Transcript electronic instrumentations

ELECTRONIC
INSTRUMENTATIONS
ECE 6319
EECE 6311
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Dr. Nor Farahidah Za’bah
Room number : E2-2-13.12
Phone number : 03-6196 4562
Email address :
 [email protected]
 [email protected]
 Website: http://staff.iium.edu.my/adah510
COURSE CONTENT
Text book
• Modern Measurements: Fundamentals and Applications by
Alessandro Ferrero, Dario Petri, Paolo Carbone and
Marcantonio Catelani
Other references
• Bently, John P., (2005) Principles of Measurement Systems,
4th Edition, Pearson, Prentice Hall.
• Kalsi, H. S. (2003), Electronic Instrumentation: 7th Reprint,
Bombay, Tata Mc Graw Hill.
• Turner K. (2013) Instrumentation for Engineers, Springer.
1. Measurement Models and Uncertainty
2. Basic Instruments
• Multi-meters
• Oscilloscope
3. Basic Architecture of Electronics Instrumentation
Measurement System
4. Sensors and Transducers
5. AD and DA Conversion
6. Digital Signal Processing in Measurement
ASSESSMENT
Assessments
Percentage
Quizzes
10%
Assignment
*it can be a simulation or a
lab assignment
10 %
Midterm
30%
Final Exams
50%
WHY DO WE PERFORM
MEASUREMENTS?
1. To establish the validity of the design
2. To predict the limit of capacity
3. To provide information needed to supplement further
work
INSTRUMENTS
The type of instruments depends of the types of data
•
Steady state data
•
Transient data
•
Dynamic data
Steady state data:
If the data varies in the range of 0-5 Hz i.e value is not changing or changing
very slowly
Transient data:
If the parameter variations is at much higher rate >> 5Hz.
Dynamic data:
The parameter variation is periodic
INSTRUMENTS
Measurement involves using an instrument is a physical
means to determine the value or quantity of a variable
Different instruments are compared and analysed based on
performance characteristic parameters which are divided into
two:
i.
Static Characteristic
ii.
Dynamic Characteristic
STATIC CHARACTERISTIC OF
AN INSTRUMENT
• Instruments which are used to measure an unvarying
process condition
• Static performance characteristic are obtained by a
process known as calibration
• Related definitions that are associated to Static
Characteristics are:
i.
Accuracy
ii.
Precision
iii. Sensitivity
iv. Reproducibility
v.
Drift
vi. Dead Zone
vii. Error
i.
ii.
Static
Limiting
ACCURACY
Closeness with which an instrument reading approaches the
true value
It indicates the maximum error which will not be exceeded as
assured by manufacturer
For example: If the accuracy of 100V voltmeter is  1%,
hence the maximum error will not exceed  1V
PRECISION
It is the measure of order to which a particular parameter is
measured – simply referred to the decimal places of the a
measured value
For example: 75.2375 V is a precise value.
PRECISION vs ACCURACY
Precision DOES NOT necessarily guarantee accuracy.
For example:  = 3.14 is a correct value or true value. This is an
accurate value.
3.14285 is a precise value AND an accurate value.
However, 3.24285 is a precise value but it is NOT accurate
SENSITIVITY
Indicates the capacity of the instrument to respond truly to
the change in the output corresponding to the change in the
input.
For example: For a voltmeter, the ratio vo /  vi is referred to
sensitivity
REPRODUCIBILITY
This is the scale reading over a period of time when the input
is constantly applied.
If the reading fluctuates and changes, then the
reproducibility of the instrument is poor
DRIFT
Drift is the change in output with change in the input. There
are three types of drift
Zero Drift – the whole calibration shifts by the same amount
Span Drift –the drift is not constant, but increases gradually
with the deflection of the pointer
Combined Drift – Both drift occurs simultaneously
ZERO
SPAN
COMBINED
DEAD ZONE
The max value of input up to which the input does not respond due
to hysteresis of the instrument. In other words, insensitivity of a
instrument in a specific range of input signals
ERROR
Static Error:
Defined as the difference between the true value and the measured
value. If the error is constant, it is referred to as static error
Limiting Error:
The limit of deviation from the specified value. For example, a
resistor has rated value 100  5%. The limiting error is  5%.
Limiting Error Example
A 600 V voltmeter is specified to be accurate within  2% at
full scale.
Calculate the limiting error when the instrument is used to
measure voltage of 250 V
600 V   12 V deviation
(12/250) x 100% = 4.8 %
DYNAMIC CHARACTERISTIC
OF AN INSTRUMENT
• Related definitions that are associated to Static
Characteristics are:
i.
Lag
ii.
Fidelity
iii. Speed of Response
iv. Dynamic Error
LAG
The speed of response of the instrument is referred to as lag.
