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TASK 1 PLC and its types

Programmable Logic Controller (PLC) is a digital computer used for the automation of various electro-mechanical processes in industries. These controllers are specially designed to survive in harsh situations and shielded from heat, cold, dust, and moisture etc. PLC consists of a microprocessor which is programmed using the computer language.

Unitary. (Aries, n.d.)

A unitary PLC is the simpler type of controller, and contains all of the basic system components within a single housing, or box. These components typically include the processor, which runs the software program, in addition to ports for input and output connections. Unitary PLCs are typically attached directly to the device or application that is being controlled.

Advantages. (E.Moorley, 2015)

It includes on-board memory for storing programs, 32 digital input and output ports, and a communications port used to program the unit.

Disadvantage.

Whole beast needs replacing if any component becomes damaged.

Can only run relatively small programmers.

Unable to expand.

Example- The Micrologix 1000

Modular.

A modular PLC contains several different modules that can be coupled together to build a customized controller. Typically, a base module contains core functions such as electrical power regulation, the computer processor, and input connections. Additional modules, including analogue to digital signal converters or additional outputs, can be added to this core unit as needed.

Advantages

This modular design allows a PLC to be customized and changed easily. This unit is able to handle between 23 and 40 inputs and outputs. This provides a wide range of flexibility and is typical of a modular PLC.

Disadvantages

Requires separate power supply, Whole Module needs replacing if any component becomes damaged. Limited expansion is allowed depending on type of brand

Example- The Allen Bradley Micrologix 1200.

Rack- Mounted.

The rack mounting type of PLC is similar to the modular concept, but is implemented differently. Whereas each module in a modular PLC connects to the base unit directly, a rack mounting PLC keeps each module separate. All extra modules are connected through a network, and modules are held in organized racks. This approach allows for larger systems to be built without becoming overly cluttered and complicated. Modules are well organized on the rack and can be removed and reinserted as needed.

Advantages.

There are essentially no limits on the number of modules that can be added to this system, each mounted on a standard rack chassis. This setup allows large, scalable automation solutions to be built and is common in factory settings.

Disadvantages.

Overwhelming initial cost.

Example- SLC 500

TASK 2. Typical switching voltages and sensors in PLC.

Typical switching voltages applied to the input of PLC.

In smaller PLCs the inputs are normally built in and are specified when purchasing the PLC. For larger PLCs the inputs are purchased as modules, or cards, with 8 or 16 inputs of the same type on each card. The list below shows typical ranges for input voltages, and is roughly in order of popularity. (engineeringonadisk,com, 2015)

• 12-24 Vdc.

• 100-120 Vac.

• 10-60 Vdc.

• 12-24 Vac/dc.

• 5 Vdc (TTL).

• 200-240 Vac.

• 48 Vdc.

• 24 Vac.

• DC voltages are usually lower, and therefore safer (i.e., 12-24V)

• DC inputs are very fast; AC inputs require a longer on-time. For example, a 60Hz wave may require up to 1/60sec for reasonable recognition.

• DC voltages can be connected to larger variety of electrical systems.

• AC signals are more immune to noise than DC, so they are suited to long distances, and noisy (magnetic) environments.

• AC power is easier and less expensive to supply to equipment.

• AC signals are very common in many existing automation devices.

Types of switch

• N.O. pushbuttons.

• N.C. Stop switch.

Types of sensors.

PH Sensors. (sensorland.com, n.d.)

The PH sensor detects the acidity of any substance, it includes a measuring electrode, a reference electrode, and a temperature sensor; a preamplifier; and an analyser or transmitter. PH is an important parameter to be measured and controlled.

Digital Temperature Sensor (Jon, 2010)

Digital temperature sensor that provides temperature of device with 9-bit temperature readings. It acts a thermostat with its three thermal alarm outputs. If the temperature of device is greater than or equal to user defined temperature TH, then THIGH is driven high. If the temperature of the device is less than or equal to user defined temperature TL, then the TLOW is driven high. If the temperature of the device exceeds TH and remains high until it falls below that of TL, then the TCOM is driven high.

