P a p e rs
`
`May 7 1 1 1995
`
`- CHI ' 95 M O S A IC OF CREATIVITY
`
`Applying Electric Field Sensing to
`Human-Computer Interfaces
`
`Thomas G. Zimmerman, Joshua R. Smith, Joseph A. Paradise, David Allport', Neil Gershenfeld
`
`MIT Media Laboratory - Physics and Media Group
`20 Ames Street E15-487
`Cambridge, Mass 02176-4307
`(617) 253-0620
`tz,jrs,joep,dea,neilg @media.mit.edu
`
`ABSTRACT
`A non-contact sensor based on the interaction of a person
`with electric fields for human-computer
`interface is
`investigated. Two sensing modes are explored: an
`external electric field shunted
`to ground
`through a
`human body, and an external electric field transmitted
`through a human body
`to stationary receivers. The
`sensors are
`low power (milliwatts), high
`resolution
`(millimeter) low cost (a few dollars per channel), have
`low latency (millisecond), high update rate (1 kHz), high
`immunity to noise (>72 dB), are not affected by clothing,
`surface texture or reflectivity, and can operate on length
`scales from microns to meters. Systems incorporating the
`sensors include a finger mouse, a room that knows the
`location of its occupant, and people-sensing furniture.
`Haptic feedback using passive materials is described.
`Also discussed are empirical and analytical approaches to
`transform
`sensor measurements
`into
`position
`information.
`
`input device, gesture
`interface,
`KEYWORDS: user
`interface, non-contact sensing, electric field.
`
`INTRODUCTION
`(EF) based human-
`field
`Our research on electric
`computer
`interfaces (HCI) grew out of a project to
`instrument Yo-Yo Ma's cello [8]. We needed to measure
`bow position in two axes with minimum impact on the
`instrument and its playability. In this paper we discuss
`two types of EF sensing mechanisms: shunting, where an
`external EF is effectively grounded by a person in the
`field; and transmitting, where low frequency energy is
`coupled into a person, making the entire Ixxly an EF
`emitter. The benefits of each sensing mechanism are
`
`1. Visiting scientist from HP Labs, Bristol, England.
`
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`CHI' 95, Denver, Colorado, USA
`© 1995 ACM 0-89791-694-8/95/0005...$3.50
`
`280
`
`to other sensing
`presented along with comparisons
`means. We
`report on
`several EF
`systems
`and
`applications, designed by arranging the size and location
`of EF transmitters and receivers, and suggest some future
`applications.
`
`Since electric fields pass through non-conductors, passive
`materials that apply force and viscous friction may be
`incorporated into EF sensing devices, providing haptic
`feedback. We have constructed a pressure pad and a
`viscous 3-D workspace based on this principle.
`
`PREVIOUS WORK
`The first well-known use of EF sensing for human-
`machine
`interface was Leon Theremin's musical
`instrument. Two omnidirectional antennas were used to
`control the pitch and amplitude of an oscillator. Body
`capacitance detunes a resonant tank circuit [7]. The effect
`of body capacitance on electric circuits was well known
`to radio's pioneers, who saw the effect as an annoyance
`rather than an asset.
`
`As the need for electronic security and surveillance
`increases, there is growing use of remote (non-contact)
`occupancy and motion detectors. Sensing mechanisms
`include capacitance, acoustic, optoelectronic, microwave,
`ultrasonic, video, laser, and triboelectric (detecting static
`electric charge) [5]. Many of these mechanisms have
`been adapted to measure the location of body parts in
`three dimensions, motivated by military cockpit and
`virtual reality (VR) applications [15].
`
`Acoustic methods are line-of-sight and are affected by
`echoes, multi-paths, air currents,
`temperature, and
`humidity. Optical systems are also line-of-sight, require
`controlled lighting, are saturated by bright lights, and
`can be confused by shadows. Infrared systems require
`significant power to cover large areas. Systems based on
`reflection are affected by surface texture, reflectivity, and
`incidence angle of the detected object Video has a slow
`update rate (e.g., 60 Hz) and produces copious amounts
`of data that must be acquired, stored, and processed.
