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EM Instruments (DC)
Chapter Two
Basic Test and
Measurement Instruments 1:
Electromechanical
Instruments
(DC)
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Fall 2012
ME-3504
Sensors, Actuators and
Instrumentation
5th Semester (Mechatronics)
SZABIST, Karachi
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Engr. Humera Rafique
Assistant Professor
(Department of Mechatronic Engineering)
humera.rafique@szabist.edu.pk
Office: 100 Campus 4th Floor R-404 (Ext. 138)
Course Support: Zabdesk
Alternate: https://sites.google.com/site/zabistmechatronics/home/fall-2012/sai
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Course Outline
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1. Fundamentals
2. Basic Test & Measurement Instruments
–
Electromehcanical Instruments (DC)
3. Sensors
4. Actuators
5. Mechatronic Systems
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Chapter Contents
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1. PMMC Instruments
2. Galvanometer
3. DC Ammeter
4. DC Voltmeter
5. Ohmmeter
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PMMC
Instruments
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PMMC
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Permanent Magnet Moving Coil:
•
Light weight copper coil
•
Permanent magnet
•
Pointer
•
Calibrated dial
•
Current source
Modifications: Essentially a low level DC ammeter:
•
|| resistor increases current measurement capability
•
Series resistor with coil: dc volt meter
•
Precision resistor + PMMC + batteries: Ohmmeter
•
PMMC + rectifier circuit: AC (ammeter + voltmeter)
•
PMMC + stationary coil (instead of permanent magnet): Electrodynamic
instrument (used in e.g., wattmeters)
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PMMC
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Working Principle of Deflection Instruments
Forces
Deflecting
Controlling
Damping
Magnet
Spiral spring
Aluminum coil frame
Deflects pointer from 0
Retains coil at 0
Minimizes
oscillations
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PMMC
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PMMC
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PMMC
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Deflecting:
• P-magnet applies force on current carrying conductor
• coil rotates on its pivot
• pointer moves on the scale
Controlling:
• non-magnetic spiral spring, low resistance spring makes electrical connection
with coil
• works for 0 current spring winds up under deflecting force
• stops when τdef = τcontr
Damping:
•
•
•
•
works when coil moves
Al-coil frame non-magnetic conductor
flux due to induced eddy-currents opposes the coil motion
=> oscillations damped
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PMMC
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Lack of damping
let the pointer to
oscillate
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PMMC
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PMMC
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PMMC
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Moving System Support:
1.
Pivot & Jewel bearing suspension
2.
Taut-band suspension
•
The pointed ends of the shafts or
pivots fastened to the coil are
inserted into cone-shaped cuts in
jewel bearing
•
Coil rotates freely with least
possible friction
•
Some bearings are spring
supported as shock absorber
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PMMC
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Moving System Support
•
Much
JBS
•
Flat metal ribbons
under tension
•
Provide
controlling
force & electrical
connections to the
moving coil
•
More sensitive than
JBS i.e., FSD of JBS:
25 uA while for TBS:
as small as 2 uA
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tougher
than
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Construction (D’ Arsonval Instrument):
•
Two soft iron pole shoes – permanent magnet
•
Soft iron core (cylindrical)
•
Light weight moving coil
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PMMC
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PMMC
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Torque Equation & Scale:
. Coil diameter
Magnetic flux
density (tesla)
# of turns
Length of coil current (amp)
(meters)
∘
Deflecting angle proportional to the coil current
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PMMC
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Torque Equation & Scale:
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Galvanometer
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Example 3-1: (Bell):
A PMMC instrument with a 100-turn coil has a magnetic flux density in its air
gaps of B = 0.2 T. The coil dimensions are D = 1 cm and l = 1.5 cm. Calculate
the torque on the coil for a current of 1 mA.
