Functions of Programmable Logic Controller

Describe about the Programmable Logic Controller for Programming?

A programmable logic controller is used to control machines and processes. To store specific functions and instructions, a programmable memory is used by it so it can be called as a specialized computer or digital computer also. Instructions and specific functions include timing, ON/OFF control, sequencing, counting, arithmetic and data handling. It is used for automation of typically industrial electromechanical processes. It is used in many industries and machines. Extended temperature ranges, digital and analog inputs and outputs can be arranged easily by programmable logic controller. Battery- backed- up store the programs which are used to control the machine operations. Battery-backed-up is also known as non-volatile memory. PLCs have the facility for extensive input / output arrangements. PLCs are connected to sensors and actuators by inputs and outputs. Positions of complex positioning systems, analog process variables and limit switches are read by programmable logic controller. Process variables include temperature and pressure. Pneumatic or hydraulic cylinders, electric motors, solenoids, magnetic relays and analog outputs are operated by programmable logic controller.

For placing and interconnecting the components, system layout is the perfect approach. It is the approach for not only to satisfy the application, but also to ensure the trouble free operation of the controller in its environment. System layout consists the other components that form the total system. Safety control relays, auxiliary power supply, incoming line noise suppressors and isolation transformers are included in these components. These components are easy to access and maintain, in a carefully constructed layout. System productivity can be increased by careful installation planning. The programmable logic controller should be located near the machine because temperature, electrical noise and humidity can be controlled easily by it. When designing the system layout, the effect of humidity, temperature, vibration and electrical noise are important. Actual placement of the controller is decided by these factors. The placement of the programmable logic controller depends on the number of components, modularity and physical design of each component of the system. Mounting and spacing requirements are different for each controller. Local power disconnects isolation and constant voltage transformers and surge suppressors should be located near the top of the devices and beside the power supply. These devices are known as incoming line devices.

PLC Cables-

There are various companies which manufacture PLC cables. Some of these are- Siemens, Allen Bradely, Mitsubishi, Panasonic, Omron, Fuji, GE Fanuc, Delta, Hitachi, Schneider.

Set up of Programmable Logic Controller

Siemens PLC cables-

Express card CP5512  is suitable for laptop express card slot. PROFIBUS/ MPI/ PPI/ FWL communication is achieved between PC and SIMATIC net by it. Communication speed is 9.6 Kbps – 12 Mbps. After the conversion card, it can be used for desktop computer also. Cables fully support the USB to PROFIBUS/ MPI/ PPI communication. Special protocol processing chip is used by it. It contains more powerful functions and it is 100 % compatible with Siemens original PC adapter USB. Largest speed is 1.5 Mbps. It supports the long distance communication.

PLC Cables Adapter-

Adapter should be compatible with Siemens HMI adapter and PC adapter. RS232 port of HMI and PC is fully supported to Profit bus/ MPI communication by this adapter. Siemens special protocol processing chip is used by it. This adapter can be used for Siemens S7- 300/400 series PLC and other equipments with MPI interface/ profit bus. Optical isolated power supplies are RS232 – RS485. It is suitable for the strong communication interface. To establish the communication between Siemens S7 – 200 PLC and Ethernet, Siemens S7 – 200 PLC Ethernet adapter is used. It is used to set up the communication between Siemens S7 – 200 PLC and internet or 3G wireless network by the programing interface of S7 – 200 PLC. RS 485 interface is used as programing interface. There is no need to revise the software, communication protocol and support dynamic DNS. User can remote control the PLC by internet. In order to suit harsh environment in the industrial scene, the RS 485 interface, RJ 45 port and power supply ports are isolated. To establish the communication between S7 – 300 / 400 PLC and Ethernet, Siemens S7 – 300 / 400 PLC Ethernet adapter is used.

Higher data rates are transmitted by coaxial cables. Coaxial cable does not require the shielding of steel conduit.  There is an advantage of small size, resistance to noise and flexibility in fiber optic cables. Mostly, the PLC communications is done by twisted pair cables and RS 232C. RS 232 port can communicate with printers, host computers, terminals and other devices. The maximum transmission speed achieved by it is 19.2 Kbps. For longer transmission distances, RS 422 is used. There are two wires in each of its primary signals. With respect to signal ground,  these wires are always at opposite logic levels. So, the longer transmission distance can be achieved by the interface (4000 feet) and higher data transmission rates (up to 90 Kbps). Data transfer can reach 10 Mbps in shorter runs ( less than 50 feet). Data exchange between a computer and a printer or between two computers can be done by RS 232 port. A path is used for data exchange between the devices. For this path, the voltage is defined by RS 232. For the purposes of cancelling out electromagnetic interference (EMI) from external sources, two conductors are wound together in a form of wiring. This form of wiring is known as twisted pair cabling. The loop area between the wires is reduced so the interference is decreased in twisted wires. Equal and opposite signals are carried by two wires. Because of phase shift, the signals are neutralized. Electromagnetic interference from external sources is prevented by twisted pair cables. They prevent the cross talk from external wires also. They are used in data networks, telephone networks and cable shielding. Twisted pair cables protect cable from external electromagnetic interferences. These cables are used with token ring networks. These cables are long lasting in nature and highly flexible. These cables are used in many telephone systems and Ethernet networks. 

