Data processing: measuring – calibrating – or testing – Measurement system – Orientation or position
Reexamination Certificate
2002-10-11
2004-06-22
Barlow, John (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system
Orientation or position
C702S152000, C702S158000, C324S207170, C324S207120, C600S437000, C342S463000
Reexamination Certificate
active
06754609
ABSTRACT:
The domain of the invention is the measurement of the position and the orientation of a mobile body, which moves in translation and in rotation with respect of a fixed body or structure.
In particular, the invention relates to the determination of the posture of the helmet of a pilot of military aircraft, in which the angular position of a target is determined by aiming, through a system comprising the pilot's helmet visualisation display unit.
The operation of such a system is recalled briefly below: via an ancillary collimator device, the pilot sees, through his semi-reflecting visor secured to the helmet, on the viewing axis, the image of a reticle projected to infinity superimposed with the outside scene. When he wishes to designate a target, the pilot makes this reticle coincide with the target and signals that coincidence is achieved, by means for example of a push-button control provided for this purpose.
Provided that the exact angular position of the helmet is referenced at the moment that coincidence is signaled, it is possible to determine, with respect to the aircraft, the direction of aim and to designate the objective to a weapon system, or to point an optical system for example in this direction.
A device for measuring orientation and position of the helmet of the pilot in a reference frame tied to the aircraft can consist of an orientation and position sensor made up of three orthogonal electromagnetic coils and placed on the helmet, and of an emitter, situated at a fixed point of the cabin, and made up of three other electromagnetic coils.
The process then consists in passing an electric current through each coil of the emitter (forming a substantially orthogonal fixed trihedron). These currents engender three magnetic fields which are sensed by the coils of the sensor (forming a substantially orthogonal moving trihedron tied to the helmet). The analysis of these magnetic fields makes it possible to determine the position and the orientation of the moving trihedron with respect to the fixed trihedron.
In this domain of application in particular, it is vital to obtain an accurate measurement of the magnetic fields emitted by the fixed emitter, and detected by the sensor tied to the helmet, so as to accurately designate, to a weapon system, the objective selected by the pilot.
The existence has long been known of magnetic phenomena liable to arise in conducting bodies and in bodies of ferromagnetic type. It is thus known that in the presence of an external magnetic field, induced currents, of the type of eddy currents, appear in the conducting elements. It is also known that magnetic inductions arise in permeable materials, such as steels or materials of magnetic susceptibility of iron
ickel type.
When one wishes to evaluate a magnetic field in an environment exhibiting conductors and/or bodies of ferromagnetic type, it is therefore very difficult to obtain an accurate measurement, by reason of the considerable field disturbances caused by these induced fields.
Now, the cockpits of fighter planes or combat helicopters comprise numerous conducting and/or ferromagnetic elements which, in particular in the presence of magnetic fields emitted by the three coils of the emitter, create strong magnetic disturbances, and falsify the measurement.
It is therefore very difficult, in these disturbed environments, to obtain operation of the helmet viewfinder which satisfies the specifications of static accuracy throughout the volume swept by the orientation and position sensor.
It has therefore been envisaged to compensate for the effects of the magnetic disturbances in the measurements, by attempting to obtain a modeling of the magnetic disturbances, according to several complex methods of processing, on the basis of a very accurate map of the actual environment of the system.
A drawback of these prior art techniques, which implement a modeling of the disturbing fields, is that the products, and in particular the helmet viewfinders obtained according to these techniques, are very difficult to implement. Specifically, these products must be able to adapt to carriers of any type. Moreover, the sensitivity of the errors with respect to the stability of the magnetic environment remains very considerable.
A first method of compensating for the magnetic disturbances consists in producing a magnetic map of the environment of the position and orientation sensor, so as to construct a reference table encompassing the values of the disturbing fields (otherwise known as a “lookup table”) or a reference model. This table is then used to correct the sensor's position and orientation measurements, during operation. It is therefore necessary to immobilize the apparatus on the ground for several days so as to be able to compile this map, for each pilot station, this being unacceptable in numerous situations, and in particular in the case discussed hereinabove of military equipment.
A drawback of this prior art technique is therefore that it is tedious, and very lengthy to implement.
Another drawback of this prior art technique is that it is not suited to possible variations in the environment of the sensor. In particular, the reference table (“lookup table”), or the reference model, loses some of its value should an object be moving within the cockpit (such as the movement of the pilot's adjustable seat, for example) or should a piece of equipment be added subsequently to the fighter plane or the compact helicopter, since the accuracy rating then decreases.
Several other techniques have been developed for compensating for the magnetic disturbances created by the eddy currents induced in conductors.
A first method, described in particular in patent document U.S. Pat. No. 4,829,250, consists in emitting magnetic fields alternating at harmonic frequencies, and in determining by extrapolation to low frequency, where the disturbances of the eddy effects are negligible, the value of the orientation from the orientations determined previously for each of the harmonic frequencies, as if there were just a single frequency for each estimation. Specifically, the magnetic disturbances due to eddy currents depend on the frequency of the magnetic field, and on the conductivity of the material in which they arise. It has been noted that these disturbances decrease with frequency, vanishing at zero frequency.
A drawback of this prior art technique is that it is not suited to the presence of ferromagnetic materials in the environment of the position and orientation sensor. In particular, it does not allow efficient filtering of the magnetic disturbances due to ferromagnetic materials, which may be considerable at low frequency.
A second method for compensating for the disturbances due to eddy currents, described in particular in patent documents U.S. Pat. Nos. 4,849,692 and 4,945,305, consists in emitting continuous magnetic fields (pulses) at the level of the emitter. These may have a permanent component, and the technique then consists in waiting for the eddy currents to attenuate over time in order to perform the measurement at the level of the sensor. The pulse must be sufficiently long over time for the eddy currents to have time to attenuate before the measurement is performed, or for the estimation of the steady state value to be accurate.
A drawback of this prior art technique is that the measurements are performed at rates which do not allow the steady state to be reached and the accuracy is insufficient. If one were to wait for the steady state, this would excessively limit the dynamic measurement range.
Another drawback of this prior art technique is that it is not suited to the presence, in the environment of the position and orientation sensor, of bodies of ferromagnetic type. Specifically, the disturbances of ferromagnetic origin depend on the magnetic permeability of the material and on the frequency of the excitation field, so that the magnetic disturbances decrease as the frequency increases, according to an inverse behavior to that of the eddy currents.
Yet another drawbac
Barlow John
Bhat Aditya S
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Thales Avionics S.A.
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