Abstract:
A device for measuring the movement of a self-guiding vehicle, that has an enhanced measuring reliability, in particular during an adhesion loss and independently from the travel profile of the vehicle in terms of slope, turn and slant. To this end, the device for measuring the movement of a self-guiding vehicle includes on board thereof two accelerometers coupled to a movement calculator, wherein each accelerometer includes two measurement axes on which are measured projections of a vehicle acceleration resultant. The four measurement axes of the accelerometers are adjusted so that the calculator provides, from the four projection measures, at least one very accurate longitudinal acceleration value of the vehicle at each point of a route including both slopes and turns.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a device for measuring the movement of a self-guided vehicle which comprises, on-board the vehicle, an accelerometer having two measurement axes in a longitudinal plane defined by a first longitudinal axis according to a principal movement of the vehicle, assumed to be rectilinear, and of a second axis perpendicular to the floor of the vehicle, and a computer connected to receive an output signal associated with each measurement axis. Each output signal comprises a protection measurement of a total acceleration resultant of the vehicle on the associated measurement axis. 
     Numerous methods or devices for measuring movement, speed or acceleration of a vehicle are known nowadays, in particular for vehicles intended for public transport such as a carriage of a train, of a metro train, a trolleybus, a tramway car, a bus or any other vehicle driven in traction by at least one trackway or a rail such as a guide rail. In particular, in the case of a vehicle which is self-guided by a traffic system (railway signals, onboard and/or remote autopilot system of the vehicle, etc.), measures to provide self-guidance which is reliable (against breakdown) and safe (for the passengers or goods) are indispensable whatever the nature of the route of the vehicle. In this sense, it is essential to be accurately informed in real time about the position, the speed (and the acceleration) of the vehicle, in particular in situations where the vehicle incurs unavoidable loss of adhesion, such as during slipping (during acceleration of the vehicle) or wheel locking (during braking of the vehicle) of the free-running measurement axle or drive axle. 
     When the guided vehicle has an axle which is free of any tractive or braking force, the movement of the vehicle is directly provided by the rotation of the axle (or one of the wheels associated with this axle). 
     However, this solution reduces the tractive or braking power and thus the performance of the vehicle, and this is why the majority of systems do not have free-running axles. 
     In the absence of a free-running axle and to overcome the consequences associated with slipping/wheel locking, with loss of adhesion of one of its wheels, several devices exist and use:
         either measuring means which are completely independent of the wheels permitting a measurement of speed by optical means or even by means of a Doppler effect radar system. These costly devices generally use, however, an additional tachometer for operation at low speed and when the vehicle is stationary, said tachometer making it possible to obtain the angular speed of a wheel or the number of revolutions of the wheel per unit of time;   or inertial units combining accelerometers, gryometers and terrestrial localization systems such as a GPS. Said systems, however, remain very costly due to their high-level technology, frequently used in applications for aeronautical systems;   or, such as in EP 0 716 001 B1, a single tachometer arranged on an axle and a means for taking into account a safety margin for the values measured on one wheel or on the wheels in order to attempt to compensate for the effects of possible slipping/wheel locking which impairs the performance for measuring movement as it still remains too approximate. This also results in an anti-lock system as compensation which may be very abrupt for a vehicle and its passengers or goods;   or, such as in US 2005/0137761 A1, an accelerometer fitted in the vehicle and a tachometer on an axle, the measurement signals of which are linked to an appropriate central computer, even if not specifically disclosed, to take into account errors which occur in the event of loss of adhesion and to provide the speed and the position of the vehicle on its route. In particular, the accelerometer comprises two measurement axes in order respectively to determine an acceleration in one direction of the trajectory of the vehicle and in order to determine and thus take into account, in the calculation of movement, a slope of the vehicle relative to a horizontal plane. Values of the measurement signals of the accelerometer and of the tachometer are also compared to threshold speed values which, if they exceed a threshold, make it possible to indicate the presence of loss of adhesion (slipping/wheel locking) of the vehicle. Although the effects of slope sustained by the vehicle are taken into account, other effects associated with the trajectory of the vehicle are inevitable depending on the position of the accelerometer (and of the positioning of the two measurement axes thereof) in the vehicle, as a railway transport unit frequently has an elongate geometry along which a single accelerometer and a tachometer placed upstream of the vehicle may not provide a measurement means which shows the effects acting on the complete assembly of the vehicle such as, for example, the effects of turn or lateral acceleration.       

