Shengnan Lu

Design and Testing of a Highly Reconfigurable Fixture With Lockable Robotic Arms

The paper describes an innovative fixture created within the AUTORECON project of the European Commissions 7th Framework Program. The fixture is designed to respond to the automotive industrys needs of high modularity and full flexibility, by allowing the secure multishape grasping of a very wide range of (often large and heavy) metal workpieces typical for car-assembly operations. The fixture is used as an end-effector of an industrial robot, which in turn acts in cooperation with other machines, such as other robots performing machining or other processes on the part. The fixturing device is both a highly reconfigurable structure and a robotic mechanism: moving as a programmable robot to reconfigure and acting as a supporting structure once a hold on the part has been established. This dual functioning has been made possible by the development of a key component, a stepless lockable revolute joint, described herein. In order to get a readily modifiable system, all parts of the fixture are designed as modules. Prototypes of the main parts and the whole fixture have been fabricated and experiments validating the design are reported.

Novel Deployable Mechanisms With Decoupled Degrees-of-Freedom

A novel family of deployable mechanisms (DMs) is presented. Unlike most such devices, which have one degree-of-freedom (DOF), the proposed DM can be deployed and compacted independently in two or three directions. This widens the range of its potential applications, including flexible industrial fixtures and deployable tents. The mechanisms basic deployable unit (DU) is assembled by combining a scissor linkage and a Sarrus linkage. The kinematic properties of these two components and of the combined unit are analyzed. The conditions under which the unit can be maximally compacted and deployed are determined through singularity analysis. New 2DOF DMs are obtained by linking the DUs: each mechanisms shape can be modified in two directions. The relationship between the degree of overconstraint and the number of DUs is derived. The magnification ratio is calculated as a function of link thickness and the number of DUs. The idea of deployment in independent directions is then extended to three dimensions with a family of 3DOF mechanisms. Finally, kinematic simulations are performed to validate the proposed designs and analyses.

A Network of Type III Bricard Linkages

Among Bricard’s overconstrained 6R linkages, the third type has two collapsed configurations, where all joint axes are coplanar. The paper presents a one-degree-of-freedom network of such linkages. Using the two coplanar states of the constituent Bricard units, the network is able to cover a large surface with a specific outline when deployed, and can be folded compactly into a stack of much smaller planar shapes. Five geometric parameters describing each type III Bricard mechanism are introduced. Their influence on the outline of one collapsed configuration is discussed and inverse calculation to obtain the parameter values yielding a desired planar shape is performed. The network is built by linking the units, either using scissor linkage elements, if the thickness of the panels can be ignored, or with hinged parallelograms, for a thicker material. Two case studies, in which the Bricard network deploys as a rectangle and a regular hexagon, respectively, are presented, validating the analysis and design methods.Copyright

A Novel Deployable Mechanism With Two Decoupled Degrees of Freedom

This paper presents a novel deployable mechanism. Unlike most deployable structures, which have one degree of freedom, the proposed device can be deployed and compacted independently in two directions. This widens the range of its potential applications, including flexible industrial fixtures and deployable tents. The mechanism’s basic deployable unit is assembled by combining a scissor linkage and a Sarrus linkage. The kinematic properties of the two component linkages and the combined unit are analyzed. The new deployable mechanism is obtained by linking the deployable units. The Mobility and kinematics are analyzed. The relationship between the degree of overconstraint and the number of deployable units is derived. The magnification ratio is calculated as a function of the geometry of the link and the number of deployable units. Finally, kinematic simulations are performed to validate the proposed design and analysis.Copyright

Reconfigurable Chains of Bifurcating Type III Bricard Linkages

This paper presents the construction of a family of reconfigurable mechanisms composed of an unlimited number of doubly collapsible (type III) Bricard linkages. First, the geometries of these overconstrained six-hinge spatial loops are parameterized and their kinematics is investigated. The configuration-space curve is computed; its bifurcation behavior is analyzed and illustrated by projections. It is then shown that type III Bricard linkages can be connected in series in a one-degree-of-freedom chain. Such a multi-loop mechanism has the ability to reconfigure in multiple ways due to the bifurcations of the individual Bricard units. Consequently, the chain has multiple states where all joint axes are coplanar. In each such configuration, the physical links, every one realized as a planar figure, spread out to cover a curving stripe in the plane. Several simulations and case studies are performed.

