4 ms·

SMA wires are not particular cheap. Rather, due to mass production of cellphones, small "pager" motors are incredibly cheap (tens of cents). From the video, it

by quanto 2y ago

SMA wires are not particular cheap. Rather, due to mass production of cellphones, small "pager" motors are incredibly cheap (tens of cents). From the video, it seems that the SMA wires used for precise control & counting of the gear teeth. Thanks for the awesome video.

geor9e 2y ago
It's pretty cheap https://www.sparkfun.com/muscle-wire-0-005-diameter-1-foot.html https://www.sparkfun.com/muscle-wire-0-005-diameter-1-foot.h... It's not the wire that's precise here - it just provides enough force to get a the stamped metal piece over the tooth of the plastic gear, then it slams into a hard stop, then the button batteries dump current into the other wire, which pulls it the other way, pushing the gear one tooth width, and slamming into the other hard stop. Each back and forth ratchet of the tooth turns the worm gear on the plunger the precise 0.05 units. Another cool part of these is the initial insertion - there's a huge torsion spring that stabs a hollow needle into the skin then fully retracts it, leaving the rubber tube. Then there's a little corner of the PCB with a cutout and power trace, you can stab with a paper clip to disable the pod in an emergency. Then there's this membrane that equalizes the air pressure in the pod if you're flying with it. It lets air through but not water. So you can dive in a lake with it too. I love clipping these things open and just marveling at the clever engineering for cheap mass production of such a sophistocated device.
- quanto 2y ago
  I agree. The wire itself is not where the precision is. It's the reciprocating mechanism done by the wire. Regarding price, my point still stands that wire is not cheap enough to motivate its use. Your link actually proves my point (1.6 USD/30cm). Pager/toy motors are costing cents.
  - geor9e 2y ago
    It seems absurd that hair thin thread costs more than a whole assembly of permanent magnets and copper coiled in a stator. Just thinking from first principles. Whats driving the cost? The processing? For scale, Omnipod only makes this one product and they're on the S&P 500 because they sell so many of them. I'd guess 10 cents in bulk but who knows. It's a SparkFun link for DIY hobbyists. They're not charging the bulk rate. They have to make a margin. But maybe it is more expensive. Then they probably choose it for the reliability, and a simple and quiet design. Or perhaps the button batteries don't have the energy to drive a pager motor.
  - archi42 2y ago
    The margin on this stuff can easily be 1000% when sold to hobbyists. Or rather costs of cutting the wire to length, packaging it, warehousing, etc. For a small number of orders, I wouldn't be surprised if this was done mostly by hand. Plus there is some risk the stock sells too slow, since it is so niche (compared to ESP32 or sensors). That's all priced in there, plus some healthy margin. While the pump manufacturer just puts a 1000m spool in his assembly line every few hours.
  - londons_explore 2y ago
    At high enough volumes, cost becomes the cost of the raw materials. NiTi wires are obviously made of Ni and Ti, both of which are ~3x the cost of copper. If you can design the wire to require 3x less material than the motor approach, it will win.

by quanto 2y ago

geor9e 2y ago

It's pretty cheap https://www.sparkfun.com/muscle-wire-0-005-diameter-1-foot.html https://www.sparkfun.com/muscle-wire-0-005-diameter-1-foot.h... It's not the wire that's precise here - it just provides enough force to get a the stamped metal piece over the tooth of the plastic gear, then it slams into a hard stop, then the button batteries dump current into the other wire, which pulls it the other way, pushing the gear one tooth width, and slamming into the other hard stop. Each back and forth ratchet of the tooth turns the worm gear on the plunger the precise 0.05 units. Another cool part of these is the initial insertion - there's a huge torsion spring that stabs a hollow needle into the skin then fully retracts it, leaving the rubber tube. Then there's a little corner of the PCB with a cutout and power trace, you can stab with a paper clip to disable the pod in an emergency. Then there's this membrane that equalizes the air pressure in the pod if you're flying with it. It lets air through but not water. So you can dive in a lake with it too. I love clipping these things open and just marveling at the clever engineering for cheap mass production of such a sophistocated device.

quanto 2y ago
I agree. The wire itself is not where the precision is. It's the reciprocating mechanism done by the wire. Regarding price, my point still stands that wire is not cheap enough to motivate its use. Your link actually proves my point (1.6 USD/30cm). Pager/toy motors are costing cents.
- geor9e 2y ago
  It seems absurd that hair thin thread costs more than a whole assembly of permanent magnets and copper coiled in a stator. Just thinking from first principles. Whats driving the cost? The processing? For scale, Omnipod only makes this one product and they're on the S&P 500 because they sell so many of them. I'd guess 10 cents in bulk but who knows. It's a SparkFun link for DIY hobbyists. They're not charging the bulk rate. They have to make a margin. But maybe it is more expensive. Then they probably choose it for the reliability, and a simple and quiet design. Or perhaps the button batteries don't have the energy to drive a pager motor.
- archi42 2y ago
  The margin on this stuff can easily be 1000% when sold to hobbyists. Or rather costs of cutting the wire to length, packaging it, warehousing, etc. For a small number of orders, I wouldn't be surprised if this was done mostly by hand. Plus there is some risk the stock sells too slow, since it is so niche (compared to ESP32 or sensors). That's all priced in there, plus some healthy margin. While the pump manufacturer just puts a 1000m spool in his assembly line every few hours.
- londons_explore 2y ago
  At high enough volumes, cost becomes the cost of the raw materials. NiTi wires are obviously made of Ni and Ti, both of which are ~3x the cost of copper. If you can design the wire to require 3x less material than the motor approach, it will win.