Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR MEASURING TEMPERATURE USING A REMOTE,
PASSIVE, CALIBRATED RF/RFID TAG INCLUDING A METHOD FOR
CALIBRATION
The present invention relates to a system that includes a passive,
remote radio frequency identification (RFID) tag, and a control device, such
as an active smart RFID tag or RFID reader/writer that can communicate
with the passive tag giving it instructions and receiving data from it and
that
can analyze and store data so received. In the case of this control device
being an active smart RFID tag, the system would further require a
reader/writer that can instruct the active RFID tag and can read data created
during use of the tags. The system will be utilized primarily in the transport
and storage field, or any other field of use where detecting changes in
temperature at distance without the use of a direct probe is desirable. The
system utilizes a unique means for determining temperature, which means
forms part of this patent application. This invention further claims a
methodology of calibration to provide accurate temperature measurements
(in the range of +/- 0. 1 degree C).
BACKGROUND OF THE INVENTION
Devices to measure temperature in transported or stored goods are
widely known. Inexpensive temperature sensing devices are generally
designed to record only a single excursion outside a preset temperature
window. More expensive temperature logging devices are typica!!y bulkier
and too costly for single use or disposable applications. A further limitation
of prior art is the requirement to retrieve a tag from a shipping box to take
a
reading or download data from it.
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There are many instances where it is desirable to determine and
record the temperature at many points during the transportation or storage
of a product or load. Electronic sensing and recording devices that can do
this are available; they include thermistors or other temperature sensors, a
clock, a battery, a memory in which temperature data are recorded, and
some form of output mechanism (such as a plug-in port) whereby the
recorded data can be read for interpretation by an interested party at a later
time. These devices are large in size compared to the disposable tags, and
significantly more expensive. They are generally reusable. In some cases,
for example where the container's size is small relative to the size of the
sensing and recording device or extreme temperatures that would affect
battery performance are of interest, they require probes containing a
temperature sensor that destroys the integrity of the container. Where the
container is large enough to accommodate a sensing and recording device,
the device must be removed from the container when it reaches its
destination to allow it to be reused.
Passive RFID tags are in wide use for identifying and tracking all
manner of packages and items. RFID technology uses electromagnetic or
electrostatic coupling in the radio frequency (RF) range of the
electromagnetic spectrum to identify uniquely an object of interest. RFID is
increasingly supplementing or replacing bar coding for identification purposes
and providing electronic packing slip functions in the shipping and logistics
industry as it does not require direct contact or line-of-sight scanning.
A passive RFID system comprises an antenna and transceiver
(usually combined as a reader) and a transponder (RFID tag). The antenna
generates a RF signal that activates the RFID tag. The activated tag then
transmits data back to the antenna. The data may be used to notify a logic
controller to initiate an actian, or stored for subsequent retrieved by an
interested party.
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In response to the size and cost issues, Petersen and Wilson
(Environment Monitoring and Recording Tag and Reading System - U.S.
Patent Pending 2002) have described an active or smart RFID Tag system for
monitoring and recording environmental conditions including temperature,
that is small in size, relatively inexpensive, and can transmit its data to a
reader via wireless means. However it is sufficiently expensive in its
manufacture that it could be uneconomical as a single use or disposable
device for most applications, necessitating its removal from the container or
package at its destination, for the purpose of later re-use. A further
limitation is the adverse effect of decreasing temperature on the battery,
making the device impractical for very cold applications (such as monitoring
dry-ice packaged goods) without the addition of a temperature probe
containing a thermistor, which destroys the integrity of the package, or the
use of expensive low temperature battery technology.
SUMMARY OF THE INVENTION
The invention combines a low cost passive RFID tag (slave tag) with
an active RFID master tag and a RF writer/reader in a system that senses
and records temperature a preprogrammed intervals. The low cost of the
slave tag makes it feasible to be used as a disposable temperature sensor.
A further implementation of this invention combines a low cost passive
RFID tag (slave tag) with an RF writer/reader in a recording system, omitting
the master tag.
The slave tag is inserted in a package or container whose internal
temperature is of interest. The master tag polls the slave tag at pre-
programmed intervals via RF signal, and the slave tag transmits data (such
as its unique ID and/or manufacturer code) back to the master tag by RF
signal. The master tag can then determine the temperature of the
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environment into which the siave tag has been inserted by way of calculating
the frequency shift caused by such ambient temperature from its nominal
calibrated frequency (see below for method of calibration).
The master RFID tag comprises an integrated circuit with
programmable or re-programmable procedure memory, battery, antenna,
clock, high-stability oscillator such as piezo-electric, quartz or ceramic
resonator, and volatile or non-volatile data memory. The master tag can be
pre-programmed at the time of manufacture or can be programmed prior to
use using the RF writer/reader. At programmed intervals, the temperature
data calculated from the frequency shift during data transmission sessions by
the slave tag are stored in the master tag's data memory. The stored
temperature data can be downloaded via RF writer/reader by someone
interested in the temperature to which the container contents have been
exposed.
