NL9301541A - Antenna for electromagnetic detection system. - Google Patents

Description

Antenna for electromagnetic detection system

The invention relates to an electromagnetic detection system, comprising a transceiver which is coupled to at least one antenna element to form an electromagnetic interrogation field and a number of responders which can generate a detectable signal under the influence of the interrogation field. Such electromagnetic detection systems are designed to detect and recognize certain persons, objects, vehicles, animals and the like, or the class or group to which persons, objects, vehicles, animals and the like belong. Such a system comprises responders which generate an encoded signal in a suitable interrogation field. Such a responder is described, for example, in applicant NL 176 404. This responder is widely used in, among other things, automatic feed systems. For example, a cow or a pig wears a responder on a collar. When this responder comes near a headpiece with which an electromagnetic interrogation field is emitted, the responder responds by transmitting a unique code. This code is received by an antenna, which may be the same as the transmitting antenna, detected in an electronic device and fed to a computer. The computer then determines how much feed the animal still has credit, after which this amount can be automatically deposited in a feeding trough in the vicinity of the animal. A fence construction ensures that only the intended animal receives its portion and that the animal is not disturbed by other animals when eating. In practice, it is often the case that, in order to detect the responder of an animal, the animal is, as it were, run through the antenna. In that case, the antenna coil has the appropriate dimensions, for example 2x1 meters.

It is an advantage that the antenna coil resonates at the system frequency. After all, the current in the circuit then becomes much greater than the control current due to the oscillation, so that a certain field strength can be achieved even with a low control current. In addition, the resonant coil forms a filter for the received signals, which mainly filters out disturbances with low frequencies.

Traditionally, the antenna coil consists of several turns. In addition, the coil must be robust (resistant to eating by animals and the stable environment) and it must be enclosed for protection. This casing, for example a tube, must be treated again and sealed against, inter alia, moisture penetration.

In practice, these requirements often lead to complicated and objectionable constructions of the antenna coil, which also require a great deal of time to install.

Said drawbacks can be overcome by using an antenna coil with only one turn. The antenna can be constructed from a strong metal conductor, for example, stainless steel. This provides a robust antenna and treatment against moisture and mechanical protection is no longer necessary.

It is also possible, as described in EP 0 331 269, to use part of the physically present construction as a single winding antenna coil.

To achieve the desired impedance matching between the antenna coil and the transceiver, an impedance transformer is used. The electromagnetic detection system then has an electrical configuration as outlined in figure 1. A transceiver (1) is coupled by means of an impedance transformer (2) to a one-winding antenna coil (3). Given the relatively high currents that can flow in coil (3) and the low permissible impedance of coil (3), the transformer (2) is preferably mounted as close as possible to coil (3).

The detection system is brought to the correct tuning frequency by means of an adjustable capacitor, which is mounted on the primary side of the antenna transformer (2).

The drawback of this tuning method is that the tuning capacity is very dependent on the transforming ratio of the transformer (2). Since the transformer is part of the resonant circuit, the losses in the transformer (2) will also affect the quality factor Q of the tuned antenna.

The object of the invention is to overcome these drawbacks. To this end, the circuit is changed in such a way that resonance only occurs in the secondary circuit of the transformer. The one-winding antenna coil must then resonate with the tuning frequency of the system with a tuning capacitor. This step is not obvious, since the self-inductance of a one-winding antenna is very small, so that a very large value for the tuning capacitor must be chosen. When realizing an antenna circuit with a high quality factor, the bandwidth is small and the tuning must be carefully arranged. Continuous tuning is then highly desirable. Making a very large capacitor continuously adjustable is technically difficult to achieve.

Tuning to the desired frequency can also be done by making the inductance of the resonant circuit adjustable. An adjustable inductance as a tuning element in series with the antenna coil makes this possible.

The invention is described in more detail below with reference to figures.

Figure 1 schematically shows a conventional configuration of a one-winding antenna of an electromagnetic identification system. Figure 2 schematically shows the electrical configuration of a detection system according to the invention.

Figure 3 schematically shows the electrical configuration of the automatic tuning.

Figure 4 schematically shows another configuration for the automatic tuning.

Figure 5 schematically shows the unwanted coupling between two antennas and the compensation circuit used.

