Broadband Ultrasonic Location Systems for Improved Indoor Positioning
Mike Hazas and Andy Hopper
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Abstract—Ultrasonic location systems are a popular solution for the provision of fine-grained indoor positioning data.Applications include enhanced routing for wireless networks,computer-aided navigation,and location-nsitive device behavior.However,current ultrasonic location systems suffer from limitations due to their u of narrowband transducers.This paper investigates the u of broadband ultrasound for indoor positioning systems.Broadband ultrasonic transmitter and receiver units have been developed and characterized.The utilization of the units to construct two positioning systems with different architectures rves to highlight and affirm the concrete,practical benefits of broadband ultrasound for locating people and devices indoors.
Index Terms—Location-dependent/nsitive,pervasive and ubiquitous computing,support rvices for mobile computing.
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1I NTRODUCTION
I N the past15years,there has been an increasing rearch focus on developing systems which can perform location nsing.Positioning data of nodes can be ud in wireless network deployments to improve dynamic routing or to allow location-stamping of nsor data.By additionally tracking urs,location systems also enable a diver range of context-aware applications,allowing devices to more effectively rve the people using them.
The Global Positioning System(GPS)allows mobile receiver units to compute their position by measuring radio signal times-of-arrival from satellites[1].Typical accuracies are within a few meters,but GPS performance is riously hampered indoors.To address this shortcoming,a number of dedicated indoor location systems have been developed. All involve gathering data by nsing a real-world physical quantity and using it to calculate or infer a position estimate. Some of the earliest systems rely upon periodic infrared light puls nt between transmitter and receiver units[2], [3],[4].The infrared signal uniquely identifies the transmit-ting unit and,using the known location of the fixed units in the system,the position of mobile units can be estimated with room-scale granularity.More recently,location sys-tems bad upon radio signals have been introduced.Some utilize visibility or signal strength measurements of wireless LAN access points[5],while others rely upon specialized tags and infrastructure to perform radio signal time-of-flight measurements[6].Both types of systems have typical accuracies of three meters or more.
In contrast,fine-grained location systems are capable of supplying positioning information with centimeter-level accuracy,which can be ud to enhance existing applications as well as enable new ones.A number of fine-grained location-aware applications exist for workplace,home,and public spaces.Examples include moving maps,mobile desktop control,follow-me teleconferencing and multimedia streaming,activity annotation,ubiquitous ur interfaces, augmented reality,indoor environmental control,phone call forwarding,monitoring and support systems for the elderly, and muum tour guides.
Ultrasonic location systems have proven to be a relatively simple,effective solution for fine-grained indoor positioning.However,existing ultrasonic location systems for mobile computing all utilize narrowband ultrasound, forcing compromis in a number of aspects of system performance.
This paper explores the u of broadband ultrasound as the basis for fine-grained location nsing.First,previous fine-grained ultrasonic location systems are reviewed and their common limitations identified.Broadband ultrasonic transmitter and receiver units appropriate for u in prototype indoor positioning systems are then described and characterized.A suitable spread spectrum signaling method is outlined and its capacity for ranging is evaluated. This paper then reports experimental results taken using a polled,centralized location system and a privacy-oriented l
ocation system.The performance analysis of the two systems allows general conclusions to be drawn about the benefits of broadband ultrasonic location systems.
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A variety of fine-grained indoor location nsing solutions exist for mobile computing.In the HiBall system,ur-worn lateral effect photo diodes n arrays of flashing infrared LEDs mounted in the ceiling[7].Ultra-wideband radio location systems developed by Ubin Ltd.utilize receivers deployed in a building to track urs carrying small tags which emit ultra-wideband signals[8].The TRIP system employs computer vision techniques to track 2D circular bar code tags on urs and devices[9].
Although highly effective,fine-grained infrared and ultra-wideband systems employ specialized hardware and
.M.Hazas is with the Computing Department,Infolab,South Drive,
Lancaster University,LA14WA,United Kingdom.
E-mail:hazas@comp.lancs.ac.uk.
. A.Hopper is with the Computer Laboratory,University of Cambridge,
William Gates Building,JJ Thomson Ave.,Cambridge,CB30FD,United
Kingdom.E-mail:ah12@cam.ac.uk.
