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Which hazards are represented by the blue section on both the NFPA and HMIS labels?
Hazards to human health
What is dis advantage of Human Machine Interface?
There really aren't any disadvantages of an HMI. Typically, HMIs are used in factories, manufacturing process lines, and actually places like Sheetz to order food. HMIs have greatly advanced efficiency & safety in manufacturing. A worker will input, or enter data on a screen and an automated process could function for an entire shift without human involvement. The only disadvantage would be; 'there aren't enough choices on the screen', for ex. You might want a hamburger cooked 'medium' instead of well done.
What is profibus communicatio protocol?
Profibus is a field bus standard that contains a set of protocols (located in three levels of the OSI model) that make possible communication between PLCs and distributed I/O modules, HMIs, drives, et cetera. It was developed in the late 80s by several companies (including Siemens).
What is the function of human machine interface?
A human machine interface (HMI) allows users to interact with and control machines or systems. It provides a user-friendly way for operators to monitor processes, input commands, and receive feedback. HMIs play a crucial role in enhancing efficiency, safety, and productivity in industrial and technological settings.
What are some seven letter words with 4th letter H and 5th letter M and 6th letter I and 7th letter S?
According to SOWPODS (the combination of Scrabble dictionaries used around the world) there are 1 words with the pattern ---HMIS. That is, seven letter words with 4th letter H and 5th letter M and 6th letter I and 7th letter S. In alphabetical order, they are: mishmis
What is a DH 485 port?
The DH-485 port is a communication interface used primarily in Allen-Bradley programmable logic controllers (PLCs) for network communication. It operates on a peer-to-peer network and allows multiple devices to communicate over a single twisted-pair cable, supporting various protocols for data exchange. This interface is commonly used in industrial automation systems to connect devices like PLCs, HMIs, and sensors, enabling efficient data transfer and control.
What kinds of control equipment are there?
There are various types of "control", therefore, there are various types of control equipment. There is birth control equipment, livestock control equipment, air pollution control equipment, etc.
What are the major components of the modern automated factory?
Equipment and facilities component: such as; CNC machines and industrial robots. Production methods component: procedures, production methods, quality assurance, and production control, such as co-ordinate measuring machines. Material handling component: material handling or material moving systems such as peripheral devices like conveyors. The above are plugged into a communications adapter that is attached to a network. Last there is the Labor component: people.
What is hmi?
A Human-Machine Interface (HMI) is a user interface or dashboard that connects a person to a machine, system, or device. While the term can technically be applied to any screen that allows a user to interact with a device, HMI is most commonly used in the context of an industrial process. Although HMI is the most common term for this technology, it is sometimes referred to as Man-Machine Interface (MMI), Operator Interface Terminal (OIT), Local Operator Interface (LOI), or Operator Terminal (OT). HMI and Graphical User Interface (GUI) are similar but not synonymous: GUIs are often leveraged within HMIs for visualization capabilities.
What is the use of PLC in an industry and why its important?
Programmable Logic Controllers (PLCs) are crucial for various industries for several reasons: Automation: PLCs automate industrial processes by controlling machinery and equipment. This automation improves efficiency, reduces human error, and increases productivity. Industries ranging from manufacturing to energy rely on PLCs to automate tasks such as assembly lines, conveyor belts, and chemical processes. Flexibility: PLCs offer flexibility in programming, allowing for easy customization and reconfiguration of control logic to adapt to changing production needs. This flexibility enables industries to quickly adjust to new product specifications or process requirements without significant downtime. Reliability: PLCs are designed to operate in harsh industrial environments and are built to withstand extreme temperatures, humidity, vibration, and electrical noise. Their robustness ensures reliable operation even in challenging conditions, minimizing downtime and maintenance costs. Data Collection and Analysis: PLCs are often equipped with input/output modules to gather data from sensors and equipment. This data can be used for real-time monitoring of processes, predictive maintenance, quality control, and performance optimization. PLCs can also communicate with higher-level systems such as SCADA (Supervisory Control and Data Acquisition) or MES (Manufacturing Execution Systems) for comprehensive data analysis and reporting. Safety: PLCs play a crucial role in ensuring the safety of industrial operations. They can be programmed to implement safety interlocks, emergency shutdown procedures, and other safety protocols to protect personnel, equipment, and the environment from accidents and hazards. Scalability: PLCs support scalability, allowing for easy expansion or modification of control systems as production requirements evolve or new equipment is added. This scalability enables industries to efficiently grow their operations without the need for extensive reengineering or investment in new control infrastructure. Integration: PLCs can integrate with other automation technologies, such as robotics, motion control systems, and human-machine interfaces (HMIs), to create comprehensive control solutions. This integration streamlines operations, enhances coordination between different components of the production process, and improves overall system performance. Overall, PLCs are indispensable tools for modern industrial automation, providing the foundation for efficient, reliable, and flexible control of manufacturing and production processes across various industries.
