Sensor Introduction
MAS S62 Crafting Material Interfaces
Oct 18 2011
Nan-Wei Gong
Responsive Environments Group
MIT Media Lab
A sensor is often defined as ~
a device that receives and responds to a signal or stimulus.
Fig. 1.1. Level-control system. A sight tube and operator’s eye form a sensor
(a device which converts information into electrical signal).
Jacob Fraden, Handbook of Modern Sensors – physics, designs and applications.
So… exactly what do we need to know about “sensors” in this class?
Input
Output
THE Black Box!!!
Da Bird cat teaser
Sensors
Electronics
Active: sonar, FSR…
Passive: photodiodes, piezo microphone
Analog / Digital electronics
http://www-scm.tees.ac.uk/users/a.clements/DSP/ADintro.htm
Output devices
Leds / speakers / displays
/computer ….
So… exactly what do we need to know about “sensors” in this class?
Input
Output
THE Black Box!!!
Da Bird cat teaser
Sensors
Electronics
Active: sonar, FSR…
Passive: photodiodes, piezo microphone
Analog / Digital electronics
http://www-scm.tees.ac.uk/users/a.clements/DSP/ADintro.htm
Output devices
Leds / speakers / displays
/computer ….
Before we begin, here’s a super quick overview about THE black box…
•
Resistors
–
–
–
–
THE Black Box!!!
•
Capacitors
–
–
Passive component:
Capable of operating without an external power.
Ex. Resistors, capacitors, inductors..etc
Active component:
Gauss’s Law
Capacitor in parallel / series
•
Inductors
•
Diodes
–
•
•
•
Zener diodes
Transistors
Op Amps
–
–
–
–
–
Requiring a source of power to operate
Ex. Transistors, Op-Amps, ICs.
More details can be found here:
{simonetti.media.mit.edu/MASS62/}
Ohm’s Law
Resistor in parallel / series
Voltage divider
Wheatstone bridge
Ideal model
Comparator / Schmidt trigger
Voltage follower
Non-inverting Amp / Inverting Amp
Summing / Differential Amplifier
and more!
Fig. 1.3. Positions of sensors in a data acquisition system. Sensor
1 is noncontact, sensors 2 and 3 are passive, sensor 4 is active,
and sensor 5 is internal to a data acquisition system.
Jacob Fraden, Handbook of Modern Sensors – physics, designs and applications.
Back to input devices…. All sensors may be of two kinds:
1. passive (directly generates electric signal) – piezo, photodiode…
2. active (need to apply external stimulus) – thermistor, strain gauge
A quick demo with a scope
Things that you want to know before deciding which sensor to use
- what “phenomena” you are interested in and what is the “range” of
the signal. Does the signal need “conditioning*” or “amplification**”??
Tables borrowed from Fraden’s sensor book
*,** see my note on basic electronics
What can we learn from a datasheet
Phenomena : Force
Range : several hundred gs for detecting touch
Passive? Active?
http://www.media.mit.edu/resenv/classes/MAS836/Readings/fsrguide.pdf
Dimension of the available sensors
Example Circuit Design
Example Circuit Design
Strong
response in
the lower
force(0~200g)
region
Most linear
and widest range
Sensing on a Surface – Pressure and Force
Force, Strain, and Tactile
a.
a. Piezoresistivity
b. Strain into Force
Strain is defined by s = dL/L
c. Displacement into pressure
E.g., F = -kx, and P = F/A (force per area)
http://media.digikey.com/photos/Measurement%20Specialties%20Photos/0-1004308-0.jpg
b.
http://www.openmusiclabs.com/learning/sensors/fsr/
http://www.omega.com/literature/transactions/volume3/strain.html
Some FSR-Bendy-Sensor Gloves
Mattel’s Power Glove
1989
FSR bendy sensor
Laetitia Sonami’s Lady’s Glove Immersion’s Cyber Glove
(STEIM, 1997)
Images from http://www.media.mit.edu/resenv/classes/MAS836/
Strain Gauges – measuring the strain of an object
need to be bonded onto a hard surface, so they can be forced into strain when the surface is
deflected. Soft materials won’t strain the gauge enough
Simple strain gauge
http://en.wikipedia.org/wiki/Strain_gauge
Remote Sensing – Detecting “the Field”
Remote Sensing
•
the acquisition of information about an object or phenomenon, without
making physical contact with the object.
•
Again, passive and active.
