Abstract:
Inertial measurement systems for measuring accelerations and angular speeds are currently widely used in vehicles where they serve as sensors for safety devices and navigation systems. The required reliability and precision, however, can be obtained only with extremely accurate sensors and precise finishing. The invention therefore provides for a method according to which an inertial measurement system comprising economical, mass-produced sensors is first gauged overall in a testing device in one calibration operation. During gauging any offset, mounting position and scaling errors are detected as coefficients of coupling matrices of the sensors and converted into compensation values which are then used to improved the accuracy of the sensors&#39; measurement values when the measurement system is in operation. The above method is suitable for inertial measurement systems in land, air and water craft.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method for improving measurement values of an inertial measurement system in accordance with the preamble to claim  1 . 
     Such methods are used in inertial measurement systems that are currently widely used. For instance, they are used in regulation and control systems in which it is necessary to know the precise acceleration and angular speeds relative to an inertial coordinate system (fixed star system). Generally the earth-fixed system is used in a first approximation for the inertial reference system. Such measurement systems, also called inertial platforms, are used in very technically complex systems in the design and control of weapons or are employed in tilt regulation of high speed trains. Such systems will also soon be mass-produced as motor vehicles increasingly are equipped with airbags and side-impact protection and also with pitch and yaw stabilizers or navigation systems and use such systems to trigger and control these safety devices. Such devices are also required in the airline industry and naval engineering. 
     Thus, known from German Offenlegungsschrift 44 16 586 is a measurement system with angular speed and acceleration sensors and a computer unit, in which system the measurement values of the sensors are converted to output values that are related to an inertial coordinate system. 
     The sensors currently available for these measurement systems are still quite expensive when they have the necessary reliability and precision. Although sensors such as gyroscopes and accelerometers can be produced cost effectively as microelectronic components, when built-in they have substantial errors so that for constructing the measurement systems substantial production expenses, including the required alignment and compensation, are required to achieve acceptable error and offset tolerances. 
     It is therefore the object of the present invention to provide a method for improving the measurement values in which a measurement system built with inexpensive sensors for angular speeds and accelerations can be aligned and compensated in a simple manner. 
     SUMMARY OF THE INVENTION 
     This object is achieved by the features of claim  1 . 
     The inventive method is particularly advantageous when employed in the manufacture of mass-produced articles since only limited personnel are required to set up the measurement system and the testing equipment. Complex mechanical alignment work is not necessary, nor are the alignment apparatus and aids required for such work. The inventive method is very suitable for a fully-automatic, economical in-process inspection of inertial measurement systems since it makes possible the use of simple sensors for angular speeds and accelerations that are inexpensive to produce and takes into account their production tolerances in the alignment or calibration of the measurement system so that the accuracy of the inertial measurement system is substantially improved thereby. 
     One of the most important requirements for the calibration or alignment of the sensors is the availability of calibration standards with great precision, that is, high accuracy of the absolute values for angular speed and acceleration as well as optimum reproducibility of excitation and angular position. For this, the measurement system is mounted on a precision rotary table, the axis of rotation of which is precisely horizontal. The assembly occurs such that coordinate axes of the measurement system and of the rotary table are parallel; parallel displacements are inconsequential for the result. Thus the measurement system can be subjected in a simple manner to two reference excitations. A dynamic excitation of the gyroscope by precise, constant rotation and a static excitation of the accelerometer can be realized in this simple manner by the gravitational acceleration. 
     The inventive method is also advantageously further developed in the subordinate claims. 
     It has proved advantageous for optimum testing, before the measurement on the rotary table, to perform measurement of the angular speed offset of the gyroscope and the acceleration offset of the accelerometer as a function of temperature, since these measurements are generally time-consuming. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained in more detail in the following with respect to the exemplary embodiment illustrated in the drawings, in which: 
     FIG. 1 is a basic drawing of the testing equipment with a measurement system; 
     FIG. 2 is a circuit diagram of a measurement system; 
     FIG. 3 is a functional diagram of the measurement system. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     As shown in FIG. 1, used for the inventive method is a precision rotary table  1  with a rotary mounting surface  2  rotatable about at least one axis and upon which the inertial measurement system  10  is mounted. For controlling the measurement process, the rotary table  1  is connected via a control cable  7  and the measurement system  10  is connected via a control and power cable  6  to a computer  5 . Not only does the interface  11  of the measurement system  10 , which interface is connected to the power cable  6 , supply current, it also transmits data for control commands for activating the alignment and operational mode of the measurement system  10 . 
