LOADING EFFECTS IN MEASUREMENT
WE ALL IDEALLY WANT THE MEASUREMENT SYSTEM WHOSE PRESENCE IN THE SYSTEM DOES NOT CAUSE ANY TRANSIENT ERROR. ANY MEASURING DEVICE MAY CONSISTS OF ELEMENTS USED FOR SIGNAL SENSING, CONDITIONING, TRANSMISSION OR DETECTION AND IDEALLY ORIGINAL SIGNAL SHOULD REMAIN UN-DISTORTED WHEN THESE ELEMENTS ARE INTRODUCED.
BUT PRACTICALLY ALL ELEMENTS HAVE THEIR INDIVIDUAL PROPERTIES WHICH CONSISTS OF RESISTANCE, INDUCTANCE AND CAPACITANCE WHICH EFFECTS THE SYSTEM AS THEY INTRODUCES TRANSIENT EFFECT, DISTORTION OF INPUT SIGNAL AND HENCE ATTENUATION, WAVEFORM DISTORTION, PHASE SHIFT AND MANY UNDESIRABLE FEATURES ARE INTRODUCED IN THE SYSTEM.
THUS THE SYSTEM IS INCAPABLE TO FAITHFULLY MEASURE, RECORD OR CONTROL THE INPUT SIGNAL IN UN-DISTORTED FORM IS CALLED LOADING EFFECT.
ANY MEASUREMENT SYSTEM CONSISTS OF THREE STAGES :
- DETECTOR-TRANSDUCER STAGE
- SIGNAL CONDITIONING STAGE
- SIGNAL PRESENTATION STAGE
THE LOADING EFFECT MAY NOT OCCUR IN THE FIRST STAGE BUT MAY OCCUR IN ANY TWO SUBSEQUENT STAGES WHILE THE FIRST STAGE LOADS THE INPUT SIGNAL, SECOND STAGE LOADS THE FIRST STAGE AND FINALLY THE THIRD STAGE LOADS THE SECOND STAGE.
REASON FOR LOADING EFFECT :
LOADING EFFECT OCCURS DUE TO IMPEDANCE OFFERED BY VARIOUS ELEMENTS CONNECTED IN A SYSTEM.
IMPEDANCE MAY BE OF ANY FORM AS ELECTRICAL, MECHANICAL, THERMAL, FLUID.
WE TALK ABOUT GENERAL IMPEDANCE AND CATEGORIZE IN TWO TYPES
- INPUT IMPEDANCE
- OUTPUT IMPEDANCE
INPUT IMPEDANCE
THE MAGNITUDE OF THE IMPEDANCE ELEMENT CONNECTED ACROSS THE SIGNAL SOURCE IS CALLED INPUT IMPEDANCE.
ZI = SOURCE VOLTAGE / INPUT CURRENT
THUS INSTANTANEOUS POWER EXTRACTED IS
P = VI = V2 / ZI
- LOW INPUT IMPEDANCE DEVICE CONNECTED ACROSS THE VOLTAGE SIGNAL SOURCE DRAWS MORE CURRENT AND DRAINS MORE POWER FROM THE SIGNAL SOURCE THEN HIGH INPUT IMPEDANCE DEVICE.
- LOW INPUT IMPEDANCE DEVICE CONNECTED ACROSS VOLTAGE SIGNAL SOURCE LOADS THE SOURCE MORE HEAVILY THAN A HIGH INPUT IMPEDANCE DEVICE.
THUS WHENEVER ANY MEASURING INSTRUMENT IS INTRODUCED IN A SYSTEM,
THE SYSTEM CONDITION CHANGES FROM THE ORIGINAL CONDITIONS ON ACCOUNT OF THE POWER DRAWN FROM THE SYSTEM AND THE AMOUNT BY WHICH THEY DEVIATE FROM ORIGINAL CONDITIONS DEPEND UPON THE INPUT IMPEDANCE OF THE DEVICE.
LOWER THE INPUT IMPEDANCE, HIGHER THE POWER EXTRACTED AND HENCE HIGHER IS THE DISTORTION OF ORIGINAL SIGNAL.
SINCE THE VOLTMETER IS CONNECTED ACROSS THE SYSTEM FOR THE MEASUREMENT, IT SHOULD HAVE HIGH INPUT IMPEDANCE.
