DOLPi Camera Chassis

thingiverse

This is a simple chassis for the Dolpi-Mech polarization imager developed by David Prutchi. The imager consists of a Raspberry Pi and Pi Camera to collect images, a printed filter wheel to hold polarizing films, a servo and Adafruit ServoHat to rotate the filter wheel, and a mounting plate to hold it all together. I won't document usage for the camera itself. David Prutchi has done an amazing job of that in his blog write-up. One thing I will mention is that the software in David's white paper won't work without some tinkering. In particular, the Adafruit ServoHat now uses libraries called ServoKit, so you'll need to adapt the software to use it. ServoKit requires Python 3, and the imager software also needs OpenCV, which takes a bit of work to get running on Python 3. It's not too tough to sort out. The camera base plate has standoffs for the Raspberry Pi and the Pi Camera. The holes are sized for M2 screws. I use 6 mm screws and drive them right into the plastic. I used a TowerPro SG92R micro-servo, and made a small printed spacer for it. To attach the servo, use 12 mm M2 screws. I use 6 mm M2 screws and washers on the filter wheel, both to hold polarizer film and to attach the servo horn. You'll probably need to drill out holes on the servo horn to accept M2 screws. You can use something like a #52 drill and the screws will cut their own threads in the horn. The polarizer film should be cut to just a bit less than 3/4 inch square. At the bottom of the camera base plate, there is a tab with a hexagonal pocket sized for a 1/4-20 nut. You can drop a nut in this pocket and use some electrical tape to keep it in place, then the base plate can be mounted to most standard photography tripods. You also would have the option to construct your own base with a 1/4-20 mounting stud. There are a couple of possible improvements that might need to be made. I found that the weight of the filter wheel caused the servo to oscillate uncontrollably when it tried to turn the wheel. I took care of this by using some double stick tape to stick makeup sponge on the camera base, between the filter wheel and the base. This added just enough drag to stop the oscillation. The 6mm screws used to hold polarizer film penetrate the filter wheel, and can get hung up on the base, causing rotation problems. Shorter screws, perhaps 4 mm, might fix this. Finally, I made the base look like Swiss cheese, with a bunch of holes, to reduce the amount of plastic used, and to hopefully mitigate warping of ABS. It should be pretty easy to edit the FreeCAD file to eliminate these holes if you think they are too silly.