     All these devices make it possible, therefore, to calculate the movement of a guided vehicle, which does not have axles which are free of any braking and tractive force, and which runs on a track of any profile but with an accuracy which is much lower than that of an “ideal” system with a free-running axle, as they may not completely overcome losses of adhesion (slipping and wheel locking caused by tractive/braking forces) in addition to errors caused by lateral acceleration (turn, slant) and even vertical acceleration (slope). 
     BRIEF SUMMARY OF THE INVENTION 
     One object of the present invention is to propose a device for measuring the movement of a self-guided vehicle that has enhanced measuring reliability, in particular during an adhesion loss and independently from the travel profile of the vehicle in terms of slope, turn and slant. 
     To this end, a device for measuring the movement of a self-guided vehicle comprising two on-board accelerometers, each including two measurement axes and of which the measurement signals are coupled to a computer for calculating the movement, is proposed as claimed in claim  1 . 
     As one possibility, at least one tachometer may be mounted on one of the axles of the vehicle and also coupled to the computer for processing data thus provided from all the sensors (accelerometers and tachometer). The measurement signals delivered by the tachometer may be utilized to improve the accuracy of the device. 
     The device according to the invention, based on accelerations measured on the measurement axes, provides data regarding the speed and longitudinal movement of the vehicle (for example along a railway track). It may be associated with any type of on-board device likely to require an accurate and continuous measurement of the speed and of the movement of the vehicle, irrespective of the conditions of rail/wheel adhesion and whatever the profile of the route in terms of slope, turn and slant. 
     The accelerometers and their measurement axes are arranged such that, based on measurements taken on the different measurement axes, they permit longitudinal acceleration, lateral acceleration and slope acceleration of the vehicle to be calculated in order to determine subsequently the speed and the longitudinal movement of the vehicle by the integration of time onto the acceleration values. 
     The device according to the invention also advantageously makes it possible to detect in a reliable manner an immobilization of the vehicle on its route and produces to this end information about zero speed from information delivered by the sensors. 
     The device comprises a means for auto-calibration and auto-testing which makes it possible, when the vehicle is immobile, to verify the correct functioning of the sensors and as a result to guarantee with a high degree of reliability data made available by other on-board systems. 
     One appropriate use of the device according to the invention covers the field of guided vehicles whatever their type of guidance (mechanical or intangible, i.e. without a mechanical connection between the ground and the vehicle) in particular trains, metro trains, tramway cars or buses and whatever the type of operation (axles, bogies) with iron wheels or tires. It is noteworthy here that for this category of vehicle with an elongate geometry/chassis, the effects of turn and slope are not negligible, depending on the position (or the offset) of the accelerometers on-board the vehicle. The invention thus advantageously permits these effects to be overcome in order to determine the movement of the vehicle more accurately. 
     The device according to the invention thus makes it possible to calculate the movement of a guided vehicle which does not have axles free of any braking and tractive force, and which runs on a track of any type of profile, maintaining an accuracy which is equivalent to that of a system with free-running axles, whilst overcoming loss of adhesion (slipping and wheel locking caused by tractive/braking forces) and errors caused by lateral acceleration (turn) and vertical acceleration (slope). 
     A set of sub-claims also presents advantages of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       Exemplary embodiments and examples of application are provided with reference to the figures described, in which: 
         FIG. 1  shows a vehicle provided with a device for measuring movement of the self-guided vehicle according to the invention, 
         FIG. 2  shows a diagram for defining the planes associated with the vehicle in motion, 
         FIG. 3  shows a diagram for taking into account the effect of slope on the device, 
         FIG. 4  shows a diagram for taking into account the effect of turn on the device. 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a vehicle VEH provided with a device for measuring the movement of the self-guided vehicle according to the invention and, possibly associated with  FIG. 2 , clarifying how the planes associated with the vehicle in motion are defined according to the acceleration sustained by the vehicle and measured by two accelerometers  101 ,  102 .  FIGS. 3 and 4  show the arrangement of measurement axes Acc 1 , Acc 2 , Acc 3 , Acc 4  of the accelerometers according to the planes selected according to the type of acceleration Gx, Glat, Gpes (longitudinal movement, effect of turn or/and of slope) sustained by the vehicle as a co-ordinate (X, Y, Z) centered on the accelerometers and of which the axis X indicates the direction of the longitudinal trajectory of the vehicle. 
     The device for measuring movement (real-time position Dx) of the self-guided vehicle VEH comprises on-board thereof:
         an accelerometer  101  provided with two measurement axes Acc 1 , Acc 2 , in a longitudinal plane Py defined by a first longitudinal axis X according to a principal movement VEx, assumed to be rectilinear, of the vehicle and a second axis Z perpendicular to the floor of the vehicle,   a computer  103  connected to an output signal S 1 , S 2  associated with each measurement axis Acc 1 , Acc 2  where each output signal S 1 , S 2  includes a measurement as an orthogonal projection Gacc 1 , Gacc 2  of a total acceleration resultant of the vehicle on the associated measurement axis Acc 1 , Acc 2 ,   a second accelerometer  102  being provided with at least two measurement axes Acc 3 , Acc 4  in a horizontal plane Pz defined by the first axis X and a third axis Y perpendicular to the first and to the second axis X, Z,   the computer  103  is connected to an output signal S 3 , S 4  associated with each measurement axis Acc 3 , Acc 4 , where each output signal S 3 , S 4  includes a projection measurement Gacc 3 , Gacc 4  of the total acceleration resultant of the vehicle on the associated measurement axis Acc 3 , Acc 4 ,   all the measurement axes Acc 1 , Acc 2 ; Acc 3 , Acc 4  of the first and of the second accelerometer  101 ,  102  have in their respective plane Py, Pz an adjustable relative angle A 1 +A 2 , A 3 +A 4  which is thus adjusted so that the computer  103  provides from the four projection measurements Gacc 1 , Gacc 2 , Gacc 3 , Gacc 4 , at least one instantaneous value of longitudinal acceleration Gx of the vehicle at each point of a route including both slopes and turns. In other words, the value of longitudinal acceleration Gx is an exact acceleration value, taking into account the effects of slope and turn. Similarly, a loss of adhesion leading to the falsification of an acceleration measurement which would be deduced from the rotation of the axles, may be ideally compensated here.       

