NASA Technical Reports Server (NTRS) 20120001632: Coded acoustic wave sensors and system using time diversity

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(12) United States Patent (io) Patent No.: US 8,094,008 B2
Solie et al. (45) Date of Patent: Jan. 10, 2012
(54) CODED ACOUSTIC WAVE SENSORS AND (56) References Cited
SYSTEM USING TIME DIVERSITY
U.S. PATENT DOCUMENTS
(75) Inventors: Leland P. Solie, Chetek, WI (US);
7,100,451 132 * 9/2006 Solie ............................... 73/703
Jacqueline H. Hines, Annapolis, MD
7,268,662 132 * 9/2007 Hines et al . ..................... 338/34
(US)
* cited by examiner
(73) Assignee: Applied Sensor Research &
Primary Examiner Jeffery Hofsass
Development Corporation, Arnold, MD
(74) Attorney, Agent, or Firm Schiff Hardin LLP
(US)
(57) ABSTRACT
(*) Notice: Subject to any disclaimer, the term of this
patent is extended or adjusted under 35 An apparatus and method for distinguishing between sensors
U.S.C. 154(b) by 537 days. that are to be wirelessly detected is provided. An interrogator
device uses different, distinct time delays in the sensing sig-
(21) Appl. No.: 12/267,153
nals when interrogating the sensors. The sensors are provided
with different distinct pedestal delays. Sensors that have the
(22) Filed: Nov. 7, 2008
same pedestal delay as the delay selected by the interrogator
(65) Prior Publication Data are detected by the interrogator whereas other sensors with
different pedestal delays are not sensed. Multiple sensors
US 2009/0121847 Al May 14, 2009 with a given pedestal delay are provided with different codes
so as to be distinguished from one another by the interrogator.
Related U.S. Application Data
The interrogator uses a signal that is transmitted to the sensor
(60) Provisional application No. 60/986,721, filed on Nov. and returned by the sensor for combination and integration
9, 2007. with the reference signal that has been processed by a func-
tion. The sensor may be a surface acoustic wave device hav-
(51) Int. Cl. ing a differential impulse response with a power spectral
G08B 21100 (2006.01) density consisting of lobes. The power spectral density of the
(52) U.S. Cl . ............... 340/539.1; 340/572.1; 310/313 D differential response is used to determine the value of the
(58) Field of Classification Search ............... 340/539.1, sensed parameter or parameters.
340/539.22, 539.26, 572.1; 310/311-313 D
See application file for complete search history. 14 Claims, 8 Drawing Sheets
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CODED ACOUSTIC WAVE SENSORS AND and a "stop" reflector to indicate when the interrogator can
SYSTEM USING TIME DIVERSITY start to read the response, and when the response is complete.
Between these two reflectors are multiple reflectors that are
CROSS-REFERENCE TO RELATED either present or absent (or in other variations the positions
APPLICATION 5 are modified to alter the phase of the reflection) to create a
code, along with error checking bits. The start bit in this case
This application claims the benefit of U.S. Provisional effectively "turns on" the signal processing in the interrogator
Patent Application Ser. No. 60/986,721, filed Nov. 9, 2007, to initiate code identification. Such single frequency tags
which is incorporated herein by reference. have been used successfully in applications that can utilize
10
scanning of a single tag at a time, at relatively short range,
STATEMENT REGARDING FEDERALLY
such as monitoring identification of livestock or automobile
SPONSORED RESEARCH OR DEVELOPMENT
toll booths. Such approaches are not well suited to use in a
system where multiple sensors would respond to interroga-
This invention was made with Government support under
15 tion in a given time window, as the signals cannot be
Contract NNK07EA38C awarded by NASA. The Govern-
adequately resolved from one another.
ment has certain rights in this invention.
