Field of the invention

The present invention relates to a label for use in a positioning system, which label is provided with a transmission and reception unit for sending and receiving positioning signals, the transmission and reception unit comprising a plurality of coil antennas. Further, the invention relates to a positioning system, a livestock management system, and a method for determining a position of at least one first label of a plurality of labels.

Background

A label as described above is known per se for use in stock farming, for managing, e.g., herds of animals. The position of an individual animal, such as a cow in a herd of cows in a barn, can be determined when the distance between the animals mutually and the distance to a small number of beacons is known. If the animal carries a device with which the distance to neighboring animals can be measured, eventually, the position of the animal in the herd can be determined. In practice, this measuring instrument, and the measuring method used, should be resistant to the conditions in a barn, in consequence of which not all measuring methods are usable. In addition, the measuring instrument must regularly carry out a measurement, preferably in the order of a few seconds, and, because of battery supply, must therefore consume power economically. Furthermore, in a livestock management system, for a large number of cows, the distance to a considerable number of other cows is to be measured at a high pace.

Distance measuring can be done in different manners. For measuring the distance between two cows, a contactless system is necessary. Known manners include measuring with light (lidar), with sound (sonar) or with microwaves (radar). These measuring methods are not suitable or less suitable for measuring the distance between two cows. Especially with regard to the propagation of the measuring signal through the body of a cow, these systems are not properly usable. To a greater or lesser extent, the accuracy of the different technologies is also limited by the transmission and reception range, the amount of sensitivity to reflection of transmitted signals, as well as signal attenuation due to walls and other objects in the indoor space of a barn.

Summary of the invention

It is an object of the present invention to provide a label as described hereinabove with which a position, for instance relative to other labels, can be determined with sufficient accuracy and which is suitable in an indoor space such as a barn and among other animals.

To this end, the invention provides, according to a first aspect thereof, a label for use in a positioning system, which label is provided with a transmission and reception unit for sending and receiving positioning signals. The transmission and reception unit comprises a plurality of coil antennas. Each coil antenna is located in the label such that a longitudinal axis through the coil antenna extends, in use, parallel to a surface of the earth. The plurality of coil antennas comprises at least one first coil antenna and at least one second coil antenna. The at least one first coil antenna and the at least one second coil antenna are mutually differently oriented. The transmission and reception unit further comprises, for transmitting an outgoing positioning signal, an energization circuit. The energization circuit is configured for energizing the transmission and reception unit with an electrical signal, for generating a first magnetic alternating field with the first coil antenna and a second magnetic alternating field with the second coil antenna. The transmission and reception unit further comprises a readout circuit which is configured for determining a voltage potential of both an alternating voltage in the first coil antenna and an alternating voltage in the second coil antenna. The readout circuit is configured in this way for receiving an incoming positioning signal. The insight underlying the invention is that measuring with magnetic fields can generally take place unhindered by ambient factors. In consequentce, the use of magnetic fields for the present application provides a high position resolution and a sufficiently large distance range to enable the measurements to be made. Moreover, magnetic fields can effortlessly propagate through a large body such as that of a cow, and are consequently much less sensitive to, e.g., signal reflections.

To be able to make use of magnetic fields for accurately making distance measurements, such a label should be configured such that the measurement is disturbed as little as possible by fields originating from interaction of the environment with the generated magnetic alternating fields. The chief source of disturbance in the surroundings is formed by the earth: the ground reacts for instance with eddy currents generated therein, when changing magnetic fields have a field direction that is transverse to the ground/earth. The label according to the invention is configured, with the aid of two coils (coil antennas), to generate a horizontal magnetic alternating field. An alternating field is not influenced by any static magnetic fields present. Further, due to the horizontal direction of the generated fields, the measurement is insensitive to the presence of the bottom/ground. Moreover, with the aid of a magnetic alternating field, a large distance range can be gained.

The label according to the invention further makes use of horizontally directed magnetic fields. This is meant to say that the magnetic fields are generated by coil antennas of which the longitudinal axis through the turns of the coil extends (in use) parallel to the earth’s surface. Theoretically, also use can be made of vertically directed magnetic fields (fields of which the north-south axis of the magnetic field is located transversely to the earth’s surface). Advantage of a vertically directed magnetic field is that such a field propagates around the coil symmetrically and so, for every angle in the horizontal plane, provides the same field strength for the same distance. In this way, on an imaginary circle around the label, the same signal is measured throughout, so that, theoretically, this is a suitable measure of the distance to this animal. Even so, from a practical point of view, this implementation is less suitable, because the earth forms a magnetic short-circuit for alternating magnetic fields. The proximity of the earth’s surface disturbs the generated alternating field, in consequence of which, already at a good two meters’ distance, the value of the measured field strength can start to deviate from the theoretically expected value, and a measuring error arises. The field to be measured will, at a distance, become rapidly weaker and change direction, which limits the measuring distance. Because each ground varies in composition, this cannot be corrected for in advance. For this reason, this measuring method provides a reliable measurement only at a great height above the earth’s surface, or near the earth’s surface only over short measuring distances up to about three meters, so that the methodology is not suitable for the present application in a barn.

