Some interesting npvel ideas on how the compliance needs of manipulators may be met by changing the stiffness of a structure have been presented in a paper by Ang and Andeen [1]. The purpose of the present note is to outline a novel concept for another variable-stiffness mechanism that may be useful in automating the process of driving nails or piles. It is common knowledge that, when manually hammering a nail into wood, care must be exercised not to use excessive impact energy. Starting with a smaller than necessary impact based on judgement, the impact would normally be increased until a satisfactory rate of penetration is achieved. Therefore, to automate the process of nail driving, the appropriate level of impact loading needs to be determined. However, this may vary significantly owing to the variability in the strength and penetration resistance of wood and the properties of the nail. While very few articles dealing with research into cracks caused by nailing are available in the literature [2,3] (the present author has read the abstracts only as the articles are in Japanese), the author could not find any work on attempts to minimize such damage. Soft thin wood requires only an impact of small magnitude, and extra impact may damage the wood and/or the nail. On the other hand, thick hard wood may require a very large impact to penetrate the wood. There is a similar problem in driving piles in soil. The soil disturbance due to pile driving has been the subject of several recent papers [4-8]. Since soil strength and resistance vary considerably, the impact energy needed to drive a pile without causing soil disturbance is difficult to predict.
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