Ultrasonic Single Element and Sectorized Array Transducers with Omnidirectional 2 D Field Distribution for Non-Contact Human-Machine Interface and EchoLocation

This work discusses the possibilities of some novel applications for the high frequency (200 kHz–500 kHz), high sensitivity (-20 dB– -35 dB) and wide band (> 50 % @ -20 dB) air-coupled transducers that have been used so far for non-destructive testing and materials characterization at CSIC. These applications refer to non-contact human-machine interfaces, gesture recognition and echolocation. Compared with other technologies, the relatively higher frequency and bandwidth permit to achieve better resolution and the higher sensitivity permit to reach longer distances. In addition, this later feature also permits to reduce the number of transducers. This is achieved by using a conical reflector that provides an omnidirectional (2D) acoustic field. Examples of applications and realizations are shown by using single element 250 kHz transducers; and 8-elements 400 kHz sectorized array transducers. This later type of air-coupled array is introduced here for the first time. DOI: http://dx.doi.org/10.5755/j01.eie.23.4.18722

1 Abstract-This work discusses the possibilities of some novel applications for the high frequency (200 kHz-500 kHz), high sensitivity (-20 dB--35 dB) and wide band (> 50 % @ -20 dB) air-coupled transducers that have been used so far for nondestructive testing and materials characterization at CSIC.These applications refer to non-contact human-machine interfaces, gesture recognition and echolocation.Compared with other technologies, the relatively higher frequency and bandwidth permit to achieve better resolution and the higher sensitivity permit to reach longer distances.In addition, this later feature also permits to reduce the number of transducers.This is achieved by using a conical reflector that provides an omnidirectional (2D) acoustic field.Examples of applications and realizations are shown by using single element 250 kHz transducers; and 8-elements 400 kHz sectorized array transducers.This later type of air-coupled array is introduced here for the first time.

Index
Terms-Piezoelectric transducers; ultrasonic transducer arrays; impedance matching; robot sensing systems; sensor systems and applications.

I. INTRODUCTION
There is a technology for bridging the human-computer barrier, a revision of different technologies can be seen in [1].Air-coupled ultrasound is a very interesting technology for non-contact human machine interfaces and gesture recognition systems.These systems are of interest in different fields, the best example is provided by the gaming industry, yet there are also some other fields where avoiding contact between the user and the machine (computer, smartphone, etc.) may present an advantage.Examples appear in different robotic applications, in the use of machines and interfaces by medical doctors during different medical procedures, etc. Technologies based in laser and image processing have been widely used and there are also some applications based in air-coupled ultrasound [2]- [5].In this case, the main problems are to find efficient ultrasonic transmitter/receivers, to reduce the number of transmitters/receivers and to achieve good spatial accuracy over the whole volume of the space of human-machine interaction for the application under consideration.The selection of the technology to be used largely depends on the minimum and maximum distances, the volume of interaction, the size of the object to be detected (the tip of a pen, a finger, a hand a head or a whole human body), its movement, the required accuracy and other restrictions in terms of size, volume, weight and consume of the device.

II. TRANSDUCERS AND ELECTRONIC EQUIPMENT
Two different types of air-coupled transducers have been employed in this work, all of them designed and fabricated at CSIC: single element transducers and sectorized array transducers [6]- [9].The key design elements are: the centre frequency and the bandwidth (relatively high to achieve a good spatial resolution) and the sensitivity (high enough so that it is possible to generate a 2D omnidirectional field and detect small objects at large distances).All transducers have been driven by using a Panametrics pulser-receiver, either the 5077 or the 5058, the former provides a semicycle of square wave that can be tuned to the transducer centre frequency, with amplitude between 100 V and 400 V; while the later provides a wide band spike excitation with peak amplitude in the range 50 V-900 V. Display and digitalization of the received echoes were performed by a DPO 5054 Tektronix oscilloscope.
In order to compare the obtained results with those that can be obtained by using commercial air-coupled ultrasonic transducers, two other transducers were also used: the 40 kHz Murata transducer (MA 40S4S) and the 200 kHz Multicomp transducer (MCUSD19A200B11RS).

