Curved ultrasonic array transducer for AGV applications

Curved ultrasonic array transducer for AGV applications J.P.

Huissoon

Department N2L 3Gl

and

of Mechanical

Received 5 October

D.M.

Moziar

Engineering,

University

1988; revised 29 March

of Waterloo,

Waterloo,

Ontario,

Canada

7989

An electrostatic curved phased array transducer for airborne ultrasonic imaging has been developed for use in a hybrid sensor system in the guidance control of an autonomous AGV. The cylindrical transducer employs 32 elements distributed over 120” giving a wide field of view while enabling a small aperture to be used. The transducer and interface electronics for the controlling microprocessor are described and both theoretical and experimental data of the transmission characteristics are given. Keywords:

transducers;

design

parameters;

For the many experimental AGVs in existence, sonar is frequently used as the primary means of detecting the boundaries within which the vehicle must operate. A ‘ring’ of transducers, operating in pulse echo mode, has been implemented on numerous vehicles e.g. Neptune’, Hermies IIz. This relatively straightforward technique of obtaining ‘all round vision’ suffers from the drawback that the angular resolution is modest; limited by the beam width of the transducers (typically 15’ for the most commonly used electrostatic transducers). Stereo reception of echo pulses has been described3 which overcomes this problem to a large extent, improving angular resolution to better than f l”, but with a limited field of view. Focussing the ultrasonic beam by means of a parabolic reflector has also been described as a means of improving resolution4 but the time required to obtain one ‘scan’ places severe limitations on the speed at which the vehicle can safely travel. To circumvent these restrictions, considerable attention has recently been given to phased array transducer systems 5*6. Phased arrays are well established in the medical field as well as in non-destructive testing, where operating frequencies in the megahertz range are used’,‘. However, for airborne ultrasonic uses, the frequencies must be much less, typically 50 kHz, to enable a useful range to be obtained, due to the increasing attenuation with frequency. One major disadvantage in using phased arrays is the presence of side lobes in the transmitted beam pattern. These side lobes become more pronounced as the beam axis is steered away from the normal to the plane of the transducer face, and may dominate over the ‘main’ lobe at the deflection limits9, effectively rendering the transducer as less than satisfactory. This is because the returned echoes from the side lobes will dominate over that from the main lobe. To overcome the inherent problems in the techniques described above, a transducer has been designed that allows the beam pattern to be directed electronically over 0041-624X/89/040221 @ 1989 Butterworth

-05 $03.00 & Co (Publishers)

Ltd

performance

characteristics

a wide angle by the selection of the array elements to be driven. Due to the curvature of the transducer face, the relative phase between the elements used is such that the main lobe width and strength is optimized with respect to the relative strength of the dominant side lobe(s). Transducer

design

Having established a basic concept for the physical appearance of the transducer and approximate dimensions and operating parameters, the response was simulated using a numerical technique to compute the theoretical beam pattern. A cross-section of the transducer model is shown in Figure 1. Six array elements were chosen as providing a sufficiently large active transmission area while keeping the curvature of the active area small to minimize side lobe generation. The numerical solution for the sound pressure level (SPL) in the far field of the transducer is obtained as a summation of the incremental pressures generated by elemental line sources over the surface of each array element. Referring to Figure 2, this is obtained as

Figure

1

Transducer

cross-section

Ultrasonics

1989 Vol 27 July

221

Curved ultrasonic array transducer

for AGV applications:

J. P. Huissoon

-10

Figure

Figure

2

-10

-20

-20

3

Computed

4

Computed

SPL (normalized),

all elements in phase

SPL (normalized),

optimum

dB

Transducer array model (not to scale)

P(r, 0) =fl(f2 cos cb-A

sin4)

-10

where:f, = p,clJ,khAx/2zr; f2 = IZr=1Cy= I cos(kArij(B)); f3 = Cl= rEcjm_ I sin(kArij(0)); 4 = arctan( -f&; h = element height; n = number of elements; k = wave number (271/I_); c = velocity of sound in medium; p. = density of transmission medium; U, = amplitude of oscillation of element surface; Ax = element of width b divided into m equal strips; Arij(0) = difference between r and the distance from point (r, 19)to the centre of the strip (or elemental line source) j on element i. The numerical solution generated for the case of a very large radius of curvature (giving an essentially flat array) corresponds very closely to that obtained from an exact solution for this case, which is computed as P(r, 8, t) = (jpcU,kbh/r)e”“‘-k”

[ si;;“,it;

e)]

where k is the wave number. Figure 3 shows the theoretical polar SPL pattern obtained when all six elements are driven in phase. As would be expected from the geometry of the array, a phase delay between elements will be required to reinforce the main lobe and possibly reduce side lobe magnitudes. Figure 4 shows the result of advancing the phase of elements 2a and 2b (see Figure 2) by 20” and that of elements 3a and 3b by 60”. It is clear that the width of the main lobe has been increased but is accompanied by a significant reduction in the side lobes. It was found that the effect of altering the relative phase between elements

222

and D. M. Moziar

Ultrasonics

1989 Vol 27 July

Figure

-20

-20

-10

relative phase

by about 10” produced only minor changes in the computed SPL pattern. However, reinforcement of the main lobe with outer element phase was found, as expected, to be significant in the on-axis pressure, as shown in Figure 5. The conclusion drawn from this simulation was that a useful beam pattern could be obtained by selectively driving a set of array elements on a curved transducer face.

