Navvab Kashiri

WALK‐MAN: A High‐Performance Humanoid Platform for Realistic Environments

In this work, we present WALK-MAN, a humanoid platform that has been developed to operate in realistic unstructured environment, and demonstrate new skills including powerful manipulation, robust balanced locomotion, high-strength capabilities, and physical sturdiness. To enable these capabilities, WALK-MAN design and actuation are based on the most recent advancements of series elastic actuator drives with unique performance features that differentiate the robot from previous state-of-the-art compliant actuated robots. Physical interaction performance is benefited by both active and passive adaptation, thanks to WALK-MAN actuation that combines customized high-performance modules with tuned torque/velocity curves and transmission elasticity for high-speed adaptation response and motion reactions to disturbances. WALK-MAN design also includes innovative design optimization features that consider the selection of kinematic structure and the placement of the actuators with the body structure to maximize the robot performance. Physical robustness is ensured with the integration of elastic transmission, proprioceptive sensing, and control. The WALK-MAN hardware was designed and built in 11 months, and the prototype of the robot was ready four months before DARPA Robotics Challenge (DRC) Finals. The motion generation of WALK-MAN is based on the unified motion-generation framework of whole-body locomotion and manipulation (termed loco-manipulation). WALK-MAN is able to execute simple loco-manipulation behaviors synthesized by combining different primitives defining the behavior of the center of gravity, the motion of the hands, legs, and head, the body attitude and posture, and the constrained body parts such as joint limits and contacts. The motion-generation framework including the specific motion modules and software architecture is discussed in detail. A rich perception system allows the robot to perceive and generate 3D representations of the environment as well as detect contacts and sense physical interaction force and moments. The operator station that pilots use to control the robot provides a rich pilot interface with different control modes and a number of teleoperated or semiautonomous command features. The capability of the robot and the performance of the individual motion control and perception modules were validated during the DRC in which the robot was able to demonstrate exceptional physical resilience and execute some of the tasks during the competition.

Development and control of a series elastic actuator equipped with a semi active friction damper for human friendly robots

Compliance is increasingly being incorporated in the transmission of robotics actuation systems to cope with unpredictable interactions, improve the robustness of the robot and in some cases its efficiency. However, compliance also introduces some drawbacks as e.g.?reduced bandwidth of the controlled system and typically underdamped vibration modes which decrease the accuracy and stability margin of the controlled system. To tackle these issues, variable physical damping has recently been incorporated in such actuation systems. This paper presents the analysis, development, control, identification and experimental evaluation of a novel actuation system which embodies transmission characteristics such as passive compliance and variable physical damping. The first part of this paper introduces an analysis on how these two physical properties affect the performance of the actuation system with the second part analysing the mechatronic design and control in detail. Furthermore, a novel damping estimation method is presented. Results are presented to validate the results obtained in the analysis section advantages gained by employing such actuation approach and to show the effectiveness of the actuation unit in replicating and estimating desired mechanical impedance values. Whole realization process of a successful implementation of a variable impedance actuator.Comprehensive analysis on the effects of compliance and variable physical damping.Mechatronic implementation of the variable impedance actuator.Introduction of a novel mechanical impedance estimator for measuring physical damping.Experimental results validate the analysis and the whole mechatronic system.

Time-optimal trajectory planning of robot manipulators in point-to-point motion using an indirect method

In this study, an iterative method for computing the time-optimal point-to-point control of robot manipulators is studied, subject to limits on the actuator torques and actuator jerks. This method uses the indirect solution of an open-loop optimal control problem so that the optimal control problem is translated to a non-linear two-point boundary value problem (TPBVP). Since there are many difficulties in finding the switching points and solving the TPBVP, a simple iterative method is proposed. This method is based on solving the minimum-energy problem without the necessity of finding the switching point. So, the resultant TPBVP can be solved using the usual algorithms. In addition, since the solution of the TPBVPs is sensitive to the initial guess, a simple procedure for making proper initial guesses is introduced. Furthermore, since the inverse kinematic of redundant manipulators is an ill-posed problem, the general form for boundary conditions corresponding to the final time is considered. In addition, it is shown that the proposed method has no sensitivity for specifying the time-optimal trajectory of manipulators that may contain singular arcs.

On the stiffness design of intrinsic compliant manipulators

The incorporation of intrinsic compliance in robotic actuation systems has attracted the attention during recent years due to the considerable benefits which is not possible to achieve with conventional “stiff” actuation systems. However, despite the numerous compliant robots developed, a systematic method for tuning the passive elasticity of the individual joints is still missing. This tuning is typically performed using experimental trial and error processes and very little information on the criteria and methodologies used is available. This work studies the effects of passive compliance on the key parameters of the robotic systems including natural frequency, damping ratio, Cartesian stiffness and energy storage capacity. Criteria are then defined based on the desired performance of the system; and a method for the selection of the passive stiffness of compliant actuated arms is introduced. The proposed method is evaluated on a four degrees of freedom (DOF) compliant arm and the compliance of its joints is tuned. The sensitiveness of the main dynamic and static parameters of the robot with respect to the stiffness of joints is illustrated to show the effect of the compliance of each individual joint.