     Chiefly, therefore, the device according to the invention uses two bi-axial accelerometers  101 ,  102  fixed to the body of the vehicle and intended to measure a longitudinal acceleration and a lateral acceleration of the vehicle. The vehicle is subjected to three forces producing a longitudinal acceleration Gx (movement of the vehicle subjected to tractive/braking forces), a lateral acceleration Glat (the turn of the trajectory causing centrifugal acceleration) and a vertical acceleration Gpes due to the gravity which is exerted in the presence of a slope (the slope of the trajectory). The first accelerometer  101  of which the two axes Acc 1 , Acc 2  are located in the vertical plane Py and the second accelerometer  102  of which the two axes Acc 3 , Acc 4  are located in the horizontal plane Pz, make it possible to measure a resultant of the accelerations (longitudinal, lateral, gravity) projected on each of the four measurement axes. The angles between the different measurement axes of the accelerometers are known and fixed after adjustment. The computer  103  solves a system composed of four equations in order to determine four unknowns at the position Dx of the vehicle, namely a slope angle Ax of the trajectory, a lateral acceleration angle Ay (resultant of the centripetal force due to the speed of the vehicle and dependent on the radius of curvature R of the trajectory in addition to the offset of the accelerometer relative to the center of the vehicle), a value of lateral acceleration Glat and the value of longitudinal acceleration Gx. By successive integration over the duration of the journey, the computer  103  determines the longitudinal speed Vx and the longitudinal movement Dx of the vehicle VEH over its route for any slope and turn COURB. 
     If required, the device according to the invention is complemented by a tachometer  108  to improve the accuracy of the above measurement of the speed Vx and of the distance Dx covered. The tachometer  108  is fixed to one of the axles R 1   a , R 2   a , R 1   b , R 2   b  of the vehicle VEH and its output signal(s) STb (is) are transmitted to the computer  103 . The computer  103  evaluates a movement DxT and a speed VxT based on measurement signal(s) of the tachometer. The computer carries out a comparison between the results of the measurement of movement from the tachometer and those from the accelerometers. When for these measured values, a difference in measurement is lower than a threshold, the measurement values are reset to those of the tachometer. In the opposite case (value greater than a threshold) there is no correction of the results from the measurement of the accelerometers. 
     As shown in  FIG. 1 , zero speed information Op may also be reliably provided by the computer  103  from information Im originating from equipment of the vehicle (immobilization signal, zero speed indicator, etc.) or be determined by the device according to the invention itself. To determine this information, the computer  103  processes the information from the tachometer and the accelerometers. 
     When the device determines zero speed and due to the specifics of the proposed mounting of the accelerometers, the device also advantageously has the capacity to implement an auto-test function. This auto-test function makes it possible to evaluate the corrections which have to be made to the measurements from the accelerometers (after auto-calibration) and to identify faults in the operation of the accelerometers. The multiplicity of the measurement axes provides a redundancy which is very advantageous for several measurements (due to the two bi-axial accelerometers) and makes it possible by a periodic verification of reliability of the accelerometers (for example at each stop at a station) to guarantee test measurements (and thus subsequent movement) with a very low probability of error, making them compatible with the safety demands of a reliable system as required in the railway field. 
     In the remainder of this description, reference is made to the two  FIGS. 3 and 4 . 
     Considering the measurement axes Acc 1 , Acc 2  of the first accelerometer  101  (see  FIG. 3  where, for reasons of clarity, the lateral acceleration Glat has deliberately been omitted), the components of the projection measurements Gacc 1 , Gacc 2 , by adding the projections of the accelerations Gx, Glat, Gpes on each of the axes Acc 1 , Acc 2  of the accelerometer  101  are:
         on the axis Acc 1 
 