OFC (orthogonal frequency coded) sensors attempted to
BACKGROUND OF THE INVENTION address this issue by providing codes within the sensors that
are inherently orthogonal. The mathematical relationship
1.Field of the Invention 20 between codes used in a sensor system is such that each code
The present invention relates generally to a surface acous- will correlate well with itself, producing a clear correlation
tic wave sensor and an interrogator apparatus for wireless peak (or in the case of differential delay line sensors, produc-
communication with the surface acoustic wave sensor and to ing a pair of correlation peaks), while interacting with differ-
a method for distinguishing between multiple sensors. ent codes to produce cross correlations that are much lower
2.Description of the Related Art 25 and without clear peaks. In theory this should allow for clear
The simultaneous interrogation of uniquely identifiable sensor discrimination. However, in practice this approach
wireless sensors is recognized as a highly desirable goal. To encounters significant problems. Specifically, the orthogo-
facilitate achieving this goal, active sensor nodes and passive- nality of the codes is only maintained if they are operating in
acting sensor nodes containing active components often con- a synchronous system. Because of the random RE propaga-
tain memory that stores a code to identify each sensor. The 30 tion delays experienced by signals from each sensor, and due
interrogating system can interact with these sensors using a to changes in delay due to the sensed parameter, even ideal
"handshake" process that instructs the sensor when to take a orthogonal codes will necessarily produce non-orthogonal
measurement and return its information. Thus, sensors can be interfering signals. This issue, which has been widely recog-
instructed to delay their return signal by a specified delay in nized in the realm of CDMA (code division multiple access)
order to reduce collisions between signals from multiple sen- 35 communication systems, makes the use of orthogonal codes
sors, or they can be instructed to re-send the measurement, alone as a basis for passive sensor discrimination ineffective.
among other options. This level of bi-directional interaction, It is well known in the wireless communication industry
with the sensor modifying its response to interrogation based that there are several possible approaches to achieve multi-
on specific instructions, is only possible in systems where the user access to shared bandwidth. The ability of each approach
sensors contain active components. In a wireless sensor sys- 40 to provide robust, reliable demodulation of multiple ran-
tem utilizing purely passive sensors such as acoustic wave (or domly occurring signals without interchannel interference is
surface acoustic wave SAW) sensors, the sensor acts as a a measure of the efficacy of the approach. Well known
passive reflector for the radio frequency (RF) interrogation approaches include Code Division Multiple Access
signal. When a broadcast interrogation signal is being used (CDMA), Time Division Multiple Access (TDMA), and Fre-
(as compared to broadcasting a coded signal specific to one of 45 quency Division Multiple Access (FDMA). CDMA systems
the sensors), all of the SAW sensors within the range of a assign different codes (or signature waveforms) to each user,
particular interrogator will reflect back sensor responses, and the interrogation system is responsible for identifying the
each containing information on the sensor identification and sensor from the collective responses received. TDMA sys-
the measured (or sensed) parameter. Since there is no way to tems utilize signature waveforms that do not overlap in the
select a single sensor in such a broadcast system, these 50 time domain—one user utilizes the channel, then another, and
response signals will arrive at the receiver at slightly varying so forth. FDMA systems separate users in the frequency
times based on their physical locations due to the small RE domain by allocating specific bandwidth to each user, so that
signal propagation delay. In order to resolve the individual signature waveforms do not overlap in the frequency domain.
sensor signal responses, it is necessary to achieve discrimi- Strictly speaking, TDMA and FDMA systems do not need to
nation between sensor responses, which has historically been 55 utilize coded waveforms unless the resource (time or fre-
achieved for SAW sensors using sensor coding. quency slot) allocation is dynamic, although the inclusion of
Well known wireless coded SAW sensors include single such codes increases system capacity.
frequency "tag" sensors such as the multiple tap reflective When reviewing wireless SAW sensor systems developed
delay line temperature sensor shown in 1981 by Vaughan to date, it is clear that in those systems where multiple simul-
(U.S. Pat. No. 4,399,441), and the orthogonal frequency 60 taneous sensor interrogation has been attempted, one of these
coded (OFC) SAW sensor developed recently by Malocha. standard approaches to multiple access has been utilized. For
Other RFID tags using SAW also use such encoding tech- example, OFC sensors used a CDMA approach, while SAW
niques, including various coding approaches (on-off keying, resonator sensor systems with frequency diversity utilize
phase coding, etc.). These devices incorporate a code in the FDMA. Not considered here are sensor systems that broad-
reflector structures on the SAW surface to produce reflections 65 cast coded signals to interrogate a specific sensor, but rather
that occur at specific times. In the traditional single frequency systems that interrogate a group of sensors simultaneously
reflective tag approach, there is generally a "start" reflector and interpret the resulting sensor responses. It is clear that