AUTOMATION AND ROBOTICS:CLASSIFICATION OF ASSEMBLY TYPES AND CHOICE OF ASSEMBLY METHODS

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CLASSIFICATION OF ASSEMBLY TYPES AND CHOICE OF ASSEMBLY METHODS

Investigations into automation assembly procedures should first try to ascertain whether and in what scope automation assembly systems can be applied efficiently. This requires a very careful analysis of the assembly tasks as well as a thorough examination of possible alternatives and their profitability.

The total assembly cost of a product is related to both the product design and the assembly system used for its production. Assembly costs can be reduced to a minimum by designing the product so that it can be assembled economically by the most appropriate assembly system. Assembly systems can be classified into three categories:

1. Manual assembly

2. Special-purpose assembly

3. Flex-link (programmable) assembly

In manual assembly, the control of motion and the decision making capability of the assembly operator, assuming that the operator is well trained, are far superior to those of existing machines or artificial intelligence systems. Occasionally, it does make economic sense to provide the assembly operator with mechanical assistance, such as fixtures or a computer display detailing assembly in- structions, in order to reduce the assembly time and potential errors.

Special-purpose assembly systems are machines built to assemble a specific product. They consist of a transfer device with single-purpose work heads and feeders at the various workstations. The transfer device can operate on a synchronous indexing principle or on a free-transfer nonsynchronous principle.

Flex-link assembly systems, with either programmable work heads or assembly robots, allow more than one assembly operation to be performed at each workstation and provide considerable flexibility in production volume and greater adaptability in designing changes and different product styles.

Capital-intensive assembly, such as when automatic assembly systems are applied, produces the required low unit costs only when relatively large quantities are produced per time unit and a long amortization time is selected. Due to its flexibility, however, an assembly system able to assemble various products is more economical despite its larger investment volume. A flexible assembly system is also capable of assembling smaller batches cost-effectively, thus meeting the increasing market demands for product variety and shorter product life.

The type of automated assembly system is essentially determined by the workpieces to assemble. The most influential factors are the workpiece’s total weight, size, amount, and number of types and variants.

Assembly systems are mainly automated for small-volume products or components with a total weight of a few kilograms. The cycle times for automated assembly systems vary between tenths of seconds and minutes.

The various types of automated assembly systems arise due to the combination of the different types of assembly stations and work transfer devices, which, in turn, are dependent on the require- ments of the workpieces to assemble. Flexibility, that is, being adaptable to different conditions and assembly tasks, should also be a characteristic of assembly systems. An adequate configuration of the assembly station as well as respective work transfer devices will help meet the different flexibility requirements.

Assuming that the product design is compatible with automated assembly, there are several dif- ferent ways to characterize the operation and configuration of the automated system. One way consists of classifying the system by the type of work transfer used by the system. These types are shown in Figure 2.

In transfer systems linked to rigid work transfer systems, the workpieces and / or partly assembled components run through the assembly process on workpiece carriers in clocked cycles. The assembly stations are placed on a circuit or next to each other parallel to the transfer device. Rigid work transfer systems have the following features:

• Joint or jointly controlled handling and / or passing-on devices

• A uniform cycle for all assembly stations set by the pace of the station with the longest cycle time

• Assembly of the manufacturing equipment in specified uniform intervals

• The shutdown of one assembly station results in the stoppage of the whole assembly installation

Automation and Robotics-0113

For transfer systems to flex-link systems, flexibility is achieved in the way the individual stations of an assembly system are linked—for example, for a junction in the material flow or the evening out of capacity fluctuations and / or technical malfunctions of the individual assembly stations.

Flex-link systems also allow a different level of automation at each of the assembly stations. Depending on the respective assembly tasks, the automation level can be stipulated according to technical and economic factors. In addition, step-by-step automation of the assembly system is pos- sible because individual manual assembly equipment can still be replaced by automatic assembly station later on.

Some features of flex-link assembly stations are:

• Independent handling and manufacturing equipment that is linked to the interfaces for control purposes

• Single disengagement of the processing and handling cycles that allows different cycle times at each of the assembly stations

• Magazines as malfunction buffers between the assembly stations for bridging short stoppage times

• Freedom of choice in the placement and sequence of the assembly stations.

For longitudinal transfer systems, the transfer movement usually results from the friction between the workpiece carrier and the transfer equipment (belt, conveyor, etc.). The workpiece carriers trans- port the workpieces through the assembly system and also lift the workpieces in the assembly stations. The workpiece carrier is isolated and stopped in the assembly station in order to carry out the assembly process. The coding of the workpiece is executed by mechanical or electronic write / read units on which up-to-date information about the assembly status or manufacturing conditions can be stored.

Due to flex-linking, the number of linked assembly stations can be very high without impairing the availability of the whole system. Flexible modular assembly systems with well over 80 assembly stations for automated assembly of complex products are therefore not uncommon.

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