Low cost and high precision cobot for automated pipetting
The goal of this project is to design a low cost 6 DOF robotic arm. The project is to include everything from harmonic drive design to task planning. The most notable feature of this robot will be its impedance control with low cost hardware.
The idea for this project came about when I was working on a backdrivable high gear ratio hand crank fan for a class project where my team was designing 1-D electromechanical Flappy Bird. Shortly after prototyping a harmonic drive for the fan, I realized that it was quite an elegant design due to its low part count and therefore cost.
In the second week of this project I worked on refining the gearbox design and realized that it would my life much easier if most joints were modular. I also decided that the robot arm would be driven by nema 17 stepper motors with magnetic encoders.
My concerns about the strain wave gear creeping were validated. After leaving the actuator assembled overnight, I observed a noticeable 'speed bump' effect every time the input shaft rotated from the deformation of the strain wave gear. However, there was a path forward: I printed a TPU gear and the stiffness is so low that creep does not affect the operation of the gearbox.
In addition to discovering and solving this creep issue, I also spend some time knocking out the arm's base and also started to explore different end effectors.
I decided in Week 2 that I would use the FOC library to drive all the steppers because I happen to find this example of a torque controlled stepper with the super common L298N motor driver. Using this setup works out to be about 12 dollars per motor with encoder and driver which is a whole lot cheaper than using BLDC's (100+ per motor/driver combo) and there is no custom pcb design needed.
