26W11 — Coupler Tester and Fun with Flexures

The last week or so has been all about gettin distracted! I have been continuing to run test parts in my tumble polisher, and while I waited, I decided to get started on sketching out a plan for my coupler testing.

But diving into that led me to a whole rabbit hole of fun playing around with some diaphragm flexure design concepts.

Below is a bit of my meandering through all of that. I’m quite curious if anyone is interested in these ‘rough sketch’ and brainstorming aspects of my design journey. So I’d love any feedback ya’ve got for me!

Coupler Tester

My current thinking is a 3 module design.

1. Torque Sensor module
2. Load Module
3. Axial Load Sensor Module

As I’m currently envisioning it, this will not allow for dynamic measurements, but I think I’m ok with just some static testing…at least to start.

By separating them into individual modules, I can use adjustments at the interfaces between them to vary the alignment and axial preload between tests.

Here is my first rough sketch of the ‘rotating’ portion of the system. One change I already know I’ll need is the addition of a second radial roller for each sensor module. With the single one each I’ve included in the sketch, they will not be properly constrained in rotation about X and Y. On the load cell side, I’m going to need to figure something out to not have the roller bearing interface influence load measurements.

I’m thinking I’ll make custom sensors for both the load cell and the torque sensor (although for the former I could just use some of the cheapo load cells I have on the shelf). My biggest hesitation is that if I use plastic-printed flexures for these, they will have horrible drift behavior and require regular recalibration. But arguably unnecessary custom sensors is exactly the sort of scope creep I find irresistible, so I’m at least going to take a stab at some design concepts.

For the Load Module, I am not going to include any axial support, like thrust bearings. My assumption is that the axial force will be balanced between the two coupler interfaces and no net axial force needs to be supported by the Load Module’s housing. So for the below, simplified free body diagram, I’m assuming F1=F2 and therefore FR=0. In fact, it occurs to me as I’m writing this, if FR does not equal 0 it will directly impact my axial load measurements. So I will need to take a more detailed look at this in the design.

…then I started thinking about the Axial Load Sensor, and it’s making me want to revisit an old project I have been eager to take another stab at….and now you see why it takes me months for each video.

Diaphragm Flexure-based Sensor

The project was the displacement sensor whose model is shown below.

The design has a razor blade attached to the moving element in the center (attached to the rod with a ball) and an opto-interrupter attached to the rigid outer frame.

This cross section shows the configuration in there a little more clearly.

I did some basic calibration and repeatability tests on it, and I was really happy with how it performed in terms of stability and repeatability.

But seeing it here alongside these 1–2–3 blocks, you can see that it’s definitely not a small sensor. And the arms that have to wrap around the opto to connect to the ‘rear’ flexure get in way when trying to actually hold the thing.

So I ended up bailing on that sensor at the time and going with the double compound flexure design I shared in this video.

But I really like the axisymmetric nature of the diaphragm flexure, and so I always wanted to come back to it. I think now’s as good a time as any.

You may be wondering why I’m so hung up on a displacement sensor for my load cell needs, but the same basic design can work for both. By tuning the stiffness of my flexures, I can have the range of displacement equal to my range of load values. But I find it easier to think about it as a displacement sensor, cause it’s more in my comfort zone.

My main goal with this new revision is something smaller with a cylindrical, grippable outer surface. To do this, I plan to move the opto and razor (or equivalent) outside of the pair of flexures. This opens up the interior for some hijinks.

And that is what I was having some fun tinkering on this week. I was/am looking at some options for adding ‘gear reduction’ elements.

Because the sensing elements, like opto-interupters, have a finite sensing range, they directly set the range of the full sensor if driven directly. Or, as my super artistic sketch below shows, x1=x2. I represent the diaphragm flexures as resistor-esque squiggles. Even though I show and upper and lower, I’m treating them as a single, axial spring.

So what I’m wanting to explore are designs where x2 does not equal x1.

The first concept I took a look at, and the one that started me down this path, was basically just to nest a second pair of diaphragm flexures inside another pair (please ignore the math, I stand by none of it…I was lounging in a hammock ¯*(ツ)*/¯).

This is also the only one I’ve modeled in Fusion thus far

…but I really just tossed it together to see what it would look like in a simulation.

The input (maxing out at ~3.5mm) is on the right, and the output (~1.5mm) on the left. So this config is giving a little better than 2:1 reduction. But this ‘model’ is really just for helping me to visualize the motion and sanity check that it should behave roughly like I’d expected.

So instead of tackling those difficult issues, I returned to the hammock to sketch up a second concept.

This one is basically just stacking three pairs in a line. There are a few things about this option that I like. Because they’re arranged in a line, the outer diameter can be kept smaller. And also because they are in a line, the flexures can all be of the same nominal stiffness, which makes the math much easier…I like this part very much.

The next variant I want to take a look at is something that will give a reversed ratio, where x2 > x1. I have a couple of ideas, but I haven’t put any of them to paper just yet. So I’ll have to update ya on those next time.