A. Single Element 250 kHz Transducers
The single element 250 kHz transducers were fabricated using a 1-3 connectivity piezoelectric composite disk (65 % volume fraction of PZT5A in epoxy resin matrix), 25 mm diameter, poled in the thickness direction and with the first thickness mode at 250 kHz.Matching to the air is achieved by a layered impedance matching layer made of a stack of five sublayers attached to the radiating surface.Peak sensitivity is -25 dB at 290 kHz and -20 dB bandwidth is 46 %.Impulse response and sensitivity vs frequency of these transducers can be seen in [6].

B. Sectorized 8-Elements 450 kHz Transducers
The sectorized 8 elements 450 kHz transducer is introduced for the first time in this work.It was fabricated using a 1-3 connectivity piezoelectric composite disk similar to the ones used before but with the first thickness mode located at 400 kHz.Matching to the air is achieved by following a similar procedure.The sectorized array is kerfless, hence, the isolation between the different elements is obtained by removing the metallization along the edges of the sector elements only on the back surface, while the metallization in the front surface is continuous and ground connected, see Fig. 1.All 8 elements are identical.Figure 2 shows the response of one element of the sectorized array in pulse-echo mode with a flat reflector at 30 mm.
Figure 3 shows a picture of some of the monolithic 250 kHz air coupled transducers and the 450 kHz sectorized 8-elements transducer.

C. Conical Reflector (or Field Spreader) to Generate A 2D Omnidirectional Field
The role of the conical reflector is to spread the acoustic field so that the number of transmitters/receivers can be minimized and the potential volume of interaction maximized.Two main configurations have been used: i) cantered conical reflector, to achieve a 360º acoustic field distribution and ii) Laterally displaced conical reflector to achieve a 180º acoustic field distribution.These two configurations are shown in Fig. 4.
Figure 5 (right) shows the reduction of the sensitivity of the transducers due to the use of the conical reflectors.Case 1 is obtained for two 250 kHz transducer in pitch-catch mode without conical reflector, while cases 2 and 3 represent the 250 kHz transducers with conical reflectors (180º and 360º configurations, respectively).For comparison purposes the SNS band of the Murata and the Multicomp transducers without conical reflector is also shown.The different configurations of transducers and conical reflectors used for these measurements is also shown on Fig. 5 (left).
Figure 6 shows the experimental set-up for through transmission measurements using two transducers (one transmitter, Tx, and one receiver, Rx) with two conical reflectors in the 180º configuration and the received signal measured using the 250 kHz and the 40 kHz transducers.

A. Two Single Element Transducers in Pulse-echo Operation Mode
Figure 8 shows the configuration to determine the location of an object (cylindrical reflector) in a half space using two transducers with conical reflectors in 180º configuration, both transducers operate in pulse-echo mode.Two time of flights (tof) between transducer and reflector are measured (tof1 and tof2) and hence two distances are obtained (D1 and D2).The position of the object is worked out as the intersection of the two circumferences having radii equal to D1 and D2 and centred at Tr/Rx1 and Tx/Rx2, respectively.

B. Three Single Element Transducers in Pitch-catch Mode: One Transmitter (Tx) and Two Receivers (Rx)
Fig. 9 shows the configuration to determine the location of an object (cylindrical reflector) in a half space using three transducers with conical reflectors in 180º configuration.In this case there is only one transmitter and the two receivers.Two time of flights (tof) between the transmitter and the two receivers are measured (tof1 and tof2) and hence two distances are obtained (D1 and D2).The position of the object is worked out as the intersection of the two ellipses whose foci are located at the transducers positions, that is, Tx and Rx1 for one ellipse and Tx and Rx2 for the other.

C. One Sectorized Array Transducer in Pulse-echo Mode and Conical Reflector in 360º Configuration
Figure 10 shows the configuration using one sectorized array transducer with a conical reflector in 360º configuration.Location of objects is directly determined from tof (distance) and the number of the element that received the echo (direction).

