WO2009055839A1

WO2009055839A1 - Object singulation process

Info

Publication number: WO2009055839A1
Application number: PCT/AU2008/001412
Authority: WO
Inventors: Anthony Laviale; David Malcolm Hall
Original assignee: Tagsys Sas
Priority date: 2007-10-30
Filing date: 2008-09-25
Publication date: 2009-05-07

Abstract

A method is disclosed for singulation of at least two moving RFID tag populations in an RFID application. The method comprises the steps of transmitting a radio frequency interrogation signal to read tags and collecting raw data from or along with tag replies. The method further comprises determining for a given tag whether the tag is in motion or stationary; and whether the tag belongs to one of the at least two moving RFID tag populations. If a tag is stationary, it is assigned to a non-moving population and if a tag is in motion but does not belong to one of the at least two moving RFID tag populations, it is assigned to a moving population other than the two moving RFID tag populations. The raw data collected from or along with the tag replies may include received signal strength indication (RSSI) data and a time-stamp or time-sequence. The raw data collected from or along with the tag replies may include tag excitation level (TEL) data and a time-stamp or time-sequence.

Description

OBJECT SINGULATION PROCESS

FIELD OF THE INVENTION

The present invention relates to an object management system wherein information bearing electronically coded radio frequency identification (RFID) tags are attached to objects which are to be identified, sorted, controlled and/or audited. In particular the present invention relates to a method for singulation of at least two moving tag populations in an RFID application to determine if the tag belongs to one of the two moving tag populations wherein each tag is read in the presence of other tags which may or may not be associated with the RFID application.

BACKGROUND OF THE INVENTION

When reading tags with a reader using an ultra-high frequency (UHF) carrier signal, it can be difficult to contain the reading zone to a well defined reading volume due to the UHF signal reflecting from surrounding surfaces and reinforcing with a direct path signal which would otherwise be too weak for a successful tag reading but after the reinforcement may be strong enough for a reading.

Placing reader antennae in a metallic enclosure or tunnel goes some way to confining the reading volume, but some signal leakage still occurs at tunnel entry and exit regions which leads to reading of tags outside the tunnel.

Lengthening the reading tunnel may ensure that even if reinforcement has occurred from reflections off the tunnel structure, the signal leaking from the tunnel entry and exit regions is attenuated or weakened due to propagation loss experienced by the signal. Similarly due to propagation loss of the tag reply, the tag reply level in the reader is also weakened. Reply threshold techniques may be used to filter tags not within the reading volume. This technique works to a point, beyond which further increases in tunnel length means that two successive containers of tag populations may be simultaneously present within the tunnel, and the sequence of containers cannot be further separated in their relative distances to each other as this would slow down the throughput of the RFID application.

By using a reader which is able to assign to each tag reply a received signal strength indication (RSSI), successive readings of a tag may allow a map of RSSI vs time to be built up. Processing of an RSSI vs time map or information, may allow tags to be discriminated between those that are in motion and belonging to the RFID application, from stray tags outside the tunnel that are either themselves in motion or stationary. It may also allow a tag read in a first container to be assigned to the population of the first container, and a tag read in a second container to be assigned the population of the second container.

An alternative to RSSI data may be data bits which are included in a tag reply that represent the tag excitation level (TEL) experienced by the tag from the reader field. The tag reply, which is generated by reflecting back to the reader varying amounts of the reader's field at the tag position, has a level in the reader's receiver circuitry which is proportional or related to the TEL data bits. The TEL data bits may be generated by an analogue to digital conversion of one or more control signals within the tag's chip or micro-circuit. The control signals may perform a combination of tag detuning and power diversion when the tag is in a strong reader field to collect less energy or dissipate energy in excess of what is needed for tag operation.

A further but less reliable method may be to use a frequency component of the tag reply of an UHF tag, since the reply is formed from a self-timed or asynchronous to the reader operating carrier frequency. This asynchronous frequency varies with tag excitation but its characteristics vary between chip or micro-circuit vendors in applications using open standard protocols. A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission that the document or matter was, in Australia, known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims.

