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Sensors 2
KON-C2004 Mechatronics BasicsRaine Viitala 21.11.2019
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Lecture topics
Strain based sensors – force, torque, pressure
Magnetic sensors
Temperature sensors
Inertial sensors
Microelectromechanical systems
Sensor fusion etc.
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Force & torque
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Strain gauge
Resistance changes when
the gauge deforms
Result affected by temperature
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http://en.wikipedia.org/wiki/Strain_gauge
𝑹 = 𝝆𝒍
𝑨
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Wheatstone bridge for strain gauges
Zero centered, more sensitive output
Temperature compensating possible
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http://www.allaboutcircuits.com/vol_1/chpt_9/7.htmlhttp://www.allaboutcircuits.com/vol_1/chpt_9/7.html
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Piezoelectricity
Mechanical strain generates
electric charge in material- Or charge generates strain
Very high dynamics
Sensors and actuators
Oscillators- Quartz clock
- Microprocessors
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http://en.wikipedia.org/wiki/Piezoelectricity
http://upload.wikimedia.org/wikipedia/commons/thumb/3/3a/PiezoBendingPrinciple.gif/640p
x-PiezoBendingPrinciple.gif
Piezo ~ "pressure," from
Greek piezein "to press tight, squeeze,"
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Piezoresistivity
Mechanical strain induces a change in semiconductor’s
electrical resistivity- Highly temperature sensitive
• Can be compensated for
- Larger change than in normal strain gages
• More sensitive sensors
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Force sensors (load cell)
Strain gauge- Measures deformation
- Stable, linear
Piezoelectric load cells- High dynamics, very sensitive
- Small size
- Charge leaks -> measurement drifts
• No static measurements
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http://www.appmeas.co.uk/load-cells-and-force-sensors.html
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Torque sensors
Often strain gage based
Also encoder based measurement
of axle torsion
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http://www.brighthubengineering.com/machine-design/65871-how-to-
measure-torque-requirements-for-gear-box/
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Pressure sensors
Piezoresistive- Most common
Piezoelectric- High dynamics. Drifting due to leakage.
Strain gauge
Capacitive
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Hall effect
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Hall effect
External magnetic field causes
a voltage in the sensor
- Current sensors
- Magnetic field sensors
Very high dynamics
Robust sensors since no contact- Integrated into one chip
Also magnetoresistivity- Change in external magnetic field induces
a change in material’s electrical resistivity
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Magnetic sensors
Magnetic switch- Mechanical or electrical
- Magnetic field intensity
- On/off
• Motor commutation
Magnetometer- Senses magnitude and direction of
external magnetic field
- Electric compass
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Magnetic encoders
• Hall sensors sense the direction or the change of the
magnetic field
• E.g. gear tooth recognition
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Temperature & imaging
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Temperature
Thermocouple- Change in voltage, a few µV per ̊ C
- Temperature up to 2000 ̊ C, fast response
Resistance temperature sensor/detector (RTD)- Change in resistance (Pt100 100 ohm at 0 ̊ C )
- High accuracy, linear response
- Usually for temperatures below 600 ̊ C
Thermistor (NTC/PTC) (negative/positive temperature coefficient)
- Resistance changes. Exponential response -> limited range
- Temperature up to 1700 ̊ C
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http://sensorwiki.org/doku.php/sensors/temperature
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Pyroelectricity
Change in temperature (infrared
radiation) causes
voltage difference in certain
materials
Application: Passive Infrared
(PIR) motion sensors
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https://en.wikipedia.org/wiki/Passive_infrared_sensor#/media/File:Motion_detector.jpg
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Contactless temperature measurement
All objects emit black body radiation
Benefits from contactless sensing- Moving objects
- Short response time
- Temperatures over 1300°C
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Thermal imaging
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Hyperspectral imaging
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https://hbr.org/2014/02/what-drones-and-crop-
dusters-can-teach-about-minimum-viable-product
https://en.wikipedia.org/wiki/Hyperspectral_imaging
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Misc.