Transducer

construction

The transducer array is fabricated by etching the required element pattern on a piece of epoxy glass copper clad printed circuit board, the board having been machined to a thickness of 0.5 mm to provide the necessary flexibility in taking up the required curvature. This technique enabled a number of array patterns and dimensions to be tested using the same transducer body and simply replacing the array. The body of the transducer consisted of a hollow perspex cylinder, o.d. 115 mm, machined to facilitate the attachment of the element board and the tensioning device for the gold clad polyester film*, used as the common cathode for the array * Courtesy of the Polaroid Corporation

dB

Curved ultrasonic array transducer for AGV applications: J. P. Huissoon and D. M. Moziar

li

20

40

80

so

100

9wff)

Figure 7

Photograph of transducer

Figure 5 Computed on-axis SPL versus relative phase angle. 42: /I, 0”; 0, 5”; v, IO”; 0.15”; .,20”; V. 25”

foil \

array

\

Figure

6

Photograph of array elements

electrode elements. Figures 6 and 7 show the array elements and the transducer respectively. fn the design of Aat electrostatic transducers, it has been reported that foil tension has little effect on performance’. This is not so for the curved array design since tensioning the foil prevents it from being electrostatically repelled from the array elements. To reduce this effect, stand-off strips, or rails, were incorporated onto the array by double etching of the array pattern to produce a ridge between the elements, as iliustrated in Figure 8. These rails performed a double function in that the film between adjacent strips could be displaced radially with less effort and that an increased air volume is created between the film and the array face. Variations on this design were tested prior to arriving at this configuration, including arrays without rails, arrays machined (rather than etched) to give deeper grooves between the elements, and perforated arrays where a series of 1 mm diameter holes were drilled in each element. The cross-section design shown in Figure 8 was found to give the best results. Each array element is individually driven by a class B

Ftgure 8

element

base

Array cross-section

high voltage amplifier, the schematic of which is shown in Figure 9. This approach to generating the driving signals to the elements was taken to provide minimum variation in applied signal phase and amplitude between elements to that generated by the controlling circuitry. The generation of the three phased driving signals was implemented using the circuit shown schematically Figure 10. The 10 MHz clock frequency enables phase differences in 1.8” increments between adjacent elements to be generated at 50 kHz transmission frequency. The selection of the elements in the array to be driven is accomplished via a digital circuit consisting of five programmable array logic integrated circuits, between the phase generating circuitry and the amplifiers. This circuit decodes the selected elements from a digital input provided by the controlling microcomputer and generates the signals for the array on six of the 32 outputs.

Test equipment The transducer’s polar far field SPL pattern was obtained by rotating the transducer, issuing short durations of

Ultrasonics

1989 Vol 27 July

223

Curved

ultrasonic

array transducer

for AGV

applications:

TTL i/p

Figure

9

Element

driver circuit

1oMHz

Switchee

J. P. Huissoon

and D. M. Moziar

A Polaroid electrostatic transducer was used as the receiving element in the tests. The front face of the transducer was masked to give a 12 mm wide strip ‘aperture’ corresponding to a 0.7” angle subtended at the transmitter. The relationship between the measured electrical signal amplitude from the receiver circuitry and the expected SPL was obtained by computing the ratio of the measured output at 1.65 m and 2.15 m spacing between transmitter and receiver with that measured at 1.15 m, for the same transmitter output. These ratios were then fitted to the inverse distance relationship by a quadratic function. This was repeated for a number of transmitter power output levels, adjusted by altering the excitation voltage amplitude, the measured data being tabulated in Table I. The consistent ratio of the measured data for the various transmitter output levels indicates that any measured value may be related to relative SPL by referring this to a reference value, taken to be the ‘on axis’ measurement. The coefficients for the quadratic function were found to be a, = 0.042, a, = 1.14, a2 = -0.18 for the relationship SPL = a, + a,( V,/ V,) + a*( V,/ V,)’

Relative

where V, is an arbitrary measured value and V, is that value measured on the transmitter axis for the same driving signal amplitude.

