mechanical measurement and control system(basics)

MEASUREMENT AND ITS TYPE

MEASUREMENT:

MEASUREMENT IS USED TO TELL US THE LENGTH, THE WEIGHT, THE TEMPERATURE, OR A CHANGE IN ONE OF THESE PHYSICAL ENTITIES OF A MATERIAL. IT IS THE RESULT OF AN OPINION FORMED BY ONE OR MORE OBSERVERS ABOUT THE RELATIVE SIZE OR INTENSITY OF SOME PHYSICAL QUANTITY. THE OPINION IS FORMED BY THE OBSERVER AFTER COMPARING THE OBJECT WITH A QUANTITY OF SAME KIND CHOSEN AS A UNIT, CALLED STANDARD.

THE RESULT OF MEASUREMENT IS EXPRESSED BY A NUMBER REPRESENTING THE RATIO OF THE UNKNOWN QUANTITY TO THE ADOPTED STANDARD.

FIG NO: -1 FUNDAMENTAL MEASURING PROCESS

FOR EXAMPLE 10 CM LENGTH OF AN OBJECT IMPLIES THAT THE OBJECT IS 10 TIMES AS LARGE AS 1 CM; THE UNIT EMPLOYED IN EXPRESSING LENGTH.

TYPE OF MEASUREMENT

MEASUREMENTS ARE DIVIDED INTO THREE CATEGORIES

DIRECT AND INDIRECT MEASUREMENTS

DIRECT MEASUREMENTS:

THE VALUE OF THE PHYSICAL PARAMETER (MEASURAND) IS DETERMINED BY COMPARING IT DIRECTLY WITH REFERENCE STANDARDS. THE PHYSICAL QUANTITIES LIKE MASS, LENGTH, AND TIME ARE MEASURED BY DIRECT COMPARISON.

INDIRECT MEASUREMENTS:

THE VALUE OF THE PHYSICAL PARAMETER (MEASURAND) IS MORE GENERALLY DETERMINED BY INDIRECT COMPARISON WITH SECONDARY STANDARDS THROUGH CALIBRATION.

PRIMARY, SECONDARY AND TERTIARY MEASUREMENTS

PRIMARY:

IN PRIMARY MODE, THE SOUGHT VALUE OF A PHYSICAL PARAMETER IS DETERMINED BY COMPARING IT DIRECTLY WITH REFERENCE STANDARDS. THE REQUISITE INFORMATION IS OBTAINABLE THROUGH SENSES OF SIGHT AND TOUCH. E.G. ARE MATCHING OF TWO LENGTHS WHEN DETERMINING THE LENGTH OF AN OBJECT WITH A RULER.

SECONDARY: - 

THE INDIRECT MEASUREMENTS INVOLVING ONE TRANSLATION ARE CALLED SECONDARY MEASUREMENTS. THE CONVERSION OF PRESSURE INTO DISPLACEMENT BY BELLOWS (FIG:-2) IS A SIMPLE EXAMPLE OF THE SECONDARY MEASUREMENT.

FIG NO: -2 BELLOWS CONVERT PRESSURE INTO DISPLACEMENT

TERTIARY:

THE INDIRECT MEASUREMENTS INVOLVING TWO CONVERSIONS ARE CALLED TERTIARY MEASUREMENTS. THE MEASUREMENT OF THE SPEED OF A ROTATING SHAFT BY MEANS OF AN ELECTRIC TACHOMETER IS THE EXAMPLE OF THE TERTIARY MEASUREMENTS.

FIG NO: -3 MEASUREMENT OF ANGULAR SPEED BY AN ELECTRIC TACHOMETER

•  CONTACT AND NON-CONTACT TYPE MEASUREMENTS

CONTACT MEASUREMENT: - WHERE THE SENSING ELEMENT OF THE MEASURING DEVICE HAS A CONTACT WITH THE MEDIUM WHOSE CHARACTERISTICS ARE MEASURED.

NON-CONTACT: - WHERE THE SENSOR DOES NOT COMMUNICATE PHYSICALLY WITH THE MEDIUM.

Elements of a measurement system

    A measuring system exists to provide information about the physical value of some variable being measured. In simple cases, the system can consist of only a single unit that gives an output reading or signal according to the magnitude of the unknown variable applied to it. However, in more complex measurement situations, a measuring system consists of several separate elements as shown in Figure :-

1.The first element in any measuring system is the primary sensor: this gives an output that is a function of the measured (the input applied to it). 

2.Variable conversion elements are needed where the output variable of a primary transducer is in an inconvenient form and has to be converted to a more convenient form. In some cases, the primary sensor and variable conversion element are combined, and the combination is known as a transducer. 

3.Signal processing elements exist to improve the quality of the output of a measurement system in some way. 

4.In addition to these three components just mentioned, some measurement systems have one or two other components, firstly to transmit the signal to some remote point and secondly to display or record the signal if it is not fed automatically into a feedback control system. 

5.The final optional element in a measurement system is the point where the measured signal is utilized.

METHODS OF MEASUREMENT

Direct comparison:

•Compare unknown quantity (measurand) against a known quantity (standard).  Eg: tape measure.

•Direct comparison not always possible or practical.  Eg: measuring sound levels.

Indirect comparison (calibrated system):

•Makes use of a sensor or transducing device (transducer) to transform the measurand into an analogous form.

•The sensor is connected to a series of instruments which convert the output of the sensor into a useful analogous form which presents the measurand in a useful and practical format.