The invention, for that reason, makes use of a horizontally directed alternating magnetic field, and in particular a magnetic field rotating with a constant rotation speed, that is generated with two horizontally directed coil antennas placed at an angle to each other. Because the label makes use of these two alternating magnetic fields in the horizontal plane (the plane parallel to the earth’s surface), there arises a magnetic field rotating or moving elhptically or circularly (depending on the frequency, direction and mutually relative phase of the magnetic fields). The magnetic field strength of this field can be determined with one or more further coil antennas of a receiving label - which receiving coil antennas are likewise horizontally directed with respect to the earth’s surface. The distance to the sending label is determinable by determining the amplitude of the received magnetic field. The magnetic field strength decreases in the vicinity of the coil antennas with the third power of the distance (distance cubed), and at a greater distance with the second power of the distance (distance squared) (the transition from third to second power takes places at a relatively great distance (order of magnitude 200 meters) in comparison with the measuring distance (a few meters to a few tens of meters)). The amplitude of the generated magnetic alternating field is accurately determinable. Thus, the label is able to determine the distance between two individual animals, independently of the angle at which these two animals are positioned, by starting from the dependency on the decreases of the magnetic field with distance. In this manner, the mutual distance between animals can be determined in a barn with randomly placed cows.

In principle, the methodology is applicable with two magnetic alternating fields oriented horizontally in the above manner; of which a few parameters are known or are fixed. For instance, according to some embodiments, the at least one first coil antenna and the at least one second coil antenna are oriented at an angle with respect to each other. A random angle between the coil antennas in the horizontal plane provides, when the frequencies of the two fields are not equal to each other, a magnetic field that in the horizontal plane changes direction and strength continuously. However, for each direction, it holds that periodically the sum of the two fields provided by the two coil antennas adds up to a fixed maximum that depends on the distance. So, by receiving the signal sufficiently long and measuring the field strength, this maximum can be established and the distance is known. A similar connection also exists between the average magnetic field strength and the distance, as can be established on the basis of statistics, so that the maximum need not necessarily be looked for. While this is a possible implementation of the invention, it is not the most advantageous embodiment thereof. This is due to the continuously changing resultant of the magnetic field. At equal frequency of the two fields, the result of such a measurement becomes more definite, hence better, but the shape of the rotating magnetic field, as a result of the random (but fixed and known) mutual angle between the two fields, may be an elliptical path. If both the angle and the phase between the two fields are known, the shape of this ellipse is fixed and the distance can be determined with knowledge of the transmitted amplitude. In a preferred embodiment, the at least one first coil antenna is oriented transversely with respect to the at least one second coil antenna. Because of the mutually transversely oriented coil antennas, on the receiving side the mutually perpendicular components of the field can be analyzed.

According to a further embodiment, the at least one first coil antenna and the at least one second coil antenna are located in proximity to each other, preferably adjoining each other. In these embodiments, it is achieved that, as a result of either mutual induction or capacitive coupling (or through a combination of both), energization of one coil antenna will also lead to the generation of a magnetic field with the other coil antenna. In particular, this occurs efficiently given a sufficiently high coupling factor between the coils, e.g., a coupling factor of 2.5% or more. The alternating magnetic fields, as a result of this reciprocally inductive or capacitive coupling, will have a mutual phase difference of n/2 radians and, in perpendicular orientation relative to each other, provide a circularly rotating magnetic field. This makes the distance simply determinable in all directions. For this reason, according to some embodiments, the energization circuit is configured for energizing the transmission and reception unit for generating the magnetic alternating field in the at least one first coil antenna. Of course, this is only one of the possibilities, and the energization circuit may also be configured to generate a field in both coils, or two energization circuits may be present.

According to a further embodiment, the first and the second magnetic alternating field are equal to each other in frequency. As indicated earlier, the shape of the magnetic rotating field (circle or ellipse) is thereby fixed and is predictable when the phase is known (starting from known amplitude). Further, according to a further embodiment, the first and the second magnetic alternating field have a mutual phase difference of a quarter of their wavelength ( /2 radians). Given equal frequency and with this phase difference, mutually perpendicular horizontal magnetic fields provide a circularly rotating magnetic alternating field, so that the field strength of the received magnetic field at the receiver becomes independent of the direction to the sending label. The distance is then directly determinable from the amplitude of the signal. Because the receiving label also has two mutually perpendicular receiving antennas, determining the distance in this manner becomes simple and accurate.

