High speed measurement

1. Presented By:
Renu Verma (162517)
Rohit Kumar (162519)
ME- I&C (R)
Presented To:
Dr. (Mrs.) Lini Mathew
Associate Professor
NITTTR, Chandigarh

2. Introduction
 An avalanche photodiode (APD) is a highly sensitive, high speed
semiconductor light sensors, electronic device that exploits the
photoelectric effect to convert light to electricity.
 APDs are photodetector that provide a built-in first stage
of gain through avalanche multiplication.
 By applying a high reverse bias voltage (100–200 V in silicon),
APDs show an internal current gain effect due to impact
ionization.
 The higher the reverse voltage, the higher the gain.

3. Principle of Avalanche Multiplication

4. Working of Avalanche Photodiode
 When the electron-hole pairs are generated in the depletion
layer of a photodiode with a reverse voltage applied to the PN
junction.

5.  The electrons drift towards the N+ side while the hole drift
towards the P+ side due to electric field developed across the
PN junction.
 The thickness of 𝝅/I-region is large as compared to other
region so that the incoming light is quickly passes through n+
and p-layers and spend major time in this region creating large
electrons and hole pairs.

6. Characteristic of APD
Operated with much higher reverse bias, which allows each photo
generated carrier to be multiplied by avalanche breakdown.

7. Dark Current Vs Reverse Voltage
 The dark current is mainly created by diffusion, generation-
recombination and tunneling.
 In an APD dark current is generated both from leakage at the
surface of the diode and also from electron – holes thermally
generated within the bulk of the silicon which are then
multiplied in the gain region.

8.  By increasing the gain of the APD, by increasing the external
bias, also increases this dark current.

9. Advantage:
 APD includes greater level of sensitivity.
 Fast response time.
 High performance.
Disadvantage:
 The APD multiplication process also produces an additional
noise component, known as "excess noise”.
 APD include much higher operating voltage may be required.

10. Application
 APDs are widely used in applications such as distance
measurement, data transmission ( over fibre or through free
space ), range finding, high speed industrial inspection (
including colour measurement ) and in various other medical
and scientific instrumentation

12. Introduction
High precision speed measurement techniques are a
very important subject in development of Physics. It
impacts also in the applied, industrial and commercial
fields around the world. It is required to have
standards that allow keeping congruent measurements
in different parts of the world.

13. Device for indicating the angular (rotary) speed of a
rotating shaft. Either by registering the total no. of
revolution during the period of contact or by
indicating the number of revolution per min.
Tachometer

14. What Are the Different Types of
Tachometers?
 Tachometers can be classified
1.On the basis of data acquisition
 Contact
 Non contact types
2. Classified as data type
 Analog
 Digital
3. On the basis of power .
 Mechanical
 Electrical

15. Comparison
Contact Type
1) The tachometer has to be in
physical contact with the
rotating shaft.
2) Preferred where the
tachometer is generally
fixed to the machine.
3) Generally, optical encoder /
magnetic sensor is attached
to shaft of tachometer.
Non Contact Type
1) The tachometer does not need
to be in physical contact with
the rotating shaft.
2) Preferred where the
tachometer needs to be
mobile.
3) Generally, laser is used or an
optical disk id attached to
rotating shaft and read by a
IR beam or laser.

16. How To Choose a Tachometer?
 Accuracy
 Precision
 Range
 Acquisition Time
 Contact type / Non Contact type
 Portable / Fixed
 Digital / Analog
 Cost

18. Optical techniques are most suitable for speed
measurement of fast moving projectile, particularly when
the projectile is of small size and speed is to be measured
in the vicinity of the projectile. Optical techniques have
distinct advantage of being non-contact, non-destructive in
nature and are suitable for practically all sizes of objects.

20. Working
Transmitting Part:
• Laser Gun emits laser light
• Laser beam falls on collimating(minimize divergence) optics
lance
Optical Screen:
• Its have a rotating projectile
• Laser beam cut by this projectile
Receiving Part:
• When laser beam fall on Photodiode, produce pulses
• Detector count number of pulses passes

21. Doppler Effect
The relativistic Doppler effect is the change
in frequency (and wavelength) of light, caused by the
relative motion of the source and the observer (as in
the classical Doppler effect), when taking into account
effects described by the special theory of relativity.

