EP2313739A1 - Method for the improved estimation of an object orientation and attitude control unit implementing said method - Google Patents

Abstract

The invention relates to a method for estimating the spatial orientation of an object at a moment k using the global acceleration (yA), magnetic field (yM), and rotation speed (yG) measurements of said object along three spatial axes, and which comprises: A- the pre-processing (200) of said measurements (yA, yM> yG) at a moment k for detecting the existence of an interference in said measurements and estimating interference-free measurements at the moment k; B- the estimation of the orientation (formula (I)) at the moment k by an observer from the interference-free measurements (formula (II)) at the moment k resulting from step A.

Description

 IMPROVED ESTIMATING METHOD OF OBJECT ORIENTATION AND ATTITUDE CENTER USING SUCH A METHOD

The present invention relates to a method for estimating the orientation of an object in space, with or without its own acceleration and with or without magnetic disturbance, and to a device capable of allowing the estimation of the orientation. implementing such a method. The obtaining of the orientation generally requires the implementation of several sensors, forming part of a set designated by motion capture device, also designated by central attitude. MEMS sensors ("Micro-Electro-Mechanical Systems" or electronic microsystems) can be used to build this plant, they have the advantage of being small and low cost. The use of such MEMS sensors makes it possible to envisage the use of attitude control units in various fields of application, in particular the biomedical field, for the surveillance of elderly people at home, functional rehabilitation, in the sports field, for the analysis of the movements of athletes, in the automotive, robotics, virtual reality, animation in three dimensions and more generally in any field in which we seek to determine or observe a movement. However, compared to non-MEMS sensors (used for example in the field of navigation), these MEMS sensors have the disadvantages of being relatively noisy and biased.

Moreover, it is known that there are attitude centers that use together accelerometers and magnetometers, which make it possible to reconstruct the movements with three degrees of freedom, that is to say movements whose own accelerations and Magnetic disturbances are respectively negligible compared to the Earth's gravity field and the Earth's magnetic field. On the other hand, when this hypothesis is not respected, that is to say that one can not neglect the own acceleration or magnetic disturbances, movements have six or nine degrees of freedom. It is therefore impossible, using an attitude center using only accelerometers and magnetometers to estimate the orientation of the object in motion. However, the diversification of the applications of motion capture imposes to overcome these constraints, It was then envisaged to use additional sensors, particularly to jointly use gyrometers, accelerometers and magnetometers. The measurements from these sensors are composed of two parts: an informative part directly related to the orientation of the object in motion and a disturbing part whose nature depends on the sensor considered. In the first order, these are clean accelerations for the measurements provided by the accelerometers, magnetic disturbances for the measurements delivered by the magnetometers and the bias for the gyrometers. These disturbances lead to an erroneous estimation of the orientation.

Currently there are several methods to obtain, from measurements provided by accelerometers, magnetometers and gyrometers, an estimate of the orientation of the object. There are methods, called optimization, implementing one or more optimization criteria, but these are relatively expensive in computing time. In addition, when the problem becomes complex, the definition of the optimization criteria is difficult.

There are also methods implementing neural networks, which require a substantial learning phase, particularly with respect to the size of the database and the calculation time, to obtain an accurate estimate.

Moreover, optimization methods and those implementing neural networks hardly take into account the notion of temporal evolution of states, which makes them less robust. There are also methods implementing an observer, which, unlike the methods mentioned above, make it possible to merge a double information: that coming from the measurements provided by the sensors and that coming from a model of evolution, and this while maintaining a computation time compatible with a real time implementation. The known methods implementing an observer rely mainly on the use of a Kalman filter. The advantage of this technique is to allow data fusion while taking into account the quality of the information provided by the measurements provided by the sensors and the quality of the model. Several types of Kalman filters exist and are well known to those skilled in the art:

the extended Kalman filter ("Extended Kalman Filter" or EKF), which is fast and easy to implement, one of its applications to motion capture is described in particular in the document "Quaternion Kalman filter for determining orientation by inertial and magnetic sensing ", SABATINI AM, IEEE Transactions on Biomedical Engineering, 2006, 53 (7).

- The Kalman filter UKF ("the Unscented Kalman Filter" in English terminology) is dedicated to highly non-linear problems. But in the context of motion capture, the problems encountered are weakly non-linear; therefore it is less appropriate for the estimation of the orientation. Moreover, the calculation cost is greatly increased compared to an EKF filter. Its interest is therefore reduced compared to an EKF filter. For example, the paper "Portable Guidance Based on Accelerometers, Magnetometers and Gyroscopes Sensors for Sensor Network" by HARADA T., UCHINO H., MORI T., SATO T. IEEE Conference on Multisensor Fusion and Integration for Intelligent Systems, 2003 discloses a portable attitude estimator in which the fusion of the absolute orientation by means of an accelerometer and a magnetometer and the rotational speed by means of a UKF filter is performed. - the Kalman complementary filter: in this case, the aim is to estimate the error on the states and not the states themselves, its implementation is very complex.