Material safety data sheet for Licorice extract root?
Material Data Safety Sheets (MSDS) are required for any chemical which may have an effect on a human. When searching for Material Data Safety Sheets in the internet, specific word order is important. In this case, the search should be for Licorice Root extract. Here is a link to one of the MSDS for this item: http://www.sciencelab.com/xMSDS-Licorice_root_extract-9924491 Here is the text: Material Safety Data Sheet Licorice root extract MSDS Section 1: Chemical Product and Company Identification Product Name: Licorice root extract Catalog Codes: SLL1270 CAS#: 8008-94-4 or 68916-91-6 RTECS: MD2007000 TSCA: TSCA 8(b) inventory: No products were found. CI#: Not available. Synonym:Glycyrrhiza Glabra; Powdered Licorice Root; Licorice extract; Licorice Root; Licorice root extract Chemical Name:Glycyrrhiza Extract Chemical Formula: Not available. Contact Information: Sciencelab.com, Inc. 14025 Smith Rd. Houston, Texas 77396 US Sales: 1-800-901-7247International Sales: 1-281-441-4400 Order Online: ScienceLab.com CHEMTREC (24HR Emergency Telephone), call:1-800-424-9300 International CHEMTREC, call: 1-703-527-3887 For non-emergency assistance, call: 1-281-441-4400 Section 2: Composition and Information on IngredientsComposition: Name CAS # % by Weight Licorice root extract 8008-94-4 or 100 68916-91-6 Toxicological Data on Ingredients: Not applicable. Section 3: Hazards Identification Potential Acute Health Effects: Slightly hazardous in case of skin contact (irritant), of eye contact (irritant), of ingestion, of inhalation. Potential Chronic Health Effects: CARCINOGENIC EFFECTS: Not available. MUTAGENIC EFFECTS: Not available. TERATOGENIC EFFECTS: Not available. DEVELOPMENTAL TOXICITY: Not available. Repeated or prolonged exposure is not known to aggravate medical condition. Section 4: First Aid Measures p. 1 Eye Contact: Check for and remove any contact lenses. In case of contact, immediately flush eyes with plenty of water for at least 15 minutes. Cold water may be used. Get medical attention if irritation occurs. Skin Contact: Wash with soap and water. Cover the irritated skin with an emollient. Get medical attention if irritation develops. Cold water may be used. Serious Skin Contact: Not available. Inhalation: If inhaled, remove to fresh air. If not breathing, give artificial respiration. If breathing is difficult, give oxygen. Get medical attention. Serious Inhalation: Not available. Ingestion: Do NOT induce vomiting unless directed to do so by medical personnel. Never give anything by mouth to an unconscious person. Loosen tight clothing such as a collar, tie, belt or waistband. Get medical attention if symptoms appear. Serious Ingestion: Not available. Section 5: Fire and Explosion Data Flammability of the Product: May be combustible at high temperature. Auto-Ignition Temperature:Not available. Flash Points: Not available. Flammable Limits: Not available. Products of Combustion: Not available. Fire Hazards in Presence of Various Substances:Slightly flammable to flammable in presence of open flames and sparks, of heat. Explosion Hazards in Presence of Various Substances: Risks of explosion of the product in presence of mechanical impact: Not available. Slightly explosive in presence of open flames and sparks. Fire Fighting Media and Instructions: SMALL FIRE: Use DRY chemical powder. LARGE FIRE: Use water spray, fog or foam. Do not use water jet. Special Remarks on Fire Hazards: When heated to decomposition it emits acrid smoke and fumes. Special Remarks on Explosion Hazards:Not available. Section 6: Accidental Release MeasuresSmall Spill: Use appropriate tools to put the spilled solid in a convenient waste disposal container. Finish cleaning by spreading water on the contaminated surface and dispose of according to local and regional authority requirements. Large Spill: Use a shovel to put the material into a convenient waste disposal container. Finish cleaning by spreading water p. 2 on the contaminated surface and allow to evacuate through the sanitary system. Section 7: Handling and Storage Precautions:Keep away from heat. Keep away from sources of ignition. Empty containers pose a fire risk, evaporate the residue under a fume hood. Ground all equipment containing material. Do not ingest. Do not breathe dust. If ingested, seek medical advice immediately and show the container or the label. Storage: Keep container tightly closed. Keep container in a cool, well-ventilated area. Section 8: Exposure Controls/Personal ProtectionEngineering Controls: Use process enclosures, local exhaust ventilation, or other engineering controls to keep airborne levels below recommended exposure limits. If user operations generate dust, fume or mist, use ventilation to keep exposure to airborne contaminants below the exposure limit. Personal Protection:Safety glasses. Lab coat. Dust respirator. Be sure to use an approved/certified respirator or equivalent. Gloves. Personal Protection in Case of a