Method and acoustic device for measuring surface movements
A method for detecting movements of a plurality of points (P) of a surface ( 21 ), comprising a measuring step during which an incident ultrasonic wave is emitted into the air towards the surface and an ultrasonic wave reflected into the air by the surface ( 21 ) is detected. During the measuring step, each measuring point is illuminated by the incident ultrasonic wave at a multiplicity of angles of incidence, and the reflected ultrasonic wave is detected by a network of receiving transducers ( 3 ) comprising a plurality of ultrasonic receiving transducers ( 3 a ). The movements of the surface are determined at a measuring point by determining a delay and/or a phase shift between two beam-forming signals for said measuring point.
1. A method for detecting movements of a surface ( 21 ) reflecting ultrasonic waves, comprising several successive measuring steps during each of which at least one incident ultrasonic wave is emitted into the air towards the surface ( 21 ) using an ultrasonic wave emitting device ( 2 ) and reflected signals representative of at least one ultrasonic wave reflected in the air by said surface ( 21 ) from said at least one incident ultrasonic wave are detected, characterized in that, during each measuring step:
the movements of a plurality of measuring points (P) belonging at least to said surface are measured by illuminating each measuring point (P) with said at least one incident ultrasonic wave at a multiplicity of angles of incidence,
the reflected signals are detected using a network of receiving transducers ( 3 ) comprising a plurality of ultrasonic receiving transducers ( 3 a ) and a beam-forming signal is determined for each measuring point (P), by at least beam-forming in reception from said reflected signals,
and in that it further comprises at least one movement determination step during which said movements of the surface ( 21 ) at the considered measuring point (P) are determined by determining at least one delay or phase shift between two beam-forming signals for this measuring point (P).
2. The method according to claim 1 , wherein during each measuring step, each measuring point (P) of the surface ( 21 ) is illuminated with said at least one incident wave at angles of incidence extending over a range of angles of incidence of at least 20 degrees.
3. The method according to claim 1 , wherein, during each measuring step, the movements are measured at substantially any point of the surface ( 21 ) over an area greater than 10 cm 2 .
4. The method according to claim 1 , wherein the ultrasonic wave emitting device ( 2 ) and the network of receiving transducers ( 3 ) are two-dimensional.
5. The method according to claim 4 , wherein the network of emitting transducers ( 2 ) has an aperture (OE Y , OE Z , OE′ Y , OE′ Z ) at least equal to an aperture (OR Y , OR Z ) of the ultrasonic wave receiving device ( 3 ) in two substantially perpendicular directions.
6. The method according to claim 5 , wherein the aperture (OE Y , OE Z , OE′ Y , OE′ Z ) of the network of emitting transducers ( 2 ) is equal to at least three times the aperture (OR Y , OR Z ) of the ultrasonic wave receiving device ( 3 ), in at least one of the directions.
7. The method according to claim 4 , wherein the aperture (OE Y , OE Z , OE′ Y , OE′ Z ) of the ultrasonic wave emitting device ( 2 ) is at least equal to 20 cm in each direction.
8. The method according to claim 1 , wherein the ultrasonic wave emitting device ( 2 ) comprises a network of emitting transducers ( 2 ) comprising a plurality of ultrasonic emitting transducers ( 2 a ).
9. The method according to claim 8 , wherein the ultrasonic emitting transducers ( 2 a ) are divided into several groups and during said measuring step, a same signal is simultaneously emitted by the ultrasonic emitting transducers ( 2 a ) belonging to a same group.
10. The method according to claim 1 , wherein the ultrasonic wave emitting device ( 2 ) comprises at least one ultrasonic emitting transducer ( 2 a ) so arranged as to emit into a mixing cavity ( 4 ) adapted to cause multiple reflections of said at least one incident ultrasonic wave before sending it towards the surface ( 21 ).
11. The method according to claim 1 , wherein the ultrasonic waves have a frequency of less than 100 kHz.
12. The method according to claim 1 , wherein the ultrasonic waves are emitted at a rate above 500 shots per second.
13. The method according to claim 1 , wherein, during each step of measuring index k, a beam-forming signal S k in reception is calculated at said point (P), at least at different points (P) of the surface ( 21 ),
and in each movement determination step, the movement of each point (P) of the surface ( 21 ) is determined by determining a delay or phase shift between the beam-forming signals S k in reception at said point (P), for two different k values.
14. The method according to claim 13 , wherein during each step of measuring index k, the ultrasonic wave emitting device ( 2 ) emits a non focused incident ultrasonic wave towards the surface ( 21 ).
15. The method according to claim 13 , wherein, during each step of measuring index k, the ultrasonic wave emitting device ( 2 ) successively emits focused incident ultrasonic waves towards the different points (P) of the surface ( 21 ) and the beam-forming signal S k in reception at said point (P) is determined from the reflected signals corresponding to the focused incident ultrasonic wave at said point (P).
