Flexure material

[nst]

Hi everybody,
I am designing a flexure bearing for a small motion application (+/- 1.5 mm) using a small voice coil motor. Room temperature operation. Load barely 15 gm. Compact.
Required life: 4 years at 1 stroke every 2 seconds.
Critical specifications: At any point in the range of motion (i)repeatability on the lateral position of axis better than 1 micron (ii) repeatability on tilt of axis, better than 2 arc minutes. Most of the times the actual motion will be not more than 1 mm towards one end of the entire range ( i.e. motion between 0.5 mm and 1.5 mm from undeformed position). So the flexures would not be fully reverse stressed. I am thinking of 0.1 mm thick discs of AISI 420(Hardness HRC 60).
Questions:
i) Any comments on this or any other flexure material, hardness, heat treatment, fatigue data (SN curves) etc. ?
ii) Know some good sources of high quality flexure material(AISI 420 or other)?
ii) Instead of flexures, how about sliding bearing with a controlled clearance around a cylindrical tube (around 10 mm) which can be preloaded from outside the cylinder with a permanent magnet? Feasible?
Low friction coating?: Diamond? Graphite? Ceramic?
PTFE based?
I guess PTFE wear rate may be high. Any experience with
any of these or other coatings?
How about storming brains?
Thanks a lot
-nstcook

 

[LTD]

Hmmm, 126 million cycles...

Carbon Fiber will give you exceptional life, and will not affect your voice coil...

unequal length "A arm" suspensions like in sports cars come to mind - with precision "swiss" watch maker type jewel bearings. If you can tolerate radial drift while maintaining the angular pointing tolerance.

Ruby balls (they are actually cheap)running in a carbide cylinder with no lube might work fine.

(check with a metrology company supplying probes, and get the balls before they drill holes in them to mount the shanks!)

There are many "inch worm" piezo actuators available too.

Many micro-stages or linear actuators. You mentioned load, but not acceleration or speeds, which are important!!!

Larry

 

Thanks Larry,

I have already started designing with a small voice coil motor with a circular symmetry and flexure bearings. Initial design with flexure discs looks promising although not very compact.

For position feedback two inexpensive options :
1) Linear hall effect sensor 2) PSD (position sensitive device)
I am testing the hall sensors (brand:Allegro) over long term and they seem to be stable but are very sensitive and are affected by stray magnetic fields so I will have to do some shielding with “mu-metal” or some similar material. I have no previous experience with PSD either.

Questions:
Anybody with experience with any of the sensors? Any other cheap option? I thought of LVDT or inductive sensor or capacitive but they are expensive.
Do you know any sources of Hall effect or PSD ?

Thanks for your time and attention.
-nstcook

 

Sorry, I forgot to mention the motion spec.
Its 0.5 mm in 30 milliseconds, anywhere in the range.
-nstcook

 

Geez, if you want CHEAP, chop up a CD into strips... and break apart the mechanism. use filled epoxy for stability, and build yourself a gadget that gives you DIRECT read-outs... You'll have to calibrate it, but 1mm travel won't be too bad.

There are "optical Scales" complete with readers for a couple hundred bucks commercially available beyond your resolution.

How many of these gadgets are you building?

microns are small, but not that small... you could do this easy and cheap with a flex / jewel bearing lever from a larger very cheap motion too.

What'cha buildn?