According to a further embodiment, the transmission and reception unit is provided with at least one first resonance circuit comprising the first coil antenna and at least one second resonance circuit comprising the second coil antenna, and wherein the energization circuit is switchably connected with at least one of the first and the second resonance circuit for energizing the transmission and reception unit for transmitting the outgoing positioning signal. In this embodiment, the energization circuit is switchably and selectively connectible with the resonance circuits of the first or second coil antenna so that one of the antennas can be energized for transmitting an alternating magnetic field. The first and second coil antenna are preferably sufficiently close to each other to be coupled by reciprocal induction, capacitive coupling, or both, as described above. This, however, is not a requirement. Alternatively, it is also possible to bring the energization circuit selectively - and, for instance, time -dependently with the aid of a controller - into contact with both resonance circuits, for instance when there is no or insufficient coupling involved between the coil antennas.

According to some of the above embodiments, the at least one first resonance circuit or second resonance circuit is a resonance circuit having a high quality value, such as a quality value of at least 100, preferably at least 200; such as, e.g., a quality value of 300 or 400. In this way, the resonator can oscillate independently for many tens of periods while only a small part of the energy is lost in the resonator. The two right-angled coil antennas of the resonators are coupled with each other inductively, capacitively, or through a combination of both.

According to some further embodiments, the label is provided with one single first coil antenna and one single second coil antenna. The use of more than two coil antennas is entirely possible, but a sufficiently efficient and optimally effective embodiment can be obtained through the use of two coil antennas.

Furthermore, according to some embodiments, the readout circuit comprises one or more switching units for selectively effecting and breaking a connection between at least one of the first or the second coil antenna and an earth potential, for selectively discharging the at least one coil antenna upon effectuation of the connection, and for selectively determining the voltage potential of the at least one coil antenna upon breaking of the connection. For receiving a sent transmitted signal from a sending label, it is desired to remove the energy present in the coil antenna, by, for instance, discharging any built-up charge therein. By selectively connecting the coil antennas briefly with an earth potential, the receiving label can be rendered currentless and voltageless (dead). Upon breaking of the connection with the earth potential, the buildup of the signal will be mainly determined by the received transmitted signal, and disturbance by any remaining residual signals can be prevented. Better yet than being discharged via an earth potential, the coil antenna, according to some embodiments, can be briefly connected with a capacitor in a supply circuit, the accumulated energy in the coil thereby flowing partly back again to the supply (i.e., to the battery or accumulator).

In further embodiments, the label comprises a management unit for operating the switchable connection between the energization circuit and the at least one of the first and the second resonance circuit, for energizing the transmission and reception unit for transmitting the outgoing positioning signal. The management unit can operate the switchable connections in the label, and in that way switch the label to a transmit mode or receive mode. In the receive mode, using the management unit, optionally also the distance can be determined, or characteristic signal values can be determined by the management unit and be stored in, e.g., an optional working memory of a label. Also, additional wireless communication means can be optionally present, for exchanging measuring data with a data network. In this last embodiment, the management unit may be configured to switch the label periodically into a transmit mode, and then into a listen or receive mode.

In some of these embodiments, the management unit is furthermore configured for managing the switching units for selectively effecting and breaking the connection between the at least one coil antenna and the earth potential /capacitor (or other discharge solution). Furthermore, according to some embodiments, the management unit may be configured for operating the switching units, for determining the voltage potential of the plurality of coil antennas after a predetermined minimum time span from effectuation and breaking of the connection between the at least one coil antenna and the earth potential. The magnetic alternating field may then be, for instance, a periodic alternating field and the predetermined minimum time span in such an embodiment may be between 15 and 25 periods, and preferably amount to 20 periods. After reopening of the connection with the earth potential /capacitor (or other discharge solution), and during the reception of a transmitted signal of another label from the vicinity, it is desired to wait for a number of periods before measuring the voltage across the coil antennas. The coupling between transmitter and receiver is relatively small and the waiting time is desired to have the voltage across the two coil antennas build up at the receiver. The above specified range of 15 to 20 periods is a fine compromise between the bandwidth, coupling and collected energy for a largest possible detection distance.

According to some further embodiments, the management unit is configured for receiving a synchronization signal and for, on the basis of the synchronization signal, establishing timeslots for operating the energization circuit and the readout circuit. Such embodiments provide an important advantage, making it possible to synchronize the labels mutually to switch them, depending on the timeslots, into a desired operative mode (transmit mode or receive mode). In this way, for instance, in a group of labels, e.g. 500 labels in a herd of 500 cows, the labels can be switched into a transmit mode by turns, so that in the other timeslots listening can be done by each of the labels by bringing them into the receive mode. Also, it is possible, at the beginning of each timeslot, for the receiving labels (i.e. , the labels that are switched in the receive mode in that timeslot) to be briefly discharged via the earlier-mentioned earth potential so as to be able to make an accurate measurement in each timeslot. The measurement may thereafter, as discussed hereinabove, e.g. a number of periods after the reopening of the connection with the earth potential, be determined by measuring the voltage on both coil antennas of the receiving label.