In addition to the choice of the filter, the quality of the measurements injected into the filter is of great importance, and in particular the confidence that one gives to their value. Indeed, as mentioned above, the measurements include an informative part directly related to the orientation of the moving object and a disturbing part whose nature depends on the sensor considered. In the first order, these are clean accelerations for the measurements provided by the accelerometers, magnetic disturbances for the measurements delivered by the magnetometer and the bias for the gyrometers. It is also necessary to take into account the noise of measurement, however this one is conventionally treated in the filter. There are currently several methods for dealing with disturbances. One of these is to consider disturbances as negligible and to provide the filter with the measurements as delivered by the sensors, as is the case in the documents "Design, implementation and experimental results of a quaternion-based Kalman filter for Human body motion tracking, YUN X., BACHMANN ERIEEE Transactions On Robotics, 2006, 22 (6), and "Application of MIMU / Magnetometer integrated system on the attitude determination of micro satellite," SU K., REN DH, YOU Z , ZHOU Q., International Conference on Intelligent Mechatronics and Automation, August 2004, Chengdu, China. Therefore, in the case where a disturbance actually occurs on one of the sensors, the measurement provided to the filter is distorted but it will consider it as accurate. The estimation of the orientation then becomes erroneous. It is therefore not possible to neglect disturbances to obtain an estimate with a desired accuracy, regardless of the number of degrees of freedom. There is therefore a strong degradation of the performances, since the disturbed measurements are provided such as to the observer. Estimation methods then make it possible to take into account the point imperfection of certain measurements by detecting the presence of disturbances and by changing the confidence given to these measurements, this is for example described in the document "Portable orientation estimation device based on accelerometers, magnetometers and gyro sensors for sensor network ", HARADA T., UCHINO H., MORI T., SATO T. IEEE Conference on Multisensor Fusion and Integration for Intelligent Systems, 2003. This method provides an additional step of detection of the disturbance in the measurements. When a disturbance is detected, the confidence in the corresponding measurement is minimized.

The information provided by the disturbance measure is therefore not taken into account for the estimation of the orientation. The orientation estimate is based only on the measurements provided by the other sensors. However, in the case where the measurements of several sensors simultaneously present a disturbance, the observer does not have enough information to propose a correct estimate of the orientation. Finally, there is a last method described in the documents "Inertial and magnetic sensing of human motion", ROETENBERG D., Ph.D. thesis, University Twente, Netherlands, 2006, and "Measuring orientation of human body segment using miniature gyroscopes and accelerometers ", PhD Thesis, Inertial sensing of human movement, LUINGE HJn 2002b, in which it is expected to detect the presence of disturbances and estimate them, thanks to the observer. For this, the state vector is increased and disturbances occur in the measurement model that becomes closer to reality. This technique seems, in principle, adapted to the case of movements with six or nine degrees of freedom. However, joint estimation of disturbance and orientation is tricky due to a lack of observability. It also requires the adjustment of a large number of parameters, which complicates its implementation. It is therefore an object of the present invention to provide a method of estimating orientation providing an accurate estimate of the orientation, in the presence or absence of clean accelerations and magnetic disturbances, and this in a simplified manner compared to existing methods.

The purpose stated above is achieved by a method of estimating the orientation on the basis of measurements along the three axes of the space of the acceleration, the magnetic field and the speed of rotation, comprising:

a step of pretreatment of these measurements to detect the existence of a disturbance and to estimate undisturbed measurements, and

a step of estimating the orientation on the basis of the measurement values resulting from the preprocessing step.

The process does not neglect disturbances, which does not distort the estimate; he estimates them permanently. If they exist, it does not reject the associated measurement or measurements, as is the case in other estimation methods. Moreover, it does not integrate them in the state vector or in the measurement model, which simplifies the model and does not lead to situations where estimation becomes impossible. It is therefore expected to estimate the orientation, and possibly to estimate the disturbances, in two successive stages. The observer is thus provided with measurements of accelerometers, magnetometers and gyrometers as close as possible to ideal conditions for estimating the orientation: that is to say without proper accelerations, without magnetic disturbances and without bias, respectively. For this, it is expected to use the orientation estimated at the previous instant as additional information to perform preprocessing measures.

The estimation method according to the invention therefore makes it possible to extract measurements from the sensors, the orientation of the object in an optimal manner, whatever the movement considered. This method is also simple to implement and has only a small number of adjustment parameters. The observer is advantageously an extended Kalman filter. It is possible to estimate the disturbances, in particular the natural accelerations, which makes it possible, by integration and double integration, to go back to the speed and to the position of the object respectively. The main subject of the present invention is therefore a method for estimating the orientation of an object in space at the instant k using the measurements of the total acceleration, of the magnetic field and the speed of rotation of said object. along three axes of space, comprising the steps of: A- preprocessing said measurements at a time k to detect the existence of a disturbance in said measurements, said perturbation belonging to a group comprising a specific acceleration of the object, a magnetic field adding to the earth's magnetic field and a bias in the measurement of the speed of rotation, and for estimating undisturbed measurements at time k, B- estimating the orientation at time k by an observer from the undisturbed measurements estimated from step A. Step A advantageously comprises: A1- a pretreatment of the rotational speed measurements, A2- a detection of the existence or not of a disturbance at the insta nt k in said measurements of the total acceleration and magnetic field,

A3- if there is no disturbance at time k, the undisturbed measurement estimated at time k is equal to the measurement at time k. In the event of a disturbance, the undisturbed measure estimated at time k is computed on the basis of the estimated direction at the previous instant k-1. Step A1 consists in subtracting from the rotational speed measurements an average bias determined during a preliminary initialization step. This average bias can be obtained by immobilizing the means providing the rotational speed measurements for a given time and calculating the average of the values of the rotational speed measurements on each axis. In the case of an attitude center carried by a person, this immobilization is to remove the central of the person to overcome the inevitable tremors of the person.

