Ultrasonic distance meter
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.
A.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 accomplished utilizing mechanical devices such as calipers micro meters 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.
B.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 electro acoustic 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 are 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 becomes insensitive to such variations.
C.DETAILED DESCRIPTION
A. principle of ultrasonic distance measurement
1.the principle of piezoelectric ultrasonic generator
Piezoelectric ultrasonic generator is the use of piezoelectric crystal resonators to work. Ultrasonic generator the internal structure as shown in Figure 1 it has two piezoelectric chip and a resonance plate. When its two plus pulse signal the frequency equal to the intrinsic piezoelectric oscillation frequency chip the chip will happen piezoelectric resonance and promote the development of plate vibration resonance ultrasound is generated. Conversely if the two are not inter-electrode voltage when the board received ultrasonic resonance it will be for vibration suppression of piezoelectric chip the mechanical energy is converted to electrical signals then it
becomes the ultrasonic receiver.
2.the principle of ultrasonic distance measurement
Ultrasonic transmitter in a direction to launch ultrasound in the moment to launch the beginning of time at the same time the spread of ultrasound in the air obstacles on his way to return immediately the ultrasonic reflected wave received by the receiver immediately stop the clock. Ultrasound in the air as the propagation velocity of 340m / s according to the timer records the time t we can calculate the distance between the launch distance barriers that is: s=340t / 2
B .Ultrasonic Ranging System for the Second Circuit Design
System is characterized by single-chip microcomputer to control the use of ultrasonic transmitter and ultrasonic receiver since the launch from time to time single-chip selection of 8751 economic-to-use and the chip has 4Kof ROM to facilitate programming. Circuit schematic diagram shown in Figure 2. Draw only the front range of the circuit wiring diagram left and right in front of Ranging Ranging circuits and the same circuit it is omitted.
1.40 kHz ultrasonic pulse generated with the launch.
Ranging system using the ultrasonic sensor of piezoelectric ceramic sensors UCM40 its operating voltage of the pulse signal is 40kHz .Ranging in front of single-chip termination circuit P1.0 input port single chip implementation of the above procedure the P1.0 port in a 40kHz pulse output signal after amplification transistor T the drive to launch the first ultrasonic UCM40T issued 40kHz ultrasonic pulse and the continued launch of 200ms. Ranging the right and the left side of the circuit respectively then input port P1.1 and P1.2 the working principle and circuit in front of the same location.