According to some of these embodiments, the management unit is configured for, in a predetermined first timeslot, operating the energization circuit for causing the outgoing positioning signal to be transmitted, and for, in a predetermined second timeslot, operating the readout circuit for receiving an incoming positioning signal coming from a further label. In a herd of animals, in this way, each label can be assigned an own timeslot to send, while in the remaining timeslots, listening to the remaining labels is done. When all labels have had their turn to generate a transmission signal, the process can start anew again, so that the labels are each brought into the transmit mode periodically. According to some other or further ones of these embodiments, the label is provided with a clock system for establishing the timeslots on the basis of the synchronization signal, wherein the clock system comprises a first and a second clock unit, wherein the first clock unit is configured to provide a first clock signal with a first time resolution and wherein the second clock unit is configured to provide a second clock signal with a second time resolution, wherein the first time resolution is smaller than the second time resolution, and wherein the management unit is configured to switch on the second clock unit on the basis of the clock signal of the first clock unit and at a moment in time depending on the synchronization signal. By making use in the above manner of a clock with a low resolution and a clock with a high resolution, where the high resolution clock, depending on the low resolution clock, in each case is switched on at a predetermined moment, an accurate and continuously adjustable mutual synchronization can be gained without the high resolution clock needing to remain switched on continuously. The energy consumption can be limited efficiently in this manner.

In some embodiments, the synchronization signal comprises synchronization triggers which are received with a fixed time interval, and wherein the management unit is configured to switch on the second clock unit on the basis of the first clock signal at a fixed length of time prior to the reception of a first synchronization trigger, wherein the second clock unit is switched off synchronously with the beginning or end of the first synchronization trigger, and wherein a clock value of the switched-off second clock unit is used by the management unit for calibrating the first clock signal, for making accurately predictable a reception of a second synchronization trigger after the first synchronization trigger.

In the above described embodiments, for instance, at fixed periodic points in time a synchronization signal is centrally transmitted, which can be received by all labels. The time span between two synchronization signals is consequently known at the labels. Utilizing the first low resolution clock signal, it is consequently possible in each label to switch on the high resolution clock unit in a timely manner prior to reception of the synchronization signal, so that the high resolution clock unit is active at the moment of reception of the synchronization signal. For instance, the time span - which can be used as reference value - can be counted down with the low resolution clock signal, and at a fixed moment prior to the expected reception, the second clock unit can be switched on. With the high resolution clock signal of the second clock unit, the first clock unit can now be tuned. For instance, related to the above-mentioned time span, as at a fixed moment after the switch-on moment of the second clock, on the basis of the high resolution clock signal, a counter may be started which counts the number of elapsed cycles in the high resolution time signal until the beginning or end of the synchronization trigger - this moment being a centrally determined fixed moment that can be synchronized very accurately. The counted number of cycles is then used to accelerate or decelerate the clock signal of the first clock unit, in order to adjust it and accurately tune it with respect to the synchronization signal. The number of counted cycles may for instance be stored as a value in a buffer or other memory for some time, and be used as a reference value upon reception of a next synchronization signal. If more cycles are counted, the low resolution clock signal runs a bit too fast, leading to the second clock being switched on a bit too soon, and deceleration can be applied. Conversely, if fewer cycles are counted, the low resolution clock signal runs a bit too slow, leading to the second clock being switched on a bit too late, and acceleration can be applied.

In some of these embodiments, the management unit is furthermore configured for, on the basis of the second clock signal, selectively switching on at least one of the readout circuit or the energization circuit. For instance, the management unit can simultaneously activate the readout circuit (receiver circuit) and also start the above- mentioned counter.

In some embodiments of the present invention, the management unit is configured for operating the readout circuit for receiving an incoming positioning signal coming from a further label, and for, on the basis of the voltage potentials of the alternating voltages of the first and second coil antenna, generating positioning details associated with the further label. In these embodiments, on the basis of the determined voltage across the coil antennas of the label, with the readout circuit and the management unit the positioning details relative to the sending label are determined. These positioning details comprise, for instance, the distance to the sending label.

In some embodiments of the present invention, the label is configured for wirelessly sending a data signal comprising positioning details of the label and/or positioning details of one or more further labels to at least one of: a receiver of the positioning system, and the one or more further labels. In this way, the positioning details can be exchanged, so that together with positioning details of remaining labels the mutual positions of all labels can be determined. The positioning details, in this way, can for instance be processed centrally by sending them to a receiver of a central livestock management unit. Also, it is possible for the positioning details to be (wholly or partly) processed locally in the labels themselves, for instance in that each label determines the distances to a number of directly neighboring (or possibly even all) labels.

According to a second aspect, the invention furthermore provides a positioning system for determining positions of a plurality of labels according to any one of the above discussed embodiments, wherein the system comprises at least one receiver for receiving data signals comprising positioning details of one or more of the plurality of labels, further comprising a synchronization unit configured for transmitting a synchronization signal to the plurality of labels. According to a third aspect, the invention provides a livestock management system comprising a positioning system such as described above, with each of the labels being configured to be worn by an animal.

