所谓电力拖动方案是指根据零件加工精度、加工效率要求、生产机械的结构
当所有的指令执行完毕时，集中把输出状态寄存器的状态通过输出部件转换成被控设备所能接受的电压或电流信号，以驱动被控设备，这就是输出刷新阶段。PLC经过这五个阶段的工作过程，称为一个扫描周期。完成一个周期后，又重新执行上述过程，扫描周而复始地进行。扫描周期是PLC的重要指标之一，在不考虑第二个因素（与编程器等通信）时，扫描周期T为：
（读入一点时间×输入点数）+（运算速度×程序步数）＋（输出一点时间×输出点数）十故障诊断时间
显然扫描时间主要取决于程序的长短，一般每秒钟可扫描数十次以上，这对于工业设备通常没有什么影响。但对控制时间要求较严格，响应速度要求快的系统，就应该精确的计算响应时间，细心编排程序，合理安排指令的顺序，以尽可能减少扫描周期造成的响应延时等不良影响。
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