µBOT v3 – project update

v3 was really really close to working.

The PCB arrived and I assembled it:

This picture was taken towards the end of the day. There are actually three steppers attached here. The one on the bottom left is different to the rest as I ran out of the threaded rod type stepper. The stepper on the top right is missing because it broke off as I was trying to remove a wheel.

These micro steppers are very easy to break. A small movement of the pins causes an internal connection break so that is something I will have to keep in mind for the next iteration. Some kind of mechanical support is neccesary, just soldering the pins on is asking for trouble. Maybe a dab of hot glue would do the trick.

I forgot to route the rx and tx between the usb-serial chip and the esp32 hence the two jumper wires. Apart from that it wasn’t too long until all of the steppers were soldered and moving:

(give the video a minute to load – this server provides many opportunities to practice patience :D)

The wheels were kind of hard to come by. After racking my brain for several minutes I settled on milk bottle caps as they are circular and have a dimple in the middle which makes it easy to drill a hole. This was the result:

The steppers did not have quite enough torque. A couple of points worth mentioning here. First thing is that the wheel diameter is quite large. By reducing that diameter down to the minimum physical requirement the torque requirement could be reduced by maybe 50% or more.

The second thing is the battery is probably double or triple the weight it needs to be. By reducing the battery weight and the wheel diameter I think this thing could propel itself.

3d printed wheels are really the way to go. Putting the wheel on before soldering is also a must because putting mechanical stress on the stepper while it is soldered is risking a pin disconnection like this:

That actually happened when I was trying to take the wheels off but it just was luck it hadn’t occured earlier.

The odd stepper out (bottom right in the pic below) got really hot. Way hotter than the other steppers and it took me a while to figure out why. The coil resistances are different. Some steppers are 40 ohms and others are 20. The threaded rod variety are all 40 ohm so they don’t get as hot.

This is a useful thing to know. The 40 ohm ones would be preferable as they stay at a reasonable temperature even with continuous operation. That being said the 20 ohm ones probably have more torque but they get too hot to touch which is not ideal.

Here are some more pics :

So v3 has been a useful test. Even getting this far is cool. For a few minutes it looked like the ESP32 was dead so to get things moving and tested was a nice jump forward.

v3 is not finished yet. The next step is to buy more steppers, 3d print some wheels and buy a new tiny lipo battery.

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