【正文】 is the partial pressure of the water vapor, H is the barometric pressure, w and a are the ratio of constant pressure specific heat to constant volume specific heat for water vapor and dry air, respectively. Thus, although the repetition rate of the trigger pulses is measured very accurately by the counter 46, the sound velocity is influenced by temperature and humidity so that the measured distance D cannot be determined accurately. In accordance with the principles of this invention, a reference unit 12 is provided. The reference unit 12 is of the same construction as the measuring unit 10 and therefore includes an electroacoustic transmitter 50 which includes piezoelectric material 52 sandwiched between a pair of electrodes 54 and 56, and an electroacoustic receiver 58 which includes piezoelectric material 60 sandwiched between a pair of electrodes 62 and 64. Again, transducers other than the piezoelectric type can be utilized. The transmitter 50 and the receiver 58 are spaced apart a known and fixed reference distance DR . The electrodes 62 and 64 are coupled to the input of the amplifier 66, whose output is coupled to the input of the detector 68. The output of the detector 68 is coupled to the pulse former 70 which generates trigger pulses. The trigger pulses are applied to the pulse generator 72 which controls the modulator 74 to pass bursts from the continuous wave oscillator 76 to the transmitter 50. The trigger pulses from the pulse former 70 are also applied to the counter 78. Preferably, all of the transducers 16, 18, 50 and 58 have the same resonant frequency. Therefore, the oscillators 42 and 76 both operate at that frequency and the pulse generators 40 and 72 provide equal width output pulses. In usage, the measuring unit 10 and the reference unit 12 are in close proximity so that the sound velocity in both of the units is the same. Although the repetition rates of the pulses in the measuring unit 10 and the reference unit 12 are each temperature and humidity dependent, it can be shown that the distance D to be measured is related to the reference distance DR as follows: i D=DR (1/tR )/(1/t) where tR is the propagation time over the distance DR in the reference unit 12. This relationship is independent of both temperature and humidity. Thus, the outputs of the counters 46 and 78 are provided as inputs to the microprocessor 90 in the utilization means 14. The microprocessor 90 is appropriately programmed to provide an output which is proportional to the ratio of the outputs of the counters 46 and 78, which in turn are proportional to the repetition rates of the respective trigger pulse trains of the measuring unit 10 and the reference unit 12. As described, this ratio is independent