MANIPULATOR

Grinding, wire brushing, and similar operations. Most of these operations use a

rotating spindle as the end effector to drive a grinding wheel, wire brush, polishing

wheel, or similar tool at high speed to accomplish finishing and deburring operations on the workpiece. In an alternative approach described in [13], the robot is

equipped with a gripper to hold and manipulate the workpiece against a rotating

deburring head.

• Waterjet cutting. This is a process in which a high-pressure stream of water is forced

through a small nozzle at high speed to cut plastic sheets, fabrics, cardboard, and

other materials with precision. The end effector is the waterjet nozzle that is directed to follow the desired cutting path by the robot.

• Laser cutting. The function of the robot in this application is similar to its function

in waterjet cutting. Laser beam welding is a similar application. The laser gun is attached to the robot as its end effector. In an application described in [6], robots are

used to trim excess sheet metal from parts produced in hot stamping operations.

The hot-stamped sheet metal is too hard to trim with conventional cutting dies, so

laser cutting must be used.

8.4.3 Assembly and Inspection

In some respects, assembly and inspection are hybrids of the previous two categories:

material handling and processing. Assembly and inspection can involve either the handling

of materials or the manipulation of a tool. For example, assembly operations typically involve the addition of components to build a product. This requires the movement of parts

from a supply location in the workplace to the product being assembled, which is material

handling. In some cases, the fastening of the components requires a tool to be used by the

robot (e.g., driving a screw). Similarly, some robot inspection operations require that parts

be manipulated, while other applications require that an inspection tool be manipulated.

Traditionally, assembly and inspection are labor-intensive activities. They are also

highly repetitive and usually boring. For these reasons, they are logical candidates for

robotic applications. However, assembly work typically involves diverse and sometimes

difficult tasks, often requiring adjustments to be made in parts that don’t quite fit together. A sense of feel is often required to achieve a close fitting of parts. Inspection work

requires high precision and patience, and human judgment is often needed to determine

whether a product is within quality specifications or not. Because of these complications

in both types of work, the application of robots has not been easy. Nevertheless, the potential rewards are so great that substantial efforts have been made to develop the necessary technologies to achieve success in these applications.

Assembly. Assembly involves the combining of two or more parts to form a new

entity, called a subassembly or assembly. The new entity is made secure by fastening the

parts together using mechanical fastening techniques (e.g., screws, bolts and nuts, rivets) or joining processes (e.g., welding, brazing, soldering, or adhesive bonding). Welding

applications have already been discussed.

Because of the economic importance of assembly, automated methods are often

applied. Fixed automation is appropriate in mass production of relatively simple products,

such as pens, mechanical pencils, cigarette lighters, and garden hose nozzles. Robots are

usually at a disadvantage in these high-production situations because they cannot operate

at the high speeds that fixed-automated equipment can. The most appealing application

f industrial robots for assembly involves situations in which a mix of similar models are

produced in the same work cell or assembly line. Examples of these kinds of products include electric motors, small appliances, and various other small mechanical and electrical

products. In these instances, the basic configuration of the different models is the same,

but there are variations in size, geometry, options, and other features. Such products are

often made in batches on manual assembly lines. However, the pressure to reduce inventories makes mixed-model assembly lines (Appendix 15A.2) more attractive. Robots can

be used to substitute for some or all of the manual stations on these lines. What makes

robots viable in mixed-model assembly is their capability to execute programmed variations in the work cycle to accommodate different product configurations.

Industrial robots used for the types of assembly operations described here are typically small, with light load capacities. The most common configurations are jointed arm,

SCARA, and Cartesian coordinate. Accuracy and repeatability requirements in assembly

work are often more demanding than in other robot applications, and the more precise

robots in this category have repeatabilities of {0.05 mm 1{0.002 in2. In addition, the

requirements of the end effector are sometimes difficult. It may have to perform multiple

functions at a single workstation to reduce the number of robots required in the cell.

These functions may include handling more than one part geometry and performing both

as a gripper and an assembly tool.

Inspection. There is often a need in automated production to inspect the work

that is done. Inspections accomplish the following functions: (1) making sure that a given

process has been completed, (2) ensuring that parts have been assembled as specified,

and (3) identifying flaws in raw materials and finished parts. The topic of automated

inspection is considered in more detail in Chapter 21. The purpose here is to identify the

role played by industrial robots in inspection. Inspection tasks performed by robots can

be divided into the following two cases:

1. The robot performs loading and unloading to support an inspection or testing machine. This case is really machine loading and unloading, where the machine is an

inspection machine. The robot picks parts (or assemblies) that enter the cell, loads

and unloads them to carry out the inspection process, and places them at the cell

output. In some cases, the inspection may result in sorting of parts that must be

accomplished by the robot. Depending on the quality level of the parts, the robot

places them in different containers or on different exit conveyors.

2. The robot manipulates an inspection device, such as a mechanical probe or vision

sensor, to inspect the product. This case is similar to a processing operation in which

the end effector attached to the robot’s wrist is the inspection probe. To perform

the process, the part is delivered to the workstation in the correct position and orientation, and the robot must manipulate the inspection device as required.