In the preface, it introduces the origin of selecting the topic, limits the object to be studied and briefly narrates the key points and difficulties in the paper.

绪言部分概要介绍了本文的选题缘起、界定了研究对象，并简要说明了本文的重点、难点。

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Here the "timely", including the timely review after class, three minutes of time to close your eyes to review the key points and difficulties, noon lunch break to review again, up to five minutes.

这里的“及时”，包括下课后及时复习，三分钟的时间闭上眼睛回顾本课重点和难点，中午午休要再次复习，至多五分钟。

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For the ROV status and difficulties of subsea manipulators and underwater tools, key points are discussed and analysed, and trends and Suggestions are given for the future research work.

然后分别针对水下机械手的研制思路和水下作业工具的研究状况及难点等进行了分析与评述，给出了相关建议。

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