 Gacc 1=projection( Gx )−projection( Gpes )−projection( Glat )
 
 Gacc 1 =Gx  cos( Ay )cos( A 1)+ Gpes  sin( A 1 −Ax )− Glat  sin( Ay )cos( A 1)  (1)
   on the axis Acc 2 
 
 Gacc 2=projection( Gx )−projection( Gpes )−projection( Glat )
 
 Gacc 2 =Gx  cos( Ay )cos( A 2)− Gpes  sin( A 2 +Ax )− Glat  sin( Ay )cos( A 2)  (2)
       

     Similarly, considering the measurement axes Acc 3 , Acc 4  of the second accelerometer  102  (see  FIG. 4  where, for reasons of clarity, the slope acceleration Gpes has deliberately been omitted), the components of projection measurements Gacc 3 , Gacc 4  by the addition of the projections of the accelerations Gx, Glat, Gpes on each of the axes Acc 3 , Acc 4  of the accelerometer  102  are:
         on the axis Acc 3 
 
 Gacc 3=projection( Gx )−projection( Glat )−projection( Gpes )
 
 Gacc 3 =Gx  cos( A 3 +Ay )− Glat  sin( A 3 +Ay )− Gpes  sin( Ax )cos( A 3)  (3)
   on the axis Acc 4 
 
 Gacc 4=projection( Gx )−projection( Glat )−projection( Gpes )
 
 Gacc 4 =Gx  cos( A 4 −Ay )+ Glat  sin( A 4 −Ay )− Gpes  sin( Ax )cos( A 4)  (4)
       

     With for the equations (1) to (4):
         the angle A 1  in the plane Py between the axis X and the axis Acc 1     the angle A 2  in the plane Py between the axis X and the axis Acc 2     the angle A 3  in the plane Pz between the axis X and the axis Acc 3     the angle A 4  in the plane Pz between the axis X and the axis Acc 4     the angle Ax of the trajectory of the vehicle in the plane Py (i.e. the angle between the horizontal and the axis X)   the offset distance Dx between the center of the vehicle and the fixing point of the accelerometers  101 ,  102  installed on the vehicle   the angle Ay associated with the radius of curvature R in the plane Py. The angle Ay is calculated by Arctg (Lx/R), thus in a first approximation Lx/R, given that the value of the radius of curvature R is usually greater than the offset distance Lx.       

