Wheeled Mobile Manipulator Research Articles

Stability Improvement of Two Wheel Driven Mobile Manipulator Using Nonlinear PD Controller

This paper proposes the stability improvement of two wheeled mobile manipulator using nonlinear Proportional Derivative (PD) controller. Nonlinear PD controller is a controller which automatically adjusts its gains according to the stability conditions on running. Lyapunov function based on kinetic energy of the system and position error are used to re-schedule the gains of the nonlinear PD controller. This method will expand the stability margin of the two wheeled mobile manipulator. Further advantages of this method are reducing the parameter uncertainty and difficulty of adjusting gains due to nonlinear nature of system. In the proposed approach, virtual double inverted pendulum is introduced for the control of the two wheeled mobile manipulator. The model based on the double inverted pendulum, makes it easy to design the controller. Here, wheel control and manipulator control are designed separately and unified by using two nonlinear PD controllers. Two nonlinear PD controllers are implemented for better stability. The method proposed in this paper has used nonlinear PD controller in work space of the manipulator controller together with null space control. Here the null space control increases the control stability of manipulator configuration. The effectiveness of the proposed method was confirmed by the experiment of two wheel mobile manipulator.

Robust Tracking Control For A Wheeled Mobile Manipulator With Dual Arms Using Hybrid Sliding‐Mode Neural Network

ABSTRACTIn this paper, a robust tracking controller is proposed for the trajectory tracking problem of a dual‐arm wheeled mobile manipulator subject to some modeling uncertainties and external disturbances. Based on backstepping techniques, the design procedure is divided into two levels. In the kinematic level, the auxiliary velocity commands for each subsystem are first presented. A sliding‐mode equivalent controller, composed of neural network control, robust scheme and proportional control, is constructed in the dynamic level to deal with the dynamic effect. To deal with inadequate modeling and parameter uncertainties, the neural network controller is used to mimic the sliding‐mode equivalent control law; the robust controller is designed to compensate for the approximation error and to incorporate the system dynamics into the sliding manifold. The proportional controller is added to improve the system's transient performance, which may be degraded by the neural network's random initialization. All the parameter adjustment rules for the proposed controller are derived from the Lyapunov stability theory and e‐modification such that uniform ultimate boundedness (UUB) can be assured. A comparative simulation study with different controllers is included to illustrate the effectiveness of the proposed method.

Control of two Wheeled Welding Mobile Manipulator

A three-linked manipulator mounted on a two-wheeled mobile platform is used to weld a long curved welding path. A welding torch mounted at the end of a manipulator of the welding mobile manipulator (WMM) must be controlled for tracking a welding path with constant velocity and constant welding angle of torch. In this paper, a decentralized control method is applied to control the WMM considered as two separate subsystems such as a mobile platform and a manipulator. Two decentralized motion controllers are designed to control two subsystems of WMM, respectively. Firstly, based on a tracking error vector of the manipulator and a feedback motion of the mobile platform, a kinematic controller is designed for manipulator. Secondly, based on an another tracking error vector of the mobile platform and a feedback angular velocities of revolution joints of three-link, a sliding mode controller is designed for the mobile platform. These controllers are obtained based on the Lyapunov's function and its stability condition to ensure for the tracking error vectors to be asymptotically stable. Furthermore, simulation and experimental results are presented to illustrate the effectiveness of the proposed algorithm.

Neuro-adaptive compliant force/motion control of uncertain constrained wheeled mobile manipulators

In this paper, neuro-adaptive force/motion control of mobile manipulators in the presence of unknown dynamic model and uncertain constraints is studied. The working surface is deformable, and the g...

Kinematic and dynamic model-based control of wheeled mobile manipulators: a unified framework for reactive approaches

SUMMARYThe work presented in this paper aims at providing a unified modelling framework for the reactive control of wheeled mobile manipulators (WMM). Where most work in the literature often provides models, sometimes simplified, of a given type of WMM, an extensive description of obtaining explicit kinematic and dynamic models of those systems is given. This modelling framework is particularly well suited for reactive control approaches, which, in the case of mobile manipulation missions, are often necessary to handle the complexity of the tasks to be fulfilled, the dynamic aspect of the extended workspace and the uncertainties on the knowledge of the environment. A flexible reactive framework is thus also provided, allowing the sequencing of operational tasks (in our case, tasks described in the end-effector frame) whose natures are different but also an on-line switching mechanism between constraints that are to be satisfied using the system redundancy. This framework has been successfully implemented in simulation and on a real robot. Some of the obtained results are presented.

Dynamic redundancy resolution in a nonholonomic wheeled mobile manipulator

SUMMARYWheeled Mobile Manipulators (WMM) possess many advantages over fixed-base counterparts in terms of improved workspace, mobility and robustness. However, the combination of the nonholonomic constraints with the inherent redundancy limits effective exploitation of end-effector payload manipulation capabilities. The dynamic-level redundancy-resolution scheme presented in this paper decomposes the system dynamics into decoupled task-space (end-effector motions/forces) and a dynamically consistent null-space (internal motions/forces) component. This simplifies the subsequent development of a prioritized task-space control (of end-effector interactions) and a decoupled but secondary null-space control (of internal motions) in a hierarchical WMM controller. Various aspects of the ensuing novel capabilities are illustrated using a series of simulation results.

