Flexible Mobile Manipulator Research Articles

Edge Computing Framework for Enhanced Robotic Adaptivity in Line-Less Mobile Assembly Systems

In response to the shortage of skilled workers and the increasing diversity of products and variants, integrating adaptive and flexible mobile manipulators into line-less mobile assembly systems (LMAS) stands out as a promising solution. To fully benefit from these robotic systems in LMAS, offloading computationally intensive tasks from mobile manipulators to edge computing systems is essential. This offloading of computational tasks not only lightens the processing load on individual robots but also enhances AI based perception, improving the overall system capabilities in dynamic manufacturing environments. Achieving these goals requires a robotic edge computing framework that serves as the key to flexible and adaptive mobile manipulation in LMAS. To construct such an edge computing framework, a comprehensive set of building blocks is required. These building blocks include a powerful edge system with high processing power, sensor-equipped mobile manipulators with the ability to perceive the environment, and a robust edge-to-robot connectivity. A comprehensive software stack, including software modules for localization and navigation in dynamic assembly environments, AI perception for precise pose estimation of assembly components, and motion planning for interaction between manipulators and assembly components, is crucial. Furthermore, virtualization techniques, a comprehensive deployment strategy, and a detailed description of robot hardware, site, and resources are essential. The proposed edge computing framework presents a solution that addresses mobile manipulators in LMAS and paves the way towards advanced industrial automation. Through implementation in a research assembly environment, the realized proof-of-concept showcased the feasibility of the proposed edge computing framework in a real-world pick-and-place scenario, highlighting its potential to enhance adaptivity and flexibility. There is a potential to refine and expand the framework to accommodate a broader range of industrial applications and streamline diverse manufacturing processes through the integration of mobile manipulators and edge computing within industrial settings.

Trajectory tracking problem for a Flexible Mobile Manipulator: A Flatness Based Approach combined with Active Disturbance Rejection Control

This article proposes a suitable combination of Differential Flatness and Active Disturbance Rejection Control (ADRC) for the trajectory tracking problem on a flexible link mobile manipulator. The robot is composed of a flexible link (appendage) mounted on a cart and actuated at its base. A simplified virtual rigid link model, equipped with a rotational joint flexibility at the base, is proposed for modeling the nonlinear dynamics. The proposed model allows, quite realistically, to develop a robust controller on the closed loop behavior of the system. A finite element software (FEM software) is used to validate the proposed approach which simulates the dynamics of the real system. The robustness and effectiveness of the proposed control method is tested via realistic computer simulations with the FEM software, using physical parameters values obtained from an actual experimental platform. The obtained results are quite satisfactory in spite of un-modeled disturbances. Experiments, which are currently under way on an laboratory platform, will be reported elsewhere.

Towards robotic laboratory automation Plug & Play: The "LAPP" framework.

Increasing the level of automation in pharmaceutical laboratories and production facilities plays a crucial role in delivering medicine to patients. However, the particular requirements of this field make it challenging to adapt cutting-edge technologies present in other industries. This article provides an overview of relevant approaches and how they can be utilized in the pharmaceutical industry, especially in development laboratories. Recent advancements include the application of flexible mobile manipulators capable of handling complex tasks. However, integrating devices from many different vendors into an end-to-end automation system is complicated due to the diversity of interfaces. Therefore, various approaches for standardization are considered in this article, and a concept is proposed for taking them a step further. This concept enables a mobile manipulator with a vision system to "learn" the pose of each device and - utilizing a barcode - fetch interface information from a universal cloud database. This information includes control and communication protocol definitions and a representation of robot actions needed to operate the device. In order to define the movements in relation to the device, devices have to feature - besides the barcode - a fiducial marker as standard. The concept will be elaborated following appropriate research activities in follow-up papers.

Motion equations of cooperative multi flexible mobile manipulator via recursive Gibbs–Appell formulation

