basic instructions of siemens plc

7/29/2019 Basic Instructions of Siemens PLC

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Njoroge M James, Centurion Systems. Tel 0710869963 Page 1

PROGRAMMABLE LOGIC CONTROLLERSDefinition

A PLC (Programmable Logic Controllers) is an industrial computer used to monitor inputs, and

depending upon their state make decisions based on its program or logic, to control (turn on/off)

its outputs to automate a machine or a process.

PLC History

PLC development began in 1968 in response to a request from an US car manufacturer (GM).

The first PLCs were installed in industry in 1969.

Communications abilities began to appear in approximately 1973. They could also be used in the

70s to send and receive varying voltages to allow them to enter the analog world.

The 80s saw an attempt to:

standardize communications with manufacturing automation protocol (MAP), reduce the size of

the PLC, and making them software programmable through symbolic programming on personalcomputers instead of dedicated programming terminals or handheld programmers.

The 90s have seen a gradual reduction in the introduction of new protocols, and the

modernization of the physical layers of some of the more popular protocols that survived the

1980s.

The latest standard IEC 1131-3 has tried to merge plc programming languages under oneinternational standard. We now have PLCs that are programmable in function block diagrams,

instruction lists, C and structured text all at the same time.

Advantages of PLC control1. Rugged and designed to withstand vibrations, temperature, humidity and noise2. Have interfacing for inputs and outputs already inside the controller3. Easily programmed and have an easily understood programming language.

4. Enhanced Reliability

5. Ease in logic modification6. Interactive operator interface

7. On line repair facility

8. I/O forcing through Software9. On line monitoring of user logics

10. Finalization of control logic and fabrication of PLC can be done simultaneously

11. Capability of self-diagnostics to find failure in the equipment12. Flexibility while setting and changing the logic during commissioning and operation13. Reliable components make these likely to operate for years before failure

14. Trouble shooting aids make programming easier and reduce downtime

15. Computational abilities allow more sophisticated control

Disadvantages of PLC control- Too much work required in connecting wires.
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- Difficulty with changes or replacements.

- Difficulty in finding errors; requiring skillful work force.- When a problem occurs, hold-up time is indefinite, usually long.

PLC Hardware

Hardware Components of a PLC System

These include the Processing unit (CPU), Memory, Input/Output, Power supply unit,

Programming device and other devices.

Central Processing Unit (CPU)CPUMicroprocessor based, may allow arithmetic operations, logic operators, block memory

moves, computer interface, local area network, functions, etc.CPU makes a great number of check-ups of the PLC controller itself so eventual errors would be

discovered early.

System BussesThe internal paths along which the digital signals flow within the PLC are calledbusses.

The system has four busses:- The CPU uses the data bus for sending data between the different elements,

- The address bus to send the addresses of locations for accessing stored data,- The control bus for signals relating to internal control actions,

- The system bus is used for communications between the I/O ports and the I/O unit.

MemorySystem (ROM) to give permanent storage for the operating system and the fixed data used by theCPU.

RAM for data. This is where information is stored on the status of input and output devices and

the values of timers and counters and other internal devices. EPROM for ROMs that can beprogrammed and then the program made permanent.
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I/O SectionsInputs monitor field devices, such as switches and sensors.Outputs control other devices, such as motors, pumps, solenoid valves, and lights.

Power Supply

Most PLC controllers work either at 24 VDC or 220 VAC. Some PLC controllers have electricalsupply as a separate module, while small and medium series already contain the supply module.

Programming DeviceThe programming device is used to enter the required program into the memory of the processor.

The program is developed in the programming device and then transferred to the memory unit ofthe PLC.

PLC Programming

Programming Languages

A program loaded into PLC systems in machine code, a sequence of binary code numbers torepresent the program instructions.

Assembly language based on the use of mnemonics can be used, and a computer program called

an assembler is used to translate the mnemonics into machine code.High level Languages (C, BASIC, etc.) can be used.

