1、外文资料 What is a PLC? Programmable Logic Controllers (PLCs) consist of input modules or points , a Central Processing Unit(CPU) , and output modules or points . It is actually a control device that consists of a programmable microprocessor , and is programmed using a specialized computer language . A
2、PLC monitors inputs , makes decisions based on its program , and controls output to automate process or machine . An input accepts a variety of digital or analog signals from various field devices (sensors) and converts them into a logic signal that can be used by the CPU. The CPU makes decisions an
3、d executes control instructions based on program instructions from the CPU into a digital or analog signal that can be used to control various field devices(actuators).A programming device is used to input the desired instructions.These instructions determine what the PLC will do for a specific inpu
4、t.An operator interface device allows process information to be displayed and new control parameters to ve entered. Pushbuttons(sensors),in this simple examole,connecting to PLC input ,can be used to start and stop a motor which is in connection with a PLC through a motor starter(actuator). Before,a
5、 programmable logic controller would have been programmed in ladder logic, which is similar to a schematic of relay logic.A modern programmable logic controller is usually programmed in any one of several languages,ranging from ladder logic to Basic or C.Typically, the program is written in a develo
6、pment environment on a personal computer(PC),and the is downloaded onto the programmable logic controller directly through a cable connection.The program is stored in the programmable logic controller in non-volatile memory. PLCs are used in commercial and industrial fieds.They have made significant
7、 contributions to factory automation.Earlier automation systems had to use thousands of individual relays and cam timers,but all of the relays and timers within a factory system can often be replaced with a single programmable logic controller.Today, programmable logic controllers deliver a wide ran
8、ge of functions, including basic relay control,as well as being used in distributed control systems. Siemens makes several PLC product lines in the SIMATIC(r)S7 family.They are:S7-200,S7-300,and S7-400. (1) S7-200 The S7-200 is called as a micro-PLC because of its small size.The S7-200 can be used o
9、n smaller,stand-alone applications,such as elevators,car washes,or mixing machines.It can also be used on more complex industrial field, such as bottling and packaging machines. (2) S7-300 and S7-400 The S7-300 and S7-400 PLCs are used in more complex fields that support a greater number of I/O poin
10、ts. Both PLCs are modular and expandable.The power supply and I/O consist of separate modules connected to the CPU. Programming Devices The program is created in a programming device(PG) and then transferred to the PLC.The program for the S7-200 can be created by using a dedicated Siemens SIMATIC S7
11、or PG 720 or PG 740 ,if STEP7 Micro/WIN software is installed. Connector Cables PPI(Point-to-Point Interface) Connector cables are required to transfer data from the programming device to the PLC.Communication can only take place when the two devices speak the same language or protocol.Communication
12、 betweent a Siemens programming device and the S7-200 uses a 9-pin,D-connector.This is a straight-through serial device that is compatible with Siemens programming devices (MPI port) and is a standard connector for other serial interfaces. A special cable,referred to as a PC/PPI cable, is needed whe
13、n a personal computer is used as a programming device.This cable allows the serial interface of the PLC to communicate with the RS-232 serial interface of a personal computer.DIP switching on the PC/PPI cable are used to select an appropriate speed (bit rate) at which information is passed betweed t
14、he PLC and the computer. Development At a lot of situations, the list is is a smooth movement that cant guarantee the equipments by the control of the single machine, but pass the information exchanges of the equipments and equipments to attain the result that we want. For example fore pack and the
15、examination of the empress work preface, we will arrive wrapping information feedback to examine the place, and examine the information of the place to also want the feedback to packing. Pass the information share thus to make both the chain connect, becoming a total body, the match of your that thu
16、s make is more close, at each other attain to reflect the result that mutually flick. The PLC correspondence has already come more more body now its value, at the PLC and correspondence between PLCs, can pass the communication of the information and the share of the datas to guarantee that of the eq
17、uipments moderates mutually, the result that arrive already to repair with each other. Data conversion the adoption RS232 between PLC connect to come to the transmission data, but the RS232 pick up a people and can guarantee 10 meters only of deliver the distance, if in the distance of 1000 meters w
18、e can pass the RS485 to carry on the correspondence, the longer distance can pass the MODEL only to carry on deliver. The PLC data transmission is just to be called a form to it in a piece of and continuous address that the data of the inner part delivers the other party, we, the PLC of the other pa
19、rty passes to read data in the watch to carry on the operation. If the data that data in the watch is a to establish generally, that is just the general data transmission, for example today of oil price rise, I want to deliver the price of the oil price to lose the oil ally on board, that is the sha
20、re of the data； But take data in the watch for an instruction procedure that controls the PLC, that had the difficulty very much, for example you have to control one pedestal robot to press the action work that you imagine, you will draw up for it the form that a procedure combine with the data send
21、s out to pass by. The form that information transport contain single work, the half a work and the difference of a workses .The meaning of the single work also is to say both, a can send out only, but a can receive only, for example a spy he can receive the designation of the superior only, but cant
22、 give the superior reply； A work of half is also 2 and can can send out similar to accept the data, but cant send out and accept at the same time, for example when you make a phone call is to cant answer the phone, the other party also； But whole pair works is both can send out and accept the data,
23、and can send out and accept at the same time. Be like the Internet is a typical example. The process that information transport also has synchronous and different step cent: The data line and the clock lines are synchronous when synchronous meaning lie in sending out the data, is also the data signa
24、l and the clock signals to be carry on by the CPU to send out at the same time, this needs to all want the specialized clock signal each other to carry on the transmission and connect to send, and is constrained, the characteristics of this kind of method lies in its speed very quick, but correspond
25、 work time of take up the CPU and also want to be long oppositely, at the same time the technique difficulty also very big. Its request lies in canting have an error margins in a datas deliver, otherwise the whole piece according to compare the occurrence mistake, this on the hardware is a bigger difficulty. Applied more and more extensive
