Industrial assembly of parts with dimensional variations. Case study: Assembling vehicle headlamps

https://doi.org/10.1016/j.rcim.2011.05.004Get rights and content

Abstract

Presently, it is quite common, because of their benefits, to find plastic parts as the main elements of diverse commercial products. However, plastic components also present some drawbacks, such as their typical dimensional variations (especially in parts with complex geometry or with considerable size), making difficult, or even impossible, the correct assembly of the parts. This paper introduces a new methodology of dynamic assembly for an industrial application that requires an adaptive positioning of the parts that are to be assembled. In addition, this work presents a successful example of an industrial prototype where different technologies, which aim to solve different problems, have to be analyzed and tested. In particular, different approaches were studied: surface measurement sensors for transparent and deformable objects, actuation systems that could modify the assembly position of the parts, and control algorithms that could carry out this adaptive assembly automatically. The final goal of this work was to obtain a robust industrial prototype for vehicle headlamp assembly that could solve the high dimensional variations of the main plastic parts. To demonstrate the performance of the proposed solution, a wide set of experimental test was carried out in both a research lab and in the assembly line of a vehicle headlamp factory. The new prototype solves the problem of assembling vehicle headlamps, achieves a final product with minimum dimensional errors and offers an example of a solution to the problem of the assembly of pieces with dimensional errors.

Highlights

► A new methodology of dynamic assembly, that requires an adaptive positioning of the parts that are to be assembled, is proposed. ► This work presents a successful example of an industrial prototype where different technologies were analyzed and tested. ► A robust industrial prototype for vehicle headlamp assembly has been built using this methodology. ► This prototype achieves a final product with minimum dimensional errors and offers an example of a solution to the problem of the assembly of pieces with dimensional errors.

Introduction

In the rapidly changing and highly competitive global economy, automated manufacturing systems are crucial. In fact, manufactured products are mainly produced in automated systems [1]. However, the dramatic changes that are occurring in today's integrated manufacturing environment and the necessity of using new applications are continuously increasing the need to find other alternatives to improve the productivity [2]. One strategy to reach this goal is the utilization of plastic components in the production of commercial products. However, this use can also present some drawbacks, and one of the most widespread is the dimensional variations of the plastic parts, especially in ones with complex geometry or with a considerable size. Certainly, the automatic assembly of these parts creates problems such as mechanical inconsistencies, poor quality of the final products or even the loss of the customer.

A representative example of this kind of assembly problem can be found in the production of vehicle headlamps in which one of the main stages is the assembly of the cover lens, which is made of polycarbonate, over a black housing made of polypropylene (Fig. 1). The other components, the reflector, lighting system and bezel, are placed into the housing. Once the headlamp is assembled, the car manufacturer demands that the final geometry of the headlamp fulfills certain dimensional tolerances.

Today, the current assembly technology is based on placing the lens over a channel around the perimeter of the housing. This channel is first filled with gum to create a watertight compartment and also to fix the lens to the housing. Later, a set of fixing elements and wedges are placed into the channel, forcing the lens into a fix position. Then, a dimensional machine is used to determine if the headlamp is faulty. Using this methodology, because it does not correct the possible physical deviations of the lens, a suitable final geometry of the headlamp cannot be guaranteed. However, the faulty headlamps are not reusable, entailing an economic cost for the manufacturer.

The dimensional deviation of the pieces to be assembled is due to several reasons: both elements are made of different kinds of plastics with different physical properties, and they have complex geometries, and size variations. These deviations are not insignificant because they can vary between ±2mm. This problem makes it necessary to find a new solution to satisfy the dimensional requirements of the customer and to eliminate the production costs by reducing the number of faulty headlamps.

This paper introduces a new methodology of dynamic assembly [3] for an industrial application that requires an adaptive positioning of the parts that are to be assembled. It also presents an example of an industrial prototype where different technologies, which aim to solve different problems, have to be analyzed and tested. In particular, several different approaches were studied: surface measurement sensors for transparent and deformable objects, actuation systems that could modify the assembly position of the parts, and control algorithms that could carry out this adaptive assembly automatically. The final goal of this work was to obtain a robust industrial prototype for vehicle headlamp assembly that could resolve the high dimensional variations of the main plastic parts. To demonstrate the performance of the proposed solution, a wide set of experimental test was carried out in both a research lab and in the assembly line of the vehicle headlamp factory.

The remainder of this paper is as follows. First, the definition of the problem is introduced in Section 2. Second, the design of the prototype, differentiating between the hardware and the control algorithm, is described in Section 3. Third, the experimental results, which validate the proposal, are presented in Section 4. Finally, the conclusions of this work are given in Section 5.

Section snippets

Definition of the problem

Consider the following elements of a conventional headlamp (Fig. 1): housing, bezel and lens. When they are assembled, the housing is placed over a plate in such a way that it represents the reference base system. Subsequently, the bezel is located over the housing, and finally, the lens is seated over the gum channel. Because of the nature of the production process, the dimensional variability of both the housing and the lens is relatively high if it is compared with the position requirements

Design and construction of the prototype

As referred to in the Introduction, the final goal of this work was to obtain a robust industrial prototype for vehicle headlamp assembly that could solve the high dimensional variations of the main plastic parts. Different approaches were studied to carry out the design and construction of this prototype:

  • (i)

    To design a system of actuator/s that can move the lens over the gum channel following the instructions indicated by the control algorithm.

  • (ii)

    To choose a precise measurement system that can

Experimental results and validation

The validation of the final prototype was carried out in two parts: in the lab and in the production line of the Opel 4400 headlamp. A total of more than 5000 headlamps were assembled obtaining deviations at each position sensor of less than 0.7 mm and satisfying the geometric conditions. One example of the final deviations of one of the control point sensors, sensor 8, is shown in Fig. 11. In all cases, the final position deviation was lower than the maximum value required by the manufacturer.

Conclusions

This paper presented a contribution to the dynamic assembly of an industrial application that required an adaptive positioning of the parts that were to be assembled. These parts were elements of a vehicle headlamp. It also presented an example of an industrial prototype where different technologies, which aim to solve different problems, are analyzed and tested. In particular, different aspects were solved such as: selecting and configuring actuators capable of moving the lens over the gum

Acknowledgments

This work was partially supported by the projects DPI2008-05798 of the National Research Program CYCIT, TEP2009-5363 of the Regional Research Program and the Flush&Gap project supported by VALEO LIGHTING SYSTEMS (MARTOS).

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