Under this condition, drive cycles Munit necessary for satisfying total drive cycles Ntotal written tn eq. (1) is
Munit = Ntotal / (1150Hz × 60second) = 39992time (3).
The driving unit was operated about 40000 times for one minutes operation which corresponds to the equivalent cycle time for ten years. The strain of the driving unit was set to be 350 ppmrms. This is correspondent to the strain necessary for generating 198dB transmitting level, when 70Hz sound source is set up by these units. The input current into the unit was 1.6 Arms. Five driving units were divided into two parts of two and three series due to the capacity (2 kVA) of used power amplifier, and they were put in the bath which filled with the silicone oil and kept to -11℃ in the constant temperature oven. The data of the temperature change measured by the thermocouple attached on the giant magnetrostrictive rod are shown in Fig. 4. It is proven that the temperature of the rod did not reach at the Curie point (about 380℃), since the maximum temperature in the operation was about 180℃. The oil temperature changed between 2.7℃ and 4.8℃ in this operation. The displacement was measured by an acceleration pickup installed on the vibrating face of the driving unit, and the impedance was measured by a gain-phase analyzer. These measurements were made at the timing afier drive start, 4540, 10080, 11868, 20623, 30428 and 40233 time drives. The session of the experiment started at April 1996 and ended at the end of March 1997. The measurement was interrupted at around 11868 time drives because the constant temperature oven suspended between the middle of July and the end of October 1996.
2-3. Results
The variation of strain converted from the data of the acceleration pickup which attached at 5 driving units is shown in Fig. 5. The amplitude varied around 350 ppm, and the aging variation could not be observed. Frequency characteristics of real and imaginary part of the impedance and the displacement-current sensitivity of the No. 1 unit are shown in Figs. 6-8. The aging variation was not observed in these figures. The similar results were obtained on other units. Finally, the aging variation of impedance and displacement-current sensitivity at the 1150Hz resonant frequency are shown in Figs. 9-11 on five driving units. The trend of change could not be viewed on real and imaginary part of the impedance. Though No. 1 unit shows the high value of about 2dB from other units on the displacement-current sensitivity, it seems to be dispersion of the prestress in the driving unit assembly without observing the aging variation.
