instrumentation and controls-intro

8/3/2019 Instrumentation and Controls-Intro

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Instrumentationand ControlsDefinition and Examples of

Systems


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Introduction

}Measurement and control involve a

system in which there are several physicalvariables.


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Introduction} In a control system or measuring system, there

exist some energy converting processes.

} In each process, there is one or more thanone physical variables to be measured.

} Measuring process is to determine quantity ofsuch physical variables.

} For a process control system, it is veryimportant to know the quantity of a physicalvariable to be controlled at every time, andcompare the measured value to thedesirable value.


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Introduction

} If there is a difference between the

measured value and the desirable one,the operator will determine to adjust thecontrolled variable as desired.

} However, no measuring system is perfect.The operator should know the limits of the

measured variablesand characteristics ofthe measuring equipment.


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Systems (SISO, MIMO)

} System: A system in general is referred toas a composition of elements in which

there exists a relationship between theinput signal(s) and output signal(s).

}A system with a single input signal and asingle output signal is called single input

single output (SISO) system. A system withinput signals and output signals is calledmulti-input multi-output (MIMO) system.


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Process

} Process: Process is the manipulation ofthe signals within a system. Process

involves in an energy conversion.

} The physical system to be controlled maybe electrical, thermal, hydraulic,pneumatic, gaseous, mechanical, or any

other physical type.


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Process

} There are several types of processes.

} (a) electrical, (b) hydraulic, (c) pneumaticand (d) thermal.


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Open and Closed-loopControl Systems} Open-loop Control Systems (Non-feedback Systems):

Those systems in which the output has no effect onthe control action are called open-loop controlsystems.

} In other words, in an open-loop control system theoutput is neither measured nor fed back forcomparison with the input.

} One practical example is a washing machine.Soaking, washing, and rinsing in the washer operate

on a time basis. The machine does not measure theoutput signal, that is, the cleanliness of the clothes.

} In any open-loop control system the output is notcompared with the reference input.


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Open and Closed-loopControl Systems} Closed-loop Control Systems (Feedback Systems): A

system that maintains a prescribed relationshipbetween the output and the reference input bycomparing them and using the difference as ameans of control is called a feedback controlsystem.

} An example would be a room-temperature controlsystem.

} By measuring the actual room temperature and

comparing it with the reference temperature(desired temperature), the thermostat turns theheating or cooling equipment on or off in such a wayas to ensure that the room temperature remains atacomfortable level regardless of outside conditions.


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Definition and Needs forInstrumentation

} Instrumentation is the application ofinstruments for monitoring, sensing, and

measurement. Its purpose may be:} product testing and quality control;

}monitoring in the interest of health, safety,or costing;

} part of a control system;}maintenance and repair; or

} research and development.


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General Structure of aMeasuring System

} Transducer: is an energy converter whichreceives the physical quantity being

measured (referred to as the measurand)and converts it into some other physicalvariable; e.g. flow to pressure, speed tovoltage, strain to resistance.

} Signal Conditioner: rearranges thetransduced signal into a form which canbe readily recorded or monitored.


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General Structure of aMeasuring System

} Recorder or Display: is the recorder,display or indicating device.


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Static Performance

} Sensitivity: have a wide variety of units,depending on the instrument or

measuring system being considered.

} The platinum resistance thermometer, forexample, gives a change of resistancewith increase of temperature and

therefore its sensitivity would have units ofohms/oC.


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Static Performance

} Linearity: is a term associated from alinear input/output relationship, usually

expressed as a percentage of full scale.

} In other word, it is the ratio of themaximum deviation from a linearrelationship between the input and

output to the full scale.


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Static Performance} Scale Factor: is derived by taking the span of the

variable being measured and dividing it into (or

by) the span of the converted value, eachexpressed in its own appropriate engineering units.

} Range: Therange of a variable is simply anumerical statement of the minimum andmaximum values that the variable may assume.

} Span: is simply the numerical difference between

two range values. For example, if the range oftemperature for a given period were 18 to 23oC,the span would be 5 Celsius degrees.


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Static Performance

} Precision: The term precision, whileassociated with accuracy, does not

mean the same thing.

}Where, for the same input, applied on anumber of occasions, an instrumentprovides reading which are very close in

value, it is said to have high precision.


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Static Performance} Reproducibility: is a general term used with

regard to precision and provides a measure

of the closeness of readings given for aconstant input.

} Repeatability: refers to reproducibility when aconstant input is repeatedly applied for shorttime intervals under fixed conditions.

} Stability: concerns repeatability when the

constant input is applied for a long timecompared with the time required to take areading under fixed conditions.


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Static Performance} Constancy: refers to reproducibility when the

constant input is provided continuously but

the conditions during the measurement arepermitted to vary within specified limits.

} Tolerance: is the term used for maximumpossible error, in particular with regard tomeasurements of length.

} Resolution: The smallest change of input to an

instrument which can be detected withcertainty, expressed as a percentage of fullscale.


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Static Performance

} Dead-band: The largest change of inputto which the system does not respond

due to friction or backlash effects,expressed as percentage of full scale.

} Hysteresis: The maximum differencebetween readings for the same inputwhen approached from oppositedirections i.e. when increasing anddecreasing the input expressed as apercentage of full scale.


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Summary

instrumentation and controls-intro

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