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Wednesday, 17 October 2012

CLASSIFICATION OF INSTRUMENTS


 CLASSIFICATION OF INSTRUMENTS
THE CLASSIFICATIONS OF INSTRUMENTS ARE GIVEN BELOW  

AUTOMATIC AND MANUAL INSTRUMENTS: -
THE MANUAL INSTRUMENTS REQUIRE THE SERVICES OF AN OPERATOR WHILE THE AUTOMATIC TYPES DO NOT. FOR EXAMPLE, THE TEMPERATURE MEASUREMENTS BY MERCURY-IN-GLASS THERMOMETER IS AUTOMATIC AS THE INSTRUMENT INDICATES THE TEMPERATURE WITHOUT REQUIRING ANY MANUAL ASSISTANCE. HOWEVER, THE MEASUREMENT OF TEMPERATURE BY A RESISTANCE THERMOMETER INCORPORATING A WHEATSTONE BRIDGE IN ITS CIRCUIT IS MANUAL IN OPERATION AS IT NEEDS AN OPERATOR FOR OBTAINING THE NULL POSITION.
AUTOMATIC INSTRUMENTS ARE PROFFERED BECAUSE OF THEIR FAST DYNAMIC RESPONSE AND LOW OPERATIONAL COST.

SELF-GENERATING AND POWER-OPERATED INSTRUMENTS:-
IN A SELF-GENERATING INSTRUMENT, THE OUTPUT ENTIRELY OR ALMOST ENTIRELY BY THE INPUT SIGNAL. THE INSTRUMENT
DOES NOT REQUIRE ANY OUTSIDE POWER IN PERFORMING ITS FUNCTION. FOR EXAMPLE, THE MOTIVE POWER IN A MERCURY-IN-GLASS THERMOMETER IS SUPPLIED WHOLLY BY THE THERMAL EXPANSION OF MERCURY AND AS SUCH IT CONSTITUTES A SELF-OPERATED DEVICE. LIKEWISE, THE DIAL INDICATOR IS AN ACTIVE DEVICE SINCE ALL THE ENERGY REQUIRED TO OPERATE THE INSTRUMENT IS FURNISHED BY THE SYSTEM WHOSE DISPLACEMENT IS BEING MEASURED.

SOME OTHER COMMON EXAMPLES OF ACTIVE INSTRUMENTS ARE:-
  • ·         EXPOSURE METER OF A CAMERA WHICH IS ESSENTIALLY A PHOTO VOLTAIC CELL.
  • ·         BOURDON GAUGE FOR THE MEASUREMENT OF PRESSURE.
  • ·         PITOT-TUBE FOR THE MEASUREMENT OF VELOCITY.
  • ·         TACHO-GENERATOR FOR ROTATIONAL SPEED MEASUREMENT.

SOME INSTRUMENTS REQUIRE SOME AUXILIARY SOURCE OF POWER SUCH AS COMPRESSED AIR, ELECTRICITY, HYDRAULIC SUPPLY ETC. FOR THERE OPERATION AND HENCE ARE CALLED EXTERNALLY POWERS INSTRUMENTS. IN THESE DEVICES, THE INPUT SIGNAL SUPPLIES ONLY AN INSIGNIFICANT PORTION OF THE OUTPUT POWER. FOR EXAMPLE, IN THE ELECTRO-MECHANICAL MEASUREMENT SYSTEM AN EXCITER HAS BEEN INCORPORATED TO SUPPLY ELECTRIC ENERGY FOR THE TRANSDUCER. LIKEWISE, THE DIGITAL REVOLUTION COUNTER IS A PASSIVE DEVICE. THE POWER TO DRIVE THE SOLENOID COMES FROM THE A.C. POWER LINES AND NOT FROM THE ROTATING SHAFT.

SOME OTHER COMMON EXAMPLES OF PASSIVE INSTRUMENTS ARE:-
·         
  • ·         LVDT USED IN THE MEASUREMENT OF DISPLACEMENT, FORCE, PRESSURE.
  • ·  VOLTAGE-DIVIDING POTENTIOMETER WHICH CONVERTS ROTATION OR DISPLACEMENT INTO POTENTIAL DIFFERENCE.
  • ·      PHOTO-CONDUCTIVE TRANSDUCER WHICH TRANSLATE LIGHT INFORMATION TO RESISTANCE INFORMATION.
  • ·         STRAIN-GAUGE LOAD CELL USING WHEAT STONE BRIDGE CIRCUIT.
  • ·         RESISTANCE THERMOMETERS AND THERMISTERS.

SELF-CONTAINED AND REMOTE INDICATING INSTRUMENTS: -
THE DIFFERENT ELEMENTS OF A SELF-CONTAINED INSTRUMENT ARE CONTAINED IN ONE PHYSICAL ASSEMBLY. IN A REMOTE INDICATING INSTRUMENT, THE PRIMARY SENSING ELEMENT MAY BE LOCATED AT A SUFFICIENTLY LONG DISTANCE FROM THE SECONDARY INDICATING ELEMENT. IN THE MODERN INSTRUMENTATION TECHNOLOGY, THERE IS A TREND TO INSTALL REMOTE INDICATING INSTRUMENTS WHERE THE IMPORTANT INDICATIONS CAN BE DISPLAYED IN THE CENTRAL CONTROL ROOMS.