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Galvanometer
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Galvanometer
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Construction:
•
Essentially a PMMC
•
Sensitive to extremely low current
•
Centre-zero scale: µA
•
Current sensitivity: µA/mm
•
Same torque relationship as PMMC
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Galvanometer
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Construction:
•
Most sensitive Galvanometers use Taut-band suspension assembly (ribbon’s
twist produces the controlling torque)
•
Eddy current damping may be provided by the coil in non-magnetic coil frame
(as PMMC)
•
For controlling torque: Critical damping is recommended (quick + less/no
oscillations)
•
To control level of Eddy currents generated by the coil movement:
shunt resistance is used
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Galvanometer
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Light Beam Deflection System
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DC Ammeter
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DC Ammeter
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Ammeter always connected in series with the test circuit
•
To avoid affecting original current levels of test circuit for a ↓ Rm , range of
ammeter ↑
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Extending the range:
•
PMMC : ammeter,
deflection ∝ Icoil
•
Normal PMMC: low current level
meter (Small FSD)
•
↑ Icoil can destroy the coil
•
Shunt (very low ohm) resistance
in parallel
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pointer’s
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Ammeter Scale:
•
At FSD, Icoil = Imax = 150 mA
•
Meter scale calibration:
–
0.5 FSD = 75 mA
–
0.25 FSD = 37.5 mA
–
0.33 FSD ≅ 50 mA
m
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Galvanometer
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Example 3-3: (Bell):
An ammeter has a PMMC instrument with a coil resistance of Rm = 99 Ω and
FSD current of 0.1 mA. Shunt resistance Rs = 1 Ω. Determine the total current
passing through the ammeter at (a) FSD, (b) 0.5 FSD, and (c) 0.25 FSD.
Shunt resistance:
Example 3-4: (Bell):
A PMMC instrument has FSD of 100 µA and a coil resistance of 1 kΩ.
Calculate the required shunt resistance value to convert the instrument into
an ammeter with (a) FSD = 100 mA. (b) FSD = 1A.
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DC Ammeter
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Multiragne Ammeter:
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DC Ammeter
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Multiragne Ammeter:
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Galvanometer
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Example 3-5: (Bell):
A PMMC instrument has a three-resistor Ayrton shunt connected across it to
make an ammeter. R1= 0.05 Ω, R2 = 0.45 Ω, and R3 = 4.5 Ω.
The meter has Rm = 1000 Ω and FSD = 50 uA.
Calculate the three ranges of the ammeter.
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DC Voltmeter
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DC Voltmeter
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•
•
•
Pointer’s deflection ∝ I coil
I coil ∝ Vcoil
Scale of PMMC could be calibrated to indicate voltage
A series resistance Rs with PMMC
•
Extending the range of PMMC voltmeter:
– Rm ↓, therefore Vcoil ↓
– Rs: multiplier resistance (V = I x R)
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DC Voltmeter
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Example 3-7: (Bell):
A PMMC instrument has FSD of 50 µA and a coil resistance of 1.7 kΩ, is to be
employed as a voltmeter with ranges of 10 V, 50 V and 100 V. Calculate the
required values of the multiplier resistors for the circuits of figure.
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Ohmmeter
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Ohmmeter
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Series Ohmmeter:
•
Normally part of multifunction meters (e.g., VOM)
•
Consists of
–
Low-current PMMC
–
Standard resistance (R1)
–
Unknown resistance (Rx) terminals
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Ohmmeter
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Ohmmeter Calibration:
•
Rx = 0 (A-B shorted) & R1 is adjusted for FSD => zero ohms
•
Rx = max (A-B open) => infinity
Example 3-14: (Bell):
A series ohmmeter is made up of
a 1.5 V battery, a 100 µA meter
and a resistance R1 which makes
(R1 + Rm) = 15 kΩ.
a.
b.
Determine the instrument indication when Rx = 0.
Determine how the resistance scale should be marked at 0.5 FSD, 0.25 FSD
and 0.75 FSD.
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Ohmmeter
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Ohmmeter with Zero Adjust:
•
The ohmmeter described works fine as for as battery voltage remains the
same (usually 1.5 V)
•
If NOT, meter’s current will be proportional to new value of (Rm + Rs) =>
meter is no longer calibrated correctly
•
A parallel resistor (zero control) is connected with PMMC device
||
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Ohmmeter
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Ohmmeter with Zero Adjust:
Example 3-15: (Bell):
The ohmmeter circuit in figure has Eb = 1.5 V, Rm = 50 Ω, R1 = 15 kΩ , R2 = 50 Ω,
and meter FSD = 50 µA. Determine the ohmmeter scale reading at 0.5 FSD, and
determine the new resistance value that R2 must be adjusted to when Eb falls to 1.3 V.
Also recalculate the value of Rx at 0.5 FSD when Eb = 1.3 V.
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References
1.
Electronic Instrumentation & Measurement (2e) David A. Bell
2.
Berlin and Getz
3.
Wikipedia Encyclopedia
4.
Picture Credits (pic crdt.pdf)
5.
neutron.com, physics.edu
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