`
`MICROSOFT EXHIBIT 1006
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`C H I ' 95 M O S A IC OF CREATIVITY
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`P a p e rs
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`regulation
`Microwaves pose potential health and
`problems. Simple pyroelectric systems have very slow
`response times (>100 msec) and can only respond to
`ch^ging signals. Lasers must be scanned, can cause eye
`damage, and are
`line-of-sight. Triboelectric sensing
`requires the detected object to be electrically charged.
`
`Mathews [14] developed an electronic drum that detects
`the 3-D location of a hand-held
`transmitting baton
`relative to a planar array of antennas by using near-field
`signal-strength measurements. Lee, Buxton, and Smith
`[13] use capacitance measurement to detect multiple
`contacts on a touch-sensitive tablet. Both systems require
`the user to touch something.
`
`Capacitive sensors can measure proximity without
`contact. To assist robots to navigate and avoid injuring
`humans, NASA has developed a capacitive reflector
`sensor [22] that can detects objects up to 30 cm away.
`The sensor uses a driven shield to push EF lines away
`from grounding surfaces and towards the object. Wall
`stud finders use differential capacitance measurement to
`locate wood behind plaster boards by sensing dielectric
`changes [6]. Linear capacitive reactance sensors are used
`in industry to measure the proximity of grounded objects
`with an accuracy of 5 microns [4]. Electrical impedance
`tomography places electrode arrays on the body to form
`images of tissue and organs based on internal electric
`conductivity [21].
`
`Weakly electric fish (e.g., Gymnotiformes, sharks, and
`catfish) are very sophisticated users of electric fields [1].
`These fish use amplitude modulation and spectral
`changes to determine object size, shape, conductivity,
`distance, and velocity. They use electric fields for social
`communication, identifying sex, age, and dominance
`hierarchy. They perform jamming avoidance when they
`detect the beating of their field with an approaching fish:
`the fish with the lower transmit frequency decreases its
`frequency, and the fish with the higher frequency raises
`its frequency. Some saltwater weakly electric fish have
`adapted their sensing ability to detect EF gradients as low
`as 5nV/cm.
`
`Given this long history of capacitive measurement, one
`might wonder why EF sensing is not common in human-
`computer
`interfaces. But
`it
`is only
`recently
`that
`inexpensive
`electronic
`components
`have
`become
`available to measure the small signals produced by EF
`sensors. Also non-uniform electric fields have made it
`difficult to transform these signals into linear position
`coordinates. Our research addresses these issues to help
`make EF sensing more accessible to interface designers.
`
`It will be shown that EF sensors provide ample resolution
`and that converting the EF signal strength into position
`is the more challenging task.
`
`MODES OF OPERATION
`The Human Shunt
`An electrical potential (voltage) is created between an
`oscillator electrode and a virtual ground electrode (Figure
`1). A virtual ground is an electrical connection kept at
`zero potential by an operational amplifier, allowing
`current Ir
`to ground to be measured. The potential
`difference induces charge on the electrodes, creating an
`electric field between the electrodes. If the area of the
`electrodes is small relative to the spacing between them,
`the electrodes can be modeled as point charges producing
`dipole fields. The dipole field strength varies inversely
`with distance cubed. In practice the measurable field
`strength extends approximately
`two dipole
`lengths
`(distance
`between
`the
`transmitter
`and
`receiver
`electrodes). As the electrodes are moved farther apart, a
`larger electrode area is required to compensate for the
`decrease in signal strength.
`
`When a hand, or other body part, is placed in an electric
`field the amount of displacement current h reaching the
`receiver decreases. This may seem counter-intuitive since
`the conductive and dielectric properties of the hand
`should increase the displacement current. However, if an
`object is much larger than the dipole length, the portion
`of the object out of the field serves as a charge reservoir,
`which is what we mean by "ground". The hand intercepts
`electric field lines, shunting them to ground, decreasing
`the amount of displacement current h
`reaching
`the
`receiver.
`
`± .
`
`Ih
`
`Figure 1. An electric dipole field created between an
`oscillating transmit electrode and virtual ground receiver
`electrode is intercepted by a hand. Displacement current
`to ground decreases as the hand moves further into the
`dipole field.
