LEGO-compatible case for OpenMV AE3 AI camera (for LEGO Technic / Spike Prime)

The camera

https://openmv.io/ has a pretty nifty Python-programmable AI/MV-capable camera that can be used for embedded applications, it's tiny (30mmx30mm with its original case) and uses very little power (so it can be used in conjunction with a LEGO® SPIKE™ Prime Hub without extra power source). It's called the AE3 and is currently rather pricey at $150, but it could be had for $80 during the Kickstarter campaign and for $100 shortly thereafter. The camera itself has no useful mounting points, save for four M1.6 screw holes.

The holder / case

Mounting geometry

Since I intend to use the camera with a LEGO® kit, I need something to hold and mount the camera using LEGO® standard pieces.

I have designed two variants of a housing:
...one with 'ears' that are spaced 7 units (studs) apart, five holes each:

...and another variant with holes in the back, 5 studs apart, three holes per row:

The actual camera 'cell' and the lens cap are identical in both cases (okay, almost, but you shouldn't care for the tiny details here), so even the lens caps are interchangeable between the variants.
Pick what is easier to integrate into your robot.

Lens alignment

In the case with the side 'ears' the camera lens' axis is exactly aligned with the second-from-top LEGO® pin holes and is centered between them, so if you lay a seven-unit beam with the middle hole centered on top of the lens, the leftmost and the rightmost holes of the beam will match the second-from-top mounting holes.

In the case with the holes on the back the camera lens' axis is exactly aligned with the topmost hole pair.

Note, that the actual camera axis may turn out to be slightly skewed or offset from that perfect centering, but this is mechanical assembly tolerances for you. Also, the camera module isn't perfectly centered inside the blue case it came with.

See also photos in the chapter below, the pivot axis going through the IMU center goes also through the lens axis.

IMU alignment

The accelerometer (IMU) is:

• for the case with 'ears': exactly mid-plane of the mounting ears. If side-looking holes were printable, the second-from-top holes should've been horizontal and then their axis would be the IMU pivot. Such, as it is, you need to add some LEGO geometry - see photo, the tan cross-axle pins are the pivot points going through the IMU

  

• For the case with rear-mounted pins the IMU is mid-plane one stud above the mounting holes layer. If you add two 7-hole beams across and then add two 'biscuit-connectors' facing front, you will have pivots coincident with the IMU nodal axis - see photo, the tan cross-axle pins are the pivot points going through the IMU

  

The button

The camera has a side button. Both versions of the case have an integrated pusher to actuate the button. It is designed in such a way, that it should flex and push the actual button without breaking. It is also designed in such a way, that if it breaks off, it remains captive and fixed in such a position, as to remain useful. However, I have not had one break off yet, so I could not validate that this design actually works as intended, pls. let me know in the comments should your button-pusher break - I'd love to hear if it remained functional after breaking.

QWIIC / UART Port

The cases are purposedly designed so, as to 'trap' the QWIIC-compatible cable. I wanted to prevent students from unplugging/replugging the cable at the camera, so as to protect the camera-side connector. Plug the cable into the camera first, then insert the camera into the holder, the cable gets locked in (see photos).

The USB Port

The protective cover is built so, that the USB-C cable can remain connected while the lens cap / protective cover is attached. The USB-C cable should not interfere with the case unless you have really big USB-C connectors, but then you'd have issues without the case too.

The lens cap

is fully symmetrical (save for the text on the cap) and the case base has a special protrusion to close off the other USB-C cable hole. If you mounted the holding magnets properly, the cap should attach whichever way around you put it on.

Printing

I have used PLA, best is black, as the camera itself is backlight-sensitive, the OpenMV own provided case helps with it, but black backing helps more. To be honest I haven't seen much of a difference between various plastic colours, so just pick your favourite.

Print with 0.2mm layer height (if you want ultra-smooth, use 0.1mm, but except for wasting your time you won't gain much, unless you are trying to avoid using a 4.8mm reamer on the mounting holes... YMMV).

Don't use supports. You will curse the day you were born if you do, especially for the case variant with pins on the back.

You may want to smoothen the mating surfaces between the cap and the body, the ones where the magnets are (all other surfaces have 0.2mm gap, so roughness there doesn't matter).

After printing try pushing the side button actuator, it may stick a bit at first, so just push it full in and let go, this should take care of it.

If your printer is not calibrated properly, especially if it is over-extruding, you will have hard time with this case. Please calibrate before printing.

Non-printable 'spices'

You will need

• four square magnets 5x5x2mm size.

• A 4.8mm reamer or drill bit (if you want the perfect LEGO® feel or if your printer isn't calibrated perfectly)

• An adapter cable to connect to Spike Prime / Technic LPF2 port. I have made a few of these and I am in the process of setting up an eBay store to sell them.

Pay attention to assembly orientation of the magnets, especially if you intend to make more cases: keep the orientation consistent between cases or you will not be able to swap the caps around. 

Insert both magnets in the base with north poles facing out and insert both magnets in the cap with north poles facing in (or the other way around, but keep it consistent or you will curse me and everyone else).

Since mistakes happen, I have designed small holes (should come out at least 1mm diameter when printed) to push the magnets out using a thin drill bit - just in case you need to get the magnets out again... for reasons...

Use the reamer (or drill bit) to bring the LEGO®-compatible holes to exact size and finish.