• The model of time-varying multi flexible mobile manipulators is proposed. • The cooperative mobile manipulator with the closed kinematic chain is illustrated. • The Gibbs–Appell formulation has been upgraded by using Lagrange multiplayer. • The maximum dynamic load caring capacity has been evaluates for a common object. In this paper, the developed model of an N-flexible-link mobile manipulator with revolute-prismatic joints is presented for the cooperative flexible multi mobile manipulator. In this model, the deformation of flexible links is calculated by using the assumed modes method. In additions, non-holonomic constraints of the robots’ mobile platforms that bound its locomotion are considered. This limitation is alleviated through the concurrent motion of revolute and prismatic joints, although it results in computational complexity and changes the final motion equations to time-varying form. Not only is the proposed dynamic model implemented for the multi-mobile manipulators with arms having independent motion, but also for multi-mobile manipulators in cooperation after defining gripper's kinematic constraints. These constraints are imported to the dynamic equations by defining Lagrange multipliers. The recursive Gibbs–Appell formulation is preferred over other similar approaches owing to the capability of solving the equations without the need to use Lagrange multipliers for eliminating non-holonomic constraints in addition to the novel optimized process of obtaining system equations. Hence, cumbersome simultaneous computations for eliminating the constraints of platform and arms are circumvented. Therefore, this formulation is improved for the first time by importing Lagrange multipliers for solving kinematic constrained systems. In the simulation section, the results of forward dynamics solution for two flexible single-arm manipulators with revolute-prismatic joints while carrying a rigid object are presented. Inverse dynamics equations of the system are also presented to obtain the maximum dynamic load-carrying capacity of the two-rigid-link mobile manipulators on a predefined path. Two constraints, namely the capacity of joint motors torque and robot motion stability are considered as the limitation criteria. The concluded motion equations are used to accurately control the movement of sensitive bodies, which is not achievable through the use of one platform.

Gesture-Based Extraction of Robot Skill Parameters for Intuitive Robot Programming

Despite a lot of research in the field, only very little experience exists with Teaching by Demonstration (TbD) in actual industrial use cases. In the factory of the future, it is necessary to rapidly reprogram flexible mobile manipulators to perform new tasks, when the need arises, for which a working system capable of TbD would be ideal. Contrary to current TbD approaches, that generally aim to recognize both action and where it is applied, we propose a division of labor, where the operator manually specifies the action the robot should perform, while gestures are used for specifying the relevant action parameter (e.g. on which object to apply the action). Using this two-step method has the advantages that there is no uncertainty of which action the robot will perform, it takes into account that the environment changes, so objects do not need to be at predefined locations, and the parameter specification is possible even for inexperienced users. Experiments with 24 people in 3 different environments verify that it is indeed intuitive, even for a robotics novice, to program a mobile manipulator using this method.

Vibration analysis of a Timoshenko beam on a moving base

In this paper free vibration of a Timoshenko beam with a tip payload, which is mounted on a cart (referred to as TBC) is studied. The cart (base) can only have lateral displacement and the tip payload has both mass and mass moment of inertia. The center of mass of the payload does not coincide with the point where the beam connects to the payload. Therefore, the tip of the beam is exposed to an extra bending moment due to the inertial force of the payload. By employing Hamilton’s principle, the governing equations of motion and the associated boundary conditions for the TBC are first derived and then transferred into dimensionless forms. By using these governing equations and their associated boundary conditions, the closed-form frequency equation (characteristic equation) of the TBC is derived. This closed-form frequency equation is validated both analytically and numerically. The closed-form expressions for the mode shapes of the TBC and their orthogonality are also presented. By using the closed-form characteristic equation, a sensitivity study is performed and the changes in the natural frequencies versus changes in the physical parameters are investigated. The results presented in this paper are valuable for precise dynamic modeling and model-based control of flexible mobile manipulators; a flexible mobile manipulator is a flexible link manipulator with a moving base.

Mathematical modeling and trajectory planning of mobile manipulators with flexible links and joints

This paper is concerned with mathematical modeling and optimal motion designing of flexible mobile manipulators. The system is composed of a multiple flexible links and flexible revolute joints manipulator mounted on a mobile platform. First, analyzing on kinematics and dynamics of the model is carried out then; open-loop optimal control approach is presented for optimal motion designing of the system. The problem is known to be complex since combined motion of the base and manipulator, non-holonomic constraint of the base and highly non-linear and complicated dynamic equations as a result of the flexible nature of both links and joints are taken into account. In the proposed method, the generalized coordinates and additional kinematic constraints are selected in such a way that the base motion coordination along the predefined path is guaranteed while the optimal motion trajectory of the end-effector is generated. This method by using Pontryagin’s minimum principle and deriving the optimality conditions converts the optimal control problem into a two point boundary value problem. A comparative assessment of the dynamic model is validated through computer simulations, and then additional simulations are done for trajectory planning of a two-link flexible mobile manipulator to demonstrate effectiveness and capability of the proposed approach.

Path Planning of Mobile Elastic Robotic Arms by Indirect Approach of Optimal Control

Finding optimal trajectory is critical in several applications of robot manipulators. This paper is applied the open-loop optimal control approach for generating the optimal trajectory of the flexible mobile manipulators in point-to-point motion. This method is based on the Pontryagin's minimum principle that by providing a two-point boundary value problem is solved the problem. This problem is known to be complex in particular when combined motion of the base and manipulator, nonholonomic constraint of the base and highly non-linear and complicated dynamic equations as a result of flexible nature of links are taken into account. The study emphasizes on modeling of the complete optimal control problem by remaining all nonlinear state and costate variables as well as control constraints. In this method, designer can compromise between different objectives by considering the proper penalty matrices and it yields to choose the proper trajectory among the various paths. The effectiveness and capability of the proposed approach are demonstrated through simulation studies. Finally, to verify the proposed method, the simulation results obtained from the model are compared with the results of those available in the literature.

