NPTEL >> Mechanical Engineering >> Modeling and Control of Dynamic electro-Mechanical System
Introduction to Photo Sensors
D Bishakh
Dr.
Bi h kh Bhattacharya
Bh tt h
Professor, Department of Mechanical Engineering
IIT Kanpur
Joint Initiative of IITs and IISc - Funded by MHRD
Module 6- Lecture 39
NPTEL >> Mechanical Engineering >> Modeling and Control of Dynamic electro-Mechanical System
This Lecture Contains
 Different types of Optical Sensors
 Light Dependent Resistors
Photovoltaic Cells
Photoelectric sensing systems
Optical fibres
Joint Initiative of IITs and IISc - Funded by MHRD
Module 6- Lecture 39
NPTEL >> Mechanical Engineering >> Modeling and Control of Dynamic electro-Mechanical System
Module 6- Lecture 39
Types of Optical Sensors (light to electricity etc.)
etc )
 Photoresistor
 Photoemitter
 Photovoltaic & Photodiodes
 Optical Fibre
 Sensors with negligible sensor dynamics
NPTEL >> Mechanical Engineering >> Modeling and Control of Dynamic electro-Mechanical System
Light Dependent Resistors (LDR)
Photoresistor
ORP12 – A typical photoresistor
The response of a photoresistor is as shown below:
Module 6- Lecture 39
NPTEL >> Mechanical Engineering >> Modeling and Control of Dynamic electro-Mechanical System
Module 6- Lecture 39
Light Dependent Resistors (LDR)
Photoresistor
An LDR or a photoresistor
A
LDR
h
i
h
has a resistance which changes based on the i
hi h h
b d
h
amount of visible light that falls on it.
The light falling on the zigzag lines on the sensor (usually made of Cadmium Sulphide), causes the resistance of the device to fall. This is known as a negative co‐efficient LDR. There are some LDRs that work in the opposite way i.e. their resistance There
are some LDRs that work in the opposite way i e their resistance
increases with light (called positive co‐efficient LDR). NPTEL >> Mechanical Engineering >> Modeling and Control of Dynamic electro-Mechanical System
Module 6- Lecture 39
Use of LDR
+
L
D
R
L
D
V
R
The figire above shows how to measure the change of light by using LDR ‐
This needs two resistors in series. You need to measure the voltage across the LDR that is subjected to light. The whole system works like a potential divider,
LDR that is subjected to light. The whole system works like a potential divider, and the voltage across the LDR is proportional to the current.
NPTEL >> Mechanical Engineering >> Modeling and Control of Dynamic electro-Mechanical System
Module 6- Lecture 39
Photoemitters
 Photoemissive cells ‐ dominant type of sensors ‐ used in sound tracking of films
 It is a vacuum device ‐ with Cesium as cathode, Nickel as anode
 Current
Current flow between the electrodes is proportional to the energy carried flow between the electrodes is proportional to the energy carried
by the light
 A threshold frequency exists
A threshold frequency exists
 Limited range of response ‐ UV easy to sense, Infra difficult most
 Used in series with a resistor, supply voltage 25‐100V
NPTEL >> Mechanical Engineering >> Modeling and Control of Dynamic electro-Mechanical System
Module 6- Lecture 39
Photovoltaic Cells
The original and still the most common semi‐conducting material used in PV cells is single crystal silicon. Most efficient type of PV cells, conversion up to 23%. Growing large crystals of silicon and then cutting them into thin (0.1‐0.3 mm) wafers is slow and expensive. Alternatives to single crystal silicon cells include poly‐crystalline silicon cells, a variety of "thin film" PV cells, and concentrating collectors. Poly‐crystalline silicon cells are less expensive to manufacture because they do not require the growth of large crystals. Unfortunately they are less efficient than single crystal cells (15 17%)
than single crystal cells (15‐17%). NPTEL >> Mechanical Engineering >> Modeling and Control of Dynamic electro-Mechanical System
Module 6- Lecture 39
Photovoltaic Cells
"Thin films" (0.001‐0.002 mm thick) of "amorphous" or uncrystallized silicon are another PV alternative. Amorphous silicon thin film PV cells are widely used in commercial electronics, powering watches and calculators. The problem with these cells is that they are not very efficient (12% in the lab, 7% for commercial cells), and they degrade with time.