2.reception and processing of ultrasonic
Used to receive the first lau
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超声波测距仪
摘要：提出了一种可以抵消温度的影响和湿度的变化的新型超声波测距仪，包括测量单元和参考标准。在每一个单位，重复的产生一系列脉冲，每一个重复直接关系到发射机和接收机之间的距离。脉冲串提供给各级的计数器，然后利用计数器所测得的数据进行距离的测量。
出版日期： 1995年8月15日
主审查员：罗保.伊恩j.
一、背景发明
本发明涉及到仪器的测量距离，更特别的是，这种仪器传送超声波于两点之间。精密机器必须校准。在过去，这已经可以利用卡钳、微米等工具来校准机械设备。不过，使用这种工具并不容易实现自动化。据了解，两点之间的距离可以通过测量波在两点之间的传播时间来确定。这样一个类型的波可以是一种超声波、或声、或波。当超声波传播于两点之间时，两个点之间的距离可以通过由超声波波速乘以它的传播时间，在合适的分离的两点。因此，此发明提供了一个通过仪器利用超声波准确测量两点之间距离的方法。
当任意两点之间的介质是空气时，声音的速度取决于温度和空气的相对湿度。因此，它是进一步的研究对象。本次的发明，提供的是独立于温度和湿度的变化的新型仪器。
二、综述发明
这项距离测量仪器发明是根据上述的一些条件和额外的一些基础原则完成的，其中包括一个参考单元和测量单元。参考和测量单元是相同的，每个单元都包括一个超声波发射机和一个接收机。参考单元的发射器和接收器之间的间隔是一个固定的参考距离，而测量单元的发射器和接收器之间的距离才是我们所要测量的部分。在每一个单元中，发射器和接收器都耦合了一个反馈回路，以使发射机产生有接收器接收的超声波脉冲，然后由接收器转换成一个电脉冲反馈到发射机，使产生一系列重复脉冲的结果。脉冲重复率是发射器和接收器之间的距离，成反比关系。在每一个单元，脉冲提供一个反馈。由于参考的距离是众所周知的声速，反比例产出利用数学期望的距离来衡量。由于这两方面都是相同的影响，温度和湿度的变化，采取的比例相同，由此产生的测量变得准确。
三、详细说明
一 超声波测距原理
1、压电式超声波发生器原理
压电式超声波发生器是利用压电石英晶体谐振器的工作。超声波发生器的内部结构，它有两个压电晶片和一个共振板如图1所示。当它的两个加脉冲信号的频率等于内在压电振荡频率芯片，该芯片将发生压电谐振，促进发展板振动共振超声产生。相反，如果两个电极间的电压时，共振极板会接受超声波共振，将振动抑制压电晶片的机械能转换为电信号，这时它就成为超声波接收器了。
2、超声波测距原理
超声波发射器向某一方向发射超声波，在发射时刻的同时开始计时，超声波在空气中传播，途中碰到障碍物就立即返回来，超声波接收器收到反射波就立即停止计时。超声波在空气中的传播速度为340m/s，根据计时器记录的时间t，就可以计算出发射点距障碍物的距离s，即：s=340t/2
二 超声波测距系统的电路设计
系统的特点是利用单片机控制超声波的发射和对超声波自发射至接收往返时间的计时，单片机选用8751，经济易用，且片内有4K的ROM，便于编程。电路原理图如图 2 所示。其中只画出前方测距电路的接线图，左侧和右侧测距电路与前方测距电路相同，故省略之。
1、40kHz 脉冲的产生与超声波发射
测距系统中的超声波传感器采用UCM40的压电陶瓷传感器，它的工作电压是 40kHz 的脉冲信号。前方测距电路的输入端接单片机 P1.0 端口，单片机执行上面的程序后，在 P1.0 端口输出一个 40kHz 的脉冲信号，经过三极管 T 放大，驱动超声波发射头 UCM40T，发出 40kHz 的脉冲超声波，且持续发射200ms。右侧和左侧测距电路的输入端分别接 P1.1 和 P1.2 端口，工作原理与前方测距电路相同。
2、超声波的接收与处理
接收头采用与发射头配对的UCM40R，将超声波调制脉冲变为交变电压信号，经运算放大器IC1A和IC1B两极放大后加至IC2。IC2是带有锁定环的音频译码集成块LM567，内部的压控振荡器的中心频率f01/1.1R8C3，电容C4决定其锁定带宽。调节R8在发射的载频上，则LM567输入信号大于25mV，输出端8 脚由高电平跃变为低电平，作为中断请求信号，送至单片机处理。前方测距电路的输出端接单片机INT0 端口，中断优先级最高，左、右测距电路的输出通过与门IC3A的输出接单片机INT1端口，同时单片机P1.3和P1.4接到IC3A的输入端，中断源的识别由程序查询来处理，中断优先级为先右后左。
3、计算超声波传播时间
在启动发射电路的同时启动单片机内部的定时器T0，利用定时器的计数功能记录超声波发射的时间和收到反射波的时间。当收到超声波反射波时，接收电路 输出端产生一个负跳变，在INT0或INT1端产生一个中断请求信号，单片机响应外部中断请求，执行外部中断服务子程序，读取时间差，计算距离。
四、超声波测距系统的软件设计
软件分为两部分，主程序和中断服务程序，如图3（a）（b）（c）所示。主程序完成初始化工作、各路超声波发射和接收顺序的控制。
定时中断服务子程序完成三方向超声波的轮流发射，外部中断服务子程序主要完成时间值的读取、距离计算、结果的输出等工作。
五、结论
对所要求测量范围30cm~200cm内的平面物体做了多次测量发现，起最大误差为0.5cm，且重复性好。可见基于单片机设计的超声波测距系统具有硬件结构简单、工作可靠、测量误差小等特点。因此，它不仅可用于移动机器人，还可用在其它检测系统中。
思考：至于为什么接收不用晶体管做放大电路呢，因为放大倍数搞不好，CX20106集成放大电路，还带自动电平增益控制，放大倍数为76dB，中心频率是38k到40k，刚好是超声波传感器的谐振频率。
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