如图1所示门形支架的一个立柱上用转轴固定一根U型导轨，导轨的另一端可由固定在顶梁上的电机控制其上下运动，使小球在导轨上按要求灵活滚动或移位。导轨以转轴处为原点，以厘米（cm）为单位标注位置（见图中放大部分）-As shown in Figure 1 a column of the portal type bracket with a fixed rotating shaft U-type guide rail and the other end of the guide rail can be by motor fixed on the top beam control the movement, the ball on the rail according to requirements for flexible rolling or shift. The guide rail to the axis of the origin, in order to centimeter (CM) as the unit of the location (see Figure).