Seek and Geek 7: Inchworm motor

While looking for actuators that could get both nm-level precision and mm-range length of travel I stumbled across inchworm actuators.

An animation of the inchworm concept can be found at:

https://en.wikipedia.org/wiki/Inchworm_motor

What kinds of things would I be concerned about if I were designing an inchworm motor?

Functional requirements

  • Step size
  • Stiffness
  • Holding force
  • Not destroying the piezo crystals
  • Frequency of operation

Design parameters

  • Step size
    • Depends on the expansion of the central piezo element
    • Depends on how the central piezo element is controlled (are we doing on/off control, or trying to carefully control its expansion?)
  • Stiffness
    • Shear stiffness of piezos
    • Any other elements in series with the piezos (what are they mounted to, for example?)
  • Holding force
    • Based on friction
  • Not destroying the piezos
    • Kinematic mounting, look out for edge loading
      • Flexures
      • Spherical tips
    • Not overloading the piezos, pay attention to mfger maximum compressive loads
  • Frequency of operation
    • Higher = more power dissipated?
    • Consult mfger literature

Also of concern:

  • Geometric goodness of the rod being gripped – if it changes diameter for example, this will impact the clamping strength
  • Surface finish of the rod being gripped – needs to be consistent for consistent stepping and clamping
  • Rod bearings – what’s guiding the rod? Will it exert a moment on the actuator?