Furthermore, the invention provides, according to a fourth aspect thereof, a method for determining a position of at least one first label of a plurality of labels as described hereinabove, in a positioning system as described hereinabove, wherein each of the plurality of labels is provided with a transmission and reception unit comprising a plurality of coil antennas, wherein each of the coil antennas is located in the label such that a longitudinal axis through the coil antenna extends, in use, parallel to a surface of the earth, wherein the plurality of coil antennas of each label comprises at least one first coil antenna and at least one second coil antenna, and wherein the at least one first coil antenna and the at least one second coil antenna are mutually differently oriented; the method comprising the steps of: transmitting, with the first label, a positioning signal through energizing of the transmission and reception unit for generating a first magnetic alternating field with the first coil antenna and a second magnetic alternating field with the second coil antenna of the first label; and determining, with a readout circuit of a second label of the plurality of labels, a voltage potential of both an alternating voltage in the first coil antenna of the second label and an alternating voltage in the second coil antenna of the second label, for receiving an incoming positioning signal.

According to some embodiments, the at least one first coil antenna and the at least one second coil antenna are oriented at an angle with respect to each other. In a particular embodiment thereof, the at least one first coil antenna is oriented transversely (in particular, for instance, perpendicularly) with respect to the at least one second coil antenna. Furthermore, in some embodiments, the first and the second magnetic alternating field can be equal to each other in frequency, and the first and the second magnetic alternating field can have a mutual phase difference of a quarter of their wavelength. In such a case, there arises as a result of the coil antennas a circular magnetic rotating field, whereby the decrease of the measurable amplitude upon reception only depends on the distance alone. In particular, the amplitude decreases in the vicinity of the coil antennas with the third power of the distance, and at a greater distance with the second power of the distance.

Brief description of the figures

The invention will be discussed hereinbelow on the basis of specific embodiments thereof not intended as limiting, with reference to the appended figures, in which:

Figure 1 schematically shows the field of a label provided with a coil antenna oriented in vertical direction (z direction);

Figure 2 schematically shows the field of a label according to an embodiment of the invention;

Figure 3 schematically shows the buildup of a magnetic rotating field using a label according to an embodiment;

Figure 4 schematically shows a potential curve in a coil antenna with which a magnetic alternating field is received, as generated with a label according to an embodiment;

Figure 5 schematically shows a transmission circuit of a label according to an embodiment;

Figure 6 schematically shows a transmission-reception circuit of a label according to an embodiment, the label receiving a signal from another label according to an embodiment;

Figure 7 schematically shows a synchronization process as implemented in a label according to an embodiment;

Figure 8 schematically shows a livestock management system based on labels according to an embodiment of the invention; Figure 9 schematically shows a label according to an embodiment of the invention.

Detailed description

Figure 8 schematically shows a livestock management system 2 based on labels 4 according to an embodiment of the present invention. In the system 2, inter alia, the positions of cows 3 are monitored. Further, the labels 4 may be configured for integrating therein a number of other functions for the purpose of livestock management, such as health condition monitoring of the animals 3, operating of farm functions such as for example: an automatic feeder, a separation gate, a milking machine, scales, et cetera. The system monitors a plurality of animals, among which the cows

3-1, 3-2 and 3-3. Each animal is provided with a label 4; for the animals 3-1 through 3-3 the labels are 4-1, 4-2 and 4-3, respectively. With each of the labels 4 an alternating magnetic field can be generated, as will be further described hereinbelow. Furthermore, the labels 4-1 through 4-3 may be configured to send data to station 8 which, via network 9, is connected with livestock management server 12. The data can comprise the distance details of each of the labels 4-1 through 4-3 relative to each of the other labels 4-1 through 4-3. The livestock management server 12 can process these details and determine the mutual positions of all labels 4 relative to each other. In an alternative implementation, it is possible that the labels 4 themselves have a processor and memory and are able to mutually determine their mutual positions 4-1 through 4-3 relative to each other. In the labels 4, of course, also a partial processing of the data may take place, with the server 12 performing the remaining analysis and data processing.

In the system 2, in accordance with the present invention, use is made of magnetic fields for determining the mutual distances of the labels

4-1 through 4-3. For this purpose, the labels 4 have coil antennas 10 and 11 with which a horizontal magnetic alternating field can be generated. The labels 4-1 through 4-3 are able to determine therewith the distance between the individual cows 3-1 through 3-3 independently of the mutual orientation and position of the animals. By measuring with the aid of alternating magnetic fields, a good measuring system 2 can be built up. Static magnetic fields are less well applicable for different reasons, inter alia because of the terrestrial magnetic field. High-frequency signals can propagate insufficiently in large watery objects, and for that reason are less suitable. However, alternating magnetic fields having a frequency of less than 5 megahertz (MHz), preferably less than circa 2 MHz, are highly suited for this purpose. The wavelengths associated with these frequencies, for that matter, are typically (much) longer than the measuring distance to be bridged of a few meters up to at most a few tens of meters.