Classification of instruments

•Active and passive instruments

•Null-type and deflection-type instruments

•Analogue and digital instruments

•Indicating instruments and instruments with a signal output

Smart and non-smart instruments

SELECTING AN MEASURING INSTRUMENT

THE SELECTION OF AN INSTRUMENT FOR A SPECIFIC MEASUREMENT APPLICATION REQUIRES THE CONSIDERATION OF SEVERAL FACTORS THE FOLLOWING BEING CERTAINLY FUNDAMENTAL TO ENTIRE PHILOSOPHY OF MEASUREMENT AND INSTRUMENT SELECTION.

• MAXIMUM AND MINIMUM VALUES THE INPUT VARIABLES ARE EXPECTED TO ASSUME 

THE INSTRUMENT RANGE MUST EQUAL TO THE EXPECTED RANGE OF VARIABLE TO BE MEASURED .EXCESSIVE INSTRUMENT RANGE MEANS A LOT OF READABILITY.FOR A WIDE RANGE IN THE VARIABLE,MULTIPLE RANGE OF INSTRUMENT ARE DESIRED.

•  NATURE OF THE INPUT SIGNAL - 

IS IT CONSTANT OR TIME DEPENDENT? IF TIME VARIANT, THEN WHETHER VARIATION IS PERIODIC, TRANSIENT OR RANDOM. FOR TRANSIENT MEASUREMENT, IT IS TO BE ENSURED THAT NATURAL FREQUENCIES AND TIME CONSTANTS ARE PROPERLY SELECTED.

•  NON INTERFERENCE WITH MEASURING SYSTEM -

THE BASIC PRINCIPAL OF MEASURING IS THAT THE ACT SHOULDN'T AFFECT THE QUANTITY BEING MEASURED. OUR JOB IS TO SEE THAT THE INSTRUMENT OFFERS MINIMUM INTERFERENCE.

•  COST CRITERION –

THE DEMAND FOR A HIGH ACCURACY WOULD REQUIRE THE USE OF HIGHLY SOPHISTICATED AND SPECIALIZED INSTRUMENTS HAVING SPECIAL ANCILLARIES TO COMPENSATE FOR AMBIENT CONDITION. THE MORE ACCURATE INSTRUMENTS ARE MORE DIFFICULT TO OPERATE.

NEVER DEMAND AN ACCURACY OF MEASUREMENTS HIGHER THAN THAT WHICH IS REALLY NEEDED, AND NEVER FORGET THAT EACH DEGREE OF ACCURACY, IF DEMANDED, IS LIKELY TO HAVE A DIS PROPORTIONATE EFFECT ON THE COMPLEXITY AND COST F THE MEASURING APPARATUS

•  EASY CALIBRATION -

THE INSTRUMENT SHOULD BE EASY TO CALIBRATE AND CALIBRATION CURVE MUST BE STABLE FOR TIME REQUIRE TO COMPLETE A TEST.

•  CONVENIENCE AND EASE IN READING THE INSTRUMENT – 

SCALES THAT ARE PROPERLY RULED AND NUMBERED FOR EASY LEGIBILITY, PROPER ILLUMINATION, AND READING STRAIGHT DIALS ARE FEATURE THAT WILL HAVE A DECISIVE INFLUENCE ON THE SELECTION OF AN INSTRUMENT.      OFTEN ENGINEER RELIES WITH EXPERIENCE, WHEN MAKING A COMPARISON OF MANY FACTORS IN THE COURSE OF PROPER SELECTION OF INSTRUMENTS.

ERROR

DESPITE UTMOST CARE AND PRECAUTIONS AN EXPERIMENTER MAY TAKE TO ELIMINATE ALL POSSIBLE ERRORS, THE HAPPY GOAL IS SELDOM ATTAINED AND CERTAIN ERRORS ARE BOUND TO CREEP IN FOR EXAMPLE, EVEN IN AN APPARENTLY  SIMPLE MEASUREMENT OF FLOW VELOCITY WITH A PITOT TUBE ANY MISALIGNMENT OF THE PROBE, LEAKS IN THE PRESSURE TUBING, CHANGES IN THE BORE AND SURFACE CONDITIONS OF THE MANOMETER, ANY FLUCTUATIONS IN THE ATMOSPHERIC AND STREAM PRESSURE ARE LIKELY TO AFFECT THE PROBE READINGS AND GIVE RISE TO UNCERTAINTIES .ERRORS AND UNCERTAINTIES ARE INHERENT IN THE PROCESS OF MAKING ANY MEASUREMENT AND IN THE INSTRUMENT WITH WHICH THE MEASUREMENT ARE MADE. A STUDY OF ERROR IS IMPORTANT AS A STEP IN FINDING WAYS OF REDUCING THEM, AND ALSO AS MEANS OF ESTIMATING THE RELIABILITY OF FINAL RESULTS. THIS CHAPTER PROVIDES A BRIEF DISCUSSION ABOUT THE NATURE OF ERRORS AND STATISTICAL ACCURACY OF TEST RESULTS.

TYPES OF ERRORS

ERRORS  MAY  ORIGINATE IN  A  VARIETY  OF  WAYS  AND   THE FOLLOWING  SOURCES   NEED  EXAMINATION

• INSTRUMENT  ERROR  :-   

THERE   ARE  MANY  FACTORS  IN THE  DESIGN  AND CONSTRUCTION  OF   INSTRUMENTS  THAT  LIMIT  THE  ACCURACY  ATTAINABLE   . INSTRUMENTS    AND   STANDARDS  POSSESS INHERENT  INACCURACIES   AND  CERTAIN  ADDITIONAL INACCURACIES  DEVELOP  WITH  USE  AND  THE  TIME . EXAMPLES ARE  :

-         IMPROPER   SELECTION   AND   POOR   MAINTENANCE   OF INSTRUMENTS.

