about UPS (Uninterruptible power supply ) ??  

An uninterruptible power supply (UPS), sometimes called an uninterruptible power source, is a device which maintains a continuous supply of electric power to connected equipment by supplying power from a separate source when utility power is not available.  

A UPS is inserted between the source of power (typically commercial utility power) and the load it is protecting. When a power failure or abnormality occurs, the UPS will effectively switch from utility power to its own power source almost instantaneously.  

Low Level UPS : While not limited to any particular type of equipment, a UPS is typically used to protect computers, telecommunication equipment or other electrical equipment where an unexpected power disruption could cause injuries, fatalities, serious business disruption or data loss. UPS units come in sizes ranging from units which will backup a single computer without monitor (around 200 VA) to units which will power entire data centers or buildings (several megawatts). Larger UPS units typically work in conjunction with generators.  

Historically, UPS units were very expensive and were most likely to be used on expensive computer systems and in areas where the power supply is interrupted frequently. However, UPS units are now more affordable, and have become an essential piece of equipment for data centers and business computers, but are also used for personal computers, entertainment systems and more.  

In North America in particular, the electrical grid is under increasing strain particularly during heavy demand periods such as summer when air conditioning use is at its highest. In order to prevent blackouts, electrical utilities will sometimes use a process called load shedding, which involves cutting the power to large groups of customers for short periods of time. The single biggest event that brought attention to the need for UPS power backup units was the 2003 North America blackout in the north-eastern US and eastern Canada.  

A UPS is not to be confused with a standby generator, which does not provide protection from a momentary power interruption and may result in an interruption when it is switched into service, whether manually or automatically. However, such generators are typically placed before the UPS to provide cover for lengthy outages.   





PLC features : 


The main difference from other computers are the special input/output arrangements. These connect the PLC to sensors and actuators. PLCs read limit switches, temperature indicators and the positions of complex positioning systems. Some even use machine vision. On the actuator side, PLCs drive any kind of electric motor, pneumatic or hydraulic cylinders or diaphragms, magnetic relays or solenoids. The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a proprietary computer network that plugs into the PLC.  

PLCs were invented as less expensive replacements for older automated systems that would use hundreds or thousands of relays and cam timers. Often, a single PLC can be programmed to replace thousands of relays. Programmable controllers were initially adopted by the automotive manufacturing industry, where software revision replaced the re-wiring of hard-wired control panels.  

The earliest PLCs expressed all decision making logic in simple ladder logic inspired from the electrical connection diagrams. The electricians were quite able to trace out circuit problems with schematic diagrams using ladder logic. This was chosen mainly to reduce the apprehension of the existing technicians. 

The functionality of the PLC has evolved over the years to include typical relay control, sophisticated motion control, process control, distributed control systems and complex networking. Today, the line between a general purpose programmable computer and a PLC is thinning. The data handling, storage, processing power and communication capabilities of some modern PLCs are approximately equivalent to desk-top computers. PLC-like functionality, combined with remote I/O hardware, allow a general-purpose desktop computer to overlap some PLCs in certain applications.  

With the IEC 61131-3 standard, it is now possible to program PLCs using structured programming languages, and logic elementary operations. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers.  






PLC compared with other control systems :  


PLCs are well-adapted to a certain 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 everything needed to handle high power loads right out of the box; very little electrical design is required and the design problem centers on expressing the desired sequence of operations in ladder logic (or function chart) notation. PLC applications are typically highly customized systems so the cost of a PLC is low compared to the cost of contracting a designer for a specific, one-time only design. On the other hand, in the case of mass-produced goods, customized control systems quickly pay for themselves due to the lower cost of the components, which can be optimally chosen instead of a "generic" solution.  

However, it should be noted that some PLCs no longer have a very high cost. Modern PLCs with full capabilities are available for a few hundred USD 

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 and input/output hardware) 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 busses economically use PLCS instead of custom-designed controls, because the volumes are low and the development cost would be uneconomic.)  

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 controls may also require customized solutions; for example, aircraft flight controls.  


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