In this paper we describe the design and implementation of a new versatile, scalable and cost-effective sensate surface. The system is based on a new conductive inkjet technology, which allows capacitive sensor electrodes and different types of RF antennas to be cheaply printed onto a roll of flexible substrate that may be many meters long. By deploying this surface on (or under) a floor it is possible to detect the presence and whereabouts of users through both passive and active capacitive coupling schemes. We have also incorporated GSM and NFC electromagnetic radiation sensing and piezoelectric pressure and vibration detection. We report on a number of experiments which evaluate sensing performance based on a 2.5m x 0.3m hardware test-bed. We describe some potential applications for this technology and highlight a number of improvements we have in mind.

1. Leveraging Conductive Inkjet Technology
to Build a Scalable and Versatile Surface
for Ubiquitous Sensing
Nan-Wei Gong1,2, Steve Hodges2, Joseph A. Paradiso1,2
1MIT Media Lab, Responsive Environments Group, Cambridge, USA
2Microsoft Research Cambridge, Sensors and Devices Group, Cambridge, UK
13th ACM International Conference on Ubiquitous Computing
September 17-21, 2011

2. Sensate Surface - Electronic skins as Dense
Sensor Networks
Sensate Media - Multimodal Electronic Skins as Dense Sensor Networks, Paradiso,
J.A., Lifton. J., and Broxton, M., BT Technology Journal, Vol. 22, No. 4, October 2004,
pp. 32-44.
ChainMail – A Configurable Multimodal Lining to Enable Sensate Surfaces and Interactive
Objects, Mistree, B.F.T., and Paradiso, J.A., in Proc. of TEI 2010, Cambridge MA,
January 25-27, 2010, pp. 65-72.
S.N.A.K.E.: A Dynamically Reconfigurable
Artificial Sensate Skin (MS) August 2006
J. Lifton et al. “Experiences and Directions in Pushpin Computing”
Symposium on Information Processing in Sensor Networks (IPSN05).
Works from the Responsive Environments Group at MIT Media Lab

3. Flexile and stretchable electronics
Roll-to-roll process, widely used on LCD/OLED connector circuit
for providing extra elasticity to the connection and embeds/holds the back panel ICs
JainK,Klosner M,Zemel M,Raghunandan S “FlexibleElectronics and Displays: High-‐Resolution, Roll-to-Roll, Projection
Lithography and Photoablation Processing Technologies for High-‐Throughput Production “(2005) Proc IEEE93:1500–1510

4. Low-cost Flexible Electronics
(a) Copper-on-Kapton substrate (allflexinc.com)
(b) Conductive inkjet flex technology (conductiveinkjet.com/)
(c) Inkjet printed electronics using metallic nanoparticle ink (T-ink.com)
~＄100 USD (30 cm2) ~＄10 USD (30 cm2) Depending on the
Material used

5. Using the body for signal transmission
Your noise is my command: sensing gestures using the body as
an antenna. Gabe Cohn, Daniel Morris, Shwetak N. Patel, and
Desney S. Tan. CHI 2011.
EMS Synthi AKS, 1971
(picking up electric hum coupled in human body)
Zimmerman, T. G., Personal Area Networks: Near-field
intrabody communication in IBM Systems Jour-nal, vol. 35,
nos. 3&4, 1996, pp. 609-617.
DiamondTouch: a multi-user touch technology.
Paul Dietz and Darren Leigh. UIST 2001.
Passive signal detection
Active signal transmit and receive

6. Sensate Floor Systems
Lee Middleton et al., A Floor Sensor System for Gait
Recognition, AUTOID '05. Washington, DC, USA, 171-176.
a prototype floor sensor as a gait recognition system:
1536 individual sensors arranged in a 3 x 0.5 m
strip with an individual sensor area of 3 cm2.
6 x 10 foot mat surface atop a
matrix of 64 pressure-sensitive
piezoelectric (PVDF) wires,
measures the position and
intensity of footsteps, turning
them into MIDI note events.
Paradiso, J., Abler, C., et al., The Magic
Carpet: Physical Sensing for Immersive
Environments. CHI 1997. ACM Press, pp.277-
278.
Z-tiles: an array of force-sensitive
resistors on each node to detect
pressure, and that pressure
information is output by way of a
self-organized network formed by the
floor nodes
Richardson, et.al., “Z-Tiles: building blocks for modular, pressure-sensing floor spaces.
Human factors and computing systems, Vienna, Austria, pp. 1529–1532 (2004)

