Browsing by Subject "Series elastic actuators"

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Control strategies for series elastic, multi-contact robots
(2019-09-16) Thomas, Gray Cortright; Sentis, Luis; Djurdjanovic, Dragan; Bakolas, Efstathios; Chen, Dongmei
As robots designed for physical interaction with humans---humanoids, exoskeletons and beyond---make their entrance into society, understanding the limitations of their interaction behavior will be key to their effective use. The state of the art method for allowing such systems to be both compliant and force sensitive is to introduce mechanical springs into the joints of these robots, making them "series elastic". But this complicates the control of these robots, making it hard to separate truth from optimism in what they will be able to accomplish using feedback control. Robots are programmed in hierarchical layers, and each layer makes assumptions about the layer below it. The planning layer assumes the plan will be followed. The whole body controller layer assumes the actuators will supply whatever torque it specifies. And the actuator control layer assumes the actuator behaves like a linear system. This dissertation studies the interfaces between these layers as they are influenced by the choice to include series elastic actuation, hoping to resolve the mismatch between assumptions and guarantees that arise from this choice. These questions lead it naturally to the lowest of the layers, where a new system identification system allows the actuator to assume a bounded uncertainty model. The dissertation then refines the insights from studying uncertain SEA models into a simpler model that explains the most important factors. It uses this to design SEA controllers that go beyond the traditional limits of passivity. These insights also apply to the problem of strength augmentation exoskeleton control. Factor of 3 amplification results are reported on a tethered, 12 degree of freedom, powered, lower body exoskeleton with four passive joints using a simplified version of the controller and a far more advanced whole body control framework. These ideas are introduced in the context of the authors's work with various testbeds and state of the art robots including a point foot biped, the DARPA virtual robotics challenge simulator, the NASA R5 Valkyrie Humanoid, and the Apptronik Sagittarius Lower Body Exoskeleton.
Development of the ACT hand with a broad and precisely adjustable stiffness spectrum via series elastic actuators
(2024-08) Bae, Jung Hyun ; Deshpande, Ashish D.; Raul Longoria
Tendon-driven systems offer significant advantages for robotic manipulators, including low inertia and improved force control. The Anatomically Correct Testbed (ACT) robotic hand is one such system, developed to understand the intrinsic biomechanical and control features of human hands. However, this system has limited compliance, and controlling it is challenging due to its inherent nonlinearities and nonholonomic constraints. In this research, we integrated Series Elastic Actuators (SEAs) into the ACT Hand to address these challenges. SEAs were chosen for their ability to improve compliance by reducing environmental impact forces and significantly expanding the stiffness range spectrum, enabling easier achievement of the desired stiffness while maintaining stability. Unlike the original ACT Hand, where stiffness was passively determined, the SEA configuration allows the stiffness of the robot finger's end-tip in Cartesian space to be actively adjusted through both the physical manipulation of each SEA's stiffness and the positional control of the motors connected to the tendons. In the ACT Hand structure, the six tendons of the index finger, each serially connected to a spring for compliance, actuate four degrees of joint freedom to achieve the desired end-tip position. By implementing SEAs, we aimed to enhance adaptability and enable precise adjustment of the end-tip stiffness in the ACT Hand. To validate the effect of SEAs on adjustable stiffness, we conducted experiments by varying the stiffness of the SEAs and altering tendon tension via positional control. We then compared the stiffness ellipses of the ACT Hand index finger's end-tip with and without SEA implementation. The comparison revealed that SEAs enhanced the precision and control of adjustable stiffness across a broad spectrum. This improvement demonstrates that the implementation of SEAs in the ACT Hand enables the passivity bound for conservative stability to be easily maintained through active stiffness variation via position control. Additionally, SEAs enhance the ACT Hand's capability to adapt to various environments by fine-tuning stiffness in each direction within the Cartesian space.
Dynamically consistent trajectory planners and human-aware controllers for human-centered robots
(2019-05) Schlossman, Rachel May; Sentis, Luis
For robots to successfully be deployed as human assistants in a variety of applications, it is critical that the robots' controllers and planners are designed with the considerations of both the robots' and humans' abilities and needs. In space applications, where energy is a finite and limiting resource in missions, it may prove necessary to exploit the energy storing-component of series elastic actuators to meet the efficiency needs, while operating in harsh and varied environments. In human-occupied workplaces, robots can only provide the needed support to humans if the robot controller can properly reason about and react to humans' requirements and capabilities. This thesis presents and assesses strategies to address these kinds of scenarios. In Chapter 2, we present a trajectory optimization scheme based on sequential linear programming to leverage the energy-storing capabilities of series elastic actuators for high-performance tasks. We discuss the current limitations in optimization strategies for series elastic actuated robots. One of the difficulties of this planning problem is respecting all relevant, low-level actuator constraints and handling system nonlinearities in a computationally efficient manner. Our simulation and hardware experiments demonstrate the leveraging of compliance for faster motions as compared to those that are achieved by the compliant systems' rigid counterparts. Chapter 3 addresses the need for reactive synthesis to be employed to automatically devise human-aware robot controllers for scenarios in which humans and robots continuously collaborate. Through this approach, it is possible to synthesize high-level control policies that are formally guaranteed to meet human requirements. We present a case study in which a robot seeks to deliver work to a human so that the human is productive, but not stressed by her work backlog. We demonstrate the achievement of a human productivity-informed controller using a mobile manipulator robot that picks up and delivers work based on work backlog. One of the challenges of this problem is devising human productivity models that are practical and accurate. We explore a toy scenario in the hope that this research will introduce methodologies that can be generalized for more practical cases
High-performance series elastic actuation
(2014-08) Paine, Nicholas Arden; Sentis, Luis; Vishwanath, Sriram
Mobile legged robots have the potential to restructure many aspects of our lives in the near future. Whether for applications in household care, entertainment, or disaster response, these systems depend on high-performance actuators to improve their basic capabilities. The work presented here focuses on developing new high-performance actuators, specifically series elastic actuators, to address this need. We adopt a system-wide optimization approach, dealing with factors which influence performance at the levels of mechanical design, electrical system design, and control. Using this approach and based on a set of performance metrics, we produce an actuator, the UT-SEA, which achieves leading empirical results in terms of power-to-weight, force control, size, and system efficiency. We also develop general high-performance control techniques for both force- and position-controlled actuators, some of which were adopted for use on NASA-JSC's Valkyrie Humanoid robot and were used during DARPA's DRC Trials 2013 robotics competition.