Designing issues in PLC

When we were going to design our PLC, we found choices for hardware design and software architecture. The main issue was, on what criteria should we choose to develop among a variety of possible designs.
Following are the few points that we decided to keep in consideration during the designing of our PLC.
    Our PLC should fulfill the current industrial requirements.
    Our PLC should be flexible and cheap.
    Our PLC should be compact and repairable.
    It should follow the most common standards of PLC.
    Programming method should be easy and graphical one.
    It should provide the enhanced features of PLC.
    It should have expandability feature for the future needs.
 
Modular or Non-Modular  
There are two types of PLC’s Modular and Non-Modular. Modular PLC gives more flexibility in adding and removing modules and also provides enhanced features of programming. Therefore we decided to fabricate a modular PLC with five types of prefab modules.
i.    PC Module
is.    ADC Module
iii.    Input Module
iv.    Output Module
v.    DAC (Optional)    
The number of input modules and output modules attached to the PC module (CPU) depend up on the requirements. Our PLC has a module expandability feature up to 32 modules of each input, output and ADC modules.
Due to this feature of multiple modules system, our PLC has become more flexible and powerful.




PC Based PLC
The main module or the Central Processing Unit of a PLC can be designed by using micro controllers and microprocessors based systems.
We decided to use a complete PC and Central Processing Unit of our PLC and named it, as “PC Module” following are the reasons for this choice:
I.    PC Processor can support multiple module processing and attachment without sacrifice of processing speed.
II.    PC Module provides Processing, Debugging, Programming and Visual Display feature with in a single module. This feature makes our PLC a versatile one in the era of PLC’s.
III.    Just changed the processor can increase the number of attachable modules and the processing speed of our PLC.
Cost of PLC
When we go through the prices of available PLC’s we find that the cost of straightforward and cheap PLC range is from at least few hundred US dollars to thousands of US dollars depending upon the types and features provided. We choose to design a PC based PLC to reduce the cost, skill in designing and labor in the fabrication of our PLC. This is why our PLC is cheaper than any other PLC’s available in the market with the same set of features as our PLC.  
Method of Programming
There are three standard methods of PLC programming which have been discussed earlier in the previous chapter.
But the programming method used by us is new, unique and a versatile one by providing a user-interface, a large instruction set, and an enhanced capability of programming.
This programming method is named as “PLC Visual Script.” We designed software for VB Script editing, compiling, debugging and printing. This software is powerful and easy to handle. It fulfills all the requirements of PLC programming.
Hardware of our PLC
Our PLC Hardware consist of six types of modules which are given below:
1)    PC Module                             (Main processing unit)  
2)    Input Module
3)    ADC Module                  

4)    Output Module
5)    DAC Module (Optional)


6)    MAX-232 Serial Interface
The main processing unit module (PC Module) for our PLC is only one but the number of input and output may vary as per requirement up to 32 Module of each one. Physical timer and counter Modules are replaced by the built-in software timers and counters.
Software of our PLC
The software of our PLC consists of many separate programs each program is written for the specific purpose and specific place (Module) depending upon logic.
We can categorize our software into two parts: -
1.    CPU (PC Module) related program
2.    I/O Module similar program
CPU (PC Module) related programs are given as: -
I.    PLC Hardware and Abstraction Layer Program (PLC server)
II.    PLC Debugger Program
III.    PLC VB Script compiler
I/O Module related programs
I.    Input Module Micro controller Programs
II.    ADC Module micro controller Programs
III.    Output Module Micro controller Programs
     In the CPU related program, all the source codes are written in a Visual Basic language. Hardware abstraction layer program enables the CPU to communicate with I/O Module while Debugger program helps and provides the useful information about the changing states of input and output Module according to the terms and condition are given in the Visual Script program. The other part of a program is a low-level programming used for Microcontrollers operations being used as controller units of different input and output Modules.

programming methods of plc part 2

Unlike circuit diagram, the ladder diagram is constructed schematically and does not allow how the various components are arranged. This type of programming was developed from the writing (circuit) diagram. If a circuit diagram exists for a control problem, the simplest method of programming would, therefore, be to convert this into a ladder diagram.




