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1、1,The Programmable Logic Controller,John R.Hackworth,Frederick D.Hackworth,Jr.,Programmable Logic Controllers:Programming Methods and,Applications.2.52.8,The Programmable Logic Controller,John R.Hackworth,Frederick D.Hackworth,Jr.,First Edition.London:Prentice Hall,2003.3550,2.5 System Block Diagram
2、A Programmable Controller is a specialized computer.Since it is a computer,it has all thebasic component parts that any other computer has;a Central Processing Unit,Memory,InputInterfacing and Output Interfacing.A typical programmable controller block diagram is shown inFigure 2.5.,Figure 2.5,Progra
3、mmable Controller Block Diagram,The Central Processing Unit(CPU)is the control portion of the PLC.It interprets theprogram commands retrieved from memory and acts on those commands.In present day PLCsthis unit is a microprocessor based system.The CPU is housed in the processor module ofmodularized s
4、ystems.Memory in the system is generally of two types;ROM and RAM.The ROM memory containsthe program information that allows the CPU to interpret and act on the Ladder Logic programstored in the RAM memory.RAM memory is generally kept alive with an on-board battery so,that ladder programming is not
5、lost when the system power is removed.,This battery can be a,standard dry cell or rechargeable nickel-cadmium type.Newer PLC units are now available withElectrically Erasable Programmable Read Only Memory(EEPROM)which does not require abattery.Memory is also housed in the processor module in modular
6、 systems.Input units can be any of several different types depending on input signals expected asdescribed above.The input section can accept discrete or analog signals of various voltage andcurrent levels.Present day controllers offer discrete signal inputs of both AC and DC voltagesfrom TTL to 250
7、 VDC and from 5 to 250 VAC.Analog input units can accept input levels suchas 10 VDC,5 VDC and 4-20 ma.current loop values.Discrete input units present each input tothe CPU as a single 1 or 0 while analog input units contain analog to digital conversion circuitryand present the input voltage to the C
8、PU as binary number normalized to the maximum countavailable from the unit.The number of bits representing the input voltage or current depends2,3,upon the resolution of the unit.This number generally contains a defined number of magnitudebits and a sign bit.Register input units present the word inp
9、ut to the CPU as it is received(Binaryor BCD).,Output units operate much the same as the input units with the exception that the unit iseither sinking(supplying a ground)or sourcing(providing a voltage)discrete voltages orsourcing analog voltage or current.These output signals are presented as direc
10、ted by the CPU.The output circuit of discrete units can be transistors for TTL and higher DC voltage or Triacsfor AC voltage outputs.For higher current applications and situations where a physical contactclosure is required,mechanical relay contacts are available.These higher currents,however,aregen
11、erally limited to about 2-3 amperes.The analog output units have internal circuitry whichperforms the digital to analog conversion and generates the variable voltage or current output.,2.6 Update-Solve the Ladder-Update,When power is applied to a programmable logic controller,the PLCs operation cons
12、ists oftwo steps:(1)update inputs and outputs and(2)solve the ladder.This may seem like a verysimplistic approach to something that has to be more complicated but there truly are only thesetwo steps.If these two steps are thoroughly understood,writing and modifying programs andgetting the most from
13、the device is much easier to accomplish.With this understanding,thethings that can be undertaken are then up to the imagination of the programmer.,You will notice that the“update-solve the ladder”sequence begins after startup.The actualstartup sequence includes some operations transparent to the use
14、r or programmer that occurbefore actual PLC operation on the user program begins.During this startup there may beextensive diagnostic checks performed by the processor on things like memory,I/O devices,communication with other devices(if present)and program integrity.In sophisticated modularsystems,
15、the processor is able to identify the various module types,their location in the systemand address.This type of system analysis and testing generally occurs during startup beforeactual program execution.,2.7 Update,The first thing the PLC does when it begins to function is update I/O.This means that
16、 alldiscrete input states are recorded from the input unit and all discrete states to be output aretransferred to the output unit.Register data generally has specific addresses associated with it forboth input and output data referred to as input and output registers.These registers are availableto
17、the input and output modules requiring them and are updated with the discrete data.Since this,is input/output updating,it is referred to as I/O Update.The updating of discrete input and outputinformation is accomplished with the use of input and output image registers set aside in the PLCmemory.Each
