Turing Machiene

"A Turing machine is a device that manipulates symbols on a strip of tape according to a table of rules. Despite its simplicity, a Turing machine can be adapted to simulate the logic of any computer algorithm, and is particularly useful in explaining the functions of a CPU inside a computer."                                                                                                          

In its simplest form, a Turing machine is composed of a "tape", a ribbon of paper of indefinite length. There is a "head" that can read the symbol, chose to write a new symbol in place, and then move left or right. The Turing machine is said to be in a certain "state". Finally, the program is a list of "transitions", that is a list that says, given a current state and a symbol currently under the head, what should be written on the tape, what state the machine should go, and whether the head should move left or right.
The tape is used to store data. In addition, it can also store a series of transitions (a small programs) and thus, the head can run "sub-programs". 

By analogy with modern computers, the tape is the memory and the head is the microprocessor. 
Although it is composed of pretty simple capabilities, Turing argued that this simple machine could performed any computation, that is, could realize anything that results from operations.

Schematic Diagram of Unit Auxilaries- Continuation of the previous article.

Principle of Operation of Thermal Station

In the some Thermal Stations, feed water is pumped with the aid of boiler filling pump into the drum; the water from the boiler drum flows downward through the down comers by gravity to the common headers which supply each of the tubes of which the furnace wall is made up of. The water is allowed to rise in the tubes above the burner level and the burners are lit off in an arranged order from the central control room.  The feed water gains heat energy up to the saturation temperature and the wet steam moves to the drum. As the drum is designed to be half filled with water, the upper part is for the wet steam.

A device called cyclone separator (with the aid of centrifugal system) separates the wet from dry steam before the dry steam leaves for the primary superheater.  The steam leaves the primary superheater for the secondary superheater, at this point the pressure is about 12.5Mpa at 541⁰C.  Just before the steam reaches the turbine blades, it losses about 1⁰C and steam at 12.5Mpa, 540⁰C impinges on the HP turbine blade. The heat energy is converted to mechanical energy on the HP turbine and the turbine spins.  The weak steam from the HP turbine exhaust is piped back to a heating system called reheater.  This is designed to raise the temperature back to 540⁰C but at full load (220MW) the pressure will be 3.3Mpa.

The steam from the Reheater is then sent to the intermediate pressure turbine.   (IPTBN) As the three stages of the turbine are linked via one shaft connected together with the generator a speed of 3000rpm is achieved at the generator end.

At the     LP turbine exhaust, the steam has lost almost all the energy and the LP exhaust temperature is as low as 45⁰C at the point of condensation! The makes it easier for condensation to take place in the condenser.  The steam condenses and settles beneath the condenser.  This is called “hot well”.  The pressure is far lower than atmospheric pressure and hence a large pump is required to pump the water because it has to overcome the negative pressure, take it to zero pressure (atmospheric pressure).  This is done with the aid of condensate extraction pump (CEP). The water is taken via steam jet Air Ejector, in order to remove all dissolved 0₂, to the condensate polishing plant.  The Booster pump increases the pressure as the water passes through LP HTR 1----> LPHTR 2-------->LP HTR3 and then to the Dearator.

The Dearator removes any remaining 0₂ in the feed water and serves as the HTR 4. The boiler feed pump (BFP) takes the feed water and increases the pressure to the boiler pressure to the boiler pressure.  The water is taken through HP HTR 5------>HP HTR 6------>he Economiser and then to the boiler Drum and the cycle begins again.

705tons/hr of dry steam (Dryness fraction of about 0.9) is required for 220MW.

Note: The following article describes one of the ways incorporated by thermal Power Plant.The following article is a piece from a reoport of Lagos Thermal Station by P. B. Osofisan.

Turbine Control Block Diagram

Turbine Control

The below figure gives a block diagram of power generation control.  On receiving turbine governor demand signals from the Automatic Boiler Control (ABC), the governor is put into operation, and main steam flow changes.  The flow rate of main steam fed to the turbine is converted into generated energy by the turbine generator.

Conventional governor control has been obtained by driving it directly with the turbine according to the control signals.  These signals are amplified by a proportional plus integral controller from the load dispatching values and the deviation signal of generator energy.  To further improve overall control in this case the main steam flow control has been added.  Also the cascaded control method that incorporates the above main steam flow control, has been included as a sub unit of generator energy control.Fig 4.1 gives a block diagram of power generation control.  On receiving turbine governor demand signals from the Automatic Boiler Control (ABC), the governor is put into operation, and main steam flow changes.  The flow rate of main steam fed to the turbine is converted into generated energy by the turbine generator.

