Programmable Logic Controller - Assignment Example

• Task 1:
  • Unitary PLC
    • Design and Characteristics

Unitary PLC devices have all the characteristics of a simple system in a consolidated unit form. The characteristics include a unit for power supply, input and output modules and the main module having the CPU (central processing unit). This type of PLC’s is fixed directly on the module or the machine under its control.

• • • Advantages of Unitary PLC

They unitary PLCs are small, but carry all the essential elements of the system into a single unit. Additionally, they are easily portable and accessible. Thirdly, they are the most cost effective PLC types.

• • • Disadvantages

One of the disadvantages is that they not expandable. Secondly, failure in a single unit leads to the need to replace the entire unit. The third disadvantage is that they are very basic and have limited performance.

• • • Application of Unitary PLC

Unitary PLC devices are used in any application, which does not need much input or output.

• • Modular PLC
    • Design and Characteristics

Modular PLC is a set of modules, positioned together to construct a system. The simple modules of the device have power supply units, the main module carrying the CPU, the input and the output module.

• • • Advantages of Modular PLC Systems

The capacities of the input and output terminals are expandable, making it easy for them to cope with the transformations in the hardware system. Any failure in one component does not lead to failure in the entire system, so the cost of recovery from the failure is reduced.

• • • Disadvantages

The main disadvantage of modular PLC’s is the high cost of their installation.

• • • Application of Modular PLCs

Modular PLC’s devices are required in applications with more input and output. It is widely applied in the industrial sector for regulating the process lines.

• • Rack-Mounted PLC
    • Design and Characteristics

The rack mounted PLC system similar to the modular system, except that these units (modules) are on normal cards. The modules are able to relay information to each other through the rack.

• • • Advantages of a Rack-Mounted PLCs

It is easy to modify and expand the rack mounted PLC. Secondly, they possess more input and output points compared to other PLC types. Any failure in one feature means that only the affected part should be changed, this reduces the cost of repair.

• • • Disadvantages

They are the most expensive PLC types.

• • • Applications

This PLC type is extensively applied in the industrial sector. This is primarily due to capacity of input and output points, which the PLC system can carry, and because of it is able to communicate across networks.

Task 2:

• • Input and Output Devices
    • Mechanical Switches

The PLC devices display switches and wait for the sending of a signal from specific switches. The switches usually work in two approaches, normal opening, and normal closing. With the normal open switches, there is a signal sent to the switch but is not received by the PLC because of the existing open circuit. When the circuit is closed, the signal reaches the PLC, and the PLC is able to process the data. In the normal closed switch, a signal is continually received at the PLC and as the switch is formed, the circuit is made, and there is no more receipt of signals in the PLC. An example of this is the mechanical switch in the pneumatic piston with limit switches.

• • • Non-mechanical Digital Sources

This is the sensor inputs, which requires no direct mechanical procedure to work like the powered switch. These switches are advancements over the mechanical switches. They lack moving parts in their structures and are faster compared to the mechanical switches, so they are applicable in computing, for example, a transistor. The mechanism of a transistor operation is adding electrical charges to close a switch to enable the flowing current. With the removal of the electrical charge, the switch opens up so no current flows. The second example is the optical sensors. They send out optical signals to the reflector, which the PLC device interrupts to process and act. The optical sources use Infrared lights instead of the ordinary as light spectrum. This prevents natural sources of light from interfering.

• • Transducers

A transducer is a common type of sensor naturally applied in taking measurements. The main use of a transducer is to converts energy from one form to another. They naturally convert energy from mechanical to electrical. The electrical energy is then used for reporting to the PLC devices.

• • Relays

The relays are items used as output units for the PLC systems. The PLC only uses the information given to it from the available inputs. It executes a program to activate the output. An example of this is the thermostat, which controls the temperature within a specific range. When a temperature reaches the limit, the PLC processes and sends signal to the relay to the switch. This turns off the A/C unit.