Large Spill: Splash goggles. Full suit. Dust respirator. Boots. Gloves. A self contained breathing apparatus should be used to avoid inhalation of the product. Suggested protective clothing might not be sufficient; consult a specialist BEFORE handling this product. Exposure Limits:Not available. Section 9: Physical and Chemical PropertiesPhysical state and appearance: Solid. (Powdered solid.) Odor: Characteristic. Sweetish. Acrid. Slightly Spicy. Taste: Characteristic. Sweet. Acrid. Molecular Weight: Not available. Color: Brownish-Yellow to light yellow. pH (1% soln/water): Not available. Boiling Point: Not available. Melting Point: Not available. Critical Temperature: Not available. Specific Gravity: Not available. Vapor Pressure: Not applicable. Vapor Density: Not available. Volatility: Not available. Odor Threshold: Not available. Water/Oil Dist. Coeff.: Not available. p. 3 Ionicity (in Water): Not available. Dispersion Properties: See solubility in water. Solubility: Soluble in cold water, hot water. Poor solubility in alcohol. Section 10: Stability and Reactivity Data Stability: The product is stable. Instability Temperature: Not available. Conditions of Instability:Excess heat, ignition sources Incompatibility with various substances: Not available. Corrosivity: Non-corrosive in presence of glass. Special Remarks on Reactivity: Not available. Special Remarks on Corrosivity: Not available. Polymerization: Will not occur. Section 11: Toxicological Information Routes of Entry: Inhalation. Ingestion. Toxicity to Animals: Acute oral toxicity (LD50): >7500 mg/kg [Mouse]. Chronic Effects on Humans: Not available. Other Toxic Effects on Humans: Slightly hazardous in case of skin contact (irritant), of ingestion, of inhalation. Special Remarks on Toxicity to Animals: Not available. Special Remarks on Chronic Effects on Humans: Not available. Special Remarks on other Toxic Effects on Humans: Acute Potential Health Effects: Skin: Not likely to cause skin irritation. Eyes: Dust may cause mechanical eye irritation. Inhalation: Dust may cause respiratory tract irritation. Breathing small amount of this material during normal handling is not likely to be harmful. Ingestion: May cause gastrointestinal tract irritation with hypermotility and diarrhea. Swallowing this material during normal handling is not likely to be harmful. However swallowing large amount may be harmful. It may affect behavior/nervous system (muscle weakness, spastic paralysis, tetany, convulsions), heart, spleen. The toxicity is manifested primarily as hypokalemia. Chronic Potential Health Effects: Ingestion: Prolonged or repeated ingestion may affect the liver, and urinary system (kidneys) by binding to enzymes in the liver and kidney and alter the deactivation of glucocorticoids and other steroid hormones. It may also affect the blood, and metabolism (weight loss). Section 12: Ecological Information Ecotoxicity: Not available. p. 4 BOD5 and COD: Not available. Products of Biodegradation: Possibly hazardous short term degradation products are not likely. However, long term degradation products may arise. Toxicity of the Products of Biodegradation: Not available. Special Remarks on the Products of Biodegradation: Not available. Section 13: Disposal Considerations Waste Disposal: Waste must be disposed of in accordance with federal, state and local environmental control regulations. Section 14: Transport Information DOT Classification: Not a DOT controlled material (United States). Identification: Not applicable. Special Provisions for Transport: Not applicable. Section 15: Other Regulatory Information Federal and State Regulations: No products were found. Other Regulations: EINECS: This product is on the European Inventory of Existing Commercial Chemical Substances. Other Classifications: WHMIS (Canada): Not controlled under WHMIS (Canada). DSCL (EEC): This product is not classified according to the EU regulations. Not applicable. HMIS (U.S.A.): Health Hazard: 1 Fire Hazard: 1 Reactivity: 0 Personal Protection: E National Fire Protection Association (U.S.A.): Health: 1 Flammability: 1 Reactivity: 0 Specific hazard:p. 5 Protective Equipment: Gloves. Lab coat. Dust respirator. Be sure to use an approved/certified respirator or equivalent. Safety glasses. Section 16: Other InformationReferences: Not available. Other Special Considerations: Not available. Created: 10/09/2005 05:58 PM Last Updated: 11/06/2008 12:00 PM The information above is believed to be accurate and represents the best information currently available to us. However, we make no warranty of merchantability or any other warranty, express or implied, with respect to such information, and we assume no liability resulting from its use. Users should make their own investigations to determine the suitability of theinformation for their particular purposes. In no event shall ScienceLab.com be liable for any claims, losses, or damages of any third party or for lost profits or any special, indirect, incidental, consequential or exemplary damages, howsoever arising, even if ScienceLab.com has been advised of the possibility of such damages. p. 6