IV. RESULTS
Figure 11 and Fig. 12 show the received echo from flat and cylindrical obstacles, using single element 250 kHz and 40 kHz Murata transducers in pitch-catch mode and with conical reflectors in 180º configuration.It can be clearly seen that the 40 kHz transducers can hardly distinguish the echo from the obstacle from the direct transmission from Tx to Rx.This problem does not appear in the 250 kHz transducer thanks to the higher centre frequency and wider band.In addition, signal amplitude is higher for the 250 kHz in spite of the larger attenuation in the air.Figure 13 shows the received echo from flat and cylindrical (45 mm diam.)obstacles located at 20 cm from the 450 kHz sectorized array transducer, using one element, with a conical reflector (360º configuration).It can be seen that the amplitude and SNR of the echo obtained from the flat obstacle is larger, but that signal shapes is almost the same.V. CONCLUSIONS This preliminary work analyses the possibility of using air-coupled ultrasonic transducers in the frequency range 200 kHz-500 kHz for precise echolocation that could be further applied to gesture recognition and non-contact human-machine interfaces.The key elements are the extremely high sensitivity (between -19 and -30 dB) and the relatively high centre frequency (between 200 kHz and 500 kHz) and wide band response (> 50 % at -20 dB).These features make possible to obtain a much better resolution that that obtained with conventional lower frequency transducers (40 kHz), making possible to resolve a reflector at a close (< mm), well as small spherical reflectors (8 mm diam.).In addition, the very high sensitivity makes possible the use of conical reflectors to spread the beam in either 180º or 360º directions, while keeping a reasonable SNR and sensitivity (between -30 dB and -40 dB).This makes possible to reduce the number of transducer required.An alternative, based on a novel sectorized air-coupled array, presented here for the first time, has been also shown.Its response is similar to that of the single element counterparts, but these arrays enable a reduction of the number of transducers required.

Fig. 3 .
Fig. 3. Picture of the 250 kHz single element air coupled transducers (three transducers on the left) with BNC connector and the 8 elements 400 kHz sectorized array transducer (right) without any connector.

4 .
Schematic representation of the two configurations employed for the conical reflector (a).250 kHz transducer with the conical reflector in the 360º field distribution configuration (b).

Figure 7 (Fig. 5 .
Figure 7(a) shows the directivity pattern of the sectorized array with the conical reflector in the 360º configuration obtained by operating each element of the array in pulseecho mode (using the 5077 PR) and measuring the peak to peak amplitude of the echo originated by a 20 mm cylindrical reflector located at 200 mm away from the

Fig. 6 .
Fig. 6.Comparison of received signals using 250kHz single element transducer and Murata 40 kHz transducers with conical reflectors 180º configuration and in through transmission (shown above the figure).

Fig. 7 .
Fig. 7. Directivity pattern of the 8 elements sectorized 400 kHz array measured with the conical reflector in the 360º configuration and in pulseecho mode using a cylindrical reflector (20 mm diameter) located at 200 mm distance and rotated 360º around the transducers in steps of 5º (a).and c) lateral and upper view of the experimental configuration (b).

Fig. 8 .
Fig. 8. Schematic representation of the transducer configuration for echolocation of a cylindrical object using two transducers with conical reflector in 180º configuration.

Fig. 9 .
Fig. 9. Schematic representation of the transducer configuration for echolocation of a cylindrical object using three transducers with conical reflector in 180º configuration.

Fig. 10 .
Fig. 10.Schematic representation of the transducer configuration for echolocation of a cylindrical object using one sectorized array transducer with a conical reflector in 360º configuration.

Fig. 11 .
Fig. 11.Received echo from a flat obstacle located at 120 cm from the transducers using: a) two 250 kHz transducers element and b) two 40 kHz Murata transducers with a conical reflector in 180º configuration.The signal at 0.85 ms is the direct transmission from Tx to Rx while the signal at 1.15 ms correspond to the echo from the cylindrical reflector/obstacle.

Fig. 12 .
Fig. 12. Received echo from a cylindrical obstacle located at 120 cm from the transducers with a conical reflector in 180º configuration using: a) two 250 kHz transducers element and b) two 40 kHz Murata transducers.Signal at 0.85 ms is the direct transmission from Tx to Rx. Signal at 1.15 ms correspond to the echo from the cylindrical reflector/obstacle.

Fig. 13 .
Fig. 13.Received echo from a flat and a cylindrical (45 mm diameter) obstacle located at 20 cm from the transducer using element #1 of the sectorized array with a conical reflector in 360º configuration.