Abstract

This talk explores the possibility to construct two-terminal mechanical devices (one-ports) which are lossless and adjustable. To be lossless, the device must be passive (i.e. not requiring a power supply) and non-dissipative. To be adjustable, a parameter of the device should be freely variable in real time as a control input. For the simplest lossless one ports, the spring and inerter, the question is whether the stiffness and inertance may be varied freely in a lossless manner. It will be shown that the typical laws which have been proposed for adjustable springs and inerters are necessarily active and that it is not straightforward to modify them to achieve losslessness, or indeed passivity. By means of a physical construction using a lever with movable fulcrum device laws will be derived for adjustable springs and inerters which satisfy a formal definition of losslessness. A construction method will be described which does not require a power supply for physically realisable translational and rotary springs and inerters. The analogous questions for lossless adjustable electrical devices will be discussed.