3. Contents
1. Dimension measurement
• Why to measure dimensions
2. Thickness measurement
• Thickness gauges
3. Length measurement
• Laser based length measurement
4. Width measurement
• Camera based width measurement
5. Diameter measurement
• Laser based diameter measurement
REFERNECES 3
4. 1. Dimensionmeasurement
The determination of linear and angular magnitudes of particular
features or mechanical parts of a prototype or machine itself is
called as Dimension Measurements.
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5. Why to measure dimensions???
Dimension measurement and regulations are among the quality
control parameters of industrial machinery design. This practice is
done to
• Meet the customer’s requirements
• Avoid any deviation in dimensional tolerances
• Avoid rejection of the product by the customers
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7. 2. Thickness measurement
In some industrial productions the control or measurement of
thickness of coating, applied to a base material is essential e.g.
• Deposition of zinc on steel
• Application of wax, clay bitumen or plastic on paper etc.
This base or sheet material thickness measurement with and
without coating material is done with the help of a device known
as ‘thickness gauge’.
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9. Thickness gauges
a. Contact type thickness gauges
i. Inductive type
• Reluctance transducer
• Eddy current transducer
ii. Capacitive type
b. Non-contact type thickness gauges
i. Radiation type
ii. Laser based
c. Coating thickness gauges
i. Differential beta thickness gauge
ii. Beta backscatter thickness gauge
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10. a. Contact type thickness gauges
Here we find that
• Thickness of the object is found through electrical methods.
• The gauges make measurements upon a physical contact with
the test piece
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11. i. Inductive type thickness gauge
Two kinds of transducers are utilized:
• Reluctance variation transducers: These devices measure change
in inductance of a magnetic coil caused by the test object.
• Eddy current transducers: They use eddy currents to find the
thickness of test object.
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12. Reluctance variation transducer
• It consists of a U-shaped magnetic core over which a coil is
wound .The stock or a test piece, whose thickness is to be
measured, completes the magnetic circuit.
• Inductance of the coil depends upon the reluctance of the
magnetic circuit, which in turn depends upon the thickness of
the test object
15. Reluctance transducer
• The non-magnetic test piece is placed on ferromagnetic base; the
base is thick such that the reluctance of the magnetic circuit is
determined by the distance between the ends of the U-core and
the base plate.
• An increase in thickness of the test piece causes an increase in
reluctance and a decrease in inductance of the magnetic circuit.
16. Eddy current transducer* (Conducting material)
• It has a coil wounded on an insulated core, excited by an
alternating current supply. The alternating field produced as a
result generates eddy current in the test piece.
• The opposition created by the magnetic field of eddy current
against the magnetic field of coil, reduces the inductance of the
coil.
• So, higher the thickness of the test piece, higher will be the eddy
current, lower would be the inductance of the coil.
18. Eddy current transducer (Non-Conducting)
• For non-magnetic, non-conducting materials thickness
measurement is done by depositing it on a metal backing.
• If the thickness of the test piece is large, the eddy current
transducer head and the metal backing are separated by a larger
distance and, therefore, the eddy currents are small and
consequently the inductance of the coil is large.
20. Inductive type gauges
• Advantages
- Measurement of thickness of aluminum sheet 125-500 pm
- Easy to align
- Excellent repeatability
• Disadvantages
- Physical contact causes error in measurement
- It shows temperature sensitivity
- Inherently low power output
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21. ii. Capacitive type thickness gauge
• This method is used for the thickness measurement of insulators.
C = capacitance in farad
Ɛ0= 8.85 x 10-12 farad/m2
Ɛr = dielectric constant (relative permittivity)
A= area of each plate (m2)
d= distance between the plates (m)
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23. Capacitive type gauges
• Advantages
- Their frequency response is good.
- They require little force for operation.
• Disadvantages
- They are sensitive to temperature changes,
- They become contaminated with dust particles
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24. b. Non-contact thickness gauges
They measure the thickness without physical contact and are of
following three types
i. Radiation type
ii. Laser based
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25. i. Radiation type non-contact gauge
• These gauges employ alpha, beta, gamma or X-ray radiation
obtained from a radioactive element for thickness measurement.
• The radiation source is shielded all around except in the direction
of the detector.
• The radiation source radiates to the detector through the test
object.
• The radiation received at detector are amplified and calibrated to
find the thickness of object.
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27. Radiation type non-contact gauge
• Advantages
- It gives the thickness measurement of sheets in rapid motion without physical
contact.
• Disadvantages
- These are subjected to error because of radioactive decay of source and
dependence of absorption coefficient on the composition of material
- Frequent calibration checks are required for better accuracy.
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28. ii. Laser based non-contact gauge
• These gauges employ a laser beam.
• The beam is directed at a rotating (or oscillating) mirror to
collimate in to a straight ribbon.
• As the collimated beam scans across an object, it blocks the light
detector on the other side and measures the time for which the
light was blocked. Thus, scan time across the shadow is used to
measure the thickness of an object.
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30. c. Coating thickness gauges
To measure the thickness of coating materials, two types of gauges
are used
i. Differential beta gauge
ii. Capacitive sensor for measuring coating thickness
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31. i. Differential beta gauge
This technique involves two beta gauges.
• The readings are obtained by passing the un-coated sheet through
one measuring head and the coated from the other. The intensity
of radiation emitted is a measure of total thickness.
• They found application in the measurement of thickness of wax
and plastic coatings applied to paper and aluminium sheet when
the weight of coating is 25% or more of the weight of substrate.
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33. 3. Length measurement
Laser Doppler Velocimeters (LDVS)
• The laser Doppler velocimeter sends a monochromatic laser beam
towards the target and collects the reflected radiation.
• According to the Doppler Effect, the change in wavelength of the
reflected radiation is a function of the targeted object's relative
velocity.
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35. 4. Width measurement
Camera based width measurement is an online non contact
technique, using two configurations.
• Stereoscopic vision based:
It is used where the strip thickness changes substantially.
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37. 5. Diameter measurement
• It is a laser based system to measure diameter of any type of
opaque rod or rounds.
• The laser beam emitted from the laser diode in the transmitter is
converged into a parallel beam by the projecting lens unit.
• The laser beam is then directed through the slit on the receiver
and focused on the light receiving element.
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38. Laser Diameter gauge*
• Construction and working
- The gauge consists of two diameter measuring sensors which are
kept such a way to measure axes of the rod at tow angles of
interest.
- The laser transmitter-receiver pairs are kept facing each other at
some distance. When a test rod is passed through the
measurement area, the light falling on the receiver will reduce, in
the amount of diameter of the rod.
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41. Summary
• Dimension measurement is required to assure top quality of
products as per customer specifications.
• Thickness gauges can be of Contact and Non-contact type and use
inductance, capacitance, laser technology and beta radiations for
measuring the thickness of base materials and applied coating
surfaces.
• The length of an object can be found by subjecting it to a laser
source and thus noticing the shift in frequency of laser beam.
42. Summary
• The width of object can be found by employing Stereoscopic vision
principle.
• The diameter of an object is found using laser transmitter and
receiver arrangement, the decrease in intensity of radiations
received is a direct measure of diameter.