The pretreatment step A2 of the acceleration and magnetic field measurements may comprise: a step A2.1 consisting of a comparison test of the standard of the measurements of the total acceleration with that of the gravitational field, if the absolute value the difference between the standard of accelerometric measurements at time k and that of the gravitational field is less than a predetermined threshold, it is considered that the accelerometric perturbation is zero, otherwise it is considered that there is a disturbance, the perturbation being equal on each axis, the difference between the measurement of the total acceleration on each axis at time k and the undisturbed accelerometric measurement estimated at time k, and - a step A2.2 consisting of a comparison test from the standard of magnetic field measurements to that of the Earth's magnetic field, if the absolute value of the difference between the standard of magnetic field measurements and that of the magnet field If the earthquake is below a predetermined threshold, it is considered that the magnetic disturbance is zero, otherwise the magnetic disturbance is considered to be equal, on each axis, to the difference between the magnetic field measurement on each axis at time k and the undisturbed magnetic field measurement estimated on each axis at time k.

Advantageously, step A2.1 is provided with an additional test on the estimated disturbance at instant k-1: in the case where the absolute value of the difference between the standard of the measurements of the total acceleration and that of the gravitational field at the moment k is below the predetermined threshold, it is checked whether the norm of the accelerometric perturbation estimated at time k-1 is below a predetermined threshold, if this test is positive, it is considered that the accelerometric perturbation is effectively zero at time k, and / or at step A2.2, an additional test is provided on the magnetic disturbance estimated at time k-1: in the case where the absolute value of the difference between the standard of the magnetic field measurements and that of the earth's magnetic field is lower than the predetermined threshold, it is checked whether the absolute value of the magnetic disturbance estimated at time k-1 is below a predetermined threshold, if this test is positive it is considered that the magnetic disturbance is effectively zero at time k. This additional step makes it possible to improve the accuracy of the estimation method. In unfavorable employment cases where it is known that the estimated orientation can be derived, however, the robustness of these comparison tests of steps A2.1 and A2.2 is ensured only over a limited time window, different according to cases. The comparison tests are therefore advantageously carried out over time ranges as indicated later in the description. Thus, by noting y _{A k} , y _{M k} , y _GΛ the measurements obtained at the end of the preprocessing step which are called estimated undisturbed measurements: - the detection of clean accelerations is carried out solely through the standard of accelerometric measurement; If this norm is different from the Go (Earth Gravity) norm on at least one of the measurements of a sliding window of duration T _A then the measurement at the current time is considered to be disturbed; - the detection of magnetic disturbances is carried out in a similar way: o if the norm of the magnetometric measurement is different from the norm of Ho (terrestrial magnetic field) on at least one of the measurements of a sliding window of duration T _M o OR if the angle between the magnetometric measurement and the opposite of the undisturbed accelerometric measurement - y _A is different from the angle between the vectors Go and H ₀ , then the measurement at the current time is disturbed magnetically. TA may be a parameter of constant value while the value of T _M may be related to the speed of movement.

Thanks to the use of this variant, which can be installed on the same equipment and activated on the initiative of the user, we do not have to go to search for the values of the measurements at the moment k-1 and one thus avoids the drift.

The observer used in step B is preferably an extended Kalman filter, which is fast and simple.

Step B of estimating the orientation from the measurements estimated at time k may comprise:

estimating the a priori state vector at time k from the estimated state vector a posteriori at time k-1, estimating a priori measurements at time k from the a priori estimation of the prior state vector at time k, the so-called a priori estimation of the measurements,

calculating the gain of the extended Kalman filter and the innovation by calculating the difference between the undisturbed measurements estimated at time k and the measurements estimated a priori, calculating the estimated orientation at time k by correction of the state vector estimated a priori at time k by gain and innovation.

The state vector used in the Kalman filter can contain the elements of angular velocity and orientation quaternion. The state vector used in the extended Kalman filter advantageously contains only the elements of the orientation quaternion, which makes it possible to simplify the structure of the state and measurement models. The present invention also relates to an attitude center comprising means capable of providing acceleration measurements, magnetic field measuring means, and means for measuring the speed of rotation along three axes of the space, said means being intended to be integral in motion of an object, and means for estimating an orientation at time k on the basis of measurements provided by said measuring means, said estimating means comprising:

means for pretreatment of said acceleration, magnetic field, and rotational speed measurements, said pretreatment means being able to detect the existence of a disturbance in said measurements; and delivering estimated undisturbed accelerometric measurements, estimated undisturbed magnetometer measurements, and unbiased rotational velocity,

means for estimating the orientation at an instant k by an observer from the measurements provided by the preprocessing means. This observer can be an extended Kalman filter.