Where use PLC systems?
Siemens Simatic S7-400 system at rack, left-to-right: power supply unit PS407 4A,CPU 416-3, interface module IM 460-0 and communication processor CP 443-1.A Programmable Logic Controller, PLC or Programmable Controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. The abbreviation "PLC" and the term "Programmable Logic Controller" are registered trademarks of the Allen-Bradley Company (Rockwell Automation). PLCs are used in many industries and machines. Unlike general-purpose computers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed-up or non-volatile memory. A PLC is an example of a hard real time system since output results must be produced in response to input conditions within a limited time, otherwise unintended operation will result.HistoryBefore the PLC, control, sequencing, and safety interlock logic for manufacturing automobiles was mainly composed of relays, cam timers, drum sequencers, and dedicated closed-loop controllers. Since these could number in the hundreds or even thousands, the process for updating such facilities for the yearly model change-over was very time consuming and expensive, as electricians needed to individually rewire relays to change the logic. Digital computers, being general-purpose programmable devices, were soon applied to control of industrial processes. Early computers required specialist programmers, and stringent operating environmental control for temperature, cleanliness, and power quality. Using a general-purpose computer for process control required protecting the computer from the plant floor conditions. An industrial control computer would have several attributes: it would tolerate the shop-floor environment, it would support discrete (bit-form) input and output in an easily extensible manner, it would not require years of training to use, and it would permit its operation to be monitored. The response time of any computer system must be fast enough to be useful for control; the required speed varying according to the nature of the process.[1]In 1968 GM Hydra-Matic (the automatic transmission division of General Motors) issued a request for proposals for an electronic replacement for hard-wired relay systems based on a white paper written by engineer Edward R. Clark. The winning proposal came from Bedford Associates of Bedford, Massachusetts. The first PLC, designated the 084 because it was Bedford Associates' eighty-fourth project, was the result.[2] Bedford Associates started a new company dedicated to developing, manufacturing, selling, and servicing this new product: Modicon, which stood for MOdular DIgital CONtroller. One of the people who worked on that project was Dick Morley, who is considered to be the "father" of the PLC.[3] The Modicon brand was sold in 1977 to Gould Electronics, and later acquired by German Company AEG and then by French Schneider Electric, the current owner.One of the very first 084 models built is now on display at Modicon's headquarters in North Andover, Massachusetts. It was presented to Modicon by GM, when the unit was retired after nearly twenty years of uninterrupted service. Modicon used the 84 moniker at the end of its product range until the 984 made its appearance.The automotive industry is still one of the largest users of PLCs.DevelopmentEarly PLCs were designed to replace relay logic systems. These PLCs were programmed in "ladder logic", which strongly resembles a schematic diagram of relay logic. This program notation was chosen to reduce training demands for the existing technicians. Other early PLCs used a form of instruction list programming, based on a stack-based logic solver. Modern PLCs can be programmed in a variety of ways, from the relay-derived ladder logic to programming languages such as specially adapted dialects of BASIC and C. Another method is State Logic, a very high-level programming language designed to program PLCs based on state transition diagrams.Many early PLCs did not have accompanying programming terminals that were capable of graphical representation of the logic, and so the logic was instead represented as a series of logic expressions in some version of Boolean format, similar to Boolean algebra. As programming terminals evolved, it became more common for ladder logic to be used, for the aforementioned reasons and because it was a familiar format used for electromechanical control panels. Newer formats such as State Logic and Function Block (which is similar to the way logic is depicted when using digital integrated logic circuits) exist, but they are still not as popular as ladder logic. A primary reason for this is that PLCs solve the logic in a predictable and repeating sequence, and ladder logic allows the programmer (the person writing the logic) to see any issues with the timing of the logic sequence more easily than would be possible in other formats.ProgrammingEarly PLCs, up to the mid-1980s, were programmed using proprietary programming panels or special-purpose programming terminals, which often had dedicated function keys representing the various logical elements of PLC programs.