【正文】 M567 輸入信號大于 25mV，輸出端 8腳由高電平躍變?yōu)榈碗娖?，作為中斷請求信號，送至單片機(jī)處理 。其部分源程序如下： RECEIVE0： PUSH PSW PUSH ACC CLR EX0 ； 關(guān)外部中斷 0 MOV R7, TH0 ； 讀取時間值 MOV R6, TL0 CLR C MOV A, R6 SUBB A, 0BBH； 計(jì)算時間差 MOV 31H, A ； 存儲結(jié)果 MOV A, R7 SUBB A, 3CH MOV 30H, A SETB EX0 ； 開外部中斷 0 POP ACC POP PSW RETI 對于一個平坦的目標(biāo)，距離測量包括兩個階段 :粗糙的測量和精細(xì)測量。 第四步：設(shè)置回波放大器的所得來規(guī)格輸出，假定是 3伏。測量過零時間與回波的振幅。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。 五、結(jié)論 對所要求測量范圍 30cm~200cm 內(nèi)的平面物體做了多次測量發(fā)現(xiàn)，其最大誤差為 ，且重復(fù)性好。 附件 2：外文原文（復(fù)印件） 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. 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. 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 d