Design Considerations for Vibration Assisted Compliant Assembly

This paper presents design considerations for vibration assisted compliant assembly involving peg-in-hole insertion. We propose a feasible parts mating assembly model, based on the positional uncertainty and tolerance of an assembly task. For an infeasible task, whose tolerance set does not contain the uncertainty set, it is proposed to introduce a relative motion between the two mating parts, so as to enlarge the task tolerance relative to its uncertainty. A specific type of such motion, viz vibration in two orthogonal directions, is studied in detail. The amplitudes and frequencies of vibrations are determined for given tolerance, uncertainty, and other assembly parameters. A numerical procedure is devised to select the ratio of the two orthogonal vibration frequencies, for minimum search time of parts engagement. Criteria on suitable compliances for assembly are proposed, with consideration of insertion failure.

A Method of Position Sensing for Precision Assembly

The problem of part mating and assembly with close tolerances or fits has been addressed in the past by active or passive compliance and by force/position control. In this paper a position-sensitive wrist is described. A method is offered to ascertain the position of a peg during an unsuccessful attempt at assembly. The experimental and simulated results of this approach are presented. This work is a part of a research effort in precision assembly using a fuzzy control model.

Orientation and Insertion of Randomly Presented Parts Using Vibratory Agitation

In this article, we propose a new approach to automated assembly. Currently, automated assembly is expensive and difficult because all of the machines required for its implementation impose organization on the parts being assembled. It is this organization that is costly. We investigate an approach that requires less organization and results in assembly as a bulk process. We describe an initial implementation of this approach with a particular parts presentation system. Experiments are performed to determine the part characteristics that promote the success of the implementation. The experimental results are described and future work is suggested.

An Approach to Dynamic Distance Calculations for Obstacle Avoidance Problems

This paper presents a distance calculation method which can be used in machine motion planning optimizations where interference is a concern. Dynamic distance calculations are discussed which use the quadratic programming method combined with an approximate swept volume approach. Distance-to-contact calculations can be obtained for both interference and non-interference situations. The swept volume of a moving polygon is constructed through a series of overlapped swept volume segments. Each of the swept volume segments is efficiently developed by checking the inner products of polygon outward boundary normals with velocity vectors for polygon vertices. Two dimensional examples of distance-to-contact computations and robot path planning problems are presented for a sample three link robot with three rotational joints.

On the Automatic Generation of Assembly Sequences for Polyhedral Assemblies

We consider the problem of automatically generating all sequences for assembling a product. The individual components of the product are restricted to be polyhedra. Each node of the assembly tree represents a component and every branch from a leaf node to the root of the tree represents a feasible assembly sequence. The capability to generate all feasible assembly sequences, based purely on geometry, is necessary for deriving a good sequence based upon other criteria such as robot motion, fixturing and part stability.

Redundancy Compensation in Control of Multiple Arms

During manipulation of an object grasped at N locations by end-effectors of N robots, relaxation of assumption on invariant grasping locations of end-effectors Is too realistic. When coordinating end-effectors take infinitesimal displacements at the contact grasping locations with a rigid object, a slippage occurs which undesirably induces some redundancy in closed form kinematics/dynamics formulation of the entire robotic system. Particularly, in force/position control of multiple coordinating robot arms such an affect produces Inevitable impulsive reaction forces/moments which need to be compensated for in global control strategy of the system. In this paper, we have presented a control strategy with a Dynamic Redundancy Compensator (DRC) for cartesian space control of the coordinating multiple robots manipulating a common object. The proposed control scheme embeds dynamics of the individual coordinating robot arms and dynamically is capable to compensate for the kinematic/dynamic redundancies while preserving optimum forces/torques distribution between the end-effectors of robot arms. The results of study has been demonstrated on control of two robot arms manipulating a common object through prescribed coordinated motions.

Modeling and Analysis of Parts Mating in Vibration Assisted Compliant Assembly

This paper presents the theoretical modeling and parameter analysis of vibration assisted robotic assembly involving peg-in-hole insertion. Transitions among various mating configurations during the vibration assisted insertion are studied. Mating forces and insertion motions are analyzed and numerically simulated. Insertion failure is classified into several types, with the characteristic of each type studied. The types of insertion failure include jamming due to insufficient assembly forces, jamming due to robot deflection, jamming due to assembly geometry restriction, wedging due to compression forces, and peg sliding opposite to the insertion direction. Experiments are performed, where vibration is applied to the hole piece when a SCARA robot is performing peg-in-hole insertion, to verify the analytical results.

An Improved Kinematic Model for Robots and Spatial Mechanisms: Part I — Model Development

In this paper an improved kinematic modeling method is developed which is applicable to both open and closed kinematic chain topology mechanisms. This methodology is based on relative coordinate frames assigned to the individual links and joints, and the 4 × 4 homogenous transformation matrices between these relative coordinate frames. The homogenous transformation matrices can accommodate the full six degrees-of-freedom necessary in 3-D space. Therefore, this method enables one to develop a kinematic model that corresponds to the actual mechanism. In doing so, the effect of the links and joints are considered separately which will aid one in conceptual and actual development of the model. The method is applied to a Cincinnati-Milacron T3 robot which is a six degree-of-freedom robot with a 3-D spatial serial configuration mechanism made of binary links and one degree-of-freedom joints connecting the links. The results obtained from the methodology developed here are compared to the results of a popular method developed by another researcher. The methodology developed in this paper is applicable to higher degree-of-freedom joints, up to the full six degrees-of-freedom. It can also be applied to multi-loop mechanisms with the accompanying increase in the complexity of the model. This method helps to reduce the complexity of the problem when one uses the kinematic model of a mechanism in an interference checking, dynamic modeling and simulation, and link flexibility problem. Finally, it is shown that the use of 4 × 4 homogenous transformation matrices do not increase the calculational complexity of the problem appreciably.

The Effects of JIT Manufacturing Philosophy on Machine Utilization and Tool Life

This paper examine how the applications of Just-in-Time (JIT) manufacturing philosophy create excess capacity if the rate of production in a machining center is greater than the rate of the demand for the products at a subsequent machining center. The excess machining capacity created can be reduced by operating the machine(s) with excess capacity at a lower speed up to a certain limit (in order to maintain efficiency). By operating the machine(s) with excess capacity at a lower speed, the machine utilization is increased as well as the tool life of the machine(s).

A Two-Encoder Finger Position Sensing System for a Two-Degree-of-Freedom Robot Hand

As part of a robot hand with two independently controlled fingers each having one degree of freedom, a novel two-encoder position sensing system was designed for each of the fingers. In this system, a combination of a linear encoder and a rotary encoder is used to indicate finger position. The linear encoder provides coarse measurements while the rotary encoder provides fine measurements between two adjacent linear encoder counts. This two-encoder system permits more precise measurements than a system with only the linear encoder. The two encoders are connected to an IBM PC through an interface system. This paper presents the complete design and implementation of this two-encoder position sensing system.