| 摘要 |
PROBLEM TO BE SOLVED: To accurately detect the clogging of a filter by detecting the clogging of the filter based on the change in resonance frequency which is an eigen value of the filter. SOLUTION: A fitting part 17c for fitting an ultrasonic generator 30 is integrally formed in the inside of a frame body 17a of a filter 17. When dust stuck to the filter 17, the weight (m) of the filter is increased. On the other hand, the resonance frequency of the filter 17 is expressed by an equation f=1/2π(k/m)<1/2> . However, (k) is a constant in the equation. Therefore, if the weight (m) of the filter 17 is changed, the resonance frequency (f) is also changed. The ultrasonic generator 30 is applied to detect the change in the resonance frequency (f) and the changes in the input or the output voltage of the ultrasonic generator 30 caused by the deviation of the oscillation characteristics are detected so that the clogging of the filter part, 17b is detected. This constitution can accurately detect the clogging of the filter 17.
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