Figure 6 shows an exemplary embodiment of the compensation circuit.

Figure 2 schematically shows the electrical configuration of a detection system according to the invention. The transceiver (1) is coupled to an impedance transformer (2) that provides the correct impedance matching. The transformer (2) is connected to the antenna coil (3) by means of a tuning element (4), which ensures that the antenna coil (3) is tuned to the correct frequency. Tuning on the secondary side of the transformer (2) by means of a tuning element (4) has the advantage that the tuning does not depend on the transform ratio of the transformer (2) but only on the impedance of the antenna coil (3) . The losses in the transformer (2) are also much lower because it is no longer part of the resonant circuit.

Tuning the resonant frequency of the antenna circuit to the system frequency can be achieved by controlling a direct current in the adjustable inductance. Pre-magnetizing the core then results in a change in the permeability of the coil core and the self-inductance will change accordingly. A circuit can even be devised in which this coordination is automatically realized.

Figure 3 shows such an embodiment schematically. A phase comparator (7) compares the phase of current and voltage and controls the direct current through the tuning element until there is no longer a phase difference between current and voltage: the circuit resonates. Current and voltage must be measured to properly set the resonant frequency. At resonance not only current and voltage are in phase, but both are also maximum.

Another embodiment, shown in Figure 4, uses this. One-winding antennas are often very large, so that the magnetic coupling between two such antennas, which belong to different transmitters / receivers, is by no means always 0. If two such antennas are in close proximity, then there is crosstalk of signals from one antenna to the other and vice versa. If this is limited to only the transmission signal, this is not a major problem. Because the frequency differences of different transmitters are small, beatings occur at this very low frequency. These beats can be easily filtered in the receiver. Respondent signals are also passed on via this unwanted link. For example, a responder activated in the field of an antenna (3) will generate a signal in antenna (3). The signal that the responder generates directly in another not too distant antenna is very weak. However, due to the magnetic coupling of the large antenna loops, a signal is still passed on in this indirect manner and the responder can be detected in the wrong place.

In order to counteract this coupling, a transformer with an adjustable coupling is connected parallel to the antenna coils, which may show this effect.

This is shown in figure 5. The added transformer (8) is connected with such a phase that, when correctly adjusted, the coupling present between the large coils is correctly compensated.

A practical embodiment of this transformer is shown in figure 6. The transformer (8) here consists of two pot core halves (9) which are mounted on a shaft (10) in a slidable manner. Sliding the pot core halves (9) along this axis (10) towards each other increases the magnetic coupling between the two halves and vice versa.

Claims (7)

An electromagnetic identification system, in which the transmit and receive antenna is designed as a loop with only one winding, which is coupled to the transmit / receive device by means of a transformer, characterized in that the secondary circuit of the transformer, to which the one-winding antenna is connected in series with a capacitor, resonates at the operating frequency of the identification system. An electromagnetic identification system according to claim 1, characterized in that a coil with adjustable self-inductance is connected in series with the one-winding antenna, so that the circuit can be tuned to the resonant frequency. An electromagnetic identification system according to one or more of the preceding claims, characterized in that the inductance of the coil is determined by pre-magnetizing the core of the coil by flowing a direct current of a certain value into a coil of the coil. An electromagnetic identification system according to one or more of the preceding claims, characterized in that a magnetic part combines the function of controllable coil and of transformer. An electromagnetic identification system according to one or more of the preceding claims, characterized in that the tuning of the resonant frequency of the circuit by means of the controllable coil is realized automatically by including the coil in a control loop, with such a current in the control winding of the adjustable inductance is controlled, so that current and voltage in the resonant circuit receive the same phase. An electromagnetic identification system according to one or more of the preceding claims, characterized in that the tuning of the resonant frequency of the circuit by means of the controllable coil is automatically realized by including the coil in a control loop, with such a current in the control winding of the adjustable inductance is controlled, so that current or voltage in the resonant circuit is maximized. An electromagnetic identification system according to one or more of the preceding claims, characterized in that a winding of a transformer with an adjustable coupling is connected in parallel to the one-winding antenna coil, wherein a second winding of this transformer is parallel to a second antenna coil of a second transceiver and whereby the adjustable coupling of the transformer is adjusted in such a way that the magnetic coupling present between the two antenna coils is compensated by this.