Proximity Sensors.

Proximity Sensor Ideal for the Food and Beverage Industry. Applicable to 120°C (with DC 3-wire connection) (Heat resistance to 1,000 hours.), Water resistant under high-temperature, high-pressure cleaning based on DIN 40050-9. (Pressure: 8,000 to 10,000 kPa, Water temperature: 80°C,) it detects the position of the object to stop or continue the running process.

Directional Sensors.

When drilling vertical, horizontal, or extended reach wells, obtaining accurate measurements of inclination and azimuth directional sensor is required.

Broken Beam Sensor (Dale R. Patrick, 2009)

Usually it is use for counting and detecting objects or for checking levels in a container. It works on the basis of photodetector but in this it has to parts transmitter and receiver when the beam between beaks it sent information to the PLC. To control the process.

TASK 3 digital information communication parts of PLC.

PLCs have built-in communications ports, usually 9-pin RS-232, RS-422, RS-485, Ethernet. Various protocols are usually included. Many of these protocols are vendor specific.

Most modern PLCs can communicate over a network to some other system, such as a computer running a SCADA (Supervisory Control and Data Acquisition) system or web browser.

PLCs used in larger I/O systems may have peer-to-peer (P2P) communication between processors. This allows separate parts of a complex process to have individual control while allowing the subsystems to co-ordinate over the communication link. These communication links are also often used for HMI devices such as keypads or PC-type workstations.

Formerly, some manufacturers offered dedicated communication modules as an add-on function where the processor had no network connection built-in.

TWISTED PAIR. (rose, n.d.)

Twisted pair is the ordinary copper wire that connects home and many business computers to the telephone company. To reduce crosstalk or electromagnetic induction between pairs of wires, two insulated copper wires are twisted around each other. Each connection on twisted pair requires both wires. Since some telephone sets or desktop locations require multiple connections, twisted pair is sometimes installed in two or more pairs, all within a single cable. For some business locations, twisted pair is enclosed in a shield that functions as a ground. This is known as shielded twisted pair (STP). Ordinary wire to the home is unshielded twisted pair (UTP).

Advantages.

Less expensive, in a telephone system, signals can travel several kilometres without amplification when twisted pair wires are used. These media can be used for both analog and digital data transmissions. The bandwidth depends on the thickness of the wire and the distance travelled, but several megabits per second can be achieved for a few kilometres in many cases.

Disadvantages.

Easily pick up noise signals which results in higher error rates when the line length exceeds 100 meters. Being thin in size, it is likely to break easily. It can support 19,200 bps up to 50 feet on RS-232 port.

Coaxial cables (raw, n.d.)

Coaxial cables are a type of cable that is used by cable TV and that is common for data communications. Data is transmitted through the centre wire, while the outer braided layer serves as a line to ground. Both of these conductors are parallel and share the same axis.

Just like all electrical components, coaxial cables have a characteristic impedance. This impedance depends on the dielectric material and the radii of each conducting material.

Advantages.

It has better shield against electromagnetic interference than twisted pair cable, so it can span longer distance at higher data bits per second (bps).

It can be used for both analog and digital data transmissions. For analog data transmission, 75-ohm broadband coaxial is used and for digital transmission, 50 – ohm baseband cable is used.

It is inexpensive as compared to twisted pair wires and UTP cables but easy to handle.

Disadvantage.

It tends to be more ridged and more difficult to bend around tight corners.

It has a tendency to pick up signals around it unless shielded. Power transformers, high signal cables, home power lines can all cause interference on the cable. Imperfect shielded can cause a grounded loop. Proper installation with good grounded will limit this.