`
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`- C HI ' 95 M O S A IC OF C R E A T I V I TY
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`The Human Transmitter
`Low frequency energy is capacitively coupled into a
`person's body, making the entire person an EF emitter
`(Figure 2). The person can stand on, sit on, touch, or
`otherwise be near the oscillator electrode. One or more
`receiver electrodes are placed about the person. The
`displacement current into a receiver Ir increases as the
`person moves closer to that receiver. At close proximity,
`the person and the receiver electrode are modeled as ideal
`flat plates, where displacement current varies with the
`reciprocal of distance. At large distances, the person and
`the receiver electrode are modeled as points, where
`displacement current varies with
`the reciprocal of
`distance squared.
`
`Figure 2. Energy from an oscillator Is coupled into a
`person standing on the transmit electrode making the
`person an electric field emitter. As the person moves any
`body part closer to the grounded receive electrode, the
`displacement current into the receiver !„ increases.
`
`Mode Crossover
`When a hand (or any large object relative to the dipole
`length) approaches the dipole field of Figure 1 (shunt
`mode), the displacement current h decreases. When the
`hand gets very close (much less than a dipole spacing)
`the displacement current h begins to increase; the system
`changes from shunt mode to transmit mode. Actually
`both modes occur simultaneously, the hand is always
`coupling some field to the receiver (transmit mode) but
`until the hand is very close to the electrodes, the amount
`of displacement current shunted away from the receiver
`exceeds the amount coupled into the receiver.
`
`SYSTEiVI HARDWARE
`Signai Detection Strategy
`Many capacitance detection schemes [5, 6, 13] measure
`the charging time of a resistor-capacitor (RC) network.
`The capacitance and displacement currents for EF
`sensing are on the order of picofarads (10"'^ farad) and
`nanoamps (10"' amps), requiring more sophisticated
`detection strategies. A synchronous detection circuit
`(Figure 3) is used to detect the transmitted frequency and
`reject all others [10], acting as a very narrow band-pass
`filter. Other detection methods
`include
`frequency-
`
`282
`
`modulation chirps (as used in radar), frequency hopping,
`and code modulation (e.g., spread spectrum).
`
`The displacement current can be measured with
`approximately 12 bits accuracy (72 dB) using
`the
`components shown in Figure 3. There is a trade-off
`between update rate (sample rate/number of samples
`averaged) and accuracy
`(signal-to-noise ratio). The
`signal-to-noise ratio increases as the square root of the
`number of samples averaged. For example, averaging 64
`samples increases the signal-to-noise a factor of eight
`(+18 dB), with a corresponding 1/64 update rate.
`
`Information can be coded in the modulated transmitter
`signal. A multitude of small EF sensing devices can be
`scattered about a room, like eels in a murky pond,
`transmitting measurements to neighboring devices with
`the same EF used to measure proximity. The jamming
`avoidance mechanism of weakly electric fish [1] suggests
`that
`such devices can adjust
`their
`transmission
`frequencies autonomously when new devices are
`introduced into the sensing space.
`
`lOOkH!
`
`cumm
`symhronais
`ampl^tr
`deleclor
`Figure 3. Synchronous detection circuitry.
`
`low-passftller
`output gain
`
`Small displacement currents require good shielding,
`however the capacitance of shielded coaxial cable is
`orders of magnitude greater that the capacitance between
`electrodes. Cable capacitance low-pass filters the received
`signal, typically limiting the operating frequency to 30
`kHz, and introduces a phase shift that is compensated for
`in the synchronous detector (not shown). Placing the
`current amplifier at the receiver electrode allows higher
`frequencies, limited by the amplifier's slew rate. For
`example, attaching the receive electrode directly to the
`TL082 current amplifier allows an operating frequency of
`220 kHz.
`
`Transmitter Power
`The frequency range we use for EF HCI is 10 kHz to 200
`kHz. Below this range, displacement currents and update
`rates are too small. Above this range FCC power
`regulations become more stringent [3]. The distance
`between electrodes is a fraction of a wavelength, so no
`appreciable energy is radiated. The only power consumed
`by the transmitter is the energy required to charge the
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`capacitance of the transmitter electrode to the oscillating
`voltage.
`
`In practice the transmitter power is less than a milliwatt.