[2] Some propreitary programming terminals displayed the elements of PLC programs as graphic symbols, but plain ASCII character representations of contacts, coils, and wires were common. Programs were stored on cassette tape cartridges. Facilities for printing and documentation were minimal due to lack of memory capacity. The very oldest PLCs used non-volatile magnetic core memory. More recently, PLCs are programmed using application software on personal computers, which now represent the logic in graphic form instead of character symbols. The computer is connected to the PLC through Ethernet, RS-232, RS-485 or RS-422 cabling. The programming software allows entry and editing of the ladder-style logic. Generally the software provides functions for debugging and troubleshooting the PLC software, for example, by highlighting portions of the logic to show current status during operation or via simulation. The software will upload and download the PLC program, for backup and restoration purposes. In some models of programmable controller, the program is transferred from a personal computer to the PLC through a programming board which writes the program into a removable chip such as an EEPROM or EPROM.FunctionalityThe functionality of the PLC has evolved over the years to include sequential relay control, motion control, process control, distributed control systems and networking. The data handling, storage, processing power and communication capabilities of some modern PLCs are approximately equivalent to desktop computers. PLC-like programming combined with remote I/O hardware, allow a general-purpose desktop computer to overlap some PLCs in certain applications. Regarding the practicality of these desktop computer based logic controllers, it is important to note that they have not been generally accepted in heavy industry because the desktop computers run on less stable operating systems than do PLCs, and because the desktop computer hardware is typically not designed to the same levels of tolerance to temperature, humidity, vibration, and longevity as the processors used in PLCs. In addition to the hardware limitations of desktop based logic, operating systems such as Windows do not lend themselves to deterministic logic execution, with the result that the logic may not always respond to changes in logic state or input status with the extreme consistency in timing as is expected from PLCs. Still, such desktop logic applications find use in less critical situations, such as laboratory automation and use in small facilities where the application is less demanding and critical, because they are generally much less expensive than PLCs. Programmable Logic Relay (PLR)In more recent years, small products called PLRs (programmable logic relays), and also by similar names, have become more common and accepted. These are very much like PLCs, and are used in light industry where only a few points of I/O (i.e. a few signals coming in from the real world and a few going out) are involved, and low cost is desired. These small devices are typically made in a common physical size and shape by several manufacturers, and branded by the makers of larger PLCs to fill out their low end product range. Popular names include PICO Controller, NANO PLC, and other names implying very small controllers. Most of these have between 8 and 12 discrete inputs, 4 and 8 discrete outputs, and up to 2 analog inputs. Size is usually about 4" wide, 3" high, and 3" deep. Most such devices include a tiny postage stamp sized LCD screen for viewing simplified ladder logic (only a very small portion of the program being visible at a given time) and status of I/O points, and typically these screens are accompanied by a 4-way rocker push-button plus four more separate push-buttons, similar to the key buttons on a VCR remote control, and used to navigate and edit the logic. Most have a small plug for connecting via RS-232 or RS-485 to a personal computer so that programmers can use simple Windows applications for programming instead of being forced to use the tiny LCD and push-button set for this purpose. Unlike regular PLCs that are usually modular and greatly expandable, the PLRs are usually not modular or expandable, but their price can be two orders of magnitude less than a PLC and they still offer robust design and deterministic execution of the logic. PLC topicsFeaturesControl panel with PLC (grey elements in the center). The unit consists of separate elements, from left to right; power supply, controller, relay units for in- and output The main difference from other computers is that PLCs are armored for severe conditions (such as dust, moisture, heat, cold) and have the facility for extensive input/output (I/O) arrangements. These connect the PLC to sensors and actuators. PLCs read limit switches, analog process variables (such as temperature and pressure), and the positions of complex positioning systems. Some use machine vision.