2.    Function Chart:
The Function table may be used both for just scanning programs and for the representation of sequence programs. In its schematic version (with commentary), it can be used as a flow chart; if on the other hand, a flow chart is presented along with a problem this can be easily converted into a Function Chart. It is also called “Control System Flow Chart (CSF).”
Logic elements shown by a basic rectangular symbol a function designation and negated inputs are indicated by a circle in front of the symbol.
If a flow chart of several steps is available, the function table should be divided into levels areas (boxes). Such an area contains the level member (starting at the top with 0) and a freely selectable commentary. The step area connects all inputs belonging to it (combination of data) to the corresponding executive elements. When a step has been fulfilled, it passes on automatically to the next Function Chart. This programming has been developed from the logic diagrams of electronics. However, these do not include a direct representation of sequence steps. Thus for PLC programming of course programs, it was necessary to introduce a chronological sequence with levels.
Statement List
Unlike LAD and CSF, the report list (STL) does not present the program graphically but describes it textually.
The announcement list is made up of individual instruction lines. It is possible to write a comment (in everyday language) to the right of each instruction line, giving a more precise description of the switching elements. The direction lines in the statement list are numbered consecutively. The set of instructions (statement) comprise various conditional and executive instructions.
The instruction (command) are listed in abbreviated form. L (“Load”) signifies the start of instruction; the logic elements AND, OR and NOT are shortened to A, O, and N respectively. The assignment “set, otherwise reset” is expressed by ‘=.' “Set” otherwise reset” signifies: this output should be set at one if the logical signal is present and at 0 if the 0 logic signal is present.


The DIN standard for the statement list does not recognize any steps but must, in the case of sequence programs work with level flags (with ladder diagrams, too, such projects must be programmed using step colors). However today there is also a form of STL programming which lists the individual stages of the program and their related instructions verbally in chronological order. With this STL even complex control problems can be presented. Boolean equations for control task and displacement step diagrams can be written with the case as STL.
Flags (Memory Relays)
Flags are internal 1-bit memories. These functions units are of extreme importance in the PLC. They store results, which are later required by the program sequence. Flags are treated as if they were simultaneously inputs and outputs. Thus they can be placed at any point instead of inputs or outputs and interrogated. Outputs can be interviewed in the same way. Flags are outputs without cards, i.e., without connection between the internal electronics of the PLC and the actuators.  
PLC VISUAL SCRIPT
A newly developed technique for programming a PLC in which each module is allotted a specific Alphabetical ID. For example Input module = X Output = Y and Analog module is = ADC. Condition codes and instructions, for performing different tasks, can be written in any of Windows based language. This idea provides the facility of language platform independence for a PLC system. Example.
if
        X. 2.1 = 1  &  X 2.3 = 1 then
        Else
    Y 2.4 = 1


PLC Basic  operations :
These operations are used in SIEMENS S5 series PLCs.

Programming Methods Of PLC

Methods of Programming

While using SIEMENS PLCs, Programs are written by using PGs (Programmers) and then transferred to the CPU module of PLC.

PGs are used to write the control programs for programmable controllers, and can also be used to test these programs when the system is put into operation.

Extensive user-friendly testing and debugging functions support troubleshooting and error recovery and a wide variety of documentation functions are available for adding program commentary, and documenting and printing out user programs.  

Programming Languages:

The Programming languages serve as a mean of communication between the user and automation and are the language in which user formulates his control task.

There are four possible programming languages:

1.                 The Ladder diagram (LAD)

2.                 The control system flowchart (CSF) or Function Chart

3.                 The statement list (STL)

4.                 PLC Visual Script

1.                 Ladder Diagram:

Like a ladder, the diagram is made up of two vertical lines. The left one represents the connection to the voltage source, the right one the connection to the earth. The various current paths (rungs) run horizontally from left to right between the two Inputs are shown by these symbols:

-] [-    :         contact; normally open contact

-] / [-  :         negated contact; normally closed contact

AND-elements from inputs are actuated by the switching of contacts in series, OR-elements by parallel switching. In a ladder diagram, a negated input is designated by a normally closed contact.

Output has the symbols  –( )–  at the right-hand end of the current path in question. When programming, each symbol is accorded a real PLC address or a short form (symbolic address).

The ladder diagram (LAD) is suitable for some control tasks, particularly if a circuit diagram is available.  

Components Of PLC

The Components of a PLC

Component
Solutions to control problems realize technically with PLC applications and may vary in their complexity. However, the following building blocks are always necessary:
Hardware:  
By hardware, we understand the electronic modules, through which all functions of the installation or machine to be controlled are addressed and actuated in the logical sequence.
Software:  
By software we mean the programs in which logic operations and the related triggering of components within the installation or machine are precisely specified. The software is filed in particular hardware memory and can be modified if required. The control sequence changes with the new program. It is not necessary to modify the hardware.
Sensors (Sensors):  
By attaching these components directly to the machine or installation to be controlled, information regarding the current status of these devices can be transmitted to the PLC. Sensors are, e.g., limit or proximately switches.
Actors (Actuators):  
These are the components fitted directly to the machine or installation to be controlled, which can be used to change status via the PLC. In other words, sequences can be influenced, or changes in status can be announced. Examples of Actuators are buzzers and solenoid valves.
Programmer:  
The developer is used to create the software and to transfer it to the PLC Memory. In most cases, it can also be used for testing software Sensorics and Actorics.