`This allows the design of very low power systems with no
`radio interference. By adding an inductor, the transmitter
`can be driven
`into
`resonance, decreasing energy
`dissipation and increasing the transmitter potential, for
`example 60 volts from a 5 volt supply. A
`larger
`transmitter potential
`increases
`the strength of
`the
`received signal and therefore the signal-to-noise ratio,
`producing greater spatial resolution. Transmit signal
`strength can be increased until the current amplifier is
`saturated.
`
`"Fish" Evaluation Board
`To assist researchers in exploring EF HCI, our group has
`produced a small microprocessor-based EF sensing unit,
`supporting one transmitter and four receivers (Figure 4).
`It is called a "fish" after the amazing EF abilities of
`weakly electric fish, and because fish can navigate three
`dimensions while a mouse can navigate only two. The
`evaluation board supports MIDI, RS-232, and RS-485
`serial communication protocols. We are currently
`designing a "smart
`fish," a second generation EF
`evaluation board utilizing a digital signal processor to
`allow automatic calibration and the exploration of more
`complex detection strategies, such a spread spectrum.
`The smart fish also measures the power loading of the
`transmitter to disambiguate mode crossover. Transmitter
`loading
`is monotonic;
`the current drawn from the
`transmit electrode always
`increases as an object
`approaches the transmit electrode.
`
`ELECTRiC FIELD GEOMETRY
`The value returned by a sensor is unfortunately not
`directly proportional to the distance between a hand and
`
`Figure 4. Fish electric field sensing evaluation board.
`
`the sensor. Recovering information such as the (x,y,z)
`position of a hand from three sensor values (r,s,t) is a
`non-trivial problem. Solving the problem requires a
`
`model of the electric field geometry. The absolute signal
`strength depends on the coupling of the person to a
`reference (ground for shunt mode and the
`transmit
`electrode for transmit mode). This coupling acts as a
`global system gain. The relative signal strength of the
`sensors contains the position information. For this reason
`normalized sensor readings are used to calculate position
`information.
`
`There are two basic strategies for creating this model. In
`the analytical approach, knowledge of electrostatics
`(Laplace equation) is used to derive, for a given sensor
`geometry, an expression for the signals received as a
`function of hand position. The expression is then
`inverted analytically or numerically.
`In the empirical
`approach, signals are measured for a variety of known
`hand positions, and a function (e.g., radial basis
`function) that converts sensor values to hand positions is
`fit to the resulting data set. The analytical approach
`provides insight into the behavior of the sensors and does
`not require a training phase. However, any given
`analytical solution is applicable only for a particular
`sensor geometry, and different sensor geometries require
`new solutions. The empirical approach is more flexible,
`because changes in the sensor layout or environment can
`be acconunodated by retrainmg.
`
`Since our measurements occur within a fraction of a
`wavelength, we are in the near-field limit where the
`electric field is the gradient of the potential across the
`electrodes, so we can
`treat
`the situation as an
`electrostatics problem [12]. The same physics applies for
`electrode spacing that ranges from microns to meters.
`Small electrode spacing has been used
`to measure
`position with micron resolution [19]; large electrode
`spacing has been used to measure the location of a person
`in a room. We are not interested in the absolute values of
`sensor values; we care only about
`their functional
`dependence on the position of the body part we are
`measuring.
`Since the human body is covered with
`conductive, we treat the body as a perfectly conducting
`object. The hand is treated as a grounded point in space.
`In practice, the finite area of a hand and its connection to
`an arm serves to blur or convolve the ideal point
`response. But this point approximation usually works
`well as long as the real hand is a constant shape, the
`same convolution is being applied everywhere, and so the
`basic functional form of the hand response will be the
`same as that of the point response.
`
`In-Plane Measurements
`Figure 5 shows a contour plot of the predicted received
`signal, calculated using the classic dipole field expression
`[12] for a hand moving around a Z plane 0.9 dipole units
`above the dipole axis. A dipole unit is the distance
`
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`between the transmit electrode and receive electrode. The
`predicted contour compares well to data collected ly
`moving a grounded cube (2.5 cm on each side) across the
`plane.