The attitude control unit according to the present invention may also comprise means for calculating an average bias of the means for measuring the speed of rotation during an initialization step of the control unit. The pretreatment means may comprise means for detecting the existence of an own acceleration in the acceleration measurements and means for detecting the existence of magnetic disturbances in the magnetic field measurements. The attitude unit according to the invention may also comprise means for estimating the proper acceleration and for calculating the speed and the position of the object.

The means capable of providing measurements of the total acceleration, magnetic field measurements, and measurements of the speed of rotation along three axes of the space are advantageously MEMS sensors.

The present invention will be better understood with the aid of the description which follows and the attached drawings, in which:

FIG. 1 is a flowchart of the method according to the present invention at time k,

FIGS. 2A to 2C show detailed flowcharts of a step of pretreatment of the measurements of an accelerometer, a gyrometer and a magnetometer respectively, according to the present invention.

It is desired to obtain the orientation of an object moving in space, for example the orientation of a person. For this, we use a central attitude comprising sensors able to provide measurements of the total acceleration, magnetic field and rotation speed along the three axes of the space. The sensors are advantageously MEMS sensors offering a reduced cost and a small footprint. It may be, for the measurement of acceleration, for example a tri-axis accelerometer or three single-axis accelerometers providing a measurement on each of the axes.

Similarly for magnetic field measurements, it may be a tri-axis magnetometer or three single-axis magnetometers. For the measurement of the speed of rotation, it may be, for example, three single-axis gyrometers or preferably two bi-axis gyrometers. The triaxes can be aligned or not, in the latter case the relative orientation between the axes must be known. In the remainder of the description, for purposes of simplicity, we will designate the accelerometer or accelerometers, by an accelerometer, the magnetometer (s) by a magnetometer and the gyrometer (s) by a gyrometer. These sensors are attached to the object whose orientation is desired. We only have y measures, which we model by:

with YA: tri-axis measurement of the total acceleration provided by the accelerometer,

YM: tri-axis measurement of the magnetic field supplied by the magnetometer,

YG: tri-axis measurement of the speed of rotation provided by the gyrometer,

R: rotation matrix, G ₀ : earth gravity field (vector 3x1), H ₀ : Earth's magnetic field (vector 3x1), ω: angular velocity, a: natural accelerations, d: magnetic disturbances, b: gyrometer bias, v _A : accelerometer measurement noise, v _M : measurement noise of the magnetometer,

VQ: measurement noise of the gyrometer.

The orientation is estimated with respect to a reference frame, entirely defined by the data of the vectors Go and Ho. For example, the geocentric reference is defined by the vectors

G ₀ (0; 0; 1) and H ₀ (0.5; 0; ₂ )

For the sake of simplicity, we will not distinguish the measurements according to the three directions of space.

As is clear from the mathematical definition (I) of the measures, each of these measures includes a first part

"-RGo", "R-Ho" and ω, which contains the information to obtain an estimate of the orientation, a second part a, d and b which represents the possible disturbances which can appear, at random in the measurements, and finally a third part v _A , v _M , VQ representing the measurement noise at each sensor.

In the method according to the present invention, after the collection of measurements, a pretreatment step is provided before use in a processing step for providing an estimate of the orientation.

In Figure 1, a general flowchart of the method according to the present invention can be seen. In the following description of the method, the estimation of the orientation at time k is taken as an example, where k is a natural integer greater than or equal to 2.

The method according to the present invention comprises a step 100 of initialization of the attitude center, a step 200 of preprocessing the measurements provided by the sensors and a third processing step 300 by the observer. Each of the steps will be described in detail in the following description.

Before describing in detail the different steps of the method according to the invention, we will describe the observer. Advantageously, the observer used in the measurement processing step is an extended Kalman filter, which is simple, robust and quick to implement.

This filter is widely known to those skilled in the art and will not be described in detail. We will only give the mathematical expressions of the state model and the measurement model.

A Kalman filter includes a state model defining the temporal and dynamic evolution of states, and a measurement model that allows to link sensor measurements and states.

The state vector of the Kalman filter is composed according to a first modeling of the three elements of the angular velocity and the four elements of the quaternion defining the orientation.

The associated state and measurement models can be respectively:

with x: state vector ω: angular velocity q: quaternion

T: time constant of the evolution model of the angular velocity y: measurements

^Wχ : modeling noise For the purpose of simplification, we identify the vector quaternion [0, Go] of dimension 4x1 with the vector Go of dimension 3 * 1, and also for the vector quaternion [0, H ₀ ] of dimension 4x1, the vector H ₀ also being of dimension 3 * 1.

