The vision for this project is to have a cute snail robot which crawls around on the fridge. Currently, I'm planning for it to be powered by a rechargeable battery. I would want it to last for roughly a month per charge, at least. I'd like for it to always be in motion, and move very slowly.

I made a prototype of the motion system. For actual locomotion, I'm planning to use a wheel with a TPU tire. To make it look more snail-licteke, it will first extend the head forward, and then move the body forward while retracting the head. The head will move using a crank attached to the wheel. To make this a bit more organic looking, I'll add a slot to the crank, so that there is some dwell time on extension and retraction, following movement 166.

I had some smaller steppers lying around, so I prototyped this movement:

This works well! Some parts need to be tuned - the TPU doesn't grip the fridge all that well, silicone or a softer rubber would probably be better. But, I'm satisfied that this mechanism will work.

The above test uses a small stepper motor, an Elegoo 28byj-48. It's pretty big and heavy, and way overkill for this project. I bought some of these small geared motors. They seem almost ideal for this case, since they're small, low-voltage, and are already geared down to run slowly. Many small motors run at very high RPM, which is the opposite of what we want here.

I was considering two ways for the robot to move. The first, and my preferred method, is for it to always be in very slow motion. The second is for it to periodically move at roughly a snail's pace. For the first method to be practical, I'll need to be able to run the motor for short increments. Regardless of which method I go with, it needs to be pretty quiet so as not to be annoying.

I did a test with the small geared motors. First I verified that I could run them for a short period (~10ms), and that would be enough for the output gear to turn a little, and to have decent torque. This was pretty good, but a bit jerk-y, which could make some noise. So, I tried PWMing the motor at a high frequency (187kHz), and ramping the duty cycle up and down to smooth things out. In thisI ramp the duty cycle up to 37.5% max. It's barely audible with the naked motor (although it will be a bit louder with things connected to the motor). It has pretty good torque, I think that this will work well.

I'm pretty confident that these motors will work, so next I'm going to work on testing the movement mechanism using this motor.