Reconfigurable Parallel Robot Research Articles

Depth-Dependent Control in Vision-Sensor Space for Reconfigurable Parallel Manipulators.

In this paper, a control approach for reconfigurable parallel robots is designed. Based on it, controls in the vision-sensor, 3D and joint spaces are designed and implemented in target tracking tasks in a novel reconfigurable delta-type parallel robot. No a priori information about the target trajectory is required. Robot reconfiguration can be used to overcome some of the limitations of parallel robots like small relative workspace or multiple singularities, at the cost of increasing the complexity of the manipulator, making its control design even more challenging. No general control methodology exists for reconfigurable parallel robots. Tracking objects with unknown trajectories is a challenging task required in many applications. Sensor-based robot control has been actively used for this type of task. However, it cannot be straightforwardly extended to reconfigurable parallel manipulators. The developed vision-sensor space control is inspired by, and can be seen as an extension of, the Velocity Linear Camera Model-Camera Space Manipulation (VLCM-CSM) methodology. Several experiments were carried out on a reconfigurable delta-type parallel robot. An average positioning error of 0.6 mm was obtained for static objectives. Tracking errors of 2.5 mm, 3.9 mm and 11.5 mm were obtained for targets moving along a linear trajectory at speeds of 6.5, 9.3 and 12.7 cm/s, respectively. The control cycle time was 16 ms. These results validate the proposed approach and improve upon previous works for non-reconfigurable robots.

Optimum design of the reconfiguration system for a 6-degree-of-freedom parallel manipulator via motion/force transmission analysis

This research introduces a 6-degree-of-freedom parallel manipulator with a reconfigurable fixed base; the manipulator performance has been evaluated to select the most suitable reconfiguration system. The optimal design of the parallel manipulator refers to the enhancement of robot behavior achieved by the geometrical variation of the fixed platform. Motion/force transmission, with the aid of the principle of power conservation, defines the manipulator capacity to transmit its force from the actuated joints to the end-effector from different reconfiguration approaches and vice versa considering the stiffness analysis. Results allow inferring that reconfiguration of the fixed base enhances the performance of this parallel manipulator. The proposal of the reconfiguration system allows obtaining a reconfigurable parallel robot with the minimum number of actuators.

Rotational Workspace Expansion of a Planar CDPR with a Circular End-Effector Mechanism Allowing Passive Reconfiguration

Cable-Driven Parallel Robots (CDPR) operate over a large positional workspace and a relatively large orientation workspace. In the present work, the expansion of the orientation Wrench Feasible Workspace (WFW) in a planar four-cable passive reconfigurable parallel robot with three degrees of freedom was determined. To this end, we proposed a circular-geometry effector mechanism, whose structure allows automatic mobility of the two anchor points of the cables supporting the End Effector (EE). The WFW of the proposed circular structure robot was compared with that of a traditional robot with a rectangular geometry and fixed anchor points. Considering the feasible geometric and tension forces on the cables, the generated workspace volume of the robot was demonstrated in an analysis-by-intervals. The results were validated by simulating the orientation movements of the robot in ADAMS software and a real experimental test was developed for a hypothetical case. The proposed design significantly expanded the orientation workspace of the robot. The remaining limitation is the segment of the travel space in which the mobile connection points can slide. Overcoming this limitation would enable the maximum rotation of the EE.

Uso de Plataformas para el Desarrollo de Aplicaciones Virtuales en el Modelado de Robot Manipuladores

This paper describes the use of platforms for the development of virtual applications as tools for modeling of robot manipulators. The proposal is based on take advantage of the potential that these platforms currently have for solving the rigid body dynamics, which easily allows modeling the mechanical aspects of the manipulator. On the other hand, the possibility offered by these platforms of incorporate programming code in conventional languages allows to modeling the dynamic behavior of real physical systems, such as sensors and actuators, which allows implementing the development of the instrumentation and control stage of an industrial robot in the same way as a real one. Using these platforms allows the modeling from the bases of any manipulator robot. The modeling of a reconfigurable parallel robot is presented as a case study.

AN APPLICATION OF SCREW THEORY FOR THE IDENTIFICATION OF SINGULARITIES IN A NOVEL RECONFIGURABLE PARALLEL ROBOT

This paper reports the application of the screw theory as a tool for the determination of the singular configurations of a reconfigurable parallel robot composed of two parallel sub-manipulators. The Jacobian matrices of the robot, key elements for the identification of singularities, are easily determined when the input-output equation of velocity of the robot is obtained by the application of some screw theory basic operations. Through this application, the inverse, direct and combined singularities are clearly identified, and their graphical representations can be obtained almost intuitively.