Advantageously, according to a second modeling, it is possible to use a state vector containing only the elements of the quaternion, which is only of dimension 4, whereas it is of dimension 7 in the first modeling. The gyrometric measurement is then injected directly into the state model and the measurement vector contains only the accelerometer and magnetometer measurements. The state model and the measurement model can then be written: x = f (x) ≈ q = -.q yyy _G + w _x (II), and

This second modeling makes it possible to simplify the structure of the state and measurement models since their dimension is directly reduced. Moreover, the number of adjustment parameters, in particular the elements of the covariance matrices of the modeling noise, the measurement noise and the estimation error of the state vector, is also restricted, which facilitates the setting of implementation of this method. The estimation results obtained in this way are of similar precision to those obtained using the first modeling.

We will now describe in detail the steps 100, 200 and 300 according to the present invention. The initialization step 100 provides for estimating the average perturbation of the gyrometer. This disturbance b, which is in fact the bias of the gyrometer, varies between two extreme values. For the initialization step (k = 1): Let we know the proper acceleration ai and the magnetic disturbance di at the initial moment, then the initialization step involves the determination of the state vector Xi: the angular velocity is assumed to be zero at the initial moment, the quaternion is determined by optimization using the acceleration and magnetic field corrected for the perturbations ai and di; Either we know the orientation in the initial state, in this case we can deduce ai and di. The initialization step 100 of the plant at k = 1 further comprises:

the setting of the covariance matrices Q, R and P ₁ respectively related to the modeling noise, to the measurement noise and to the estimation error of the initial state vector;

- the calculation of an estimate of the bias b, which is noted b _moym . the stationary attitude unit is maintained for a predetermined time, for example about one second, and the average of the output values of the gyrometer is calculated on each axis. This estimate of the average _mean bias b is subsequently subtracted from each measurement of the gyrometer during the pretreatment step 200, which makes it possible to minimize the influence of the bias and thus to improve the accuracy of the results obtained.

This estimate of the average _mean bias b preferably takes place at the beginning of each acquisition. It is also possible to renew this estimate during periods of immobility.

In FIGS. 2A to 2C, the details of the steps of the method according to the invention can be seen.

In step 200, the measurements delivered by the three sensors are preprocessed in three steps 210, 220 and 230. According to the context of use of the method and the desired accuracy, several alternative embodiments are possible.

The first step 210 is identical in all the embodiments. During this step 210, shown in FIG. 2B, the pretreatment of the measurement yβ _{, k} of the gyrometer takes place. As indicated above, this pretreatment of the measurement y0 _{, k} is obtained by subtracting the average bias b _mmen from the actual measurement, a preprocessed measurement of the gyrometer at the instant k designated by y _{G k} is obtained.

In the description, the acceleration is given in a multiple of GB (Earth magnetic field), and the magnetic field is given in a multiple of Ho for simplification purposes.

In a first embodiment, two tests for detecting the existence of accelerometric (step 220) and magnetometric (step 230) perturbations are advantageously carried out in parallel. During step 220, the pre-processing of the measurements delivered by the accelerometer y _Alk at time k takes place. This step 220 comprises a first substep 220.1 for detecting the existence or not of a disturbance, ie of an own acceleration a, and a second substep 220.2 for constructing a pretreated measurement of the acceleration y _{A k} on the basis of the orientation estimated at the previous instant k-1.

In the step 220.1, to detect the existence or not of an own acceleration a, the norm of the measure y ^ _k is compared to the norm of the gravitational field (as a reminder, one works in a multiple of GB), one makes a comparison with 1: If y * I ~ l <a A>

Advantageously, in the case of a positive test, the following test is added:

AI <β 'A The comparison of the norm of the eigenaccess estimated at time k-1, _kA , at βA, advantageously makes it possible to exclude particular cases for which the first test would not suffice. Indeed, it is considered that if at the moment k-1 the acceleration owns a high value, ie greater than βA, it is unlikely that at the moment k the acceleration is less than β _{A. A} and β _A are for example equal to 0.04 and 0.2 respectively. This second test therefore improves the accuracy of the estimation of the undisturbed measurement y _{Λ k} and thus the estimation of the orientation.

If the two tests above are positive, then we decide that the proper acceleration _k is zero at time k. In step 220.2, the estimated measurement is then equal to y ^ _λ - and is directly usable by the observer. _k = oy ₄ , _k = y _Λ , _k

Otherwise, during step 220.2, a new acceleration measurement is constructed using the estimated orientation q _kA , estimated at the previous instant. The undisturbed accelerometric measurement estimated at time k is then written using the measurement model: .

We can also deduce a value of the acceleration own _k at time k (step 220.3), which allows integration and double integration respectively to deduce the speed and position of the object. The proper acceleration is equal to: at _k = yu + £ * -i ® G _Q <S> q _k _ _ι , since y ₄ = y _AΛ + â _k

During step 230, similar to step 220, the preprocessing of the measurements delivered by the magnetometer YMM at time k takes place. This step 230 comprises a first substep 230.1 for detecting the existence or not of a magnetic disturbance d, and a second substep 230.2 of constructing a pretreated measurement of the magnetic field y _{M k} on the basis of the orientation estimated at the previous instant k-1.