Conventional governor control has been obtained by driving it directly with the turbine according to the control signals.  These signals are amplified by a proportional plus integral controller from the load dispatching values and the deviation signal of generator energy.  To further improve overall control in this case the main steam flow control has been added.  Also the cascaded control method that incorporates the above main steam flow control, has been included as a sub unit of generator energy control.

1.       Dispatching Value of Main Steam Flow Rate

The selection of main steam flow rate values in the turbine following mode is different from that of the coordinated control mode.

a)    Coordinated control mode (Not turbine following mode)

The dispatching values of the main steam flow rate are decided by adding advance signals based on the load dispatching values to corrected values of generated energy control.

b)    Turbine following mode

Dispatching values of the main steam flow rate are decided by correcting the program setting point based on the total fuel flow rate of boiler input using the deviation signals of the main steam pressure.

2.    Upper and Lower Limits Using Main Steam Pressure

Stable plant operation cannot be achieved if there is wide variation of main steam pressure, which exceeds the specified value.  Pressure should be immediately restored to its specified value.

Large variation in main steam pressure when operating under coordinated control is caused by a response delay of the boiler.  In order to lessen this variation, main steam pressure must be controlled by the turbine using a turbine governor which provides the most rapid effect on the main steam pressure.

3.   Turbine Governor Control

Main steam flow rate deviation signals are given to the proportional plus integral controller through a deviation limiting circuit for main steam pressure.  Governor opening signals, amplified by the proportional plus integral controller, will send turbine governor control signals after having passed through a manual/auto changing circuit.

Schematic Diagram of Steam Power Station

Components of Conventional Power Plant

The principal items of conventional or steam power plant are the boiler and the turbine. For these units a number of auxiliary units are also required.  A boiler uses coal, oil or gas as the fuel.  For this purpose, the fuel is stored in the coolant, a fuel handing plant is necessary to maintain a regular supply of fuel to the boiler.  This may again involved a number of smaller units.

Since the total weight of the fuel is not decomposed during combustion in the furnace while firing the boiler, thus a certain percentage of the fuel is collected as waste.  In case of coal fired boiler, almost 10-15% of the total weight of coal fired is collected in the form of ash.  Thus a station using 200,000 tonnes of coal/annum will produce in the average about 25,000 tonnes of ash. Such a huge quantity of ash from the furnace requires an ash handing equipment which will transfer ash from boiler furnace to ash storage.

Besides, air heaters may be used in plant to make use of the heat of flue gases which are always at elevated temperature.  Flue gases go to the atmosphere through the chimney, induced or forced draft fans may be used to create the necessary draft.

Steam from the boiler is supplied to the turbine where it expands thereby doing work.  Exhaust steam is passed to the condenser where it is condensed.  The condensate (i.e. H₂0) is pumped to the boiler by passing through high and low pressure heaters.

Condensers use H₂O for condensing steam. H₂O at the outlet of the condenser is relatively hot and may be cooled in the cooling tower and then recirculated.

Turbine is couple to the generator.  Output of the generator is supplied to consumer is relatively hot and may be cooled in the cooling tower and then recalculated.

Turbine is coupled to the generator.  Output of the generator is supplied to consumers through circuit breakers, transformers etc.

PLC Disadvantage

(a)           Newer Technology: It is difficult to change some personnel’s thinking from ladders and relays to the PLC computer concepts.

(b)           Fixed Program Applications: Some programs are single function application.  It does not pay to use a PLC that includes multiple programming capabilities if they are not needed.  One example is in the use if drum controller/ sequencers.

(c)           Environmental Considerations: Certain process environment, such as high heat and vibration, interfere with the electronic devices in PLC’s which limits their use.

(d)           Fixed-Circuit Operation: If the circuit in operation is never altered, a fixed control system (such as a mechanical drum) might be less costly than a PLC.  The PLC is most effective when periodic changes in operation are made.

(e)           Fail-Safe Operation: In relay systems, the stop button electrically disconnects the circuit; if the power fails, the system stops.  Furthermore the relay system does not automatically restart when power is restored.  This, of course, can be programmed into the PLC; however, in some PLC programs, you may have to apply an input voltage to cause a device to stop.  These systems are not fail-safe.  This disadvantage can be overcome by adding safety relays to a PLC system.

PLC Advantages

PLCs have been gaining popularity on the factory floor and will probably remain predominant for some time to come.  Most of this is because of the advantages they offer which are :

(a)           Flexibility: In the past, each different electronically controlled production machine required its own type of controller.  But now one model of a PLC can serve as the controller for any of the machines.

