Applications Of Programmable Logic Controllers

Published Date: 02 Nov 2017

An opto-isolator is a device that uses short optical transmission paths to transfer an electronic signal between elements of a circuit, typically a transmitter and a receiver, while keeping them electrically isolated.

Another effect that an opto-isolated transistor has is that helps eliminate triggering of the plc, caused by noise inputs by having only an on/off state and trigger voltage.

The opto-isolator is simply a package that contains both an infrared light-emitting diode (LED) and a photo detector such as a photosensitive silicon diode, transistor Darlington pair, or silicon controlled rectifier (SCR). The wave-length responses of the two devices are tailored to be as identical as possible to permit the highest measure of coupling possible.

The photo transistor component is attached to the input of the PLC, when the photo transistor is activated by the led it allows a signal to be sent to the PLC, as the input isn’t physically attached to the sensor output allowing this device to isolate thousands of volts.

Thyristors

A thyristor is a solid state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current pulse, and continue to conduct for as long as they are forward biased.

The main terminals are labelled Anode, cathode and control terminal.

The control terminal acts as a gate, no voltage can travel through the thyristor until a pulse or signal reaches the gate.

When the gate is activated the thyristor allows voltage to pass across from the anode and cathode.

The Thyristor is similar to that of a diode, in the fact it will only operate in a forward bias, if placed in a circuit in reverse it will block voltage from passing across the thyristor.

The thyristor is best suited for high power applications.

Task 2:

Investigate and document the different types of cable used in network communications on PLC systems giving their advantages and disadvantages.

Coaxial

Coaxial is an electrical cable that has a solid inner copper cable sheathed by a flexible insulator. The flexible insulator has a stranded copper woven copper shield which is covered by an insulating jacket.

The advantages and disadvantages are:

Advantages:

It has a sufficient frequency range to support multiple channels, which allows for much greater throughput.

The cable’s shielding reduces noise and crosstalk.

It provides greater bandwidth system wide than the twisted cable.

Lower error rates, this is because the internal core is in a faraday shield.

Disadvantages:

Signal leakage, is the passage of electromagnetic field through the cables shield this can happen on both the reverse and forward sections.

Inductance, the external current sources from a switched mode power supply can create an inductance on the outer sheath between the sender and receiver.

Ground loops, the imperfection in the shield can cause when a continuous current is passed through that section visible and audio interference.

Fibre-Optics

An optical fiber cable is a cable containing one or more optical fibers. The optical fiber elements are typically individually coated with plastic layers and contained in a protective tube suitable for the environment where the cable will be deployed.

The advantages and disadvantages are:

Advantages

Increased bandwidth and capacity.

Overall better system performance.

Immune to electromagnetic interference.

Signal security.

No cross talk.

Lower bit rate error.

Disadvantages:

Lack of standardisation.

Expensive costs.

Twisted pairs

Twisted pair cabling is a type of wiring in which two conductors (the forward and return conductors of a single circuit) are twisted together for the purposes of canceling out electromagnetic interference (EMI) from external sources.

The advantages and disadvantages are:

Advantages:

Its thin, flexible and can be easily positioned.

Twisted cable cost less per meter than any other cable available.

Due to size more wires can be passed through same wiring ducts.

Disadvantages:

Have susceptibility to Electro magnetic frequency, which means the wires have stringent on pulling tensions and minimum bend radius.

No mechanical protection.

If video is being transferred using twisted cable, the video is delayed, which is known as skew, the skew effect cause colours defect and ghosting.

Task 3:

Please compare and document two different types of net work used on PLC systems.

AS-I

AS-I, actuator sensor interface (AS-I or as interface) is a system for networking binary devices such as sensors. Until recently, extensive parallel control wiring was needed to connect sensors to controlling devices. AS-I replaces complex wiring and more cables with a simple 2 core cable. The information goes back and forth the wire between the plc and the slave units (I/O)

Advantages

Cheaper to make

Cheaper to manage/install into PLC System

Easy to install insulation

Only uses 2 core cables between PLC and slave unit

Disadvantages

Certain Environments can affect the cables such as spikes from high voltage so they need to be shielded or another method of control will have to be used.

Ethernet/LAN

Ethernet is a family of frame-based computer networking technologies for local area networks (LANs). PLC systems use this to connect to each other broadly over a large network such as one country to another. A local area network (LAN) is a computer network covering a small physical area, like a home, office, or small groups of buildings, such as a school, or an airport. PLC networks use this type of connections to keep the system on a small basis.

Advantages

Cheap to make

Cheap to install and maintain

Easy/Quick to install Can be used on a large or small scale of PLC in vast range

Can also be used to connect and communicate with computers and PLC

Disadvantages

If one of the wires fails or device shuts down then the network shuts down

Difficult to change the PLC system if it shuts down

Task 4:

Investigate and document the growing developments in PLC network technology indicating how the use of smart sensors, wireless applications and the developing PC integrated systems will improve their capabilities and their uses in the next generation of computers.

The progression of Programmable logic controllers is primary improving the operation speed that the controller can operate at, they are doing this by upgrading hard drive memory from a dram style system to using a flash based memory chip that gives greater read and write speeds than previous systems. The way in which the PLC is communicating to the outside world is also being upgraded, this is being done by using super fast Ethernet chipsets, which allows for the Plc to communicate to devices such as Desktop PC’s and over industrial products.

The Present generation of sensors are being upgraded with components that give a status update to the plc which can give a prior warning of a fault or issue with the sensor. The bonus of Smart sensors is that they allow the streamlining which eliminates the over excessive data from the sensor and allowing concentration on data that is actually relevant to the process needs. With the information given a confidence value can be calculated, this confidence value can represent the health of the sensor, by establishing the rate of change and the trend of the CV’s can indicate the likely hood of failure. Using the CV the health of each sensor can be shown clearly and identified so that it can be documented and clearly targeted by maintenances.

Wireless applications, are very similar to the standard wiring solutions accept that they transmit a high burst of Wi-Fi based information, the positives is that the components system can be placed anywhere on a production line and isn’t limited by the placement of cabling, all the sensors available are similar except that they have a radio frequency transmitter attached. The latest generation of Wireless based systems uses a select high frequency which is unique and gives a relative high chance of interference not affecting the system. The highest quality systems can operate up to ten miles from the RF hub, as long as the sensor is within its line of sight. The PLC can also be made wireless which allows it to communicate with the sensors and also allowing a maintenance person to communicate on the move or from a remote location along the production line by a laptop.

A new introduction is the developing of pc interaction with the plc, this can allow for a more in-depth software to be used in both running and programming the PLC, which in turn allows the plc to accomplish more difficult tasks and multi task easier. The introduction of desktop integration also allows for greater access to the plc allowing for modification and backing up of programs on different sources.