Where the terms "comprise", "comprises", "comprised" or "comprising" are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other feature, integer, step, component or group thereof.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method for singulation of at least two moving RFID tag populations in an RFID application, comprising the steps of:

(a) transmitting a radio frequency interrogation signal to read tags;

(b) collecting raw data from or along with tag replies; and

(c) for a given tag determining: (i) whether the tag is in motion or stationary; and

(ii) whether the tag belongs to one of said at least two moving RFID tag populations; and: if a tag is stationary, assigning it to a non-moving population; and if a tag is in motion but does not belong to one of the at least two moving RFID tag populations, assigning it to a moving population other than said two moving RFID tag populations.

In some embodiments the method of the present invention may include repeating steps (a), (b) and (c). The method may be performed by a tag reader, and may be initiated by activating software trigger controls.

The raw data collected from or along with the tag replies may include received signal strength indication (RSSI) data and a time-stamp or time-sequence. The raw data collected from or along with the tag replies may include tag excitation level (TEL) data and a time-stamp or time-sequence.

The method may further include processing characteristics of signal values of the RSSI data or TEL data using mathematical and/or statistical processing tools. The mathematical and/or statistical tools may include average of time weighted by RSSI data and maximum of absolute value of time derivative of

RSSI data. The mathematical and/or statistical tools may include average of time weighted by TEL data and maximum of absolute value of time derivative of TEL data.

In some embodiments the method of the present invention may be performed by means of a neural network. The method of the present invention may further include a genetic algorithm to adapt the method to changes over time of application parameters.

When using a reader with one or more mono-static antennae, wherein each antenna fulfils the dual roles of reader-to-tag and tag-to-reader communication, the level of the field at the tag position is proportional or related to the level of the tag reply in the reader's receiver circuitry due to reciprocity. Thus either RSSI data or TEL data may be used as the raw data input to the processing tools. Multiple antennae may be used to produce fields in different directions to cover probable tag orientations that may be found in the RFID application.

When using a reader with bi-static or separate antennae for reader-to-tag and tag-to-reader communication, reflections at UHF carriers impact tracking of RSSI and TEL data, such that a tag with a high TEL may have a low RSSI. Either data can still be utilised, however the map of RSSI or TEL vs time will show more variations in level, which can be considered as noise in the data, and is somewhat more susceptible to reflections of changing surrounds.

For simplicity, the RSSI from a reader with a mono-static antenna will be described below to illustrate the present invention. In the example containers of tag bearing items move sequentially past a reader antenna, and the objective is to capture for each tag a map of RSSI vs time, and to assign each tag to its correct container. The containers may include containers of items on a conveyor or the like.

A virtual or desired reading volume may be set-up by defining a volume surrounding the reader antenna within the physical or possible reading volume. The boundary between the virtual and physical volumes may be reduced either by using a shielding enclosure around the reader antennae, or by using radio frequency absorbing material in the immediate vicinity of the reader antennae to eliminate reflections from surrounds thus eliminating reinforcement of direct and reflected signals to and from tags.

Additional aids may be used to prevent stray tags from interfering with the process, such as light beam sensors or triggered micro-switches so the reader searches for application tags when application tags are expected. Stray stationary tags that are read continuously or during non-expected times may be filtered out.

A series of at least two filters may be applied to the collected RSSI vs time data. A first filter may be applied to discriminate between tags in motion on the conveyor and tags outside the reading volume, the latter tags being either stationary or moving. A second filter may be applied to discriminate between tags belonging to a first container and tags belonging to a second container.

By analysing RSSI vs time data from successive tag readings, a tag in a first container may have a peak in RSSI level vs time at an approximate centre of the reading volume, or when it is closest to the reader antenna, whereby the tag may be assigned a time centre within a time period, the time period being a time domain approximation to the container length (time being container length divided by conveyor speed). Since tags within the container are spaced in the direction of motion, the time periods of each tag of a container will have a slightly different time centre, so that the analysis may allow these different time centres to be associated with its closest neighbouring time centre, with the time periods (length of the container) as an upper limit on the time domain distance to a neighbour. Thus the container may be represented by a first ideal time period centred at the centre of the container and by second and third time periods extending from the leading and trailing edges of the container as it moves through the reading volume.

