small robot arm with ball race

• disclaimer: this robot barely works, please read the details down below before thinking about making this project

Video: https://youtu.be/KbjHq6FzcFs

things needed:

-resin printer

-Adruino Micro/Uno/Leonardo or something similar

-1.5mm drill bit to round out bearing surfaces

-small linear actuators with 6mm diameter stepper motors and 9mm travel

-thin enamel wire (e.g. salvaged from a broken 5V power supply transformer coil)

-electronics/soldering experience, some electronic parts for the magnet gripper (see LTspice diagram)

-lots of patience assembling this thing

This project had mainly two goals: feeling out the limits of miniature MSLA resin printing and testing small linear stepper motors directly attached to the microcontroller. While you "should" never attach a motor directly to an Arduino, those small stepper motors probably wont instantly kill your microcontroller. Both current draw and the body diodes used as freewheeling diodes are used out of spec, your mileage may vary.

Elements that work well:

-printed axles with printed "c-clips" work pretty good, using the 1.5mm drill bit on the bearing holes to smoothen the operation.

-the ball gripper consists of a small nail with enameled wire would around it as a coil and a permanent magnet placed on top wtih sticky tape. This way without power the ball attaches and a small pulse from a tiny perfboard pcb is enough to let the ball drop (see LTspice diagram, electric doorstops work similarly). The strength of the magnet force was weakend a bit by adding spacers between the nail and the magnet. The custom pulse circuit is optional, if you have a 9V battery and can make sure to only shortly pulse the coil this would also work.

-the basic kinematics work pretty well, the ball gripper is "passively" linked to the base, which keeps it in a horizontal position. The rest is pretty "standard" robot arm design. The maximum reach from a center point is about +/- 15 mm in both reach and height. Currently simple forward kinematics are used https://www.youtube.com/watch?v=M2geGw83Rt8

Elements that barely work:

-while "sturdy" for its size, tiny resin prints still only can handle so much force. Some of the thinner details can and will break when not being careful. Printing some spares is a must

-the electrical connections to the stepper motors are very fragile. Directly soldering enemal wire to the solder "pins" of the motors is possible, but multiple motors were trashed by breaking the even smaller enemal wire coming from the stepper motor coils

-the motors, even with the linear attachment are extremley weak, losing steps constantly and seizing even in seemingly low force situations. Also there seems to be quite some variation motor to motor

-while AliExpress has loads of those "6mm mini linear stepper motors" my guess is that this specific model will not be around for long, being probably surplus from some proprietary model

The ball race was added more as an afterthought, since the forces capable by the robot are so small, only the most simple, "least frictionary" tasks are able to be performed. I also added a "sump" for dropped balls and a lever that helps placing them in the ball race again.

A basic Arduino sktech for controlling the movement of the robot, as well as a Processing script for sending keyboard inputs via serial to the Arduino are provided (only on Thingiverse, since printables does not allow *.pde). I also made it optional to hook up a microswitch control panel using one of the analog pins (see infos in the Arduino code).

While I plan to add a bit more documentation in the future I don't think I will evolve the design much further, it is very frustrating working on such a fragile project.

For now please see the Arduino sketch, SketchUp model and photos as reference, I may add a more comprehensive documentation in the future. Feel free to ask in the comments if there are things that need more information.

Still I think it is worth sharing and I'm hoping some people find some inspiration form this project. Maybe some different, more beefy and less frustratring actuators are available in the future and someone wants to work on a remix, who knows ;)

Category: Robotics