[4] On the actuator side, PLCs operate electric motors, pneumatic or hydraulic cylinders, magnetic relays, solenoids, or analog outputs. The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a computer network that plugs into the PLC.Scan timeA PLC program is generally executed repeatedly as long as the controlled system is running. The status of physical input points is copied to an area of memory accessible to the processor, sometimes called the "I/O Image Table". The program is then run from its first instruction rung down to the last rung. It takes some time for the processor of the PLC to evaluate all the rungs and update the I/O image table with the status of outputs.[5] This scan time may be a few milliseconds for a small program or on a fast processor, but older PLCs running very large programs could take much longer (say, up to 100 ms) to execute the program. If the scan time was too long, the response of the PLC to process conditions would be too slow to be useful. As PLCs became more advanced, methods were developed to change the sequence of ladder execution, and subroutines were implemented.[6] This simplified programming could be used to save scan time for high-speed processes; for example, parts of the program used only for setting up the machine could be segregated from those parts required to operate at higher speed.Special-purpose I/O modules, such as timer modules or counter modules, can be used where the scan time of the processor is too long to reliably pick up, for example, counting pulses and interpreting quadrature from a shaft encoder. The relatively slow PLC can still interpret the counted values to control a machine, but the accumulation of pulses is done by a dedicated module that is unaffected by the speed of the program execution.System scaleA small PLC will have a fixed number of connections built in for inputs and outputs. Typically, expansions are available if the base model has insufficient I/O. Modular PLCs have a chassis (also called a rack) into which are placed modules with different functions. The processor and selection of I/O modules are customized for the particular application. Several racks can be administered by a single processor, and may have thousands of inputs and outputs. A special high speed serial I/O link is used so that racks can be distributed away from the processor, reducing the wiring costs for large plants.User interfaceSee also: User interface and List of human-computer interaction topics PLCs may need to interact with people for the purpose of configuration, alarm reporting or everyday control. A human-machine interface (HMI) is employed for this purpose. HMIs are also referred to as man-machine interfaces (MMIs) and graphical user interface (GUIs). A simple system may use buttons and lights to interact with the user. Text displays are available as well as graphical touch screens. More complex systems use programming and monitoring software installed on a computer, with the PLC connected via a communication interface.CommunicationsPLCs have built in communications ports, usually 9-pin RS-232, but optionally EIA-485 or Ethernet. Modbus, BACnet or DF1 is usually included as one of the communications protocols. Other options include various fieldbuses such as DeviceNet or Profibus. Other communications protocols that may be used are listed in the List of automation protocols. 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.ProgrammingPLC programs are typically written in a special application on a personal computer, then downloaded by a direct-connection cable or over a network to the PLC. The program is stored in the PLC either in battery-backed-up RAM or some other non-volatile flash memory. Often, a single PLC can be programmed to replace thousands of relays.[7] Under the IEC 61131-3 standard, PLCs can be programmed using standards-based programming languages. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers. Initially most PLCs utilized Ladder Logic Diagram Programming, a model which emulated electromechanical control panel devices (such as the contact and coils of relays) which PLCs replaced. This model remains common today.IEC 61131-3 currently defines five programming languages for programmable control systems: function block diagram (FBD), ladder diagram (LD), structured text (ST; similar to the Pascal programming language), instruction list (IL; similar to assembly language) and sequential function chart (SFC).[8] These techniques emphasize logical organization of operations.