Fixed-base Robots Research Articles

Multi-expert synthesis for versatile locomotion and manipulation skills.

This work focuses on generating multiple coordinated motor skills for intelligent systems and studies a Multi-Expert Synthesis (MES) approach to achieve versatile robotic skills for locomotion and manipulation. MES embeds and uses expert skills to solve new composite tasks, and is able to synthesise and coordinate different and multiple skills smoothly. We proposed essential and effective design guidelines for training successful MES policies in simulation, which were deployed on both floating- and fixed-base robots. We formulated new algorithms to systematically determine task-relevant state variables for each individual experts which improved robustness and learning efficiency, and an explicit enforcement objective to diversify skills among different experts. The capabilities of MES policies were validated in both simulation and real experiments for locomotion and bi-manual manipulation. We demonstrated that the MES policies achieved robust locomotion on the quadruped ANYmal by fusing the gait recovery and trotting skills. For object manipulation, the MES policies learned to first reconfigure an object in an ungraspable pose and then grasp it through cooperative dual-arm manipulation.

Single Swept Volume Reconstruction by Signed Distance Function Learning: A feasibility study based on implicit geometric regularization

In automated production using collaborative robots in a manufacturing cell, a crucial aspect to ensure the safety of workers and robots in human-robot interaction is to avoid collisions or detect unsafe situations. When detecting collisions, one approach is the use of swept volume to identify safe protective space for the operation or maintenance. To reduce the cost of collision checking due to the complexity of swept volume geometries, we propose to use Implicit Geometric Regularization (IGR) to train the signed distance function of the precomputed swept volume as a continuous collision detector during task trajectory execution. We validate the accuracy of the reconstruction given by Hausdorff distance and memory budgets over three trajectory instances followed by planar 2-link and PUMA-like fixed-base robot arms with polyhedral links and rotary joints. The simulation results and the error analysis of test scenarios suggest that it is feasible to model the signed distance function of a precomputed swept volume by IGR.

On-Line Precision Calibration of Mobile Manipulators Based on the Multi-Level Measurement Strategy

Autonomous industrial mobile manipulator (AIMM), which is composed of a mobile platform and a robot arm of six degrees of freedom (DoFs), can move autonomously and perform dexterous tasks independently in the industrial environment. A novel on-line calibration system for AIMM in real manufacturing workshop is proposed in this paper. Compared to calibration scenarios of fixed-base robot, we focus on the large-space autonomous calibration for AIMM. To cope with the issue of high precision calibration in large space, we propose a multi-level guided-based measurement strategy, which includes the low-cost vision positioning and guidance system, and the high-precision distributed base station (BS) measurement system. The corresponding experiment proves that the total uncertainty of measurement system is less than 0.1 mm. Based on the measured data of end-effector installed at the end of robot arm, the AIMM can perform on-line calibration tasks anywhere automatically without taking into account the expensive devices for the high-precision location of mobile platform and the downtime for the robotic off-line calibration. The mean positioning error of AIMM after calibration reduces 93% based on the calibration experiment.

Coordinated Control of a Dual-Arm Space Robot: Novel Models and Simulations for Robotic Control Methods

Space robots have attracted increasing attention for performing, autonomously or telerobotically, on-orbit servicing missions such as repairing, refueling, and upgrading spacecraft; reusing space assets; and on-orbit assembly. The extension of robot application to space can release astronauts from risky, time-consuming, and expensive extravehicular activities [1]. However, in the microgravity environment, the floating base of a space robot will be disturbed by the robot's arm motion when it approaches or manipulates a target. The motion of the spacecraft base resulting from this disturbance will, conversely, affect the motion of end effectors (known as coupling dynamics), making control of space robots more complicated than that of fixed-base robots. In addition, such a disturbance of spacecraft attitude may result in a communication interruption between the spacecraft and the ground station or a failure of energy accumulation caused by disorientation of solar panels [2].

On the dynamics and control of flexible joint space manipulators

Space manipulator systems are designed to have lightweight structure and long arms in order to achieve reduction of fuel consumption and large reachable workspaces, respectively. Such systems are subject to link flexibilities. Moreover, space manipulator actuators are usually driven by harmonic gear mechanisms which lead to joint flexibility. These types of flexibility may cause vibrations both in the manipulator and the spacecraft making the positioning of the end-effector very difficult. Here, both types of flexibilities are lumped at the joints and the dynamic equations of a general flexible joint space manipulator are derived. Their internal structure is highlighted and similarities and differences with fixed-base robots are discussed. It is shown that one can exploit the derived dynamic structure in order to design a static feedback linearization control law and obtain an exact linearization and decoupling result. The application of such controllers is desired in space applications due to their small computational effort. In case of fixed-base manipulators, the effective use of a static feedback controller is feasible only if a simplified model is considered. Then, the proposed static feedback linearization control law is applied to achieve end-effector precise trajectory tracking in Cartesian space maintaining a desirable non-oscillatory motion of the spacecraft. The application of the proposed controller is illustrated by a planar seven degrees of freedom (dof) system.