In step 230.1, in order to detect the existence or not of a magnetic disturbance d, the norm of the measurement y _Mtk is _compared with the norm of the magnetic field (as a reminder, one works in a multiple of H ₀ ), on therefore makes a comparison with 1:

If Vi MJk -1 <a M '

Advantageously, in the case of a positive test of non-existence of a disturbance of the magnetic field, the following test is added: d * -i <β M

The comparison of the standard of the magnetic disturbance estimated at time k-1, d _k _ _λ , to PM. advantageously makes it possible to exclude particular cases for which the first test would not suffice. Indeed, it is considered that if at the time k-1 the magnetic disturbance has a high value, ie greater than PM, it is unlikely that at the moment k the magnetic disturbance is less than βy. a _M and β _M are for example equal to 0.04 and 0.2 respectively.

This second test therefore improves the accuracy of the estimation of the undisturbed measurement y _{M k} and thus the estimation of the orientation. If the two tests above are positive, then we decide that the magnetic disturbance d _k is zero at time k. In step 230.2, the estimated measurement is then equal to y ^^ and is directly usable by the observer. d _k = 0 y M, k ~ y M j

Otherwise, during step 230.2, a new magnetic field measurement is constructed using the estimated orientation q _kA , estimated at the previous instant.

The undisturbed magnetometric measurement estimated at time k is then written using the measurement model: > W =? _t -i ® #o ®? JM.

We can also deduce a value of the magnetic disturbance at time k (step 230.3), equal to: d _k = JV * -? * _, ® H ₀ ®? _t _, since _M j _t = d _k + J ^ A second embodiment is particulèrement application in cases where it is known that the estimation of the orientation can drift significantly. In these cases, it is advantageous to define time windows in which the tests for the existence of accelerometric and magnetometric disturbances are carried out. In this variant, for the accelerometric measurement test, the comparison provided for in step 220 is carried out on a window ending at the instant f0: if a proper acceleration is detected (norm of the accelerometric measurements different from the standard of Go to a ₄ on at least one of the measurements of the window [fc - T _A ; t _k ]) then the undisturbed accelerometric measurement is constructed thanks to the orientation estimated at the previous instant; otherwise, the undisturbed accelerometric measurement is equal to the measurement entering the preprocessing phase (sensor measurement). The value of the own acceleration is calculated from the sensor measurement. Typical values given by way of example only for a _A and TA are 0.2 g and 0.4 s.

The absence of a clean acceleration test is then written:

If L-P, \ "x _A , Vt _k e [ _tk - T ₄ ; tj

= 0 y AI - y,

Otherwise y _{4 k} = -q _k . _ι ® G ₀ ® g _k _ _ι

Advantageously, the own acceleration can be calculated systematically, even if thresholds are not exceeded by the same formula as in the first variant embodiment:

Then, or in parallel (this second option being advantageous since it saves time), the disturbance detection test is carried out of the magnetometric signal provided in step 230: if a magnetic disturbance is detected (standard of the magnetometric measurements different from the norm of H ₀ to a _M on at least one of the measurements of the window [t _k -

TM; t _k ]) then the undisturbed magnetometric measurement is constructed by the orientation estimated at the previous instant. Otherwise, the undisturbed magnetometric measurement is equal to the measurement entering the pre-processing phase (sensor measurement). The value of the magnetic disturbance is then calculated from the sensor measurement. Typical values given only as an example for a _M , and TM are respectively 0.1h and 0.5s.

The test of non-existence of a perturbation is then written:

If y _M ^ omk -T _M ; t _k ): d _k - = 0

5v k ~ y _M , u otherwise y _M , k ~ S ⁾ H ₀ ® <7 * -.

End if

Advantageously, the magnetic disturbance can be calculated systematically, even if thresholds are not exceeded by the same formula as in the first variant embodiment:

In these first two embodiments, steps 202 and 203 are advantageous to be performed in parallel.

On the other hand, a third embodiment makes it possible to improve the accuracy of the detection, when this is necessary, and that the device is provided with calculation and storage means that are sufficient to use trionometric functions. In this case, the advantage provided by the parallelism of the detection calculations is dispensed with and it is more advantageous to carry out the magnetic disturbance test after the presence test of a proper acceleration. At the output of this first step calculation 202, u _k = angle (-y _{Λ k} , y _uk ) is also calculated and the measured angle between the Go and HQ vectors is recorded as Uo. This parameter can be calculated during step 100 of initialization. Then the magnetic disturbance detection test is carried out as follows: if a magnetic disturbance is detected (standard of the magnetometric measurements different from the norm of H ₀ to or near _M or angle u _k different from M ₀ to a _u close to at least one of the measurements of the window [Î _H -T _M ; t _k ]), then the undisturbed magnetometric measurement is constructed by virtue of the orientation estimated at the previous instant. Otherwise, the undisturbed magnetometric measurement is equal to the measurement entering the pre-processing phase (sensor measurement). The value of the magnetic disturbance is then calculated from the sensor measurement. Advantageously, two different values of TM will be used depending on whether there is or no presence of an own acceleration (respectively T _M j _as t and Typical values given by way of example only for a _M , a _u , TMjast and TM_SIOW are respectively 0.1 h, 10 °, 0.5 s and 3 s.