[7]While the fundamental concepts of PLC programming are common to all manufacturers, differences in I/O addressing, memory organization and instruction sets mean that PLC programs are never perfectly interchangeable between different makers. Even within the same product line of a single manufacturer, different models may not be directly compatible.PLC compared with other control systemsAllen-Bradley PLC installed in a control panel PLCs are well adapted to a range of automation tasks. These are typically industrial processes in manufacturing where the cost of developing and maintaining the automation system is high relative to the total cost of the automation, and where changes to the system would be expected during its operational life. PLCs contain input and output devices compatible with industrial pilot devices and controls; little electrical design is required, and the design problem centers on expressing the desired sequence of operations. PLC applications are typically highly customized systems, so the cost of a packaged PLC is low compared to the cost of a specific custom-built controller design. On the other hand, in the case of mass-produced goods, customized control systems are economical. This is due to the lower cost of the components, which can be optimally chosen instead of a "generic" solution, and where the non-recurring engineering charges are spread over thousands or millions of units.For high volume or very simple fixed automation tasks, different techniques are used. For example, a consumer dishwasher would be controlled by an electromechanical cam timer costing only a few dollars in production quantities.A microcontroller-based design would be appropriate where hundreds or thousands of units will be produced and so the development cost (design of power supplies, input/output hardware and necessary testing and certification) can be spread over many sales, and where the end-user would not need to alter the control. Automotive applications are an example; millions of units are built each year, and very few end-users alter the programming of these controllers. However, some specialty vehicles such as transit buses economically use PLCs instead of custom-designed controls, because the volumes are low and the development cost would be uneconomical.[9]Very complex process control, such as used in the chemical industry, may require algorithms and performance beyond the capability of even high-performance PLCs. Very high-speed or precision controls may also require customized solutions; for example, aircraft flight controls. Single-board computers using semi-customized or fully proprietary hardware may be chosen for very demanding control applications where the high development and maintenance cost can be supported. "Soft PLCs" running on desktop-type computers can interface with industrial I/O hardware while executing programs within a version of commercial operating systems adapted for process control needs.[9]Programmable controllers are widely used in motion control, positioning control and torque control. Some manufacturers produce motion control units to be integrated with PLC so that G-code (involving a CNC machine) can be used to instruct machine movements.[citation needed]PLCs may include logic for single-variable feedback analog control loop, a "proportional, integral, derivative" or "PID controller". A PID loop could be used to control the temperature of a manufacturing process, for example. Historically PLCs were usually configured with only a few analog control loops; where processes required hundreds or thousands of loops, a distributed control system (DCS) would instead be used. As PLCs have become more powerful, the boundary between DCS and PLC applications has become less distinct.PLCs have similar functionality as Remote Terminal Units. An RTU, however, usually does not support control algorithms or control loops. As hardware rapidly becomes more powerful and cheaper, RTUs, PLCs and DCSs are increasingly beginning to overlap in responsibilities, and many vendors sell RTUs with PLC-like features and vice versa. The industry has standardized on the IEC 61131-3 functional block language for creating programs to run on RTUs and PLCs, although nearly all vendors also offer proprietary alternatives and associated development environments.In recent years "Safety" PLCs have started to become popular, either as standalone models or as functionality and safety-rated hardware added to existing controller architectures (Allen Bradley Guardlogix, Siemens F-series etc.). These differ from conventional PLC types as being suitable for use in safety-critical applications for which PLCs have traditionally been supplemented with hard-wired safety relays. For example, a Safety PLC might be used to control access to a robot cell with trapped-key access, or perhaps to manage the shutdown response to an emergency stop on a conveyor production line. Such PLCs typically have a restricted regular instruction set augmented with safety-specific instructions designed to interface with emergency stops, light screens and so forth. The flexibility that such systems offer has resulted in rapid growth of demand for these controllers.Discrete and analog signalsDiscrete signals behave as binary switches, yielding