历史是人们所创造的，但人们并非有意识、有目的地去创造历史，即他们的历史活动是一种无动机的行为。

History is created by people, but people do not create history consciously and purposely, which means that their historical activity is a sort of unmotivated behavior.

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在无刷直流电动机高性能速度跟踪系统中，由于PI控制的滞后性，无法根据未来动态行为产生较为理想的PWM波形。

In the environment of high-performance drives, the PWM controller can't produce a fine PWM waveform according to the future dynamic behavior, due to the lag of PI regulator.

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