The test for the existence of a perturbation is then written: If presence of a proper acceleration

TM ⁼ TM fast else

End if

SSii JyM _M J. _k \ - HH _r0 1>> α " _uM oOuU uM _k4 - uM ₀₀ > α _t for at least one value tk such that t _k e [t _k -τ _M ; h}: otherwise y M, k ~ y Mk

End if d _k -

In step 300, the preprocessed measurements y _AΛ , y _{M k} , y _{G k} are used by the observer. For example, an extended Kalman filter is used in its factored form.

We will now explain the calculation steps performed by the filter. We will assume that we are using the second modeling. But it is understood that the first model can be used in a similar way. Step 300 comprises the following steps: a) A priori estimation of the state vector, b) A priori estimation of the measurements, c) Calculation of the gain K _k of the Kalman filter and the innovation l _k , d) Correction of the state estimated a priori.

Steps a) to d) will be described in detail below. During step a), a priori estimation of the state vector at time k is performed from the a posteriori estimation of the state vector at time k-1. The estimation of the prior state vector is given by: with X ^ ₁ : a posteriori estimation of the state vector at time k-1,

% l: a priori estimation of the state vector at time k,

T _e : sampling period, During step b), using the measurement model (III), an estimation of the measurements is carried out using the state vector estimated in step a ):

91 = ^h ( ^χ D

In step c), the gain K _k is calculated and the innovation l _k , the innovation is obtained by subtracting the measures estimated a priori from the pre-processed measures.

We obtain :

During step d), the estimated state is corrected a priori with the gain and the innovation. This correction gives a posteriori estimate of the orientation q _k at time k.

Advantageously, during a subsequent step e), the estimated quaternion is normalized q _k = - ^ -, which avoids a drift at each stage of the calculation.

_9k

The method according to the present invention offers the advantage of providing the observer with measurements close to undisturbed measurements, consistent with the measurement model. The influence of disturbances on the estimation of the orientation is therefore greatly reduced.

It also makes it possible to continuously provide the observer with the information coming from each of the sensors even in the presence of disturbances, while maintaining a constant confidence in each measurement. Indeed, the joint use of the accelerometer, magnetometer and gyrometer measurements makes it possible to reduce the influence of measurement errors (measurement noise, remaining disturbances, remaining gyrometer bias) on the estimated orientation. The method according to the present invention makes it possible to estimate the orientation, but also the natural accelerations and the magnetic disturbances at each sampling step, whatever the movement achieved up to nine degrees of freedom. The implementation of this method is very simple since it relies on the use of elementary bricks: value tests, analytical calculations, extended Kalman filter.

