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　　原文： 　　Ultrasonic distance meter 　　Document Type and Number:United States Patent 5442592Abstract:An ultrasonic distance meter cancels out the effects of temperature and humidityvariations by including a measuring unit and areference unit. In each of the units, a repetitive series of pulses isgenerated, each having a repetition rate directly related to therespective distance between an electroacoustic transmitter and anelectroacoustic receiver. The pulse trains are provided to respectivecounters, and the ratio of the counter outputs is utilized to determinethe distance being measured. 　　Publication Date:08/15/1995 　　Primary Examiner:Lobo, Ian J. 　　BACKGROUND OF THE INVENTION 　　This invention relates to apparatus for the measurement of distanceand, more particularly, to such apparatus which transmits ultrasonicwaves between two points. 　　Precision machine tools must be calibrated. In the past, this has beenaccomplished utilizing mechanical devices such as calipers,micrometers, and the like. However, the use of such devices does notreadily lend itself to automation techniques. It is known that thedistance between two points can be determined by measuring thepropagation time of a wave travelling between those two points. Onesuch type of wave is an ultrasonic, or acoustic, wave. When anultrasonic wave travels between two points, the distance between thetwo points can be measured by multiplying the transit time of the waveby the wave velocity in the medium separating the two points. It istherefore an object of the present invention to provide apparatusutilizing ultrasonic waves to accurately measure the distance betweentwo points. 　　When the medium between the two points whose spacing is being measuredis air, the sound velocity is dependent upon the temperature andhumidity of the air. It is therefore a further object of the,presentinvention to provide apparatus of the type described which isindependent of temperature and humidity variations. 　　SUMMARY OF THE INVENTION