In other words, the time delay in the output to the change in
input
FIDELITY
Quality of indication by the instrument with regard to
changes in input is known as fidelity
SPEED OF RESPONSE
It is defined as the rapidity with which a measurement
system responds to changes in measured quantity.
DYNAMIC ERROR
Defined as the difference between the true value and the
measured value. But the error is not constant.
STATISTICAL EVALUATION OF
MEASUREMENT DATA
•
Arithmetic Mean (Average)
•
•
The most probable value of measured
variable is the arithmetic mean of the
number of readings taken.
Deviation
•
•
Deviation is departure of the observed
reading from the arithmetic mean of the
group of readings.
Standard Deviation
•
The standard deviation of an infinite
number of data is defined as the square
root of the sum of the individual
deviations squared divided by the
number of readings.
Question:
The following 10 observation were recorded when measuring
a voltage: 41.7, 42.0, 41.8, 42.0, 42.1, 41.9, 42.0, 41.9, 42.5,
41.8 volts. The true value is 42 V
Calculate Mean, Standard Deviation, and Range.
Standard deviation is a number used to tell how measurements for a
group are spread out from the average (mean), or expected value.
A low standard deviation means that most of the numbers are very
close to the average.
A high standard deviation means that the numbers are spread out.
BASIC INSTRUMENTS
- MULTIMETER
• capable of measuring DC and AC voltages as well as current
and resistance
• contains the following elements:
• Balanced-bridge DC amplifier and indicating meter
• Input attenuator or RANGE switch, to limit the magnitude of
the input voltage to the desired value
• internal battery and additional circuitry, to provide the
capability of resistance measurement
• Rectifier section, to convert an AC input voltage to a
proportional DC value
• FUNCTION switch, to select the various measurement
functions of the instrument
Balanced-bridge DC amplifier and indicating meter
Without input signal, the
gate terminals of the
FETs are at GND
potential and the
transistors operate
under identical
quiescent conditions.
Ideally no current
should flow through the
meter
With a positive input signal applied to the gate of input transistor Q1 its drain current
increases causing the voltage at the source terminal to rise. The resulting unbalance
between the two transistors Q1 and Q2 source voltages is shown by the meter
movement, whose scale is calibrated in terms of the magnitude of the applied input
voltage.
Input attenuator or RANGE switch
The range of input voltages can easily be
extended by an input attenuator or RANGE
switch.
The unknown DC input voltage is applied
through a large resistor in the probe body to a
resistive voltage divider.
Thus, with the RANGE switch in the 3-V
position as shown, the voltage at the gate of
the input FET is developed across 8 M of the
total resistance of 11.3 M  and the circuit is
so arranged that the meter deflects full scale
when 3 V is applied to the tip of the probe.
With the RANGE switch in the 12-V position,
the gate voltage is developed across 2 M  of
the total divider resistance of 11.3 M and an
input voltage of 12 V is required to cause the
same full-scale meter deflection.
Resistance Measurement
Rx = the unknown resistance
When unknown resistor Rx is connected to the multimeter, the 1.5-V
battery supplies current through one of the range resistors and the
unknown resistor to ground.
Voltage drop Vx is then applied to the input of the bridge amplifier and
causes a deflection on the meter. Since the voltage drop across Rx is
directly proportional to its resistance the meter scale can be
calibrated in terms of resistance.
Rectifier using Diodes
This combination circuit will
produce the peak voltage.
Initially during positive cycle,
the capacitor is charged until
peak value.
Then, capacitor discharges
through the load resistor.
The problem here is the forward voltage drop of the rectifying diodes. If
we are measuring large voltages, this voltage loss may be negligible.
The lowest AC voltage is normally limited to 10 V. Lower range
voltages cannot be achieved because of the diode forward voltage
Rectifier using Op-amps
To capacitor and
resistor circuitry
inverting amplifier
Summing amplifier
ASSIGNMENT
Simulate the precision circuit using PSPICE and evaluate the
output.
Explain in detail the operation of the circuitry.
A
B
1. Use sine wave of 10 V peak value
2. Don’t forget the power supply for the op-amp (normally it is 5 V)
3. Please include the output graphs at node A and node B
A multimeter when it is used as an ammeter is connected in
SERIES with the measured device It will consist of a low
internal impedance, but not zero
A multimeter when it is used as a voltmeter is connected in
PARALLEL with the measured device. It will consist of a very
high internal impedance, but not infinity
EXAMPLE
To measure a current that circulate through a load of about 8
connected to a voltage source of 12 V, a DMM is inserted as
ammeter in the 2 A range. The manufacturer specifications state
that the maximum voltage drop under full scale is 700 mV.
What will be the error in the measurement due to the insertion of
the ammeter?
• The multimeter is used as an AMMETER
• Connected in series with the resistor
EXAMPLE
A multimeter is used to measure a DC voltage in a circuit.
The internal impedance is 10 M. What is the load effect if
the Thevenin impedance is 350 k.