Positioning device using negative spring-rate tensioning means

摘要

A positioning device for controlling a member such as a pen of a chart recorder, wherein the positioning motion is produced by a wire formed of temperature-responsive material (Nitinol) exhibiting a martensitic transformation which contracts the wire when its temperature is increased within a predetermined temperature range, and relaxes the wire when cooled within that range. The wire is maintained under tension by a spring having a negative spring-rate. This spring cooperates with internally generated forces and the elasticity of the wire material to produce contraction/expansion length changes of the wire in response to changes in its temperature. Electrical circuitry for controlling the wire temperature is disclosed, and includes means for inductively developing a frequency-modulated pulse-current through the wire.