ON THE DYNAMIC TIP-OVER STABILITY OF WHEELED MOBILE MANIPULATORS

Due to excessive maneuverability, mobile manipulators which consist of one or more manipulators mounted on a mobile base have attracted much of interest. Tipping over is one of the most important problems in mobile manipulators especially in manipulating heavy objects, also during maneuvers in unknown environment or on rugged terrains. Therefore, estimation and evaluation of dynamic stability with appropriate easy-computed measure throughout the motion of such systems is a challenging task. In this study, a new tip-over stability measure named as moment--height stability (MHS) measure is presented for wheeled mobile manipulators. The suggested MHS measure can be effectively used for both legged robots and mobile manipulators. The required computational effort of the MHS for a given system is compared to other measures, which reveals the efficiency of the MHS over the others. Finally, various case studies are presented to demonstrate the new MHS measure performance compared to the results of other measures. All calculations for system dynamics modeling have been performed using a symbolic code developed in Maple VI, and the obtained models were transferred to another code in Matlab VII to complete numerical simulations. The obtained results show the merits of the new proposed MHS measure, in terms of prediction of the exact time of instability occurrence, without extra unnecessary precautions which may confine the maneuverability of the system and its operation.

Dynamic Modeling and Tracking Control of a Nonholonomic Wheeled Mobile Manipulator with Dual Arms

This paper presents methodologies for dynamic modeling and trajectory tracking of a nonholonomic wheeled mobile manipulator (WMM) with dual arms. The complete dynamic model of such a manipulator is easily established using the Lagrange's equation and MATHEMATICA. The structural properties of the overall system along with its subsystems are also well investigated and then exploited in further controller synthesis. The derived model is shown valid by reducing it to agree well with the mobile platform model. In order to solve the path tracking control problem of the wheeled mobile manipulator, a novel kinematic control scheme is proposed to deal with the nonholonomic constraints. With the backstepping technique and the filtered-error method, the nonlinear tracking control laws for the mobile manipulator system are constructed based on the Lyapunov stability theory. The proposed control scheme not only achieves simultaneous trajectory and velocity tracking, but also compensates for the dynamic interactions caused by the motions of the mobile platform and the two onboard manipulators. Simulation results are performed to illustrate the efficacy of the proposed control strategy.

A kinematically compatible framework for cooperative payload transport by nonholonomic mobile manipulators

In this paper, we examine the development of a kinematically compatible control framework for a modular system of wheeled mobile manipulators that can team up to cooperatively transport a common payload. Each individually autonomous mobile manipulator consists of a differentially-driven Wheeled Mobile Robot (WMR) with a mounted two degree-of-freedom (d.o.f) revolute-jointed, planar and passive manipulator arm. The composite wheeled vehicle, formed by placing a payload at the end-effectors of two (or more) such mobile manipulators, has the capability to accommodate, detect and correct both instantaneous and finite relative configuration errors. The kinematically-compatible motion-planning/control framework developed here is intended to facilitate maintenance of all kinematic (holonomic and nonholonomic) constraints within such systems. Given an arbitrary end-effector trajectory, each individual mobile-manipulator's bi-level hierarchical controller first generates a kinematically-feasible desired trajectory for the WMR base, which is then tracked by a suitable lower-level posture stabilizing controller. Two variants of system-level cooperative control schemes--leader-follower and decentralized control--are then created based on the individual mobile-manipulator control scheme. Both methods are evaluated within an implementation framework that emphasizes both virtual prototyping (VP) and hardware-in-the-loop (HIL) experimentation. Simulation and experimental results of an example of a two-module system are used to highlight the capabilities of a real-time local sensor-based controller for accommodation, detection and corection of relative formation errors.

Manipulability-based configuration evaluation of cooperative payload transport by mobile manipulator collectives

In this paper, we focus on the development of a quantitative performance analysis framework for a cooperative system of multiple wheeled mobile manipulators physically transporting a common payload. Each mobile manipulator module consists of a differentially driven wheeled mobile robot (WMR) with a mounted planar three-degree-of-freedom (DOF) revolute-jointed manipulator. A composite cooperative system is formed when a payload is placed at the end-effectors of many such modules. The system possesses the ability to change its relative configuration as well as to accommodate relative positioning errors of the wheeled agents. However, the combination of nonholonomic constraints due to the mobile bases, holonomic constraints due to the closed kinematic loops, and the different joint-actuation schema (active/passive/locked) within the system requires careful quantitative evaluation to efficiently realize the payload manipulation task. Hence, in this paper, we extend the differential kinematic model for treatment of constrained articulated mechanical systems to formulate a framework to include both the mixture effect of holonomic/nonholonomic constraints and the different possible joint-actuation schema in our system. The system-level performance is then examined quantitatively by the manipulability measure in terms of isotropy index with representative case studies.

