Advice on transforming stepper motor to wheel motion on sun-chasing plant car?

Short version: how do I get my plant-mobile to be less jittery?

Long version:

Hi folks, I am new to robotics and hobby electronics, and could use some advice on hooking up a motor for linear motion in a project involving a robot car platform.

My goal is to move a small potted plant back and forth along a narrow windowside table to keep it in the sun and avoiding the daily shadow that sweeps along from a mid-window frame. I have chosen a small robot car chassis kit and a 28BYJ-48 stepper motor in order to enable fine-grained linear motion and relative position tracking.

The mechanics have turned out to be the trickiest part of the project so far (and something I’m a complete noob at). Currently the robot looks like this, with the stepper motor mounted on the side. After a lot of googling + trial and error I’ve coupled the Double-D 5mm shaft of the motor to a 2mm shaft on which I’ve mounted a rubber band pulley, connected to the axle and wheels which I’ve sourced from this kit. These replace the larger wheels connected to the dc (servo?) motors in the original kit, and also make for a lower clearance which is nice since lower down = less shadow in my window setup.

The pulley hookup underneath. Pretty simple! Unfortunately this results in imprecise and jittery motion. The motor turns smoothly, but I suppose the tension from the stretchy rubber band overcomes the friction in the wheel only intermittently.

How do I make this better? I’m trying to come up with preferably lower-effort solutions that ideally require less machining… else I’ll need to buy some tools and replace more parts. I have a little hand drill I’ve been using for the plastic and a small wood saw and file for aluminum chassis tweaks. Some ideas below.

  • Maybe I can find some sort of timing belt or chain that’s less stretchy, like this but different size? Seems tricky to get something sized right or mounted with decent tension, but ideally could just swap out the rubber band/pulley in my current design.
  • Maybe a plastic chain could work too? Not a ton of room for a big sprocket down there though…
  • Some sort of worm gear? Perhaps I can mount the motor on top of the platform right above the axle and get the fit just right, but may require custom mounting.
  • Are there low-tech suggestions for customizing my platform (e.g. tune the position of the motor up an inch) that don’t involve 3D printing or woodworking? A pile of sticky note pads perhaps?
  • Perhaps I can buy a second motor and connect the wheels directly to them (with the couplers)? This would probably work but (a) 2x the power drain (?) and (b) seems like it would require a lot of sawing or modification to mount it to my existing platform.
  • Similar to above but a single wheel, with the motor turned inward? Easier to build and no new parts, but maybe less balanced?
  • Maybe an alternate platform/chassis would be a better fit?
  • I can give up on the stepper motor… most of the motion is to opposite sides of the table which seems feasible without precise positional control. I’m also building an alternate DC-motor version I was going to compare with stepper one. The downside of this is I wouldn’t be able to graph the light intensity/shadow as a function of position and time which I was hoping to do.

Would love your ideas or feedback! I’m very new to robotics so I’m likely overlooking some things.

submitted by /u/LunarLob
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