The peak of RSSI vs time data may be found by examining the time derivative of RSSI vs time, rather than simply using a maximum data value. This technique may assist in removing noise of RSSI vs time data which arises from spatial nulls in the field due to surrounding reflections and path-length issues of the reader which occur as a predominant phase of the tag reply varies from phases which are used in data recovery circuits or down-conversion. Multiphase detectors or down-converters are common to help keep a tag reply detectable as the tag moves through the field, but there may be some ripple in RSSI vs time data as a reader switches between detectors with the best output. A combination of detector outputs of a multi-phase reader may result in some smoothing of RSSI ripple.

The peak may also be quite flat in the case of a strongly excited tag or a tag close to a reader receiver antenna, wherein reply signal strength compression may occur from a tag which is hard limited or greatly detuned to prevent over- voltage, or the reader's receiver circuitry is saturated. In such a case the time centre of the tag may be found by an average of time weighted by RSSI.

A tag in a second container may exhibit a similar peaked trace in RSSI vs time data, and while it may have had a valid read at a similar time as a tag in the first container, when its time centre is associated with its nearest neighbouring time centre it may be assigned to the second container.

A stray or parasitic tag, being a tag not within a container of the RFID application, may be found by observing: a) a trace without a peak in RSSI, from a stationary or static tag; or b) a trace of RSSI with an above threshold time period and hence container size dissimilar to others of the application or a different RSSI trace shape, from a moving tag but not belonging to the RFID application.

It may thus be possible to assign tags to corresponding containers even while tags other than those in the container of interest are simultaneously present.

The filters may be applied by means of software running on a host computer connected to the reader or if hardware permits inside the reader itself.

Statistical tools may be used to discriminate application tags moving along the conveyor from other tags groups. The statistical tools may include: variance of RSSI; variance of the square of RSSI weighted by time; variance of RSSI divided by the average of RSSI; variance of RSSI weighted by (time - average time); average deviation of RSSI; average deviation of RSSI weighted by (time- average time); maximum of the absolute value of the time derivate of RSSI(t); covariance of RSSI vs time compared to a Gaussian function; correlation function of RSSI compared to a Gaussian function; and a Jarque Berra test.

Statistical tools may be used to determine the approximate time of the tag passing through the centre of reading volume or by the reader antenna. The statistical tools may include: average of time weighted by RSSI; median of time weighed by RSSI; and time derivative of RSSI(t).

In the above functions, a term may be raised to a power n, or transformed through a logarithmic, exponential or Fourier transform function. For example, variance of RSSI may be replaced by variance of the square of RSSI, variance of Iog10(RSSI), or variance of 10 raised to the power of RSSI.

Once the data is represented in a two dimensional grid, for example max(abs(dRSSI/dt)) vs time, analysis involving association of time centres with neighbouring time centres may involve use of a neural network to group tags into common populations. If the application or application parameters change over time a genetic algorithm may allow the sorting criterion to adapt to changes in the application.

Also data grids of three or more dimensions may be implemented by combining additional data with the time centre. Such additional data may include TEL data in addition to RSSI data, or vice versa, or time within a physical volume whose boundary is defined by light beams or triggered micro- switches, or a tag on the container itself with a different application identifier represented within the tag data, a different protocol, a different operating frequency, a different technology such as bi-statics (electrostatic or low frequency electric field coupling), magneto-acoustics, hard-material-soft- material magnetic strip or surface acoustic wave (SAW), or a non-radio- frequency technology such as a barcode or optical alignment marker.

The process may be tuned for various applications with various system input parameters such as the number of tags per container, length of the container, spacing between successive containers, and speed of the conveyor or the like. The threshold technique used may also vary for various applications with input parameters such as number of antennae used and relative positioning of such antennae between themselves and the containers passing through the reading volume.

DESCRIPTION OF A PREFERRED EMBODIMENT

A preferred embodiment of the present invention will now be described with reference to the accompanying drawings wherein: Figure 1 shows containers on a conveyor which passes through a reading structure or tunnel;

Figure 2 shows traces of RSSI level vs time for three tags; and

Figure 3 shows read tags grouped into populations.

Figure 1 shows a first tag 10 belonging to a first tag population 1 1 associated with container 12 such as a carton, a second tag 13 belonging to a second tag population 14 associated with container 15 such as a carton, and a stray tag 16 not associated with container 12 or 15. The containers 12, 15 belong to an RFID application and are on a conveyor 17 which passes through an antenna housing 18. Antenna housing 18 has a length 19, an entry point 20 and an exit point 21. Figure 1 shows one antenna 22 of a possible plurality of antennae forming a reading volume associated with antenna housing 18. The process of the present invention may assign each tag to a tag population 1 1 associated with container 12 of length 23 and length uncertainties 24 and 25.