     The indicated coordinate system x, y, z matches the physical axes of the measurement system  10  according to which the sensors of the measurement system  10 , the accelerometer and angaular speed sensors, i.e., gyroscopes, are oriented. The x-axis in the drawing plane, or the longitudinal axis, of the measurement system  10  is parallel to the axis of rotation and oriented to the mounting surface  2 . The rotation in the mathematically positive sense therefore occurs around the x-axis in the direction of the arrow  8 . The y-coordinate corresponds to the transverse axis and the z-coordinate to the vertical axis of the measurement system  10 . 
     For determining the alignment data, all of the sensors in the measurement system  10  must undergo a precise predetermined excitation. The gravitational acceleration g=9.80665 m/sec 2  is provided for exciting the accelerometer. In the illustrated position of the measurement system  10 , the gravitational acceleration acts in the direction of the z-axis. In order to activate the direction of action for the y-axis, the measurement system must then be rotated exactly 90°. Rotating an additional 90° achieves the excitation of the accelerometer in the negative z-direction and the third 90° rotation makes possible the excitation in the negative y-coordinate axis. Furthermore, in this position the gyroscope that measures the angular speed is excited in the x-axis by a constant, uniform rotation around a predetermined angular range. 
     For measuring acceleration in the direction of the x-coordinate, the measurement system  10  is re-mounted so that the x-axis runs in the vertical and parallel to the gravitational vector g. In this position, the y-axis points to the rotary table so that the gyroscope oriented to the y-axis can be measured dynamically. Another change in mounting is required for measuring the gyroscope in the x-axis and the x-axis is oriented to the rotary table. 
     The measurement system must be mounted several times in a testing apparatus in accordance with FIG. 1 that has a mounting surface  2  with only one axis of rotation. The housing surfaces are the reference surfaces and must be finished in a strict orthogonal manner. However, since the reference surfaces are always formed by the housing surfaces, even when the measurement system  10  is in the latter usage position, this does not impose any limitation on precision. 
     FIG. 2 illustrates the basic structure of the measurement system  10 . The measurement system  10  has a computer unit  12  with integrated memory  14  for data and programs and is connected to the computer  5  via the interface  11 . 
     The computer unit  12  is also connected to a signal processor  16 , an inertial value computer  50 , and a data output  60  and controls the internal data processing of the measurement system  10  using the programs stored in the memory  14 . The angular speeds and accelerations in an alignment unit  20  and a compensating unit  40  measured by a sensor unit  15 , depending on alignment or operational mode, are prepared in a signal processor  16  for further processing in the inertial value computer  50 . 
     FIG. 3 illustrates the basic functions of the inventive measurement system  10  in a functional diagram. The sensor data, angular speed, acceleration, and temperature recorded by the sensor unit  15  are forwarded via the data bus  16  either to the alignment unit  20  or to the compensating unit  40 . The selection of the function is performed from outside via the interface  11  in FIG. 1 by the control computer  5 . The subsequent method steps are then triggered, program-controlled, by the computer unit  12 . For the sake of simplification there are no lines drawn in from the computer unit  12  to the function blocks in FIG.  3 . 
     The acceleration and angular speed measurement values recorded by the sensors fundamentally include errors. Offset, mounting position, and scaling errors are differentiated. Sensors that can now be produced in large numbers as microelectronic components must now be aligned with good precision when being integrated into the complete measurement systems and must be oriented to the coordinate axes x, y, z of the measurement system  10 . Although the aforementioned errors do directly affect the measurement results, in the inventive method they are determined by reference measurement in the production test and are compensated when in operational use. As illustrated in FIG. 3, for this it is necessary to acquire and process separately the sensor data for the accelerometer and gyroscope for each coordinate axis x, y, z in the measurement system  10 . 