-         MECHANICAL   FRICTION  AND  WEAR  ,  BACKLASH  , YIELDING   OF  SUPPORTS  , PEN OR  POINTER  DRAG ,  AND  HYSTERESIS  OF  ELASTIC   MEMBERS  DUE  TO  AGING .

-         UNAVOIDABLE   PHYSICAL   PHENOMENON DUE TO FRICTION,   CAPILLARY   ATTRACTION  AND  IMPERFECT RAREFACTION.

• ENVIRONMENTAL  ERRORS  :- 

THE  INSTRUMENT  LOCATION  AND  THE  ENVIRONMENT   ERRORS ARE  INTRODUCED  BY  USING  AN  INSTRUMENT  IN  CONDITIONS DIFFERENT   FOR   WHICH   IT  HAS  BEEN  DESIGNED ,ASSEMBLED AND  CALIBRATED  . THE  DIFFERENT   CONDITIONS  OF  USE  MAY BE  TEMPERATURE  ,PRESSURE  , HUMIDITY  AND  ATTITUDE  ETC.

• TRANSLATION AND  SIGNAL  TRANSMISSION  ERRORS:

THE  INSTRUMENT  MAY  NOT  SENSE  OR TRANSLATE  THE MEASURED  EFFECT  WITH  COMPLETE   FIDELITY  .  THE  ERRORS ALSO  INCLUDES  THE  NON  CAPABILITY   OF  THE  INSTRUMENT TO  FOLLOW  RAPID  CHANGES  IN   THE  MEASURED  QUANTITY DUE TO  INERTIA  AND  HYSTERESIS  EFFECTS .  THE   TRANSMISSION ERRORS  CREEP  IN WHEN  THE  TRANSMITTED SIGNAL  IS RENDERED FAULTY  DUE TO  ITS  DISTORTION  BY  RESONANCE.

• OPERATIONAL ERRORS:- 

A   PER-REQUISITE  TO  PRECISE  AND   METICULOUS  MEASUREMENTS  IS  THAT  THE  INSTRUMENTS  SHOULD  BE PROPERLY  USED . QUITE OFTEN, ERRORS ARE CAUSED BY POOR OPERATIONAL   TECHNIQUES.  

EXAMPLES   ARE:-

- A  THERMOMETER  WILL  NOT READ   ACCURATELY  IF THE SENSITIVE  PORTION   IS  INSUFFICIENTLY  IMMERSED  OR  IS RADIATING   HEAT  TO A   COLDER   PORTION  OF   THE  INSTALLATION.

-  A  PRESSURE  GAUGE  WILL  CORRECTLY   INDICATE  PRESSURE  ONLY   WHEN  IT  IS  EXPOSED  ONLY  TO  THE  PRESSURE  WHICH  IS TO BE  MEASURED .

LINEARITY

THE WORKING RANGE OF MOST OF THE INSTRUMENT PROVIDES A LINEAR RELATIONSHIP BETWEEN THE OUTPUT (READING TAKEN FROM THE SCALE OF THE INSTRUMENT) AND INPUT (MEASURED, SIGNAL PRESENTED TO THE MEASURING SYSTEM). THE ASPECT TENDS TO FACILITATE A MORE ACCURATE DATA REDUCTION. LINEARITY IS DEFINED AS THE ABILITY TO REPRODUCE THE INPUT CHARACTERISTICS SYMMETRICALLY, AND THIS CAN BE EXPRESSED BY THE STRAIGHT LINE EQUATION 

                                                                               Y=MX+C

WHERE Y IS THE OUTPUT, X IS THE INPUT, M IS THE SLOPE AND C IS THE INTERCEPT.APPARENTLY, THE CLOSENESS OF THE CALIBRATION CURVE TO A SPECIFIED STRAIGHT LINE IS THE LINEARITY OR THE INSTRUMENT.

                        

  

ANY DEPARTURE FROM THE STRAIGHT-LINE RELATIONSHIP IS NON-LINEARITY.

THE NON-LINEARITY MAY BE DUE TO:

* NON-LINEARITY ELEMENTS IN THE MECHANICAL DEVICE

* MECHANICAL HYSTERESIS

* VISCOUS FLOW OR CREEP,AND

* ELASTIC AFTER EFFECTS IN THE MECHANICAL SYSTEM

IN A NOMINALLY LINEAR MEASUREMENT DEVICE, THE NON-LINEARITY MAY TAKE DIFFERENT FORMS AS ILLUSTRATED IN FIG.

1. THEORETICAL SLOPE LINEARITY:

MAXIMUM DEPARTURE "A" FROM THE THEORETICAL STRAIGHT LINE OA PASSING THROUGH THE ORIGIN. THE LINE OA REFERS TO THE STRAIGHT LINE BETWEEN THE THEORETICAL END POINTS, AND IT IS DRAWN WITHOUT REGARD TO ANY EXPERIMENTALLY DETERMINED VALUES.

2. END POINT LINEARITY:

MAXIMUM DEPARTURE "B" FROM THE STRAIGHT LINE OB PASSING THROUGH THE TERMINAL READING (EXPERIMENTAL END POINT - ZERO AND FULL SCALE POSITION).