Hardware  
The most important components of the PLC device is the central control unit (CCU) which corresponds to a computer in structure. The processes taking place in the central control unit are designed processors (processing).  
The data processed and stored in the CCU are in the form of binary signals, each as a rule composed of a single bit (with status zero or one).
The input and output modules make the connection between the central control unit and the actuators/sensors. Each of these modules contains a fixed number of inputs and outputs which can assume the binary values, Zero and one. A critical component of the central control unit is the flag memory. Flag is one-bit memories, through which the PLC can “Note” the status of binary signals.
Software  
PLC programs have a concrete structure, determined in the Central Control Unit. They are created by the programmer from source programs, in principle, the programmer can create these programs by four different methods:
     As a statement list program (STL)
     As a ladder diagram (LAD)
     Function Chart or Control System Flow Chart (CSF)
     PLC Visual Scripts

Sensors
Sensors are signal generators, which can be used by the PLC to integrate the status of the installation or machine to be controlled. As the PLC works “Electrically,” the sensors must convert non-electric signals into electric ones so that they can be understood by the input module. Examples of sensors are:
 I) Limit Switches
Normally closed contacts, normally open contacts or changeover contacts.
II) Initiators
Contacting and contactless switches, which emit one or zero signal when approached by an object, inductive generators react to metal parts, capacitive generators also to other materials.
III) Light Barriers
Contacting and contactless switches, which emit one or zero signal when an optical connection is interrupted.
IV) Temperature Sensors
Contacting and contactless switches emitting one or zero signal on attainment of a set temperature value.

Actors
Actors are executive elements used to amplify binary signals from the output modules to create switching signals or to convert them into signals of another energy type.
The distinction is made between electric, electro hydraulic and electro pneumatic actuators. They can be used to create switching processes and linear or rotary movements. Examples of actuators are:
     Alarms
     Lamps, Buzzers, Bell

     Pneumatic Cylinders (with value systems)
 Single and double acting cylinders, cylinders with through rod, tandem cylinders, multi positions cylinders.
           Electro Motors
DC motors, stepping motors, synchronous, AC motors, rotating vane motors.
           Hydraulic actuators (with value system)  
Single or double acting cylinders, flow control valves, hydraulic motors.
            Electro Hydraulic Motors  
           Stepper motors, servo drives etc.

Programmer
The developer is used to write and correct programs to translate them into the PLC machine code to transmit them to the PLC and to test them. It can be just like a simple or unique personal computer.
Personal Computer
In the past, this was mainly carried out using specialized tools, tailor made for individual manufacturers control system. Today conventional personal computers are increasingly being used as programming tools loaded for the task with programming software specific to the control system involved.
Programming System
Programming Systems based on the personal computer mostly offer several methods of programming, i.e., the programmer creates a source program graphically or in plain text, which the programming system translates into machine code, understood by the PLC being used. If the personal computer processes a data interface suitable for the PLC, programs can be transmitted directly into the program memory of the PLC.
To gain a better overall view and for the purpose of programming methods documentation, it is possible to print out the programs created.

PLC Definition and Introduction

A Programmable Logic Controller is a user-friendly electronic computer that carries out control functions of many types and levels of complexity. It can be calculated, monitored and operated by a person unskilled in operating computers. The programmable logic controllers mostly draw the lines and devices of ladder diagrams. The resulting drawing in the computer replaces much of the external wiring required for control of a process. The programmable logic controllers can operate any system that has output devices that go ON and OFF. It can also run any system with analog outputs. The Programmable Logic Controllers can be performed on the Input side by ON / OFF devices or by analog input devices.
In 1978, the National Electrical Manufacturers Association (NEMA) released a standard for PLC’s. This specification, NEMA Standard ICS3-1978, was the result of four years of work by a committee made up of representatives from PC manufacturers, Part ICS3-304 of the standard defines a PLC as:

“A digitally operating electronic apparatus, which uses a programmable memory for the internal storage of instructions for implementing specific

