A sensor is a transducer whose purpose is to sense (that is, to detect) some characteristic of its environs. It detects events or changes in quantities and provides a corresponding output, generally as an electrical or optical signal; for example, a thermocouple converts temperature to an output voltage. But a mercury-in-glass thermometer is also a sensor; it converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube. Sensors are used in everyday objects such as touch-sensitive elevator buttons (tactile sensor) and lamps which dim or brighten by touching the base, besides innumerable applications of which most people are never aware. With advances in micromachinery and easy-to-use microcontroller platforms, the uses of sensors have expanded beyond the more traditional fields of temperature, pressure or flow measurement. Moreover, analog sensors such as potentiometers and force-sensing resistors are still widely used. Applications include manufacturing and machinery, airplanes and aerospace, cars, medicine and robotics. A sensor's sensitivity indicates how much the sensor's output changes when the input quantity being measured changes. Some sensors can also have an impact on what they measure; for instance, a room temperature thermometer inserted into a hot cup of liquid cools the liquid while the liquid heats the thermometer. Sensors need to be designed to have a small effect on what is measured; making the sensor smaller often improves this and may introduce other advantages. Technological progress allows more and more sensors to be manufactured on a microscopic scale as microsensors using MEMS technology. In most cases, a microsensor reaches a significantly higher speed and sensitivity compared with macroscopic approaches. The sensitivity is then defined as the ratio between output signal and measured property. For example, if a sensor measures temperature and has a voltage output, the sensitivity is a constant with the unit [V/K]; this sensor is linear because the ratio is constant at all points of measurement. For an analog sensor signal to be processed, or used in digital equipment, it needs to be converted to a digital signal, using an analog-to-digital converter.

1. Seminar on
PHYSICAL EFFECTS FOR
SENSORS
By
Sabeel Irshad B

2. Contents:
Introduction
Resistive effects
Inductive effects
Capacitive effects
Charge Generating Effects

3. INTRODUCTION
 A sensor is a transducer whose purpose is to sense (that is,
to detect) some characteristic of its environment.
 Output of a sensor is generally as an electrical or optical
signal
 It is the effects which causes the changes that the sensor is
going to measure.
 Effects is nothing but the changes in the environment
which helps in measuring a quantity
 Property or quantity to be measured is called measurand.

4. Resistive Effects:
 The measurand directly or indirectly alters the
electrical resistance of a resistive element.
 Resistive sensors: A resistive sensor is a transducer or
electromechanical device that converts a mechanical
change such as displacement into an electrical signal.
 Resistivity is resistance stated in terms of length and
cross-sectional area as shown in the equation
Resistance = (Resistivity * Length)/Area
 Changing the value of one of the above factor
changes the resistance

5. Influence of temperature
• Thermistor is a type of resistor whose resistance varies
significantly with temperature.(Thermal + resistor).
• There are two categories made depending on resistance will
either increase or decrease
– NTC: Resistance will decrease with the increase in
temperature.
• Rise in temperature increases the charge carriers promote the electrons
into the conduction band decreases the resistance.
– PTC: Resistance will increase with the increase in
temperature.
• Potential barriers are formed causing the resistance to increase .
• At high temp material reverts to NTC behavior.

6. Thick and thin film resistors
Thin film: (0.1um or smaller)
Resistive layer is sputtered on to the ceramic
base.
Laser trimming is used to create patterns to
increase the resistive path and calibrate the
resistance values

7. Thick Film: (100um)
 Most used resistors
 Come usually as chip resistors(SMD)
 The resistance material is a special paste with a mixture of
binding, a carrier & metal oxide is deposited.
 Paste is deposited at 850 C, becomes glass like after cooling.
 Layers are added to increase the resistance value.
 All the above three are used to measure the engine temperature.

8. Length and angular proportionality of
resistors:
Potentiometer Sensors:
 Most commonly used as position sensor
 It has a wiper contact linked to a mechanical shaft that can be either angular
(rotational) or linear (slider type) in its movement
 Which causes the resistance value between the wiper/slider and the two end
connections to change giving an electrical signal output.
 The output voltage signal is taken from the wiper terminal of the sliding
contact
 Accelerator pedal, throttle valve, fuel tank level

9. Piezoresistive
Piezoresistivity in metals and semiconductors
 Piezoresistive effect: The Piezoresistive effect is the
change in the electrical resistivity of a semiconductor
or metal when mechanical strain is applied.
 Strain changes the inner atomic spacing affecting the
band gap.
 Electrons are raised to conduction band, this results in
change in the resistivity of material.
 Piezoresistors are resistors made from a piezoresistive
material and are usually used for measurement of
mechanical stress.

10. Strain-gage resistors
A strain-gage resistor is a resistor in which the
resistance changes with strain
It is literally glued on to the device where you
want to measure strain.
When the device is under stress it may elongate or
compress changing the resistance.
It is sensitive to smaller change in geometry.
Pressure sensors(ABS) ,force sensors

11. Magnetic field dependency
Magnetoresistance
 Magnetoresistance is the property of a material to change the value of its
electrical resistance when an external magnetic field is applied to it.
 The magnetoresistive sensors are based on the magnetoresistive effect. This
effect changes the resistivity of a current carrying ferromagnetic material
due to a magnetic field.
 The resistance depends on the angle formed by the internal magnetization
vector (M) of the ferromagnetic material and the direction of the current (I)
flow
 Speed or delivery angle measurement in injection pumps.