Manuscript received14Apr.2004;accepted23Jan.2005;published online
15Mar.2006.
For information on obtaining reprints of this article,plea nd e-mail to:
tmc@computer,and reference IEEECS Log Number TMC-0136-0404.
1536-1233/06/$20.00ß2006IEEE Published by the IEEE CS,CASS,ComSoc,IES,&SPS
are currently relatively expensive for many mobile comput-ing scenarios.Computer vision-bad approaches which u commodity cameras are potentially lower cost,but they have yet to be deployed on large scales.Thus,their true cost has not been assd and their ability to perform wide-area tracking of many people and devices in everyday environ-ments has not been confirmed.
A number of other rearch efforts have been directed toward ultrasonic location systems.Ultrasonic transducers are relatively inexpensive,and their signals typically have lower processing requirements than the solutions men-tioned above,resulting in simple,lower-cost systems which have proven to be effective for wide-area fine-grained positioning indoors.
2.1Ultrasonic Location Systems
In the Bat system[10],urs wear small badges which emit an ultrasonic pul when radio-triggered by a central controller.The system determines pul times-of-flight from the badges to a network of receivers on the ceiling and calculates the3D positions of the badges using a multi-lateration algorithm.The system yields location information with an accuracy of approximately3cm and an aggregate location update rate of150Hz is theoretically possible.
The Cricket system[11]employs beacons distributed throughout a building which each nd an RF signal describing the surrounding space while simultaneously nding an ultrasonic pul.The beacons broadcast at random times in order to minimize signal collisions. Devices called listeners,carried by mobile urs,receive the RF and ultrasonic signals and calculate their approx-imate distance from the beacon using time-of-flight meth-ods.The first implementation of the system
could localize urs to within an area of about one square meter,but by using a greater density of beacons,the authors were able to achieve between5and25cm accuracy[12].Since25distance samples were gathered from each beacon to minimize the effects of reflections and ultrasonic noi,a single location update would take an average of over5conds to produce. The listeners can independently locate themlves,avoiding the potential compromis of ur privacy involved with centralized systems.
Similarly,Randell and Muller describe a system[13] which allows wearable and mobile computers to autono-mously compute their position.Four ultrasonic transducers are placed at the corners of a square on the ceiling and are wired to a controller.The controller nds a radio trigger and then issues an ultrasonic pul from each of the four transducers in succession.A mobile receiver unit, synchronized by the radio trigger,measures the ultrasonic pul times-of-flight from which it estimates its location with3D accuracies between10and25cm.The update rate of the system is veral hertz.
2.2Limitations of Narrowband Ultrasonic Location
Systems
The above three systems all utilize narrowband ultrasonic transducers for their ranging measurements.Three inherent limitations ari becau of this:
1.Single-ur access.If multiple colocated transmit-
ters nd signals at the same time,the signals can
interfere with one another,corrupting reception.
消防演讲2.Lack of identification encoding.Narrowband sys-
tems characteristically have slow data rates,which
make it nearly impossible to encode a unique
identifier in the short-duration ranging signal;thus,
it is difficult for a receiver to distinguish between the
signals from different tags.1
3.Noi nsitivity.The systems exhibit poor perfor-
mance in the prence of ultrasonic noi.
The ultrasonic location systems described above avoid the first problem by attempting to ensure that colocated transmitters nd their signals one-at-a-time.This solution comes at the expen of a slower update rate;allowing only one transmitter to nd at a time constrains the number of location updates possible for a given time interval.
The cond problem is often addresd by using a wider bandwidth technology,such as radio or infrared,to indicate the identification of a transmitting unit.However,provid-ing a dedicated communications channel for this purpo increas system complexity,requires more static infra-structure,and rais the power consumption of the mobile devices.
The third problem has not been addresd.Ultrasonic noi is created through peoples’everyday actions.Exam-ples include the clink of a pencil dropping onto a desk,the clacking of someone typing on a computer keyboard,or the rustling of a bag of potato chips being opened.The sounds occur frequently in typical indoor environments. For the duration of such an occurrence,the above ultrasonic location systems are prohibited from generating accurate, up-to-date position estimates for mobile units colocated with the noi source.