Programming DevicesPLC can be reprogrammed through an appropriate programming device:

Programming Console

PC

Hand Programmer

Introduction to Ladder LogicLadder logic uses graphic symbols similar to relay schematic circuit diagrams.

Ladder diagram consists of two vertical lines representing the power rails. Circuits are connected

as horizontal lines between these two verticals.

Ladder diagram features

Power flows from left to right.Output on right side cannot be connected directly with left side.

Contact cannot be placed on the right of output.

Each rung contains one output at least.

Each output can be used only once in the program.A particular input a/o output can appear in more than one rung of a ladder.

The inputs a/o outputs are all identified by their addresses, the notation used depending on the

PLC manufacturer.
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Introduction to Statement listStatement list is a programming language using mnemonic abbreviations of Boolean

logic operations. Boolean operations work on combination

of variables that are true or false.A statement is an instruction or directive for the PLC.

Statement List Operations

* Load (LD) instruction

* And (A) instruction

* Or (O) instruction* Output (=) instruction

Function Block DiagramsFunction block is represented as a box with the function name written in


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Example

please note:

LD: load

O: or

AN: and not (and a normally closed contact)

ALD: AND the first LD with second LD

Basic Instructions of Siemens PLC

Series connection
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Figure 4.21 Ladder and mnemonic code for series connection on Siemens PLC

Parallel connection

Figure 4.22 Ladder and mnemonic code for parallel connection on Siemens PLC
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The union of parallel connection

Figure 4.23 Ladder and mnemonic code for the union of parallel connection on Siemens PLC

the union of series connection

Figure 4.24 Ladder and mnemonic code for the union of series connection on Siemens PLC

Fault detection techniques

For any PLC controlled plant, by far the greater percentage of the faults are likely to be withsensors, actuators, and wiring rather than with PLC itself. The faults within the PLC most are

likely to be in the input/output channels or power supply than in the CPU.
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Case 1

Consider a single output device failing to turn on though the output LED is on.If testing of the PLC output voltage indicates that it is normal then the fault might be a wiring

fault or a device fault.

If checking of the voltage at the device indicates the voltage there is normal then the fault is the

device.

Case 2Failure of an input LED to illuminate as required could be because:

*Input device is not correctly operating,

*Input device is not correctly powered,*Incorrect wiring connections to the input module or LED or input module is defective.

Many PLCs provide built-infault analysis procedures which carry out self-testing and display

fault codes, with possibly a brief message, which can be translated by looking up code in a list togive the source of the fault and possible method of recovery.

Troubleshooting

Program troubleshootingThere are several causes off alteration to the userprogram:

- extreme environmental conditions- electromagnetic Interference (EMI)

- improper grounding- improper wiring connections and unauthorized tampering

If you suspect the memory has been altered, check the program against a previously saved

program on an EEPROM, UVPROM or flash EPROM module.

Hardware troubleshootingTips for troubleshooting control systemIf installation and start-up procedures were followed closely, controller will give reliable service.

If a problem should occur, the first step in the troubleshooting procedure is toidentify the

problem and its source.Do this by observing your machine or process and by monitoring the diagnostic LED indicators

on the CPU, Power Supply and I/O modules.

By observing the diagnostic indicators on the front of the processor unit and I/O modules, the

majority of faults can be located and corrected. These indicators, along with error codes

identified in the programming device user manual and programmers monitor, help trace the

source of the fault to the users input/output devices, wiring, or the controller.

Troubleshooting Controller

In identifying the source of the controllers operation problem use troubleshooting considerationstable including status indication, trouble description, probable causes and recommended action.

To receive the maximum benefit, follow these steps:

-Identify Power Supply and CPU LED status indicators
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-Match processor LEDs with the status LEDs located in troubleshooting tables

-Once the status LEDs are matched to the appropriate table, simplymove across the tableidentifying error description and probable causes

-Follow the recommended action steps for each probable cause untilthe cause is identified

-If recommended actions do not identify the cause, contact manufacturer or distributor for

assistance.

