NL1029590C2 - Radio frequency positioning system for animals, uses tag with antenna impedance modulated by signal from subcarrier wave generator - Google Patents

Abstract

A transmitter sends a stepped frequency request signal to a tag (13) with an antenna (14) and the impedance of the antenna is modulated by a signal produced by a sub-carrier wave generator (15, 16). The modulated response signal sent by the antenna is received by a receiver.

Description

- 1 - i i

System for detection, location and identification by means of frequency hopping and phase measurement.

The invention relates to applications in which the localization of animals is central, possibly extended with an identification function. A first application concerns the detection of free-running small pets, such as cats. A second application is in livestock farming where, for example, the movements of cows in a stable or in a pasture have to be registered. A further application is in biological research, the so-called biological telemetry, in which it must be possible to follow the movements of certain animals in the free field.

In all these applications it is about locating animals, goods and people, whereby it must be possible to measure wirelessly, inter alia, the distance of the object to one or more interrogation units.

15

The invention uses a distance measurement method which is based on measuring frequency-dependent phase differences, see figure 1. A continuous carrier wave 1 is thereby transmitted by transmitting antenna 2, part of interrogation unit 3. Carrier wave 1 varies in frequency. This variation can be done in 20 different ways, for example by means of a frequency sweep. However, the preferred method is by means of frequency steps, the so-called frequency hopping.

If the signal is reflected on an object or animal 4, the interrogation unit 3 will receive a signal 5 back. Transmitter circuit 6 generates the carrier wave signal which is frequency controlled by frequency synthesizer 7.

Both signals are multiplied with each other in a detector circuit 9, so that an output voltage is created which is related to the phase difference. This phase difference is proportional to the duration of the signal, from transmitter to object and from object 30 back to the receiver, thus proportional to the distance between transmitter / receiver and the object. Measuring circuit and microprocessor 10 thus determines the distance, controls the interrogation unit and communicates the results to the outside.

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If the object has a radial speed relative to the transceiver, the phase will be constant, which means that a difference frequency will occur. This difference frequency is equal to the doppler shift, and is proportional to that speed.

The above method is suitable for objects that have a highly reflective capacity, such as ships and aircraft. However, the reflective capacity of the body of an animal, or human, or that of most goods is low, and is not distinctive with respect to materials 11 in the immediate vicinity. That means that a reflected signal from such an object drowns in signals 12, reflected by the environment, in the radar terminology indicated by the word 'dutter'.

The invention is further described with reference to figures.

Figure 1 gives a schematic picture of the known distance measuring principle.

Figure 2 gives a schematic view of a first embodiment of a label as used in the invention.

Figure 3 shows a block diagram of a first embodiment of an interrogation transmitter / receiver according to the invention.

Figure 4 gives a schematic view of a second embodiment of a label as used in the invention.

Figure 5 shows a block diagram of a second embodiment of an interrogation transmitter / receiver according to the invention.

Figure 6 gives a schematic view of a third embodiment of a label as used in the invention.

Figure 7 shows a block diagram of an extensive interrogator.

25

It is the object of the invention to make such a modification in the known method that a clearly recognizable signal is reflected on the animal. To this end, the animal is provided with an electronic label. This label, schematically represented in Fig. 2 with number 13, consists of an antenna 14 functioning in the frequency band of the interrogation signal 30, for example a dipole, a label modulator 15, and a signal source 16. This signal source generates a subcarrier fsub, for example 66.5 i 1029590 kHz, 3 kHz, which is then applied to the label modulator. The modulator 15, for example, consists of a diode 17, which connects the two halves of the dipole antenna. By applying a forward voltage to the diode in turn, and

J

then again a voltage in the reverse direction, the diode for the high-frequency voltage, received by the dipole antenna, starts to conduct or inhibit. The impedance of the antenna is thus modulated to the rhythm of the subcarrier frequency. Battery 18 powers the electronic circuit in the label.