A LOW IMPEDANCE VOLTMETER LOADS THE SYSTEM HEAVILY AND THUS RESULTS IN WRONG MEASUREMENTS.
IDEALLY A VOLTMETER SHOULD HAVE INFINITE INPUT IMPEDANCE AND HENCE SHOULD NOT DRAW ANY POWER.
WE ALL IDEALLY WANT THE MEASUREMENT SYSTEM WHOSE PRESENCE IN THE SYSTEM DOES NOT CAUSE ANY TRANSIENT ERROR. ANY MEASURING DEVICE MAY CONSISTS OF ELEMENTS USED FOR SIGNAL SENSING, CONDITIONING, TRANSMISSION OR DETECTION AND IDEALLY ORIGINAL SIGNAL SHOULD REMAIN UN-DISTORTED WHEN THESE ELEMENTS ARE INTRODUCED.
BUT PRACTICALLY ALL ELEMENTS HAVE THEIR INDIVIDUAL PROPERTIES WHICH CONSISTS OF RESISTANCE, INDUCTANCE AND CAPACITANCE WHICH EFFECTS THE SYSTEM AS THEY INTRODUCES TRANSIENT EFFECT, DISTORTION OF INPUT SIGNAL AND HENCE ATTENUATION, WAVEFORM DISTORTION, PHASE SHIFT AND MANY UNDESIRABLE FEATURES ARE INTRODUCED IN THE SYSTEM.
THUS THE SYSTEM IS INCAPABLE TO FAITHFULLY MEASURE, RECORD OR CONTROL THE INPUT SIGNAL IN UN-DISTORTED FORM IS CALLED LOADING EFFECT.
ANY MEASUREMENT SYSTEM CONSISTS OF THREE STAGES :
IDEALLY A VOLTMETER SHOULD HAVE INFINITE INPUT IMPEDANCE AND HENCE SHOULD NOT DRAW ANY POWER.
WE ALL IDEALLY WANT THE MEASUREMENT SYSTEM WHOSE PRESENCE IN THE SYSTEM DOES NOT CAUSE ANY TRANSIENT ERROR. ANY MEASURING DEVICE MAY CONSISTS OF ELEMENTS USED FOR SIGNAL SENSING, CONDITIONING, TRANSMISSION OR DETECTION AND IDEALLY ORIGINAL SIGNAL SHOULD REMAIN UN-DISTORTED WHEN THESE ELEMENTS ARE INTRODUCED.
BUT PRACTICALLY ALL ELEMENTS HAVE THEIR INDIVIDUAL PROPERTIES WHICH CONSISTS OF RESISTANCE, INDUCTANCE AND CAPACITANCE WHICH EFFECTS THE SYSTEM AS THEY INTRODUCES TRANSIENT EFFECT, DISTORTION OF INPUT SIGNAL AND HENCE ATTENUATION, WAVEFORM DISTORTION, PHASE SHIFT AND MANY UNDESIRABLE FEATURES ARE INTRODUCED IN THE SYSTEM.
THUS THE SYSTEM IS INCAPABLE TO FAITHFULLY MEASURE, RECORD OR CONTROL THE INPUT SIGNAL IN UN-DISTORTED FORM IS CALLED LOADING EFFECT.
ANY MEASUREMENT SYSTEM CONSISTS OF THREE STAGES :
- DETECTOR-TRANSDUCER STAGE
- SIGNAL CONDITIONING STAGE
- SIGNAL PRESENTATION STAGE
THE LOADING EFFECT MAY NOT OCCUR IN THE FIRST STAGE BUT MAY OCCUR IN ANY TWO SUBSEQUENT STAGES WHILE THE FIRST STAGE LOADS THE INPUT SIGNAL, SECOND STAGE LOADS THE FIRST STAGE AND FINALLY THE THIRD STAGE LOADS THE SECOND STAGE.
REASON FOR LOADING EFFECT :
LOADING EFFECT OCCURS DUE TO IMPEDANCE OFFERED BY VARIOUS ELEMENTS CONNECTED IN A SYSTEM.
IMPEDANCE MAY BE OF ANY FORM AS ELECTRICAL, MECHANICAL, THERMAL, FLUID.