simply an On or Off signal (1 or 0, True or False, respectively). Push buttons, Limit switches, and photoelectric sensors are examples of devices providing a discrete signal. Discrete signals are sent using either voltage or current, where a specific range is designated as On and another as Off. For example, a PLC might use 24 V DC I/O, with values above 22 V DC representing On, values below 2VDC representing Off, and intermediate values undefined. Initially, PLCs had only discrete I/O. Analog signals are like volume controls, with a range of values between zero and full-scale. These are typically interpreted as integer values (counts) by the PLC, with various ranges of accuracy depending on the device and the number of bits available to store the data. As PLCs typically use 16-bit signed binary processors, the integer values are limited between -32,768 and +32,767. Pressure, temperature, flow, and weight are often represented by analog signals. Analog signals can use voltage or current with a magnitude proportional to the value of the process signal. For example, an analog 0 - 10 V input or 4-20 mA would be converted into an integer value of 0 - 32767.Current inputs are less sensitive to electrical noise (i.e. from welders or electric motor starts) than voltage inputs.ExampleAs an example, say a facility needs to store water in a tank. The water is drawn from the tank by another system, as needed, and our example system must manage the water level in the tank by controlling the valve that refills the tank. Shown is a "ladder diagram" which shows the control system. A ladder diagram is a method of drawing control circuits which pre-dates PLCs. The ladder diagram resembles the schematic diagram of a system built with electromechanical relays. Shown are: Two inputs (from the low and high level switches) represented by contacts of the float switchesAn output to the fill valve, labelled as the fill valve which it controlsAn "internal" contact, representing the output signal to the fill valve which is created in the program.A logical control scheme created by the interconnection of these items in softwareIn the ladder diagram, the contact symbols represent the state of bits in processor memory, which corresponds to the state of physical inputs to the system. If a discrete input is energized, the memory bit is a 1, and a "normally open" contact controlled by that bit will pass a logic "true" signal on to the next element of the ladder. Internal status bits, corresponding to the state of discrete outputs, are also available to the program.In the example, the physical state of the float switch contacts must be considered when choosing "normally open" or "normally closed" symbols in the ladder diagram. The PLC has two discrete inputs from float switches (Low Level and High Level). Both float switches (normally closed) open their contacts when the water level in the tank is above the physical location of the switch.When the water level is below both switches, the float switch physical contacts are both closed, and a true (logic 1) value is passed to the Fill Valve output. Water begins to fill the tank. The internal "Fill Valve" contact latches the circuit so that even when the "Low Level" contact opens (as the water passes the lower switch), the fill valve remains on. Since the High Level is also normally closed, water continues to flow as the water level remains between the two switch levels. Once the water level rises enough so that the "High Level" switch is off (opened), the PLC will shut the inlet to stop the water from overflowing; this is an example of seal-in (latching) logic. The output is sealed in until a high level condition breaks the circuit. After that the fill valve remains off until the level drops so low that the Low Level switch is activated, and the process repeats again. | | | Low Level High Level Fill Valve | |------[/]------|------[/]----------------------(OUT)---------| | | | | | | | | | | Fill Valve | | |------[ ]------| | | | | |A complete program may contain thousands of rungs, evaluated in sequence. Typically the PLC processor will alternately scan all its inputs and update outputs, then evaluate the ladder logic; input changes during a program scan will not be effective until the next I/O update. A complete program scan may take only a few milliseconds, much faster than changes in the controlled process.Programmable controllers vary in their capabilities for a "rung" of a ladder diagram. Some only allow a single output bit. There are typically limits to the number of series contacts in line, and the number of branches that can be used. Each element of the rung is evaluated sequentially. If elements change their state during evaluation of a rung, hard-to-diagnose faults can be generated, although sometimes (as above) the technique is useful. Some implementations forced evaluation from left-to-right as displayed and did not allow reverse flow of a logic signal (in multi-branched rungs) to affect the output.