Mechanisms with Decoupled Freedoms Assembled from Spatial Deployable Units

The chapter presents a family of new deployable mechanisms obtained by conjoining an indefinite number of copies and variants of the same basic spatial linkage. The unit mechanism is an assembly of scissor and Sarrus linkages able to move in two or three independent directions. Unlike most deployable structures, which have one degree of freedom, the proposed devices can be deployed and compacted independently in two or three directions. Moreover, the physical boundary of the mechanism can be used to approximate and control the shape of some geometric curves. Kinematic simulations are performed to validate the proposed designs.

Approximation and Control of Curvilinear Shapes via Deployable Mechanisms with Two Degrees of Freedom

This paper proposes a multi-unit mechanism, which can be used to approximate, with two independent degrees of freedom, the shape of the geometric outline of an arbitrarily large area. The new device is a variant of a recently introduced planar deployable mechanism with two uncoupled degrees of freedom, built from identical units, each combining Sarrus and scissor linkages. Similar units, but with varying sizes, are used in the new device, which is able to change its elliptical physical boundary by varying independently the two parameters in the standard ellipse equation. The size and placement of the deployable units and their links are analyzed and selected for getting the expected geometric shape. The relationships between the number of dividing lines and the approximating accuracy and the degree of overconstraint, respectively, are calculated. A similar deployable mechanism controlling a hyperbola is also outlined. Kinematic analysis and simulated models show that the mechanisms can be used to approximate geometric curves, as desired.© 2014 ASME

Generalized Construction of Bundle-Folding Linkages

A mechanism which is able to fold into a bundle is of particular interest: minimal size facilitates storage and transport. The paper presents a simple and general geometric method to design bundle-folding linkages based on one-degree-of-freedom spatial overconstrained loops. The so designed mechanism can be folded into a line bundle and deployed into a spatial shape. The geometric conditions, under which an overconstrained linkage can be folded into a bundle, are discussed. Case studies of bundle-folding designs are presented and validated using simulations.

Reconfigurability and Flexibility in a Robotic Fixture for Automotive Assembly Welding

On an automotive assembly line, different workpieces are jointed by respot welding. Modern car manufacturers need to be highly adaptive and able to switch to new products frequently within a short time frame. Hence, there is a high and constantly growing demand for the development of flexible and autonomous fixturing systems. A novel highly reconfigurable fixture is created by the Autorecon project within the European Commission’s 7th Framework Programme. The proposed system is a structural robot composed of lockable arms, a frame, and double tool changers. The reconfigurability and flexibility of this fixture can improve the manufacturing process while lowering costs and expediting new-model setup and product delivery.

Design and Testing of a Novel Stepless Lockable Joint for Use in Reconfigurable Fixtures

Reconfigurable or flexible fixtures, an important element in automotive body-welding assembly lines, can be designed as mechanisms with lockable joints. This paper proposes a new flexible fixture using stepless lockable joints allowing infinitely many configurations, and hence a broad application range and potential. Several conceptual designs of the key component, the lockable joint, are presented. Unlike existing mechanisms, which can lock the joint in a finite number of positions, the proposed solution is a stepless device. Once locked, the joint can resist high torque in both directions. These characteristics of the lockable joint guarantee the performance of the reconfigurable fixture that employs it. The load testing has been performed on the prototype of a revolute joint that can sustain 800 Nm, and the stiffness of a lockable arm which employs two lockable joints is detected.Copyright