EP0733408B1 - Ultrasonic sensor and detection method using such a sensor - Google Patents

Description

The present invention relates to a method detection and / or recognition of objects in the air, in particular for a guidance system of a mobile robot.

It is known, for example in ultrasound, to use ultrasound in liquid media to "see" objects.

For this purpose, use is made of sensors constituted by transducer arrays.

Thanks to an appropriate phase shift between the different transducers of the matrix, we generate by interference a wave directional acoustics whose echo informs about a possible obstacle encountered by this wave in its direction of fire.

By performing a point-by-point scan of a field of observation to be analyzed, one can thus reconstruct an image and the reproduce on a screen.

We understand that the point-by-point scanning of a surface requires a considerable number of shots.

Liquid media are particularly suitable for use of this technique, due to the fact that the ultrasonic waves spread easily and quickly.

On the other hand, in the air, this scanning technique does not could not be implemented under satisfactory conditions, due to the low speed of sound movement which would impose too long scan times. However, such durations would prohibitive, especially for robotics applications.

It follows that the sensors made up of matrices of transducers known to date do not allow to recognize effectively objects in the air.

The present invention aims to provide a new method which solves in particular the problems recalled above.

The present invention relates to a method according to claim 1.

According to the invention, the term “set of frequencies” means a single frequency, a superposition of random frequencies forming a noise, or a superposition of fixed frequencies.

The sensor used in the method of the invention is suitable, because of its structure, to provide an ultrasonic wave with directivity variable.

Indeed, the directivity of the shooting is obtained by interference, by superimposition of waves emitted simultaneously by a variable number transducers, it being understood that the number and the position of the excited transducers determine the directivity of the resulting wave issued.

In addition, the direction of fire can be adjusted, in the manner known, by creating a phase shift between the transducers of the sensor.

According to the invention, the network can be planar, spherical, or arranged on a tablecloth of any other shape.

Being a flat network, an advantage resulting from such sensor is that it is easy and economical to manufacture.

A sensor arranged on a spherical cap could have the advantage of being more suited to an emission of acoustic waves directional.

Generally speaking, the choice of a particular form of sheet on which the transducers of the sensor are arranged can be done, depending on the type of acoustic wave that we want to generate from the sensor.

In a first embodiment of the invention, the network regular at the nodes of which the transducers are placed is a network with square mesh.

In another embodiment, the network is a network with triangular mesh.

The sensor can cover a field observation with a fairly low number of transducers.

Indeed, we know that to increase the directivity of a sensor, we can add transducers to it, so as to increase the overall dimensions of its matrix. The directivity of the sensor is then equivalent to that of a transducer with a diameter equal to the large dimension of the matrix.

Now, the dimensions of the matrix of the sensor can be increased by adding a number limited transducers.

In fact, in a preferred embodiment of the invention, the sensor consists of a plurality of alignments of transducers, alignments whose axes cross at the center of a common central transducer.

In this embodiment, the transducers can be arranged in a cross in the case of a square mesh network, or following a six-pointed star in the case of a mesh network triangular, the nodes of the network located outside the branches do not with no transducer.

The large dimension of the matrix, determining to fix the directivity of the sensor, is then equal to the length of the alignments.

As a result, the directivity of the sensor is equivalent to that of a sensor of such large diameter but which would include a transducer at each node of its matrix.

In addition, by using a smaller number of sensors, everything by placing them on intersecting axes of a regular network, in particular with triangular mesh, we reduces the energy of secondary waves emitted in directions different from the direction of fire, within volumes of spread called side lobes.

In accordance with the invention, the transducers making up the sensor may be of the type capable of both transmitting and receiving an acoustic wave.

The method according to the invention has the advantage of providing information on high level with each emission of an ultrasonic wave, which is advantageous considering the low speed of sound movement in the air.

In other words, if the use of ultrasonic waves to scanning point by point an observation field to analyze is not reasonably possible in the air for the reasons explained above, the method according to the invention nevertheless makes it possible to obtain quickly information on an object to recognize, minimizing the number of shots to be taken.