3. LEAST SQUARE LINEARITY:

MAXIMUM DEPARTURE "C" FROM THE BEST FIT STRAIGHT LINE OC DETERMINED BY THE LEAST SQUARE TECHNIQUE.

IN MOST INSTRUMENTS, THE LINEARITY IS TAKEN TO BE THE MAXIMUM DEVIATION FROM A LINEAR RELATIONSHIP BETWEEN INPUT AND OUTPUT, I.E FROM A CONSTANT SENSITIVITY AND IS OFTEN EXPRESSED AS A PERCENTAGE OF FULL SCALE.

THE CALCULATION OF MEASUREMENT ERROR REQUIRES NUMERICAL VALUES OF ACCURACY, RESOLUTION, AND LINEARITY ETC. FOR THE INSTRUMENT BEING USED. FOR THE MAJORITY OF LABORATORY INSTRUMENTS, THIS DATA IS GIVEN IN A MANUFACTURER HAND BOOK. HOWEVER FOR SOME INSTRUMENTS SUCH AS MICROMETERS, VERNIER CALLIPERS, THERMOMETERS AND TESTING EQUIPMENT, THE DATA IS GIVEN IN THE STANDARDS MAINTAINED BY THE COUNTRY. 

SENSITIVITY: SENSITIVITY OF AN INSTRUMENT OR AN INSTRUMENTATION SYSTEM IS THE RATIO OF THE MAGNITUDE OF THE RESPONSE (OUTPUT SIGNAL) TO THE MAGNITUDE OF THE QUANTITY BEING MEASURED (INPUT SIGNAL), I.E.,                

                       

STATIC SENSITIVITY, K= CHANGE OF OUTPUT SIGNAL/CHANGE OF INPUT SIGNAL.

SENSITIVITY IS REPRESENTED BY THE SLOPE OF THE CALIBRATION CURVE IF THE ORDINATE ARE EXPRESSED IN THE ACTUAL UNITS. WITH A LINEAR CALIBRATION CURVE, THE SENSITIVITY IS CONSTANT. HOWEVER, IF THE CALIBRATION CURVE IS NON- LINEAR THE STATIC SENSITIVITY IS NOT CONSTANT AND MUST BE SPECIFIED IN TERMS OF THE INPUT VALUE AS SHOWN IN FIG.

                           

IN CERTAIN APPLICATION, USE IS MADE OF RECIPROCAL OF SENSITIVITY AND THAT IS TERMED AS INVERSE SENSITIVITY OR DEFLECTION FACTOR.

SENSITIVITY HAS A WIDE RANGE OF UNITS, AND THESE DEPEND UPON THE INSTRUMENT OR MEASUREMENT SYSTEM BEING INVESTIGATED.

LET A DIFFERENT ELEMENTS COMPRISING A MEASUREMENT SYSTEM HAVE STATIC SENSITIVITIES OF K1, K2, K3 ... ETC. WHEN THESE ELEMENTS ARE CONNECTED IN SERIES, THEN THE OVERALL SENSITIVITY IS WORKED OUT FROM THE FOLLOWING RELATIONS:-

THE ABOVE RELATION IS BASED UPON THE ASSUMPTION THAT NO VARIATION OCCURS IN THE VALUE OF INDIVIDUAL SENSITIVITY K1, K2, K3 ...ETC. DUE TO LOADING EFFECTS.

WHEN THE INPUT TO AND OUTPUT FROM THE MEASUREMENT SYSTEM USED WITH ELECTRICAL/ELECTRONIC EQUIPMENT HAVE THE SAME FORM, THE TERM GAIN IS USED RATHER THAN SENSITIVITY. LIKEWISE AN INCREASE IN DISPLACEMENT WITH THE OPTICAL AND MECHANICAL INSTRUMENT IS DESCRIBED BY THE TERM AMPLIFICATION. APPARENTLY THE TERM SENSITIVITY, GAIN AND MAGNIFICATION ALL MEAN THE SAME AND THEY DESCRIBED THE RELATIONSHIP BETWEEN THE OUTPUT AND INPUT. FURTHER WHEN THE INPUT OR OUTPUT SIGNAL IS CLANGING WITH TIME , THE TERM TRANSFER FUNCTION OR TRANSFER OPERATOR IS USED RATHER THAN THE SENSITIVITY, GAIN OR AMPLIFICATION.

                                 

CONTROL SYSTEMS

INTRODUCTION

THE KEY CHARACTERISTIC OF CONTROL IS TO INTERFERE, TO INFLUENCE OR TO MODIFY THE PROCESS. THIS CONTROL FUNCTION OR THE INTERFERENCE TO THE PROCESS IS INTRODUCED BY AN ORGANIZATION OF PARTS (INCLUDING OPERATORS IN MANUAL CONTROL) THAT, WHEN CONNECTED   TOGETHER IS CALLED THE CONTROL SYSTEM.  DEPENDING ON   WHETHER A HUMAN   BODY (THE OPERATOR) IS PHYSICALLY INVOLVED IN THE CONTROL SYSTEM, THEY ARE DIVIDED INTO   MANUAL CONTROL AND AUTOMATIC CONTROL. DUE TO ITS EFFICIENCY, ACCURACY AND RELIABILITY,   AUTOMATIC CONTROL IS WIDELY USED IN CHEMICAL PROCESSED.