Use of Learning Methods to Improve Kinematic Models

Abstract Kinematic model is the basic aspect in robot design and motion planning. Kinematic models are idealized, however there exist certain specific aspects of particular robot or environment, so that during navigation, the robot can significantly deviate from the planned trajectory. To increase the accuracy of motions, kinematic model can be improved and to achieve that the artificial intelligence methods can be used. In case of fixed base robots different approaches are used to train kinematics, at the same time, for the mobile base robots it proves to be a more complicated task. The reason is that a mobile robot can move unbound with respect to environment thus it is difficult to control the platform without deviation from the target position, which leads to inaccuracy in the position estimate. This paper presents the method meant for improvement of the accuracy of motion of differential drive platform. Genetic programming is used to obtain the wheel velocity function, from which the coefficient, which describes different factor influence on motion, is obtained. As a result, the kinematic model of a particular platform for a particular task is obtained. This method is effective because the developed kinematic model is more specific than the general one.

Reaction Null Space of a multibody system with applications in robotics

Abstract. This paper provides an overview of implementation examples based on the Reaction Null Space formalism, developed initially to tackle the problem of satellite-base disturbance of a free-floating space robot, when the robot arm is activated. The method has been applied throughout the years to other unfixed-base systems, e.g. flexible-base and macro/mini robot systems, as well as to the balance control problem of humanoid robots. The paper also includes most recent results about complete dynamical decoupling of the end-link of a fixed-base robot, wherein the end-link is regarded as the unfixed-base. This interpretation is shown to be useful with regard to motion/force control scenarios. Respective implementation results are provided.

Dynamics and Cooperative Object Manipulation Control of Suspended Mobile Manipulators

Dynamics and control of mobile manipulators is obviously a more challenging problem compared to fixed-base robots. Including a suspension system for these mobile platforms increases their maneuverability, but considerably adds to their complexity. In this paper, a suspended wheeled mobile platform with two 6-DOF Puma-type manipulators is used to manipulate an object along a given path. To apply a model-based control algorithm, it is required to have an explicit dynamics model for such highly nonlinear system. This model should be as concise as possible to include fewer mathematical calculations for online computations. Therefore in this paper, a detailed set of dynamics equations for a multiple arm wheeled mobile platform equipped with an effective suspension system is presented. The method is based on the concept of Direct Path Method (DPM), which is extended here for such challenging type of robots. The obtained dynamics model is then verified with a dynamical analysis study using software ADAMS. Then, Natural Orthogonal Complement Method is used to include the non-holonomic constraint of the wheeled platform in a more concise dynamics model. Next, an impedance control law is applied for cooperative manipulation of an object by the two manipulators. The obtained results for a suspended wheeled platform equipped with two 6-DOF Puma-type manipulators reveal a successful performance for moving an object along a mixed circular-straight path, even in the presence of unexpected disturbing forces, system/end-effector flexibility and impacts due to contact with an obstacle.

Impedance Control of Free-Flying Space Robot for Orbital Servicing

One of the most important phases of orbital servicing by a space robot is capturing a target satellite. In this phase, there is the risk that contact will push the target and robot away from each other. Controlling the impedance of the manipulator effectively prevents this. For a free-flying space robot, however, conventional methods used for fixed base robots cannot be used because the motion of the base interferes with the manipulator motion. An impedance control method for a space manipulator arm is proposed, where the end tip of the manipulator is controlled as if a mass-damper-spring system fixed in inertial space. Possible applications in orbital servicing are also discussed.

An Approach To Manipulator Path Planning

A path planning technique for fixed-base robot manipulators consists of a Mapping Phase and a Path Planning Phase. The Mapping Phase employs a closed-form relationship to map from object space to C-Space for a manipulator with revolute joints. The Path Planning Phase consists of develop ing a potential field around each object in C-Space and using the resulting field to choose an appropriate path. To accom plish this, a trial path is chosen and then modified under the influence of the potential field. Considering the entire path greatly reduces the probability of becoming trapped in a local minimum, making the method effective for global planning.