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 d