If the transmission signal from the interrogation unit is picked up by the antenna in the label, this antenna will radiate this signal again. This phenomenon is called "backscatter." The extent to which that occurs depends on the impedance of the antenna. Because the impedance is modulated with the subcarrier frequency, the retransmitted signal will also be modulated with it. That means for the spectrum of that 'backscatter' signal that it consists of the original carrier frequency, with two sideband frequencies on either side thereof, flsb and fusb at a distance equal to the subcarrier frequency of the carrier.

The receiver receives these two sideband components, and multiplies them with the carrier signal fc in a so-called quadrature mixer. This quadrature mixer consists of two mixer circuits, in which the carrier signal is injected with a 90-degree phase difference. Each mixer has its own output, with one output being called the I channel (In phase), and the other the Q channel (Quadrature). A 90 degree phase shift is then made in the Q channel, after which the signal in the I channel and the phase shifted signal in the Q channel are added and subtracted in a parallel channel. This method, known as the so-called phase-method for detecting single-sideband, or independent sideband, modulated signals, separates the signals in the I and Q channels into a signal originating from the upper sideband and into a signal originating of the lower sideband of the received signal from the tag.

30

It is part of the invention that the phase difference between the detected upper side band signal, hereinafter also referred to as USB, and the detected lower side band signal, hereinafter also referred to as LSB, is determined. Furthermore, it is a part of the invention that the carrier frequency is varied, and the above-mentioned phase difference is determined at each carrier frequency.

5 The calculation of the phase shift and distance relationship takes place as follows.

The transmitter transmits an interrogation signal with carrier wave frequency cuc, where a> c = Ijtfc- After having traveled a distance d \ between the transmitter and label, the phase shift of that interrogation signal amounts to ψ \. In the tag, the interrogation signal is modulated in amplitude with a subcarrier frequency cosc.

10 This AM modulation at the location of the label results in two sideband components, USB and LSB: S (t) = 2Atag · sin (a) sct) · cos (a> c + φχ) = Atag [sin ((a> sc) + a> c) t + ψχ) + sin ((cosc - a> c) t - <? i)] = Atag [sin ((tt) c + a> sc) t + ψχ) - sin ((ω0 - a) sc) t + ς? ι)] 15 USB LSB (1)

After having traveled a distance d2 between the label and the receiver, the phase shift of that response signal is .2. Upon receipt in the receiver, the retransmitted signal S (t) from the label in the receiver mixer is multiplied by 20 cos (ωεή (I-channel), and sin (a> ct) (Q-channel), and with omission of the sum frequency terms that yields:

Carrier wave, I channel:

Sj (/) = A · cos {(üct + φχ + φ2) · cos (a) ct)

a

= - · cos (<p1 + <p2) (2) 25

Carrier wave, Q channel:

ScQ (t) - A · cos (a) ct + <pi + φ2) · sin (wct) i (3) 1029590 - 5 - - sin (? L + ^ ¾) USB, I channel:

a

Susbl (t) = 2 · sin ((ö> c + <xv) i + <pi + <? W>) · cos (mct) A. , = - · sin (u) sct + <pi + cpusb) (4) 5 USB, Q channel:

a

SusbQ (t) = - · sin ((o> c + a> sc) t + <ρι + φ ^) · sin (ωεή = ^ · COS (ajsct + φι + <Pusb) (5)

In Q-channel shift 90 degrees phase: ^ 4 S ^ bQ '(t) = - · sin (ojsct + φι + cpusb) (6) 10 Sum of I- and shifted Q-channel:

Susb (t) = 0

Difference from I and shifted Q channels:

a

$ usb (0 ~ ^ * COS (0) sct + ψ \ + (Push) (7) 15 LSB, I channel:

Sisbl (0 = 2 * ~ sin ~ 1 + + Φι * ώ * cos (ω «0 A., = - · -sin (-ω * + + φι + ψι, φ)

a

= - · sin (a) sct - φι - φι *) (8) 20 LSB, Q channel:

a

^ Q (/) = - · sin ((cuc - cüiC) / + 99, + 0> ω) · sin (cocr) 1 029590

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= - · -COS ((Osct + Ψ \ + <Plsb) = - ^ · COS ((Dsct - <pi - (plsb) (9)

