超声波传感器工作原理：
①采用I/O触发测距，给至少10us的高电平信号；　　
②模块自动发送8个40kHZ的方波，自动检测是否有信号返回；
③有信号返回，通过I/O输出一高电平，高电平持续的时间就是超声波从发射到返回的时间。
　　　　　　　　 测试距离=(高电平时间*声速(340M/S))/2。(Ultrasonic sensor working principle:
(1) trigger the ranging with I/O to give at least 10us high level signal.
Module automatically sends 8 40kHZ Fang Bo to automatically detect whether there is a signal return.
(3) a signal is returned, and a high level is output through I/O. The duration of high level is the time from ultrasonic emission to return.
Test distance = (high level time * sound speed (340M/S)) /2.)