Abstract:
The method of aligning an inertial unit having an axially symmetrical vibrating sensor that generates vibration comprises the step of establishing the vibration in a position for which the sensor presents variation in drift error that is the smallest relative to variation in drift error for other positions of the vibration.

Description:
[0001]    The present invention relates to a method of aligning an inertial unit having an axially symmetrical vibrating sensor, and to a corresponding inertial unit including an axially symmetrical vibrating sensor. 
       BACKGROUND OF THE INVENTION 
       [0002]    In order to determine the orientation of a carrier, in particular in order to navigate an airplane, it is common practice to use an inertial unit that includes axially symmetrical vibrating sensors such as hemispherical-bell vibrating sensors or Quapasons. 
         [0003]    It is also known that an axially symmetrical vibrating sensor is adapted to operate either in free-gyro mode or in rate-gyro mode. In free-gyro mode, the vibration is sustained but position is left free; when the orientation of the carrier varies, the position of the vibration relative to the base is representative of the angle through which the carrier has turned. 
         [0004]    In rate-gyro mode, vibration is sustained and its position relative to its base is kept unchanging by applying suitable electronic control. The control required is then representative of the rate of rotation of the carrier in inertial space. 
         [0005]    In practice, axially symmetrical vibrating sensors that are used for navigation in free-gyro mode benefit from a scale factor (Bryan coefficient) of excellent quality. 
         [0006]    Navigating a journey comprises two main stages: 
         [0007]    a preparatory or “alignment” stage for defining the initial geographical orientation of the unit; and 
         [0008]    the journey proper during which the initial geographical orientation serves as a reference for calculating the instantaneous geographical orientation and the position of the carrier. 
         [0009]    The quality of the resulting navigation, and in particular the accuracy with which heading is calculated, depends directly on the drift error of the gyroscope fitted to the navigation unit. 
         [0010]    Axially symmetrical vibrating gyroscopes used in free-gyro mode present the feature of presenting drift error that is a function of the position of the vibration relative to the base of the sensor. This drift error is naturally modeled and compensated, but such compensation is never perfect. There thus remains drift error that is a function of position. When such gyroscopes are used in navigation units, this drift error constitutes one of the sources of error in the unit. 
         [0011]    In addition, the drift error also varies as a function of temperature and as a function of time. 
         [0012]    The invention is based on two observations: 
         [0013]    1) the drift error of gyroscopes during the alignment stage has a particularly large influence on navigation performance; and 
         [0014]    2) during the alignment stage, the carrier does not change its orientation. 
         [0015]    When a navigation unit is started, the initial position of the vibration is a priori arbitrary, and it can vary from one start-up to another; the drift error associated with the position of the vibration is thus variable and can disturb the quality of the alignment. 
       OBJECT OF THE INVENTION 
       [0016]    An object of the invention is to minimize the effects of drift error on the navigation performance of the inertial unit. 
       SUMMARY OF THE INVENTION 
       [0017]    In order to achieve this object, the invention provides a method of aligning an inertial unit having an axially symmetrical vibrating sensor that generates a vibration, the method including the step of establishing the vibration in a position for which the sensor presents drift error variation that is minimized relative to the drift error variations for other positions of the vibration. 
         [0018]    This procedure makes it possible: 
         [0019]    to minimize alignment errors resulting from the drift error of the sensor; and 
         [0020]    to improve the repeatability of drift error from one alignment to another, thus making it possible to observe it and therefore to compensate it, since the position of the vibration remains the same. 
         [0021]    In an advantageous implementation of the method of the invention, the position selected for the vibration during the alignment stage takes account essentially of variations in drift error as a function of temperature. 
         [0022]    In another preferred implementation of the invention, the vibration position selected for the alignment stage takes account initially of variations in drift error as a function of temperature and then of variations in drift error as a function of time. 