functions such as logic, sequences, timing, counting and arithmetic to control, through digital or analog Input / Output modules, various types of machines of a process.”
Evaluation to the present PLC  
The first PLC system evolved from the conventional computers in the late 1960s and early 1970s. The first PLC’s were mostly installed in automotive plants. Traditionally, the auto plants had to be shut down for up to a month at a change over time. The first PLC’s we used along with other new automation techniques to shorten the changeover time.
Through late 1970, the improvements were made in PLC programs to make them somewhat more user-friendly. In 1978, the introduction of the microprocessor chips increased computer power for all kind of automation and lowered the computing cost. Robotics, automation devices, and equipment of all types, including the PLC, consequently underwent many improvements. PLC programs became more understandable to people. PLC’s became more affordable, as well.    
In the 1980s with more computer power available per Dollar, the use of PLC rises exponentially. Some electronics and computer companies and some different corporate electronic divisions found that the PLC became their greatest valued product. The market for PLC’s grew from a volume of $80 million in 1978 to 1 billion dollars per year by 1990 and is still growing.
Advantages of PLC  
The following are some of the major benefits of using a Programmable Logic Controller.
Flexibility
In the past, each another electronically controlled production machine required its controller; fifteen devices might require fifteen drivers. Now it is possible to use just model of a PLC to run anyone of the fifteen machines. Furthermore, you would probably need fewer than fifteen controllers, because one PLC can easily run many machines. Each of the fifteen devices under PLC control would have its distinct program.
Implementing changes and correcting errors
With a wired relay type panel, any program alterations require time for rewiring of panels and devices. While a PLC program can be modified from a Keyboard sequence in a matter of minutes. No extra rewiring is required for a PLC controlled system. Also, if a programming error has occurred in a PLC control ladder diagram, change can be typed in quickly.
Large quantities of contacts
The PLC has a lot of contacts for each coil available in its programming. Suppose that a panel wire relay has four connections and all are in use when a design change, requiring three more contacts, is made. It would mean that time must be taken to procure and install a new relay or relay contact block. Using a PLC, however, would only require that three more contacts be typed in. These three connections would be automatically available in the PLC. Indeed, hundreds of contacts can be used from one relay if sufficient computer memory is available.
Lower Cost
Increased technology makes it possible to compact more functions into smaller and less expensive packages. In the 1990s, you can purchase a PLC
with numerous relays, timers, counters, a sequencer and other features for a few hundred dollars.
Pilot Running
A PLC programme circuit can be pre-run and evaluated in the office or lab. The record can be typed in, tested, observed and modified if needed, saving valuable factory time. In contrast, conventional relay system has been best tested on the plant floor, which can be very time-to consume.

Visual Observation
A PLC circuit’s operation can be seen during operation directly on a CRT screen. The operation or miss-operation of a circuit can be observed as it happens. Logic paths light up on the screen as they are energized. Troubleshooting can be done quicker by visual observation.    
Speed of Operation
The relay can take an un-expectable amount of time to actuate. The operational speed for the PLC programming is breakneck. The rate of PLC logic operation is determined by scan time, which is a matter of milli seconds.
Ladder or Boolean Programming Method
The PLC programming can be accomplished in the Ladder mode by an electrician or technician, alternately, a PLC programmer who works in digital or Boolean Control systems can also easily perform PLC programming.        

Reliability
Solid-state devices a more reliable, in general, than mechanical or electrical relays and timers, the PLC is built up of solid state electronic components with very high-reliability rates.


Simplicity of Ordering Control System Components
A PLC is a device with one delivery date when the PLC arrives; all the counters relay another component also arrive. In designing a relay panel, on the other hand, you may have twenty different relays and timers from twelve different suppliers. Obtaining the parts on time involve various delivery dates and availability. With a PLC you have one product and one lead-time for delivery. In a relay system, forgetting to buy one component would mean delaying the startup of the control system until that part arrives with the PLC one more relay is always available providing you an ordered PLC with enough extra computing power.  
Documentation
An immediate printout of the real PLC circuit is available in minutes if required. There is no want to look for the blue print of the course in remote files. The PLC print out actual circuit in operation at a given moment often the file prints for relay panels are properly kept up to date. PLC print out is the course at present; no wire tracing is needed for verification.
Security
A PLC program change can’t be made unless PLC is properly unlocked and programmed. Relay panels tend to undergo undocumented changes. People on the late shift don’t always record panel alternations made when the office area is locked up for the night.
Ease of Changes by Reprogramming
Since the PLC can be reprogrammed quickly, so a mixed production processing can be accomplished. For example, if a part B comes down the assembly line while part A is still being processed, a schedule for part B’s processing can be reprogrammed into the production machinery in a matter of seconds.







Disadvantages of the PLC
Following are some of the disadvantages of or perhaps precautions for using PLC’s.
Newer Technology
It 's hard to change some personnel’s thinking from ladders and relays to the PLC computer concepts.
Fixed Program Application
Some applications are single function application. It can’t play to use a PLC that includes multiple programming capabilities if they are not needed.
Environmental Consideration
Individual process environments such as high heat and vibrations interfere with electronic devices in PLC’s, which limits their use.
Fail Safe Operations
In relay systems, the stop button electrically cuts, the circuit; if the power fails the operation ends. Furthermore, the relay system doesn’t automatically restart when power is restored. This, of course, can be programmed into the PLC however in some PLC plans you may have to implement an input voltage to produce a device to stop. These systems are not “Fail Safe.” These disadvantages can be overcome by adding safety relays to a PLC system.
Fixed Circuit Operations
If the circuit in operation never altered, a fixed control system might be less costly than a PLC. The PLC is most useful when periodic changes in operations are made.