     The resolution of the system formed by the four equations (1) to (4) falls within the scope of mathematical techniques which are not disclosed here and of which the object is to calculate the four variables Gx, Glat, Ax and Ay according to the measurements of acceleration values Gacc 1 , Gacc 2 , Gacc 3 , Gacc 4  of which the computer  103  makes use. 
     However, the resolution of the system is advantageously simplified in certain specific hypotheses for the arrangement of the accelerometers  101 ,  102 . 
     From these hypotheses may be selected the relative angles A 1 +A 2 , A 3 +A 4  each defining a right angle, i.e.: A 1 +A 2 =90° and A 3 +A 4 =90°. Thus, the device according to the invention may provide that at least one of the relative angles A 1 +A 2 , A 3 +A 4  is a right angle. 
     The device according to the invention is implemented such that each relative angle A 1 +A 2 , A 3 +A 4  is in fact subdivided (or subdivisible) into a first and a second angle A 1 , A 2  and respectively A 3 , A 4  corresponding to projection angles between the four measurement axes Acc 1 , Acc 2 , Acc 3 , Acc 4  of the first and of the second accelerometer  101 ,  102  and the first axis X (longitudinal axis according to a principal movement of the vehicle, assumed to be rectilinear). 
     In this regard, it is also very advantageous to select the angles A 1 , A 2 , A 3 , A 4  so that A 1 =A 2  and A 3 =A 4 , and in particular so that A 1 =A 2 =A 3 =A 4 =45°. 
     Regarding the choice of angles A 1 , A 3 , it is also possible to attribute to them adjustable values making it possible to estimate in the best possible manner the effects of slope or turn without impairing the accuracy of the measurement of longitudinal acceleration. 
     By way of example, if the option is selected in which the projection angles A 1 , A 2 ; A 3 , A 4  of each accelerometer are equal, i.e. A 1 =A 2  and A 3 =A 4 , the above system of equations becomes:
 
 Gacc 1 =Gx  cos( Ay )cos( A 1)+ Gpes  sin( A 1 −Ax )= Glat  sin( Ay )cos( A 1)  (1)
 
 Gacc 2 =Gx  cos( Ay )cos( A 1)− Gpes  sin( A 1 +Ax )− Glat  sin( Ay )cos( A 1)  (2)
 
 Gacc 3 =Gx  cos( A 3 +Ay )− Glat  sin( A 3 +Ay )− Gpes  sin( Ax )cos( A 3)  (3)
 
 Gacc 4 =Gx  cos( A 3 −Ay )+ Glat  sin( A 3 −Ay )− Gpes  sin( Ax )cos( A 3)  (4)
 
     The resolution of this system makes it possible to determine easily the four unknowns which are sought and defined by the variables Gx, Glat, Ax, Ay, then by integration over a duration of movement to deduce therefrom the longitudinal speed Vx and the associated position Dx over the route of the vehicle:
 
 Vx =∫( Gxdt )
 
 DX =∫( Vxdt )
 
     The device according to the invention thus permits the computer  103  to provide a value of the slope angle Ax, of a lateral acceleration angle Ay (i.e. representing the rotation of the lateral acceleration at the fixing point of the mounting of the accelerometer relative to which it would be at the center of the vehicle for the radius of curvature R) at each point of the route which includes both slopes and turns. 
     By extension, the computer  103  provides a speed Vx and a position Dx at each point of the route which includes both slopes and turns by integrating successively the value of longitudinal acceleration Gx of the vehicle. 
     As disclosed above, the device may also comprise:
         a tachometer  104  arranged on at least one axle of the vehicle and providing a tachymetric value of speed VxT and position DxT of the vehicle,   the tachymetric values VxT, DxT and the speed and position values Vx, Dx obtained and respectively delivered by the computer  103  are provided to a comparator  106  incorporated in the computer  103 ,   the comparator  106  determines the differences between the categories of speed and position values, and if said differences are below a predefined threshold, a resetting of the speed and position values Vx, Dx provided by the computer  103  at each point of the route which includes both slopes and turns is implemented on the tachymetric values VxT, DxT. If the difference is above the threshold, the resetting is inhibited.       