FIBRE OPTICS

A fiber optic cable consists of a bundle of glass threads, each of which is capable of transmitting messages modulated onto light waves. Fiber optics has several advantages over traditional metal communications lines: Fiber optic cables have a much greater bandwidth than metal cables.

Advantages

It can handle much higher bandwidth than copper. Due to the low attenuation, repeaters are needed only about every 30 Km on fiber lines, versus about every 5 km for copper.

Fiber is not affected by power surges, electromagnetic interference, or power failures. Nor is it affected by corrosive chemicals in the air, making it ideal for factory environments where electrical interference is very high.

Fiber is lighter than copper. One thousand twisted pairs copper cables of 1 km long weigh 800 kg. But fibers have only 100 kg.

Fibers do not leak light and are quite difficult to tap. This gives them excellent security against potential wire-tappers.

Disadvantages

Fibber is an unfamiliar technology requiring skills which may not be easily available.

Since optical transmission is inherently unidirectional, two-way communication requires either two fiber cables or two frequency bands on one fiber.

Fiber interfaces cost more than electrical interfaces.

NETWORKS

A network is a group of two or more computer systems linked together. There are many types of computer networks, including the following: local-area networks (LANs): The computers are geographically close together (that is, in the same building). As control systems become more complex, they require more effective communication schemes between the system components. Some machine and process control systems require that programmable controllers be interconnected, so that data can be passed among them easily to accomplish the control task.

Advantages

Files can be stored on a central computer (the file server) allowing data to be shared throughout an organization.

Files can be backed up more easily when they are all on a central fileserver rather than when they are scattered across a number of independent workstations.

Networks also allow security to be established, ensuring that the network users may only have access to certain files and applications.

Disadvantages of Networks.

The main disadvantage of networks is that users become dependent upon them. For example, if a network file    server develops a fault, then many users may not be able to run application programs and get access to shared data. To overcome this, a back-up server can be switched into action when the main server fails. A fault on a network may also stop users from being able to access peripherals such as printers and plotters. To minimize this, a network is normally segmented so that a failure in one part of it does not affect other parts.

 Another major problem with networks is that their efficiency is very dependent on the skill of the systems manager. A badly managed network may operate less efficiently than non-networked computers. Also, a badly run network may allow external users into it with little protection against them causing damage. Damage could also be caused by novices causing problems, such as deleting important files.

TASK 4 Storing, executing of a program in a PLC

Mechanically connected PLC diagram

A Programmable Logic Controller (PLC) is simply a specialized type of computer for controlling industrial processes. It usually is set to store programs in a battery-backed electronic memory, or non-volatile RAM.

To execute a program, a computer needs to be powered up and started. Once this happens the CPU loads an instruction from a portion of the memory space occupied by ROM (read-only memory) where a loader program resides and also the basic control programs for the busses that lead to input-output registers and buffers. Once the basic hardware is initialized to a known state the program in ROM executes a jump to the program in RAM.

Instructions in RAM are loaded into the CPU and processed. The CPU and the IO Registers have flags that specify the outcome of certain instructions or results from the Arithmetic and Logic Unit (ALU) or certain external events (Transmission complete output register empty, or Input waiting, for example) and either the program polls for these events or the events interrupt the CPU which then stores its current state and loads from an interrupt vector to call specialized code to deal with the event.

That is pretty much the course without getting into specifics on the PLCs on the market. A PLC is basically worthless without sensors and switches connected to it to actually control something.

Finally, the PLC is in most ways very much like the personal computer. The PC fetches its initial program from ROM or EPROM, then gathers a saved state (settings for various options such as the device to boot from first) from battery-backed non-volatile RAM before seeking to load the operating system and its programs from some boot device. The PLC has non-volatile RAM as its boot device in many cases

ALU (Kruge, 2005)

An arithmetic-logic unit (ALU) is the part of a computer processor (CPU) that carries out arithmetic and logic operations on the operands in computer instruction words. In some processors, the ALU is divided into two units, an arithmetic unit (AU) and a logic unit (LU). Some processors contain more than one AU - for example, one for fixed-point operations and another for floating-point operations. (In personal computers floating point operations are sometimes done by a floating point unit on a separate chip called a numeric coprocessor.)