Task 3:

• • Twisted Pair Cables

The twisted pair cable used in the PLC’s is made up of two insulated wires. The wires are intertwined together, to help minimize the noise from external sources. Twisted pair is suitable when moving balanced differential signals in the PLC’s. This gives valuably wide bandwidth system. There are two types of twisted pair cable, shielded and unshielded twisted-pair (STP) and (UTP) respectively (Laughton & Warne, 2003). The shielded pair is the most common, having two insulated wires intertwined together. This includes the data communication wires and the typical telephone wires.

• • • Preferred Qualities

It has a foil jacket to prevent cross talks and noise from the PLC environment and to eliminate the inductive and the capacitive coupling.

• • Coaxial Cable

Coaxial cable is a cable for high-frequency data transmission in PLC’s, which carries one solid-copper core. The cable has a high transmission capability, about 80 times that of the twisted-pair according to Cox (2006). It is applied in the PLC devices to deliver television signals. It is also used in the transmission of data in computers networks. It is cost effective for short connections within a radius of 10 m in home based network.

• • • Preferred Qualities

Coaxial cables are widely applied in feed lines, which are the links between various PLC devices such as radio transmitters and receivers. They are also used in computer networking, digital audio broadcast, and the spread of television signals. Furthermore, they prevent interference with the PLC input and output signals due to outer electromagnetic effect.

• • Fiber Optic Cables

Fiber optic is an assembly of fibers used in PLC data communication. It carries a number of optical fibers used to transmit data. Each fiber component is separately covered with a plastic covering. It is then carried in a protected tube. Data is transmitted in the fiber optic cable as pulses of light, going through tiny glass tubes with a high transmission capacity, about 26,000 times the speed of transmission in the twisted-pair cable (Petruzella, 2005).

• • • Preferred Qualities

Fiber optic cables are lighter and more reliable in the data transmission through the PLC devices. They convey information through beams of light. This implies that they use the speed of light instead of electrical pulses.

• • Ethernet

Ethernet is a network cabling system used in the PLC connectivity has a well signaling specification. It is a technology of a physical and data link layer for local area network.

• • • Preferred Qualities

Ethernet minimizes electrical noise apart from providing electrical isolation. Some networks stress on delivery of transmitted information through PLC, but Ethernet detects collision. The future safety and reliability of Ethernet is guaranteed.

Task 4

• • Internal Architecture

The architecture of a typical PLC is formed with a CPU, storage media (devices), memory, opto-isolators, input, and output units, flags and shift registers.

Figure 1: Internal Architecture of PLC

The CPU is the position of the main processing in the PLC device. The PLC uses the storage devices for storing the processes information. The storage devices include hard drive (Laplante, 2001). The memory of the device (RAM) is used for temporary memory for the running of programs. It enables the CPU to access random portions of the memory upon demand. The Opto-isolator in the PLC device is a protective device for transferring electrical signals from between input and output devices (Petruzella, 2005). It protects the internal circuit in the PLC against the possible quick change in voltage levels. The input and the output ports are connected to the input and output devices. The flag is the data type applied in the PLC device, referring to on and off'. The shift register is a set of information from the former cycles of program kept in the PLC, which later reflects on other running programs.

Task 5:

• • Operational Characteristics

Scanning refers to the process, which the PLC undergoes, beginning from the input and to the output. The scanning cycle runs as shown below:

Reading input  running the program  processing the information running the self-diagnosis writing output.

• • • Scanning Input

This is where the PLC continuously checks for the input signal.

• • • Executing Logic Operation

The PLC makes the program ready but it does not send the program.

• • • Continuous Update of Outputs

The PLC reads the program after which it passes the program on. In continuous updating of output, the CPU scans the inputs in a specific order using a build-in delay.

• • • Scan Time

The PLC checks how the program operates. The PLC finally sends signals to the appropriate outputs. The CPU does scanning of each input before it determines the program (Kandray, 2010). This enables the CPU to process only the valid input without a negative effect on amount of time taken to process while there are many input data, each having a delay.