16. The method according to claim 13 , wherein the beam-forming signal S k in reception is determined by the following formula:
S
k
(
t
)
=
∑
j
=
1
N
r
j
(
t
-
d
j
C
)
where:
r j is the signal detected by the ultrasonic receiving transducer ( 3 a ) of index j,
t is time,
d j is a distance between the point P and the ultrasonic receiving transducer ( 3 a ) of index j,
c is the speed of the ultrasonic wave in the air.
17. The method according to claim 1 , wherein the ultrasonic wave emitting device ( 2 ) comprises a network of emitting transducers ( 2 ) comprising a plurality of ultrasonic emitting transducers ( 2 a ),
during each k-index measuring step, respective impulse responses are measured between each ultrasonic emitting transducer ( 2 a ) and each ultrasonic receiving transducer ( 3 a ), and then at least at different points (P) of the surface ( 21 ) a beam-forming signal S′ k in emission and in reception is calculated at said point (P),
and during each movement determination step, a movement of each point (P) of the surface ( 21 ) is determined by determining a delay or phase shift between the beam-forming signals S′ k in emission and in reception at said point (P), for two different k values.
18. The method according to claim 17 , wherein each beam-forming signal in emission and in reception at said point (P) is determined by the following formula:
S
k
′
(
t
)
=
∑
j
=
1
N
∑
i
=
1
M
h
i
j
k
(
t
-
d
i
j
c
)
,
where:
i is an index between 1 and M referring to an ultrasonic emitting transducer ( 2 a ),
j is an index between 1 and N referring to an ultrasonic receiving transducer ( 3 a ),
h ijk is the impulse response between the ultrasonic emitting transducer ( 2 a ) of index i and the ultrasonic receiving transducer ( 3 a ) of index j,
t is time,
d ij is a distance travelled by an ultrasonic wave from the ultrasonic emitting transducer ( 2 a ) of index i to the ultrasonic receiving transducer ( 3 a ) of index j by reflecting at the considered point (P) of the surface ( 21 ),
c is the speed of the ultrasonic waves in the air.
19. The method according to claim 1 , wherein, during at least some measuring steps, a beam-forming signal is determined for each measuring point (P) of a predetermined observation area and the measuring points (P) belonging to the surface ( 21 ) are determined as those which maximize the beam-forming signal.
20. The method according to claim 1 , wherein, during the movement determination step, a delay dt between beam-forming signals corresponding to two measuring steps at the same measuring point (P) is determined and:
a travel δ at the measuring point (P) as being proportional to dt·c
and/or a speed at the measuring point (P) as being proportional to dt·c/Δt,
where c is the speed of the ultrasonic waves in the air and Δt is a time interval between said two measuring steps.
21. The method according to claim 1 , wherein, during the movement determination step, a phase shift φ is determined between beam-forming signals corresponding to two measuring steps at the same measuring point (P) and:
a travel φ at the measuring point (P) as being proportional to c·φ/(2·π·f)
and/or a speed at the measuring point (P) as being proportional to c·φ/(2·π·f)/Δt,
where c is the speed of the ultrasonic waves in the air, f is the frequency of the ultrasonic waves and Δt is a time interval between said two measuring steps.
22. A device for detecting movements of a surface ( 21 ) reflecting ultrasonic waves, comprising an ultrasonic wave emitting device ( 2 ), an ultrasonic wave receiving device ( 3 ), a control device ( 5 ) controlling the ultrasonic wave emitting device ( 2 ) and receiving signals detected by the ultrasonic wave receiving device ( 3 ), with the control device ( 5 ) being adapted to carry out several successive measuring steps during each of which the ultrasonic wave emitting device ( 2 ) emits at least one incident ultrasonic wave into the air towards the surface ( 21 ) and the ultrasonic wave receiving device ( 3 ) receives reflected signals representative of at least one ultrasonic wave reflected in the air by said surface ( 21 ) from said at least one incident ultrasonic wave,
characterized in that an ultrasonic wave emitting device ( 2 ) is adapted to illuminate a plurality of measuring points (P) belonging at least to said surface ( 21 ) by said at least one incident ultrasonic wave at a multiplicity of angles of incidence,
in that the ultrasonic wave receiving device ( 3 ) is a network of receiving transducers ( 3 ) comprising a plurality of receiving ultrasonic transducers ( 3 a ),
in that the control device ( 5 ) is adapted to determine, during each measuring step, a beam-forming signal for each measuring point (P), by at least beam-forming in reception from said reflected signals,
and in that the control device ( 5 ) is adapted to determine said movements of the surface ( 21 ) at the considered measuring point (P) by determining at least one delay or phase shift between two beam-forming signals for that measuring point (P).