Troubleshooting Input modulesAn input circuit responds to an input signal in the following manner:

An input filter removes false signals due to contact bounce or electrical interference; Optical

isolation protects the backplane circuits by isolating logic circuits from input signals; Logiccircuits process the signal; An input LED turns on or off indicating the status of the

corresponding input device.

The processor receives the input status for use in processing the program logic.

Troubleshooting Output modules

An output circuit controls the output signal in the following manner:The processor determines the output status; Logic circuits maintain the output status. An outputLED indicates the status of the output signal, Optical isolation separates logic and backplane

circuits from field signals; The output driver turns the corresponding output on or off.

Troubleshooting II

Power distribution

The master control relay must be able to inhibit all machines motion by removing power to the

machine I/O devices when the relay is de-energized. The DC power supplyshould be powered

directly from the fused secondary of the transformer. Power to the DC input, and output, circuitsis connected through a set of master control relay contacts. Interrupt the load side rather the AC

line power. This avoids the additional delay of power supply turn-on and turn-off.

Power LED

The POWER LED on the power supply indicates that DC power is being supplied to the chassis.

This LED could be off when incoming power ispresent when the:-Fuse is blown

- Voltage drops below the normal operating range

- Power supply is defective

Safety Considerations

Actively thinking about the safety of yourself and others, as well as the condition of your

equipment, is of primary importance.
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When troubleshooting, pay careful attention to these general warnings:

1. Have all personnel remain clear of the controller and equipment when power is appliedThe problem may be intermittent and sudden unexpected machine motion could result in

injury.

2.

Have someone ready to operate an emergency-stop switch in case it becomes necessary toshut off power to the controller equipment.

3. Never reach into a machine to actuate a switch since unexpected machine motion can occurand cause injury.

4. Remove all electrical power at the main power; disconnect switches before checkingelectrical connections or inputs/outputs causing machine motion.

5. Never alter safety circuits to defeat their functions. Serious injury or machine damage couldresult.

Calling for assistance

If you need to contact manufacturer or local distributor for assistance, it is helpful to obtain thefollowing (prior to calling):

1. Processor type, series letter

2. Processor LED status

3. Processor error codes4. Hardware types in system (I/O modules, chassis)

5. Revision of programming device (HHT or APS)

System documentation

The documentation is the main guide used by the users and for troubleshooting and fault finding

with PLCs.The documentation for a PLC installation should include:

- A description of the plant- Specification of the control requirements

- Details of the programmable logic controller

- Electrical installation diagrams- Lists of all inputs and outputs connections

- Application program with full commentary on what it is achieving

- Software back-ups- Operating manual, including details of all start up and shut down

- Procedures and alarms

Applications

Conveyor system

This simple application is for a conveyor (moving material machine) and how we implement it

using ladder diagram and instruction list.
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System requirements:

1. A plc is used to start and stop the motors of a segmented conveyor belt, this allows only belt

sections carrying a copper plate to move.

2. The system have three segmented conveyor belts, each segment runs by a motor.3. A proximity switch located at the end of each segment to detect the position of the plate.

4. The first conveyor segment is always on.

5. The second conveyor segment turns on when the proximity switch in the first segment detects

the plate.

6. When the proximity switch at the second conveyor detects the plate, the third segmentconveyor turns ON.

7. The second conveyor is stopped, when the plate is out of detection range of the secondproximity switch, after 20 seconds.

8. The third conveyor is stopped after 20 seconds, when the proximity switch located at the

segment doesnt detect the plate.


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Real life application examples:

1. Elevator2. Wind generator3. Level crossing4. Seven segment display5. Three cylinders6. Washing machine7. Filling two tanks

basic instructions of siemens plc

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