THE AIM OF THIS SECTION IS TO INTRODUCE THE CONCEPT OF CONTROL SYSTEMS, WHAT THEIR FUNCTION IS AND WHAT HARDWARE AND SOFTWARE IS REQUIRED BY THEM.

MANUAL CONTROL SYSTEM:

FIRST START WITH A SIMPLE MANUAL CONTROL SYSTEM, TO EXAMINE HOW CONTROL IS INTRODUCED, HOW THE CONTROL SYSTEM IS CONSTRUCTED AND HOW IT WORKS.

A DIAGRAM OF THE SYSTEM IS SHOWN BELOW.

TO BEGIN WITH THE SHOWER IS COLD. TO START THE HEATING   PROCESS THE VALVE IN THE HOT WATER LINE IS OPENED.  THE OPERATOR CAN THEN   DETERMINE THE EFFECTIVENESS OF THE CONTROL PROCESS BY STANDING IN THE SHOWER. IF THE WATER   IS TOO HOT, THE   VALVE SHOULD   BE CLOSED A LITTLE OR EVEN TURNED OFF. IF THE WATER IS NOT HOT ENOUGH THEN THE VALVE IS LEFT OPEN OR OPENED WIDER.

FUNCTIONS OF A CONTROL SYSTEM:

IT CAN BE SEEN THAT THIS CONTROL SYSTEM, COMPLETED BY THE OPERATOR, POSSESSES THE FOLLOWING FUNCTIONS:

·         MEASUREMENT

THIS IS ESSENTIALLY AN ESTIMATE OR APPRAISAL OF THE PROCESS   BEING CONTROLLED BY THE SYSTEM. IN THIS EXAMPLE, THIS IS ACHIEVED BY THE RIGHT HAND OF THE OPERATOR.

·         COMPARISON

THIS IS AN EXAMINATION   OF THE   LIKENESS   OF THE   MEASURED VALUES AND THE DESIRED VALUES. THIS IS CARRIED OUT IN THE BRAIN OF THE OPERATOR.

·         COMPUTATION

THIS IS A CALCULATED JUDGMENT THAT INDICATES HOW MUCH THE MEASURED VALUE AND THE DESIRED VALUES   DIFFER AND WHAT   ACTION AND    HOW MUCH SHOULD BE TAKEN. IN THIS EXAMPLE,   THE  OPERATOR   WILL   CALCULATE   THE   DIFFERENCE  BETWEEN   THE   DESIRED TEMPERATURE AND THE ACTUAL ONE. ACCORDINGLY  THE  DIRECTION AND AMOUNT  OF  THE ADJUSTMENT OF THE VALVE ARE WORKED OUT AND THE ORDER FOR THIS ADJUSTMENT IS SENT TO THE LEFT HAND FROM THE BRAIN  OF  THE  OPERATOR.  IF THE OUTLET WATER TEMPERATURE IS LOWER, THEN THE BRAIN OF THE OPERATOR WILL TELL THE LEFT HAND TO OPEN THE STEAM VALVE WIDER. IF THERE IS ANY DISTURBANCE, OR VARIATION OF FLOW RATE IN WATER TO THE SHOWER INLET, SOME ADJUSTMENT MUST BE MADE TO KEEP THE OUTLET WATER TEMPERATURE AT A DESIRED VALUE.

·         CORRECTION

THIS IS ULTIMATELY THE MATERILISATION OF THE ORDER FOR THE ADJUSTMENT.   THE LEFT HAND OF THE OPERATOR TAKES THE NECESSARY ACTIONS FOLLOWING THE ORDER FROM BRAIN.

THEREFORE, FOR A   CONTROL SYSTEM   TO OPERATE SATISFACTORILY, IT MUST HAVE THE ABILITIES OF MEASUREMENT, COMPARISON, COMPUTATION AND CORRECTION.

OF COURSE, THE MANUAL OPERATION HAS OBVIOUS DISADVANTAGES E.G.  THE ACCURACY   AND   THE CONTINUOUS INVOLVEMENT OF OPERATORS.  ALTHOUGH ACCURACY OF THE MEASUREMENT COULD BE IMPROVED BY USING AN INDICATOR, AUTOMATIC CONTROL MUST BE USED TO REPLACE   THE   OPERATOR. IN INDUSTRY, IT IS AUTOMATIC CONTROL THAT IS WIDELY USED.

AUTOMATIC CONTROL SYSTEM:

BASED ON THE ABOVE PROCESS, WE CAN EASILY SET UP AN  AUTOMATIC CONTROL SYSTEM AS SHOWN IN THE NEXT FIGURE.

• FIRSTLY,    WE CAN   USE A  TEMPERATURE MEASUREMENT  DEVICE TO  MEASURE  THE  WATER TEMPERATURE,  WHICH   REPLACES  THE  RIGHT  HAND OF THE OPERATOR. THIS ADDITION TO THE SYSTEM WOULD HAVE IMPROVED ACCURACY.
• INSTEAD OF MANUAL VALVES, WE USE A SPECIAL KIND OF VALVE, CALLED A CONTROL VALVE, WHICH IS DRIVEN BY COMPRESSED AIR OR ELECTRICITY.  THIS WILL REPLACE THE LEFT HAND OF THE OPERATOR.
• WE  PUT  A  DEVICE  CALLED  A  CONTROLLER,  IN THIS CASE A  TEMPERATURE  CONTROLLER,  TO REPLACE THE  BRAIN  OF  THE  OPERATOR.  THIS HAS THE FUNCTIONS OF COMPARISON AND COMPUTATION AND CAN GIVE ORDERS TO THE CONTROL VALVE.
• THE SIGNAL AND ORDER CONNECTIONS BETWEEN THE MEASUREMENT DEVICE, CONTROL VALVE AND CONTROLLER ARE TRANSFERED THROUGH CABLES AND WIRES, WHICH REPLACE THE NERVE SYSTEM IN THE OPERATOR.

