【文章內(nèi)容簡(jiǎn)介】 CLR C MOV A, R6 SUBB A, 0BBH； 計(jì)算時(shí)間差 MOV 31H, A ； 存儲(chǔ)結(jié)果 MOV A, R7 SUBB A, 3CH MOV 30H, A SETB EX0 ； 開(kāi)外部中斷 0 POP ACC POP PSW RETI 四、超聲波測(cè)距系統(tǒng)的軟件設(shè)計(jì) 軟件分為兩部分，主程序和中斷服務(wù)程序，如圖 3（ a）（ b） (c) 所示。主程序完成初始化工作、各路超聲波發(fā)射和接收順序的控制。 定時(shí)中斷服務(wù)子程序完成三方向超聲波的輪流發(fā)射，外部中斷服務(wù)子程序主要完成時(shí)間值的讀取、距離計(jì)算、結(jié)果的輸出等工作。 五、結(jié)論 對(duì)所要求測(cè)量范圍 30cm~200cm 內(nèi)的平面物體做了多次測(cè)量發(fā)現(xiàn)，其最大誤差為， 且重復(fù)性好?？梢?jiàn)基于單片機(jī)設(shè)計(jì)的超聲波測(cè)距系統(tǒng)具有硬件結(jié)構(gòu)簡(jiǎn)單、工作可靠、測(cè)量誤差小等特點(diǎn)。因此，它不僅可用于移動(dòng)機(jī)器人，還可用在其它檢測(cè)系統(tǒng)中。 思考：至于為什么接收不用晶體管做放大電路呢，因?yàn)榉糯蟊稊?shù)搞不好， CX20216集成放大電路，還帶自動(dòng)電平增益控制，放大倍數(shù)為 76dB，中心頻率是 38k 到40k，剛好是超聲波傳感器的諧振頻率 Ultrasonic distance meter Document Type and Number:United States Patent 5442592 Abstract:An ultrasonic distance meter cancels out the effects of temperature and humidity variations by including a measuring unit and a reference unit. In each of the units, a repetitive series of pulses is generated, each having a repetition rate directly related to the respective distance between an electroacoustic transmitter and an electroacoustic receiver. The pulse trains are provided to respective counters, and the ratio of the counter outputs is utilized to determine the 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 distance and, more particularly, to such apparatus which transmits ultrasonic waves between two points. Precision machine tools must be calibrated. In the past, this has been acplished utilizing mechanical devices such as calipers, micrometers, and the like. However, the use of such devices does not readily lend itself to automation techniques. It is known that the distance between two points can be determined by measuring the propagation time of a wave travelling between those two points. One such type of wave is an ultrasonic, or acoustic, wave. When an ultrasonic wave travels between two points, the distance between the two points can be measured by multiplying the transit time of the wave by the wave velocity in the medium separating the two points. It is therefore an object of the present invention to provide apparatus utilizing ultrasonic waves to accurately measure the distance between two points. When the medium between the two points whose spacing is being measured is air, the sound velocity is dependent upon the temperature and humidity of the air. It is therefore a further object of the,present invention to provide apparatus of the type described which is independent of temperature and humidity variations. SUMMARY OF THE INVENTION The foregoing and additional objects are attained in accordance with the principles of this invention by providing distance measuring apparatus which includes a reference unit and a measuring unit. The reference and measuring units are the same and each includes an electroacoustic transmitter and an electroacoustic receiver. The spacing between the transmitter and the receiver of the reference unit is a fixed reference distance, whereas the spacing between the transmitter and receiver of the measuring unit is the distance to be measured. In each of the units, the transmitter and receiver are coupled by a feedback loop which causes the transmitter to generate an acoustic pulse which is received by the receiver and converted into an electrical pulse which is then fed back to the transmitter, so that a repetitive series of pulses results. The repetition rate of the pulses is inversely related to the distance between the transmitter and the receiver. In each of the units, the pulses are provided to a counter. Since the reference distance is known, the ratio of the counter outputs is utilized to determine the desired distance to be measured. Since both counts are identically influenced by temperature and humidity variations, by taking the ratio of the counts, the resultant measurement bees insensitive to such variations. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing will be more readily apparent upon reading the following description in conjunction with the drawing in which the single FIGURE schematically depicts apparatus constructed in accordance with the principles of this invention. DETAILED DESCRIPTION Referring now to the drawing, there is shown a measuring unit 10 and a reference unit 12, both coupled to a utilization means 14. The measuring unit 10 includes an electroacoustic transmitter 16 and an electroacoustic receiver 18. The transmitter 16 includes piezoelectric material 20 sandwiched between a pair of electrodes 22 and 24. Likewise, the receiver 18 includes piezoelectric material 26 sandwiched between a pair of electrodes 28 and 30. As is known, by