18、 discrete input point has associated with it one bit of an input imageregister.Likewise,each discrete output point has one bit of an output image register associatedwith it.When I/O updating occurs,each input point that is ON at that time will cause a 1 to be setat the bit address associated with th
19、at particular input.If the input is off,a 0 will be set into the bitaddress.Memory in todays PLCs is generally configured in 16 bit words.This means that one,word of memory can store the states of 16 discrete input points.,Therefore,there may be a,number of words of memory set aside as the input and
20、 output image registers.At I/O update,thestatus of the input image register is set according to the state of all discrete inputs and the status,of the output image register is transferred to the output unit.,This transfer of information,typically only occurs at I/O update.It may be forced to occur a
21、t other times in PLCs which havean Immediate I/O Update command.This command will force the PLC to update the I/O at othertimes although this would be a special case.,One major item of concern about the first output update is the initial state of outputs.,This,is a concern because their may be outpu
22、ts that if initially turned on could create a safety hazard,particularly in a system which is controlling heavy mechanical devices capable of causing bodilyharm to operators.In some systems,all outputs may need to be initially set to their off state toinsure the safety of the system.However,there ma
23、y be systems that require outputs to initially beset up in a specific way,some on and some off.This could take the form of a predetermined setupor could be a requirement that the outputs remain in the state immediately beforepower-down.More recent systems have provisions for both setup options and e
24、ven a combinationof the two.This is a prime concern of the engineer and programmer and must be defined as thesystem is being developed to insure the safety of personnel that operate and maintain theequipment.Safety as related to system and program development will be discussed in a laterchapter.2.8
25、Solve the LadderAfter the I/O update has been accomplished,the PLC begins executing the commandsprogrammed into it.These commands are typically referred to as the ladder diagram.The ladderdiagram is basically a representation of the program steps using relay contacts and coils.The,ladder is drawn wi
26、th contacts to the left side of the sheet and coils to the right.,This is a,holdover from the time when control systems were relay based.This type of diagram was usedfor the electrical schematic of those systems.A sample ladder diagram is shown in Figure 2-6.4,Figure 2.6,Sample Ladder Diagram,The sy
27、mbols used in Figure 2.6 may be foreign at this point,so a short explanation will benecessary.The symbols at the right of the ladder diagram labeled CR1,CR2,CR3 and CR4 andare circular in shape are the software coils of the relays.The symbols at the left which look likecapacitors,some with diagonal
28、lines through them,are the contacts associated with the coils.Thesymbols that look like capacitors without the diagonal lines through them are normally opencontacts.These are analogous to a switch that is normally off.When the switch is turned on,thecontact closes.The contact symbols at the left tha
29、t look like capacitors with diagonal linesthrough them are normally closed contacts.A normally closed contact is equivalent to a switchthat is normally turned on.It will turn off when the switch is actuated.As can be seen in Figure 2.6,contact and coil position is as described above.Also,one cansee
30、the reason for the term ladder diagram if the rungs of a stepladder are visualized.In fact,eachcomplete line of the diagram is referred to as one rung of logic.The actual interpretation of thediagram will also be discussed later although some explanation is required here.The contactconfiguration on
31、the left side of each rung can be visualized as switches and the coils on the rightas lights.If the switches are turned on and off in the proper configuration,the light to the rightwill illuminate.The PLC executes this program from left to right and top to bottom,in thatorder.It first looks at the s
32、witch(contact)configuration to determine if current can be passed tothe light(coil).The data for this decision comes from the output and input image registers.Ifcurrent can be passed,the light(coil)will then be turned on.If not,the light(coil)will be turned,off.This is recorded in the output image r
33、egister.,Once the PLC has looked at the left side of,therung it ignores the left side of the rung until the next time it solves that particular rung.Once5,the light(coil)has been either turned on or off it will remain in that state until the next time thePLC solves that particular rung.After solving