Figure 1 schematically shows a cow 3 provided with a label 4 having therein a vertically directed coil antenna. The coil antenna provides a vertically directed magnetic field B, directed in the direction of the longitudinal axis 5, the vertical direction being indicated in the figure as the z direction. The field lines 7 extend omnidirectionally from pole to pole as indicated in Figure 1, running partly through the ground beneath the animal 3. Most field fines are therefore partly in the ground on which the animal 3 is standing. The earth 1, however, is able, as a reaction to the alternating field, to generate a new alternating field. This field disturbs the desired field line pattern, in consequence of which, already at a good two meters’ distance, the value of the measured field strength can inconveniently start to deviate from the theoretically expected value, thereby introducing a measuring error. The field to be measured will, at a distance, rapidly become weaker and change direction, which limits the measuring distance. Because each ground 1 varies in composition, this issue cannot be corrected for in advance.

Figure 2 shows an embodiment according to the invention, which is provided with two horizontally oriented coils 10 and 11. A random cow (not shown) is in the center of the image in Figure 2, at the point where coil antennas 10 and 11 are schematically represented. This cow is provided with a label 4 according to an embodiment of the invention. Such a label is schematically shown in Figure 9. The label 4 is provided with a transmission and reception unit 50 for sending and receiving positioning signals. The transmission and reception unit 50 comprises a transmission circuit 40 and a readout circuit or reception circuit 60, which are both connected with coil antennas 10 and 11. The coil antennas 10 and 11 can be regarded as external parts of the transmission and reception unit 50, or as separately coupled parts; this is not relevant for the operation of the invention. The label 4 furthermore comprises a power supply 41, for example an accumulator, battery, photovoltaic cell, or a different type of supply. The supply is connected with an energization circuit which is part of the transmission circuit 40. A processor 57 is used as management unit for operating the functions of the label 4, and is furthermore configured to analyze the received signals from the reception circuit 60. Furthermore, the label 4 is optionally provided with a communication unit 59 with which the label 4 can for instance communicate with a central server 12 via station 8 (see Figure 8) or with other labels 4. The communication unit 59 is shown with an antenna of its own, which, however, may, if so desired, also be connected with the coil antennas 10 and 11. Optionally, though preferably, the label is furthermore provided with a plurality of sensors 48 with which various parameters, such as animal parameters or environmental values, can be determined for the purpose of the livestock management system.

In Figure 2, the sending label 4, in order to be able to determine its position relative to other labels worn by other animals 3-1 through 3-3, generates a magnetic rotating field. The magnetic rotating field is produced with the aid of a first magnetic alternating field 15 and a second magnetic alternating field 16. The first magnetic alternating field 15 is produced with coil antenna 10. The second magnetic alternating field 16 is produced with coil antenna 11. The manner in which, according to an embodiment, the magnetic rotating field is built up with the aid of the magnetic alternating fields 15 and 16, is represented in Figure 3 and will be further discussed hereinafter. For an understanding of Figure 2, it is sufficient, for now, to know that the two fields result in a magnetic rotating field (whose field direction turns around its axis in time). The three cows 3-1, 3-2 and 3-3 on the broken line all have the same distance to the sending label 4 in the center. The magnetic field produced by the sending label 4 couples inductively with the coil antennas in X and Y direction in the labels of the cows 3-1 through 3-3. This received field is measurable as alternating voltage on the coils of the receiving labels. In this manner, the receiving labels are able to determine the distance to the sending label 4. This is not dependent upon the orientation of each animal 3 with respect to the fields 15 and 16.

Figure 3 shows the principle of the buildup of a magnetic rotating field using two orthogonal, that is, mutually transversely oriented, magnetic alternating fields 15 and 16. The curve in time t of field 15 (Bi) in the Y direction is represented in the horizontal graph 20 above the figure. The curve in time t of field 16 (B2) in the X direction is represented in the vertical graph 21 next to the figure. In each of the graphs and figure, the time t=0 is represented. The time t=t 1 (t i>0) is represented in the graph 20 as point 23, and in the graph 21 it is represented as point 24. The magnetic alternating fields 15 and 16 have the same waveform, frequency and amplitude, but a mutual phase difference cp of a quarter wavelength: so the phase difference is cp=±7i/2 in radians. At time t=0 the field strength of field 15 (Bi) is at a maximum and positive, and the field strength of field 16 (B2) is nil.