The slave tag comprises a procedure memory programmed with
calibration data (if required, as described below) optional data such as ID,
manufacturer codes, electronic packing slip information etc., Manchester-
type phase encoder, oscillator whose frequency is temperature sensitive such
as a tick oscillator, and a resonator such as an antenna coil, printed
antenna,
ceramic antenna, etc.
In a further aspect of the system, the temperature sensitive
oscillator of the slave tag replaces a thermistor or other direct temperature
sensing device as the means of determining the temperature, eliminating the
need for a battery in the slave tag. This reduces the cost of manufacturing
the slave tag and eliminates the problem of a battery's voltage being
temperature sensitive. In principle, any ISO standard RFID tag can be used
for this function, adding little or no additional cost to logistic systems
already
utilizing such RFID tags to track shipments.
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In this system, temperature is measured by rnmparing the stable
oscillator frequency of the master tag to the frequency of the slave tag's
tick
oscillator, a semi-conductor device whose frequency is predictably dependent
on temperature. The master tag's antenna is calibrated to receive signals in
the frequency range of interest, determined by the range of temperature to
which the slave tag might be exposed and the range of frequencies that
would result from such temperatures. The temperature at the slave tag
would influence the tick count of its oscillator, which would be known to have
a predictable change in tide frequency in response to changes in
temperature.
Penc = Tf (temp)
Where Penc = period of Manchester encoder pulses; Tf = sigmoid
type function; temp = temperature
The relationship between temperature and frequency would be
calibrated prior to use and stored in the procedure memory of the master
tag. Deviation of the returning RF signal's frequency from that of the stable
oscillator in the master tag would be determined and compared to the
frequency-temperature calibration data for the slave tag's oscillator. The
resulting calculated temperature at the slave tag would then be recorded in
the master tag's temperature data memory. At a later time, a party
interested in the temperature to which the container was exposed could
download the stored temperature data from the master tag using a RF or
other type of reader.
In use, the slave tag is placed inside a container at the time of
shipping or at the time of manufacture of such container. The master tag is
placed outside the container in close proximity, such as attached to the
container by adhesive or other means. The master tag is programmed to
cause the slave tag to transmit data to it at intervals of interest to the
user.
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The master tag polls the stave tag by transmitting an RF signal that
excites the stave tag's tick oscillator and provides energy to the device. The
slave tag responds by transmitting an RF signal that can be detected by the
master tag. Because of the sensitivity of the slave tag's tick oscillator to
changes in temperature, its signal will deviate from the master tag's RF
frequency according to a predictable relationship.
Calibration data for the stave tag's temperature-frequency
relationship will be stored either in the slave tag's or master tag's memory,
and the difference in frequency from the transmitted RF signal to the
received RF signal will be applied to the calibration data to determine the
temperature at the slave tag. This will be time stamped and recorded in the
master tag's data memory.
To calibrate the slave tag's temperature-frequency relationship
Penc = Tf (temp)
Penc is determined under two or more specific temperatures tempi.....
temp". To achieve this, the slave tag could be inserted into a temperature
insulated chamber containing a thermoelectric element (effet Pettier). The
temperature could be changed rapidly and with precision, allowing Penc,.....
Penc" to be determined. The specific function can then be calculated for this
tag sample and stored in the procedure memory of the slave tag from which
it would be downloaded to the master tag or writer/reader to be used in
temperature determination. This would obviate the necessity of uniquely
pairing slave and master tags. The temperature-frequency function might
equally be stored in the procedure memory of the master tag if specific
master-slave pairs are to be used. From this function the temperature can
be computed for any value of Penc.
To provide very exact temperature measurements (+/- 0.1 degree
C), each tag would be calibrated. However, in large scale implementation
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calibration could be generalized to the design and components of a particular
type of slave tag (such as a standard RFID tag produced by a particular
manufacturer) and therefore avoid the requirement for individual calibration
of each tag. In such cases, a generic calibration coefficient would be
sufficient to provide temperature measurements in the +/- 1 degree C range,
if not better.
As described by Petersen and Wilson (2002), the temperature data
can be retrieved from the master tag by an interested party. The master tag
can be single use or multiple use and may be reprogrammable. Petersen and
Wilson (2002) describe various means by which this might be done. The
slave tag can also be single use or reusable
In a variation of the invention, the master tag is omitted from the
system and the passive RFID slave tag is inserted into a package or
container. Each time the package passes through a RF reader at a loading
station, unloading station, sorting station, warehouse or delivery point, as
is
currently in widespread use for tracking the movement of such packages, the
temperature at the RFID tag at that time could also be determined and
processed (stored, downloaded) by the reader. This would allow a central
database to keep track of the time and internal temperature of the package
at each waypoint in systems where such packages are already being scanned
and tracked using barcodes and/or RFID tags.
In a further variation of the invention, the package ID, electronic
packing slip, other logistics related information, manufacturer code (to
facilitate "generic" calibration - see above) and temperature sensitive
components could be combined in a single passive RFID tag, thereby
eliminating the requirement for additional hardware to provide the benefit of
temperature measurements, by utilizing RFID readers and passive RFID tags
already incorporated into the logistics infrastructure of shipping and courier
companies.