It is propod that broadband transducers and spread spectrum signaling techniques be ud to overcome the limitations.The aim of the rearch prented in this paper is to identify and characterize the practical benefits of broadband ultrasound for indoor location systems.
3D EVICE D ESCRIPTION AND C HARACTERIZATION Prototype broadband ultrasonic transmitters and receivers, collectively referred to as Dolphin units,have been devel-oped.This ction describes the design and characterization of the Dolphin prototypes.It is shown that channel bandwidth,as well as the transmitter and receiver angular nsitivity patterns,justify the u of the Dolphin proto-types for exploration of the capabilities of broadband ultrasonic location systems.
It should be noted that the Dolphin units were designed to facilitate flexibility,in order to fully gauge the potential of the new technology.Thus,the devices have not been optimized for size or power consumption,and signal analysis and generation are performed externally.How-ever,optimizing the designs to make the units small,lf-contained,wireless,and battery-powered is feasible,and the modifications needed in order to accomplish this are t out in Section5,alongside the discussion of Dolphin location system implementations.
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1.Although the Bat system,which us a differential-pha modulation technique,allows up to three t
ags to transmit simultaneously[10],it still cannot uniquely identify tags from their ultrasonic ranging signals alone.
3.1Transducer Selection and Mounting
The narrowband transducers typically ud in ultrasonic location systems rely on piezoelectric ceramics as their nsing elements.The kinds of transducers are inexpen-sive,small,rugged,and have a high nsitivity.However, they are highly resonant and,in most cas,have a usable bandwidth of less than5kHz.
Electrostatic transducers,on the other hand,posss high nsitivity and extremely wide bandwidth.However,they are not very rugged and are expensive,making their deployment on a large scale prohibitive.
Certain synthetic polymer films have piezoelectric properties[14]and are known as piezopolymer films or simply piezo films.The piezo films,most notably poly-vinylidene fluoride(PVDF),have been applied as ultrasonic transducers in hydrophone applications,and for high frequency(greater than200kHz)medical imaging and nondestructive testing[15],[16],[17],[18].Piezo film transducers are small,inexpensive,more rugged than electrostatic transducers,and have a wide frequency bandwidth.
Although there are off-the-shelf piezo film devices available which have appropriate bandwidth characteristics for ultrasonic location,the packages tend to be too bulky for application in mobile computing scenarios.Thus,it was necessary to create a piezo film transducer which is small and unobtrusive,but still has a bandwidth and radiation pattern appropriate for indoor location systems.
A piezo film ultrasonic transducer for air ranging has been designed by Fiorillo[19],[20]and further character-ized by Wang and Toda[21]and Toda and Tosima[22].It consists of a small,rectangular piece of piezo film mounted along two of its edges to form a half-cylinder.As depicted in Fig.1,the design works by virtue of the fact that the ends of the piezo film are firmly clamped.Any change in the radius r of the hemicylinder can be en as a change in the length l of the piezo film,since l¼ r.Due to the piezoelectric properties of the film,changes in l are approximately proportional to the voltage across the thickness of the film.Thus,the clamped piezopolymer can function in two ways:
1.as a transmitter,when voltages are applied to it,and如何挑选苹果
2.as a receiver,when airborne acoustic waves impact it.
This transducer was designed to measure short distances and has a maximum range of40cm.
A modified version of Fiorillo’s design is ud in the Dolphin prototype transmitters and receivers.Sheets of piezo film were procured from Measurement Specialties, Inc.2Small transducer elements were cut from the piezo film sheets.As shown in Fig.2,each element was placed between expod conductive pads on two pieces of printed circuit board(PCB).Using a screw and bolt adjacent to each of the pads,the two pieces of PC
B are held tightly together, rigidly clamping the ends of the piezo film element.The piezo film elements fitted on the transmitter units have a cylindrical length of approximately5mm,while tho on the receiver units are10mm,to allow greater nsitivity.
3.2Transmitter
Compared to more common ultrasonic transducers,such as piezoceramics,piezo film transducers have low efficiency.For flexibility during experiments,it was desired that the transmitter unit be capable of driving its piezo film transducer with signal levels ranging from tens to hundreds of volts peak-to-peak.As shown in Fig.3,this was accomplished by using a power op amp with some signal conditioning and a step-up transformer.
