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Load cells

Load cells

Load cells

(OP)
My question is entirely based around electronic load cells.

So I need to be able to map where pressure is applied by the foot. I know they have those DR Scholl's machines that map feet, but they claim to use "2000 sensors". I don't need to map the foot to that level of specification. What I'm looking for is more like, where pressure is being applied and then if the pressure is moved from side to side, kind of like if the person applying the pressure swayed, reapplying pressure to the outside of the foot then adjusting it back to center or maybe to the opposite outside of their foot.

Now here's where I'm having trouble I was reading about different kinds of load cells, and it seems to me like I need a Piezoelectric type load cell only because the pressure will be dynamic. But a strain gauge cell also sounds like it would be applicable, because it's malleable. Can anyone point me in the right direction of which load cell I'm looking for.

Also, out of whichever cell is suggested, I have some follow up questions. How thick can the surface material be between the person's foot, and the load cell. It has to be a malleable surface of course. However a user of this product would be using a shoe'd foot. So I'm guessing the sensor must have a high sensitivity for it to read pressure shifts of a ~ 180 lb human through a shoe and then another surface maybe a half inch thick. Any thoughts on that? Thanks guys, I really do appreciate any help!

RE: Load cells

Google "piezoresistive pressure sensor" and you will get a lot of useful hits.

A Piezo resistive sensor can be had in lots of shapess, among them thin film sensors and the electronics needed is very simple.

You do not seem to need very high precision, so the 1 - 5 % accuracy they offer should be adequate.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Load cells

The Dr. Scholl's kiosks expect the user to take off their shoes... you say you want to leave the shoes on, so accuracy of the pressure point just dropped like a rock.

From your description, it sounds like you just need to know which way a person is leaning. Four cells in a diamond pattern should be plenty adequate for such a task.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Load cells

I wouldn't worry about the dynamics if using strain gauge cells. If you want to spend a lot of time and money up front you might consider doing an FEA analysis of a thin, strain gauged sheet of metal.

RE: Load cells

Three (or four) load cells under a small platform would be able to compute the location of the center of pressure on the platform with great accuracy. The surface resiliency and type of load cell is unimportant.

RE: Load cells

Piezo electric effects will lead to the a durable accurate response to force of acceleration and stress velocity but not position.

These are typically ceramics with Lead Zirconate Titanate (PZT).

For cheap experiments try Y5V 1uF caps 10M INA's to detect differential acceleration. The SMD caps are fairly microphonic and the decay time constant limits the integration to velocity and worse position. But for broadband acoustic g sensors at 0.1 cents each, you can load up a thick FR4 as a floor board with SMD's on the bottom side and INA's to detect the motion on a floor grid, integrated as a system as required.

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