Gallium arsenide (GaAs), Copper indium diselenide (CuInSe2), cadmium telluride (CdTe) and titanium dioxide (TiO2)have all been used as thin film PV cells, with various efficiencies and production costs. Titanium dioxide thin films, just recently developed, are very interesting because they are transparent and can be used as windows. NPTEL >> Mechanical Engineering >> Modeling and Control of Dynamic electro-Mechanical System
Module 6- Lecture 39
Photoelectric Sensing System
g y
Photoelectric sensing requires an emitter (to generate light) and a receiver (which "sees" the light from the emitter). Most photoelectric sensors use LEDs (Light Emitting Diodes) as a light source ‐ a solid‐state semiconductor, similar electrically to a diode, except that it emits a small amount of light when current flows through it in the forward direction. ll
t f li ht h
t fl
th
h it i th f
d di ti
LEDs can be built to emit green, blue, blue‐green, yellow, red, or infrared light.
The LED colors most commonly used in sensing are visible red and infrared. In applications which sense color contrasts, the choice of LED color can be important. Because LEDs are solid‐state, they will last for the entire useful life of a sensor.
f
NPTEL >> Mechanical Engineering >> Modeling and Control of Dynamic electro-Mechanical System
The figure below shows different coloured LEDs that can be used in a photoelectric system.
used in a photoelectric system. Module 6- Lecture 39
NPTEL >> Mechanical Engineering >> Modeling and Control of Dynamic electro-Mechanical System
Module 6- Lecture 39
The figure above is showing how a photoemitter and receiver combination could be used for sensing an object. This is a very common sensor used in automated plants.
Photoelectric receivers typically use one of three light‐sensitive electronic elements. Photoelectric
receivers typically use one of three light sensitive electronic elements
The phototransistor is the most widely used receiver opto‐element in industrial photoelectric sensor design, because they offer the best tradeoff between light sensitivity and response speed.
sensitivity and response speed.
NPTEL >> Mechanical Engineering >> Modeling and Control of Dynamic electro-Mechanical System
Module 6- Lecture 39
Optical Fibre: Method of Sensing
When an optical fibre is subjected to perturbations of different kinds, it experiences geometrical (size, shape) and optical (refractive index, mode conversion) changes depending upon the nature and the magnitude of the
conversion) changes depending upon the nature and the magnitude of the perturbation. In fibre optic sensing, this response to external influence is magnified so that the res lting change in optical radiation can be sed as a meas re of the
the resulting change in optical radiation can be used as a measure of the external perturbation. The optical fibre serves as a transducer and converts parameters like temperature, stress, strain, rotation or electric and magnetic currents into a corresponding change in the optical radiation.
Since light is characterized by intensity, phase, frequency and polarization, any one or many of these parameters may undergo a change during the motion of a dynamic system.
NPTEL >> Mechanical Engineering >> Modeling and Control of Dynamic electro-Mechanical System
Module 6- Lecture 39
Extrinsic vs. Intrinsic Sensors
Extrinsic sensors are those which use fiber to supply light to a sensing Extrinsic
sensors are those which use fiber to supply light to a sensing
device and return signal light to a detection system, intrinsic sensors use the fiber itself as the transducer. NPTEL >> Mechanical Engineering >> Modeling and Control of Dynamic electro-Mechanical System
Module 6- Lecture 39
Intrinsic Sensors
This type of sensor may be used to measure the force being exerted between the two objects. As the pressure increases the fibre will become slightly deformed and experience increased micro‐bending losses which results in a decrease in the light i
i
d i
b di l
hi h
lt i d
i th li ht
intensity received at the detector. Diffraction grating
m = d Sin(), m=0,1,2,3..
NPTEL >> Mechanical Engineering >> Modeling and Control of Dynamic electro-Mechanical System
Module 6- Lecture 39
Special References for this lecture
 Wikipedia - http://en.wikipedia.org/wiki/
http://en wikipedia org/wiki/
 www.sensorsmag.com
 Modern Control Systems – Dorf and Bishop, Addison-Wesley
Joint Initiative of IITs and IISc ‐ Funded by MHRD
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