Resistive Sensors

This was a talk about Resistive sensors I gave at IIT Roorkee

1. Resistive Type of Sensors -
Their Analysis and Applications
Debasmit Das
10115039
Batch : E3

2. Introduction
A resistive sensor is a transducer or electromechanical device
that converts a mechanical change such as displacement into an
electrical signal that can be monitored after conditioning.
Resistive sensors are among the most common in
instrumentation.
These Transducers do NOT generate electricity. Hence, they are
called passive devices.
The simplest resistive sensor is the potentiometer.
Other resistive sensors include strain gauges, thermocouples,
photoresistors and thermistors.

3. Theory of Operation
Resistance = (Resistivity * Length)/Area
The resistance of a material depends on four factors:
· Composition
· Length
· Temperature
· Cross Sectional Area
•To change the resistance of a material, you must change the value of one of the
above factors.
•When length is modified the change in resistance is direct. If you double the
material’s length, it’s resistance doubles. When the cross sectional area is
modified the change in resistance has an inverse effect, IE R = k/A. If you
double the cross-sectional area of wire, its resistivity is cut in half.
•But, Changes in composition and temperature do not change the resistivity of a
material in such a simple way.

4. Examples of Resistive Transducers
Sliding contact devices
Wire resistance strain gauge
Thermistors
Thermocouples
Light Dependent Resistors (LDRs)
Device Action Application
Light Dependent ResistorResistance falls with increasing 
light level
Light operated switches
Thermistor Resistance falls with increased 
temperature
Electronic thermometers
Strain gauge Resistance changes with force Sensor in an electronic 
balance
Moisture detector Resistance falls when wet Damp meter

5. Sliding contact devices
There is a long conductor whose effective length is variable.
One end of the conductor is fixed, while the position of the other end
is decided by the slider or the brush that can move along the whole
length of the conductor along with the body whose displacement is to
be measured.
When the body moves the slider also moves along the conductor so
its effective length changes, due to which it resistance also changes.
These devices can be used to measured linear as well as angular
displacement.

6. Construction of rotary and slider types
 The unit consists basically of a ‘track’ having a fixed resistance and a variable contact
which can be moved along and make continuous contact with the track.
If the track resistance is proportional to the length along the track (i.e. linear track), the
output voltage will be proportional to the movement of the variable contact and the unit
is suitable for use as a position transducer.
The track may comprise a film of carbon formed on a substrate or may be a length of
resistance wire wound on an insulator former.

7. Applications: Potentiometer
The Potential divider is the most obvious application. In its simplest form it is two resistors in series
with an input voltage Vs across the ends.
If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat.
Potentiometers were formerly used to control picture brightness, contrast, and color response in
Television sets.
Low-power potentiometers, both linear and rotary, are used to control audio equipment, changing
loudness, frequency attenuation and other characteristics of audio signals.
Potentiometer
Potential Divider Circuit

8. Strain Gauge
If a strip of conductive metal is placed under compressive force
(without buckling), it will broaden and shorten.
If these stresses are kept within the elastic limit of the metal strip
(so that the strip does not permanently deform), the strip can be
used as a measuring element for physical force, the amount of
applied force inferred from measuring its resistance.
This is the principle of a Strain Gauge.

9. Gauge Factor
The gauge factor is defined as:
where
R is the change in resistance caused by strain,
RG is the resistance of the undeformed gauge,
and
is Strain

Also expression for Strain is,
= L/L
For metallic foil gauges, the gauge factor is usually a little over 2

10. Half Bridge Strain Gauge Circuit
Unlike the Wheatstone bridge using
a null-balance detector and a
human operator to maintain a state
of balance, a strain gauge bridge
circuit indicates measured strain by
the degree of imbalance, and uses a
precision voltmeter in the center of
the bridge to provide an accurate
measurement of that imbalance:

11. Strain Gauge
With no force applied to the test specimen, both strain gauges have equal
resistance and the bridge circuit is balanced. However, when a downward force is
applied to the free end of the specimen, it will bend downward, stretching gauge
#1 and compressing gauge #2 at the same time:

12. Applications : Strain Gauge
Strain gauges are used to measure force and small
displacements. They are used for analyzing the dynamic
strain of complex structures. They are used to measure
tension, torque etc.
Types of strain gauges are:
(a) Wire strain gauges
(b) Foil strain gauges
(c) Thin film
(d) Semiconductor

13. Thermistors
Thermistors work on the principle that resistance of
some materials changes with the change in their
temperature.
When the temperature of the material changes, its
resistance changes and it can be measured easily
and calibrated against the input quantity.
The commonly used thermistors are made up of the
ceramic like semiconducting materials such as
oxides of manganese, nickel and cobalt.
Thermistors can be used for the measurement of
temperature, as electric power sensing devices and
also as the controls for various processes.

14. Thermistors
The most common type of thermistor that we use has a
resistance that falls as the temperature rises.
It is referred to as a negative temperature coefficient
device(NTC).
A positive temperature coefficient(PTC) device has a
resistance that increases with temperature.

15. Thermistor Analysis
where: T is temperature (in kelvin),
 
TRef is the reference temperature, usually at room temp. 
(25 °C; 77 °F; 298.15 K),
  R is the resistance of the thermistor (W),
  RRef is the resistance at TRef,
 
b is a calibration constant depending on the thermistor 
material, usually between 3,000 and 5,000 K.
The thermistor resistance-temperature relationship can be 
approximated by,
  
 

16. Thermistor Analysis
The graph of resistance against temperature is like 
this.
The resistance on this graph is on a 
logarithmic scale, as there is a large 
range of values. 

17. Applications of thermistors
Measurement of temperature
Measurement of Difference of two temperatures
Control of temperature
Temperature compensation
Thermal conductivity measurement.
Measurement of Gas Composition
Measurement of Flow
Current-limiting devices for circuit protection as replacement for 
fuse (PTC)

18. Thermocouple
The thermocouples work on the principle of Seebeck effect, Peltier effect and
Thomson effect.
As per the Seebeck effect, when two dissimilar elements are joined at their
ends the electromotive force exists at their junction.
As per Peltier effect, the amount of electromotive force generated depends on
the temperature of the junction
While, the Thomson effect says that the amount of voltage generated depends
on the temperature gradient along the conductors in the circuit.
The voltage output from the thermocouple changes as its temperature changes
or the temperature of the body in whose contact it is changes.
The voltage output is calibrated against the temperature of the body that can be
measured easily.
Thermocouple is a very popular device used for measurement of temperature.

19. Thermocouple Internal Circuit

20. Types of Thermocouples
A thermocouple is available in different combinations 
of metals or calibrations. 
The four most common calibrations are J, K, T and E. 
There are high temperature calibrations R, S, C and 
GB. 
Each calibration has a different temperature range 
and environment, although the maximum 
temperature varies with the diameter of the wire used 
in the thermocouple.

21. Applications : Thermocouple
Steel Industry
Heating Appliance Safety
Power Production : Thermoelectric Generation
Thermoelectric Cooling
Diesel Engines
Gas Turbine Exhaust Temperature Measurement

22. Temperature Variation of Resistive Sensors

23. Light Dependent Resistor
The light dependent resistor consists of a length of material (cadmium sulphide) 
whose resistance changes according to the light level.   
Therefore the brighter the light, the lower the resistance. 

24. Principle of Operation
An LDR is made of a high resistance semiconductor. 
If light falling on the device is of high enough 
frequency, photons absorbed by the semiconductor give bound electrons 
enough energy to jump into the conduction band. 
The resulting free electron (and its hole partner) conduct electricity, 
thereby lowering resistance.

25. LDR Applications
Smoke detection
 Automatic lighting
Clock Radios
Alarm systems
Dynamic Compressors
Solar Street Lamps
Camera Light meters

26. References
Wikipedia
http://www.eecs.berkeley.edu/~culler/WEI/labs/lab7-sensing/sensing.htm
http://www.ce-transducer.com/Resistance.asp
http://forum.onestopgate.com/forum_posts.asp?TID=3536
http://www.eee.metu.edu.tr/~koray/exp1.pdf
http://www.brighthubengineering.com/hvac/53335-variable-resistance-transducers/
http://ptuas.loremate.com/beee/node/5

27. THANK YOU