NU Robotics Robot Arm

The actuators are built around common 8308 brushless motors which were left over from another club project. The goal was to develop a low cost, mostly printed actuator. The drivers were Moteus r4.11's which were also left over from a previous project, so the bulk of the new cost of the actuators would be bearings and other hardware. 

We printed most of the components, including the stator mount, for which we used a high temperature glass filled PLA which was then annealed for higher temperature resistance. In theory this should have raised it's heat deflection temperature to around 130 °C, which was high enough to handle the heat of the stator during operation. 

The output plate was secured by preloading it against a pair of large 100mm OD deep groove ball bearings. A set of shoulder bolts secured the top and bottom output plates together by sandwiching the pair of bearings between them. This pre-load was designed to remove most of the play from the bearings. The shoulder bolts also serve as the posts for the planet gears, which needed to be placed slightly asymmetrically to ensure proper meshing as a result of the specific combination of gear tooth numbers chosen. 

In order to achieve a full 6 DoF for the arm, and given the fact that the wrist was limited to a single DoF for tendon routing reasons, a spherical shoulder design was adopted. To save weight further up the arm, this was implemented with a differential using a pair of the previously developed actuators, combined with a single base rotation DoF. 

To further reduce inertia, the elbow actuator was placed lower down and torque was transmitted through a pair of 90 degree bevel gears to the elbow output. This also had the benefit of making the packaging of this joint neater with the round actuator placed within the round upper arm. 

A small off-the-shelf actuator was used for the forearm joint, although the factory driver was removed and replaced with the more user-friendly r4.11 driver. The forearm axis is offset from the elbow in order to extend the elbow range of motion and allow the hand to reach targets closer to the base of the robot. 

The implementation of the arm was relatively straightforward given that the motor drivers call all be daisy chained with power and CAN for data. Each joint can easily be addressed from Python using a CAN to serial converter to communicate with the bus. For this demo, the robot was controlled with a 3D CAD mouse which was driving the target for one of the numerical IK solvers included in the Robotics Toolbox for Python. 

This was not the full tele-operated hand+arm demo that I had hoped for, but it was sufficient to show that the prototype arm seemed to meet our requirements. The arm was shown to be relatively fast, especially compared to previous stepper motor based designs, and the custom actuators worked as expected. The spherical shoulder kinematics do result in singularity issues if the arm is upright, however, which may be a reason that this arrangement is not very common.