![]() The analog value when the handle is pulled down is approximately 273. If we've dialed in the right combination, the the servo is free to fully pull down and move to this PWM value. To detect if the safe is opened, we tell the servo to pull on the handle by going to PWM value 80. Analog feedback is important when you tell the servo head to go to position 175, did it actually get there? Doing an analogRead() lets us know if we’ve arrived. It works extremely well.Īdditionally, we modified our servo to give us analog feedback (there are good tutorials 1 and 2 on how to do this). Rob had the breakthrough: using a nautilus design we can apply increasing torque to the handle as the servo head pulls on the string. A basic 83 oz-in servo worked ok, but it didn’t have enough throw to guarantee that it could pull the handle down far enough to the open position. We really wanted to use an off-the-shelf servo for cost and ease of use. You don’t want to let the handle rest under its own weight or it might fall onto the dials and get caught in the indents on disc C. A small spring is used to bring the shroud and handle back to return position when the servo returns to the rest position. It’s basically a shroud with an idler pin to rotate around with a 40 lbs cord attached to the end of the shroud. First, we had to model the safe handle then design and print a connector. We tried a few different methods to non-permanently attach to the handle. I had a few instances where the reed switch would open when the magnet was directly under the switch and close again when the magnet moved a few millimeters away. ![]() This was a horrible idea: reed switches detect relative proximity but are bad at detecting exact location. Originally, we tried a reed switch with a magnet built into the 3D printed coupler. We tried a few different methods to calibrate the dial. Any slack in the system will cause problems later. It’s worth noting that the tolerances of the coupler to dial and the rigidness of the hub and motor mounts are important. Once we zero the dial, the step count is used to determine on which digit the dial is. The encoder uses two interrupt pins connected to the interrupt pins on the Arduino. 12V was good because we had used a similar power supply and display for our Speed Trap project. ![]() We wanted a lower cost motor that had LOTS of resolution to measure the internal indents in disc C (covered in a later section). The counts per revolution was most important. We used a motor from Pololu for $40 that has the following specs We found that hubs with set screws would loosen quickly and wreak havoc with our control algorithms. This was attached to our motor with a 6mm clamping hub. The dial on our safe was modeled, and a coupler was 3D printed. ![]()
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December 2022
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