     For each coordinate axis x, y, z the sensor unit  15  has one accelerometer each for the accelerations a x , a y , a z , and one gyroscope each for the angular speeds ω x , ω y , ω z . The gravitational acceleration g=9.80665 m/sec 2  is used for the reference for the accelerometer. However, care must be taken that the acceleration sensor that is allocated to the coordinate axis of the measurement system  10  that is directed in the direction of the gravitational vector measures the negative gravitational acceleration. 
     For determining the alignment data for the accelerometer, the acceleration values a nx , a ny , a nz  are transmitted from the sensor unit  15  to the alignment unit  20 , whereby the coordinate axis allocation is characterized with the index n of the measurement cycle and with the indices x, y, z. For noise reduction, in an acceleration data acquisition unit  31  (B-data acquisition unit  31 ), a number of acceleration values a nx , a ny , a nz  are recorded for each coordinate and the mean is found for 2000 typical measurements, for instance. A plurality of measurement cycles is conducted such that the measurement system  10  is rotated with its coordinate axes in the direction of the gravitational vector g or in the inverse direction. However, the measurement values of all of the acceleration sensors are always transmitted to the B-data acquisition unit  31 . That is, starting with FIG. 1, first the z-axis matches the gravitational vector g so that the accelerations a 1z , a 1x , and a 1y  are transmitted to the B-data acquisition unit  31 . The rotary table  2  is rotated precisely 90° in accordance with the arrow  8  and thus the y-axis is oriented to the gravitational vector g so that the acceleration values a 3y , and a 3x , a 3z  are determined. An additional 90° rotation causes the excitation of the accelerometer in the -z-axis and thus acquisition of the accelerations a 2z , and a 2x , a 2y . The third 90° rotation results in excitation in the negative y-coordinate axis and the accelerations a 4y , and a 4x , a 4z , which are also transmitted to the B-data acquisition unit  31 . For measuring the acceleration sensor of the x-axis, the measurement system must be re-mounted so that the accelerations a 5x , and a 5y , a 5z  are measured and acquired. The accelerations a 6x , and a 6y , a 6z  are obtained by reversal, that is, by two 90° rotations and the effect of the gravitational vector on the accelerometer in the x-axis. The acceleration offsetΔa x , Δa y , Δa z  for each acceleration sensor is now calculated from each of the accelerations a 1 , a 3 , a 5  when the orientation is in the direction of the gravitational vector g and the accelerations a 2 , a 4 , a 6  when the orientation opposes the direction of the gravitational vector as follows:                  Δ                   a   x       =         a     5      x       +     a     6      x         2            
            Δ                   a   y       =         a     3      y       +     a     4      y         2            
            Δ                   a   z       =         a     1      z       +     a     2      z         2               (   1   )                                
     There are cross couplings between the acceleration sensors in the coordinate axes x, y, z, which means that an acceleration excitation only acts precisely in one axis if all acceleration sensors are oriented precisely orthogonally with no installation errors. If this is not the case, the acceleration excitation also results in measurement values in the two other acceleration sensors. Furthermore, the measurement values are affected by the measurement amplification of each sensor. The fact that the measurement values a nx , a ny , a nz  depend on the excitations a x , a y , a z  and on the acceleration offsets Δa x , Δa y , Δa z  is generally described by a system of equations in accordance with equation 2 in matrix form                (           a   nx               a   ny               a   nz           )     =         (           k   11           k   12           k   13               k   21           k   22           k   23               k   31           k   32           k   33           )     ·     (           a   x               a   y               a   z           )       +     (           Δ                   a   x                 Δ                   a   y                 Δ                   a   z             )               (   2   )                                
     and can be illustrated briefly by equation 3 
     
       
         ( A   n )=( K )·( A )+(Δ A )  (3), 
       
     
     whereby the coupling matrix K with the coefficients K ij  (i, j=1, 2, 3) contains the mounting position deviations and scaling errors, that is, amplification errors. 
