AN INVESTIGATION INTO THE PERFORMANCE OF NICKEL ALLOY MAGNETOSTRICTIVE TRANSDUCERS

PREPARED FOR COMMERCIAL USE - - l

A. GRANGE

Ultrasonics Limited, Shipley, Yorks (Great Britian)

B. BROWN

Department of Pure and Applied Physics, University of Salford, Lancs (Great Britain)

(Received: 28 March , 1969)

SUMMA R Y

The vibration characteristics of a number of commercially prepared Ni-Co-Cr alloy magnetostrictive transducers have been measured. The behaviour of a single trans- ducer coupled to a stainless steel probe, resonant at 13 kHz, has been examined under unloaded conditions and it is shown that the length of the transducer is not critical to within +_ 1 in of the length calculated to be resonant at 13 kHz. Further experiments with 3 transducers attached to the probe are described. The suitability of the existing method of mounting the system at the nodal point of the probe has been investigated and the results indicate that some improvement in probe output should be possible by the use of a flexible mounting at the nodal point of the probe. Further experiments carried out under load conditions show that it is difficult to know accurately the resonant frequency of the system as the response is quite flat.

INTRODUCTION

The use of magnetostrictive transducers in ultrasonic cleaning equipment is well established t and such transducers are manufactured in relatively large numbers to specifications laid down by the manufacturers of the ultrasonic equipment. Usually the magnetostrictive transducers are attached to a stainless steel probe as shown in Fig. 1, the number of transducers used varying with the probe diameter. For instance, on a 3-in-diam. probe, 6 transducers are used, while with a 5-in-diam. probe, 16 transducers are used. The transducers are attached to the probe via a threaded stub, details of which are shown in Fig. 2. The nickel alloy laminations are brazed into the slot in the stub and the stub is then screwed into the base of the probe. The length of the transducers and probe is such that when the transducers

Applied Acoustics (2) (1969F---O Elsevier Publishing Company Ltd, England--Printed in Great Britain

I I 2 A. GRANGE, B. BROWN

are excited by an ex t e rna lb appl ied field at a par t icular frequency, then they are at mechanical resonance. The ~vork described below was carried out with the object of ob ta in ing in format ion which ~ou ld enable an improved product to be made. i.e. a more efficient, or al ternat ively, a cheaper, ul t rasonic cleaning unit.

Staintess .~ probe

Threaded

Magnetostricti transducers

Fig. I. Magnetostrictive transducers attached to a stainless steel probe.

,[ 0"710~n A/F

U/cut 0.125in wide ~----~{.,,_

X0.055indeep -I ,~r___~

• ' 0-250in

= 1-375in 1

Fig. 2. Threaded stub.

AN INVESTIGATION INTO THE PERFORMANCE OF TRANSDUCERS- - I I [3

RESONANT FREQUENCY AND AMPLITUDE OF VIBRATION OF

STANDARD TRANSDUCERS

The t ransducers used th roughou t the exper iments were made from N i - C o - C r al loy laminat ions . 0.008 in thick. The al loy consisted o f 96-3% nickel. 2 .3%

• O / ch romium and I 4 /o cobal t . The lamina t ions were annealed and then brazed into the slot in the stub. The dis tance between the end of the s tub and the laminat ions can vary due to the presence of brazing material . However , there is a min imum dis tance between the laminat ions and stub.

I

Fig• 3• Transducer vibration characteristics measuring equipment. (a) Probe arm can be locked into position to give gap of 0.020 in between probe and transducer, it is swung aside for withdrawal of transducer. (b) Locking screw. (c) Probe. (d) 0.020 in feeler gauge is swung aside after setting probe arm. (e) Transducer. (f) Nylon tube. (g) Non-metallic strap. (h) Vibrational amplitude meter. (i) Connections to generator. (j) Transducer laminations. (k) Coil on insulated former.

(I) Leaf spring to earth transducer. (m) Earthing wire.

Fif ty t ransducers were taken f rom stock and were inserted individual ly in the exper imenta l a r r angemen t shown in Fig. 3. A single excit ing coil moun ted verti- cal ly was connec ted to a Rapic lean M k III generator• The t ransducers were placed

114 A. GRANGE. B. BROWN

in turn in this coil and were excited by the osci l la t ions p rov ided by the gene ra to r opera t ing at cons tan t power. The frequency of the osci l la t ions was adjus ted until the t ransducers were v ibra t ing with max imum ampl i tude . A spring retainer system was used to prevent the t ransducers from moving as a whole when excited. It was ensured that the res t ra int thus provided d id not d a m p the vibrat ions. A W a y n e - K e r r p ick-up t ransducer , type M C I , was used to measure the ampl i tude o f v ibra t ion o f the t ransducers . This was connec ted to a W a y n e - K e r r v ibra t ion meter, type B 731 B which has a full scale range of 0.01 in. Measurements were carr ied out to an

accuracy of + 100 ttin.