The center of Figure 3 shows the resulting magnetic field B at times t=0 and t=ti. At time t=0, the field B is fully directed in the direction of Bi, as represented by vector 27. At time t=ti the field strength of B2, by then, is greater than the momentaneous field strength Bi. The momentaneous magnitude of the two fields 15 and 16, vectors Bi and B2, is represented in the left upper corner. The resulting magnetic field B at time t=t 1 is represented in Figure 3 with vector 28. As follows from the two graphs 20 and 21, and the resulting magnetic field B, magnetic field B has rotated clockwise, as represented by rotation 29. Upon elapse of a complete wave, field B will have returned again to the initial position represented by vector 27. The terminal point of vector 28 of the resulting magnetic field B in this way traverses a circle as represented in Figure 3.

On the receiving side, it makes no difference how the receiving coils 10 and 11 of the receiving label are oriented. Given an accurate phase difference of cp=±7t/2, the vector 28 of the rotating field describes a perfect circle. Actually, the distance can then already be accurately determined with the signal of one receiving coil, by determining the amplitude of the received alternating voltage signal in the coil: which is indeed equal for each orientation then. Given a random phase difference with cp =±7c/2, the resulting field vector of the field B will not describe a circle, but will describe a line when the fields 15 and 16 are in phase (cp=0 or cp=7i) or an ellipse given a random other phase difference. In that case, the signal of one coil 10 or 11 is not sufficient to determine the distance accurately, but this can be done by determining the amplitudes of the signals of both coils 10 and 11. For that matter, in the preferred embodiment, when the coils 10 and 11 of the sending label 4 are in mutual proximity and one of these coils is energized, automatically a stable equilibrium will arise where the two coils 10 and 11 will transmit with a phase difference of <p=±7i/2. In that case, therefore, the magnetic rotating field comes about automatically.

In Figure 4, the reception signal 33 is shown such as it is received on one of the coils 10 or 11 of the receiving label. The originally transmitted signal 32 is also represented. The amplitude Vmax of the received signal 33 is represented with up-down arrow 36 and with level line 37. The amplitude Vo of the originally transmitted signal 32 is represented with up -down arrow 35 and with level line 30. The ratio between the amplitude Vmax of the received signal 33 and the amplitude Vo of the transmitted signal 32 is indicative of the distance between the sending label 4 and the receiving label.

The above example implementation, depicted in Figures 3 and 4, provides only one possible implementation or class of embodiments. In a variant of this implementation, only one coil antenna 10 of the coil antennas 10 and 11 of the sending label 4 is energized, and the energy flows from the energized coil antenna 10, as a result of coupling between the coils 10 and 11, to the other coil antenna 11. The voltage amplitude across the coil 11 will thereby, in the periods following energization, be built up to the maximum value. This occurs - depending on the coupling factor - in a number of periods, for example 20 periods. Accordingly, there will be a trend between the amplitudes of the first magnetic field 15 and the second magnetic field 16. For instance, the first magnetic field 15 starts with a maximum amplitude which decreases over the following periods (for instance, in a time span equal to 20 periods); and the second magnetic field 16 starts with an amplitude which is nil and which is built up over the following periods up to the maximum (in the same time span equal to 20 periods). By averaging, integrating or cumulating the amplitudes, measured upon reception, over this time span (20 periods) in each direction, eventually for each direction the same value is obtained, from which the amplitude of the received signal, and hence the distance, can be determined. In Figures 3 and 4, such a trend, which occurs, for one thing, upon energization of only one of the coils 10 or 11, is not shown.

Figure 5 shows a transmission circuit 40 of a transmission and reception unit 50 in a label 4. The transmission circuit 40 comprises a first and a second coil antenna 10 and 11. The first coil antenna 10 is connected with a resonance circuit 47 comprising a capacity C _y and resistance R _y connected in parallel with the coil 10. Through switch Si, which is preferably operable through a controller such as processor 57 (see Figure 9), the first resonance circuit 47 can be brought into conducting connection with an energization circuit which consists of the battery 41 and the capacity 42. The second coil antenna 11 is connected with a resonance circuit 49 comprising a capacity C _x and resistance R _x connected in parallel with the coil 11. Through switch S2, which is preferably likewise operable through the controller such as processor 57 (see Figure 9), the second resonance circuit 49 can be brought into conducting connection with the energization circuit which consists of the battery 41 and the capacity 42. In fact, one of the two switches Si or S2 may also be permanently open, or one of the two resonance circuits 47 or 49 may actually not be connected with the energization circuit at all. The coming about of the field in the nonenergized coil 10 or 11 is caused by reciprocal induction, when the coils 10 and 11 are in close mutual proximity. Due to the values of the resonance circuits 47 and 49 (capacities C _y and C _x , resistances R _y and R _x , and coil properties L _y and L _x ), the signal will be able to resonate between the circuits with little loss.

The idea of the circuitry in Figure 5 is that via Si the coil L _y is charged to a current having a predetermined maximum value. After opening of Si, the L _y is then going to oscillate spontaneously with the aid of C _y . A small part of the energy is lost in R _y , but the greater part is transferred to the equivalent resonator around L _x and C _x via coupling 52. The switch S2 is only closed after about 20 periods when the current in L _x has risen to a maximum value. The remainder then flows via S2 back to the supply (through buffer C 42).

