis  71  with the same oscillation and with an amplitude proportional to the rotation rate. The RMRM rotates the gyroscope about the Input Axis at a known rate.  
         [0079]    The gyroscope of FIG. 10 is repeated in FIG. 11 with the addition of an integral Flip Member. The RMRM  65  of Gyroscope  70  is connected with flexures  72  to Flip Member  74  instead of to the case. Flip Member  74  is connected with flip mechanisms  75  to case  76 . The pointer indicates alignment between D ( 0  degree orientation) and D′ (180 degree orientation).  
         [0080]    Second Accelerometer, Tuned Flexure Accelerometer  
         [0081]    A second accelerometer  80  is illustrated in FIG. 12. See U.S. Pat. No. 6,338,274 B1, incorporated herein by reference, for a tuned flexure accelerometer. Inner member  81  is attached by flexures  82  to Outer Member  83 . Outer Member  83  is attached with flexures  84  to Acceleration Mechanical Reference Member (AMRM)  85 , that is attached with flip mechanisms  86  to case  87 .  
         [0082]    Operationally, Outer Member  83  oscillates about Tuning Axis  88 , thereby oscillating Inner Member  81  about the same axis. When subjected to acceleration along Input Axis  89 , Inner Member  81  is free to move about Output Axis  97  without restraint from flexures  82 . A closed loop is used to hold Inner Member  81  at null. Inner Member  81  is made pendulous by adding mass  91  so that it can be sensitive to acceleration. The AMRM function is to translate the accelerometer along the Input Axis to introduce known accelerations.  
         [0083]    The accelerometer of FIG. 12 is repeated in FIG. 13 with the addition of a Flip Member. The AMRM  85  of Gyroscope  90  is connected with flexures  92  to Flip Member  94  instead of to the case. Flip Member  94  is connected with flip mechanisms  95  to case  96 . The pointer indicates alignment between E (0 degree orientation) and E′ (180 degree orientation).  
         [0084]    Two Degree of Freedom Gyroscopes and Accelerometers  
         [0085]    The invention also applies to two degree of freedom instruments, although the Mechanical Reference Member is in such cases replaced with two Mechanical Reference Members to allow inputs about both input axes.  
         [0086]    Other embodiments will occur to those skilled in the art and are within the following claims: