Force, Torque, and Shaft Power
Measurement
By
Aj. Somchai Triratanajaru M.Eng.
KMUTT,
B.Eng
KMITL
Term 1/2553
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LECTURE OUTLINE
„ 1. Standards and Calibration
„ 2. Basic Methods of Force Measurement
„ 3. Characteristics of Elastic Force Transducers
„ 4. Resolution of Vector Forces
„ 5. Torque Measurement on Rotating Shafts
„ 6. Shaft Power Measurement (Dynamometers)
„ 7. Gyroscopic Force and Torque Measurement
„ 8. Vibrating-Wire Force Transducers
*Major part of this presentation are from
Measurement Systems, Ernest O. Doebelin
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1.Standards and calibration
„ Force, (F=ma); thus a standards for force
depends on standards of mass and
acceleration.
Mass standards-> cylinder of platinum-iridium
called International Kilogram.
Acceleration -> drive from standards of length
and time.
g -> sea level and 45o latitude 980.665 cm/s2
If not , must be compensated.
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Force standards
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2. Basic methods of force measurement
There are seven methods introduce in the
reference text book. Some methods are only
use in lab. We will focus on weighting and
torque measuring instrument But keep in mind
that it is still have so many methods out there.
Use this information as the guide line for further
study.
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2.1 Balancing it against the known gravitational
force on a standard mass, either directly or through a
system of levers.
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2.2 Measuring the acceleration of a body of known
mass to which unknown force is applied.
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2.3 Balancing it against a magnetic force developed
by interaction of a current-carrying coil and a
magnet.
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2.4 Transducing the force to a fluid pressure and
then measuring the pressure.
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2.5 Applying the force to some elastic member and
measuring the resulting deflection.
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Strain-Gauge Load Cell
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Strain-Gauge Beam Transducers
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Young's modulus
„ Young's modulus, E, can be calculated by
dividing the tensile stress by the tensile strain:
„ where
E is the Young's modulus (modulus of elasticity)
„ F is the force applied to the object;
„
A0 is the original cross-sectional area through
which the force is applied;
„ ΔL is the amount by which the length of the
object changes;
„ L0 is the original length of the object.
„
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Young's modulus..
Young's modulus is the ratio of stress, which
has units of pressure, to strain, which is
dimensionless; therefore Young's modulus
itself has units of pressure.
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Young's modulus…
„ Material
Modulus (GPa)
„ Aluminum
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117
207
„ Copper
„ Steel
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LVDT Force Transducer
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Piezoelectric Load Cells
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Torque Measurement of Rotating
Machines: Dynamometers
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Chassis Dynamometer
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Servo-controlled Dynamometer
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2.6 Measuring the change in precession of
gyroscope caused by an applied torque related to the
measured force.
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2.7 Measuring the change in natural frequency of a
wire tensioned by the force.
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Further reading and more exercises
Text.
Process control Instrumentation
technology
Curtis D. Johson
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