Figure 6 shows the interaction between two labels 4. The upper part of Figure 6 shows the transmission circuit 40 of Figure 5. The lower part of Figure 6 shows a complete transmission and reception unit, including a readout circuit 60 and a controller 57. The receiving coils 10 and 11 are at a distance inductively coupled with each of the sending coils 10 and 11 of the sending label. In the receive mode, switches 44 and 45 between the coils 10 and 11 and the energization circuit will remain open. When the receiving label is switched into the receive mode, switches 52 and 53, after they have been briefly closed in order for the resonance circuits of coils 10 and 11 to be discharged of interference signals, will be opened by controller 57. The received signal on each of the coils 10 and 11 is thereupon amplified by amplifiers 55 and 56, and the value of the amplitudes is established by controller 57. When the original field strength (amplitude Vo of the transmitted signal) is known, the distance can be determined. This amplitude Vo may either have been fixed at a fixed known value, or may for instance be communicated in a different manner between the labels 4 or between each label 4 and the central server 12. Various implementations are possible on this point.

To enable having the labels 4 within a group of labels monitor each other’s positions efficiently, it is useful to make use of a synchronization system. This may for instance be done by sending via station 8 (Figure 8) a synchronization signal with which the operation of all labels 4 can be synchronized. Figure 7 shows such a synchronization system. In it, a synchronizer 65 provides a periodic synchronization pulse 66, represented in the figure in time as pulses 66-1, 66-2 and 66-3. In this context, the label 4 has a clock circuit 70. It can for instance be part of the controller 57, but provision can also be made for a separate clock circuit 70. The clock circuit 70 comprises a first low resolution clock 71 and a second high resolution clock 72. The low resolution clock 71, as a result of the lower time resolution, consumes relatively little energy. The clock signal 75 of the low resolution clock, however, may contain inaccuracies, so that the clocks of different labels may start to vary mutually. Due to the synchronization pulses 66 being sent with a fixed interval, the point in time at which a next pulse is received, is known. The clock signal 75 of the low resolution clock 71 can be accurately corrected in that respect by making use of the high resolution clock 72. The clock circuit 70 switches the high resolution clock 72 on, some time before the new synchronization signal 66-1 is received, viz. at the time indicated with 79-1. The high resolution clock 72 builds up its signal 76, and at a defined moment 80-1 the clock circuit 70 switches on its receiver 73 for receiving the synchronization pulse 66-1, while simultaneously a counter is started on the high resolution clock signal 76. This counter continues to count until a moment to be universally established that is equal for all labels, such as the end 81-1 (or the beginning) of the received synchronization pulse 66-1. By comparing the value of the counter with the preceding value of the counter (in relation to the previous synchronization pulse 66), the clock signal 75 of the low resolution clock 71 can be corrected, for instance by accelerating or decelerating it. Thus, with the aid of the counter value in relation to pulse 66-1, the clock signal 75 can be corrected upon the next synchronization pulse 66-2 by comparing these counter values associated with pulses 66-1 and 66-2 with each other.

In this above manner, it can be achieved that the labels 4 can be operative on the basis of allocated timeslots, whereby, for instance, in each case one label 4 is sending while the other labels are listening. Also, the resonance circuits may be operated on the basis of the clock signal 75 such that energy from the transmission coils 10 and 11 can be recovered in buffer capacitor 42 (Figures 5 and 6), by operating the switches 44 and 45 in a timely manner.

The above-described specific embodiments of the invention are intended to illustrate the principle of the invention. It is believed that the implementation and the operation of the invention are readily apparent from the foregoing description and the appended illustrations. The invention is not limited to any embodiment described or shown herein. For the sake of clarity and conciseness of the description, features have been described herein as part of the same or of separate embodiments; it will be clear to a person skilled in the art that embodiments comprising combinations of any or all of the features described also fall within the scope of protection of the invention. Within the ability of those skilled in the art, alterations are possible which are understood to be within the scope of protection. Also, all kinematic inversions are understood to be within the scope of protection of the present invention. Expressions such as "consisting of', when used in this description or the appended claims, should be construed not as an exhaustive enumeration but rather in an inclusive sense of "at least consisting of'. Indications such as "a" or "one" may not be construed as a limitation to just a single specimen, but have the meaning of "at least a single specimen" and do not preclude plurality. Expressions such as: "means for ..." should be read as: "component configured for ..." or "member constructed to..." and should be construed to cover all equivalents of the structures described. The use of expressions such as: "critical", "advantageous", "preferably", "desired", et cetera, is not intended to limit the invention. Moreover, also features that are not specifically or expressly described or claimed in the construction according to the invention but do he within reach of the skilled person, can be encompassed without departing from the scope of protection as defined by the appended claims.