Abstract:
Many soft robots struggle to produce dynamic motions with fast, large displacements. We develop a parallel 6 degree-of-freedom (DoF) Stewart-Gough mechanism using Handed Shearing Auxetic (HSA) actuators. By using soft actuators, we are able to use one third as many mechatronic components as a rigid Stewart platform, while retaining a working payload of 2kg and an open-loop bandwidth greater than 16Hz. We show that the platform is capable of both precise tracing and dynamic disturbance rejection when controlling a ball and sliding puck using a Proportional Integral Derivative (PID) controller. We develop a machine-learning-based kinematics model and demonstrate a functional workspace of roughly 10cm in each translation direction and 28 degrees in each orientation. This 6DoF device has many of the characteristics associated with rigid components - power, speed, and total workspace - while capturing the advantages of soft mechanisms.
Discussion:
Fig. 1. Photo and Dimensions of the Soft Stewart Platform: The platform has a diameter of 40.6cm and a strut-length of 25cm. The platform-to-platform distance ranges from 25.3cm to 33.2cm depending on the system’s state.
We kicked this project off with two main goals: We wanted to build a self-resetting hardware platform for some upcoming work in sample-efficient-learning, and we wanted to try pushing towards a soft system with the kinds of performance metrics that characterize rigid systems.
As a learning platform it has been great: durable, easy to use, with dynamics complicated and non-linear enough to be interesting, but not so tough to learn that we can’t benchmark against conventional methods. I imagine I’ll have more to say about this when the work has been published.
In terms of creating a performant soft system, I have been very impressed with the TPU HSAs. The precise performance characteristics are discussed in the paper, but we are able to reliably produce large forces, fast motions, and large displacements over 100’s of hours of near-continuous operation. In several areas (chiefly reliability and movement speed) we are more limited by our XM430-W350 Dynamixel motors than we are by any mechanical characteristic of the HSAs.
While the Soft Stewart Platform (SSP) doesn’t quite yet rise to the level of a “practical” device, that is at least partially a question of matching new capabilities to new applications. Traditional Stewart-Gough platforms were developed for testing in the automotive industry, and have since found their niche in applications requiring high precision and single-face mounting. Widespread deployment of SSPs will involve finding applications that leverage characteristics like contact-safety, compliance, and mechanical simplicity.
What is an HSA?
Vid. 1. The extensions shown here are representative of the motions used in the paper, and cover about half the total range of the actuators.
A Handed Shearing Auxetic (HSA) actuator is a cylindrical soft-mechanism that acts as a compliant rotary-to-linear converter. When one side of the cylinder is rotated (with the other fixed) the structure will either extend or contract depending on the direction of rotation. HSAs are used as the vertical struts on the the platform (see figure 1). The HSAs used in this project were designed and made by our collaborators in Ryan Truby’s Lab at Northwestern.
For an example of this behavior see the video which shows the robot moving under human control.