EP0733408A1 - Ultrasonic sensor and detection method using such a sensor - Google Patents

Abstract

The ultrasonic sensor comprises several transducers (1) operating at the same frequency, about 40KHz. They are arranged at the nodes of a regular network. The transducers are circular. The distance separating the centre of two adjacent transducers in the network is, at most, the order of the wavelength in air of the waves emitted by the transducers, namely about 8. 5 mm. The sensor is directional through an angle of about 20 deg. when its overall diameter (D1) is about three times that of a single transducer.

Description

The present invention relates to an ultrasonic sensor for a system for detecting and / or recognizing objects in the air, in particular for a system for guiding a mobile robot, and to detection methods using such a sensor.

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 arrays of transducers.

Thanks to an appropriate phase shift between the different transducers of the matrix, a directional acoustic wave is generated by interference, the echo of which indicates a possible obstacle encountered by this wave in its firing direction.

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

It is understood that the point-by-point scanning of a surface requires a considerable number of shots.

Liquid media lend themselves particularly well to the implementation of this technique, because the ultrasonic waves propagate there easily and quickly.

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

It follows that the sensors made up of matrixes of transducers known to date do not allow effective recognition of objects in the air.

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

The subject of the present invention is an ultrasonic sensor for a system for detecting and / or recognizing objects in the air, in particular for a system for guiding a mobile robot, comprising a plurality of transducers designed to operate at the same together of given frequencies, arranged at the nodes of a regular network, characterized in that the transducers are circular, and that the distance which separates the centers of two adjacent transducers from the network is at most of the order of the wavelength in the air waves emitted by the transducers.

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 according to the invention is suitable, by virtue of its structure, for providing an ultrasonic wave with variable directivity.

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

In addition, the direction of fire can be adjusted, in the known manner, 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 sheet of any other shape.

As regards a flat network, an advantage resulting from such a sensor lies in the fact that it is easy and economical to manufacture.

A sensor arranged on a spherical cap could have the advantage of being more suitable for the emission of directional acoustic waves.

In general, the choice of a particular form of sheet on which the transducers of the sensor according to the invention are arranged can be made, depending on the type of acoustic wave that it is desired to generate from the sensor.

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

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

The sensor according to the invention can cover an observation field 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.

However, in accordance with the invention, the dimensions of the sensor matrix can be increased by adding a limited number of transducers.

In fact, in a preferred embodiment of the invention, the sensor is constituted by a plurality of alignments of transducers, alignments whose axes intersect at the center of a common central transducer.

In this embodiment, the transducers can be arranged crosswise in the case of a square mesh network, or along a six-pointed star in the case of a triangular mesh network, the nodes of the network located outside the branches not having a transducer.

The large dimension of the matrix, decisive for fixing the directivity of the sensor, is then equal to the length of the alignments.

It follows that 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, while having them, according to this embodiment of the invention, on intersecting axes of a regular network, in particular with triangular mesh, the wave energy is reduced secondary emitted in directions different from the direction of fire, within propagation volumes called secondary lobes.

According to the invention, the transducers making up the sensor can be of the type capable of both transmitting and receiving an acoustic wave.

The present invention also relates to a method for detecting and / or recognizing objects in the air, in particular for guiding a mobile robot, using a sensor as described above.

This process is characterized by the fact that a priori the shape of an object to be recognized is determined, which is emitted from the sensor an ultrasonic wave which propagates inside a volume having common geometrical characteristics with said a priori determined shape, which one collects the echo of the ultrasonic wave emitted, and which one analyzes the energy of this echo to determine if the geometric characteristics determined a priori have been encountered in the object to be recognized.

This method has the advantage of providing high level information on each emission of an ultrasonic wave, which is advantageous given the low speed of movement of sound in air.

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

In particular, when navigating a mobile robot in an establishment, we know a priori that the robot will have to recognize walls, doors, angles and some specific obstacles. To enable the robot to recognize these objects, it is fitted with a sensor as defined above and an electronic device for controlling the sensor is programmed so that the latter emits waves which propagate to the 'interior of specific volumes which coincide with particular forms of the objects to be recognized.

For example, to recognize an open door, the sensor emits waves which propagate substantially in a vertical plane and scans the field of observations in a horizontal direction. As long as the ultrasonic wave bounces off the wall, the energy of the echo is important. However, as soon as the wave enters the frame of the open door, the energy of the echo drops suddenly.

Thus, in its process of recognizing objects, each time the robot notices a sudden drop in the energy of the echo during a horizontal scanning of a wave emitted in a thin, substantially vertical volume, it deduces the possible presence of a door in its field of observations.

This hypothesis can then be confirmed by the emission of other ultrasonic waves.

In accordance with the invention, to emit an ultrasonic wave which propagates inside a thin volume enveloping a firing plane, a determined number of contiguous transducers is excited with the same alignment of transducers of the sensor, the axis of this alignment extending substantially perpendicular to said firing plane.

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

Thanks to the good directivity of the sensor, it is possible to obtain relevant information on the obstacle or obstacles which are inside the propagation volume of the ultrasonic wave.

