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Industrial Automation and Control Chapter 1-Programmable Logic Controllers.

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Presentation on theme: "Industrial Automation and Control Chapter 1-Programmable Logic Controllers."— Presentation transcript:

1 Industrial Automation and Control Chapter 1-Programmable Logic Controllers

2 Controllers  What type of tasks might a control system handle?  Required to control a sequence of events.  Maintain some variable constant.  Follow prescribed change.  Example 1;  Control system for an automatic drilling machine might be required to start lowering the drill when work piece in position.  Then start drilling when tool reaches the work piece.  Stop drilling when drill produced required depth of hole.  Retract the drill, move the work piece and bring in the new one.

3 Controllers  Example 2  Control number of items moving along a conveyor belt and direct them into a packing case.  Inputs may come from switches being closed or opened.  For instance the presence of the work piece might be indicated by it moving against a switch and closing it or by any other sensors.  Controller might be needed to run a motor to move an object to some position or to turn a valve etc..

4 Controllers  What form might a controller have?  For automatic drilling, we could wire up electrical circuits in which closing or opening of switches would result in motor being switch on or valves being actuated.  This might activate a relay, which in turn, switches on the current to a motor and causes the drill to rotate.  Additionally, another switch might be used to activate a relay and switch on the current to a pneumatic or hydraulic valve, which results in the work piece being pushed into the required position.

5 Programmable Logic Controller  Called PLC in short.  Special form of microprocessor based controller that uses programmable memory to store instructions and implement functions such as;  Logic  Sequencing  Timing  Counting and  Arithmetic  Designed to be operated by engineers with perhaps limited knowledge of computers and computing languages.  Not designed so that ONLY computer programmers can set up or change the programs.  The control program can be entered using a simpler, intuitive form of language.

6 Programmable Logic Controller  The term logic is used because programming is primarily concerned with implementing:  logic and  switching operations.  PLC have the great advantage that the same basic controller can be used with a wide range of control systems.  The operator can easily change the different set of instructions and there is no need to rewire.  Result is a flexible, cost effective system that can be used with control systems which vary quite widely in their nature and complexity.

7 Programmable Logic Controller  PLCs are similar to computers, but  Computers are optimized for calculation and display tasks  PLCs are optimized for control tasks and the industrial environment.  PLCs are;  Rugged and designed to withstand vibrations, temperature, humidity and noise.  Having interfacing for inputs and outputs already inside the controller.  Are easily programmed and have an easily understood programming language that is primarily concerned with logic and switching operations.

8 Hardware  A typical PLC system has the basic fundamental components;  Processor Unit or Central Processing Unit  Power Supply Unit  Programming Device  Memory Unit  Input Output Sections  Communications Interface

9 PLC System

10 Processor Unit  Unit containing microprocessor  Unit interprets the input signals and carries out the control actions according to the program stored in its memory.  Communicating the decisions as action signals to the outputs.

11 Power Supply Unit  Converts the mains AC voltage to the low DC voltage (5V).  This is necessary for the processor and the circuits in the input and output interface modules.

12 Programming Device  Used to enter the required program into the memory of the processor.  Program is developed in the device and then transferred to the memory unit of the PLC.

13 Memory Unit  It is where the program containing the control actions to be exercised by the microprocessor is stored.  Also this is where the data is stored from the input for processing and for the output

14 I/O sections  This is where the processor receives information from the external devices and communicates information to external devices.  Inputs might be from switches or other sensors such as photoelectric cells, temperature sensors, flow sensors etc.  Input and output devices can be classified as giving signals that are;  Discrete  Digital  Analog

15 I/O sections  Devices giving discrete or digital signals are ones where the signals are either off or on.  A switch is an example of a device which gives discrete signal.  Digital devices can be considered as discrete devices that give a sequence of on/off signals.  Analog devices give signals of which the size is proportional to the size of the variable being monitored.  Example: Temperature sensor may give a voltage proportional to the temperature.

16 Communications Interface  Used to receive and transmit data on communication networks from or to other remote PLCs.  It is concerned with such actions as device verification, data acquisition, synchronization between user applications and connection management.

17 Internal Architecture of PLC

18  Consists of CPU containing;  System microprocessor  Memory  Input/output circuitry  CPU controls and processes all the operations within the PLC.  Supplied with a clock with frequency typically in the range 1 and 8 MHz.  Frequency determines the operating speed of the PLC and provides;  Timing and  Synchronization for all elements.

19 Internal Architecture of PLC  Information within PLC is carried by means of digital signals.  Internal paths along which digital signals flow are called buses.  A bus is just a number of conductors along which electrical signals can flow.  These might be tracks on printed circuit boards or wires in ribbon cable.  CPU uses;  data bus for sending data between constituent elements,  Address bus to send the addresses of locations for accessing stored data,  Control bus for signals relating to internal control actions,  System bus for communications between input/output ports and the input/output unit.