Selection methodology for end-effectors of a modular system for wrist joints as parts of a reconfigurable parallel robot

This paper introduces a modular system consisting of a 3-DOF robotic local structure with revolute joints. Several different wrist joints are presented where the contact element is systematically varied. On one hand, the range of objects to be handled is analysed concerning the motion task (e.g., pick-and-place-task) and the objects properties. On the other hand, the properties of each contact principle can be considered. On a certain level, the object's properties can be related to those of the contact principles with the aim to identify a minimum set of contact principles for solving a given set of varying motion tasks. Realising these principles with one gripper each and defining general interfaces leads to a modular system, which ensures the versatility, flexibility and reconfigurability required for the introduced parallel handling system.

Selection methodology for end-effectors of a modular system for wrist joints as parts of a reconfigurable parallel robot

Selection methodology for end-effectors of a modular system for wrist joints as parts of a reconfigurable parallel robot

Modular design of parallel robots with static redundancy

Modularity in design for most mechanical systems can lead to higher reliability and reduction in cost for the design and build of the system. In this paper, the modular design of a new class of reconfigurable parallel robots is discussed. These robots can attain static redundancy without actuation redundancy and are ideal for applications that require fault tolerance and enhanced stiffness but may not necessarily require redundant actuation. Such applications can be found in the area of wing morphing for performance improvement. It is shown that a universal modular limb connectivity type exists that can be adopted for modular design of statically redundant parallel robots, regardless of the degree of static redundancy of the system. Additionally, the modular design approach is demonstrated through a case study for a morphing wing application.

Design and analysis of reconfigurable parallel robots with enhanced stiffness

Static redundancy in a parallel manipulator can enhance the stiffness of the end-effector, improve its fault tolerance, minimize its singularity loci, and reduce the internal loads experienced by the joints. Traditionally, this form of redundancy would be accompanied by actuation redundancy. Introduced in this paper is a new approach to statically enhance a manipulator without actuation redundancy. This is achieved through the use of lockable passive joints that are utilized in an alternating fashion to reconfigure the system into various isostatic and hyperstatic topologies without any external assistance. Although applicable to both kinematically non-redundant and constrained manipulators, this approach is especially effective for those with lower instantaneous mobility. The inherent redundancy in these reconfigurable robots is exploited to obtain full finite mobility with as few as one actuator through under-actuation with the use of virtual alternating constraints. The architecture design, kinematic analysis, and kinetostatic analysis of the proposed robots are addressed herein, followed by a case-study to demonstrate the effectiveness of the proposed design and analysis.

Optimal control for reducing the energy consumption of a reconfigurable parallel robot

Abstract The paper presents the control architecture of the PENTROB reconfigurable parallel robot with five degrees of freedom, pointing out several ways of reducing the electric energy consumption. Based on the implicit functions which characterize the relation between the coordinates of the active joints and the end-effector pose, the control functions are determined. The control program of the robot is described, presenting the internal architecture of the software and hardware components together with the man-machine interface which provides four possibilities of controlling the robot. The energy consumption reduction is approached through several different ways: the use of an intelligent driver, the configuration of the driver using the advanced microstepping control technique.

Module-based method for design and analysis of reconfigurable parallel robots

This paper presents a method for the design and analysis of reconfigurable parallel robots. The inherent modularity in a parallel robot lends itself as a natural candidate for reconfiguration. By taking the branches as building blocks, many modular parallel robots can be constructed, from which a reconfigurable parallel robot can be realized. Among three types of reconfigurations, namely, geometry morphing, topology morphing, and group morphing, the method presented here is for the last two reconfigurations, thereby advancing the current research that is mainly limited to geometry morphing. It is shown that the module-based method not only provides a systematic way of designing a reconfigurable parallel robot, but also offers a unified modeling for robot analysis. Two examples are provided, one showing the topology morphing and the other showing the group morphing.