DEFLECTION AND NULL OUTPUT INSTRUMENTS:-
IN NULL-TYPE INSTRUMENTS, THE PHYSICALLY EFFECT CAUSED BY THE QUANTITY BEING MEASURED IN NULLIFIED BY GENERATING AN EQUIVALENT OPPOSING EFFECT. THE EQUIVALENT NULL CAUSING EFFECT THEN PROVIDES A MEASURE OF THE UNKNOWN QUANTITY.      
           
A DEFLECTION TYPE INSTRUMENT IS THAT IN WHICH THE PHYSICAL EFFECT GENERATED BY THE MEASURING QUANTITY IS NOTED AND CORRELATED TO THE MEASURAND.

A DISTINCTION BETWEEN THE NULL AND DEFLECTION MODE OF OPERATION CAN BE MADE BY CONSIDERING THE WORKING OF A PAN BALANCE AND A PLATFORM SCALE. IN THE PAN BALANCE THE UNKNOWN WEIGHT IS PLACED OF ONE PAN OF THE BALANCE AND WEIGHTS OF KNOWN VALUE ARE PLACED IN THE OTHER PAN UNTIL A BALANCE CONDITION IS INDICATED BY ZERO OR NULL POSITION OF THE POINTER.

IN A PLATFORM SCALE, I.E. THE DEFLECTION MODE, THE WEIGHT OF AN OBJECT PLACED ON THE PLATFORM OF THE SCALE IS INDICATED BY THE RELATIVE DISPLACEMENT BETWEEN THE POINTER AND A DIAL.

THE NULL-TYPE DEVICES ARE SLOW IN OPERATION, HAVE POOR DYNAMIC RESPONSE BUT ARE MORE ACCURATE AND SENSITIVE, AND DO NOT INTERFERE WITH THE STATE OF THE QUANTITY BEING MEASURED. DEFLECTION INSTRUMENTS ARE SIMPLE IN CONSTRUCTION AND OPERATION, HAVE GOOD DYNAMIC RESPONSE. HOWEVER, THEY INTERFERE WITH THE STATE OF MEASURAND AND AS SUCH DO NOT DETERMINE ITS EXACT STATE/VALUE/CONDITION.

ANALOG AND DIGITAL INSTRUMENTS: -
THE SIGNALS OF AN ANALOG UNIT VARY IN A CONTINUOUS FASHION AND CAN TAKE ON INFINITE NUMBER VALUES IN A GIVEN RANGE. WRIST WATCH, SPEEDOMETER OF AN AUTOMOBILE, FUEL GAUGE, AMMETERS AND VOLTMETERS ARE EXAMPLES OF ANALOG INSTRUMENTS.
                                 
SIGNALS VARYING IN DISCRETE STEPS AND TAKING ON A FINITE NUMBER OF DIFFERENT VALUES IN A GIVEN RANGE ARE DIGITAL SIGNALS AND THE CORRESPONDING INSTRUMENTS ARE OF DIGITAL TYPE. FOR EXAMPLE, THE TIMERS ON A SCOREBOARD, THE CALIBRATED BALANCE OF A PLATFORM SCALE, AND ODOMETER OF AN AUTOMOBILE ARE DIGITAL INSTRUMENTS.
                                 
THE DIGITAL INSTRUMENTS CONVERT A MEASURED ANALOG VOLTAGE INTO DIGITAL QUANTITY WHICH IS DISPLAYED NUMERICALLY, USUALLY BY NEON INDICATOR TUBES. THE OUTPUT MAY EITHER BE A DIGIT FOR EVERY SUCCESSIVE INCREMENT OF THE INPUT OR BE A CODED DISCRETE SIGNAL REPRESENTATIVE OF THE NUMERICAL VALUE OF THE INPUT. THE DIGITAL DEVICES HAVE THE ADVANTAGE OF HIGH ACCURACY HIGH SPEED AND THE ELIMINATION OF HUMAN OPERATIONAL ERRORS. HOWEVER, THESE INSTRUMENTS ARE UNABLE TO INDICATE THE QUANTITY WHICH IS A PART OF THE STEP VALUE OF THE INSTRUMENT. THE IMPORTANCE OF THE DIGITAL INSTRUMENTATION IS INCREASING VERY FAST DUE TO THE APPLICATIONS OF THE DIGITAL COMPUTERS FOR DATA HANDLING, REDUCTION AND IN AUTOMATIC CONTROLS. APPARENTLY IT BECOMES NECESSARY TO HAVE BOTH ANALOG-TO-DIGITAL CONVERTERS AT INPUT TO THE COMPUTERS AND DIGITAL-TO-ANALOG CONVERTERS AT THE OUTPUT OF THE COMPUTERS.

INSTRUMENTS BASICALLY PERFORM TWO FUNCTIONS.
  • ·         COLLECTION OF DATA.
  • ·         CONTROL OF PLANT AND PROCESS.

ACCORDINGLY BASED UPON THE SERVICE RENDERED, THE INSTRUMENTS MAY ALSO BE CLASSIFIED AS INDICATING INSTRUMENTS, RECORDING INSTRUMENTS AND CONTROLLING INSTRUMENTS.


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