当执行机构需要的不是控制量的绝对值，而是控制量的增量（例如去驱动步进电动机）时，需要用PID的“增量算法”。(When the executive body needs not the absolute value of the control volume, but the increment of the control volume (for example, to drive the stepping motor), we need to use the incremental algorithm of PID.)