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Kocaeli University
Mechatronics Engineering Department
MKT 208 SENSOR TECHNIQUES
H.Metin ERTUNÇ, PhD
2012 Spring
Sensors and Transducers
• Sensor – converts physical
signals into electrical
signals.
• Transducer – converts
energy signals in one form
to another form
The term transducer is often used in place of the term sensor.
Example: A microphone is a sensor that perceive the sound.
A transducer converts sound waves into electrical current using inductor inside.
Therefore, we can accept these two words are synonyms.
Sensors
• Sensors are sensing elements that transforms
physical quantities frequently used in daily life
such as position, pressure, temperature, level,
flow, into another quantity, electrical signals.
• If we make an analogy between an industrial
system and human body, then the sensors are
the eyes and ears of the system.
Actuators
• Actuators are the devices that performs the reverse
operation of sensors.
• They convert the electrical signals into another from,
that is usually electrical signals.
• Actuators can be called as transducers, as well.
Measured quantities
•
Position, velocity, acceleration, displacement,
proximity, level, flow, pressure, temperature,
force, strain, torque, angle, light intensity, surface
roughness, density, mass, capacity,
electromagnetic flow, voltage, current, resistance,
humidity, vibration ….
Classification of sensors based on
measured quantities
• Mechanical : Length, area, quantity, mass flow, force, torque
(moment), pressure, velocity, acceleration, position, sound
• Thermal : Temperature, heat flow
• Electrical : Voltage, current, load, resistance, inductance,
capacitance, dielectric coefficient, polarization, electrical field
and frequency
• Magnetic : Field density, flux density, magnetic moment,
permeability
• Light (Optical) : Intensity, wavelength, polarization, phase
• Chemical : Density, content, pH, reaction speed
Classification of sensors based on
their outputs
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•
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Analog sensors
Digital sensors
Voltage output sensors
Current output sensors
Classification of sensors based on
their supply
• Active sensors
– They don’t need external power for their operation
– They produce signals based on the measured quantity
– Example: Thermocouple, photovoltaic cells (solar cells),
piezoelectric sensors.
• Passive sensors
– They need external power supply.
– They measure the change in the resistance, capacitance and
inductance based on the measured quantity.
– They transform and amplify the change of the quantities into
electrical signals.
A typical sensor application
Measured
quantity
Electrical
output
SENSOR
SIGNAL
CONDITIONING
voltage
A/D
CPU
Serial or
parallel
Various sensor applications
Automated sensor door
Metal detector
Basic definitions
• Range, Span: The interval between the min and max
values of measured quantities, span = max-min
• Error : Actual value– measured value
• Accuracy : The agreement between the actual value
and the measured value.
• Sensitivity: linear output/unit input
• Hysteresis error: producing different outputs for
increasing and decreasing inputs.
• Non-linearity error :.
• Repeatability/reproducibility: Variation of sensor
measurements when the same quantity is measured
several times.
Basic dfinitions (con’t)
• Stability: the output drift for constant input.
• Dead band/time: The range of input values for which
there is no output.
• Resolution: The change in measured variable to
which the sensor will respond
• Output impedance: The effect of electrical circuit
conneted to the sensor.
Resolution example
Static and Dynamic Characteristics
• Response time – time to 95% of
final value for step input
• Time constant – time to 63.2% (1e-1) of final value
• Rise time – time to rise some
specified percentage of s.s. output
• Settling time – time to get to within
2% of the s.s. value
Displacement, proximity, position,
(linear and angular)
• Proximity/limit switch (mechanical)
– Binary output
• Potentiometer (rotary or linear)
– Analog output
Microswitch™ Limit Switch
• LVDT, RVDT
– Variable transformer
LVDT
Joystick (2 pots)
Differential Transformer
Position sensors
Inductive Sensors
When a metal object approximate to a
magnetic filed, the inductive sensors
produce output.
Bottle cap control
Sliding and rotation measurement
for elevators
Revolution measurement
Position sensors
CAPACITIVE SENSORS
They can sense all the objects in the nature
Capacitive sensors are usually used for
unmetallic objects and level measurement
Uygulama alanları
Glass sensing
Object level measurement
Level measurement
from outside of the tank
Wooden sensing
(Non-Contact Sensors)
• Ultrasonic
Ultrasonic
Fluid Level Meter
• Optical
• Magnetic (Inductive, Reed, Hall Effect)
• Laser vibrometer
• Capacitive or Eddy current
– measuring vibration of rotating shafts
Encoders
for rotational position and velocity
4 Bit Absolute Encoder
50 pulses/rev Incremental
Toothed wheel (gear) + magnetic pickup + counter
500 pulses/rev,
Incremental Quadrature
Encoders
incremental
Absolute
Incremental and Absolute
Encoders
Application areas
Vibration
• Accelerometer
– Piezo-electric (AC)
– IC, Strain gage (DC)
Force/Torque
• Strain gage
Resistance  Strain
FORCE
• Piezo-electric (AC coupled)
• Piezo-resistive, piezo-ceramic
Pressure
• Microphone
(AC coupled)
• Diaphragm
(for static measurement)
• Tube, Bellows
• Manometer
Flow measurement
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•
•
•
•
•
•
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Orifice plate, venturi
Turbine meter
Float
Rotameter
Hot-wire anemometer
Laser interferometer
Pitot tube
Positive displacement meter (rotary
vane)
Temperature
• Thermometer
• Thermocouple
• Thermistor
• RTD (platinum)
•
•
•
•
Solid state sensor (thermodiodes and transistors)
Pyro-electric sensor
Bimetallic strip
Optical pyrometer
Optical sensors
• Photo-voltaic cell
• CdS sensor (R output)
• Phototransistor
Photoelectrical sensors
Through Beam sensors
Retroreflective type sensors
Diffuse reflective type sensors
Selection of sensors
• The nature of the measurement required
– The variable to be measured, its nominal value, the range of
values, the accuracy required, the required speed of measurement,
the reliability required, the environmental conditions under which
the measurement is to be made
• The nature of the output required from the sensor
– Signal conditioning
• Some factors must be accounted
– Range , accuracy, linearity, speed of response, reliability,
maintability, life, power supply requirements, ruggedness,
availability, cost.