Sensor Introduction MAS S62 Crafting Material InterfacesOct 18 2011

Nan-Wei GongResponsive 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?

THE Black Box!!!

Da Bird cat teaser

Input Output

Sensors Active: sonar, FSR…Passive: photodiodes, piezo microphone

Electronics Analog / Digital electronics

http://www-scm.tees.ac.uk/users/a.clements/DSP/ADintro.htm

Output devicesLeds / speakers / displays/computer ….

So… exactly what do we need to know about “sensors” in this class?

THE Black Box!!!

Da Bird cat teaser

Input Output

Sensors Active: sonar, FSR…Passive: photodiodes, piezo microphone

Electronics Analog / Digital electronics

http://www-scm.tees.ac.uk/users/a.clements/DSP/ADintro.htm

Output devicesLeds / speakers / displays/computer ….

Before we begin, here’s a super quick overview about THE black box…

THE Black Box!!!

• Resistors– Ohm’s Law– Resistor in parallel / series– Voltage divider– Wheatstone bridge

• Capacitors– Gauss’s Law– Capacitor in parallel / series

• Inductors• Diodes

– Zener diodes• Transistors• Op Amps

– Ideal model– Comparator / Schmidt trigger– Voltage follower– Non-inverting Amp / Inverting Amp– Summing / Differential Amplifier

• and more!

Passive component:Capable of operating without an external power.Ex. Resistors, capacitors, inductors..etc

Active component:Requiring a source of power to operateEx. Transistors, Op-Amps, ICs.

More details can be found here:{simonetti.media.mit.edu/MASS62/}

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

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**”??

A quick demo with a scope

Tables borrowed from Fraden’s sensor book

*,** see my note on basic electronics

What can we learn from a datasheet

http://www.media.mit.edu/resenv/classes/MAS836/Readings/fsrguide.pdf

Phenomena : ForceRange : several hundred gs for detecting touchPassive? Active?

Dimension of the available sensors

Example Circuit Design

Example Circuit Design

Most linearand widest range

Strong response in the lower force(0~200g)region

Sensing on a Surface – Pressure and Force

Force, Strain, and Tactile

a. Piezoresistivity

b. Strain into Force Strain is defined by s = dL/L

c. Displacement into pressureE.g., F = -kx, and P = F/A (force per area)

a.

b.

http://www.openmusiclabs.com/learning/sensors/fsr/ http://www.omega.com/literature/transactions/volume3/strain.html

http://media.digikey.com/photos/Measurement%20Specialties%20Photos/0-1004308-0.jpg

Immersion’s Cyber GloveLaetitia Sonami’s Lady’s Glove(STEIM, 1997)

Mattel’s Power Glove1989

Some FSR-Bendy-Sensor Gloves

FSR bendy sensor

Images from http://www.media.mit.edu/resenv/classes/MAS836/

Simple strain gauge

Strain Gauges – measuring the strain of an objectneed 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

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

• CdS (Cadmium Sulfide) and CdSe (Cadmium Selenide) cells are common

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...

Other photon sensors such as-photodiodes-phototransistor-Photodiode ICs and color sensors (IC)-See optical sensing note from MAS836

http://www.advancedphotonix.com/ap_products/pdfs/PDV-P9008.pdf

Non-contact Capacitive Sensing

• 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

Loading Mode (measure I )t

• Single Electrode• No cable to electrode• Couples to everything• Hard to adjust sens. area• Used for everything - Stud finders (pre MIR) Theremins, buttons...

C0

Cg

TransconductanceAmplifier

(FISH front end)

50-100 Khz25 V p-p

Ct Cr

XMITElectrode

ReceiveElectrode

Body Vout irir

ig

Equivalent circuit for all modes of electric field sensing

it

http://www.youtube.com/watch?v=w5qf9O6c20o

Theremin- capacitive sensing of users hand Invented by Leon Theremin in Russia circa 1917-1920 First “successful” electronic musical instrument

http://en.wikipedia.org/wiki/Theremin

Pitch control

Volume control

Multi-target or Multi-sensor?

Example 1 : multi-target electromagnetic sensing

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).

Passive Mode Active Shunt Mode

Capacitive Sensing for presence and gait detection

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

900/1800MHz ¼ wavelength GSM antenna

Cutouts on the electrode eliminate Eddy currents that would decrease performance.

• 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• Speed of sound in ideal gas (~air) is

• Spaced ultrasound pair in feedback loop:

• So, can compute temperature as:

sv RT

sv f 2 2

2f d nT f

R R

K

Ideally, 1-PointCalibration

MAS.836 Final Project (Martin A. Segado) 2011

Receiver (mostly hidden by tape)

Temperature Probe

Transmitter

Ultrasonic Thermometry MAS.836 Final Project (Martin A. Segado) 2011

Temp. Probe [F]

Ultrasonic Estimate [F]

75 75

91.2 79

124.5 84

77.6 75.3