In particular, for the navigation of a mobile robot in a establishment, we know a priori that the robot will have to recognize walls, doors, angles and some special obstacles. For allow the robot to recognize these objects, we equip it with a sensor as defined above and an electronic device is programmed sensor so that it emits waves that propagate inside specific volumes that coincide with particular forms of objects to recognize.

For example, to recognize an open door, the sensor emits waves which propagate substantially in a vertical plane and scan the field of observations in a horizontal direction. So much as the ultrasonic wave bounces off the wall, the energy of the echo is important. On the other hand, as soon as the wave enters the framing of with the door open, the energy of the echo suddenly drops.

Thus, in its process of object recognition, each the robot sees a sudden drop in echo energy during a horizontal scan of a wave emitted in a thin volume substantially vertical, it deduces the possible presence of a door in his field of observations.

This hypothesis can then be confirmed by issuing other ultrasonic waves.

According to the invention, to emit an ultrasonic wave which spreads inside a thin volume enveloping a plane of firing, we excite a determined number of contiguous transducers of the same alignment of sensor transducers, the axis of this alignment extending substantially perpendicular to said firing plane.

Similarly, to emit an ultrasonic wave propagating to inside a volume of revolution centered around a firing axis, all the transducers located inside a disc are excited, determined radius, centered on the central transducer of the sensor.

Thanks to the good directivity of the sensor, it is possible to obtain a relevant information on the obstacle (s) that are inside the propagation volume of the ultrasonic wave.

According to the invention, to collect the echo of the wave you can use the transducers of the sensor that emitted this ultrasonic wave.

In particular, in the case of a sensor whose transducers are aligned along intersecting axes in the center of a transducer common central, the transducers located at the ends of the branches of the sensor to receive the echo of the wave ultrasonic.

By choosing two of the transducers appropriately which allow us to collect the echo, we can thus perform a directional measurement of the received wave, in particular by measuring the phase shift between these transducers.

• la figure 1 représente schématiquement, vu de face, un capteur utilisable selon un premier mode de mise eu oeuvre de l'invention,
• la figure 2 représente schématiquement, vu de face, un capteur utilisable selon un deuxième mode de mise en oeuvre de l'invention,
• la figure 3 représente schématiquement, vu de face, un capteur utilisable selon un troisième mode de mise en oeuvre de l'invention,
• la figure 4 est une vue analogue à la figure 3, représentant le capteur en cours d'émission,
• la figure 5 est une vue en coupe selon V-V de la figure 4, représentant la directivité angulaire des ondes ultrasonores émises par le capteur,
• la figure 6 est une vue en coupe selon VI-VI de la figure 4, représentant la directivité angulaire des ondes ultrasonores émises par le capteur,
• la figure 7 est une vue analogue à la figure 3, représentant le capteur en cours d'émission,
• la figure 8 est une vue en coupe selon VIII-VIII de la figure 7, représentant la directivité angulaire des ondes ultrasonores émises par le capteur,
• la figure 9 est une vue analogue à la figure 3, représentant le capteur en cours d'émission,
• la figure 10 est une vue en coupe selon X-X de la figure 9, représentant la directivité angulaire des ondes ultrasonores émises par le capteur,
• la figure 11 représente schématiquement, vu de face, un capteur utilisable selon un quatrième mode de mise en oeuvre de l'invention.