According to the invention, to collect the echo of the ultrasonic wave, it is possible to use the transducers of the sensor which emitted this ultrasonic wave.

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

By suitably choosing those of the transducers of the sensor which make it possible to collect the echo, it is thus possible to carry out 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 selon un premier mode de réalisation de l'invention,
• la figure 2 représente schématiquement, vu de face, un capteur selon un deuxième mode de réalisation de l'invention,
• la figure 3 représente schématiquement, vu de face, un capteur selon un troisième mode de réalisation 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 selon un quatrième mode de réalisation 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 according to a first embodiment of the invention,
• FIG. 2 schematically represents, seen from the front, a sensor according to a second embodiment of the invention,
• FIG. 3 schematically represents, seen from the front, a sensor 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 according to a fourth embodiment of the invention.

As can be seen in the drawing, in the sensor of FIG. 2, the transducers are located at the nodes of a planar triangular mesh network.

As shown in FIG. 1, the distance which separates the centers of two contiguous transducers 1 is at most equal to approximately the wavelength in the 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 approximately 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 D ₂ of the sensor of FIG. 2, but which has the advantage of having a much smaller number of transducers.

In fact, the sensor in Figure 2 has 37 transducers, while the sensor in Figure 3 has only 19.

In this sensor, the transducers are aligned on three axes X ₁ , X ₂ , X ₃ forming two by two an angle of 60 ° and intersecting the center of the sensor with a transducer.

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

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

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

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

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 an extended envelope of the energy of the ultrasonic wave as a function of the angle of emission, the angle d 'average opening of the wave being approximately equal to 100 °.

On the other hand, in the V plane, the interference between the ultrasonic waves emitted by the three transducers results in a resulting wave whose directivity is finer. As can be seen in FIG. 5, the resulting wave comprises a main lobe and secondary lobes of smaller extent, which gives an average opening angle of approximately 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 the number of excited transducers is increased, as seen in FIG. 7, the resulting wave has, 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 the plane 8, as shown in FIG. 8, where it can be seen 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, it is possible to adjust the directivity of this wave, that is to say the axis of the volume of revolution inside which the ultrasonic wave propagates.

In general, it appears from the tests carried out by the inventors that good results are obtained if small diameter transducers are used, which makes it possible to widen the interference field and to vary it in a way specifies the direction of the shot 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 star arrangement shown in FIG. 3.

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

In this way, given the delay between reception of the echo by different sensors, one can easily determine the angle of the echo and thus locate an object.

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

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

It is understood that the embodiments which have just been described have no limiting character and that they can receive any desirable modification without departing from the scope of the invention.

Claims (11)

Ultrasonic sensor for a system for detecting and / or recognizing objects in the air, in particular for a system for guiding a mobile robot, comprising a plurality of transducers designed to operate at the same given set of frequencies, arranged at nodes of a regular network, characterized in that the transducers (1) are circular and that the distance which separates the centers of two adjacent transducers from the network is at most of the order of the wavelength in air waves emitted by the transducers. Sensor according to claim 1, characterized in that the network is planar. Sensor according to either of Claims 1 and 2, characterized in that the regular network at the nodes of which the transducers (1) are placed is a triangular mesh network. Sensor according to either of Claims 1 and 2, characterized in that the regular network at the nodes of which the transducers (1) are placed is a square mesh network. Sensor according to any one of Claims 1 to 4, characterized in that it is constituted by a plurality of alignments of transducers (1), alignment whose axes (X ₁ , X ₂ , X ₃ ) intersect at the center of a common central transducer (1 a). Method for detecting and / or recognizing objects in the air, in particular for guiding a mobile robot, using a sensor according to any one of claims 1 to 5, characterized in that the shape of an object to be recognized is determined a priori, that an ultrasonic wave is emitted from the sensor which propagates inside a volume having common geometric characteristics with said a priori determined shape, that the echo of the emitted ultrasonic wave is collected, and that the energy of this echo is analyzed to determine whether the geometric characteristics determined a priori have been encountered in the object to be recognized. Method according to claim 6, characterized in that having determined a priori the shape of the object to be recognized, said ultrasonic wave is emitted by scanning the field of observation of the sensor. Method according to either of Claims 6 and 7, characterized in that, in order to emit an ultrasonic wave which propagates inside a thin volume enveloping a firing plane, a predetermined number of contiguous transducers is excited. 'A single alignment of transducers of the sensor, the axis (X ₁ ) of this alignment extending substantially perpendicular to said firing plane. Method according to either of Claims 6 and 7, characterized in that, in order to emit an ultrasonic wave propagating according to a volume of revolution centered around a firing axis, all the transducers located inside of are excited. a disc, of determined radius, centered on the central transducer (1 a ) of the sensor. Method according to any one of Claims 6 to 9, characterized in that, to collect the echo of the ultrasonic wave emitted, transducers of the sensor are used. Method according to claim 10, using a sensor according to claim 5, characterized in that the transducers situated at the ends of the alignments of transducers are used to collect the echo of the ultrasonic wave emitted.

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