HARDWARE OF A CONTROL SYSTEM:

EXAMINING  THE  AUTOMATIC  CONTROL  SYSTEM,   IT  IS FOUND  THAT  IT   CONTAINS  THE  FOLLOWING HARDWARE.

• SENSOR - A PIECE OF EQUIPMENT TO MEASURE SYSTEM VARIABLES. IT SERVES AS THE SIGNAL SOURCE IN AUTOMATIC CONTROL. THESE WILL BE DISCUSSED AT LENGTH IN A LATER MODULE.
• CONTROLLER - A PIECE   OF EQUIPMENT TO PERFORM THE FUNCTIONS OF COMPARISON AND COMPUTATION. THE ACTIONS THAT A CONTROLLER CAN TAKE WILL BE DISCUSSED AT LENGTH IN A LATER MODULE.
• CONTROL ELEMENT - A PIECE OF EQUIPMENT TO PERFORM THE CONTROL ACTION OR TO EXERT DIRECT INFLUENCE ON THE PROCESS. THIS ELEMENT RECEIVES SIGNALS FROM THE CONTROLLER AND PERFORMS SOME TYPE OF OPERATION ON THE PROCESS. GENERALLY THE CONTROL ELEMENT IS SIMPLY A CONTROL VALVE.

SOFTWARE OF A CONTROL SYSTEM:

ASSOCIATED  WITH  A  CONTROL  SYSTEM  ARE  A  NUMBER OF  DIFFERENT  TYPES  OF  VARIABLES.

FIRST WE HAVE THE   CONTROLLED VARIABLE.   THIS IS THE BASIC PROCESS VALUE BEING REGULATED BY THE SYSTEM. IT IS THE ONE VARIABLE THAT WE ARE ESPECIALLY INTERESTED IN - THE OUTLET WATER TEMPERATURE IN THE EXAMPLE ABOVE.  IN FEEDBACK CONTROL THE CONTROLLED VARIABLE IS USUALLY THE MEASURED VARIABLE.

AN IMPORTANT CONCEPT RELATED TO THE   CONTROLLED VARIABLE IS THE SETPOINT.  THIS IS THE PREDETERMINED DESIRED   VALUE FOR THE CONTROLLED VARIABLE.  THE OBJECTIVE OF THE CONTROL SYSTEM IS TO REGULATE THE CONTROLLED VARIABLE AT ITS SETPOINT.

TO ACHIEVE THE CONTROL OBJECTIVE THERE MUST BE  ONE OR MORE VARIABLES WE CAN ALTER OR ADJUST. THESE ARE CALLED THE MANIPULATED VARIABLES.  IN THE ABOVE EXAMPLE THIS WAS THE INPUT HOTWATER FLOW RATE.

CONCLUSIVELY, IN THE CONTROL SYSTEM WE ADJUST THE MANIPULATED VARIABLE TO MAINTAIN THE CONTROLLED VARIABLE AT ITS SETPOINT. THIS MEETS THE REQUIREMENT OF KEEPING THE STABILITY OF THE PROCESS AND SUPPRESSING THE INFLUENCE OF DISTURBANCES.

A SYSTEM IS AN ASSEMBLAGE OF DEVICES AND COMPONENTS CONNECTED OR RELATED BY SOME FORM OF REGULAR INTERACTION OR INTERDEPENDENCE TO FORM AN ORGANIZED WHOLE AND PERFORM SPECIFIED TASKS. THE SYSTEM PRODUCES AN OUTPUT CORRESPONDING TO A GIVEN INPUT. THE THERMOMETER AND THE MASS SPRING DAMPER SYSTEM CAN BE IDENTIFIED AS SYSTEMS.

            

 A THERMOMETER HAS THE INPUT X=Q(TEMP) ANT THE OUTPUT Y= L(LENGTH OF THE MERCURY COLUMN IN THE CAPILLARY).IN THE MASS SPRING ARRANGEMENT ,THE FORCE AND THE POSITION OF THE MASS CONSTITUTE THE INPUT TO AND  OUTPUT FROM THE SYSTEM , RESPECTIVELY IN A ROTATIONAL GENERATOR OF AN ELECTRICITY, THE INPUT WOULD BE ROTATIONAL SPEED OF THE PRIME - MOVER SHAFT AND THE OUTPUT WOULD EITHER BE THE INDUCED VOLTAGE AT THE TERMINALS( WITH NO LOAD ATTACHED TO THE GENERATOR) OR THE UNIT OF ELECTRICAL POWER (WITH LOAD ATTACHED TO THE GENERATOR).

           

THE TURN CONTROL IMPLIES TO REGULATE, OR COMMAND. A CONTROL SYSTEM MAY THUS BE DEFINED AS "AN ASSEMBLAGE OF DEVICES AND COMPONENTS CONNECTED OR RELATED SO AS TO COMMAND, DIRECT OR REGULATE ITSELF OR ANOTHER SYSTEM".

            

IN A CONTROL SYSTEM, DELIBERATE GUIDANCE OR MANIPULATION IS EMPLOYED TO MAINTAIN A SYSTEM VARIABLE AT A SET POINT TO CHANGE IT ACCORDING TO PRESENT PROGRAMME.

