超音波を用いた地下風道風速計の開発

Abstract

This paper describes a measurement system using with ultrasonic transducers to monitor airflow velocity in underground airways.A flow velocity anemometer, which is able to monitor airflow rate of underground airways for a long term, is useful for ventilation managements and design.When transport vehicles or locomotives are runing through airways, an usual point measurement anemometer, such as propeller type or thermal type one, have to be set near inside walls of airways. Further more, if average velocity is estimated based on a point measured value, presumption error becomes large because flow velocity profile reduces near walls.The anemometer using with two 40kHz ultrasonic transducers set near wall of underground airways, hereinafter called an ultrasonic anemometer, has been developed. The anemometer measures the diffrence in travel time between two ultrasonic pulse-waves along and against the airflow. One transducer is located upstream of the other and transmittings of two waves are synchronized. The travel times of the ultrasonic waves in the upstream and downstream directions are measured over the same path as illustrated in Fig. 1. The diffrence in travel time is proportional to the airflow velocity. Analogue output and recording system using a simple composition circuit (Fig. 2) was used. And a digital counter circuit to measure the time differences (Fig. 3) was utilized to calibrate its analogue output as against airflow velocity.Some experiments with ultrasonic anemometer related to airflow velocity were executed compared with a thermal anemometer at a loboratory. From the results, it is presumed that its measurement error is within 3% and its resolution is minimum 0.03m/s.The ultrasonic anemometer has been investigated and examined at two metal mines from 1991.The anemometer doesn't become an obstacle for passing vehicles going through its setting area. The measurement error to estimate average velocity is relatively small and can detect flow direction.When vehicles or workers pass though the measurement point, sudden increase or decrease of output voltage occurs. Using with these output characteristics, a method is presented to recognize passings of vehicle. The ultrasonic transducer did overcome disturbances of dusts by self-cleaning of its vibrating surface. In case of the examination in dusty airflow of the Fukasawa mine, it has been operated for five months without a maintenance.