This paper describes the design of two spatial gravity compensation devices. The one is an arm-like structure with the same mobility (4dof) as the human arm, which is balanced by two springs. The other is a general suspension unit with full (6dof) mobility, balanced by a single spring. When link and spring masses are considered, rather than just the payload, the latter configuration needs an additional spring or a slight alteration of its structure for complete balance.
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Herder et al. (2000) studied this question.