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arxiv: 2606.19233 · v1 · pith:6SJAQTDMnew · submitted 2026-06-17 · 💻 cs.RO

Mobile Pedipulation for Object Sliding via Hierarchical Control on a Wheeled Bipedal Robot

Yue Qin , Yulun Zhuang , Zelin Shen , Yanran Ding This is my paper

Pith reviewed 2026-06-26 20:24 UTC · model grok-4.3

classification 💻 cs.RO
keywords wheeled bipedal robotpedipulationobject slidingnonlinear model predictive controlhierarchical controlcontact modestrajectory optimization
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The pith

A wheeled bipedal robot can slide objects with its legs by using a hierarchical NMPC on a reduced three-body model that handles hip roll and contact modes.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper establishes that wheeled bipedal robots can perform planar object sliding tasks by treating their wheeled legs as manipulators inside a hierarchical control system. It centers on a nonlinear model predictive controller built from a reduced-order three rigid bodies dynamical model that includes the hip roll degree of freedom and multiple wheel contact states. This setup lets the controller regulate both locomotion and the forces applied to the object at the same time. Experiments on hardware confirm the approach works for retrieving a 1 kg object from under a desk and sliding a 4 kg object over 0.228 m.

Core claim

The proposed approach formulates a nonlinear model predictive controller (NMPC) based on a reduced-order three rigid bodies (TRB) dynamical model that explicitly accounts for the hip roll degree of freedom and multiple wheel-environment contact modes, which is essential for lateral stepping and pedipulation tasks. Within this framework, the NMPC simultaneously regulates robot locomotion and interaction forces, allowing the robot to stably execute both rolling and object manipulation behaviors. A trajectory-optimization-based robot-object motion planner is developed to generate reference motions that incorporate stick-slip transitions in ground-object contact.

What carries the argument

Nonlinear model predictive controller (NMPC) based on a reduced-order three rigid bodies (TRB) dynamical model that includes hip roll and discrete wheel-environment contact modes.

If this is right

  • The NMPC simultaneously regulates locomotion and interaction forces to support both rolling motion and object manipulation.
  • The trajectory-optimization planner produces reference motions that include stick-slip transitions at the object-ground interface.
  • Real hardware experiments demonstrate retrieval of a 1 kg object from under a desk and sliding of a 4 kg object over 0.228 m.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same reduced-model approach could be tested on tasks that require repeated direction changes while maintaining object contact.
  • Adding explicit friction estimation to the planner might improve performance when surface conditions vary during a single slide.
  • The hierarchical split between planner and NMPC could be applied to other wheeled platforms that lack dedicated manipulators.

Load-bearing premise

The reduced-order TRB model with explicit hip roll and discrete contact modes is accurate enough to support stable combined locomotion and force regulation on real hardware.

What would settle it

Repeated loss of balance or failure to achieve the planned sliding distance with the 4 kg object during hardware trials under identical conditions would show the model is not sufficiently accurate.

Figures

Figures reproduced from arXiv: 2606.19233 by Yanran Ding, Yue Qin, Yulun Zhuang, Zelin Shen.

Figure 1
Figure 1. Figure 1: The scooting pedipulation experiment. (a) Snapshots of the first [PITH_FULL_IMAGE:figures/full_fig_p001_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: The TRB model. (a) Definition of the world frame [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Overview of the proposed hierarchical control framework. (a) User inputs are sent to the robot-object motion planner, which generates a 2D TRB state [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 5
Figure 5. Figure 5: Wheeled bipedal robot retrieves a planar object from under a desk [PITH_FULL_IMAGE:figures/full_fig_p006_5.png] view at source ↗
Figure 4
Figure 4. Figure 4: Velocity tracking experiment. Top: snapshots during the combined [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗
Figure 7
Figure 7. Figure 7: Object position and velocity along the sliding direction in the simulated [PITH_FULL_IMAGE:figures/full_fig_p007_7.png] view at source ↗
Figure 6
Figure 6. Figure 6: Maximum object mass that Fig. 1 can slide in the scooting mode [PITH_FULL_IMAGE:figures/full_fig_p007_6.png] view at source ↗
read the original abstract

