AUTOMATION AND ROBOTICS:DESIGN OF AUTOMATIC ASSEMBLY SYSTEMS

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DESIGN OF AUTOMATIC ASSEMBLY SYSTEMS

The automation scope of assembly systems includes the following priorities:

• Reduction in the cost of assembly

• Increasing productivity

• Improved product quality

Improved motivation of the staff, improved clearness, shorter processing times, and improved ergo- nomical workstations as well as better organization are further advantages.

The movement towards automation of manufacturing and assembling sequences takes place pro- gressively. Mechanized facilities can be automated only if specific prerequisites are fulfilled and if employing complex industrial goods seems profitable. Because assembly systems require maximum flexibility, the degree of automation is still relatively low.

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In principle, automatic assembly systems can be further classified into the following subcategories:

• Short-cycle assembly machines

• Flexible, modular assembly systems

• Flexible assembly systems with industrial robots

Short-cycle assembly machines are actually single-purpose machines because each of them is specially designed for one assembly task only. They work in cycles ranging from less than 1 sec up to approximately 5 sec and are also used for mass-production assembly (over 1 million units an- nually).

Short-cycle assembly machines are often constructed modularly with standardized components for the work transfer devices and the handling and joining equipment. This allows the existing assembly machines to be partially adjusted to the new situation by modifying, converting, or retooling for changes to the assembly product.

Assembly processes are monitored by means of integrated control devices (mechanical keys, inductive feeders, etc.) in the assembly stations or partly in separate stations. This is a very important function from a quality-assurance point of view. For short-cycle assembly machines, either force- moved or pneumatic-driven motion facilities are used in the assembly stations.

The essence of flexible, modular assembly systems is that all subsystems are constructed in a modular way. The modularity pertains not only to the actual assembly stations and the sequencing devices, stands, and so on, but to the work transfer devices, control and regulation. The variety of combinations made possible by the module allows the automation of a wide range of assembly tasks. The modular conception of these systems ensures

• Easy modification and supplementary possibilities

• The capability of changing to adjust to different sets of circumstances

• A high level of reusability

• Expansion of the control technology

The investment risk is thus reduced.

Flexible, modular assembly systems (see Figure 4) usually work in cycles of a few seconds and

are often used for the automated assembly of product variants for large to medium numbers of units.

The principle of the flex-link assembly stations is applied for flexible, modular assembly systems because this is the only way to realize flexibility in the linking of individual assembly stations.

Longitudinal transfer systems are used as work transfer devices. In order to prevent individual cycle times, technical malfunctions, capacity fluctuations, and other factors of certain assembly stations from affecting the next stations, a buffer function in the transfer line between the assembly sta-

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tions allows several workpiece carriers to stack up. The distance between the individual assembly stations is not determined by the transfer device but by an optimal buffer capacity.

Flexible assembly systems with industrial robots can be divided into three principal basic types with specific system structures:

1. Flexible assembly lines

2. Flexible assembly cells

3. Flex-link assembly systems

The largest number of industrial assembly robots is used in flexible assembly lines (see Figure 5). Flexible assembly lines are more or less comparable to flexible, modular assembly systems with regard to construction, features, and application. The cycle times for flexible assembly lines generally vary between 15 and 30 sec. The main area of application for these systems is the automated assembly of products with annual units of between 300,000 and 1 million.

The application of flexible assembly lines is economically viable especially for assembling prod- ucts with several variants and / or short product lives because the flexibility of assembly robots can

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be best utilized for the execution of different assembly procedures for the individual product variations or follow-up products.

Assembly robots can also execute several assembly processes within an assembly cycle. This allows a high level of utilization for the assembly system, in particular for smaller amounts of workpieces, which is important for economic reasons. Depending on the cycle time range, an assem- bly robot can be allocated with a maximum of five to six assembly procedures in a flexible assembly line.

Flexible assembly cells (see Figure 6) are complex automated assembly stations with one or two assembly robots for large work loads (larger than for assembly stations or flexible assembly lines) where an exact limitation is very difficult and seldom expedient. A certain number of periphery devices are necessary to carry out several assembly processes of complete assembly sequences or large portions of them. Task-specific assembly devices such as presses and welding stations are also integrated if required. These periphery devices significantly limit the possible workload of the flexible assembly cells, to a maximum of 8–10 different workpieces per assembly robot. There are several reasons for this:

• Only a limited number of periphery devices can be placed in the assembly robot’s workspace.

• The greater the number of subsystems, the greater the risk of interference to the whole system.

• A large number of periphery devices reduces the accessibility of the system for maintenance, repair, and troubleshooting work.

A large number of the industrial applications of flexible assembly cells are conducted as island solutions, that is, without being linked to other assembly stations. Flexible assembly cells as assembly islands are used for the automated assembly of subassembly components or simple products with usually less than 20 different parts. This system works best technically and economically with cycle times from 25–120 sec. This allows it to be applied in situations requiring an annual number of units to be assembled of between 50,000 and 500,000, depending on the scope of the assembly tasks and the number of shifts with which the system will be run.

The application of flexible assembly cells is not possible when the product or component consists of more than 20 parts. Industrial assembly very often concerns products or components composed

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of many more parts with annual units below 500,000. In order to assemble these products automat- ically, it is possible to distribute the assembly procedures among several flex-link assembly systems.

There are two typologies of linking flexible assembly systems: permanent and flex-link sequences. Linking with a permanent linking sequence means that each assembly system is linked in a set sequence to each other, such as with longitudinal transfer systems (see Figure 7). The permanent linking sequence allows these assembly systems to execute only a smaller number of assembly tasks with different assembly sequences. They are therefore suitable for the automated assembly of variants and types with comparable assembly sequences and several similar products or components with comparable assembly sequences.

Particularly for small annual workpiece amounts, it is economically necessary to assemble several different products or components in one flexible automated assembly system in order to maintain a high level of system utilization. Because sometimes very different assembly processes must be re- alized, a linking structure (flex-link sequence), independent from certain assembly processes, is nec- essary. Assembly systems of this type are able to meet the increasing demands for more flexibility.

In flex-link assembly systems, the workpieces are usually transported by workpiece carriers equipped with workpiece-specific lifting and holding devices. The transfer movement usually results from the friction between the workpiece carrier and the transfer equipment (belt, conveyor, plastic link chain, etc.). The transfer device moves continuously. However, the workpiece carriers are stopped during the execution of the assembly processes. In the assembly station, each of the workpiece carriers is stopped and indexed. If high assembly forces are required, the workpiece carriers must be lifted from the transfer device.

The coding of the workpiece carrier is executed by means of mechanical or electronic write / read units. In this way, it is possible to record up-to-date information about the assembly status or man- ufacturing procedure on every workpiece carrier and to process this information through the assembly control system. Figure 8 shows a workpiece carrier with a coding device.

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