Grasp planning for a reconfigurable parallel robot with an underactuated arm structure

Abstract. In this paper, a novel approach of grasp planning is applied to find out the appropriate grasp points for a reconfigurable parallel robot called PARAGRIP (Parallel Gripping). This new handling system is able to manipulate objects in the six-dimensional Cartesian space by several robotic arms using only six actuated joints. After grasping, the contact elements at the end of the underactuated arm mechanisms are connected to the object which forms a closed loop mechanism similar to the architecture of parallel manipulators. As the mounting and grasp points of the arms can easily be changed, the manipulator can be reconfigured to match the user's preferences and needs. This paper raises the question, how and where these grasp points are to be placed on the object to perform well for a certain manipulation task. This paper was presented at the IFToMM/ASME International Workshop on Underactuated Grasping (UG2010), 19 August 2010, Montréal, Canada.

Design and analysis of a re-configurable parallel robot

In this paper a re-configurable parallel robot is presented which consists of two base tripods. The first tripod is made of three prismatic joints with the fixed-leg length, called slide tripod. The second tripod is made of three revolute joints, called swing tripod. While the slide tripod is fixed to the moving platform, the branches of the swing tripod are designed to be detachable from the moving platform. By detaching one or two or three branches of the swing tripod separately, this parallel robot can be re-configured from 6 degrees-of-freedom to 5 or 4 or 3 degrees-of-freedom, respectively. In addition, the detached branches can be used to perform collaborative work with the parallel robot. To describe this design, first, design methodology is presented. Then a parametric model is developed to model the constraint equations resulting from re-configuration. Based on this model a unified method is developed to solve the constraint equations. Furthermore, a unified method is presented to solve the inverse kinematics considering varying configurations including 3, 4, 5 and 6 DOFs. Finally, an application case is presented to show that the proposed re-configurable parallel robot can be re-configured to perform tasks that would otherwise require two robots.

A real time framework for pc-based robot control under the QNX4 operating system

Even if highly reliable and endowed with powerful development tools nowadays industrial robot controllers don't usually allow the user to operate at the low level required in order to experiment control strategies and algorithms and to specify trajectories at the joint level. Such needs are mandatory in the robot control R & D area. Recently, the wide availability of computing power and operating systems with real-time capabilities offered the chance to develop low-cost PC-based solutions to overcome these limits. In this paper we present the design of a modular architecture for PC-Based robot control and contextually evaluate the use of QNX4 as a operating system for control applications. The main features of the designed controller have been employed for the motion control of a 4 d.o.f. reconfigurable parallel robot.

Self-Reconfigurable Parallel Robot on Vertical Plane by Coupling of Two Two-Link Robots with First Joints Unactuated

Serial link robots with unactuated (passive) joints are attracting research interest. If two of such robots can couple with each other, they can reconfigure to a parallel robot, which can make the number of actuators equal to the number of degrees of freedom. If they can couple with each other at different portions of them, they can constitute a reconfigurable parallel robot. This paper proposes this concept of a robot, presenting that two two-link robots, the first joints of which are unactuated, can reconfigure to a 5-link planar parallel configuration and a 4-link planar parallel configuration plus one actuated link. This reconfigurable parallel robot has only two actuators but can have multiple functions by reconfigurations. Due to the unactuated joints, whether or not the two-link robots can couple with each other is a non-trivial problem. We propose coupling sequences for forming the 4-link and 5-link configurations and verify those experimentally.

Self-calibration of three-legged modular reconfigurable parallel robots based on leg-end distance errors

A class of three-legged modular reconfigurable parallel robots is designed and constructed for precision assembly and light machining tasks by using standard active and passive joint modules in conjunction with custom designed links and mobile platforms. Since kinematic errors, especially the assembly errors, are likely to be introduced, kinematic calibration becomes particularly important to enhance the positioning accuracy of a modular reconfigurable robot. Based on the local frame representation of the Product-Of-Exponentials (Local POE) formula, a self-calibration method is proposed for these three-legged modular reconfigurable parallel robots. In this method, both revolute and prismatic joint axes can be uniformly expressed in twist coordinates by their respective local (body) frames. Since these local frames can be arbitrarily defined on their corresponding links, we are able to calibrate them, and yet retain the nominal local description of their respective joints, i.e., the nominal twist coordinates and nominal joint displacements, to reflect the actual kinematics of the robot. The kinematic calibration thus becomes a procedure of fine-tuning the locations and orientations of the local frames. Using mathematical tools from differential geometry and group theory, an explicit linear calibration model is formulated based on the leg-end distance errors. An iterative least-square algorithm is employed to identify the error parameters. A simulation example of calibrating a three-legged (RRRS) modular parallel robot shows that the robot kinematics can be fully calibrated within two to three iterations.