`
`proximity resolution, albeit with a corresponding slower
`update rate. The fish evaluation board used in these
`measurements has an integration tune constant of 10
`milliseconds. Figure 6 plots proximity resolution as a
`function of distance Z for shunt mode. At 85 mm
`distance, proximity resolution is 1 mm.
`
`Figure 5. Contours of electric field strength in plane Z =
`0.9 (measured in units of the dipole spacing) predicted by
`analytical model.
`
`Out-Of-Plane Measurements
`The
`relationship between hand proximity Z and
`displacement current Ir is measured using an electrical
`equivalent of a hand and arm suspended above the center
`of a dipole. The term proximity is used to emphasize that
`EF sensing measures the integrated (convolved) effect of
`an object in the electric field. When a hand is placed near
`a dipole, the hand, arm, and body attached to the arm all
`affect the field, though each contributes less as they are
`progressively farther away from the dipole.
`
`is an
`The surrogate hand and arm combination
`aluminum tube 7.6 cm in diameter and 48.3 cm long and
`is grounded through a suspending wire fw shunt mode
`and connected to an oscillator for transmit mode. The
`transmit and receiver electrode, each measuring 2.5 cm x
`2.5 cm, are 15.2 cm apart on center. A least squares fit of
`the data reveals the following functional form for both
`shunt and transmit modes;
`
`IR= A + — j= Vz
`
`where A and B are constants determined by electrode
`geometry, detection circuit gain and bias, oscillator
`frequency and voltage, and Z is distance above the dipole.
`For shunt mode B is negative since displacement current
`Ir decreases as the object moves closer to the dipole.
`
`in
`is expressed as the change
`Proximity resolution
`distance Z that produces a 6 dB change in displacement
`current IR over the noise floor (two times the noise floor).
`The resolution is dependent on the signal-to-noise ratio
`of the detection system, which is a function of integration
`time. The longer the data is averaged, the greater the
`
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`
`50
`
`150
`100
`Distance (mm)
`Figure 6. Proximity resolution of a surrogate arm in
`shunt mode plotted as a function of distance from dipole
`to arm. Resolution is the change in distance that
`produces a 6dB change in signal over noise.
`
`200
`
`250
`
`Imaging: Converting Signals To Position
`Each dipole measures a degree of freedom, either object
`position or size. A single dipole cannot distinguish a
`close small object from a large distant object, as both
`might block the same number of field lines. A second
`dipole operating on a
`longer
`length-scale
`(greater
`electrode spacing) can be used to distinguish these two
`situations, or to measure two spatial coordinates of a
`single fixed-size object. Three dipoles can measure the 3-
`D position of an object of fixed size, or determine the 2-D
`position and size of an object. Four dipoles can determine
`the size and 3-D position of an object. Five dipoles can
`determine the 3-D position, size, and elongation of an
`object We are working on the continuum limit of adding
`more dipoles, to perform low-resolution imaging.
`
`Optimal Sensor Placement
`Each receiver measurement constrains the position of a
`small object (relative to dipole spacing) to an ellipsoid
`centered on the dipole axis (see Figure 5). The dipoles
`should be oriented orthogonally in order to minimize the
`sensitivity of the solution (x,y,z) to errors in (r,s,t). The
`problem of inverting the sensor readings is equivalent to
`the geometrical problem of finding the intersection points
`of these ellipsoids. Often additional constraints (prior
`knowledge) must be imposed to select one solution from
`the many symmetric cases that are consistent with the
`data. For example, to make a two-dimensional mouse
`using only two dipoles, we must impose the constraint
`that the hand is on one side of the dipoles.
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`synthesizers and sound sources, creates a complex sonic
`terrain based on the location of the person, allowing
`navigation of a sonic environment.
`
`Smart Chair
`A chair is fitted with one transmitter in the seat and four
`receivers: two located in the headrest to measure head
`rotation, and one at each armrest to measure hand
`proximity. A person in the chair navigates multiple audio
`channels by head and hand placement [16]. The sensors
`are mounted underneath the chair fabric, so they are
`invisible to the user. Smart chairs may be used to control
`radio fimctions in a car, home audiovisual equipment, or
`simply to turn off a computer monitor when a user leaves
`a workstation.
`
`In another application, a transmitter is installed in a
`chair to allow the magicians Penn & Teller to perform
`music by waving their arms near four receivers. Hand
`position controls various sound parameters produced by
`computer-controlled sound synthesizers.
