PURPOSE:To make it possible to carry out a normal vibration test by controlling loop gain by using the negative feedback signal of a high-frequency low-level monitor signal superposed upon a control signal. CONSTITUTION:Through servo valve 5, actuator 6 is applied with a control signal of frequency (f1) with a superposed high-frequency monitor signal of frequency (f2) whose level is low enough to exert no influence upon a test. Then, a load applied upon test piece 7 is detected by load detector 8 and then negatively fed back to addition point 2, and level detector 12 detects the level of only the feedback monitor signal by way of filter 11. This detection value is compared with the set value of desired set value 13 at addition point 14 to control multiplier 3. Consepuently, the negative feedback loop gain becomes constant and a normal vibration test can be made.
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