陶瓷注塑機控制系統(tǒng)的設計畢業(yè)設計(文件)

　

【正文】 LC solve the logic in a predictable and repeating sequence, and ladder logic allows the programmer (the person writing the logic) to see any issues with the timing of the logic sequence more easily than would be possible in other formats. Programming Early PLC, up to the mid1980s, were programmed using proprietary programming panels or specialpurpose programming terminals, which often had dedicated function keys representing the various logical elements of PLC programs. Programs were stored on cassette tape cartridges. Facilities for printing and documentation were very minimal due to lack of memory capacity. The very oldest PLC used nonvolatile magnetic core memory. More recently, PLC are programmed using application software on personal puters. The puter is connected to the PLC through Ethernet, RS232, RS485 or RS422 cabling. The programming software allows entry and editing of the 3ladderstyle logic. Generally the software provides functions for debugging and troubleshooting the PLC software, for example, by highlighting portions of the logic to show current status during operation or via simulation. The software will upload and download the PLC program, for backup and restoration purposes. In some models of programmable controller, the program is transferred from a personal puter to the PLC though a programming board which writes the program into a removable chip such as an EEPROM or EPROM. 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 munication capabilities of some modern PLC are approximately equivalent to desktop puters. PLClike programming bined with remote I/O hardware, allow a generalpurpose desktop puter to overlap some PLC in certain applications. Regarding the practicality of these desktop puter based logic controllers, it is important to note that they have not been generally accepted in heavy industry because the desktop puters run on less stable operating systems than do PLC, and because the desktop puter hardware is typically not designed to the same levels of tolerance to temperature, humidity, vibration, and longevity as the processors used in PLC. In addition to the hardware limitations of desktop based logic, operating systems such as Windows do not lend themselves to deterministic logic execution, with the result that the logic may not always respond to changes in logic state or input status with the extreme consistency in timing as is expected from PLC. Still, such desktop logic applications find use in less critical situations, such as laboratory automation and use in small facilities where the application is less demanding and critical, because they are generally much less expensive than PLC. In more recent years, small products called PLR (programmable logic relays), and also by similar names, have bee more mon and accepted. These are very much like PLC, and are used in light industry where only a few points of I/O (. a few signals ing in from the real world and a few going out) are involved, and low cost is desired. These small devices are typically made in a mon physical size and shape by several manufacturers, and branded by the makers of larger PLC to fill out their low end product range. Popular names include PICO Controller, NANO PLC, and other names implying very small controllers. Most of these have between 8 and 412 digital inputs, 4 and 8 digital outputs, and up to 2 analog inputs. Size is usually about 4 wide, 3 high, and 3 deep. Most such devices include a tiny postage stamp sized LCD screen for viewing simplified ladder logic (only a very small portion of the program being visible at a given time) and status of I/O points, and typically these screens are acpanied by a 4way rocker pushbutton plus four more separate pushbuttons, similar to the key buttons on a VCR remote control, and used to navigate and edit the logic. Most have a small plug for connecting via RS232 or RS485 to a personal puter so that programmers can use simple Windows applications for programming instead of being forced to use the tiny LCD and pushbutton set for this purpose. Unlike regular PLC that are usually modular and greatly expandable, the PLR are usually not modular or expandable, but their price can be two orders of magnitude less than a PLC and they still offer robust design and deterministic execution of the logic. The main difference from other puters is that PLC is armored for severe conditions (such as dust, moisture, heat, cold) and have the facility for extensive input/output (I/O) arrangements. These connect the PLC to sensors and actuators. PLC read limit switches, analog process variables (such as temperature and pressure), and the positions of plex positioning systems. Some use machine vision. On the actuator side, PLC operates electric motors, pneumatic or hydraulic cylinders, magnetic relays, 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 puter network that plugs into the PLC. scale a small PLC will have a fixed number of connections built in for inputs and outputs. Typically, expansions are available if the base model has insufficient I/O. Modular PLC have a chassis (also called a rack) into which are placed modules with different functions. The processor and selection of I/O modules is customized for the particular application. Several racks can be administered by a single processor, and may have thousands of inputs and outputs. A special high speed serial I/O link is used so that racks can be distributed away from the processor, reducing the wiring costs for large plants.PLC may need to interact with people for the purpose of configuration, alarm reporting or everyday control. A simple system may use buttons and lights to interact with the user. Text