`
`Haptic Feedback in 3-D Space
`A foam pad is placed on top of a dipole pair. Pressing on
`the foam produces a force feedback. Since force is
`proportional
`to position
`(Hooke's
`law), and
`finger
`position is measured by EF sensing, finger force is
`measured. A passive piece of foam on an EF sensor is a
`pressure sensor.
`
`A plastic box is fitted with electrodes on three sides to
`measure hand position in 3-D. The box is filled with
`bird-seed (millet) to provide a viscous medium for haptic
`feedback. The seed allows users to rest their hand in
`space, reducing fatigue, and provides something to grab.
`Slight compression of
`the seed
`increases viscosity.
`Perhaps a computer-controlled piston, bearing on a
`movable wall of the box, could provide a simple way to
`simulate an environment with variable viscosity.
`
`FUTURE APPLICATIONS
`Researchers are currently exploring direct manipulation
`of instrumented real objects to facilitate 3-D orientation
`and manipulation [9, 17]. Electric field sensors may be
`incorporated in objects to measure object deformation,
`position, and orientation.
`
`The Tailor project [20] allows disabled individuals to run
`computer applications by mapping the unique anatomical
`movement ability of each individual to control signals.
`Combining EF sensing with such mapping techniques
`could provide a person in a wheelchair with individually
`tailored, unobtrusive, invisible, low-power, and low-cost
`computer and machine interfaces.
`
`Hermetically sealed EF sensors in a palm top could
`determine when the case is open, when the unit is being
`held, and could create a large control space around the
`small device. Foam EF buttons could provide force and
`tactile feedback, detect
`finger approach and
`finger
`pressure, and distinguish between slow and fast presses.
`
`Multiple transmitters and receivers, multiplexed in time,
`frequency, or by coding sequence, could be placed under
`a carpet to determine the number and location of people
`in a room. When an electrode under a person is activated,
`that person becomes the EF source. Smart floors can be
`used for multi-participant VR simulations without the
`burden of wires or the complexities of video cameras.
`
`Attempts have been made to instrument whiteboards
`using video cameras [11] and optoelectronics [2]. Both
`systems require rear imaging to record stylus movement.
`A conventional plastic whiteboard can be fitted with an
`array of EF sensing electrodes to measure the location of
`a metal-cased marker in the hand of a shunting or
`transmitting person.
`
`Watches have a very small woikspace and very little
`energy capacity. An EF sensor can be used to create a
`large workspace over a small watch face. Such watch
`controllers can be used
`to search
`through audio
`databases.
`
`CONCLUSION
`We have discussed some HCI systems and future
`applications of EF based sensing. The near-field nature
`of low-frequency electric fields allows the same detection
`scheme to be scaled from microns to meters. EF sensing
`provides high resolution proximity
`information. The
`difficulty is converting proximity to position. We have
`worked out an analytical method to correct for the non-
`uniform nature of dipole fields. Empirical methods may
`be used to compensate for complex field distortion caused
`by dielectrics or conductors in the field. Some of EF
`sensing's greatest qualitative appeals are the sense of
`magic, simplicity, and "naturalness" it brings to an HCI.
`The abilities of weakly electric fish to perform object
`detection, communicating, and
`jamming
`avoidance
`demonstrate what is possible with EF sensing. The
`authors know of no other sensing mechanism or system
`that can deliver non-contact sensing with millimeter
`resolution at kilohertz sample rates and millisecond lag
`times for a few dollars a chaimel. As computing power
`leaps off the desk and into a multitude of small battery-
`powered devices, the need for low-power unobtrusive
`interfaces grows. It is our belief that EF sensing can
`make a significant contribution to the sensing abilities of
`computing machines.
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`COMPARISON TO OTHER SENSOR TECHNOLOGIES
`Electric field sensors detect a bulk effect, integrating the
`body's interception of EF. Unlike optical system, the
`effect does not depend on object surface texture and
`reflectivity. The data from EF sensors is continuous with
`a resolution limited by transmission strength and noise
`rejection. There is an economy of data; only three
`channels are required to locate a hand in 3-D. In
`comparison, a video camera produces an abundance of
`data, on the order of 75 megabits per second, while
`updating at 60 Hz. An EF system operating at 100 kHz
`can average 100 samples, provide a 1 kHz update rate,
`with 1 millisecond lag time.