(b)           Implementing Changes and Correcting Errors: With a wired relay-type panel, any program alterations require time for rewiring of panels and devices. When a PLC program circuit or sequence design is made, the PLC program can be change from a keyboard sequence in a matter of minutes.  No rewiring is required for a PLC-controlled system.

(c)           Lower Cost – Increased technology makes it possible to condense more functions into smaller and less expensive packages.  Now, you can purchase a PLC with numerous relays, timers, counters and other function for a few hundred dollars.

(d)           Large Qualities of Contacts: The PLC has a large quantity of contacts for each coil available in its programming.  Time will be taken to procure and install a new relay or relay contact block when a design change is made requiring more contacts.

(e)           Pilot Running: A PLC programmed circuit can be prerun and evaluated in the officer or lab.

(f)            Visual Observation: A PLC circuit’s operation can be seen during operation directly on a CRT screen. The operation or misoperation of a circuit can be observed as it happens.  Logic paths light up on the screen as they are energized.  Troubleshooting can be done more quickly during visual observation.

(g)           Speed of Operation – Relays can take an unacceptable amount of time to actuate.  The operational speed for the PLC logic operation is determined by the scan time, which is a matter of milliseconds.

(h)           Ladder or Boolean Programming Method: The PLC programming can be accomplished in the ladder mode by an electrician or technician.  Alternately, a PLC programmer who works in digital or Boolean control systems can also easily perform PLC programming.

(i)            Reliability and Maintainability: Solid-state devices are more reliable in general than mechanical system or relays and timers.  The PLC is made of solid-state components with very high reliability rates.  Consequently, the control system maintenance cost are lower and downtime is minimal.

(j)            Documentation: An immediate print out of the true PLC circuit is available in minutes if required.

(k)           Security: A PLC program change cannot be made unless the PLC is properly unlocked and programmed. Relay panels tend to undergo document changes.

(l)            Ease of Changes by Reprogramming: Since the PLC can be reprogrammed quickly, mixed production processing can be accomplished. For example, if part B comes down the assembly line while part A is still being processed, a program for part B’s processing can be reprogrammed into the production machinery in a matter of seconds.

(m)         Simplicity of Ordering Control System Components: When the PLC arrives, all the counters, relays and other components arrive as one delivery.  With the PLC, one more relay is available – provided you ordered a PLC with enough computing power.

Factors to be considered in Purchasing a PLC

After the planning phase of the design, the equipment can be ordered. The decision will depend upon the basic criteria listed below.

·         Number of logical input and outputs.

·         Memory – often 1K and up. Need is dictated by size of ladder logic program.  A ladder element will take only a few bytes, and will be specified in manufacturers documentation.

·         Number of special I/O modules- when doing some exotic application, a large number of specual add-on cards may be required.

·         Scan Time- Big programs or faster processes will require shorter scan times.  And, the shorter the scan time, the higher the cost.  Typical values for this are 1 microsecond per simple ladder instruction.

·         Communications – Serial and networked connections allow the PLC to be programmed and talk to other PLCs. The needs are determined by the application.

·         Software – Availability of programmed software and other tools determines the programming and debugging ease.

The process of selecting a PLC can be broken into the steps listed below.

1.       Understand the process to be controlled.

·         List the number of types of input and outputs.

·         Determine how the process is to the controlled.

·         Determine special needs such as distance between parts of the process.

2.       If not already specified, a single vendor should be selected.  Factors that might be considered are, (Note: vendor research may be needed here)

·                                             Manuals and documentation

·                                             Support while developing programs

·                                             The range of products available

·                                             Shipping times for emergency replacements

·                                             Training

·                                             The track record for the company

·                                             Business practices (billing upgrades/obsolete product, etc.)

3.       Plan the ladder logic for the controls.

4.       Count the program instruction.  Use the instruction, times and memory requirements for each instruction to determine if the PLC has sufficient memory, and if the response time will be adequate for the process.

5.       Look for special program needs and check the PLC model. (e.g. PID)

6.       Estimate the cost for suitable hardware, programming software, cables manuals, training etc., or ask for a quote from a vendor.

Comparison of Control System

Characteristics	Relay Systems	Digital Logic	Computers	PLC system
Price per function	Fairly Low	Low	High	Low
Physical size	Bulky	Very compact	Fairly compact	Very compact
Operating speed	Slow	Very fast	Fairly fast	fast
Electrical noise immunity	Excellent	Good	Quite good	Good
Installation	Time – consuming to design and install	Design time-consuming	Programming extremely time-consuming	Simple to program and install
Capable of complicated operations	No	Yes	Yes	Yes
Ease of changing function	Very difficult	Difficult	Quite simple	Very simple
Ease of maintenance	Poor-large number of contacts	Poor if PLCs soldered	Poor –several custom boards	Good –few standard cards
	Design and install	Consuming	Time consuming	Install

Detailed PLC Architecture

PLC Overall System

PLC as a Computer

A PLC is a computer, but a different type form the one we are probably used to seeing and working with. Most of people are familiar with data-processing computers, especially microcomputers such as those from Apple and IBM.