WE TALK ABOUT GENERAL IMPEDANCE AND CATEGORIZE IN TWO TYPES
- INPUT IMPEDANCE
- OUTPUT IMPEDANCE
INPUT IMPEDANCE
THE MAGNITUDE OF THE IMPEDANCE ELEMENT CONNECTED ACROSS THE SIGNAL SOURCE IS CALLED INPUT IMPEDANCE.
ZI = SOURCE VOLTAGE / INPUT CURRENT
THUS INSTANTANEOUS POWER EXTRACTED IS
P = VI = V2 / ZI
- LOW INPUT IMPEDANCE DEVICE CONNECTED ACROSS THE VOLTAGE SIGNAL SOURCE DRAWS MORE CURRENT AND DRAINS MORE POWER FROM THE SIGNAL SOURCE THEN HIGH INPUT IMPEDANCE DEVICE.
- LOW INPUT IMPEDANCE DEVICE CONNECTED ACROSS VOLTAGE SIGNAL SOURCE LOADS THE SOURCE MORE HEAVILY THAN A HIGH INPUT IMPEDANCE DEVICE.
THUS WHENEVER ANY MEASURING INSTRUMENT IS INTRODUCED IN A SYSTEM,
THE SYSTEM CONDITION CHANGES FROM THE ORIGINAL CONDITIONS ON ACCOUNT OF THE POWER DRAWN FROM THE SYSTEM AND THE AMOUNT BY WHICH THEY DEVIATE FROM ORIGINAL CONDITIONS DEPEND UPON THE INPUT IMPEDANCE OF THE DEVICE.
LOWER THE INPUT IMPEDANCE, HIGHER THE POWER EXTRACTED AND HENCE HIGHER IS THE DISTORTION OF ORIGINAL SIGNAL.
SINCE THE VOLTMETER IS CONNECTED ACROSS THE SYSTEM FOR THE MEASUREMENT, IT SHOULD HAVE HIGH INPUT IMPEDANCE.
A LOW IMPEDANCE VOLTMETER LOADS THE SYSTEM HEAVILY AND THUS RESULTS IN WRONG MEASUREMENTS.
OUTPUT IMPEDANCE
IT IS THE IMPEDANCE OFFERED BY THE DEVICE AND HENCE THE CAPABILITY OF THE DEVICE TO SUPPLY TO THE LOAD DEPENDS ON THE OUTPUT IMPEDANCE.
Zo =(Vo - VL ) / IL
- A SYSTEM MUST BE HAVING LOW OUTPUT IMPEDANCE BECAUSE THE CAPACITY OF THE SYSTEM TO SUPPLY TO THE LOAD DEPENDS ON THE VOLTAGE DROP OCCURRING ACROSS OUTPUT IMPEDANCE.
- LOWER THE OUTPUT IMPEDANCE, LOWER WILL BE VOLTAGE DROP, AND HENCE MORE VOLTAGE DROP OCCURS AT LOAD IMPEDANCE AND HENCE THE CAPABILITY OF A DEVICE TO SUPPLY TO THE LOAD INCREASES WITH THE REDUCTION OF OUTPUT IMPEDANCE.
- IDEALLY A SYSTEM MUST HAVE ZERO OUTPUT IMPEDANCE AND SHOULD NOT CONTAINS LOADING EFFECT.
WHENEVER A NEW DEVICE IS INTRODUCED IN A SYSTEM, IMPEDANCE OFFERED BY IT CONSUMES POWER AND HENCE LOADING OCCURS DUE TO CHANGE IN VOLTAGE ( IF CONNECTED IN SERIES) AND CURRENT (IF CONNECTED IN PARALLEL).
IDEALLY DEVICE SHOULD NOT EFFECT THE VOLTAGE OR CURRENT AFTER THE INTRODUCTION
THIS MAY CAUSE TRANSIENT PROBLEM AND DISTORTION IN VOLTAGE WAVEFORM AND AT A BIGGER LEVEL, LOADING PROBLEMS OCCURS WHICH MUST BE CONSIDERED.
FOR UNDERSTANDING THE CONCEPT MORE DEEPLY
REFER
A COURSE IN ELECTRICAL AND ELECTRONIC MEASUREMENT BY
AK SAWHNEY
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