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Safe Data-Driven Model Predictive Control of Systems with Complex Dynamics
KTH, School of Electrical Engineering and Computer Science (EECS), Intelligent systems, Robotics, Perception and Learning, RPL.ORCID iD: 0000-0003-4933-1778
KTH, School of Electrical Engineering and Computer Science (EECS), Intelligent systems, Robotics, Perception and Learning, RPL.ORCID iD: 0000-0001-9516-6764
KTH, School of Electrical Engineering and Computer Science (EECS), Intelligent systems, Robotics, Perception and Learning, RPL. KTH, School of Electrical Engineering and Computer Science (EECS), Centres, Centre for Autonomous Systems, CAS.ORCID iD: 0000-0003-2965-2953
KTH, School of Electrical Engineering and Computer Science (EECS), Intelligent systems, Robotics, Perception and Learning, RPL.ORCID iD: 0000-0003-4173-2593
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

In this paper, we address the safety and efficiency of data-driven model predictive controllers (DD-MPC) for systems with complex dynamics. First, we utilize safe exploration of dynamical systems to learn an accurate model for the DD-MPC. During training, we use rapidly exploring random trees (RRT) to collect a uniform distribution of data points in the state-input space and overcome the common distribution shift in model learning. This model is also used to construct a tree offline, which at test time is used in the cost function to provide an estimate of the predicted states' distance to the target. Additionally, we show how safe sets can be approximated using demonstrations of exclusively safe trajectories, i.e. positive examples. During test time, the distances of the predicted trajectories to the safe set are used as a cost term to encourage safe inputs. We use a \emph{broken} version of the inverted pendulum problem where the friction abruptly changes in certain regions as a running example. Our results show that the proposed exploration algorithm and the two proposed cost terms lead to a controller that can effectively avoid unsafe states and displays higher success rates than the baseline controllers with models from controlled demonstrations and even random actions.

National Category
Control Engineering
Identifiers
URN: urn:nbn:se:kth:diva-306458OAI: oai:DiVA.org:kth-306458DiVA, id: diva2:1620620
Note

QC 20211221

Available from: 2021-12-16 Created: 2021-12-16 Last updated: 2022-06-25Bibliographically approved
In thesis
1. Safe data-driven control for robots with constrained motion
Open this publication in new window or tab >>Safe data-driven control for robots with constrained motion
2021 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Widespread deployment of robots in offices, hospitals, and homes is a highly anticipated breakthrough in robotics. In such environments, the robots are expected to fulfil new tasks as they arrive in contrast to repetitive tasks. Such environments are unstructured and may impose various constraints on a robot's motion. Therefore, robots should be able to solve new instances of complex problems where kinematic and dynamical constraints must be respected. In this thesis we investigate how to autonomously incorporate these constraints in a robotic problem specification and how to develop motion planning and control techniques that solve such a problem.

The combination of environment-imposed constraints including obstacles, with a robot's own limitations, e.g., actuation bounds, results in many scenarios where a robotic task becomes a non-convex problem. This inhibits the commonly assumed independence between motion planning and control, making various classical control approaches practically infeasible. In the first part of this thesis, we introduce our contribution toward autonomous incorporation of kinematic and dynamical limitations at planning level by representing the robot action space as a set of feedback motion primitives. We show how the dual path planning and path non-existence problems can be solved for a mobile robot even in presence of external disturbance using feedback motion primitives. We further extend the applicability of motion primitives to long-horizon planning problems and address complex tasks specified as linear temporal logic (LTL) formulas by introducing a guided search scheme. Ultimately in this part, we lay a theoretical foundation for automated construction of such feedback motion primitives for a large class of systems by decomposing their state-space into smaller regions where locally linear controllers can be synthesized.

In the second part of the thesis, we investigate the motion planning and control problem in the presence of dynamical uncertainty. In absence of accurate models (e.g., when a manipulator operates while it is in contact with the environment) we should be able to collect data from the interaction to make informed decisions toward task satisfaction. Data-driven approaches are becoming increasingly popular in robotic control under uncertainty and have enabled tackling a wide range of new tasks. However, methods developed to verify safety constraints for a controller are inherently model based. This motivated us to adopt a model-based approach to data-driven control that enables explorative data collection while ensuring system safety. Furthermore, we propose data collection policy alternatives to reduce the well-known distribution shift effect in model learning.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2021. p. 38
Series
TRITA-EECS-AVL ; 2021:80
Keywords
Data-driven control, System abstraction, Motion planning
National Category
Robotics and automation
Identifiers
urn:nbn:se:kth:diva-306460 (URN)978-91-8040-082-4 (ISBN)
Public defence
2022-01-21, Kollegiesalen, Brinellvägen 8, Stockholm, 14:00 (English)
Opponent
Supervisors
Note

QC 20211220

Available from: 2021-12-20 Created: 2021-12-16 Last updated: 2025-02-09Bibliographically approved
2. Safety Aspects of Data-Driven Control in Contact-Rich Manipulation
Open this publication in new window or tab >>Safety Aspects of Data-Driven Control in Contact-Rich Manipulation
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

A crucial step towards robot autonomy-in environments other than the strictly regulated industrial ones-is to create controllers capable of adapting to diverse conditions. Human-centric environments are filled with a plethora of objects with very distinct properties that can still be manipulated without the need to painstakingly model the interaction dynamics. Furthermore, we do not need an explicit model to safely complete our tasks; rather, we rely on our intuition about the evolution of the interaction that is built upon multiple repetitions of the same task.Accurately translating this ability in how we control our robots in contact-rich tasks is almost infeasible if we rely on controllers that operate based on analytical models of the contacts. Instead, it is advantageous to utilize data-driven techniques that approximate the models based on interactions, much like humans do, and encompass the varying dynamics with a single model. However, for this to be a feasible alternative, we need to consider the safety aspects that occur when we move away from rigorous mathematical models and replace them with approximate data-driven ones.

This thesis identifies three safety aspects of data-driven control in contact-rich manipulation: good predictive performance, increased interpretability for the models, and explicit consideration of safe inputs in the face of modelling errors or uninterpretable predictions. The first point is addressed through a model-training scheme that improves the long-term predictions in a food cutting task. In the experiments it is shown that models trained this way are able to adapt to different dynamics efficiently and their prediction error scales better with longer horizons. The second point is addressed by introducing a framework that allows the evaluation of data-driven classification models based on interpretability techniques. The interpretation of the model decisions helps to anticipate failure cases before the model is deployed on the robot, as well as to understand what the models have learned. Finally, the third point is addressed by learning sets of safe states through data. These safe sets are then used to avoid dangerous control inputs in a control scheme that is flexible and adapts to dynamic variations while effectively encouraging the safety of the system.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2022. p. 57
Series
TRITA-EECS-AVL ; 2022:3
Keywords
Robotic manipulation, model learning
National Category
Robotics and automation
Identifiers
urn:nbn:se:kth:diva-307662 (URN)978-91-8040-118-0 (ISBN)
Public defence
2022-03-04, U1, Brinellvägen 26, vån 6, Stockholm, 09:00 (English)
Opponent
Supervisors
Note

QC 20220203

Available from: 2022-02-03 Created: 2022-02-02 Last updated: 2025-02-09Bibliographically approved

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Safe DD-MPC(7543 kB)1117 downloads
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Mitsioni, IoannaTajvar, PouriaKragic, DanicaTumova, JanaPek, Christian

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