20 The CPU  Internal structure of the CPU depends on the microprocessor concerned.  In general, CPUs have the following:  An ALU (arithmetic and logic unit): responsible for data manipulation and carrying out arithmetic operations of addition and subtractions and logic operations of AND, OR, NOT, and EXCLUSIVE-OR.  Memory, termed registers, located within the microprocessor and used to store information involved in program execution.  Control unit that is used to control the timing of operations.

21 The Buses  Buses are paths used for communication within the PLC.  Information is transmitted in binary form, in a group of bits.  The term word is used for the group of bits constituting some information.  Four kinds of buses;  Data bus  Carries data used in the processing by the CPU.  Microprocessor termed as being 8-bit (for example) has an internal data bus that can handle 8-bit numbers.  Can perform operations between 8-bit numbers and deliver results as 8-bit values.

22 The Buses  Address Bus  Used to carry addresses of memory locations.  Each word can be located in memory and every memory location is given a unique address.  Each word location is given an address so that data stored at a particular location can be accessed by the CPU either to read data or to write new data.  Control Bus  Carries the signals used by the CPU for control.  This is to inform memory devices whether to receive data from an input or output data and to carry timing signals used to synchronize actions.  System Bus  Used for communications between the input/output ports and the input/output unit.

23 Memory  Several memory elements in a PLC system;  System Read only memory (ROM): gives permanent storage for the operating system and fixed data used by the CPU.  Random Access Memory (RAM): used for the user’s program and/or data.  This where information is stored on the status of input and output devices.  Values of timers and counters and other internal devices.  Data RAM is sometimes referred to as data table or register table.  Part of this memory will be assigned to; block of addresses, input/output address and the states of those inputs and outputs, for present data and storing counter values, timer values etc.  Erasable and Programmable read-only memory (EPROM)  Used to store programs permenantly.

24 Memory  Programs and data in RAM can be changed by the user.  PLCs have some amount of RAM to store programs.  However, to prevent the loss of programs when the power supply is switched off, a battery is used in the PLC to maintain the RAM contents for a period of time.  After the program has been developed in RAM, it may be loaded into an EPROM memory chip.

25 Input/Output Unit  It provides the interface between the system and the outside world.  Allows connections to be made through input/output channels to input devices such as sensors and actuators.  Through this unit programs are entered from a program and panel.  Each I/O point has a unique address that can be used by the CPU.  I/O channels provide isolation and signal conditioning features so that sensors and actuators can often be directly connected to them without the need for other circuitry.

26 Input/Output Unit  Electrical isolation from external world is usually by means of opto-isolators.  Principle of working: when digital pulse passes through the light emitting diode, a pulse of infrared radiation is produced  Pulse is detected by phototransistor and give rise to a voltage in that circuit.  Gap between light emitting diode and photo transistor gives electrical isolation.  The light transmits the digital pulses from one circuit to another.

27 Input/Output Unit  Signal conditioning in the input channel, with isolation, enables a wide range of input signals to be supplied to it.  Range of inputs might be available with a larger PLC, such as 5 V, 24 V, 110 V and 240 V discrete/digital, that is on/off, signals.  Output from I/O unit will be digital with a level of 5 V.  After signal conditioning with relays, transistors, or triacs, the output channel might be the ones shown in bottom right.

28 Input/Output Unit  Relay Type  Relay Type: signal from the PLC output is used to operate a relay is able to switch currents of the order of a few amperes in an external circuit.  Relay not only allows small currents to switch much larger currents but also isolates the PLC from the external circuit.  Relays are relatively slow to operate.  Suitable for AC and DC switching.  Can withstand high surge currents and voltage transients.  Transistor Type  Output uses a transistor to switch current through the external circuit.  Gives considerable fast switching action.  Strictly for DC switching and destroyed by overcurrent and high reverse voltage.

29 Input/Output Unit  Triac Output  With optoisolators for isolation, can be used to control external loads that are connected to the AC power supply.  Strictly for AC operation and is very easily destroyed by over current.  Fuses virtually always included to protect such outputs.

30 Sourcing and Sinking  Sourcing and sinking are used to describe the way in which DC devices are connected to a PLC.  With sourcing, using conventional current flow direction as from positive to negative, an input device receives current from the input module. ( input module is the source of current).  With Sinking, an input device supplies current to the input module, that is, the input module is the sink for the current.  If the current flows from the output module to the output load, the output module is referred to as sourcing.  If the current flows to the output module from an output load, the output module is referred to as sinking.

31 Sourcing and Sinking

32  Sensors with sourcing outputs should be connected to sinking PLC inputs  Sensors with sinking outputs should be connected to sourcing PLC inputs.


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