22. Conti….
λ Wavelength , fs frequency ,
v Velocity of observer, c Velocity of light

23. Due to the relativistic time dilation, the observer will measure
this time to be
is the Lorentz factor. The corresponding observed frequency
The ratio

24. is called Doppler factor of the source to the relative operator.
The resulting redshift can be written as:

27.  The Laser Doppler Anemometer is a very high-tech
anemometer, so it is usually more accurate.
 Laser Doppler anemometers use a beam of light from a laser
that is divided into two beams, with one propagated out of the
anemometer. The air particles caught in the beam are reflected
back into the anemometer where it’s measured.
 When the particles are in great motion, they produce a Doppler
shift for measuring wind speed in the laser light, which is used
to calculate the speed of the particles, and therefore the air
around the anemometer.

29. Advantage & Disadvantage
Advantage:
 The device does not disturb the flow during measurement
 The device measure velocity directly
 Volume of sensing part can be very small
 Highly accurate
 Can be used to measure flow of both gases and liquids.
Disadvantage:
 Requires transparent channels
 Cannot be-used on clean flows

30. Applications
 Wind tunnel studies
 Blood flow measurement
 Sensing of wind velocity
 Used in the field of combustion

32. Typical approach to using GPS for speed measurement today is
to consider a series of “track points” that record position
estimates(latitude and longitude) determined by the GPS at
regular time intervals.
The difference between the known satellite carrier frequency
and the frequency determined at the receiver is known as a
“Doppler shift”. This Doppler shift is directly proportional to
velocity of the receiver along the direction to the satellite,
regardless of the distance to this satellite.

34.  This method is also similar like previous method. Here fiber
coupling are use to communicate between transmitter and
receiver.
Drive
Circuit
Light
Source
Optical
Fiber
Detector

35. Train Speed Measurement

36. A critical safety issue is correct timing for crossing gates at
railroad highway grade crossings. If the
warning signals and crossing gates are activated too soon,
impatient motorists may try to cross in spite of
them which may result in a train-car collision. If the system is
activated too late, again a serious accident
may occur. Train presence and speed detection can also be
performed by measuring how the light signal passing through
the rail-bonded fiber sensor is affected by a train passing over
the rail.

37. Advantages & Disadvantage
Advantages:
 Optical Fiber Cables can run massive distances like 40 KM or
much more (Single Mode Fiber Cables) without having to
repeat the signal anywhere in-between.
 Optical Fiber Cables are flexible and can be laid both within
the buildings (Indoor Fiber Cables) and outside the buildings
(Shielded Fiber Cables).
Disadvantage:
 Fiber splicing is a complicated procedure and requires skilled
manpower to achieve.

38. Reference
 High-speed optical frequency-domain imaging S. H. Yun, G. J.
Tearney, J. F. de Boer, N. Iftimia and B. E. Bouma Harvard
Medical School and Wellman Laboratories for Photomedicine,
Massachusetts General Hospital 50 Blossom Street, BAR-7,
Boston, Massachusetts 02114 syun@bics.bwh.harvard.edu.
 High accuracy speed measurement using GPS (Global
Positioning System)by Tom J. Chalko, MSc, PhD*Senior
Scientist, Scientific Engineering Research P/L, Mt Best, Vic
3960, Australia.
 International Journal Of Engineering Sciences & Research
Technology Speed Violation Detection System : A Review
Sukhdeep Singh*1, Rishma Chawla2, Harpal Singh3.

39.  International Congress of Engineering Physics IOP Publishing
High precision speed measurement by using inter ferometric
techniques M A Rodríguez Ávila, R Ochoa Valiente, L A
García Trujillo Faculty de Ciencias Físico Matemáticas.
Universidad Autónoma deCoahuila. Saltillo, Coahuila de
Zaragoza, México.
 FIBER OPTICAL SENSORS FOR HIGH-SPEED
RAILAPPLICATIONS IDEA Programs Transportation
Research Board500 Fifth Street, NWWashington, DC 20001.
Reference