In order to better understand the invention, we will now describe embodiments given by way of non-limiting examples, with reference to the accompanying drawing in which:

• FIG. 1 schematically represents, seen from the front, a sensor which can be used according to a first embodiment of the invention,
• FIG. 2 schematically represents, seen from the front, a sensor which can be used according to a second embodiment of the invention,
• FIG. 3 schematically represents, seen from the front, a sensor usable according to a third embodiment of the invention,
• FIG. 4 is a view similar to FIG. 3, showing the sensor during transmission,
• FIG. 5 is a sectional view along VV of FIG. 4, representing the angular directivity of the ultrasonic waves emitted by the sensor,
• FIG. 6 is a sectional view along VI-VI of FIG. 4, representing the angular directivity of the ultrasonic waves emitted by the sensor,
• FIG. 7 is a view similar to FIG. 3, showing the sensor during transmission,
• FIG. 8 is a sectional view along VIII-VIII of FIG. 7, representing the angular directivity of the ultrasonic waves emitted by the sensor,
• FIG. 9 is a view similar to FIG. 3, showing the sensor during transmission,
• FIG. 10 is a sectional view along XX of FIG. 9, representing the angular directivity of the ultrasonic waves emitted by the sensor,
• FIG. 11 schematically represents, seen from the front, a sensor which can be used according to a fourth embodiment of the invention.

As we can see in the drawing, in the sensor of figure 2, the transducers are located at the nodes of a planar mesh network triangular.

As shown in Figure 1, the distance between the centers of two contiguous transducers 1 is at most equal to about the wavelength in air of the ultrasonic wave emitted.

In the embodiment described, the transducers are designed to operate at a frequency of approximately 40 KHz, which corresponds to a wavelength in air of around 8.5 mm.

Because they each have a diameter d close to this dimension, the transducers are contiguous.

The sensor of FIG. 1 has a directivity corresponding to an opening angle of approximately 20 ° on average, owing to the fact that its overall diameter D ₁ is approximately equal to three times that of a transducer.

The sensor of Figure 2 has an increased directivity, an average opening angle of about 8 °, because its overall diameter D ₂ is about 7 cm.

FIG. 3 shows a sensor whose directivity is substantially the same as that of FIG. 2, owing to the fact that its overall diameter D ₃ is equal to that of D ₂ of the sensor of FIG. 2, but which has the advantage of having a much smaller number of transducers.

Indeed, the sensor of FIG. 2 comprises 37 transducers, while that of Figure 3 has only 19.

In this sensor, the transducers are aligned on three axes X _1, X _2, X ₃ forming in pairs an angle of 60 ° and intersecting at the center of a transducer 1 has central sensor.

This reduction in the number of transducers is possible thanks to the arrangement according to a triangular mesh network of transducers, which are thus arranged in a star.

This star arrangement also gives the sensor a ability to emit acoustic waves with side lobes weak, compared to those that would be obtained with a square mesh.

This is explained by the fact that the star arrangement of the transducers naturally provides the equivalent of a weighting of the signal from each transducer.

However, we know that the directivity of the signal emitted by several aligned sensors is substantially the Fourier transform of the number sensors. In the case where these sensors are weighted, the transform of Fourrier therefore provides an enlarged main lobe and lobes flattened secondaries.

In use, the transducer of FIG. 3 can be supplied so as to emit an ultrasonic wave from three contiguous transducers located on the same axis X ₁ , identified by hatching in FIG. 4. In such a case, an ultrasonic wave propagating inside a thin volume is emitted, as visible in FIGS. 5 and 6 which represent the acoustic energy of the waves emitted as a function of the angle of emission.

In plane VI, the directivity is substantially equal to the directivity of a single transducer, which results in a extended envelope of ultrasonic wave energy as a function of the emission angle, the mean opening angle of the wave being approximately equal to 100 °.

On the other hand, in plane V, the interference between the waves ultrasound emitted by the three transducers result in a resulting wave with finer directivity. As we can see in Figure 5, the resulting wave has a main lobe and smaller secondary lobes, which gives an angle average opening of about 20 °.

By varying the phase shifts between the three excited transducers of the sensor, it is possible to modify the orientation of the plane 6 so as to scan a field of observation along the axis X ₁ , as indicated on the double arrow in FIG. 5.

If we increase the number of excited transducers, as we seen in figure 7, the resulting wave presents, in a plane perpendicular to the axis of alignment of the transducers concerned, an unchanged directivity substantially identical to that shown in figure 6.