In this letter, we present a hierarchical control framework that enables wheeled bipedal robots to perform planar object sliding tasks with their wheeled legs. The proposed approach formulates a nonlinear model predictive controller (NMPC) based on a reduced-order three rigid bodies (TRB) dynamical model that explicitly accounts for the hip roll degree of freedom and multiple wheel-environment contact modes, which is essential for lateral stepping and pedipulation tasks. Within this framework, the NMPC simultaneously regulates robot locomotion and interaction forces, allowing the robot to stably execute both rolling and object manipulation behaviors. A trajectory-optimization-based robot-object motion planner is developed to generate reference motions that incorporate stick-slip transitions in ground-object contact. Two representative pedipulation motions, namely scooting and lateral sliding, are validated through real-world hardware experiments, in which the robot successfully retrieves a 1 kg object from under a desk and slides a 4 kg object over a distance of 0.228 m via scooting.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 2 minor

Summary. The paper presents a hierarchical control framework for wheeled bipedal robots performing planar object sliding via pedipulation. It formulates an NMPC on a reduced-order three-rigid-body (TRB) dynamical model that includes the hip roll DOF and discrete wheel-environment contact modes, enabling simultaneous locomotion and interaction force regulation. A trajectory-optimization planner generates reference trajectories that incorporate stick-slip transitions at the ground-object interface. Hardware experiments on two tasks (scooting and lateral sliding) are reported, including successful retrieval of a 1 kg object from under a desk and sliding of a 4 kg object over 0.228 m.

Significance. If the results hold, the work advances mobile manipulation for wheeled bipeds by showing that an explicit reduced-order model with hip roll and multi-mode contacts can support stable combined locomotion and force-regulated pedipulation in real hardware. The concrete task demonstrations (object retrieval and sliding) provide direct evidence of practical utility; the absence of parameter fitting or self-referential definitions in the core formulation is a positive feature.

major comments (1)
  1. [Experiments] Experiments section: the manuscript reports task success (1 kg retrieval, 4 kg slide over 0.228 m) but supplies no quantitative metrics such as RMS position or force tracking error, success rate across trials, or comparison against a baseline controller without the hip-roll term; these data are needed to substantiate that the TRB model with explicit hip roll is load-bearing for the claimed stability.
minor comments (2)
  1. [Abstract] Abstract and introduction: the phrase 'multiple wheel-environment contact modes' is used without an early reference to the specific mode set or switching logic; a forward pointer to the relevant model section would improve readability.
  2. Notation: the TRB model is introduced as 'reduced-order' but the reduction steps (which states or contacts are eliminated) are not summarized in one location; a short table or paragraph listing retained vs. neglected dynamics would clarify the modeling choices.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the constructive feedback and positive assessment of our work. We address the single major comment below and have prepared revisions to the experiments section accordingly.

read point-by-point responses

  1. Referee: [Experiments] Experiments section: the manuscript reports task success (1 kg retrieval, 4 kg slide over 0.228 m) but supplies no quantitative metrics such as RMS position or force tracking error, success rate across trials, or comparison against a baseline controller without the hip-roll term; these data are needed to substantiate that the TRB model with explicit hip roll is load-bearing for the claimed stability.

    Authors: We agree that additional quantitative metrics would strengthen the presentation. In the revised manuscript we will report RMS position and force tracking errors computed from the logged hardware data for both the 1 kg retrieval and 4 kg sliding tasks, together with the number of successful trials out of the total attempts performed. A direct baseline comparison that disables the hip-roll DOF is not feasible without fundamentally changing the TRB model and the contact-mode formulation; the hip-roll degree of freedom is required to generate the lateral forces and stepping motions demonstrated in the experiments. We will add a short paragraph in the experiments section explaining this modeling choice and why the reported tasks cannot be executed without it. revision: partial

Circularity Check

0 steps flagged

No significant circularity

full rationale

The paper's central claim is a hierarchical NMPC formulation on an explicit TRB reduced-order model with hip-roll and discrete contact modes, validated directly by hardware experiments (1 kg retrieval, 4 kg slide over 0.228 m). No equations, fitted parameters, or self-citations are shown to reduce the claimed performance or model accuracy to the inputs by construction. The derivation chain remains independent of the target results.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract-only review yields no explicit free parameters, axioms, or invented entities; the TRB model is presented as a modeling choice rather than a new entity with independent evidence.

pith-pipeline@v0.9.1-grok · 5705 in / 1081 out tokens · 19361 ms · 2026-06-26T20:24:08.531313+00:00 · methodology

discussion (0)

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Reference graph

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