A Dolphin transmitter unit is shown in Fig.4,and measures60mmÂ94mmÂ15mm.Five of the transmitt
er units were characterized using measurements taken in an anechoic chamber.
The measured frequency respons of the five units are shown in Fig.5a.There is little difference in the respons of the individual transmitters.Fig.5b shows the averaged frequency respon for the five units.
The radiation pattern was characterized by measuring the sound pressure level at various angles in two different planes,defined in Fig.6.The averaged respon at50kHz of five transmitters for a range of the angles is shown in Fig.7.
Overall,the characteristics of the Dolphin transmitters are favorable.At one meter,the acoustic output does not vary by more than10dB for frequencies from35to100kHz and for angles of and within60degrees.
3.3Receiver
五星红旗的意义The relatively low piezoelectric efficiency of piezo films again dictated design decisions for the receiver prototype. An electrometer in a charge amplifier configuration was ud to amplify the low-level signal produced by the piezo film element,as shown in Fig.8.Becau of the nsitivity of the el
ectrometer preamplifier stage,it was necessary to electromagnetically shield the receiver circuitry(Fig.9). Otherwi,radiation from outside sources,such as the high voltages which are possible at the transmitter,can create unacceptable signal-to-noi ratio conditions.
The nsitivity characteristics of five Dolphin receivers were measured in an anechoic chamber.At one meter, the receiver units possd an average nsitivity of 1,724mV/Pa at50kHz.Fig.10a shows their respons in
Fig.1.Operation of a clamped cylindrical piezopolymer transducer[20].
2.The polymer film was28mm thick,and both sides of the sheets had
been electroded with a thin layer of vacuum-deposited copper-nickel metal
alloy.
decibels,referenced to this nsitivity,while Fig.10b shows the averaged frequency respon.
Fig.11displays the 50kHz nsitivity within the and planes (defined in Fig.6),averaged for the five receivers.The plotted nsitivities are expresd in decibels,refer-enced to the nsitivity at zero degrees.
The behavior of the receivers is not as clo to the ideal as that of the transmitters.The frequency respon varies by 25dB across the inspected frequency range.The authors suspect that the receiver frequency characteristic may be adverly affected by the metal mesh shielding,or possibly the electrometer preamplifier circuit,but further character-ization of the receiver would be necessary to determine the cau.
With regard to beam pattern,receiver nsitivity is within 10dB for j j 60 and significant sidelobes are visible at ¼Æ45 .Note that the beam pattern is within 10dB for angles between Æ60 for the transmitters,but only Æ45 for the receivers.Longer cylindrical lengths tend to focus the beam pattern
more [23].Since the length of the receiving transducers is twice that of the emitting transdu-cers,this more concentrated beam is expected.
3.4Channel Bandwidth
The average frequency respon of the channel between a Dolphin transmitter and receiver is shown in Fig.12.For on-axis ranging at a distance of one meter,the channel has 76kHz of bandwidth above the noi floor.As shown by Table 1,if the signal is attenuated by off-axis transmission or greater distances between the transmitter and receiver,there still remains significant bandwidth above the noi floor.
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With the wide bandwidth characteristic of the Dolphin units,spread spectrum signaling can be employed to achieve a processing gain,allowing ranging to be accomplished at a
variety of angles of incidence and room-scale distances.This ction discuss a signaling method appropriate for distance measurement with broadband ultrasound and prents the ranging accuracy of the method.A signal processing technique to enhance the multiur capabilities of the method is then outlined.
4.1Direct Sequence,Code Division Multiple Access In a direct quence (DS)spread spectrum system,a data-modulated carrier signal is further modulated by a pudorandom binary quence [24].The rate at which the pudorandom quence is applied is usually much faster than the data rate.This has the effect of spreading the spectrum of the signal.One can think of this as distributing the information content (i.e.,data)over a large range of frequencies instead of centering it cloly about the carrier frequency.Spreading the spectrum in this way makes the signal more robust in the prence of in-band noi.Using DS pudorandom spreading methods for ranging in a broadband ultrasonic location system would avoid the noi nsitivity limitation of existing narrowband systems.Gold codes are a particular t of pudorandom quences which have desirable auto and cross-correlation properties [25].If different Gold codes are assigned to urs in a spread spectrum system,their signals can be nt simultaneously and still be parated at the receiver.This gives the system multiple-access properties since corruption due to signal collision is minimal.A system which us
Fig.3.Transmitter block diagram.An op amp
and step-up transformer are ud to drive the piezo film element.Fig.2.PCB piezo film mounting.The piezo film element
is clamped between conductive pads on two pieces of printed circuit board.