Claims

1. Method for estimating the orientation of an object in space at time k using measurements of total acceleration (YA), magnetic field (YM) and rotation speed (ye) said object along three axes of space, characterized in that it comprises the steps: A- pretreatment of said measurements (V _A , YM, Y _G ) at a time k to detect the existence of a disturbance in said measurements, said perturbation belonging to a group comprising a specific acceleration of the object, a magnetic field adding to the Earth's magnetic field and a bias in the measurement of the rotational speed, and for estimating undisturbed measurements at the instant k, B- estimating the orientation at the instant k by an observer from the undisturbed measurements estimated {y _{A k} , y _MΛ , y _GΛ ) at time k from step A. The method of claim 1, wherein estimating the orientation of an object in space at time k uses only the measurements of the total acceleration (y _A ), magnetic field (YM) and the speed of rotation (Y _G) of said object along three spatial axes The method of claim 1, wherein step A comprises: A1- pretreatment of rotational speed (YG) measurements, A2- a detection of the existence or not of a disturbance at time k in said measurements of the total acceleration and magnetic field (y _A , YM), A3-in case of absence of disturbance at time k, the undisturbed measurement estimated at time k (y _{A k} , y _{M k} ) is equal to the measurement at time k, in case of disturbance, the undisturbed measure estimated at the moment k (y _{A k} , y _{M k} ) is calculated on the basis of the orientation estimated at the previous instant k-1. 4. Method according to claim 3, wherein step A1 consists in subtracting from the rotational speed measurements an average bias (b _average ) determined during a prior initialization step. 5. Method according to claim 4, wherein the average bias (b _average ) is obtained by immobilizing the means providing the rotational speed measurements for a given time and calculating the average of the rotational speed measurements values on each axis. . The method according to claim 3, wherein step A2 comprises: A2.1 - a comparison test of the standard of measurements of the total acceleration to that of the gravitational field (Go), if the absolute value of the difference between the standard of accelerometric measurements at time k and that of the gravitational field (Go) is less than a predetermined threshold (CIA), it is considered that the accelerometric perturbation is zero, otherwise it is considered that there is a disturbance, the disturbance being equal, on each axis, to the difference between the measurement of the total acceleration at time k and the undisturbed accelerometric measurement estimated at time k, A2.2 - a comparison test of the standard of measurements magnetic field to that of the Earth's magnetic field (Ho), if the absolute value of the difference between the standard of the magnetic field measurements and that of the earth's magnetic field is less than a predetermined threshold (CIM), then it is considered that the magnetic disturbance is equal, on each axis, to the difference between the magnetic field measurement at time k and the undisturbed magnetic field measurement estimated at time k. 7. A method according to claim 6, wherein in step A2.1 a further test is provided on the estimated disturbance ( _{k k} _ _x ) at time k-1: in the case where the absolute value of the difference between the standard of the measurements of the total acceleration and that of the gravitational field (Go) at time k is lower than the predetermined threshold (CA), it is checked whether the norm of the accelerometric perturbation estimated at time k-1 is below a predetermined threshold (β _A ), if this test is positive, it is considered that the accelerometric perturbation is effectively zero at time k, and / or is provided in step A2.2 an additional test on the magnetic disturbance estimated at time k-1 (^ _-1 ), in the case where the absolute value of the difference between the standard of the magnetic field measurements and that of the earth's magnetic field (H ₀ ) is less than the predetermined threshold (CM) , we check if the absolute value of the disturbance magnetic n estimated at time k-1 is less than a predetermined threshold (βivi), if this test is positive, it is considered that the magnetic disturbance is effectively zero at time k. 8. The method of claim 3, wherein at least one of the detections of step A2 is performed on a time window (T _A> T _M ) set by a user. The method according to claim 8, wherein the detection of the own acceleration is achieved by a processing of the form: If Bi, - T ₄ ; y A * - - G ₀ y, u = Φ G _o i Otherwise y AΛ = y Ai End if 10. The method of claim 8, wherein the detection of the magnetic disturbance is carried out by a treatment of the form: if not JV * ^~ y M, h End if d _k = y _Mk -q _{k ~ ι} ®H ₀ ®q _k ^ 11. The method of claim 9, wherein the angle u _k = angle (-y _Ak , y _Mk ) is further calculated at the output of the detection of the own acceleration and the magnetic disturbance is then detected. a treatment of the form: If presence of a clean acceleration else • M ~ I M_slow End if If JV * H ₁ > α _M or U. -U ₁ > α _u for at least one value t _k such that t _k e [t _k -T _M ; t J: if not > 'Mr M.k End if cl ≈ y M, I- -q _k ^ HH ₀ Θq _k ^ The method of one of claims 1 to 11, wherein the observer used in step B is an extended Kalman filter. 13. The method according to claim 12, wherein step B of estimating the orientation from the undisturbed measurements estimated at time k comprises: estimating the state vector a priori at time k (ij), from the state vector (% _kA ) estimated a posteriori at the instant k-1, the estimation of the measurements a priori at the instant k (j> ^~ ), from the estimate of the prior state vector at time k (£ j), calculation of the gain of the extended Kalman filter (K _k ) and the innovation (l / c) by calculating the difference between the undisturbed measurements estimated at the instant k and the measurements estimated a priori (y _k ^~ ), calculating the estimated orientation at time k (q _k ) by correcting the state vector estimated a priori at time k by the gain and innovation. The method of claim 13, wherein the state vector used in the extended Kalman filter contains the elements of angular velocity and orientation quatemion. The method of claim 14, wherein the state vector used in the extended Kalman filter contains only the elements of the orientation quatemion. 16. Central attitude comprising means capable of providing acceleration measurements (VA), magnetic field measuring means (VM), and means for measuring the rotational speed (VQ) along three axes of the space, said means being intended to be integral in motion of an object, and means for estimating an orientation at time k on the basis of measurements provided by said measuring means, said central being characterized in that said estimation means comprise: means for pretreatment of said acceleration (yA), magnetic field (yM), and rotation speed (yG) measurements, said pretreatment means being able to detect the existence of a disturbance in said measurements, said disturbance belonging to a group comprising an own acceleration of the object, a magnetic field adding to the earth's magnetic field and a bias in the measurement of the rotational speed and delivering estimated undisturbed accelerometric measurements ( ^yAΛ ), undisturbed magnetometric measurements estimated ( ^yux ), and the unbiased rotation speed ( ^{y <3Λ} ), means for estimating the orientation at an instant k by an observer from the accelerometric measurements no estimated undisturbed magnetometric measurements, undisturbed magnetometric measurements and unbiased rotational speed measurements provided by the pretreatment means. 17.. Attitude control unit according to claim 16, also comprising means for calculating a mean bias (b ") of the rotational speed measuring means during an initialization step of the central unit. 18. An attitude center according to claim 16, wherein the pretreatment means comprise means for detecting the existence of a proper acceleration in the acceleration measurements and means for detecting the existence of magnetic disturbances. in magnetic field measurements. 19. Central attitude according to claim 18, wherein the means for detecting the existence of a proper acceleration in the acceleration measurements and means for detecting the existence of magnetic disturbances in the magnetic field measurements. further include means for performing these detections in one or more time windows. 20. Central attitude according to claim 16, further comprising means for estimating the proper acceleration, the magnetic disturbance, and calculation of the speed and position of the object. 21. An attitude center according to claim 16, wherein the observer is an extended Kalman filter. 22. Central attitude according to claim 16, wherein the means capable of providing measurements of the total acceleration (y _A ), magnetic field measurements (VM), and measurements of the speed of rotation (yo). along three axes of space are MEMS sensors.