TABLE 1 RESONANT FREQUENCY AND AMPLITUDE OF VIBRATION OF STANDARD TRANSDUCERS

Amplitude range 0.7-0-8/1000 in 0.8-1/1000 in > 1/1000 in

Transducer Resonant Transducer Resonant Transducer Resonant no. frequency no. frequency no. frequency

(Hz) 1 10850 2 10925 3 11220 4 10940 5 11150 6 11185 7 11050 8 11150 9 11008

I0 11190 11 11295 12 10950 13 11150 14 11120 15 11110 16 11060 17 11180 18 10820 19 10961 20 10907 21 10960

(Hz) (Hz) 22 11130 32 11015 23 I1100 33 10930 24 11095 34 10960 25 11180 35 11072 26 11125 36 10950 27 11134 37 11050 28 11155 38 11200 29 11026 39 11170 30 11088 40 11150 31 11045 41 11005

42 10980 43 10910 44 11175

The results are summar i sed in Table I f rom which it can be seen that 21 trans- ducers had a m a x i m u m ampl i tude o f v ib ra t ion in the range 0.7/1000-0-8/1000 in, l0 t ransducers were in the range 0.8/1000 to 1/1000 in and 13 t ransducers had an ampl i tude o f v ib ra t ion greater than 1/1000 in. The 6 t ransducers missing f rom the table had very low ampl i tudes o f v ib ra t ion and, when inspected, were seen by their physical appea rance to be incorrect ly bonded . A n analysis o f the f requency character is t ics o f the t ransducers is shown in Fig. 4, where the number of t rans- ducers lying in 50-Hertz f requency bands is shown. It can be seen that the f requency d i s t r ibu t ion o f the t ransducers is asymmetr ic . This is due to the fact tha t there is a m i n i m u m dis tance between the t ransducer lamina t ions and the stub. H a d this

AN INVESTIGATION INTO THE PERFORMANCE OF TRANSDUCERS--I 1 15

positive barrier not existed, then a symmetric distribution would have been expected.

THE BEHAVIOUR OF A SINGLE TRANSDUCER COUPLED TO A

2-1N-DIAM, STAINLESS STEEL PROBE

A stainless steel probe of length 7.53 in was taken and a single nickel transducer was attached to it via the usual coupling stub. The length of the probe was such that it was mechanically resonant at 13 kHz. The initial length of the transducer

12

I0

9

t _

"o c 7 o

_ 6 o

es E

I I I I I I

10801 -I I0851 f-- 10901- 10951-11001- 11051 I- 111011 11151 I- 11201-11251- 10850 10900 10950 11000 11050 11100 11150 11200 11250 11300

F r e q u e n c y bond

Fig. 4. Frequency characterist ics o f transducers.

was 10 in. With no load on the probe, the transducer was excited withapower input of 100 watts. The frequency of the input was adjusted until the transducer-probe sysem was vibrating with maximum amplitude. The resonant frequency of the

116 A. G R A N G E , B. BROVcN

system and the ampl i tude of v ibra t ions of the probe were then measured. With the p robe length main ta ined constant , the length of the t ransducer was progress iveb shor tened in steps o f ¼ in. Fo r each t ransducer length the resonant frequency of the system and the v ibra t ional ampl i tude of the probe were measured. The results are shown in Table 2. It is evident that varying the length of the t ransducer from

TABLE 2

RESONANT FREQUENCY AND AMPLITUDE OF SINGLE TRANSDUCER COUPLED TO A 2"IN-DIAM. STAINLESS STEEL PROBE. PROBE lENGTH MAINTAINED CONSTANT AT 7-53 IN

Transducer length Resonant frequency of system Vibrational amplitude of probe (in) (Hz) (in)

10'00 9-75 8"75 8"50 8"25 7-75 7"50 7"25 7"00 6"75 6"50 6"25 6"00 5"75 5"50 5"25 5"00 4"75 4"50 4"25 4"00

12900 0"00022 12979 0-00016 13043 0-00017 13083 0-00028 13130 0-00036 13159 0.00024 13184 0-00026 13204 0.0005 13231 0.00048 3255 0.0004 3274 0.00036 3330 0.0005 3355 0.00058 3384 O.eO05 3416 0,00048 3459 0-0004

13503 0"00046 13562 0"00036 13653 0"00032 13767 0.0O026 13918 0"00024

10 in, co r re spond ing to a resonant f requency for the t ransducer a lone of 8 kHz. to 4 in, co r respond ing to a resonant frequency o f 20 kHz, had little effect on the res- onan t frequency o f the whole system which var ied only from 12.9 kHz to 13.9 kHz. This result was expected in view of the mis-match between the t ransducer area of cross-sect ion of 9 64 in 2 and the probe area of cross-sect ion of 3.14 in 2. An ill- defined max imum of v ibra t iona l ampl i tude is apparen t , but it can be seen that the length of the single t ransducer is not cri t ical to within + I in of the length calcu- lated to be resonant at 13 kHz, 6"5 in.

