Underactuated Degrees Of Freedom Research Articles

Adaptive nonlinear robust control of an underactuated micro UAV

In this article, we investigate a robust nonlinear control strategy to solve the trajectory tracking for a micro unmanned aerial vehicle, the quadrotor. The control technique is the computed torque control (CTC), this technique is widely used in the robot manipulators control. The CTC technique needs a strong knowledge of the model. In this regard, the complete dynamic model of the quadrotor has been established by the Euler–Lagrange formalism. After that, a robust nonlinear H∞ controller has been designed to stabilize the system with robustness. The proposed controller takes into account the underactuation characteristic of the quadrotor, so the controller is designed for the actuated degrees of freedom (DOF) with their coupling with the underactuated DOF. For the position tracking, we propose the backstepping controller. In both controllers, nonlinear H∞ and backstepping, the integral action is considered to get a null steady-state error. In order to improve performance quality, the control law has been reinforced by an adaptive action. This last has been performed by the linear parameterization property and the neural networks. The results show efficiency in the parametric uncertainties.

Grasp Mode and Compliance Control of an Underactuated Origami Gripper Using Adjustable Stiffness Joints

Every robotic gripper requires an equilibrated solution towards the grasp adaptability, precision, and load-bearing capacity. A versatile soft robotic gripper requires adjustable grasp mode for objects with different sizes and shapes, and adjustable compliance for switching between soft mode for small loads and delicate objects and stiff mode for larger loads and heavier objects. In this paper, we present the design of a tendon-driven robotic origami, robogami, gripper that provides self-adaptability and inherent softness through its redundant and underactuated degrees of freedom (DoF). Robogami is a planar and foldable robotic platform that is scalable and customizable thanks to its unique layer-by-layer manufacturing process. The nominally two-dimensional fabrication process allows embedding different functional layers with a high fidelity. In particular, a polymer layer with adjustable stiffness enables the independent control of the stiffness for each joint. Using this feature, we can control the input energy distribution between different joints and hence the motion of the robogami. Here, we model the behavior of a single finger, and demonstrate the compliance control of the end effector along different directions in simulations and experiments. We also validate the gripper's task versatility in soft and stiff modes by assigning model-based joints stiffness for performing different grasp modes.

Feedforward dynamics for the control of articulated multi-limb robots

We describe a general approach for using linearizing feedforward control inputs for large degree of freedom (dof) multi-limb robots operating in scenarios involving motion and force constraints, and under-actuated degrees of freedom arising from the task and the environment. Our solution is general and has low computational cost needed for real-time control loops. It supports the tuning of the feedforward term to meet multiple task objectives. Being structure-based, it is able to easily accommodate changes in motion and force constraints that often occur in robotics scenarios.

CONTROL OF A COMPLIANT HUMANOID ROBOT IN DOUBLE SUPPORT PHASE: A GEOMETRIC APPROACH

Enhancing energy efficiency of bipedal walking is an important research problem that has been approached by design of recently developed compliant bipedal robots such as CoMan. While compliance leads to energy efficiency, it also complicates the walking control system due to further under-actuated degrees of freedom (DoF) associated with the compliant actuators. This problem becomes more challenging as the constrained motion of the robot in double support is considered. In this paper this problem is approached from a multi-variable geometric control aspect to systematically account for the compliant actuators dynamics and constrained motion of the robot in double support phase using a detailed electro-mechanical model of CoMan. It is shown that the formulation of constraint subspace is non-trivial in the case of non-rigid robots. A step-wise numerical algorithm is provided and the effectiveness of the proposed method is illustrated via simulation, using a ten DoF model of CoMan.

Towards semi-autonomous operation of under-actuated underwater vehicles: sensor fusion, on-line identification and visual servo control

In this paper we propose a framework for semi-autonomous operation of an under-actuated underwater vehicle. The contributions of this paper are twofold: The first contribution is a visual servoing control scheme that is designed to provide a human operator the capability to steer the vehicle without loosing the target from the vision system's field of view. It is shown that the under-actuated degree of freedom is input-to-state stable (ISS) and a shaping of the user input with stability guarantees is implemented. The resulting control scheme has formally guaranteed stability and convergence properties. The second contribution is an asynchronous Modified Dual Unscented Kalman Filter (MDUKF) for the on-line state and parameter estimation of the vehicle by fusing data from a Laser Vision System (LVS) and an Inertial Measurement Unit (IMU). The MDUKF has been developed in order to experimentally verify the performance of the proposed visual servoing control scheme. Experimental results of the visual servoing control scheme integrated with the asynchronous MDUKF indicate the feasibility and applicability of the proposed control scheme. Experiments have been carried out on a small under-actuated Remotely Operated Vehicle (ROV) in a test tank.

The control point concept for nonlinear trajectory-tracking control of autonomous helicopters with fly-bar

An alternative approach for automatic trajectory-tracking control of small unmanned helicopters with fly-bar is proposed. This approach uses the spatial three-dimensional coordinates of a point on the helicopter's local coordinate frame other than its centre of gravity, called the control point, and the helicopter's yaw angle as four control outputs. The helicopter is assumed to have four independent control inputs. With this choice of control outputs, the helicopter's input–output model becomes a square control system, which opens the possibility of implementation of many robust nonlinear control methods that are suitable for such systems. The helicopter, which has six rigid body degrees of freedom (DOFs), has two underactuated DOFs (UA-DOFs). It is proved that the zero-dynamics of the UA-DOFs are inherently stable, leading to a stable control system. A sliding mode controller is designed for trajectory-tracking of the outputs. It is verified via simulations that the response of the control outputs and UA-DOFs are in fact stable.

Modelling of the robotic Powerball®: a nonholonomic, underactuated and variable structure-type system

The Powerball® is the commercial name for a gyroscopic device that is marketed as a wrist exerciser. The device has a rotor with two underactuated degrees of freedom, which can be actuated by the appropriate motion of human or robot wrist axes. After the initial spin, applying the appropriate motion and torques to the housing leads to a spin-up of the rotor. Finding these torques intuitively is an easy task for human operators, but a complex task for a technical consideration, for example, in robotics. This article's main contribution is a novel dynamic model that considers friction effects. The presented model includes all three working principles of the device: free rotor mode and both modes of rotor rolling in the housing. The work introduces models with one and two degrees of freedom actuation, both of which are suitable for laboratory control experiments. An estimation of the friction is discussed, and both the simulation and the experimental results are presented to evaluate the models.