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INDUSTRIAL CONTROL HANDBOOK - 2.5 SPECIAL-PURPOSE ACTUATOR SYSTEMS

Some devices used as actuators are complete automated systems in themselves. Robots are a prime example.

Off-the-shelf X-Y tables are used as components in many systems where reprogrammable position control is desired. The X-Y table, as shown in Figure 2-13, is just two servocontrolled linear actuators (often servomotors and ballscrews) with their linear axes perpendicular to each other. The X-Y table may move the table under a stationary tool (such as a glue dispenser), or may move a tool over a stationary worktable. Programmable X-Y tables are used to control water-jet cutters that are used to cut shoe leather, plastics, and floor mats for cars. Clothing manufacturers are improving efficiency by using powerful X-Y tables to move knives or lasers used to cut materials. The material cutter in Figure 2-14 can simultaneously cut many layers of material, and a computer-optimizes the nesting of components to reduce waste. Even the woodworking industry is getting into the act. X-Y tables are used in automatic panel cutters to saw7 computer-optimized patterns from four by eight feet sheets of plywood and veneer.

Numerical control (NC) equipment can cut metal under the control of a built-in digital controller instead of being operated by a human operator.

A piece of NC equipment combines X-Y motion of a workpiece or cutting tool with a third, the Z axis. In addition to linear position control, control of rotation about each of the three axes is available with some NC equipment. Additional controllable features include the rotational speed of the tool or workpiece and the flow of coolant.

Early NC lathes and mills required a program of instructions to be prepared as patterns of holes punched into paper or plastic tape. The tape was read one instruction at a time while the workpiece was machined. The program tapes were typed using equipment specifically designed to punch tape.

Figure 2-12 Ballscrew and ballnut.

Figure 2-12 Ballscrew and ballnut. (Photograph by permission, Warner Electric/Dana Corp., South eliot, Illinois.)


Figure 2-13 An X-Y table.

Figure 2-13 An X-Y table. (Photograph by permission, Panasonic Factory Automation Company, Franklin Park, Illinois.)


Later, read/write memory was added in NC controllers, so the tape had to be read only once into memory. The NC equipment became known as Computer Numerical Control (CNC) equipment. Meanwhile, advances in tape punchers allowed programmers to write whole NC programs at a standard computer terminal, then send the program to a tape puncher.

Logically, the tape was eliminated in the next evolution by having the central computer write programs directly into CNC memory. The CNC equipment became known as Direct Numerical Control (DNC) equipment.

There are systems houses that will build special-purpose automated equipment units of any kind. Each of these custom-designed systems can be used as a component in a larger system, with the larger system's controller controlling the sub-systems' controllers.

This chapter on actuators will never end if we include custom equipment. There are, however, some types of special-purpose equipment that are built in sufficient quantity and with sufficient standard features that they are worth mentioning.

Some special-purpose systems assemble electrical components onto circuit boards. These component insertion machines can be purchased to handle any of the electronic component shapes. A typical component insertion machine might remove components from their packaging in a roll, cut and bend the leads to the proper length and shape, then insert the component into the correct location on the board. Programmable component insertion machines can select components and positions.

Some systems convey assembled circuit boards through component soldering machines, where solder is applied or where pre-applied solder on the boards is heated until it flows to attach the components and then transports the circuit boards through board cleaning machines where solder flux is washed off.

Completed circuit boards are often tested by automated circuit and component testers, in which the boards are drawn down onto an array of probes, and the tester automatically runs through a series of tests, notifying the human or computer controller if a fault is detected. Chapter 9 describes an automation system that includes a circuit board tester.

Figure 2-14 An X-Y table for cutting material for clothing.

Figure 2-14 An X-Y table for cutting material for clothing. (Photograph by permission, Gerber Garment Technologies, Tolland, Connecticut.)

GO TO NEXT PAGE: INDUSTRIAL CONTROL HANDBOOK - 2.6 CONSTRUCTION OF ELECTRIC MOTORS

GO BACK TO PREVIOUS PAGE: INDUSTRIAL CONTROL HANDBOOK - 2.4 HYDRAULIC ACTUATORS AND VALVES

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