         [0023]    In another aspect, the invention provides a an inertial unit comprising an axially symmetrical vibrating sensor, means for exciting the sensor to generate vibration, and means for establishing vibration in a position for which the sensor presents variation in drift error that is the smallest compared with variation in drift error for other positions of the vibration. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0024]    Other characteristics and advantages of the invention appear on reading the following description of a particular and non-limiting implementation of the invention with reference to the accompanying figures, in which: 
           [0025]      FIG. 1  is a diagrammatic axial section view on line I-I of  FIG. 2  showing a hemispherical-bell vibrating sensor; and 
           [0026]      FIG. 2  is section view on line II-II of  FIG. 1  together with a block diagram illustrating an implementation of the method of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    With reference to the figures, the hemispherical-bell vibrating sensor shown comprises in known manner a silica bell  1  mounted on a base  2  likewise made of silica, the bell  1  being surrounded by a sealed housing  3  enabling the sensor to be evacuated. 
         [0028]    Also in known manner, the inside surface of the bell  1  is metal-plated, as is its bottom edge that extends facing two pairs of control electrodes  4  and two pairs of detection electrodes  5  suitably connected to a control and detection unit  6  for generating vibration  11  represented by a chain-dotted line in  FIG. 2 , and of position that can be controlled by the control unit  6 . 
         [0029]    Also in known manner, the control and detection unit  6  is connected to a member  7  for calculating the orientation of the sensor on the basis of detection signals received from the control and detection unit. 
         [0030]    According to the invention, the alignment stage is performed by exciting the sensor with a control signal that positions the resulting vibration in a selected position stored in a memory member  9 . 
         [0031]    In the example shown, the selected position for implementing the alignment stage is determined during a calibration stage  10  that is preferably performed in a workshop and comprises the following steps: with the sensor being held stationary, it is excited with a control signal  13  in a plurality of successive different vibration positions represented in  FIG. 2  by a chain-dotted line  11  and by bold double-headed arrows  12 . For each vibration position, a measurement  14  is performed of the variations in drift error as a function of temperature by taking the sensor successively to mutually different temperatures while keeping the vibration position control constant. The various measurements of the real positions that are obtained are stored, and then a comparison is performed between the variations in this error in order to select  15  the position that gives the smallest variations as a function of temperature. 
         [0032]    When the variations in drift error as a function of temperature are the same in a plurality of vibration positions, then, in a preferred implementation of the invention, provision is made for performing new measurements, in each of the vibration positions, which new measurements are performed at constant temperature and at mutually different instants in order to sense variations in drift error as a function of time, and to select a position that gives the smallest variations in drift error. The position finally retained is stored in the position memory member  9  for subsequent use for each alignment stage. During each alignment stage, vibration is set up directly in the stored position. 
         [0033]    Also preferably, the drift error, including variations therein, is modeled as a function of temperature and as a function of time, and the model is stored in the memory member  9  for transmission to the control and detection unit  6  in order to make corrections to measurements taken during the alignment stage. 
         [0034]    Naturally, the invention is not limited to the information described, and variant implementations can be provided without going beyond the ambit of the invention as defined by the claims. 
         [0035]    In particular, although provision is made in the preferred implementation to perform selection on a priority basis on variations in drift error as a function of temperature, since that is generally the parameter that is the most sensitive, it is possible to make provision for selecting position directly on the basis of variations in drift error as a function of time, or on the contrary to limit selection to a position in which the variations are the smallest as a function of temperature. 
         [0036]    Although the invention is described more particularly with a hemispherical-bell vibrating sensor, the invention applies to any axially symmetrical vibrating sensor used in free-gyro mode, in particular a Quapason. 
         [0037]    Although the invention is described as incorporating a prior calibration stage, it is possible for the vibration position for the alignment stage to be initially selected randomly and then to be modified after a number of journeys, thereby making it possible to perform statistical analysis on drift error for the different positions in which vibration is established during successive alignment stages, and to compare the results obtained so as to end up retaining the best position during the alignment stage.