EXAMPLE OF CONTROL SYSTEM

(1) AN ELECTRICAL SWITCH WHICH SERVES TO CONTROL THE FLOW OF ELECTRICITY IN A CIRCUIT. THE INPUT SIGNAL (COMMAND) IS THE FLIPPING OF SWITCH ON OR OFF, AND THE CORRESPONDING OUTPUT (CONTROLLED) SIGNAL IS THE FLOW OR NON FLOW OF ELECTRIC CURRENT.

(2) A THERMAL SYSTEM WHICH IT IS DESIRED TO MAINTAIN THE TEMPERATURE OF THE HOT WATER AT A PRESCRIBED VALUE .BEFORE THE OPERATOR CAN CARRY OUT HIS TASK SATISFACTORILY, THE FOLLOWING REQUIREMENT MUST BE MET:

         

(A) THE OPERATOR MUST BE TOLD WHAT TEMPERATURE IS REQUIRED FOR THE WATER. THIS TEMPERATURE, CALLED THE SET POINT OR DESIRED VALUE, CONSTITUTES THE INPUT TO THE SYSTEM.

       

(B) THE OPERATOR MUST BE PROVIDED WITH SOME MEANS OF OBSERVING THE TEMPERATURE (SENSING ELEMENT). THIS TEMPERATURE IS OUTPUT FOR THE SYSTEM AND IS CALLED THE CONTROLLED VARIABLE. THE OPERATOR WATCHES THE THERMOMETER AND COMPARES HOW THE MEASURED TEMPERATURE COMPARES WITH THE DESIRED VALUE. THIS DIFFERENCE BETWEEN THE DESIRED VALUE AND HE ACTUAL MEASUREMENT VALUE IS ERROR OR ACTUATING SIGNAL.

                                         E = R – C

WHERE R REFERS TO THE SET-POINT OR REFERENCE INPUT AND C DONATED THE CONTROLLED VARIABLE.

      
(C) THE OPERATOR MUST BE PROVIDED WITH SOME MEANS OF INFLUENCING THE TEMP AND MUST BE INSTRUCTED WHAT TO DO TO MOVE THE TEMP IN DESIRED DIRECTION.

(3) A DRIVING SYSTEM OF AN AUTOMOBILE (ACCELERATOR, CARBURETOR AND AN ENGINE VEHICLE) WHERE COMMAND SIGNAL IS TO FORCE ON THE ACCELERATION PEDAL AND THE AUTOMOBILE SPEED IS THE CONTROLLED VARIABLE. THE DESIRED CHANGE IN ENGINE SPEED CAN BE OBTAINED BY CONTROLLING PRESSURE ON THE ACCELERATOR PEDAL.

(4) AN AUTOMOBILE STEERING SYSTEM WHERE THE DRIVER IS REQUIRED TO KEEP THE AUTOMOBILE IN THE APPROPRIATE LANE OF THE ROADWAYS. THE EYES MEASURE THE OUTPUT (HEADING OF THE AUTOMOBILE), THE BRAIN AND HANDS REACT TO ANY ERROR EXISTING B/W THE INPUT (APPROPRIATE LANE) AND THE OUTPUT SIGNALS, AND ACT TO REDUCE THE ERROR TO ZERO.

(5) A BIOLOGICAL CONTROL SYSTEM WHERE A PERSON MOVES HIS FINGER TO POINT TOWARDS AN OBJECT. THE COMMAND SIGNAL IS THE POSITION OF THE OBJECT AND THE OUTPUT IS THE ACTUAL POINTED DIRECTION.

     

OTHER WELL KNOWN EXAMPLES OF THE CONTROL SYSTEMS ARE: ELECTRIC FRYING PANS, WATER PRESSURE REGULATORS, AND TOILET – TANK WATER LEVEL, ELECTRIC IRONS, REFRIGERATORS AND HOUSEHOLD FURNACES WITH THERMOSTATIC CONTROL.

Classification of Control Systems

Control systems are classified into the following two basic types:

1. Open-loop control systems (Unmonitored or non-feedback control systems)

2. Closed-loop control systems (Monitored or feedback control systems)

    Open-loop Control systems (Non-feedback Systems)

      An open-loop control system is one in which the control action is independent of the desired output. The actuating signal depends only on the input command and output has no control over it.

    The elements of an open-loop control system can usually be divided into the following two parts

(i) Controller;

(ii) Controlled process.

     Closed-loop Control System (Feedback Control System)

       A closed-loop system is one in which control action is somehow dependent on the output. In  this case the controlled output is fed back through a feedback element and compared with the reference input. Thus the actuating signal is the difference of desired output and reference input.

HYDRAULIC CONTROL VALVES

THESE ELEMENTS OF A HYDRAULIC CONTROL SYSTEM FUNCTION TO REGULATE THE FLOW OF HYDRAULIC FLUID FROM THE HIGH PRESSURE SIDE TO THE ACTUATOR, I.E. THE HYDRAULIC MOTOR. THERE ARE THREE MAIN TYPES OF VALVES.

·        THE PISTON OR SPOOL TYPE

·        THE FLAPPER AND NOZZLE TYPE

·        THE JET PIPE VALVE

IN ALL CASES, THE MECHANICAL INPUT MOTION CAN BE CONTROLLED BY MANUAL OPERATION, OR BY A LIMITED MOTION CAN BE CONTROLLED BY MANUAL OPERATION, OR BY A LIMITED MOTION ELECTRIC MOTOR, OR BY HYDRAULIC PILOT METHOD. THE OUTPUT RESULTS IN A CHANGE OF HYDRAULIC PRESSURE.

