Nicholas Paine

Design and Control Considerations for High-Performance Series Elastic Actuators

This paper discusses design and control of a prismatic series elastic actuator with high mechanical power output in a small and lightweight form factor. A design is introduced that pushes the performance boundary of electric series elastic actuators by using high motor voltage coupled with an efficient drivetrain to enable large continuous actuator force while retaining speed. Compact size is achieved through the use of a novel piston-style ball screw support mechanism and a concentric compliant element. Generic models for two common series elastic actuator configurations are introduced and compared. These models are then used to develop controllers for force and position tracking based on combinations of PID, model-based, and disturbance observer control structures. Finally, our actuators performance is demonstrated through a series of experiments designed to operate the actuator at the limits of its mechanical and control capability.

Actuator Control for the NASA-JSC Valkyrie Humanoid Robot: A Decoupled Dynamics Approach for Torque Control of Series Elastic Robots

This paper discusses the actuator-level control of Valkyrie, a new humanoid robot designed by NASAs Johnson Space Center in collaboration with several external partners. Several topics pertaining to Valkyries series elastic actuators are presented including control architecture, controller design, and implementation in hardware. A decentralized approach is taken in controlling Valkyries many series elastic degrees of freedom. By conceptually decoupling actuator dynamics from robot limb dynamics, the problem of controlling a highly complex system is simplified and the controller development process is streamlined compared to other approaches. This hierarchical control abstraction is realized by leveraging disturbance observers in the robots joint-level torque controllers. A novel analysis technique is applied to understand the ability of a disturbance observer to attenuate the effects of unmodeled dynamics. The performance of this control approach is demonstrated in two ways. First, torque tracking performance of a single Valkyrie actuator is characterized in terms of controllable torque resolution, tracking error, bandwidth, and power consumption. Second, tests are performed on Valkyries arm, a serial chain of actuators, to demonstrate the robots ability to accurately track torques with the presented decentralized control approach.

Valkyrie: NASA's First Bipedal Humanoid Robot

In December 2013, 16 teams from around the world gathered at Homestead Speedway near Miami, FL to participate in the DARPA Robotics Challenge DRC Trials, an aggressive robotics competition partly inspired by the aftermath of the Fukushima Daiichi reactor incident. While the focus of the DRC Trials is to advance robotics for use in austere and inhospitable environments, the objectives of the DRC are to progress the areas of supervised autonomy and mobile manipulation for everyday robotics. NASAs Johnson Space Center led a team comprised of numerous partners to develop Valkyrie, NASAs first bipedal humanoid robot. Valkyrie is a 44 degree-of-freedom, series elastic actuator-based robot that draws upon over 18 years of humanoid robotics design heritage. Valkyries application intent is aimed at not only responding to events like Fukushima, but also advancing human spaceflight endeavors in extraterrestrial planetary settings. This paper presents a brief system overview, detailing Valkyries mechatronic subsystems, followed by a summarization of the inverse kinematics-based walking algorithm employed at the Trials. Next, the software and control architectures are highlighted along with a description of the operator interface tools. Finally, some closing remarks are given about the competition, and a vision of future work is provided.

Stability and Performance Limits of Latency-Prone Distributed Feedback Controllers

Robotic systems are increasingly relying on distributed feedback controllers to tackle complex sensing and decision problems, such as those found in highly articulated human-centered robots. These demands come at the cost of a growing computational burden and, as a result, larger controller latencies. To maximize robustness to mechanical disturbances by maximizing control feedback gains, this paper emphasizes the necessity for compromise between high- and low-level feedback control efforts in distributed controllers. Specifically, the effect of distributed impedance controllers is studied, where damping feedback effort is executed in close proximity to the control plant and stiffness feedback effort is executed in a latency-prone centralized control process. A central observation is that the stability of high-impedance distributed controllers is very sensitive to damping feedback delay but much less to stiffness feedback delay. This study pursues a detailed analysis of this observation that leads to a physical understanding of the disparity. Then, a practical controller breakdown gain rule is derived to aim at enabling control designers to consider the benefits of implementing their control applications in a distributed fashion. These considerations are further validated through the analysis, simulation, and experimental testing on high-performance actuators and on an omnidirectional mobile base.

A new prismatic series elastic actuator with compact size and high performance

This paper discusses design and control of a prismatic series elastic actuator with high mechanical power output in a small and lightweight form factor. We introduce a design that pushes the performance boundary of electric series elastic actuators by using high motor voltage coupled with an efficient drivetrain to enable large continuous actuator force while retaining speed. Compact size is achieved through the use of a novel piston-style ball screw support mechanism and a concentrically placed compliant element. We develop controllers for force and position tracking based on combinations of PID, model-based, and disturbance observer control structures. Finally, we demonstrate our actuators performance with a series of experiments designed to operate the actuator at the limits of its mechanical and control capability.