`
`Electric field systems can be extremely small, light-
`weight and low power, as required by the ever shrinking
`real estate and energy capacity of lap, palm, and watch
`based computers. Since electric fields penetrate non-
`conductors, sensors can be hidden, providing protection
`from weather and wear, as well as adding an element of
`magic to the interface.
`
`SYSTEM CONFIGURATIONS AND APPLICATIONS
`The transmit method provides large receive signals,
`operates over large areas, and can distinguish multiple
`persons. Capacitively coupling energy into a person
`requires continuous close contact with the person. We
`have used transmit electrode ranging from 5 to 150
`square cm, depending on proximity to the person. The
`transmit electrode can be incorporated into the seat of a
`chair, a section of a floor, the back of a palm computer,
`or a wristwatch band. Direct conductive contact with the
`person's skin requires a much smaller electrode area (<5
`square cm). Asymmetric placement of receiver electrodes
`helps decouple signal strength from position calculations.
`
`The shunt method does not require close contact with a
`person. For each dimension, a minimum of one receiver
`is required. Prototyping interfaces is basically an "arts
`and crafts" project, consisting of cutting out electrodes,
`typically aluminum foil and copper tape, taping them
`down, and wiring them up to the fish evaluation unit.
`
`Figure 7. Two-dimensional finger-pointing mouse.
`
`286
`
`2-D FInger-PolntIng Mouse
`We have implemented a two-dimensional finger-pointing
`mouse on a laptop computer (Figure 7). The input device
`is activated by touching a small transmitter electrode
`with the fourth (little) finger of the left hand. Energy is
`coupled into the person, and the EF emitted from the
`pointing finger is sensed at two receiving electrodes. A
`thin uniform copper strip running across the top of the
`screen senses Y position, and a tapered strip along the
`side of the screen senses X position. The taper renders
`the electrode more sensitive to the EF emitted by the
`pointing finger and less sensitive to the field emitted by
`the arm. The shaped electrode physically implements an
`analog spatially varying signal gain. A third small
`receiving electrode, placed below the spacebar, allows the
`thumb of the left hand to generate click signals.
`
`The pointing finger does not need to be in contact with,
`or even close to the screen, thereby avoiding screen
`smudges and occlusion of the cursor by the pointing
`finger. Position sensing is easily disabled by lifting the
`forth finger off the transmitting electrode, the equivalent
`of lifting and putting down a mouse, facilitating relative
`position control.
`
`Smart Table
`To demonstrate the concept of "smart furniture," a co-
`linear dipole pair (i.e., receiver, transmitter, receiver) is
`placed underneath a wooden table to measure hand
`gestures. A computer screen displays an electronic
`newspaper whose pages are
`flipped
`forward and
`backward by sweeps of a hand across the table (X-axis).
`Placing the hand down on the table (Z-axis) advances to
`the next section, lifting the hand up displays the previous
`section. Gestures are detected by applying a threshold to
`the X and Z velocities. Position
`in
`the X-axis
`is
`approximated by differencing the two receiver signals;
`position in the Z-axis is approximated by the sum of the
`receiver signals.
`
`into a
`table
`turn a
`An array of dipoles can
`multidimensional digitizing and gesture input device.
`Such an EF sensing matrix may substitute for or augment
`a video camera for video desk applications [18]. Perhaps
`visual ambiguities and occlusions could be arbitrated by
`EF sensing, indicating hand location to the video analysis
`system.
`
`Person-Sensing Room
`In an installation piece at the MIT Media Lab, a single
`transmitter electrode covers the entire floor of a room,
`coupling energy into a person walking on the floor. Four
`receiver electrodes, located on the walls, measure relative
`signal strength, indicatmg the location of the person. A
`computer
`program,
`controlling
`a multitude
`of
`
`

`

`C H I ' 95 M O S A IC OF CREATIVITY
`
`- May 7 1 1 1995
`
`P a p e rs
`
`ACKNOWLEDGMENTS
`The authors would