船舶燃油粘度控制系统设计外文翻译资料-毕设翻译网

1. Introduction

Programmable logic controllers [PLC] are computer-based, solid-state, single processor devices that emulate the behavior of an electric ladder diagram capable of controlling many types of industrial equipment and entire automated systems . PLCs are usually a main part of automatic systems in industry . They are very efficient and reliable in applications involving sequential control and the synchronization of processes and auxiliary elenot;ments in the manufacturing, chemical and process industries [4,5]. Besides having technological advantages of using PLC, it also decreases the prices in the advanced level and complex control system [5-7]. Nowadays, most of the control elements used to execute the logic of the system was substituted by the PLCs [8].

The term logic is used because the programming is primarily concerned with implementing logic and switching operations. Input devices such as switches, and output devices such as motors, being controlled are connected to the PLC and then the controller monitors the inputs and outputs according to the machine or process [9]. Originally PLCs were designed as a replacement for hard-wired relay and timer logic control systems. (Hard-wiring means that all of the components were manually connected by wires). PLC consists of two parts i.e. the PLC hardware and pronot;gramming. Details on the hardware and programming will be discussed in the Sections 3 and 4, respectively.

PLCs were first used by the automotive industry in the late 1960s [2,10-13], its automated equipment was primarily connot;trolled by discrete inflexible circuits consisting of electronot;mechanical relays and coils hardwired on panels. General Motors developed the specifications for a programmable controller that could replace the hard-wired relay circuits [2,11,12]. The most radical idea, was the implementation of a programming language based on a relay schematic diagram, with inputs (from limit switches, pushbuttons, etc.) represented by relay contact, and outputs (to solenoids, motor starters, lamps, etc.) represented by relay coils [12]. Fig. 1(a) shows a simple hydraulic cylinder which can be extended or retracted by pushbuttons. Its stroke is set by limit switches which open at the end of travel, and the solenoids can only be operated if the hydraulic pump is running. This would be controlled by the computer program of Fig. 1(b) which is identical to the relay circuit needed to control the cylinder. These programs look like the rungs on a ladder, and were consequently called lsquo;Ladder Diagrams.

In the mid-1960s, Hydramatic, a division of General Motors Corporation, envisioned that a computer could be used to perform the logic functions then performed by relays . The engineering team wrote a list of features of the proposed computing device. GM initiated the development of the computing device by specinot;fying certain design criteria, including:

bull; The device must be durable so that it can operate in the harsh environments (dirty air, humidity, vibration, electrical noise, etc.) encountered in a factory

bull; It must provide flexibility by implementing circuit modificanot;tions quickly and easily through software changes.

bull; It must be designed to use a programming language in ladder diagram form already familiar to technicians and electricians.

bull; It must allow field wiring to be terminated on input/output terminals of the controller.emsp;

GM used this list of specifications when it solicited interested companies to develop a device that met its design requirements. Dick Morley conceived the first programmable controller on Jannot;uary 1, 1968 [14]. When his company, Gould Modicon Company developed the first PLC [2,11,14], the first model 084 PLC was installed at the Oldsmobile Division of General Motors Corporation and the Landis Company in Landis, Pennsylvania. The first PLC was large and expensive. They were capable of On-Off control only, which limited their applications to operations that required repetitive movements.

Innovations and improvements in microprocessor technology and software programming techniques have added more features and capabilities to the PLC. This enhancements enable the PLC to perform more complex motion and process control applications, and with greater speed.

Presently, more than a dozen manufacturers produce PLCs, Table 1. Most of these companies make several models that vary in size, cost and sophistication to meet the needs of specific applications.

No Manufacturer No Manufacturer

1 Siemens 9 Panasonic

2 ABB 10 Idec

3 Schneider (Modicon) 11 Keyence

4 Rockwell (Allen-Bradley) 12 Toshiba

5 Mitsubishi 13 Fuji

6 GE-Fanuc 14 Beckhoff

7 Omron 15 Bosch Rexroth

8 Koyo 16 Rockwell/Allen-Bradley

Table 1

List of PLC manufacturers in the global market.

2. Personal Computer (PC) versus PLC

The original design for the programmable logic controller was called a programmable controller, or PC [14]. The abbreviation caused no confusion until the personal computer became widely used and also adopted the PC abbreviation. To avoid confusion, the programmable controller industry added the word logic in the title, producing the new term programmable logic controller, of PLC.

A modern PLC is a computer-based device designed to control a process [1,15]. It relates information coming from sensors that monitor the state of a process, with the status of some actuators that are capable of changing it. Although PLC and Personal Comnot;puters (PC) are both computers, there are some significant differnot;ences [10,14,16].

Let us look at the similarities. The architecture of the PC and PLC systems are similar, with both featuring a motherboard, pronot;cessor, memory, and expansion slots [14].

The differences are that PLCs processor has a microprocessor chip linked to memory and I/O (input/output) chips through parnot;allel address, data,

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