Trajectory optimization of flexible mobile manipulators

A computational algorithm is developed to find a dynamic motion trajectory of a mobile manipulator with flexible links and joints that will allow the robot to carry a maximum load between two specified end positions. A compact form of the linearized state space dynamic equations is organized as well as constraint equations. Then, the problem of finding a maximum load carrying capacity on flexible mobile manipulators is formulated as a trajectory optimization problem.

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.

Control of flexible mobile manipulators: positioning and vibration reduction using an eye-in-hand range camera

In this paper, the positioning of a long flexible manipulator on a moving platform is investigated. The problem is to position the gripper at a requested relative distance in front of an object with unknown location. For this purpose, the gripper is equipped with a range camera giving the distance to surrounding objects within, ∼1% and with a sampling rate above 1 kHz. The range measurements are used in combination with internal angle measurements from joint encoders to estimate both the flexibility in the mechanical construction and the relative distance from gripper to object. This is solved satisfactorily by an extended Kalman filter (EKF). For the motion control of the manipulator, a time-scaled feedback controller is suggested. A fast inner loop is used to damp out oscillations and reject disturbances, both from the platform and the manipulator. An outer control loop, with a lower closed-loop bandwidth, then steers the gripper, based on the range measurements, to the requested final position in front of the object. This loop assumes a stationary and rigid platform and a rigid manipulator. At this moment, only simulations of a flexible manipulator on a rigid platform have been studied. However, the results show that the flexibility can be estimated from indirect measurements of the range to the object and the joint angles. Also, good damping and disturbance rejection are achieved, as long as the bandwidth of the actuators is sufficiently high compared to the oscillation. The use of range measurements of the surrounding objects makes the positioning task very robust against an uncertain platform position.

On Control of Flexible Mobile Manipulators: Positioning and Vibration Reduction Using an Eye-in-Hand Range Camera

In this paper the positioning of a long flexible manipulator on a moving platform is investigated. The problem is to position the gripper at a requested relative distance in front of an object with an unknown location. For this purpose the gripper is equipped with a range camera giving the distance to surrounding objects within, ~ 1% and with a sampling rate above 1kHz.The range measurements are used in combination with internal angle measurements from joint encoders to estimate both the flexibility in the mechanical construction and the relative distance from gripper to object.For the motion control of the manipulator, a time scaled feedback controller is suggested. A fast inner loop is used to damp out oscillations and reject disturbances both from the platform and the manipulator. An outer control loop, with a lower closed loop bandwidth, then steers the gripper, based on the range measurements, to the requested final position in front of the object. This loop assumes a stationary rigid platform, and a rigid manipulator.At this moment only simulations of a flexible manipulator on a rigid platform have been studied. However, the results show that the flexibility can be estimated from indirect measurements of the range to the object and the joint angles. Also, good damping and disturbance rejection is achieved as long as the bandwidth of the actuators is sufficiently high, compared to the oscillation.The use of range measurements of the surrounding objects makes the positioning task very robust with respect to an uncertain platform position.

Nonstationary Random Dynamics of a Mobile Robotic Manipulator

The paper presents results from a study of the structural motion of a flexible mobile manipulator, and specifically, it discusses the time-dependent statistics and the deterministic response of this motion. The manipulator is mounted atop a vehicle which is supported by a suspension system. The vehicle accelerates uniformly on a rough surface generating a nonstationary random excitation and a deterministic excitation. The system response is separated into deterministic and stochastic components. The covariance Liapunov’s equation for the manipulator stochastic joint responses is developed. Further, expressions for the covariance matrices of the manipulator tip motion are developed and the transformation necessary to relate them to various Cartesian coordinate frames is derived. Expressions for the principal variances of the tip motion are developed by means of the singular value decomposition. The effects of damping, vehicle acceleration, surface roughness coeffifients, and manipulator configuration on the tip responses are explored.

Dynamics and control of an orbiting flexible mobile manipulator

This paper presents a Lagrangian formulation for studying the dynamics and control of the proposed Space Station based Mobile Servicing System (MSS) for a particular case of inplane libration and maneuvers. The simplified case is purposely considered to help focus on the effects of structural and joint flexibility parameters of the MSS on the complex interactions between the station and manipulator dynamics during slewing and translational maneuvers. The response results suggest that under critical combinations of parameters, the system can become unstable. During maneuvers, the deflection of the MSS can become excessive, leading to positioning error of the payload. At the same time the libration error can also be significant. A linear quadratic regulator is designed to control the deflection of the manipulator and maintain the station at its operating configuration.