Feedback parameter selection for impedance control of series elastic actuators

The interest of series elastic actuators (SEAs) for legged robots has recently increased to achieve compliant interactions and efficient gaits. However, control of legged robots with SEAs is difficult due to the need to design controllers that take into account both torque and impedance feedback loops. The work presented here addresses this issue by proposing a critically-damped fourth order system gain selection criterion for a cascaded SEA control structure with inner torque and outer impedance feedback loops. Velocity filtering and feedback delays are taken into consideration for stability and impedance performance analysis. We observe and analyze the interdependence between torque and impedance feedback gains to achieve the desired closed loop performance. Based on this analysis we derive a simple gain design criterion to maximize the tracking performance of SEAs. Our final goal is to maximize the output impedance capabilities of SEAs in order to fulfill a wide range of application needs. In contrast to low impedance design studies, we focus here specifically on achieving the highest possible impedance gains of SEAs. Finally, experiments using our UT-SEA are conducted to verify our proposed approach. This study serves as a stepping stone towards utilizing and designing humanoid robots with SEA actuators for mobile behaviors and interaction with cluttered and unknown environments.

A Closed-Form Solution for Selecting Maximum Critically Damped Actuator Impedance Parameters

This paper introduces a simple and effective method for selecting the maximum feedback gains in PD-type controllers applied to actuators where feedback delay and derivative signal filtering are present. The method provides the maximum feedback parameters that satisfy a phase margin criteria, producing a closed-loop system with high stability and a dynamic response with near-minimum settling time. Our approach is unique in that it simultaneously possesses 1) a model of real-world performance-limiting factors (i.e. filtering and delay), 2) the ability to meet performance and stability criteria, and 3) the simplicity of a single closed-form expression. A central focus of our approach is the characterization of system stability through exhaustive searches of the feedback parameter space. Using this search-based method, we locate a set of maximum feedback parameters based on a phase margin criteria. We then fit continuous equations to this data and obtain a closed-form expression which matches the sampled data to within 2%-4% error for the majority of the parameter space. We apply our feedback parameter selection method to two real-world actuators with widely differing system properties and show that our method successfully produces the maximum achievable non-oscillating impedance response.

Sensitivity Comparison to Loop Latencies Between Damping Versus Stiffness Feedback Control Action in Distributed Controllers

This paper studies the effects of damping and stiffness feedback loop latencies on closed-loop system stability and performance. Phase margin stability analysis, step response performance and tracking accuracy are respectively simulated for a rigid actuator with impedance control. Both system stability and tracking performance are more sensitive to damping feedback than stiffness feedback latencies. Several comparative tests are simulated and experimentally implemented on a real-world actuator to verify our conclusion. This discrepancy in sensitivity motivates the necessity of implementing embedded damping, in which damping feedback is implemented locally at the low level joint controller. A direct benefit of this distributed impedance control strategy is the enhancement of closed-loop system stability. Using this strategy, feedback effort and thus closed-loop actuator impedance may be increased beyond the levels possible for a monolithic impedance controller. High impedance is desirable to minimize tracking error in the presence of disturbances. Specially, trajectory tracking accuracy is tested by a fast swing and a slow stance motion of a knee joint emulating NASA-JSC’s Valkyrie legged robot. When damping latencies are lowered beyond stiffness latencies, gravitational disturbance is rejected, thus demonstrating the accurate tracking performance enabled by a distributed impedance controller.Copyright

Stability and Performance Analysis of Time-Delayed Actuator Control Systems

Time delay is a common phenomenon in robotic systems due to computational requirements and communication properties between or within high-level and low-level controllers as well as the physical constraints of the actuator and sensor. It is widely believed that delays are harmful for robotic systems in terms of stability and performance; however, we propose a different view that the time delay of the system may in some cases benefit system stability and performance. Therefore, in this paper, we discuss the influences of the displacement-feedback delay (single delay) and both displacement and velocity feedback delays (double delays) on robotic actuator systems by using the cluster treatment of characteristic roots (CTCR) methodology. Hence, we can ascertain the exact stability interval for single-delay systems and the rigorous stability region for double-delay systems. The influences of controller gains and the filtering frequency on the stability of the system are discussed. Based on the stability information coupled with the dominant root distribution, we propose one nonconventional rule which suggests increasing time delay to certain time windows to obtain the optimal system performance. The computation results are also verified on an actuator testbed.

Impedance Control and Performance Measure of Series Elastic Actuators

Series elastic actuators (SEAs) have become prevalent in torque-controlled robots in recent years to achieve compliant interactions with environments and humans. However, designing optimal impedance controllers and characterizing impedance performance for SEAs with time delays and filtering are still underexplored problems. This article addresses the controller design problem by devising a critically damped gain design method for a class of SEA cascaded control architectures, which is composed of outer impedance and inner torque feedback loops. We indicate that the proposed gain design criterion solves optimal controller gains by maximizing phase-margin-based stability. Meanwhile, we observe a tradeoff between impedance and torque controller gains and analyze their interdependence in terms of closed-loop stability and overall impedance performance. Via the proposed controller design criterion, we adopt frequency-domain methods to thoroughly analyze the effects of time delays, filtering, and load inertia on SEA impedance performance. A novel impedance performance metric, defined as “Z-region,” is proposed to simultaneously quantify achievable impedance magnitude range (i.e., Z-width) and frequency range (i.e., Z-depth). Maximizing the Z-region enables SEA-equipped robots to achieve a wide variety of Cartesian impedance tasks without alternating the control structure. Simulations and experimental implementations are performed to validate the proposed method and performance metric.