Fig.4.Dolphin transmitter.In this prototype,power and analog signal generation are provided by external sources.
codes specially lected to achieve paration of over-lapping signals from different urs is described as code division multiple access (CDMA).Employing CDMA meth-ods for broadband ultrasonic ranging address the single-ur and identification coding constraints associated with narrowband systems.
DS/CDMA signal structures are appropriate for the Dolphin channel becau they make high processing gains to counter low signal-to-noi conditions more easily attainable than other multiple-access methods,such as frequency hopping.The ranging messages ud in the experiments prented in this paper consist of a 50kHz carrier wave,modulated by a Gold code using binary pha-shift keying.The Gold code has a length of 511bits and is applied at a rate of 20kHz,yielding a ranging message duration of about 25ms.Fig.13shows the frequency spreading effect when such a Gold code is ud to modulate the carrier.
In order to detect the arrival of a ranging pul,the receiver must correlate the received waveform ag
ainst a
version of the signal which is locally generated,using the known signaling parameters.A simple peak detection algorithm can then be ud on the correlation result to identify the ranging signal time-of-arrival.
4.2Ranging Accuracy
DS/CDMA signals have proven to be successful for ranging in previous systems.They are employed in GPS ranging signals [1]and Girod and Estrin have ud codes with similar properties in their acoustic ranging system,which operates in the audio frequency range [26].
To verify the ultrasonic ranging accuracy of DS/CDMA spreading using Gold codes,tests were run using a Dolphin
Fig.5.Raw transmitter frequency respon at one meter.(b)shows a corrected respon curve,which has been compensated using the known frequency characteristics of the measuring equipment and the attenuation of ultrasound in air.(a)Individual respons of five transmitters.(b)Averaged
respon.
Fig.6.Definition of transducer angles of nsitivity. describes angles in the plane normal to the transd
ucer’s cylindrical axis,while reprents angles in the plane containing the cylindrical axis.(a)Angle described by .(b)Angle described by
.
Fig.7.Transmitter radiation pattern in two perpendicular planes (e also Fig.6).(a) emission.(b) emis
sion.
transmitter and receiver.Signals were generated,captured,and analyzed using a workstation PC with data acquisition and analog output PCI cards.The speed of ultrasound in air (needed to convert a measured time-of-flight to a distance)was estimated with the aid of a temperature nsor.One thousand five hundred ranging measurements were per-formed at each of five distances between 0.5and 2.5m.The ranging error is shown in Fig.14.Variation of the
measurement error with distance is on the order of veral millimeters—no more than one might expect from the uncertainty of the hand-measured reference distances ud to compute the errors.
4.3Successive Interference Cancellation
CDMA systems in which many transmitters are nding simultaneously sometimes suffer from the problem of interference between signals,in spite of the fact that the codes chon might have near-orthogonal properties.This happens when some transmitters have a large signal power at the receiver compared to other transmitters.When trying to demodulate a signal which is many times weaker than other signals prent,the cross-correlation peaks due to tho other signals can become significant,and the detector
Fig.8.Receiver unit transducer interface electronics.The electrometer charge amplifier rves as a preamplifier stage,and the
bandpass filter removes frequencies outside of the range of interest (20-100kHz).
Fig.9.Dolphin receiver.The housing and metal mesh dome reduce electromagnetic interference.
The receiver also contains two standard 9V batteries which provide power.
Fig.10.
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Receiver frequency respon.(a)Individual nsitivities of five receivers.(b)Averaged nsitivity.
Fig.11.Receiver nsitivity pattern in two perpendicular planes (e also Fig.6).(a) nsitivity.(b) nsitivity.