of temperature and humidity and, since the reference distance DR is known, provides an accurate representation of the distance D. The utilization means 14 further includes a display 92 which is coupled to and controlled by the microprocessor 90 so that an operator can readily determine the distance D. Experiments have shown that when the distance between the transmitting and receiving transducers is too small, reflections of the acoustic wave at the transducer surfaces has a not insignificant effect which degrades the measurement accuracy. Accordingly, it is preferred that each transducer pair be separated by at least a certain minimum distance, preferably about four inches. 超聲波測距儀 文檔類型及編號： 美國專利 5442592 摘要：一個(gè)超聲波測距儀可以通過測量單元和參考單元來抵消溫度和濕度的變化。脈沖序列被提供給相應(yīng)的計(jì)數(shù)器，脈沖的計(jì)數(shù)值輸出被利用來確定距離。 精密的儀器工具必須被校對。然而，這些裝置并不易于將其使用于自動化技術(shù)中。當(dāng)一個(gè)超聲波信號在兩點(diǎn)之間傳播，兩點(diǎn)之間的距離可以通過兩點(diǎn)間傳播時(shí)間的一半乘以信號的速度來得到。 當(dāng)兩點(diǎn)之間的傳播介質(zhì)為空氣時(shí)，超聲波的傳播速度是有溫度和空氣的濕度來決定的。參考單元和測量單元都是一樣的，都包含一個(gè)電聲發(fā)射器和一個(gè)電聲接收器。在每一個(gè)單元中，發(fā)射器和接收器組成一個(gè)反饋循環(huán)，這就使發(fā)射器產(chǎn)生一個(gè)聲音信號同時(shí)這個(gè)信號被接收器接收并被轉(zhuǎn)化一個(gè)電脈沖然后反饋給發(fā)射器，這樣就產(chǎn)生了一系列的重復(fù)的系列信號。在每一個(gè)單元中，脈沖都被送到一個(gè)計(jì)數(shù)器。只要所有的計(jì)數(shù)值都受到相同的溫度及濕度影響，通過相應(yīng)的計(jì)數(shù)值比例，這個(gè)測量組合就變得對這些變量相關(guān)度降低了。 四、 詳細(xì)描述 根據(jù)現(xiàn)在的繪圖，有結(jié)果表明，測量單元 10 和參考單元 12，都聯(lián)接起來組成可以利用的單元 14。眾所周知，通過利用電極 22 和 24 之間產(chǎn)生的電場，壓電材料 20 將產(chǎn)生壓力。因此，進(jìn)一步得知，當(dāng)聲波影響到接收器 18 的時(shí)候，這時(shí)會引起接收器上的壓電材料 26 產(chǎn)生機(jī)械變變形，同時(shí)產(chǎn)生一個(gè)電信號通過 28 和 30 這一對電極，雖然已經(jīng)對壓電傳感器作了說明，但是其他電聲裝置也可以利用，例如，靜電，駐極體或電磁類型。同時(shí)，放大器的輸出端與探測器 36 相連接，脈沖發(fā) 生器 38 然后產(chǎn)生一個(gè)觸發(fā)脈沖，這是提供給脈沖發(fā)生器 40，在為了提供靈敏度，該系統(tǒng)的傳感器 16 和 18 在通常情況下都是保持運(yùn)作的。這個(gè)振蕩信號被用來童工給調(diào)制器 了使發(fā)射機(jī) 16 右線的工作，最好的做法是提供幾個(gè)周期的共振頻率信號，而不是一個(gè)單脈沖或單周期。這樣的控制脈沖給調(diào)制器 44 傳送一個(gè)周期的突破口以出發(fā)發(fā)射機(jī) ，有相當(dāng)大的噪音輸入到放大器 34，以至于其輸出脈沖發(fā)射器 40引起了正當(dāng)周期變化，這個(gè)振蕩周期是用來提供給發(fā)射器 16 的電機(jī) 22 和 24，發(fā)射器 16 因此產(chǎn)生聲波 32 并作用于接收器 18，接收器 18 然后產(chǎn)生一個(gè)電脈沖如數(shù)放大器 34，這再次觸發(fā)脈沖發(fā)生器 40.。這脈沖串被用于計(jì)數(shù)器 46，以及脈沖發(fā)生器 40。傳播時(shí)間的 T 是聲波傳播之間的距離除以速度得來的。速度實(shí)在發(fā)射機(jī) 16 和接收器 18 之間這段空氣中傳播的速度，計(jì)數(shù)器 46 測量出發(fā)脈沖的重復(fù)率，這是因?yàn)槊}沖等于 1/T。 w 和 a 是比例不同壓力下在熱水汽和干燥的空氣不同的比熱容。 根據(jù)這項(xiàng)發(fā)明的基本原理，需要利用參考單元 12.。再次，傳感器除了其他類型壓電也可以被利用。觸發(fā)脈沖應(yīng)用到脈沖發(fā)生器 72 以控制調(diào)制器 74 通過連續(xù)振蕩器 76傳送一段脈沖串傳遞至發(fā)射機(jī) 70的觸發(fā)脈沖也用于計(jì)數(shù)器 78。 按照慣例，測量單元 10 和參考單元 12 空間上很接近，使該聲速在這兩個(gè)單元上是相同的?？梢缘贸鰷y量單元和參考單元的聯(lián)系如下 iD=DR(1/tR)/(1/t)。這個(gè)關(guān)系與空氣的溫度和濕度都是無關(guān)的。這個(gè)輸出與計(jì)數(shù)器 46 和 78 的輸出是成比例的，反過來也同測量單元 10 和參考單元 12各自的觸發(fā)脈沖串成比例。這個(gè)利用方法 12 更進(jìn)一步的包括了被微處理器控制的顯示器 92，所以設(shè)備可以確定距離 D。