VIRTUAL TELEOPERATION AND REACTIVE MOTION OF A WHEELED MOBILE MANIPULATOR

The work presented in this paper is devoted to the coordinated control of wheeled mobile manipulators in the particular case where operational trajectory is not planned but generated in real-time during the mission. We present a kinematic control scheme that can be used for operational control and that also gives access to the redundancy of the system. Redundancy as well as set-point filtering and actuators' input scaling are used to take into account all the secondary constraints associated to mobile manipulation mission.

Analysis of wheeled mobile flexible manipulator dynamic motions with maximum load carrying capacities

A computational technique is developed for obtaining optimal trajectory of mobile manipulators with flexible links and joints to maximize their load carrying capacity for a given point-to-point task. In classical robotic manipulators, load carrying capacity mainly is limited by their actuators’ capacity constraints. But, this problem in flexible mobile manipulators is more complicated. This is due to extra degrees of freedom, which arises from combined motion of the base and manipulator, and limited motion accuracy as a result of link and joint flexibilities. The final dynamic equations are organized in a closed form similar to classical rigid manipulator equations. Then, the problem of dynamic load carrying capacity on flexible mobile manipulators is converted into a trajectory optimization problem, which fundamentally is a constrained nonlinear optimization problem. Dynamic equations are formulated in the state space form and linearized. Then, Iterative Linear Programming (ILP) method is developed to determine dynamic load carrying capacity of mobile manipulators. Finally, by two numerical examples involving a PUMA robot mounted on a linear tracked base and a wheeled mobile manipulator using the method is presented and the results are discussed.

車輪型作業移動ロボットにおけるグローバル/ローカルタスクプラニング(Visual Presentation Contest)

This paper investigates multiple wheeled mobile manipulators coordinating with each other under a potential collision situation. In our previous work we derived kinematic and dynamic maniuplability measures for multiple mobile manipulators transporting a common object. Main objective of this paper is to extend the previous approach to a system consisting of two mobile manipulators carrying a common object while avoiding the a collision. A newly developed simulation tool is introduced where a variety of simulation options are available. One can use such a tool for a high-level task planning as well as a low-level trajectory planning. Another objective is to verify the simulation results by conducting experiments on a real robot system. The experiment results show a good agreement with the simulation results.

From manipulation to wheeled mobile manipulation: analogies and differences

A generic formulation of kinematics and instantaneous kinematics is proposed for wheeled mobile manipulators. It is compared with the classical kinematic modelling of robotic arms. In particular, we show that many tools of classical manipulation can be re-used in this new framework. Additionally, the specificities of this emerging type of manipulation are highlighted and the arising control problems are stated together with adequate definitions. So, this article provides comprehensive bases for the study of such systems.

Manipulability of Wheeled Mobile Manipulators: Application to Motion Generation

Manipulability of Wheeled Mobile Manipulators: Application to Motion Generation

Maximum allowable load on wheeled mobile manipulators imposing redundancy constraints

This paper presents a computational technique for determining the maximum allowable load of wheeled mobile manipulators for a desired trajectory. Full load motion of mobile manipulators while carrying a load is an important consideration in their application and motion planning. The number of factors limits the maximum allowable loads, which can be achieved by a mobile manipulator during a given trajectory. Dynamic properties of mobile base and mounted manipulator, their actuator limitations and additional constraints applied to resolve the redundancy are probably the most important factors. To resolve extra degrees of freedom introduced by the base mobility, we proposed additional constraint functions directly in the task space of mobile manipulator. Finally, a numerical example involving a two-link planar manipulator mounted on a differentially driven mobile base, application of the method to determine maximum allowable load is verified. The simulation results clearly demonstrate that the value of maximum allowable load on a given trajectory directly depends on the additional constraint functions which apply to resolve the motion redundancy.

Kane's approach to modeling mobile manipulators

The paper presents a detailed methodology for dynamic modeling of wheeled nonholonomic mobile manipulators using Kane's dynamic equations. This methodology allows construction of the dynamic model simply and incrementally. The resulting model is obtained in closed form, it is computationally efficient and provides physical insight as to what forces really influence the system dynamics. Using this methodology, nonholonomic constraints can be directly incorporated within the model without introducing multipliers. The model constructed in the paper includes constraints for no slipping, no skidding and no tipover. The kinematic description of these constraints provides only necessary conditions and description is completed with dynamic expressions which is also sufficient for motion stability. The dynamic model and the constraint relations provide a framework for developing control strategies for mobile manipulators.

Mechanism and Control of Wheeled Mobile Robot Platform Which Can Travel Over a Step

We are developing a mobile robot base which works in indoor and outdoor environment. This mobile robot base has two main driving wheels and four steering wheels. The driving forces are given to steering wheels from main driving wheels via friction wheels. The front and rear steering wheels of each sidϵ are steered mechanically in the opposite phase, and can be swung up and down by DC motors to lift up the body of this vehicle. So that, the vehicle can travels over a step of 18[cm]. It has ultrasonic sensors to detect distance to step, and optical sensors to detect a negative step edge on the floor. And it has tilt angle sensors to detect the posture of its main body, to keep its posture. This vehicle will be useful to apply to wheel chair or mobile manipulator base, which has to work on various indoor and outdoor grounds.