Dynamic modeling and adaptable control of the CompAct™ arm

The introduction of physical compliance in robotic actuation systems has attracted increasing attention during recent years, due to the considerable benefits it can provide with respect to interaction safety, mechanical robustness and energy efficiency. However, the incorporation of passive compliant elements also results in systems with more complex dynamics, oscillations and limited bandwidth, requiring the development of sophisticated control strategies. Recently, variable damping mechanisms have been proposed to improve the performance of robots driven by compliant actuators. This study presents the dynamic modeling of the CompActTM actuator, a series elastic actuator equipped with a semi-active friction damper named Variable Physical Damping Actuator (VPDA) and the extension of this model to the multi-DOF case. Based on the analysed model, a control strategy is designed to modulate the clutch normal force in order to adapt the system dynamics with the task requirements; to make the system “stiff” when a precise motion is needed, and to exploit the passive compliance of the actuator to make it “soft” in the case the flexibility of the system is desirable. Finally, simulation of the arm is performed to verify the effectiveness of the proposed control scheme.

Optimal control for maximizing velocity of the CompAct™ compliant actuator

The CompAct™ actuator features a clutch mechanism placed in parallel with its passive series elastic transmission element and can therefore benefit from the advantages of both series elastic actuators (SEA) and rigid actuators. The actuator is capable of effectively managing the storage and release of the potential energy of the compliant element by the appropriate control of the clutch subsystem. Controlling the timing of the energy storage/release in the elastic element is exploited for improving motion control in this research. This paper analyses how this class of actuation systems can be used to maximize the link velocity of the joint. The dynamic model of the joint is derived and an optimal control strategy is proposed to identify optimal input reference profiles for the actuator (motor position/velocity and clutch activation timing) which permit the link velocity maximization. The effect of compliance of the joint on the performance of the system is studied and the optimal stiffness is analyzed.

Proxy-based position control of manipulators with passive compliant actuators

In this work we introduce a position control scheme which is targeted at the enhancement of the safety of compliant joint robots. In addition to the necessity for accuracy and robustness that both serve as prerequisites for the successful performance of various tasks, the ability to safely handle unexpected events, such as communication failures or unintended interactions which may endanger the robot/human safety, is a paramount requirement. To achieve a smooth motion behaviour of compliant systems under different circumstances, damping control actions are essential. To this end, a novel proxy-based approach for compliant joint robots, integrated into a passivity-guaranteed controller, is proposed. The stability analysis of the proposed scheme is presented and the global asymptotic convergence, as well as the passivity of the control scheme, are analytically proven. The performance of the proposed approach is practically evaluated by means of experiments on a spatial robotic arm with passive compliant actuators, and is compared with that of a classical PD approach. Experimental results validate the ability of the proposed approach to inject damping in order to provide smooth and damped recovery when an interruption in task execution occurs.

Physical interaction detection and control of compliant manipulators equipped with friction clutches

This work focuses on the modeling and control of robotic manipulators powered by compliant actuation systems equipped with clutches for providing friction torque on demand. A novel control scheme is proposed for modulating the clutch friction torque in this particular class of compliant actuators to make the robot operate in “Rigid mode” when it does not interact with the environment to achieve high accuracy, bandwidth and controllability; meanwhile ensuring that the robot maximum static force is constrained to a maximum threshold permitting flexible reactions in potentially risky scenarios. The robot autonomously switches to “Compliant mode” (clutches off) when it interacts with external agents to exploit the advantages of compliance during contacts. Experimental results are presented to show the effectiveness of proposed approach in improving the robot performance (tracking accuracy) while still guaranteeing an interaction-friendly behavior when contact occurs.

The effect of temperature variation on the free vibration of a simply supported curved sandwich beam with a flexible core

In this article, the free vibration of a singly curved sandwich beam with a transversely flexible core under various temperature conditions is investigated. The beam is assumed to have a constant width with simply supported end conditions. The core and face sheets are considered to be made of materials with temperature-dependent mechanical properties. In this model, faces of the sandwich beam are treated as thin beams with negligible shear strain and flexural rigidities obeying Bernoullis assumptions. The analysis is based on the high-order sandwich panel theory. The core is assumed to possess vertical normal and shear stiffness and act as a medium that transfers its inertia load to face sheets. Equations of motion and boundary conditions are derived using Hamiltons principle. The variation of free vibration frequencies and eigenmodes of the beam with temperature variation considering the temperature gradient across the thickness is studied. The effect of geometrical parameters such as the ratios of the length and the thickness to the mean radius of the beam on the vibration response of the beam is investigated. It is found that the free vibration frequency of the beam would decrease when its temperature is increased.

Model-free force tracking control of piezoelectric actuators: Application to variable damping actuator

On a new demand of safe human-robot interaction for robotic applications, the Compact Compliant Actuator, named CompActTM, is recently developed with physical compliance and active variable damping. In this mechanism, a desired physical damping behavior is realized by generating a friction force which is actively controlled by piezoelectric actuators (PEAs). However, nonlinearities such as hysteresis and creep effect make difficult to precisely control the generated piezoelectric force. This paper focuses on a development of precise force tracking controller for PEAs. A time delay estimation (TDE) using a force feedback is newly proposed to compensate a hysteretic behavior of the PEA and external uncertainties without a mathematical model. Thanks to the force-based TDE, the proposed control is accurate, computationally efficient and easily implementable on the real PEA system. The proposed control scheme is experimentally verified on the CompActTM. Root-mean-square values of the steady-state error for step commands are kept as less than error ratio of 0.13 % and the closed-loop system bandwidth for sinusoidal commands of 20 N stroke is confirmed as about 11 Hz under 100 N payload. In addition, the stability of the proposed control is proved to be bounded-input-bounded-output (BIBO) stable.