34、 a rung,the PLC moves on to solve the next rung inthe same manner and so forth until the entire ladder has been executed and solved.One rule thatis different from general electrical operation is the direction of current flow in the rung.In aladder logic,rung current can only flow from left to right
35、and up and down;never from right toleft.As an example,in the ladder shown in Figure 2.7,coil CR1 will energize if any of thefollowing conditions exist:,Figure 2.7,Illustration of allowed current flow in ladder rung,1.CR7 is off,CR6 is on.2.CR7 is off,CR2 is on,CR5 is on.3.CR7 is off,CR2 is on,CR3 is
36、 on.4.CR1 is on,CR4 is on,CR3 is on.5.CR1 is on,CR4 is on,CR5 is on.You will notice that the current flow in the circuit in each of the cases listed above is fromleft to right and up and down.CR1 will not energize in the case listed below:CR1 is on,CR4 is on,CR2 is on,CR6 is on,CR5 is off,CR3 is off
37、,CR7 is on.This is because current would have to flow from right to left through the CR2 contact.This is not allowed in ladder logic even though current could flow in this direction if we were tobuild it with real relays.Remember,we are working in the software world not the hardwareworld.To review,a
38、fter the I/O update,the PLC moves to the first rung of ladder logic.It solves thecontact configuration to determine if the coil is to be energized or de-energized.It then energizesor de-energizes the coil.After this is accomplished,it moves to the left side of the next rung and,repeats the procedure
39、.This continues until all rungs have been solved.,When this procedure is,complete with all rungs solved and all coils in the ladder set up according to the solution of eachrung,the PLC proceeds to the next step of its sequence,the I/O update.6,At I/O update,the states of all coils which are designat
40、ed as outputs are transferred from theoutput image register to the output unit and the states of all inputs are transferred to the inputimage register.Note that any input changes that occur during the solution of the ladder areignored because they are only recorded at I/O update time.The state of ea
41、ch coil is recorded tothe output image register as each rung is solved.However,these states are not transferred to theoutput unit until I/O update time.Figure2.8 Scan CycleThis procedure of I/O update and solving the ladder diagram and I/O update is referred to asscanning and is represented in Figur
42、e 2.8.The period between one I/O update and the next isreferred to as one Scan.The amount of time it takes the PLC to get from one I/O update to thenext is referred to as Scan Time.Scan time is typically measured in milliseconds and is related to,the speed of the CPU and the length of the ladder dia
43、gram that has to be solved.,The slower the,processor or the longer the ladder diagram,the longer the scan time of the system.The speed atwhich a PLC scans memory is referred to as Scan Rate.Scan rate units are usually listed inmsec/K of memory being utilized for the program.As an example,if a partic
44、ular PLC has a ratedscan rate of 8 msec/K and the program occupies 6K of memory,it will take the PLC 48 msec tocomplete one scan of the program.7,1.1 Motivation,Programmable Logic Controllers(PLC),a computing device invented by Richard E.Morley,in 1968,have been widely used in industry including man
45、ufacturing systems,transportation,systems,chemical process facilities,and many others.At that time,the PLC replaced the,hardwired logic with soft-wired logic or so-called relay ladder logic(RLL),a programming,language visually resembling the hardwired logic,and reduced thereby the configuration time
46、,from 6 months down to 6 days Moody and Morley,1999.,Although PC based control has started to come into place,PLC based control will remain,the technique to which the majority of industrial applications will adhere due to its higher,performance,lower price,and superior reliability in harsh environme
47、nts.Moreover,according to,a study on the PLC market of Frost and Sullivan 1995,an increase of the annual sales volume,to 15 million PLCs per year with the hardware value of more than 8 billion US dollars has been,predicted,though the prices of computing hardware is steadily dropping.The inventor of
48、the,PLC,Richard E Morley,fairly considers the PLC market as a 5-billion industry at the present,time.,Though PLCs are widely used in industrial practice,the programming of PLC based control,systems is still very much relying on trial-and-error.Alike software engineering,PLC software,design is facing
49、 the software dilemma or crisis in a similar way.Morley himself emphasized this,aspect most forcefully by indicating Moody and Morley,1999,p.110:,If houses were built like software projects,a single woodpecker could destroy civilization.”,Particularly,practical problems in PLC programming are to eli
50、minate software bugs and to,reduce the maintenance costs of old ladder logic programs.Though the hardware costs of PLCs,are dropping continuously,reducing the scan time of the ladder logic is still an issue in industry,so that low-cost PLCs can be used.,In general,the productivity in generating PLC
