Compact Arrangements of Cable-Pulley Type Zero-Free-Length Springs

A methodology to develop springs with zero-free-length (ZFL) characteristics is presented, and the configurations for placing the springs precisely on the manipulators are introduced. A spring–string arrangement installed between two separate links of a serial-type manipulator is employed and is divided into three regions for mounting, tensioning, and placing the string. The springs can develop ZFL characteristics if adequate length is ensured for mounting the springs. To shorten the length for placing strings, a reference length acquired from the link configurations is utilized. The minimization of the placing length can therefore be described clearly. Because the overextended springs and links occupied the workspace of the other links as a result of the long mounting length, the springs are reorganized using pulleys and wire winding configurations to shorten the mounting length. The springs can then be arranged in alignment on the links. As achieved with this additional arrangement, comprehensive spring configurations on the manipulators can be shown. Two examples are presented after deriving the spring configurations for ZFL characteristics and the configurations with wire winding, respectively.