The exper iment was repeated with three t ransducers a t tached to the 2-in-diam. probe . The t ransducers were always main ta ined at the same length and were dis- t r ibuted over the probe as shown in Fig. 5. The length of the t ransducers was varied f rom 10 in to 4 in in steps o f ¼ in and the resonant frequency and vibra t ional ampl i tude of the system were measured. The results showed that the length of the three t ransducers has little effect on the resonant frequency of the whole system.

AN INVESTIGATION INTO THE PERFORMANCE OF TRANSDUCERS--I 1 17

At the present time magnetostrictive rod transducers for commercial use are carefully constructed to be precisely a half wavelength long and hence to be a resonant length at the selected frequency. The system is normally maintained at the nodal point of the probe for longitudinal vibration. The results of the above

Fig. 5. Distribution of three transducers over probe.

experiments show that when the area of the vibrating surface of the transducers is considerably less than that of the probe then the present tolerances applied in their construction can be relaxed.

BEHAVIOUR OF THE TRANSDUCER SYSTEM MOUNTED AT THE NODAL POINT

OF THE PROBE

In view of the above results, which demonstrated that the commercial transducer system did not behave according to a simple theory of resonant lengths, a further experiment was designed in order to examine the suitability of the existing method of mounting the system at the nodal point of the probe.

A 3-in-diam. stainless steel probe calculated to be resonant at 13 kHz was welded at its nodal point to a heavy metal flange (Fig. 6). Two transducers were attached to the probe and were initially 10 in long. The transducers were maintained at the same length and this length was varied in steps of ¼ in from 10 in to 5 in. The

118 A. GRANGE, B. BROWN

resonant frequency and vibrational amplitude of the transducer system were measured and the vibrational amplitude of the metal flange was also measured. The results are shown in Table 3. It can be seen that there is a wide range of frequency over which the probe output does not vary significantly. There is also a sub- stantial output from the metal flange. The results indicate that some improvement in probe output should be possible by the use of a flexible mount at the nodal point of the probe.

Probe

WeLded .......... .,

ftange

Transducers-

~ f .......

Fig. 6. Heavy flange welded to probe.

TABLE 3

RESONANT FREQUENCY AND AMPLITUDE OF 2 TRANSDUCERS COUPLED TO A 3-IN-DIAM. STAINLESS STEEL PROBE WITH A METAL FLANGE ATTACHED TO THE NODAL POINT OF

THE PROBE

Transducer Resonant frequency Vibrational amplitude Vibrational amplitude length of system of probe of flange

(in) (Hz} (in) (in)

10'0 12524 0.00018 0'0001 9"5 12825 0-00025 0"0001 8-5 12992 0-00032 0"0004 8'0 13032 0.0004 0'00018 7'5 13108 0"0002 0'000 i 7"0 13149 0.00025 0-0001 6"5 13205 0.0002 0-00018 6'0 13226 0.00034 0"00012 5"5 i 3269 0.0003 0"00019 5"0 13326 0.00033 0"00017

OUTPUT CHARACTERISTICS OF PROBE UNDER LOADED CONDITIONS

The experiments described hitherto were carried out in air. Practically, however, probes are used under loaded conditions. An experiment was therefore designed

AN INVESTIGATION INTO THE PERFORMANCE OF TRANSDUCERS--I 1 19

to measure the vibrat ional ampl i tude of a probe under load. The appara tus is illus-

t rated in Fig. 7 which shows a brass tube of internal diameter ¼ in brazed to the

Amplitude- measuring probe

tube

uid toad

sducers

Fig. 7. Measurement of probe amplitude under load.

TABLE 4 RESONANT FREQUENCY AND AMPLITUDE OF 2 TRANSDUCERS COUPLED TO A 3"1N'DIAM.

STAINLESS STEEL PROBE UNDER A LIQL'ID LOAD

Transducer length Resonant frequency of system Vibrational amplitude of probe (in) (Hz) (in)

10'0 12461 0.00013 9"5 12593 0.00022 9"0 12730 0.00011 8"5 12760 0-00012 8"0 12855 0-00013 7"5 12902 0-0001 7"0 12910 0.0001 6.5 12970 0.00008 6.0 12970 0.00009 5'5 12950 0-00013 5-0 13000 0-00012 4'5 13080 0.000125

front face of the stainless steel probe of 3 in diam. The pick-up head of the v ibra t ion meter was inserted down the tube. The probe was mounted in a 6-in-diam. tank full of water so that the major part of the surface area of the probe was under a

120 A. GRANGE, B. BROWN

liquid load. The height of the liquid above the surface of the probe ~vas 4 in. The probe was driven by the transducers, which were maintained at the same length, and this length was varied from 10 in to 4"5 in. The resonant frequency of the system and vibrational amplitude of the probe face were measured and the results are shown in Table 4.

These results are not significant in indicating a correct transducer length. How- ever, they clearly show that under load conditions it is difficult to know accurately the resonant frequency of the system, as the response is quite fiat.

REFERENCE

1. B. BROWN and J. E. GOODMAN, High Intensity Ultrasonics, lliffe Limited, 1965.