Results

d Figure

10

Phase generator

The results of tests corresponding to the driving signal phases of Figures 3 and 4 are shown in Figures 12 and 13, respectively. The effect of phase lead on the side lobe suppression is in good agreement with that of the simulated response. The lack of definition of the boundary between the main and first side lobe, when all elements are driven in phase (Figure 12), is worse that expected. Since the rotation of the beam pattern about the transducer is accomplished by the selection of the elements to be driven rather than by altering the phase of the driving signals, the SPL pattern is not affected by ‘beam steering’. Therefore this transducer is not used as a phased array in the conventional sense, although it may be

4.1 schematic

ultrasound at predefined angular positions, and recording the observed sound pressure level synchronized (including a delay) with the transmissions. The proximity of possible reflecting surfaces was arranged such that the path length from these would result in any reflections arriving at the receiving transducer (and hence interfering with the signal to be measured) after the measurement reading had been taken. By issuing 5 ms durations of ultrasound at approximately 80 ms intervals (dependent on the speed at which the transmitter rotates and corresponding to 1.125’ rotation) and allowing a 3 ms transmission delay, the RMS readings of the SPL at a 1 m distance for 160 stations about the transducer were recorded using the circuit depicted in Figure Il. All data reported were recorded under these conditions. Averaging of the successive tests was performed on the controlling microcomputer.

Table

1

Experimental

Measured (units) 252 220 190 150 109

output

162 140 122 97 69

120 102 88 70 51

data for varying transmitter

P(r2exp)lP(f2th)

P(r3exp)/P(r3th)

0.92 0.92 0.93 0.93 0.93

0.89 0.86 0.85 0.86 0.86

Analog to Digital Converter

High Impedance

a

Amplifier

AD536 -

RMS to DC Converter Figure

11

Receiver

224

Ultrasonics

schematic

1989 Vol 27 July

AD673

output

DATA

strut convert

levels

Curved

ultrasonic

array transducer

for AGV

applications:

J. P. Huissoon

and 0. M. Moziar

characteristic of a Sell transducer, it was found that increasing the air volume behind the foil, by drilling a series of holes through the array elements, did not appreciably alter the characteristics. Furthermore, an array configuration obtained by machining the elements to create much deeper grooves (1 mm x 1 mm) did not increase the SPL, although this array did lack the rails between elements as described above. Conclusions

-10

Figure

12

-20

Experimental

SPL (normalized),

-10

dB

all elements in phase

A curved array ultrasonic transducer having electronically controlled beam steering of phased array transducers, but without the major variation in beam pattern associated with such arrays, has been described. Suitable application of the leading phase to the outer elements used in transmission, allows the main lobe to be optimized in terms of on-axis pressure and beam width, while the side lobes are reduced. The test results compare favourably with those obtained from the numerical solution for the response of the transducer. Acknowledgement The authors wish to express thanks to the Canadian NSERC for funding this research work. References 1 2

3

-10

Figure

13

-20

Experimental SPL (normalized),

-10

da

optimum relative phase

operated as such by adjusting the relative phase of the driving signals. The theoretical beam pattern when such phasing is used to deflect the main lobe by a maximum of 1.9o has been found to vary almost insignificantly from that shown in Figures 2 and 3. Moreover, since the main lobe width is much larger than the angular steering resolution, no advantage is gained by operating the transducer in this way. Also observed was the marginal effect on the beam pattern of altering the relative phase between elements by about lo”, as found in the simulation. Although the array lacked grooves, which are

7

8

9

10

Elfes, A. Sonar-Based Real-World Mapping and Navigation IEEE J Robot Automat (1987) RA-3 249-265 Weisbin, C., de Saussure, G. and Kammer, D. A Real-Time Expert System for an Autonomous Mobile Robot Comput Mech Engng (1986) 12-19 Huissoon, J.P. and Normoyle, P.D. An Ultrasonic Vision System for use on Mobile Robots and Automated Guided Vehicles UK Res Adv Manufac Proc IMechE (1986) B368/86 55-60 Crowley, J.L. Navigation for an Intelligent Mobile Robot IEEE J Robot Automat (1985) RA-1 31-41 Kay, L. Airborne Ultrasonic Imaging of a Robot Work Space Sensor Rev (1985) 8812 Kuroda, S., Jitsumori, A. and Inari, T. Ultrasonic Imaging System for Robots Using an Electronic Scanning Method Robot Sensors Vol. 2, IFS Publications Ltd., UK 271-285 Mcnab, A. and Stumpf, I. Monolithic Phased Array for the Transmission of Ultrasound in NDT Ultrasonics Ultrasonics (1986) 24 148-155 Gatzke, R., Fearnside, J.T. and Karp, S. Electronic Scanner for a Phased-Array Ultrasound Transducer, Hewlett-Packard J (1983) 13320 Higucbi, K., Suzuki, K. and Tanigawa, H. Ultrasonic Phased Array Transducer for Acoustic Imaging in Air Proc 1986 Ultrasonics Symposium (1986) 559-562 Kinsler, L.E. Fundamentals of Acoustics John Wiley and Sons, New York (1982) 1633197

Ultrasonics

1989 Vol 27 July

225