•
Examples of passive remote sensors include film photography, infrared,
charge-coupled devices, and radiometers.
Active collection, on the other hand, emits energy in order to scan objects
and areas whereupon a sensor then detects and measures the radiation
that is reflected or backscattered from the target. RADAR and LiDAR are
examples of active remote sensing where the time delay between emission
and return is measured, establishing the location, height, speed and
direction of an object.
•
http://en.wikipedia.org/wiki/Remote_sensing
Examples for the scope of this class
1.
Light sensor
Photoresistors
Photodiodes
Phototransistor
Color sensors
2.
“Range” sensor
Ultrasonic transceivers
IR proximity sensor
Acoustic transducers
3.
Electromagnetic Field Sensing
- Capacitive Sensing
- Radio Frequency Sensing
Light Detectors
http://www.advancedphotonix.com/
ap_products/pdfs/PDV-P9008.pdf
http://media.digikey.com/photos/Advanced%20Photonix%20Photos/PDV-P9203.jpg
CdS tends to like Yellow...
Photons knock electrons into conduction band. 1 photon can release 900 electrons Acceptor band keeps electron lifetime
high -> Lower Resistance with increasing light. Slow response...
•
CdS (Cadmium Sulfide) and CdSe (Cadmium Selenide) cells are common
Other photon sensors such as
-photodiodes
-phototransistor
-Photodiode ICs and color sensors (IC)
-See optical sensing note from MAS836
Non-contact Capacitive Sensing
C0
XMIT
Electrode
Receive
Electrode
Ct
Cr
Vout  ir
Body
it
50-100 Khz
25 V p-p
Cg
ig
ir
Transconductance
Amplifier
(FISH front end)
Equivalent circuit for all modes of electric field sensing
Loading Mode (measure I )
t
• User must contact transmitter
• User uniquely tagged
• Can use multiple frequencies; multiple users
• 2-object geometry
=> Best for accurate tracking
• Industrial (short range) proximity
• No contact with electrode
• 3-object geometry
=> Hard to do tracking
• Can “focus” w. tomograpy
=> Add more transceivers
• Single Electrode
• No cable to electrode
• Couples to everything
• Hard to adjust sens. area
• Used for everything
- Stud finders (pre MIR)
Theremins, buttons...
Theremin- capacitive sensing of users hand Invented by Leon
Theremin in Russia circa 1917-1920
First “successful” electronic musical instrument
Pitch control
Volume control
http://en.wikipedia.org/wiki/Theremin
http://www.youtube.com/watch?v=w5qf9O6c20o
Multi-target or Multi-sensor?
Example 1 : multi-target electromagnetic sensing
• Passive mode
Capacitive • Active mode
• Microphonic pickups
UHF / HF
• NFC square loop
antenna
• ¼ wavelength GSM
antenna
Nan-Wei Gong, Steve Hodges, and Joseph A. Paradiso.
2011. “Leveraging conductive inkjet technology to build a
scalable and versatile surface for ubiquitous sensing”
(UbiComp '11).
Capacitive Sensing for presence and gait detection
Passive Mode
Active Shunt Mode
Gait Analysis
Different signatures typically detected with the passive capacitive sensing
method. (a) Forefoot strike, (b) heel strike pattern (left feet), (c) and (d) mid-swing
between steps(right feet), detected by adjacent electrodes. The decay time is
from the RC response of the envelope detector.
Cellular signals versus localization and
identification
13.56MHz NFC square loop antenna
Cutouts on the electrode
eliminate Eddy currents that
would decrease
performance.
900/1800MHz ¼ wavelength
GSM antenna
• The pattern and signal strength of NFC are
consistent and can easily be used to determine range
by measuring peak thresholds.
• GSM signals have stronger signal response that
can infer longer distance tracking by integrating and
averaging the signal patterns.
Example 2 :Ultrasonic Thermometry
MAS.836 Final Project
(Martin A. Segado) 2011
• Speed of sound in ideal gas (~air) is
vs   RT
• Spaced ultrasound pair in feedback loop:
K
  vs f
 f  d n 2

• So, can compute temperature as:
T

f
R
R
2
2
Ideally, 1-Point
Calibration
Ultrasonic Thermometry
MAS.836 Final Project
(Martin A. Segado) 2011
Temperature Probe
Transmitter
Receiver (mostly
hidden by tape)
Temp.
Ultrasonic
Probe [F] Estimate [F]
75
75
91.2
79
124.5
84
77.6
75.3