12. Light Dependency
Semiconductor Photoresistor
 A photoresistor or light-dependent resistor (LDR)
or photocell is a light-controlled variable resistor.
 The resistance of a photoresistor decreases with increasing
incident light intensity; in other words, it
exhibits photoconductivity.
 A photoresistor is made of a high resistance semiconductor.
 In the dark, resistance is as high as a few mega ohms (MΩ).
 While in the light, resistance is as low as a few hundred ohms.
 Rain sensors, dirt sensors

13. Inductive Effects
Effects of Faraday’s law
Whenever there is a relative motion between
conductor & magnetic field, flux linkage with
the coil changes, this change in the flux
induces voltage in the coil.

14. Induced voltage sensors
Alternator
An alternator is an electrical generator that
converts mechanical energy to electrical energy in
the form of alternating current
Rotating magnet and stationary armature either
way can also be used.
Works on Faraday’s law.

15. Wiegand Effect
 It is a nonlinear magnetic effect
 Named after its discoverer John R. Wiegand
 Produced in specially annealed and hardened wire
called Wiegand wire.
 During manufacture, to give the wire its unique
magnetic properties, it is subjected to a series of
twisting and untwisting operations to cold-work the
outside shell of the wire while retaining a soft core
within the wire, and then the wire is aged.
 The result is that the magnetic coercivity of the outside
shell is much larger than that of the inner core

16. Cont’d
If a magnet is brought near the wire, the high
coercivity outer shell excludes the magnetic field
from the inner soft core until the magnetic
threshold is reached, whereupon the entire
wire — both the outer shell and inner core —
rapidly switches magnetization polarity.
 This switchover occurs in a few microseconds,
and is called the Wiegand effect.
Wiegand-wire core can increase the output
voltage of a magnetic field sensor by several
orders of magnitude as compared to a similar coil
with a non-Wiegand core.
Used in Wheel speed sensors

17. Two Modes of Magnetic Excitation
Symmetric Switching
 Symmetric switching of
Wiegand wire occurs
when alternating positive
and negative magnetic
fields of equal strength are
used to magnetize and
trigger the wire.

18. Asymmetric Switching
 Asymmetric switching
of Wiegand takes place
when the wire is
magnetized and
triggered by magnetic
fields of opposite
polarity but unequal
strength.

19. Capacitive Effects
The capacitance between two plates is determined by three things:
 Size of the plates: capacitance increases as the plate size
increases
 Gap Size: capacitance decreases as the gap increases
 Material between the plates (the dielectric): Dielectric material
will cause the capacitance to increase or decrease depending on
the material
 Acceleration sensor, yaw rate sensor, pressure sensor.

20. Charge Generating Effects
Piezoelectric Effects:
 Piezoelectric effect is the ability of certain materials to
generate an electric charge in response to applied
mechanical stress.
 Imp characteristics of piezoelectric effect/material is it is
reversible.
 Knock sensor, airbag sensor, yaw-rate sensor.

21. Pyroelectric Effects:
Pyroelectricity (from the Greek pyr, fire, and
electricity) is the ability of certain materials to
generate a temporary voltage when they are
heated or cooled.
IR sensor

22. Photoelectric charge generation
The photoelectric effect is the observation that
many metals emit electrons when light shines
upon them.
Electrons emitted in this manner can be called
photoelectrons.
electrons are dislodged by the photoelectric effect
if light reaches or exceeds a threshold frequency

23. CCD and CMOS Image sensors
 Found in Digital cameras
 Responsible for converting light into electrical signals
CCD( Charge coupled Device)
Used in earlier digital cameras to turn images from analog
light signal into digital pixels.
More expensive than its CMOS counterparts
CMOS
Uses transistors at each pixel to move charge through
traditional wire.
Offers flexibility because each pixel is treated individually.
cheaper

24. Conclusion
All the above effects are very important and are
the base of the sensor measuring technology.
In most cases, a micro-sensor reaches a
significantly higher speed and sensitivity
compared with the one with a macroscopic
approach.
Sensor must be sensitive to the measured property
Insensitive to any other property likely to be
encountered in its application

25. References:
 http://en.wikipedia.org/wiki/Capacitive_sensing
 http://en.wikipedia.org/wiki/Wiegand_effect
 http://www.nanomotion.com/piezo-ceramic-motor-
technology/piezoelectric-effect/
 http://en.wikipedia.org/wiki/Piezoelectricity
 http://www.steves-digicams.com/knowledge-center/how-tos/digital-
camera-operation/ccd-vs-cmos-whats-the-difference.html#b
 http://electronics.howstuffworks.com/cameras-
photography/digital/question362.htm
 http://en.wikipedia.org/wiki/Photoelectric_effect