Exploiting Physical Human-Robot Interaction to Provide a Unique Rolling Experience with a Riding Ballbot

TL;DR

This work addresses enabling hands-free, torso-based control of a dynamically stable riding ballbot to support seated users, including manual wheelchair users. It develops two control paradigms—hands-free impedance control (HICS) and hands-free admittance control (HACS)—built on a cascaded LQR-PI balancer and validated through duo-agent simulations and human-subject tests. Results indicate that pHRI-based HACS, particularly HACS-1 and HACS-3 variants, reduces braking effort and improves responsiveness, with novice riders achieving comparable braking performance and successful indoor navigation after modest training. The study demonstrates the viability of using torso motions to achieve safe, agile, hands-free indoor mobility, offering a personal unique rolling experience for manual wheelchair users and other riders.

Abstract

This study introduces the development of hands-free control schemes for a riding ballbot, designed to allow riders including manual wheelchair users to control its movement through torso leaning and twisting. The hardware platform, Personal Unique Rolling Experience (PURE), utilizes a ballbot drivetrain, a dynamically stable mobile robot that uses a ball as its wheel to provide omnidirectional maneuverability. To accommodate users with varying torso motion functions, the hanads-free control scheme should be adjustable based on the rider's torso function and personal preferences. Therefore, concepts of (a) impedance control and (b) admittance control were integrated into the control scheme. A duo-agent optimization framework was utilized to assess the efficiency of this rider-ballbot system for a safety-critical task: braking from 1.4 m/s. The candidate control schemes were further implemented in the physical robot hardware and validated with two experienced users, demonstrating the efficiency and robustness of the hands-free admittance control scheme (HACS). This interface, which utilized physical human-robot interaction (pHRI) as the input, resulted in lower braking effort and shorter braking distance and time. Subsequently, 12 novice participants (six able-bodied users and six manual wheelchair users) with different levels of torso motion capability were then recruited to benchmark the braking performance with HACS. The indoor navigation capability of PURE was further demonstrated with these participants in courses simulating narrow hallways, tight turns, and navigation through static and dynamic obstacles. By exploiting pHRI, the proposed admittance-style control scheme provided effective control of the ballbot via torso motions. This interface enables PURE to provide a personal unique rolling experience to manual wheelchair users for safe and agile indoor navigation.

Figures (7)

• Figure 1: (a) The riding ballbot, PURE, with its shroud. (b) A rider sitting on PURE and balancing. (c) A close-up image of omniwheel and ball utilized in the ballbot drivetrain.
• Figure 2: Modeling of rider-ballbot system in the (a) sagittal plane, (b) transverse plane, and (c) model that isolates the ballbot from the rider-ballbot model with physical human-robot interactions.
• Figure 3: Control block diagram for (a) hands-free impedance control scheme (HICS) and (b) hands-free admittance control scheme (HACS). Both schemes maintain the low-level LQR-PI balancing controller structure. HICS adjusts control gain associated with speed tracking, while HACS provides command speed utilizing measured physical human-robot interaction.
• Figure 4: (a) Simulated braking effort ($J$) for each type of control scheme subject to sensitivity parameters ($\nu$ or $\nu_{Py}$) from 0 – 1. (b) State and torque trajectories in each control scheme that produced the lowest braking effort. Top: trajectory of chassis tilt (orange) and torso lean angles (blue). Middle: drivetrain (orange) and torso torques (blue). Bottom: PURE displacement (orange), translational speed (blue), and command speed (dash blue).
• Figure 5: (a) Course layout for the braking test with a projected indicator. (b)–(c) An experienced mWCU riding PURE during the braking test.