     This possibility of resetting provides an increase in the accuracy of measuring the speed and movement based on a simple additional measurement of speed and movement which is proportional to the radius of the wheel. 
     The device according to the invention may also comprise a means for detecting zero speed  107  of the vehicle which is incorporated in or coupled to the computer  103  and to the tachometer  104 . Said tachometer comprises at least one correlator of the speed and position values Vx, Dx delivered by the computer  103  and corresponding tachymetric values VxT, DxT. 
     As a result, a very reliable function for detecting zero speed is implemented, namely:
         by taking into account information which is external to the device made available by one of the devices of the vehicle (for example by means of an internal signal of the immobilized vehicle, etc.)   by determining a stoppage of the vehicle by filtering information about speed and movement Vx, Dx provided by the computer  103 . This determination may thus be correlated with the corresponding tachymetric data VxT, DxT.   following this processing, if it is certain that the vehicle is genuinely stopped, the device provides so-called zero speed information.       

     A function known as auto-test may thus advantageously use the so-called zero speed information. When this information is legitimately provided, it means that the vehicle is immobile and as a result, the longitudinal and lateral acceleration are thus zero. 
     The associated test thus consists in checking that the measurement values delivered by the accelerometers  101 ,  102  verify the system of equations (1), (2), (3), (4) provided above, which is thus reduced to:
 
 Gacc 1 =Gpes  sin( A 1 −Ax )  (1)
 
 Gacc 2 =−Gpes  sin( A 2 +Ax )  (2)
 
 Gacc 3 =−Gpes  sin( Ax )cos( A 3)  (3)
 
 Gacc 4 =−Gpes  sin( Ax )cos( A 4)  (4)
 
     An example of resolution of this system is provided here in the particular hypothesis of the arrangement of the accelerometers, for which the projection angles A 1 , A 2 ; A 3 , A 4  are equal for each pair in each of the planes Py, Pz, i.e. that A 1 =A 2  and A 3 =A 4 : 
     From the two last equations (3) and (4) the following relations (5) and (6) may be deduced:
 
 Gacc 3 =Gacc 4  (5)
 
Sin( Ax )=− Gacc 3/( Gpes  Cos( A 3))  (6)
 
     Relative to the term Sin(Ax) in the equations (1) and (2), it is thus possible to verify the measured values of the projected accelerations Gacc 1 , Gacc 2  of the first accelerometer  101  with the above calculated results. 
     The projected accelerations Gacc 3 , Gacc 4  of the second accelerometer  102  are verified by the equation (5). In a first approximation, it is reasonable to consider that the slope has little influence on the measurement which is generally the case, for example, when parking in the garage or when stopped at the station. 
     In order to refine the verification of the projected accelerations Gacc 3 , Gacc 4  of the second accelerometer  102  it is, however, also possible to read a value of the slope from a data bank. 
     By these verifications and by selecting a filtering threshold, it is possible to determine correction factors to be made to the measurements from the accelerometers. In the case of the second accelerometer  102  it is advantageously possible to benefit from the slow process of the accelerometer drift before modifying its correction factors. These correction factors are applied following a confirmation obtained after several stops. This number of stops is adjustable according to the degree of accuracy maintained. This makes it possible to auto-calibrate the device according to the invention. 
     A second selected threshold which is higher than the first threshold may also be defined in order to declare that the device according to the invention is not in operation. 
     In order to implement the auto-test function, the device according to the invention comprises:
         a means for auto-calibration  105  of the accelerometers  101 ,  102  which may be activated if the means for detecting zero speed confirms a stoppage of the vehicle,   the means for auto-calibration processing the measurements from the accelerometers  101 ,  102  and provided by a unit for calculating accelerations  104  (itself receiving the measures from the accelerometers  101 ,  102  and being included in the computer  103 ),   the means for auto-calibration calibrates the measurements corresponding to the zero values of the longitudinal acceleration Gx and lateral acceleration Glat of the vehicle.       

     The means for auto-calibration  105  has a first control mode for verifying the equality of the measurement values Gacc 3 , Gacc 4  on the second accelerometer  102  and a means for recalculating the slope angle Ax from which the measurement values Gacc 1 , Gacc 2  of the first accelerometer  101  are verified by means of a second control mode. Thus, the verification becomes very reliable and even more so if the slope angle may be evaluated and confirmed redundantly by known information which is external to the device. 
     For this embodiment and relative to the auto-test function disclosed above, beyond a first error threshold arising from results of the auto-calibration means  105 , correction factors from the auto-calibration means  105  are thus retransmitted to the calculating unit  104  (more usually to the computer  103  for calculating the movement). 
     Similarly, beyond a second error threshold which is less safe than the first threshold arising from results of the auto-calibration means  105 , an indicator of failure of the on-board measurement is activated. 
     A simplified model of evaluating the probability of failure of the function known as auto-test may thus be implemented considering that, with the stoppage of the vehicle, measurements carried out on the measurement axes acc 1 , acc 2 , acc 3 , acc 4  of the accelerometers  101 ,  102  are obtained redundantly. 
     Assuming a time interval T between two stops of the vehicle: the probability of failure Pr of the auto-test function applied to the two measurement axes Acc 1 , Acc 21  in the plane Py is defined by:
 
