AUTOMATION AND ROBOTICS:TECHNOLOGIES FOR ASSEMBLY OPERATIONS

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TECHNOLOGIES FOR ASSEMBLY OPERATIONS

Several technologies highly relevant to assembly are discussed briefly below.

Feeding Systems

Sorting and feeding devices are an important component in the automated assembly of products. Their task is to sort out the often unsorted supplied workpieces and to deliver the right amount to the assembly station at the right time. The most commonly used feeding systems are vibratory bowl feeders, capable of sorting nearly 90% of all components automatically.

The unsorted workpieces that have been fed in are moved by triggered vibration energy to the edge of the bowl. They then run onto a sloped helical channel on the inside or outside wall of the bowl and are moved upwards over mechanical baffles. These baffles have the task of orienting theAutomation and Robotics-0140Automation and Robotics-0141

disoriented workpieces during the transfer movement by means of turning, twisting, tilting, or setting upright the workpieces and / or throwing back wrongly oriented workpieces into the bin. However, mechanical baffles are set up for only one type of workpiece and much retooling is required if the workpiece is changed. These pieces of equipment are therefore suitable only for large series assembly.

To increase flexibility, mechanical baffles are being replaced more and more with optical parts- recognition systems in the vibratory bowl feeder. The geometry of the various workpieces can be programmed, stored in, and retrieved from the control system if there is a change of product. This means that retooling can be kept to a minimum.

Image processing as a sensor for guiding robots is also being applied more and more in flexible workpiece-feeding systems. Isolation of unsorted workpieces is conducted on a circular system of servocontrolled conveyor belts. The correctly oriented workpieces on one of these conveyor bands

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are recognized by a camera and the coordinates are determined. The robot can grab only recognized workpieces; the other workpieces remain in the circular system.

The various feeding and sorting systems are depicted in Figure 29.

Magazines

Parts that cannot be easily bowl-fed can be stored in an oriented fashion in a magazine. Magazining is the storage of workpieces in a special order for stock purposes before and after the manufacturing equipment. Magazines for the simple storage of workpieces are containers with grid elements, slot- in boards, and so on. When magazines for automatic handling are used, the magazine and forwarding functions usually blend into one function. The various types of magazine are depicted in Figure 30.

Static magazines (e.g., stack magazines) are widely used especially in component manufacturing. The magazines are simply constructed and make use of the falling or rolling motion of the workpieces. Pallet magazines are used for the storage of workpieces that are mainly in sheet arrangements andAutomation and Robotics-0143

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• Single pallet

• Stacking pallet

• Drawer pallet

in a certain order. Workpiece positioning is conducted by means of a form closure. Pallet magazines are usually stackable. The pallets can be coded for controlling tasks in the material flow. They are usually used in flex-link assembly systems.

Driven and / or movable magazines are preferable when dealing with larger workpieces. These magazines are capable not only of storage but also of forwarding and / or transferring the workpieces.

Fixturing

Fixtures are the bases that securely position workpieces while the assembly system performs oper- ations such as pick-and-place and fastening. They are generally heavy and sturdy to provide the precision and stability necessary for automated assembly. They carry a variety of substructures to hold the particular workpiece. While each fixture is different, depending on the workpiece it handles, several commonsense rules for their design are suggested:

1. Design for assembly (DFA) usually results in simpler fixture design. Simple vertical stacking of parts, for example, generally allows minimal fixtures.

2. Do not overspecify fixtures; do not demand fixture tolerances not required by product toler- ances. Fixture cost is proportional to machining accuracy.

3. Fixture weight is limited by conveyor and drive capabilities. The transfer conveyor must be able to carry the fixtures. And because fixture bases get rough treatment, include replaceable wear strips where they contact the conveyor. Replacing wear strips is cheaper than replacing bases. In addition, plastic bumpers cut wear, noise, and shock.

4. Use standard toolroom components when possible. Drill bushings, pins, and clamps are avail- able from many sources. Use shoulder pins in through-holes instead of dowel pins positioned by bottoming in the hole. Through-holes are easier to drill and do not collect dirt.

5. Make sure any fragile or vulnerable parts are easily replaceable.

Sensors and Vision Systems

Sensors provide information about the automated system environment. They are the link between the technical process and its control system and can be seen as the sense organ of the technical system. They are used for a number of tasks, such as material flow control, process monitoring and regulation, control of industrial robot movements, quality inspections and industrial metrology, and for security protection and as safeguards against collisions.

The variety of sensors can be divided into technology applications as depicted in Figure 31.

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Tactile touching sensors allow simple information from the environment to be recorded by touching the objects directly. In the simplest case, the sensor is a switch that sends a binary signal when set or not set. A variety of miniaturized switches are combined with line and matrix arrangements with integrated sensor data preprocessing to enable the creation of a tactile pattern recognition similar to the human sense of touch.

Force / Torque Sensors

Force / torque sensors allow reaction forces and torques that occur during handling or processing procedures to be recorded. For example, when two parts with a low positional tolerance are joined in an assembly system, the respective motional corrections can be automatically executed due to reaction forces. A completed recording is usually conducted with rotational measurement strips and contains three forces and torques. Depending on the task, a smaller number may also be sufficient.

Video-optical Sensors

With video-optical systems, tapes of camera pictures and subsequent image processing on a computer allow statements to be made about the type, number, and position of objects at a workpiece scene. These objects are then taught to the image processing system in a specific learning process (see Figure 32).

The reflection of an illuminated workpiece passes through a lens onto the CCD chip in the camera. A 2D discrete position image is created. The camera electronically turns this image into a video signal. The image-processing computer uses a frame grabber to turn the video signal into digital images and store these in the memory. Image-processing procedures can access these image data. Despite the large amount of hardware required, the main component of image-processing systems is the image-processing software.

Image-processing systems are extremely sensitive to changes in lighting conditions. Lighting therefore plays a decisive part in the evaluation of the images. Different specific lighting procedures for gathering spatial information have been developed, such as:

• The transmitted light procedure

• The silhouette-projection procedure

• The light section procedure (triangulation)

• The structured light procedure

• The coded lighting procedure

• The Stereo image processing

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Ultrasound Sensors

An ultrasound sensor transmits a short high-frequency sound signal. If this signal is reflected by an object to the sensor and the echo is received by the sensor, it is possible to determine the distance to the object from the transfer time of the ultrasound impulse. The rate of measurement is physically limited due to the transfer time of the sound in the air. The range of sensors typically varies between 0.5 m and 6 m due to the attenuation of the sound in the air.

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