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Jack Umenberger

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8 papers

2 author rows

NeurIPS Conference 2025 Conference Paper

Constrained Diffusers for Safe Planning and Control

Jichen Zhang
Liqun Zhao
Antonis Papachristodoulou
Jack Umenberger

Diffusion models have shown remarkable potential in planning and control tasks due to their ability to represent multimodal distributions over actions and trajectories. However, ensuring safety under constraints remains a critical challenge for diffusion models. This paper proposes Constrained Diffusers, an extended framework for planning and control that incorporates distribution-level constraints into pre-trained diffusion models without retraining or architectural modifications. Inspired by constrained optimization, we apply a constrained Langevin sampling method for the reverse diffusion process that jointly optimizes the trajectory and achieves constraint satisfaction through three iterative algorithms: projected method, primal-dual method and augmented Lagrangian method. In addition, we incorporate discrete control barrier functions as constraints for constrained diffusers to guarantee safety in online implementation, following a receding-horizon control that we generate a short-horizon plan and execute only the first action before replanning. Experiments in Maze2D, locomotion, and PyBullet ball running tasks demonstrate that our proposed methods achieve constraint satisfaction with less computation time, and are competitive with existing methods in environments with static and time-varying constraints.

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NeurIPS Conference 2022 Conference Paper

Globally Convergent Policy Search for Output Estimation

Jack Umenberger
Max Simchowitz
Juan Perdomo
Kaiqing Zhang
Russ Tedrake

We introduce the first direct policy search algorithm which provably converges to the globally optimal dynamic filter for the classical problem of predicting the outputs of a linear dynamical system, given noisy, partial observations. Despite the ubiquity of partial observability in practice, theoretical guarantees for direct policy search algorithms, one of the backbones of modern reinforcement learning, have proven difficult to achieve. This is primarily due to the degeneracies which arise when optimizing over filters that maintain an internal state. In this paper, we provide a new perspective on this challenging problem based on the notion of informativity, which intuitively requires that all components of a filter’s internal state are representative of the true state of the underlying dynamical system. We show that informativity overcomes the aforementioned degeneracy. Specifically, we propose a regularizer which explicitly enforces informativity, and establish that gradient descent on this regularized objective - combined with a “reconditioning step” – converges to the globally optimal cost at a $O(1/T)$ rate.

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ICRA Conference 2021 Conference Paper

Identifying External Contacts from Joint Torque Measurements on Serial Robotic Arms and Its Limitations

Tao Pang
Jack Umenberger
Russ Tedrake

The ability to detect and estimate external contacts is essential for robot arms to operate in unstructured environments occupied by humans. However, most robot arms are not equipped with adequate sensors to detect contacts on their entire body. What many robot arms do have is torque sensors for individual joints. Through a quantitative analysis, we argue that it is fairly likely for two distinct contacts on the robot’s surface to generate almost identical joint torque measurements. When this happens, the best contact estimate achievable is the set of possible contact positions, all of which would reproduce the measured joint torque. Searching for elements of this set is equivalent to solving to global optimality a nonlinear program. By combining rejection sampling with gradient descent, we propose a contact estimation method which in practice finds all local optima of the nonlinear program at real-time rates. In addition, we propose an active contact exploration method which falsifies spurious contact estimates in the set of local optima by making small motions around the robot’s current configuration. The proposed methods highlight the caveats of contact estimation from only joint torque, which, coupled with known limitations of such estimators, suggest that a more capable sensor is probably needed for robust whole-body contact estimation.

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NeurIPS Conference 2021 Conference Paper

Stabilizing Dynamical Systems via Policy Gradient Methods

Juan Perdomo
Jack Umenberger
Max Simchowitz

Stabilizing an unknown control system is one of the most fundamental problems in control systems engineering. In this paper, we provide a simple, model-free algorithm for stabilizing fully observed dynamical systems. While model-free methods have become increasingly popular in practice due to their simplicity and flexibility, stabilization via direct policy search has received surprisingly little attention. Our algorithm proceeds by solving a series of discounted LQR problems, where the discount factor is gradually increased. We prove that this method efficiently recovers a stabilizing controller for linear systems, and for smooth, nonlinear systems within a neighborhood of their equilibria. Our approach overcomes a significant limitation of prior work, namely the need for a pre-given stabilizing control policy. We empirically evaluate the effectiveness of our approach on common control benchmarks.

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NeurIPS Conference 2019 Conference Paper

Robust exploration in linear quadratic reinforcement learning

Jack Umenberger
Mina Ferizbegovic
Thomas Schön
Håkan Hjalmarsson

Learning to make decisions in an uncertain and dynamic environment is a task of fundamental performance in a number of domains. This paper concerns the problem of learning control policies for an unknown linear dynamical system so as to minimize a quadratic cost function. We present a method, based on convex optimization, that accomplishes this task ‘robustly’, i. e. , the worst-case cost, accounting for system uncertainty given the observed data, is minimized. The method balances exploitation and exploration, exciting the system in such a way so as to reduce uncertainty in the model parameters to which the worst-case cost is most sensitive. Numerical simulations and application to a hardware-in-the-loop servo-mechanism are used to demonstrate the approach, with appreciable performance and robustness gains over alternative methods observed in both.

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NeurIPS Conference 2018 Conference Paper

Learning convex bounds for linear quadratic control policy synthesis

Jack Umenberger
Thomas Schön

Learning to make decisions from observed data in dynamic environments remains a problem of fundamental importance in a numbers of fields, from artificial intelligence and robotics, to medicine and finance. This paper concerns the problem of learning control policies for unknown linear dynamical systems so as to maximize a quadratic reward function. We present a method to optimize the expected value of the reward over the posterior distribution of the unknown system parameters, given data. The algorithm involves sequential convex programing, and enjoys reliable local convergence and robust stability guarantees. Numerical simulations and stabilization of a real-world inverted pendulum are used to demonstrate the approach, with strong performance and robustness properties observed in both.

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ICRA Conference 2014 Conference Paper

Real-time planning with primitives for dynamic walking over uneven terrain

Ian R. Manchester
Jack Umenberger

We present an algorithm for receding-horizon motion planning using a finite family of motion primitives for underactuated dynamic walking over uneven terrain. The motion primitives are defined as virtual holonomic constraints, and the special structure of underactuated mechanical systems operating subject to virtual constraints is used to construct closed-form solutions and a special binary search tree that dramatically speed up motion planning. We propose a greedy depth-first search and discuss improvement using energy-based heuristics. The resulting algorithm can plan several footsteps ahead in a fraction of a second for both the compass-gait walker and a planar 7-Degree-of-freedom/five-link walker.

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ICRA Conference 2013 Conference Paper

System identification and control of a small-scale paramotor

Jack Umenberger
Ali Haydar Göktogan

This paper presents a methodology for the system identification of a light weight, small-scale parafoil suspended motorized aircraft, known as a paramotor. The study is concerned with the acquisition of linear models describing both the lateral and longitudinal dynamics of the aircraft, with an emphasis on practical techniques that can be implemented in the real world. The mathematical models developed in this paper are first validated by comparison with real flight data, before being employed in the design of a guidance, navigation and control system, the performance of which is demonstrated by real autonomous flight tests.

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