7. Our Hybrid Approach
30 cm
30cm
Traditional rigid PCB + flexible PCB
solder points

8. Overall Architecture and Construction
-GND
-SDA
-GND
-SCL
-GND
-HEART BEAT
-GND

9. Physical topology

10. Sensing Modalities
•Passive mode
•Active mode
•Microphonic pickups
Capacitive
•NFC square loop antenna
•¼ wavelength GSM antenna
UHF / HF
•Piezoelectric pick-upPiezoelectric

11. PCB module circuitry

12. Basic operation of each slave sensor unit

13. • The simplest and the most power-efficient
mode for tracking people moving across the
surface.
• Signals can distinguish walking direction,
heal strikes and mid-swing -useful
information for gait analysis.
•The four different colors in the right-hand
figures represent the signals from the four
different electrodes in one sensing tile.
Capacitive Sensing - Passive Mode

14. Gait Signatures from Passive Mode
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.

15. Capacitive Sensing – Active Mode
• Two possible scenarios when a user’s body
comes into the electric field between transmit
and receive electrodes – transmit mode and
shunt mode.
Sample and average the charging and discharging
amplitude change (voltage) for 32 cycles
Joseph A. Paradiso and Neil Gershenfeld, Musical Applications of
Electric Field Sensing, Computer Music Journal 21(2), Summer 1997,
pp. 69-89.

16. Capacitive Sensing – Active Transmit Mode
• Transmit mode dominates when
direct contact with the transmit
electrode.
(a) The user was touching the transmit
electrode and moved from towards
electrode (1). The strength of the signal
pick-up is plotted as a function of
distance.
(b) The electrode pattern of a single tile,
where the electrode marked by the red
dot served as the transmitter.
(c) Signal pickup on all the receive
electrodes as a function of time

17. Capacitive Sensing – Active Shunt Mode
• The testing environment was set up on the
floor, with ~4cm of high-dielectric constant
foam on top of the sensors to avoid transmit
mode.
• This effect is less marked than the passive
sensing results (around 4 bits of resolution).
• During each step, the user effectively blocks
the electromagnetic field flux, hence the signal
drop: (a) heel strikes and (b) mid-swing.
• The red dots mark the transmit electrodes.

18. Piezoelectric Pickup
• Piezoelectric sensors was integrated for low
power, passive detection of pressure and
vibration.
• The signal can be used to trigger wake up of
the microcontroller from a low power sleep
node.
•Also infer the weight of a person and provide
insight into gait dynamics.
•Vibration from adjacent units is perceptible.
Images from Digikey.com

19. 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.

20. (a) Signal response versus sensing unit location when a mobile device is
held 1m from the surface. (b) Illustration of the experimental setup. (c)
Close up of the antenna. (d) signal strength versus distance
Cellular signals versus localization and identification
NFC signal GSM signal

21. Example Applications
• Localization
– Capacitive Sensing (~0.3 m), GSM (~0.8 m), NFC (~2m), Piezoelectric sensor
(~ 0.8m away)
• Identification
– Gait signature
– Signals from the cellular network
• Gesture recognition
• On-body information transmission
/exchange
– Active tags on the shoe
– Active signal transmission through
capacitive sensing

22. Conclusions and Future Works
• Conclusions
– Our work presented a low-cost scalable and versatile
distributed sensate surface based on a new conductive
inkjet printing technology.
– We demonstrated the design and implementation of
passive and active capacitive sensing, coupled with GSM
and NFC RF signal pickup – all based on copper electrodes
and antennas printed on the substrate.
– Pilot studies showed promising results which could
change the way we think about covering large areas with
sensors and associated circuitry.
• Future work
– designing minimum circuitry for direct surface mount.
– experimenting manufacturing processes for fast assembly.
– Develop a sheet of modular printed sensors and circuitry
which can be scalable and adaptive for various
applications.