     The coefficients K ij  of the rotational or coupling matrix K are calculated in a B-rotational matrix computer  32  from the acceleration values a nx , a ny , a nz  present in the B-data acquisition unit  31 , the acceleration offsets Δa x , Δa y , Δa z , and the known reference excitations by the gravitational acceleration g, whereby the excitation vectors have the values            x        -        Achse        :                     (   A   )       =     (           -   g             0           0         )       ,       y        -        Achse        :                     (   A   )       =         (         0             -   g             0         )                   and                 z        -        Achse        :                     (   A   )       =     (         0           0             -   g           )                                
     when each coordinate axis is oriented to the gravitational vector. In addition, the matrix K −1  inverse to the coupling matrix is formed with the correction values K ij  (i, j=1, 2, 3), equation 4:                  K     -   1       =     (           K   11           K   12           K   13               K   21           K   22           K   23               K   31           K   32           K   33           )       ,           (   4   )                                
     which is stored in the B-matrix memory  33  for compensating the measurement values that occur when the measurement system  10  is operated normally. 
     The alignment data for the gyroscope are determined in a manner similar to that of the accelerometer. The rotary plate  2  of the precision rotary table  1  is the reference when the gyroscope is in the static rest state and for the dynamic excitation. The three coordinate axes x, y, z of the measurement system  10  are oriented successively to the axis of rotation of the rotary table  1 , which must be done by re-mounting when a rotary table  1  such as that shown in FIG. 1 is used that permits rotational movement around only one axis. 
     First the angular speeds ω 0x , ω 0y , ω 0z  of the angular speed sensors are measured While the rotary table  1  is resting. For angular speed sensors, one gyroscope is provided in the sensor unit  15  for each coordinate axis x, y, z of the measurement system  10 . The measurement values ω 0x , ω 0y , ω 0z  of these gyroscopes are taken from the ω-offset acquisition unit  21  via the data bus  16  and stored. Then the rotary table  1  is driven and the measurement system  10  is rotated around the coordinate axis x of the measurement system  10 , which axis corresponds to the axis of rotation of the rotary table  1 , at a constant, precisely defined angular speed of ω=70°/sec. The predetermined angular speed ω x  is kept constant via a limited angle of rotation range since this angular speed ω x  forms the reference for calculating the gyroscope correction data. The measurement values ω 1x , ω 1y , ω 1z  of all three gyroscopes are measured multiple times in the ω-data acquisition unit  22  during the available measurement time, then the mean is found and stored. If, after both remounting procedures, the ω-offset is also present when oriented to the two other coordinate axes y, z in the ω-offset acquisition unit  21  and the angular speed measurement values ω nx , ω ny , ω nz  (n=2, 3) are present in the ω-data acquisition unit  22 , the angular speed values ω nx , ω ny , ω nz  (n=1, 2, 3) are corrected by the offsets ω 0x , ω 0y , ω 0z  in the ω-rotation matrix computer  23  and the rotation matrix K ω  is calculated using the coefficients k ij  in accordance with equation 5:                (           ω   nx               ω   ny               ω   nz           )     =         (           K   11   ′           K   12   ′           K   13   ′               K   21   ′           K   22   ′           K   23   ′               K   31   ′           K   32   ′           K   33   ′           )     ·     (           ω   nx               ω   ny               ω   nz           )       +     (           ω   ox               ω   oy               ω   oz           )               (   5   )                                
     The inverse matrix K ω   −1  is also formed for the angular speeds and stored as correction matrix with the correction values (coefficients) K ij (i, j=1,2,3) in the ω-matrix memory  23 . 
     In real test operations for such measurement systems  10 , the way in which the measurement program runs is optimized in terms of mounting changes. This deviates from the illustration shown here of the sequence angular speed and acceleration measurements in that, in every mounting position of the measurement system  10 , all necessary angular speed and acceleration measurements are performed independently of the previously described sequence and after the conclusion of all measurements the rotary matrices are calculated in the rotary matrix computers  23  and  24 . 
