obtained by state-of-the-art methods of the late 1940s inspired John Parsons[1], President of the Parsons Works of Traverse City, Michigan, to propose that a by- the-numbers technique (commonly used by machinists of that era) be placed under servo control with positional data generated by a computer, thereby providing much more data than would be practical by means of hand calculations. His concept was to machine to setpoints as guides for subsequent manual finishing, that is, to speed up a manual process so more points could be included. Mr. Parsons' project was enjoined by the Servo Mechanisms Laboratory of the Massachusetts Institute of Technology (MIT) and redefined as interpolative positional control that caused the cutting tool to traverse a series of straight lines between defined points at a prescribed rate of travel. Thus, the cutting tool would be almost constantly on the programmed contour and would spend very little of its time making non-cutting moves. In the MIT scheme, a contour of constantly changing curvature was represented as a poly-line with the intersections between line segments being points on the curve, and the axial coordinates of these points were listed for execution in sequential order in the part program (much like the figure which results from connecting-the-dots in an activity book). The shorter the line segments the more accurately the poly-line would approximate the actual curve. Thus, MIT retained separation of programming from operations while redefining the servo control as interpolative, rather than discretionary, positioning. MIT demonstrated the first ever NC machine tool to a select group from the military, the aerospace industry, the machine tool industry and the technical media in September, 1952. At the time when MIT was developing numerical control, engineers at General Motors were putting position transducers on the lead screws of a conventional engine lathe and recording the motion of the axes as the machinist put the machine through its paces to make a workpiece. The machine was also fitted with a servo system that took data from the recording to reproduce the same sequence of motion to produce a second, third and more parts. This technique is called record/playback and it is reminiscent of a musician playing on a piano that has been modified to record the keystrokes on a paper chart which can be read by a player piano to reproduce the music. The popular novel Player Piano was inspired by this machine. The author, Kurt Vonnegut, was exposed to the machine when he worked as a publicist for General Electric. Record/playback is different from numerical control in that the program is produced by the machinist in the process of making the first part. The Air Force wanted numerical control and not record/playback because 1) the latter put the machinists who were union members in charge of program production, thus union strikes could result in unacceptable delays in military production, and 2) numerical control demonstrated the capability of producing complex parts that were not possible by the conventional manual methods used in the record/playback technique. The Air Force used its deep pockets to get its way and while American manufacturing may have been better served by the simpler Parsons concept or by record/playback, today this is a moot issue. The electronic files used to control NC and CNC machines are often in a format called G-code, after Gerber Scientific Instruments[2], a manufacturer of photoplotters and developer of the file format. The X-Y two-dimensional motion of photoplotters was extended to include the third Z axis, and along with special codes, allows milling machines to be steered in more than three axes. Many of the lines of text in the control files start with the ASCII letter G, thus the name; however, there are other commands that start with the letter D and M, as well as X and Y for coordinates. The file format became so widely used that it has been embodied in an EIA standard.
arrangements. These connect the PLC to sensors and actuators. PLCs read limit switches, analog process variables (such as temperature and pressure), and the positions of complex positioning systems. Some even use machine vision. On the actuator side, PLCs operate electric motors, pneumatic or hydraulic cylinders or diaphragms, magnetic relays or solenoids, or analog outputs. The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a computer network that plugs into the PLC. PLCs were invented as replacements for automated systems that would use hundreds or thousands of relays, cam timers, and drum sequencers. Often, a single PLC can be programmed to replace thousands of relays. Programmable controllers were initially adopted by the automotive manufacturing industry, where software revision replaced the re-wiring of hard-wired control panels when production models changed.
logic which appeared similar to electrical schematic diagrams. The electricians were quite able to trace out circuit problems with schematic diagrams using ladder logic. This program notation was chosen to reduce training demands for the existing technicians. Other early PLCs used a form of instruction list programming, based on a stack-based logic solver. The functionality of the PLC has evolved over the years to include sequential relay control, motion control, process control, distributed control systems and networking. The data handling, storage, processing power and communication capabilities of some modern PLCs are approximately equivalent to desktop computers. PLC-like programming combined with remote I/O hardware, allow a general-purpose desktop computer to overlap some PLCs in certain applications. Under the IEC 61131-3 standard, PLCs can be programmed using standards- based programming languages. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers. |
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