On the other hand, the envelope of the energy of the ultrasonic waves is thinned in plane 8, as shown in Figure 8, where we see that the average opening angle is around 8 °.

If we excite the transducers located inside a disc centered on the central transducer 1 has, as shown in Figure 9, is obtained by interference, a resultant wave which is propagated inside a volume of revolution shown in axial section in Figure 10. The average opening angle of the wave is 20 °.

By an appropriate phase shift between the different transducers, we can adjust the directivity of this wave, i.e. the axis of the volume of revolution inside which the ultrasonic wave is spreads.

In general, it emerges from the tests carried out by the inventors that we get good results if we use small diameter transducers, which widens the field interference and precisely vary the direction of fire to facilitate the angular scans of the observation field, the sensor nevertheless having a large overall diameter.

This arrangement is obtained in particular by the arrangement in star shown in figure 3.

To receive the echo of the wave emitted by the sensor, you can use the transducers located at the ends of the alignment of sensors.

In this way, taking into account the delay between reception echo by different sensors, we can easily determine the angle of the echo and thus locate an object.

FIG. 4 shows a sensor which can be used according to a fourth embodiment of the invention, the transducers 1 of which are distributed at the nodes of a square mesh network, while being aligned on axes X ₁ and X ₂ perpendicular, intersecting at the center of a common central transducer 1 a .

Such a sensor is particularly suitable for the emission of waves ultrasound propagating inside thin enveloping volumes vertical and horizontal shots.

It is understood that the embodiments which come to be described are in no way limiting and that they will be able to receive any desirable modification without leaving for that of the scope of the invention.

Claims (9)

1. Method for detecting and/or recognizing objects in the air, especially for guiding a mobile robot, using a sensor comprising a plurality of circular transducers (1) provided to operate at a same given set of frequencies and placed at the nodes of a regular network, the distance separating the centres of two adjacent transducers of the network being, at most, about that of the wavelength in air of the waves emitted by the transducers, characterized in that the shape of an object to be recognized is determined a priori, in that an ultrasound wave, which is propagated inside a volume having geometric characteristics in common with the said shape determined a priori, is emitted from the sensor, in that the echo of the emitted ultrasound wave is picked up, and in that the energy of this echo is analysed in order to determine whether the geometric characteristics determined a priori have been encountered in the object to be recognized.
2. Method according to Claim 1, characterized in that, having determined a priori the shape of the object to be recognized, the said ultrasound wave is emitted, scanning the field of observation of the sensor.
3. Method according to either of Claims 1 and 2, characterized in that, in order to emit an ultrasound wave which is propagated inside a thin volume surrounding a firing plane, a predetermined number of adjoining transducers of the same alignment of sensor transducers are excited, the axis (X₁) of this alignment lying substantially perpendicular to the said firing plane.
4. Method according to either of Claims 1 and 2, characterized in that, in order to emit an ultrasound wave which is propagated along a volume of revolution centred about a firing axis, all the transducers located inside a disc, of predetermined radius and centred on the central transducer (1a) of the sensor, are excited.
5. Method according to any one of Claims 1 to 4, characterized in that, in order to pick up the echo of the ultrasound wave emitted, the sensor transducers are used.
6. Method according to Claim 5, using a sensor made up of a plurality of transducer (1) alignments, alignments whose axes (X₁, X₂, X₃) intersect at the centre of a common central transducer (1a), characterized in that the transducers located at the ends of the transducer alignments are used to pick up the echo of the emitted ultrasound wave.
7. Method according to any one of Claims 1 to 6, characterized in that a sensor is used whose transducer network is flat.
8. Method according to any one of Claims 1 to 7, characterized in that a sensor is used whose transducer network is a network with a triangular mesh.
9. Method according to any one of Claims 1 to 7, characterized in that a sensor is used whose transducer network is a network with a square mesh.

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