     The gyroscopes and acceleration sensors are generally temperature-sensitive. In the alignment unit  20 , therefore, offset acquisition is provided depending on temperature. As a rule, temperature cycles are very time-consuming so that equalizing processes can deteriorate when there are changes in temperature. For the temperature-dependent measurements, the measurement system  10  remains in a defined rest position without the slightest movement. With the temperature value θ of the temperature sensor in the sensor unit  15 , the measurement values of the gyroscope ω x (θ), ω y (θ), ω z (θ) are transmitted to the temperature-dependent ω-offset acquisition unit  25  and the measurement values of the acceleration sensors a x (θ), a y (θ), a z (θ) are transmitted to the temperature-sensitive B-data acquisition unit  34 . Once the mean of numerous measurement data at the same temperature θ is found, the ω-offsets are stored in a ω-temperature table  27 . The mean for the acceleration values a x (θ), a y (θ), a z (θ) is found and stored in a B-temperature table  35  as relative acceleration values. The relative acceleration values in the B-temperature table  35  are then corrected to an acceleration offset in a B-offset calculation unit  36  with the absolute acceleration values calculated in the B-data acquisition unit  31  and are stored in the B-offset temperature table  37 . 
     Once the measurement system  10  has switched to the normal operational mode due to a program control, the measurement values in the sensor unit  15 , that is, the accelerations a nx , a ny , a nz , the angular speeds ω nx , ω ny , ω nz , and the temperature θ, are transmitted via the data bus  16  to the compensating unit  40 . In the compensating unit  40 , in conformity with the temperature θ, first the offset correction of the angular speed ω nx , ω ny , ω nz  occurs with the values from the ω-temperature table  27  in a ω-summation stage  41 . Likewise, the acceleration values a nx , a ny , a nz  are corrected in the B-summation stage  42  with temperature-dependent acceleration offsets from the B-temperature table  37 . The output values for summation stages  41  and  42  thus represent the angular speeds and accelerations corrected by the offsets, whose mounting position errors and scaling errors are optimized in a compensating calculation  45  using the compensation matrices from the ω-matrix memory  24  and the B-matrix memory  33 . The measurement values thus compensated and optimized are converted in an inertial value computer  50  using the known Euler transformation method into angular speeds and accelerations of an inertial reference system and are provided in a data output  60 . They indicate the movement of the measurement system  10 , i.e., the movement of the device connected to this measurement system  10  in this inertial reference system. 
     In another embodiment of the inventive method, more measurement values are regularly acquired for acceleration a nx , a ny , a nz  and angular speed ω nx , ω ny , ω nz  by the sensor unit  15  in FIG. 2 than is necessary for calculating the coupling matrices K and K ω . The measurement system  10  can then also assume angular positions that deviate from the vertical. Since the coefficients of the coupling matrices K and K ω  are linearly independent, the matrix equations described by equations 2 and 5 are expanded according to the number of measurement results and the rotational or coupling matrices K, K ω , or matrices K −1 , K ω   −1 , which are the inverse thereto, are calculated in a known regression calculation method implemented in the computer unit  12 . 
     As a modification to the described exemplary embodiment, the alignment of the measurement system  10  can occur without remounting if, instead of a single-axis rotary table  1 , a precision rotary table is used that has a mounting surface that can be rotated around three coordinate axes. Such a solution is advantageous for a practical, automatic testing operation, but does not further improve the quality of the alignment. 
     Another modification of the measurement method for a comparable measurement system results with a reduced number of sensors. Because the method is just as well-suited for alignment and compensation when a measurement system comprises only one or two sensors, whether these are gyroscopes or accelerometers. For this, the measurement process must be adapted and the compensation matrices and memory functions must be reduced, which would be very simple for one skilled in the art who is familiar with this method. 
     The specification incorporates by reference the disclosure of German priority documents 198 58 621.3 of Dec. 18, 1998 and PCT/DE99/04037 of Dec. 15, 1999. 
     The present invention is, of course, in no way restricted to the specific disclosure of the specification and drawings, but also encompasses any modifications within the scope of the appended claims.