Figure 2 shows traces of RSSI level vs time for three tags. Trace 26 belongs to a tag in population 1 1 associated with the first container 12, while trace 27 occurring later in time belongs to a tag in population 14 associated with container 15. Trace 28 belongs to a stray stationary tag not associated with container 12 or 15. The time centres of traces 26 and 27 both have clear peaks, and can be found using an average of time weighted by RSSI. Trace 28 is associated with a stationary stray tag and shows no broad peak. Thus trace 28 may be filtered out at this stage or it may be assigned a time centre which may be related to a periodic processing time of the collected data.

Figure 3 shows each tag on a two dimensional grid wherein max(abs(time derivate of RSSI)) is plotted vs time. It may be seen that after processing RSSI vs time data that tags have fallen into three distinct populations 31 , 32, 33. Populations 31 and 32 are tags belonging to containers 12 and 15 respectively, with tags from population 31 having an assigned time earlier than those from population 32. Population 33 may be due to a stationary stray tag not belonging to the RFID application near the reading volume associated with antenna housing 18, the tag identity of which occurs persistently and has not been filtered out. Alternatively population 33 may be due to a moving group of stray tags not belonging to the RFID application associated with containers 12 and 15, again near the reading volume associated with antenna housing 18 for which persistent tag identities have been filtered out.

A subtle feature in Figure 3 is an absence of stray tags 33 during times relating to population 31 , and less than average tags of population 33 during times relating to population 32. While this may not always be the case, it is shown here to demonstrate an effect known as a small signal suppression effect. This effect occurs when two or more tags respond simultaneously and the tag with stronger reply data is read while other tags have their reply data suppressed and are unknown to the reader. Some tags in population 33 were read during times of population 32 since individual tag RSSI levels may have been lower to weak tags, tags shielded perhaps by a wet container or misplacement of a container on the conveyor. Tag excitation level data (TEL bits) was developed to assist in development of protocols which can realise that weakly replying tags can be simultaneously present in the RFID application or near the reading volume, in particular in a reader echoing the TEL bits of a tag it read in any command which may prevent the tag from replying for some time so that a strong decoded tag was commanded and any weakly replying tags were not incorrectly commanded.

Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.

Claims

1. A method for singulation of at least two moving RFID tag populations in an RFID application, comprising the steps of: (a) transmitting a radio frequency interrogation signal to read tags;

(b) collecting raw data from or along with tag replies; and

(c) for a given tag determining:

(i) whether the tag is in motion or stationary; and (ii) whether the tag belongs to one of said at least two moving RFID tag populations; and: if a tag is stationary, assigning it to a non-moving population; and if a tag is in motion but does not belong to one of the at least two moving RFID tag populations, assigning it to a moving population other than said two moving RFID tag populations.

2. A method according to claim 1 wherein said raw data collected from or along with the tag replies includes received signal strength indication (RSSI) data and a time-stamp or time-sequence.

3. A method according to claim 1 wherein said raw data collected from or along with the tag replies includes tag excitation level (TEL) data and a time- stamp or time-sequence.

4. A method according to claim 2 or 3, further comprising: processing characteristics of signal values of the RSSI data or TEL data using mathematical and/or statistical tools.

5. A method according to claim 4 wherein said mathematical and/or statistical tools include average of time weighted by RSSI and maximum of absolute value of time derivative of RSSI.

6. A method according to claim 4 wherein said mathematical and/or statistical tools include average of time weighted by TEL and maximum of absolute value of time derivative of TEL

7. A method according to claim 2, 3 or 4, further comprising repeating steps (a), (b) and (c).

8. A method according to claim 7, wherein the method is performed by a tag reader, and wherein the method is initiated by activating software trigger controls.

9. A method according to any one of the preceding claims wherein said method is performed via a neural network.

10. A method according to any one of the preceding claims further comprising a genetic algorithm to adapt the method to changes over time of application parameters.

1 1. A method for singulation of at least two moving RFID tag populations substantially as herein described with reference to the accompanying drawings.