Load cells with a Potentiometer and 3d Printer (and math!)

 

Update (08/02/20): PLA has really poor creep behavior, so these work great, but only for a week or so. After a month of use, I lost all of the throw on these flexures.  I still think this is a worthwhile project and if you’re not leaving a load applied on your 3d printed flexures, they’re still a great solution. I want to experiment with other materials or reinforced filaments, but that’s pretty far down my list at this time.

Original Post:

I am very slowly working on a self cleaning cat litter box. One of the problems with automating it is detecting when a cat has made a “deposit”. I initially thought that I could measure in increase in the weight of the whole litter box system. Now my thinking is to detect the increase in mass of the cat getting into the litter box and then leaving, since I don’t want to run the litter box with a cat inside. It also avoids needing precise calibration of the system weight.

My first thought on this was: “No Problem! I’ll just find some cheap load cells and go for it!” It turns out that “cheap load cells” are rare as hen’s teeth.

I needed a super inexpensive way of measuring force. Enter a flexure.  Flexures are awesome because they’re really easy to model with beam bending formulas that anyone can use. They also pair really nicely with a 3d printer and a parametric model. I had already built some flexure spreadsheets based on JPE Precision Point.

Flexures, spreadsheets, 3d printers, and parametric models are an amazing team. You can design your flexure for planned loads, displacements, and fatigue life in a spreadsheet pretty easily. My preference is to have a cell with the factor of safety calculated from the maximum moment and the thickness of the flexure blade with conditional formatting so I know my factor of safety stays above 2.  That lets me ignore fatigue life for most of the stuff I make.

Then you take those dimensions and put them in a parametric model like this one.

It turns a potentiometer via a live hinge in the model. I’m tempted to revise it to have the flexure push a gear rack that turns a gear on the pot, but this was also a fun case for a live hinge. It’s designed that way to make it easy to injection mold, or to make an extrusion in the event the market for custom made flexure scales takes off.

So all of that makes it possible to make a short throw scale that will turn a potentiometer to measure the load on a flexure. Neat!

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