Industrial Automation and Control Chapter 1-Programmable Logic Controllers

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.

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..

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.

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.

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.

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.

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

PLC System

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.

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.

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.

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

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

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.

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.

Internal Architecture of PLC

Internal Architecture of PLC 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.

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.

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.

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.

The Buses Address Bus Control Bus System 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.

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.

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.

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.

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.

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.

Input/Output Unit Relay Type Transistor 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.

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.

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.

Sourcing and Sinking

Sourcing and Sinking Sensors with sourcing outputs should be connected to sinking PLC inputs Sensors with sinking outputs should be connected to sourcing PLC inputs.