Shift 90 degrees in Q channel:

SlsbQ (t) = - · sin (o> sct - (pi - tpisb) (10) 5 Sum of I and shifted Q channels:

Sisb (t) = - · sin (a) sct - φι - φιΛ) (11)

Difference from I and shifted Q channels:

Slsb (t) = 0

This leaves two channels of which one carries the upper-sideband signal (USB) and the other carries the lower-sideband signal (LSB).

So after sideband filtering, the following terms remain:

a

Susb (*) = - · COS (cOsct + ψ] + (push) (7)

a

Cut) = - · sin (ajsct - φι - <pltb) (H)

The phase difference between the demodulated USB and LSB signal now becomes: 1 5 ^ (pusb - Isb - ^ -Susb if) ~ Isb (0 ψΐ "t" (pusb (ψ \ (Plsb) = 2 (p \ + (pusb + <pisb (12) 20 Now the phase angles are determined by: <Pi = y · 360 [°] - QAl · 360 [°] 300 1029590 - 7 - if cf sc) ^ z: rv roi 300 3601 1 (14)

ψΜ = i2 (f'mfx) '360 ri US

where d \ is the distance from the transmitter to the label, and d2 is the distance from the label to the receiver, ƒ in MHz.

5 Substitution in (12) results in: 360 ktpusb-isb = 2 (c / i + d2) fc · - [°] (16)

The phase difference between the upper-sideband and the lower-sideband signal is thus proportional to the sum of the distances d 1 + d 2 and to the carrier frequency f c. The frequency of the subcarrier plays no role.

For a carrier wave frequency of, for example, 865 MHz, this means that a change in d \ + d2 of 0.17 m causes a phase rotation of 360 degrees.

This is too sensitive for a positioning system. In order to be able to clearly measure distances at somewhat greater distances, we can modulate the carrier frequency a> c, and then determine the modulation in the phase shift Αφ ^ - isb.

If the carrier frequency is varied by the size Afc a change occurs in the phase shift by the size of: 360 A (Afc) (A <Pusb-isb) = 2 {dx + d2) Afc · - [] (17)! It thus appears that the distance sensitivity depends on Af, An Af = 3 MHz now gives a maximum clear distance of (d \ + ¢ / 2) 300 = 50 m, and 150 m for frequency difference of 1 MHz.

The receiver input according to the invention, see Figure 3, consists of a quadrature mixer circuit 19 in which the received signals are multiplied by carrier wave fc. Then the difference frequency products f] sb, and fusb, as well as the sum products fsc + 4b and fsc + fusb arise at the output. In phase shift networks 20 and 21, followed by the band pass filters 22, the frequency components 4b and 4b 1029590-8 are passed. The sum products are removed, as well as all low-frequency products. This also removes all dutter. The remaining signals only come from the label. These are applied to a phase comparator circuit 23. The output signal thereof is in principle a direct voltage. Low pass filter 24 filters out noise 5 and frequency components from the received signals and passes only the result of the phase measurement to DSP processor 25. In the practical solution, the phase comparison 23 and low pass filter 24 are included in the signal processing algorithm that is in DSP processor 25 is running. Microprocessor 26 calculates the distance from the measured frequency data, controls the interrogation unit, and communicates the measurement results to the outside.

Microprocessor 26 also controls the frequency synthesizer 7, and thereby determines the carrier frequency fc. Thus, changing the carrier frequency, essential in this invention, is controlled by microprocessor 26.