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Building automation
Humidity
Temperature
Light intensity
Carbon dioxide
Air speed
Radiation
Smoke detector
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Other sensors
Electrical – voltage, current
Light – intensity, spectrum
Material concentration – pH, gas detectors etc.
Flow – air or liquid
Radiation – geiger counter
Acoustic
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Indirect measurements
Strain -> Pressure
Pressure ->Hydraulic force
Force -> Mass
Acceleration -> Position/velocity
Pressure -> Fluid velocity
Back emf voltage -> Motor speed
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Inertial
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Acceleration sensors
Navigation, inclination, vibration,
input devices, screen orientation
Based on- Piezoelectricity
- Piezoresistivity
- Capacitive sensing
http://en.wikipedia.org/wiki/Piezoelectric_accelerometer
http://www.leancrew.com/all-this/2012/05/accelerometers/
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Example: wearable sensor
Accelerometer measures
body vibrations- Heart rate
- Voice
Lots of signal filtering- FFT etc.
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http://newatlas.com/heart-acoustic-sensor-monitor/46506/
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Gyroscope
Measures rate of angular rotation- Bias stability (drift), angle random
walk, maximum rate of change
Navigation, platform stabilization
Principles- Mechanical – complex, expensive, very accurate
- MEMS – low cost & power, often not very accurate (drift >10 deg/h)
- Optical – accurate, medium cost, shock tolerant
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http://www.sensorwiki.org/doku.php/sensors/gyroscope
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MEMS sensors
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MEMS
Microelectromechanical system- Part dimensions 1-100 µm
Low power
Produced like electronic circuits- Mass manufacturing -> cheap
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https://www.maximintegrated.com/en/app-notes/index.mvp/id/5418
http://rmmechanical.blogspot.fi/2010/11/micro-electro-mechanical-systems-mems.html
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Lithography
Deposit
Mask
Etch
Repeat
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http://www.tf.uni-kiel.de/matwis/amat/elmat_en/kap_6/backbone/r6_6_1.html
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MEMS gyroscopes
Cheap, small, low power
Error sources- Bias error causes drift 1-1000° /h
- Acceleration (vibration)
• Tilting causes acceleration
- Shocks
- Exceeding range
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http://www.bosch-sensortec.com/de/homepage/products_3/6_axis_sensors_2/inertial_measurement_unit_1/inertial_measurement_unit_2
http://www.memsjournal.com/2011/01/motion-sensing-in-the-iphone-4-mems-gyroscope.html
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MEMS acceleration sensor
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MEMS example: Iphone 4 gyroscope
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https://www.ifixit.com/Teardown/iPhone+4+Gyroscope+Teardown/3156
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MEMS example: Iphone 4 gyroscope
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https://www.ifixit.com/Teardown/iPhone+4+Gyroscope+Teardown/3156
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eardown/3156
http://www.memsjournal.com/2011/01/motion-sensing-in-the-iphone-4-mems-gyroscope.html
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MEMS example: Iphone 4 gyroscope
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http://www.memsjournal.com/2011/01/motion-sensing-in-the-iphone-4-mems-gyroscope.html
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Sensor fusion
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Inertial measurement unit (IMU)
2-11 degrees of freedom (DOF)- 1-3 axis accelerometer
- 1-3 axis gyroscope
- 1-3 axis magnetometer
- ”Extra” sensors
• Pressure (altitude from atmospheric pressure)
• GPS
Sensor fusion
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Sensor fusion
Combine several sensors to increase precision
http://www.edn.com/design/sensors/4402401/2/Motion-sensors-de-mystified
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Kalman filterAlgorithm to get ”optimal” results from several sensors
Left: Accelerometer, Middle: Kalman fusion, Right: Gyroscope
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Sensor fusion for positioning
Absolute measurement
(GPS)
vs.
incremental (odometry)
Odometry= data from
position sensors such as
encoders in wheels
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Summary
Everything that can be converted to voltage can be measured
Absolute vs incremental sensors
MEMS sensors are cheap and getting more accurate
Sensor fusion
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