These machines sit on your desk, or even on your lap, and have powerful systems and applications software that let you play games, do word processing, create computer-aided design (CAD) drawings, and layout spread sheets.

Such computers process reams of data, which is why they are called data-processing machines. Their input peripherals are the keyboard and mouse; their output peripherals, the video display terminal (VDT), Printer, and plotter.

There is another type of computer, however, known as a process-control computer.  Although it, of course, processes data, its main function is to control manufacturing and industrial processes (machinery, robots, assembly lines. etc).

Such computers are said to be event driven. Although they may have a keyboard input peripheral, their control inputs are switches and sensors, and although output peripherals such as VDTs and printers may be attached, the process-control computer primarily controls such devices as motors, solenoids, lights, and heaters. Such process-control computers, which number in the millions, are the control element in virtually all modern factory operations.

PLCs are a type of process-control computer: small relatively inexpensive environmentally hardened, and easy to program, operate, maintain, and repair.  They are often installed close to the machinery or process they control and are thus seen as an extension of industrial equipment.

Although PLCs are similar to ‘conventional’ computers in terms of hardware technology, they have specific features suited to industrial control:

·         Rugged, noise immune equipment

·         Modular plug-in construction, allowing easy replacement/ addition of units (e.g. input/output);

·         Standard input/output connections and signal levels;

·         Easily understood programming language (e.g. ladder diagram or function chart)

·         Ease of programming and reprogramming in-plant

These features make programmable controllers highly desirable in a wide variety of industrial-plant and process-control situations.

Cicode- SCADA

/* This function will set the passed variable high then place it low 2 seconds later.*/

FUNCTION GoPulse(STRING sTAGNAME, STRING sCLUSTER = "")

    INT     tagNameLength = StrLength(sTAGNAME);

    INT     tagHasDot       = -1;

    STRING sAssString      = "";

    STRING sOriginalTag    = "";

    INT     nError          = 0;

   

    IF (tagNameLength > 0) THEN

        // Ensure we don't do work with an empty reference

        IF StrGetChar(sTAGNAME, 0) = StrToChar("?") THEN

            // Strip the ? characters from the start and end of the super genie ass number

            sAssString = StrLeft(sTAGNAME, tagNameLength - 1);

            sAssString = StrRight(sAssString, tagNameLength - 2);

           

            sOriginalTag = sTAGNAME;

            ErrSet(1);

            sTAGNAME = AssGetProperty(sAssString, "FullName");

            nError = IsError();

            ErrSet(0);

           

            tagNameLength = StrLength(sTAGNAME);

        END

        IF (tagNameLength > 0) THEN

            // Do this check again in case the AssGetProperty() did not work   

            tagHasDot = StrSearch(0, sTAGNAME, ".");

            IF (tagHasDot >= 1) THEN

                // This tag has a Cluster reference so null any passed cluster reference

                // Assume ".tag" is not part of this logic

                sCLUSTER = "";

            END

            nError = TagWrite(sTAGNAME,1,0,TRUE,sCLUSTER); // Synchronous write

            IF (nError = 0) THEN

                Sleep(2);

                TagWrite(sTAGNAME,0,0,TRUE,sCLUSTER);      // Synchronous write

            ELSE

                // Problem in Tag write

                ErrSetHw(2, nError, 0);

                ErrLog("Error using system cicode Pulser(*) function: Tag '" + sTAGNAME + "', Cicode error " + IntToStr(nError));

            END

        ELSE

       

            // This code path means the AssGetProperty() has failed to establish the tag

            IF (nError = 0) THEN

               // Tests showed that an invalid genie tag was returning no error so correct it to something

               nError = 424;        // Tag not found

            END

            ErrSetHw(2, nError, 0);

            ErrLog("Error using system cicode Pulser(*) function: Genie Item '" + sOriginalTag + "', Cicode error " + IntToStr(nError));

        END

       

    ELSE

   

        // Passed tag name is blank

        ErrSetHw(2, 289, 0);    // Set Name does not exist error

        ErrLog("Error using system cicode Pulser(*) function: Tagname blank, Cicode error " + IntToStr(289));

    END

END