A General Formulation for Path Constrained Time-Optimized Trajectory Planning with Environmental and Object Contacts

summary

Video file (mp4)

The gist

A typical manipulation task involves computing joint torques and grasping forces for time-optimal motion while ensuring that grasp stability and all physical constraints, including dynamics,

In short

The work proposes a novel second-order cone program (SOCP) to solve time-optimal trajectory planning for robot manipulation. It integrates nonlinear friction cone constraints at both hand-object and object-environment contacts with robot dynamics and actuator limits. This formulation bridges geometric motion planning with time optimization by ensuring grasp stability during motion.

Key concepts

Second-Order Cone Program (SOCP)
A type of convex optimization problem that uses second-order cone constraints to model complex physical limitations, such as friction cones. It allows the planner to efficiently find a feasible trajectory that minimizes total time while respecting dynamic and contact constraints.
Friction Cone Constraints
These mathematical constraints define the limits on how much force can be applied at a contact point before slippage occurs. The paper uses specific approximations (PCWF and SFCE) to model these nonlinear friction behaviors accurately for both robot-object and object-environment interactions.
Time-Optimal Control Formulation
The goal is to minimize the total time $T$ taken to complete a task while satisfying all physical laws. This involves minimizing $T$ subject to the robot's equations of motion, object dynamics, and contact force limits, ensuring the fastest possible movement.
Contact Constraints (PCWF/SFCE)
These are specific mathematical models used to describe how forces interact at contacts. PCWF handles point contacts with friction on the environment, while SFCE uses an elliptic approximation for soft finger contacts with the object, providing a way to enforce grasp stability.

Terminology used across episodes

This episode discusses

The paper

A General Formulation for Path Constrained Time-Optimized Trajectory Planning with Environmental and Object Contacts · Read on arXiv

Department of Mechanical Engineering, Stony Brook University, USA · Munich Institute of Robotics and Machine Intelligence (MIRMI), Technical University of Munich

A typical manipulation task consists of a manipulator equipped with a gripper to grasp and move an object with constraints on the motion of the hand-held object, which may be due to the nature of the task itself or from object-environment contacts. In this paper, we study the problem of computing joint torques and grasping forces for time-optimal motion of an object, while ensuring that the grasp is not lost and any constraints on the motion of the object, either due to dynamics, environment contact, or no-slip requirements, are also satisfied. We present a second-order cone program (SOCP) formulation of the time-optimal trajectory planning problem that considers nonlinear friction cone constraints at the hand-object and object-environment contacts. Since SOCPs are convex optimization problems that can be solved optimally in polynomial time using interior point methods, we can solve the trajectory optimization problem efficiently. We present simulation results on three examples, including a non-prehensile manipulation task, which shows the generality and effectiveness of our approach.

DOI: 10.1109/IROS58592.2024.10801794

Transcript

Introduction to the show: ident: Robotics Radio. Generated commentary on the latest robotics and control papers.

Rosa: Today's paper: "A General Formulation for Path Constrained Time-Optimized Trajectory Planning with Environmental and Object Contacts".

Dev: A typical manipulation task involves computing joint torques and grasping forces for time-optimal motion while ensuring that grasp stability and all physical constraints, including dynamics, environment contact,

Rosa: First, who's behind it and why it matters.

Paper summary: Taro: Thinking about the authors and the title "A General Formulation for Path Constrained Time-Optimized Trajectory Planning with Environmental and Object Contacts," what do you see as the biggest practical implication of this work?

Rosa: The main implication is that we have a more mathematically rigorous framework for planning movements when you have multiple physical interactions happening simultaneously, which is something we need to do if we want robots to operate reliably in diverse, unstructured environments.

Dev: I think the title points to the fact that it offers generality; it's not just solving one specific manipulation problem but providing a general method that can be adapted for many different contact scenarios.

Taro: If this formulation is used widely, I imagine it could lead to more sophisticated autonomous systems where manipulators can interact with fragile objects or complex environments without needing extremely high-fidelity, real-time physics models for every single interaction.

Rosa: That's right; it allows us to focus our efforts on improving the fidelity of the friction cone constraints themselves rather than reinventing the core time-optimal planning structure from scratch every time we face a new setup.

Dev: From an engineering standpoint, this approach provides a solid foundation for developing online trajectory generation systems that can handle dynamic object interaction while respecting physical limits, which is critical for practical deployment.

Taro: Ultimately, this work gives us a tool to explore the space of possible optimal motions much more systematically than we could before, especially concerning those nuanced environmental contact forces.

Conclusion: Rosa: So, we've looked at how this paper tackles time-optimal trajectory planning by incorporating those friction constraints for both the hand and the object contacts, now let's wrap up what this whole thing means for us.

Dev: I think it boils down to giving us a unified mathematical way to handle all those complex physical interactions in a single optimization problem without having to write separate, messy code for every scenario.

Taro: Exactly, the authors are showing how they’ve built a framework that can manage the dynamics of the robot and its environment simultaneously using these SOC constraints. It's about making sure we don't just plan a path in empty space but a physically feasible one where everything—the robot, the object, and what it touches—moves correctly together.

Rosa: That makes sense from a field perspective; I wonder if this formulation is robust enough to handle the kinds of unexpected slips or soft impacts we see when operating outside a controlled lab setting for extended periods.

Dev: It's a crucial question for me; if we deploy this on a real robot, I need to know how quickly the solver can converge and what kind of latency we’re looking at when it has to re-plan mid-motion because something went wrong.

Taro: That brings up the point about system robustness; what happens when the environment misbehaves in an unforeseen way that violates those friction models? The paper lays out how you could potentially adapt the constraints to handle those deviations, which is important for autonomy.

Rosa: It sounds like this work gives us a solid theoretical foundation for designing safer, more adaptable robotic systems that can handle real-world complexity over longer durations.

Dev: From my side, it’s about creating a more predictable control loop where the constraints are clearly defined mathematical boundaries rather than just heuristics we have to guess at.

Taro: So the big picture here is moving from specific case studies to a general planning methodology that can be applied across different manipulation tasks with varying contact geometries.

Rosa: It seems like this paper provides a much more rigorous way for us to think about how robots should move through complex physical spaces, setting a new standard for trajectory generation.

Dev: And that rigor is what we need if we want these systems to operate reliably in demanding industrial or service settings where things get messy.

Taro: So the next big question is how this general formulation can be practically implemented and tested against those real-world scenarios we've been discussing.

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