陶瓷注塑機控制系統的設計畢業設計-資料下載頁

　　

【正文】 ecause 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 displays are available as well as graphical touch screens. More plex systems use a programming and monitoring software installed on a puter, with the PLC connected via a munication interface. PLC has built in munications ports, usually 9pin RS232, but optionally EIA485 or Ethernet. Mudbugs, Banat or DF1 are usually included as one of the munications protocols. Other options include various field buses such as Device Net or Prefabs. Other munications protocols that may be used are listed in the List of automation protocols. Most modern PLC can municate over a network to some other system, such as a puter running a SCADA (Supervisory Control and Data Acquisition) system or web browser. PLC used in larger I/O systems may have peertopeer (P2P) munication between processors. This allows separate parts of a plex process to have individual control while allowing the subsystems to coordinate over the munication link. These munication links are also often used for HMI devices such as keypads or PCtype workstations. PLC programs are typically written in a special application on a personal puter, and then downloaded by a directconnection cable or over a network to the PLC. The program is stored in the PLC either in batterybackedup RAM or some other nonvolatile flash memory. Often, a single PLC can be programmed to replace thousands of relays. Under the IEC 611313 standard, PLC can be programmed using standardsbased programming languages. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers. Initially most PLC utilized Ladder Logic Diagram Programming, a model which emulated electromechanical control panel devices (such as the contact and coils of relays) which PLC replaced. This model remains mon today. IEC 611313 currently defines five programming languages for programmable control systems: FBD (Function block diagram), LD (Ladder diagram), ST 6(Structured text, similar to the Pascal programming language), IL (Instruction list, similar to asse