What Is The Difference Between Parallel Positioners And Stacked Serial Kinematics?

When faced with a multi-axis motion application, many users stack motion stages, and in fact that is a fine approach for assemblies of just a few axes. But as applications become more complex, so do the equivalent stacks-of-stages, and very real and practical considerations begin to come into play.

• Stiffness.  Some stage manufacturers publish stiffness specifications in terms of axial deviation per unit force, but this is of little utility in estimating the dynamic performance of a stage …or a stack.  A more pertinent metric is the resonant frequency, as it integrates both the effective coefficient of stiffness of a mechanism and the summed mass of its construction.  (Accordingly, knowing Fres means you can easily estimate the possible step/settle time for a well-tuned closed-loop stage: approximately [3 Fres]-1).  In our experience, most high-quality conventional linear stages will exhibit resonant frequencies on the order of 75-120Hz, unloaded.  Stack them, and the resulting structure can have significantly limited responsiveness and long settling times.