·        PISTON OR SPOOL VALVE: THE COMMONLY USED SPOOL VALVE IS CONSTRUCTED IN EITHER A THREE-WAY OR A FOUR-WAY VALVE ARRANGEMENT AS SHOWN IN FIG. (A) 

    WHEN THE SPOOL IS IN THE NEUTRAL POSITION, THE OIL FLOW TO THE ACTUATOR IS COMPLETELY BLOCKED. DISPLACEMENT OF THE SPOOL TO RIGHT AND LEFT CAUSES ALTERNATELY PRESSURE IN ONE PORT TO BE HIGHER THAN THAT IN THE OTHER PORT. THIS IS BECAUSE WHEN ONE OF PIPE LINE COMMUNICATES WITH THE DRAIN. THE DIFFERENTIAL PRESSURE CAUSES THE HYDRAULIC MOTOR TO ROTATE IN A PARTICULAR DIRECTION. THE FLOW AND THEREFORE THE MOTOR SPEED ARE FUNCTION OF THE SPOOL-VALVE OPENING, AFFECTED SOMEWHAT BY THE LOAD PRESSURE.

FIG.(A) PISTON OR SPOOL VALVE

     FLAPPER NOZZLE VALVE: THE OPERATION OF THE UNIT BASED ON AVAILABLE LEAKAGE ARRANGEMENT, WHICH HAS THE GREAT VIRTUES OF SIMPLICITY AND RELIABILITY. THE UNIT INCORPORATES TWO ORIFICES IN SERIES, ONE OF WHICH IS A FIXED RESTRICTION AND THE OTHER VARIABLE ORIFICE CONSISTING OF A FLAPPER AND NOZZLE ARRANGEMENT SHOWN IN FIG. (B).THE NOZZLE RESTRICTION IS CHANGED AS THE FLAPPER IS POSITIONED CLOSER TO OR FARTHER FROM THE NOZZLE. THE FLUID AT CONSTANT PRESSURE PASSES THROUGH THE RESTRICTION AND A BRANCH IS LED TO THE NOZZLE. WHEN THE FLAPPER MOVES INTO A POSITION THAT COMPLETELY BLOCKS THE NOZZLE OPENING, THERE IS VERY LITTLE LEAKAGE AND THE OUTPUT PRESSURE APPROACHES THAT OF THE SUPPLY. WITH THE NOZZLE OPENING, THERE OCCURS AN INCREASING IN PRESSURE DROP ACROSS THE RESTRICTION AND CONSEQUENTLY THE OUTPUT PRESSURE DIMINISHES. THUS THE DEVICE PRODUCES A VARIABLE OUTPUT PRESSURE WITH FLAPPER POSITION.

FIG. (B). FLAPPER NOZZLE VALVE

·        JET - PIPE VALVE: THE DEVICE COMPRISE A PIVOTED NOZZLE AND TWO ADJACENT ORIFICES. THE NOZZLE CONVERTS THE STATIC PRESSURE OF THE SYSTEM INTO KINETIC ENERGY AND THEN DIRECTS THE HIGH VELOCITY JET OF HYDRAULIC FLUID TOWARDS THE ORIFICES. DURING FLOW THROUGH ORIFICES, THE KINETIC ENERGY IS RECONVERTED TO PRESSURES; THE CONVERSION BEING APPROXIMATELY 90% EFFICIENT AT MODERATE SUPPLY PRESSURE.  THIS RESULT FROM THE FACT THAT FRICTION CAN BE REDUCED TO MINIMUM BY MAKING THE SPACE BETWEEN THE NOZZLE AND ORIFICES RELATIVELY LARGE.

FIG. (C) JET - PIPE VALVE

THE FLAPPER NOZZLE AND JET PIPE VALVE ARRANGEMENT S ARE FREQUENTLY USED AS PREAMPLIFIER TO A PISTON VALVE. SHOWN IN FIG. (C)

REGULATOR:-

 A REGULATOR IS A FEED BACK CONTROL SYSTEM IN WHICH THE OUTPUT IS MAINTAINED AT A PRESET VALUE IRRESPECTIVE OF EXTERNAL LOAD ON THE PLANT .THE REFERENCE INPUT OR COMMAND SIGNAL, ALTHOUGH ADJUSTABLE, IS HELD CONSTANT FOR LONG PERIODS OF TIME. THE PRIMARY TASK IS THEN TO MAINTAIN THE OUTPUT AT THE DESIRED VALUE IN THE PRESENCE OF DISTURBANCES .EXAMPLES OF AN AUTOMATIC REGULATOR ARE; REGULATION OF STEAM SUPPLY IN STEAM ENGINES BY THE FLY BALL GOVERNOR; THERMOSTAT CONTROL OF HOME HEATING SYSTEM; CONTROL OF PRESSURE AND OF ELECTRICAL QUANTITIES SUCH AS VOLTAGE, CURRENT AND FREQUENCY.

IN GENERAL A CONTROL SYSTEM THAT REGULATES A VARIABLE IN RESPONSE TO A FIXED COMMAND SIGNAL IS KNOWN AS A REGULATOR SYSTEM WHEREAS CONTROL SYSTEM THAT ACCURATELY FOLLOWS CHANGES IN THE COMMAND SIGNAL IS REFERRED TO AS FOLLOW UP SYSTEM.