I’ve decided it’s finally time to revisit the displacement sensor project I shared a couple of years ago. Overall, that design performed pretty well (I’ve made and used several of them since), but one significant frustration I’ve had with it is the form factor/mounting.

The above picture shows one option I’ve gone with when using it, just clamping the back in a Panavise. But it’s intended to be rigidly mounted with the fastener holes showing close to camera (and their mirrored buddies on the other side). Unfortunately, that means the mounts for it have to be pretty large. On top of that, there’s no good reference surface to align the sensing axis to whatever is being measured.

All that to say, time I wanted something round.

Having a cylindrical outer body, concentric with the sensing axis means I can just mount it up in a Vee block of some sort. That will make aligning test setup MUCH easier. Not only will this make mounting easier, but it also will reduce the error that mounting will introduce into the measurements. For Gen1, the differences between the plane along the bottom of the sensor and the plane of the mount will induce some deformation in the sensor body. The impact on the sensor readings should only be a small fraction of the actual alignment error of the mount, but not nothing.

So, I needed a way to roll up my flexures into a tube, and to do that, I decided to switch up my flavor of flexure and go with some diaphragm flexures.

The below cross-section shows a bit of what’s going on inside of the new sensor.

The two diaphragm flexures and the Housing come together to make a sub-assembly. For a stylus tip, I used a 6mm airsoft BB. Honestly, I hadn’t labeled the bin they were in well enough, and designed for them thinking they were ceramic…they were not. A hardened material would be preferable. As it is, the stiffness of the Hertzian contact for that plastic ball is acting like a spring in series with the flexures. So that deformation will be contributing measurement error, since the amount of deformation will depend on the material it’s measuring against and the stiffness of the flexures. But this deformation should be on the order of 10s or low 100s of nanometers in total, and it’s really only the difference in this deformation between materials/setups that is the error. So the error should be a fraction of those 10s/100s of nanometers…I can live with that.

On the back side, the knife edge is printed into the flexure’s hub. I refer to it as a ‘knife edge’ out of habit, but I actually intentionally went with a relatively blunt edge for this version. This is really the one kind of ‘experimental’ feature I tossed into this one.

One thing I discovered thanks to the comments from the video I shared on the Gen1 build is that PETG is naturally transparent to the IR wavelength being used. From the datasheet on the opto I used in Gen2 (RPI-352), the target wavelength is 800nm. And as the plot below (from this 2023 thesis) shows, PETG is highly transmissive at 800nm. So I think any taper in the ‘knife edge’ is going to result in additional nonlinearity in my signal since it’s only attenuating, not completely blocking, the light.

As I write this, it occurs to me I really need to just test out some options for fixing this correctly….but alas…

Anywho, I went with the dull knife.

Both flexures attach to the Housing with an array of 6 M3 fasteners. The goal with the overkill collection of fasteners was to try and force the stiff outer rims of the flexures and the housing to act as a single rigid body.