Communication media to be used

There are various automation components in PROFINET component based automation (CBA). All electrical, mechanical and IT variables are covered by one component. Standard programing tools can generate the component. Using a PROFINET component description (PCD) file, a component is described in XML. These descriptions are loaded by a primary tool and logical interconnections are enabled between the individual components by the planning tool. In automation technology, PROFIBUS is a standard for field bus communication. The full form of PROFIBUS is ‘Process Field Bus’. ‘German department of education and research’ promoted it in 1989. There are two types of PROFIBUS- PROFIBUS DP and PROFIBUS PA. In production automation applications, sensors and actuators are operated by PROFIBUS DP (Decentralized Peripherals). These are operated via a centralized controller. In process automation applications, measuring equipment are monitored by PROFIBUS PA (Process Automation). A process control system is used to monitor it.  The cable (physical layer) conforms to IEC 61158-2. Over the bus, power is delivered by it. Data transmission rate in PA is of 31.25 Kbits/s. Same protocol is used in PA and DP. By using a coupler device, a link can be established between PA and DP. In hybrid applications, DP and PA can work together. In hybrid applications, process and factory automation networks are operated side by side. The communication path must be established,            to set up a communication between an IO device and the higher level controller. IO controller sets up these paths. These are set up during system start up. It is based on the configuration data in the engineering system.

Advantages-

PROFINET has been in development for about 10 years.

PROFINET ensures a future for both PROFINET and PROFIBUS. It builds on the success of PROFIBUS.

PROFIBUS is growing exponentially. Currently over 30 million devices installed.

PROFINET is currently growing at about 40% per year.

Limitations-

Due to poor layout and installation, 90% problems are caused.

Wiring problems cannot be eliminated by it.

Routing, interference pick up and grounding problems are occurred by it.

Range of protocols used is very extensive.

It is known as Ethernet for control automation technology. The protocol is suitable for soft and hard real time requirements. The addressed data is read by the Ethercat slave devices. When the telegram passes through the device, the addressed data is read. When the telegram passes, the input data is inserted. Without processing, the frame is not completely received. Optimization of Ethercat protocol is done to process data. Ethercat protocol is transported directly within the standard IEEE 802.3 Ethernet frame. Several sub telegrams are consisted in it. In the network, the data sequence is independent of the physical orders of the nodes. Addressing can be done in any order.

Advantages-

Speed

Flexible topology

Synchronization

Device profiles

Safety protocols

Development

Diagnosis and Error localization

High availability

Communication profiles

Integration of other bus systems

Limitations of PLC-

There is difficulty with replacements or changes.

There is too much work required in connecting wires.

When a problem occurs, hold – up time is indefinite, usually long.

It’s always difficult to find errors and requires skillful work force.

To solve the problems, two methods have been discussed. 1^st is the quasi broadband PLC by which the advantages of broadband and narrowband PLCs are combined. 2^nd is dual mode communication technology by which PLC and wireless are integrated together. These two technologies will be an effective solution for various smart grid applications.

References-

American Institute of Chemical Engineers (AIChE), Center for Chemical Process Safety (CCPS): Guidelines for Technical Management of Chemical Process Safety, 1989

Bishop, P.G. (editor), Dependability of Critical Computer Systems 3 – Techniques Directory – Guidelines produced by The European Workshop on Industrial Computer Systems, Technical Committee 7 (EWICS TC 7), 1990

Wichman, B.A.; Davies, M.: Experience with a compiler testing tool, NPL Report DITC 138/89, 1989

Wichmann, B.A.: Insecurities in the ADA programming language, NPL Report DITC 137/89, January 1989.

Cullyer, W.J.; Goodenough, S.J.; Wichmann, B.A.: The choice of computer languages for use in safety-critical systems, Software Engineering Journal, March 1991

International Electro technical Commission (IEC) Standard 1131-1: Programmable Controllers–Part 1; General Information

International Electrotechnical Commission (IEC) draft Standard 1508 – Functional Safety: Safety Related Systems, Parts 1 – 7, 1997

J.M. Rata: Standardisation efforts world-wide, in: Phil Bennett (ed.): Safety aspects of computer control, 1993

Ministry of Defence, The Procurement of Safety Critical Software in Defence Equipment, Interim Defence Standard 00-55, April 1991

R. E. Bloomfield: Standards for safety related computer systems: a tour through current and emerging standards, CSR 9th Annual Conference on Software Safety, Luxembourg 7-10 April 1992