 Pr=λacc 1 *Aacc 2 *T  
 
     Where the respective failure rates Aacc 1  and Aacc 2  of the measurement axes Acc 1  and Acc 2  of the bi-axial accelerometers are each assumed to be equal to a usually permitted value of 10 −5  in the following calculated example:
 
Where  T= 60 seconds,  Pr= 10 −10 *0.017=1.7*10 −12  
 
Where  T= 10 minutes,  Pr= 10 −10 *0.17=17*10 −12  
 
     It is thus apparent that if the vehicle stops periodically and frequently, the device makes it possible to guarantee a level of confidence in the measured data which is required for the safety demanded in the railway field. 
     According to this evaluation of the probability of failure of the so-called auto-test function, the device according to the invention may thus comprise a means for evaluating the probability of failure which may be activated between two stops of the vehicle and using a redundancy measurement on the measurement axes of the accelerometers. This means of evaluation may be integrated in the auto-calibration means  105  disclosed above. 
     Finally, the device according to the invention may also optionally comprise a detector of loss of adhesion of the vehicle (in the case of slipping or wheel locking) coupled to at least one of the first and second bi-axial accelerometers  101 ,  102  for which the movement measurements may be associated with external values (slope, turn from a data bank or data from a route marker system, etc.). In the case of divergence from this data, a risk of loss of adhesion of the vehicle may be detected and, by extension, complement the information provided by the system for detecting zero speed (locked wheel but vehicle in motion). 
     The detector of loss of adhesion of the vehicle may also, if required, be coupled to at least one tachometer  108  of the axis of the vehicle in addition to one of the first and second accelerometers  101 ,  102  so as to compare their data for measuring the angular movement and respectively the longitudinal movement. By this means, the function of detecting zero speed may be thus made even more secure. 
     Principal Abbreviations 
     
         
         X: longitudinal axis (of movement) of the vehicle 
         Y: axis perpendicular to the axis X and in the plane of the floor of the vehicle 
         Z: axis perpendicular to the floor of the vehicle 
         Px: plane at right angles to the axis X and determined by the axes Y, Z 
         Py: plane at right angles to the axis Y and determined by the axes X, Z 
         Pz: plane at right angles to the axis Z and determined by the axes X, Y 
         Gpes: acceleration of gravity=9.81 m/s2 
         Gx: longitudinal acceleration of the vehicle along the axis X 
         Glat: lateral acceleration of the vehicle at the location of the accelerometers in the vehicle 
         Vx: longitudinal speed along the axis X 
         Dx: longitudinal position/movement along the axis X 
         VxT: longitudinal speed provided by the tachometer 
         DxT: longitudinal movement provided by the tachometer 
         Acc 1 : first measurement axis of the accelerometer  101   
         Acc 2 : second measurement axis of the accelerometer  101   
         Acc 3 : first measurement axis of the accelerometer  102   
         Acc 4 : second measurement axis  2  of the accelerometer  102   
         A 1 : angle in the plane Py between the axis X and the axis Acc 1   
         A 2 : angle in the plane Py between the axis X and the axis Acc 2   
         A 3 : angle in the plane Pz between the axis X and the axis Acc 3   
         A 4 : angle in the plane Pz between the axis X and the axis Acc 4   
         Ax: angle of trajectory of the vehicle in the plane Py (i.e. the angle between the horizontal and the axis X) 
         Lx: offset distance between the center of the vehicle and the fixing point of the accelerometers  101 ,  102   
         Ay: angle associated with the radius of curvature in the plane Py. Ay is calculated by Arctg (Lx/R), thus in a first approximation Lx/R 
         Vx: longitudinal speed of the vehicle along the axis X