Robot Impedance Control and Passivity Analysis with Inner Torque and Velocity Feedback Loops

Impedance control is a well-established technique to control interaction forces in robotics. However, real implementations of impedance control with an inner loop may suffer from several limitations. Although common practice in designing nested control systems is to maximize the bandwidth of the inner loop to improve tracking performance, it may not be the most suitable approach when a certain range of impedance parameters has to be rendered. In particular, it turns out that the viable range of stable stiffness and damping values can be strongly affected by the bandwidth of the inner control loops (e.g. a torque loop) as well as by the filtering and sampling frequency. This paper provides an extensive analysis on how these aspects influence the stability region of impedance parameters as well as the passivity of the system. This will be supported by both simulations and experimental data. Moreover, a methodology for designing joint impedance controllers based on an inner torque loop and a positive velocity feedback loop will be presented. The goal of the velocity feedback is to increase (given the constraints to preserve stability) the bandwidth of the torque loop without the need of a complex controller.Comment: 14 pages in Control Theory and Technology (2016

Dynamics for variable length multisection continuum arms

Variable length multisection continuum arms are a class of continuum robotic manipulators that generate motion by structural mechanical deformation. Unlike most continuum robots, the sections of these arms do not have (central) supporting flexible backbone, and are actuated by multiple variable length actuators. Because of the constraining nature of actuators, the continuum sections can bend and/or elongate (compress) depending on the elongation/contraction characteristics of the actuators being used. Continuum arms have a number of distinctive differences with respect to traditional rigid arms namely: smooth bending, high inherent compliance, and adaptive whole arm grasping. However, due to numerical instability and the complexity of curve parametric models, there are no spatial dynamic models for multisection continuum arms. This paper introduces novel spatial dynamics and applies these to variable length multisection continuum arms with any number of sections. An efficient recursive computational scheme for deriving the equations of motion is presented. This is applied in a general form based on structurally accurate and numerically well-posed modal kinematics that assumes circular arc deformation of continuum sections without torsion. It is shown that the proposed modal dynamics are highly scalable, producing efficient and accurate numerical results. The spatial dynamic simulation results are experimentally validated using a pneumatic muscle actuated multisection prototype continuum arm. For the first time this enables investigation of spatial dynamic effects in this class of continuum arms

Modal kinematics for multisection continuum arms

This paper presents a novel spatial kinematic model for multisection continuum arms based on mode shape functions (MSF). Modal methods have been used in many disciplines from finite element methods to structural analysis to approximate complex and nonlinear parametric variations with simple mathematical functions. Given certain constraints and required accuracy, this helps to simplify complex phenomena with numerically efficient implementations leading to fast computations. A successful application of the modal approximation techniques to develop a new modal kinematic model for general variable length multisection continuum arms is discussed. The proposed method solves the limitations associated with previous models and introduces a new approach for readily deriving exact, singularity-free and unique MSF's that simplifies the approach and avoids mode switching. The model is able to simulate spatial bending as well as straight arm motions (i.e., pure elongation/contraction), and introduces inverse position and orientation kinematics for multisection continuum arms. A kinematic decoupling feature, splitting position and orientation inverse kinematics is introduced. This type of decoupling has not been presented for these types of robotic arms before. The model also carefully accounts for physical constraints in the joint space to provide enhanced insight into practical mechanics and impose actuator mechanical limitations onto the kinematics thus generating fully realizable results. The proposed method is easily applicable to a broad spectrum of continuum arm designs

Development of a dynamic simulator for a compliant humanoid robot based on a symbolic multibody approach

This paper reports on development of an open source dynamic simulator for the Compliant huMANoid robot, COMAN. The key advantages of this simulator are: it generates efficient symbolic dynamical equations of the robot with high degrees of freedom, it includes a user-defined model of the actuator dynamics (the passive elasticity and the DC motor equations), user defined ground models and fall detection. Users have the freedom to choose the proposed features or include their own models. The models are generated in Matlab and C languages, where the user can leverage the power of Matlab and Simulink to carry out analysis to parameter variations or optimization and also have the flexibility of C language for realtime experiments on a DSP or FPGA chip. The simulation and experimental results of the robot as well as an optimization example to tune the ground model coefficients are presented. This simulator can be downloaded from the IIT website [1]

Highly-Integrated Hydraulic Smart Actuators and Smart Manifolds for High-Bandwidth Force Control

Hydraulic actuation is the most widely used alternative to electric motors for legged robots and manipulators. It is often selected for its high power density, robustness and high-bandwidth control performance that allows the implementation of force/impedance control. Force control is crucial for robots that are in contact with the environment, since it enables the implementation of active impedance and whole body control that can lead to a better performance in known and unknown environments. This paper presents the hydraulic Integrated Smart Actuator (ISA) developed by Moog in collaboration with IIT, as well as smart manifolds for rotary hydraulic actuators. The ISA consists of an additive-manufactured body containing a hydraulic cylinder, servo valve, pressure/position/load/temperature sensing, overload protection and electronics for control and communication. The ISA v2 and ISA v5 have been specifically designed to fit into the legs of IIT’s hydraulic quadruped robots HyQ and HyQ-REAL, respectively. The key features of these components tackle 3 of today’s main challenges of hydraulic actuation for legged robots through: (1) built-in controllers running inside integrated electronics for high-performance control, (2) low-leakage servo valves for reduced energy losses, and (3) compactness thanks to metal additive manufacturing. The main contributions of this paper are the derivation of the representative dynamic models of these highly integrated hydraulic servo actuators, a control architecture that allows for high-bandwidth force control and their experimental validation with application-specific trajectories and tests. We believe that this is the first work that presents additive-manufactured, highly integrated hydraulic smart actuators for robotics