ROM

Read-Only Memory (ROM): A memory from which data can be read but not written. ROMs are often used to keep programs or data from being destroyed due to user intervention.

RAM

Random Access Memory (RAM): A memory where data can be accessed at any address without having to read a number of sequential addresses. Data can be read from and written to storage locations. RAM has volatile memory, meaning a loss of power will cause the contents in the RAM to be lost.

BUSES

The internal paths along which the digital signals flow within the PLC are called Busses.

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.

AND GATE (Anon., n.d.)

A Logic AND Gate is a type of digital logic gate that has an output which is normally at logic level “0” and only goes “HIGH” to a logic level “1” when ALL of its inputs are at logic level “1”. The output state of a “Logic AND Gate” only returns “LOW” again when ANY of its inputs are at a logic level “0”. In other words, for a logic AND gate, any LOW input will give a LOW output.

OR GATE

A Logic OR Gate or Inclusive-OR gate is a type of digital logic gate that has an output which is normally at logic level “0” and only goes “HIGH” to a logic level “1” when one or more of its inputs are at logic level “1”. The output, Q of a “Logic OR Gate” only returns “LOW” again when ALL of its inputs are at a logic level “0”. In other words, for a logic OR gate, any “HIGH” input will give a “HIGH”, logic level “1” output.

NOT GATE

The digital Logic NOT Gate is the most basic of all the logical gates and is sometimes referred to as an Inverting Buffer or simply a Digital Inverter. It is a single input device which has an output level that is normally at logic level “1” and goes “LOW” to a logic level “0” when its single input is at logic level “1”, in other words it “inverts” (complements) its input signal. The output from a NOT gate only returns “HIGH” again when its input is at logic level “0” giving us the Boolean

expression of:  A = Q.

Parts of PLC connected electrically diagram

Task 5 presentation of architecture of PLC

Task 6 application of unitary style PLC’s (Mimic., n.d.)

Application of unitary PLC

Unitary PLC: The Main Basic Types

The Unitary PLC is typically the smallest and least expensive. It would be used in a small machine or fixed application such as overhead door controls or a stand-alone parts inspection system. They are not expandable so the application is limited to on-board I/O. There are, however, some very powerful units available with built in GSM, color screens, and web servers. Most have 1 or 2 analog I/O channels as well as a high speed input and pulse train output for simple motion control.

Comparing types of PLC

Besides the unitary PLC, there’re two other types of PLC system: the modular and the rack mounted one. The unitary type is considered the most basic and simplest kind of PLC system. It consists of one box only, which accommodates processor, software, and output-input connectors. The modular type is the one consisting of several modules or several programs that can be used to adjust the users’ preference and need.

This type of PLC system is suitable for complicated processes and operation, where the overall program is more lenient and flexible; not fixed. The rack mounted type is basically almost the same with the modular one, but the implementation is different. In modular type, different modules will be connected directly to the base unit. In rack mounted type, however, the modules remain separated. People have their own reasons why they choose the unitary PLC or the other types.

Rack style PLCs are usually more expensive, expandable, and powerful than unitary or modular PLCs. The rack provides a power and communication backplane that greatly increases the communication rate between the processor and the modules as well as allowing some specialty modules to communicate with each other without the processor. In some brands, multiple processors can be in the same rack and share the inputs. Racks also allow for redundant processors for critical systems such as waste water pumps or fire control systems.

Most widely used type of PLC for industrial application.

The benefit of unitary PLC is; the system is suitable for all small to medium scale industries. It’s compact and simple. It can be attached directly to the systems being controlled. With such flexibility and easiness, the users no need to fumble with messy application or difficult procedures. It’s perfect for small business with fixed type of operations.

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