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
- Kinematic mounting, look out for edge loading
- 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?