15

In the method described above, only the distance between interrogation unit and label 13 is measured. All labels are identical therein, so that individual objects and animals cannot be distinguished. In some applications that may be sufficient, but in other applications, for example in a tracking system for a herd of cows in a pasture or stable, individual animals must be able to be followed. To this end, the circuit is provided in a comprehensive label 24 with a code generator 31, see Figure 4, which contains a memory with an identification number stored therein, and a modulator 32 which modulates the identification number in binary form on the subcarrier. In principle, all modulation methods such as ASK (amplitude modulation), FSK 25 (frequency modulation) and PSK (phase modulation) can be used. The preferred method, however, is binary differential PSK where phase jumps of 180 degrees indicate, for example, a logical 1 bit. This method gives rise to a simple circuit in the label, while a subcarrier is constantly present for the distance measurement.

30

Demodulation of the PSK modulation is possible by adding a PSK demodulator circuit 33 in the receiver circuit, which either demodulates the modulation of the LSB, or of 1029590-9, the USB signal and supplies the result to microprocessor 26. See figure 5 wherein the circuit of Figure 3 is expanded with the identification function.

Since the 180 degree phase jump occurs in both components, fkb and fusb, the phase jump will not be present in the phase difference signal produced by phase comparator 23, so that the distance measurement is not thereby disturbed.

This method for the identification function has already been described previously in patent application no. 1026338 (System for detection, location and identification according to the FM-CW principle) of Nedap.

In a further extended embodiment of the invention, data communication is provided from the interrogation unit to the label. Possible applications are, for example, the writing of information in a memory circuit in the label, the execution of a two-way communication protocol with the label, for example for a multi-label / anti-collision protocol other than according to the ALOHA principle, or for a authentication process.

Figure 6 shows a block diagram of a suitable embodiment of a label 34. The interrogation signal received by antenna 14, which is amplitude-modulated with data to be sent to the label, is demodulated by diode 17. Due to the relatively large distances between the interrogation unit and the label, the demodulated voltage applied to the modulator and demodulator circuit 35 will be small. Circuit 35 amplifies and conditions this demodulated signal, and provides it to processor and memory circuit 36. Depending on the application, circuit 36 will store data, and / or handle a communication protocol or authentication protocol.

Figure 7 shows the extensive circuit of the interrogation transmitter 37. A modulator circuit 38 is added with respect to the transmitter circuits in Figures 3 and 5, which modulates the frequency interrogation signal in amplitude 30. Microprocessor 39 generates the data stream to be sent and controls the entire communication / authentication process. Microprocessor 39 can be combined with microprocessor 26 in the receiver part in the realization.

This method of data transfer to the label has also been described in applicant's patent application 1026338 and has been repeated here to show its applicability in the method of the invention.

The said patent application also describes the method in which the distance measuring function has been extended to a positioning function using a centrally arranged transmitter and a plurality of receivers (bistatic arrangement). For reference, these receivers receive the signal transmitted by the transmitter directly, and the signal modulated by the subcarrier label, between which there is a path length difference. The position of a label can be determined with this by means of locating principles that use hyperboles, known from the existing radio location systems such as DECCA and LORAN-C. This method is usable in the same way in the invention described herein concerning a system for detection, location and identification by means of frequency hopping and phase measurement.

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Claims (3)

1. A radio location determination system, characterized in that an interrogation transmitter transmits a frequency-stepping interrogation signal to a label, which label comprises an antenna, the impedance of the antenna being modulated with a signal generated in a subcarrier generator, and wherein it is transmitted by the The antenna in the tag retransmitted signal is modulated and is received back in an interrogation receiver. A radio location determination system according to claim 1, characterized in that by multiplying the sideband frequencies of the received label signal with carrier frequency fc with the transmission signal with frequency fc two difference frequencies flsb and fusb, which mutually have a difference in phase which is proportional to the phase sum of the distances between the interrogation transmitting antenna and the label and that between the receiving antenna and the label, and with the carrier frequency fc. 3. A radio location determination system according to claim 2, characterized in that a stepwise change of the carrier frequency fc with value Af causes a change in the phase difference Δφ between the two difference frequencies 4b and 4sb, wherein Αφ is proportional to Af and the sum of the distances mentioned. 102 1029590