设计一个倒车检测系统，测量车尾与障碍物之间的距离，实现数字显示。测量精度达到20cm.车尾与障碍物之间的距离小于1m时开始报警-The design of a reverse detection system, measure the rear and the distance between obstacles and achieve the figures. Measurement accuracy of 20cm. Rear and obstacles 1m less than the distance between the alarm when