SKOOTR: A SKating, Omni-Oriented, Tripedal Robot

TL;DR

This work introduces SKOOTR, a radially symmetric tripedal robot that blends rolling and frictional contacts with a freely rotating central sphere to achieve omni-directional locomotion, obstacle traversal, and stair climbing. The authors provide an analytic inverse-kinematics model for one active leg at a time, plus a hybrid end effector design enabling multiple gaits (scooting, skating, shuffling) that can change heading rapidly without rotating the body. They validate these gaits in simulation and on a physical prototype, and quantify speed and turning performance, showing faster, tighter turning and greater stability than comparable tripedal or ballbot platforms, all at an estimated cost of ~US$500 with open-source CAD and control code. The results suggest SKOOTR as a versatile, educational, and potentially deployable platform for indoor navigation, mapping, and delivery in cluttered environments. The work highlights the value of exploring radially symmetric designs for robust, controllable locomotion beyond conventional bilateral robot forms.

Abstract

In both animals and robots, locomotion capabilities are determined by the physical structure of the system. The majority of legged animals and robots are bilaterally symmetric, which facilitates locomotion with consistent headings and obstacle traversal, but leads to constraints in their turning ability. On the other hand, radially symmetric animals have demonstrated rapid turning abilities enabled by their omni-directional body plans. Radially symmetric tripedal robots are able to turn instantaneously, but are commonly constrained by needing to change direction with every step, resulting in inefficient and less stable locomotion. We address these challenges by introducing a novel design for a tripedal robot that has both frictional and rolling contacts. Additionally, a freely rotating central sphere provides an added contact point so the robot can retain a stable tripod base of support while lifting and pushing with any one of its legs. The SKating, Omni-Oriented, Tripedal Robot (SKOOTR) is more versatile and stable than other existing tripedal robots. It is capable of multiple forward gaits, multiple turning maneuvers, obstacle traversal, and stair climbing. SKOOTR has been designed to facilitate customization for diverse applications: it is fully open-source, is constructed with 3D printed or off-the-shelf parts, and costs approximately $500 USD to build.

Figures (7)

• Figure 1: Overview of the specifications and capabilities of SKOOTR. SKOOTR has three different support modes: all feet, two feet and the center sphere, or the center sphere and the edge of the surrounding cage. Each foot has two contact modes: rolling or frictional. SKOOTR can successfully perform several tasks often associated with either rolling or legged robots. Like rolling robots, it is capable of energy efficient rolling down inclines or on level surfaces with external perturbations. Like legged robots, it is capable of obstacle traversal and stair climbing.
• Figure 2: Two different turning maneuvers. Turning with rotation keeps the heading (in green) and orientation (in black) aligned throughout the maneuver, as demonstrated by the top-down view of a dog turning. Turning without rotation maintains the same orientation of the body throughout the maneuver, but changes heading. This is demonstrated by a brittle star that simply selects a different leading leg to initiate a change in heading. The turning radius, $r$, is defined by the radius of the largest circle that can be inscribed in the turnhowland1974optimal. Turning with rotation has a much larger $r$ than turning without rotation.
• Figure 3: Kinematic chain for a single leg of SKOOTR.
• Figure 4: Sim-to-Real execution of scooting in simulation (top) and on a real robot (bottom). Scooting is quasi-static and retains all four points of contact throughout the gait cycle: 3 legs and the center sphere. The active leg can either push (forwards) or pull (backwards) with the frictional foot, then rolls in the recovery stage, while the rest of the robot remains rigid and moves along rolling contacts.
• Figure 5: Sim-to-Real execution of skating in simulation (top) and on a real robot (bottom). With skating, the robot retains rolling contact via the center sphere and two wheels with rolling contacts. The active leg maintains the frictional foot mode throughout the entire stride. The frictional foot either pushes (forward) or pulls (backward) to propel the robot, then lifts off the substrate to return back to its starting position.