Spherical Parallel Modulator Prototype

A prototype of an "agile wrist" spherical parallel modulator.

Software: Autodesk Fusion 360, Bambu Studio. Hardware: HITEC HS-85MG servos, 3 x 7 x 3 bearings, 3 & 2 mm screws, Bambu X1-Carbon. Materials: PLA.

I referenced the design of an “agile wrist” Spherical Parallel Manipulator mechanism from: An Approach for Obtaining Unique Kinematic Solutions of a Spherical Parallel Manipulator by Aibek Niyetkaliyev and Almas Shintemirov.

“The SPM consists of two pyramid-shape platforms, a base and a top mobile platforms, connected by three equally spaced legs, numbered by i = 1, 2, 3, each having two curved links. The axes of all joints, denoted by unit vectors ui, vi, and wi, intersect at a common center point, which is called the center of rotation.”

I started by creating a reference sphere with a diameter of 6 cm. From the sphere, I derived several planes and axes from which the joints would be spaced and aligned. I wanted the mechanism to be relatively small for short print times and low material usage.

I found three HITEC HS-85MG servo motors in the ECL parts bin to use for the automation of the joints. I found a model on GrabCAD and created additional models of the mounting and horn accessories I would be using to join the servos to the base by taking measurements with digital calipers.

I placed a servo at the edge of the sphere, with the axis of rotation of the horn aligned with the sphere’s centroid. I then used the circular pattern tool to replicate the servo about the sphere’s pole equidistantly.

I first designed a solid base around the aligned servos, then used the edges to generate an outline and used the pipe tool to create a wireframe version. This would allow for less material use, faster printing times, and lower weight. Future iterations will include a 1 cm thick bottom center platform for mounting.

I printed a test version of a single chassis in which the servo would be mounted to test for fit. After the test, I adjusted the radius of the wireframe pipe from 2mm to 4mm to increase stability and decrease artifacts from support removal.

From the servo horn and using the reference sphere’s axes, I designed the distal and proximal arms. I first used a model of a 3 x 7 x 3 micro-bearing from the McMaster Carr catalog, and aligned it toward the sphere centroid. I then built a housing around each and connected the housings by using the pipe tool along a three-point bezier curve which ran the reference sphere’s surface.

I adjoined bearings to those at the end of the distal arm assembly and created a housing around each. I joined the housings with a wireframe triangle and extruded a platform in the center.

I created a copy of each part individually in preparation for slicing in Fusion 360. I wanted to be particularly mindful of the orientation of each print to reduce artifacts. I exported the parts to Bambu Studio for slicing and printed the parts in a Bambu X1-Carbon printer using green basic PLA. I tested for fit, and adjusted tolerances accordingly.

When the hardware arrives, I'll finish assembling the Spherical Parallel Modulator and connect it to a Nvidia Jetson Nano for control of the servos. I'll implement the kinematics algorithm, and make note of issues and design improvement ideas for future iterations.

The next iteration will be based on the Agile Eye mechanism, designed by the Laboratoire de Robotique at Universite Laval. I would like to modify the design by giving it infinite continuous rotation enabled by a nested shaft system, with each shaft being individually belt-driven by stepper motors. I would also like to design the end effector to be able to accommodate a media assembly including a small microcontroller and media source such as a laser, light, or speaker.