GB2291303A

GB2291303A - Relative position detection

Info

Publication number: GB2291303A
Application number: GB9514409A
Authority: GB
Inventors: Brian Duffy
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-07-14
Filing date: 1995-07-14
Publication date: 1996-01-17
Also published as: GB9514409D0; GB9414256D0

Abstract

In order to monitor the distance between a transmitter 10 and receiver 12 (which may be attached to a child and parent respectively) the transmitter periodically transmits PWM coded bursts of RF from antenna 24. These are received by "zig-zag" antenna 26 (fig 3), and if they are determined 32 to be correctly coded and 30 of sufficient amplitude, timer 36 is reset. If timer 36 is not reset after n transmission periods, an alarm 40 sounds. The receiver (50, fig 4) may also incorporate a direction-finder comprising omnidirectional antenna (52) and coil antenna (54). The omnidirectional antenna (52) output gates transistor amplifier 56 which is amplifying the coil antenna (54) output to perform phase comparison; the magnitude of the resultant signal is taken to be indicative of the direction of the transmitter and indicated by LEDs (70). For monitoring purposes, oscillator (74) replaces coil antenna (54). To prevent back-sensing, coil antenna (54) is screened (72). <IMAGE>

Description

LOCATION DETECTION This invention relates to apparatus and methods for use in identifying the location of a transmitter relative to a receiver. The invention also relates to elements of transmitters and receivers.

According to one aspect of the present invention there is provided apparatus for use in indicating the orientation of a receiver relative to a transmitter, the apparatus including a transmitter and a receiver,the receiver having: an omnidirectional antenna; and a directional antenna, means for combining signals received by the antennae such that the output of the combining means is determined by the relative orientation of the directional antenna to the transmitter; and means for providing an output indicative of said orientation.

In use, the apparatus may be utilised in many applications, including child trackers: a child carries the transmitter, while the parent carries the receiver. By rotating the receiver and monitoring the orientation indicating means, the parent will be able to determine the heading to follow to find the child.

Preferably, the output of the combining means is a function of the phase difference between the antennae outputs; if the directional antenna is turned 900 from the transmitting antenna, the phase of the directional antenna output signal will be moved by 900 relative to the omnidirectional antenna output. Conveniently, the directional antenna is a loop antenna.

The combining means may be in the form of an amplifier transistor, and the output signal from the directional antenna may be used to open the gate of the transistor to control the passage of the omnidirectional antenna output signal which passes through the transistor. If the phase angle of the directional antenna output signal is "inphase" with the omnidirectional antenna output signal, the resulting output from the amplifier will exhibit some gain.

If the phase angle is "off", the amplifier will be switched on at a time when the incoming output signal from the omnidirectional antenna is at a low level, and the resulting transistor output signal will be significantly lower and distorted.

The signal to control the gate of the amplifier transistor may be switched from the directional antenna to a stable signal generator in which mode the receiver will operate as a monitor, to show that the transmitter is still within detecting range of the receiver. Of course other means for combining or mixing the antennae signals may be utilised.

The receiver may also include comparator means for determining whether or not the directional antenna is aligned with the transmitting antenna. On the comparator means receiving a high level output signal from the combining means, indicating alignment, the orientation indicating means, which may be in the form of a sounder or signal level display, is enabled to provide an appropriate audible or visual output.

Preferably, the receiver includes a shield for at least the directional antenna to prevent output of an "aligned" signal when the user is facing 1800 away from the transmitting antenna.

The apparatus may also be capable of monitoring or measuring the distance between the transmitter and the receiver. This distance monitoring aspect of the invention may be provided in the apparatus separately from the direction or orientation detection feature, and is described below.

According to another aspect of the present invention there is provided apparatus for use in measuring the distance between a transmitter and a receiver, including a receiver having means for correlating the strength of signal received from a transmitter to the distance therebetween, and means for providing an output indicative of said distance.

The output may take the form of a visual indication of distance, when the apparatus is to be used, for example, as a surveying aid. Alternatively, the output may be an alarm signal for providing an indication that the distance exceeds or is within a predetermined "safe" distance when the apparatus is used, for example, as a child alarm or a proximity sensor, respectively.

The transmitter may produce a continuous output though, preferably, the output is intermittent, thus minimising the power requirements of the transmitter.

The transmitter and receiver may be provided as a unique pair, the transmitter producing an encoded signal and this being the only signal recognised by the receiver.

Such an arrangement is necessary when the apparatus is utilised, for example, as a child alarm, to avoid the receiver detecting other receivers when used, for example, in a busy shopping mall. Alternatively, a transmitter may recognise a plurality of encoded signals for use, for example, by a parent with two or more children provided with respective transmitters.

Preferably, when used as an alarm, the receiver includes a timer which is reset each time a signal of a predetermined strength is received, failure to reset the timer within a predetermined timed period resulting in activation of alarm means. Most preferably, a plurality of signals are produced by the transmitter within each timed period, this reducing the likelihood of false alarms due to spurious single signals.

Preferably also, the transmitter produces a constant amplitude output signal such that as the distance between the transmitter and receiver increases the amplitude of the signal reaching the receiver decreases. Preferably also, the receiver includes means for demodulating and amplifying the received signal, the voltage of the demodulated and amplified signal being correlated to a corresponding distance. It has been found that the voltage/distance correlation is close to a linear relationship and thus provides higher resolution and greater accuracy than a conventional current/distance correlation.

In one embodiment the receiver may include an antenna which is insensitive to the orientation of the transmitter antenna. Preferably, the receiver antenna is linear and includes a plurality of angled portions. Most preferably, the antenna defines a "zig-zag" or saw-tooth pattern. Most preferably, the angle between each adjacent antenna portion is close to 450, greater or lesser angles leading to polarisation, that is sensitivity to the orientation of the transmitter antenna; an increase in the angle between the portions leads to a bias towards vertically oriented or polarised transmitter antennas, while decreasing the angle creates a bias towards vertically polarised antennas. The distance between the ends of the receiver antenna is dependant upon the frequency of the signal, as is the length of each antenna portion and the number of portions in the antenna.

According to another aspect of the present invention there is provided a method of determining the distance between a transmitter and a receiver, the method comprising: transmitting a constant amplitude radio signal from one location; receiving said signal at a second location; demodulating and amplifying said signal; correlating the voltage of said demodulated and amplified signal to the distance between said locations.

According to a further aspect of the present invention there is provided a receiver antenna comprising a plurality of angled portions, whereby the strength of signal received by the antenna is substantially unaffected by the orientation of the transmitter antenna.

These and other aspects of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of apparatus, comprising a transmitter and a receiver, in accordance with a first embodiment of the present invention; Figures 2a, 2b, 2c and 2d are representations of a signal reaching the receiver from the transmitter, the demodulated signal, the amplified demodulated signal, and a graph illustrating the relationship between signal voltage and transmitter\receiver distance apart, respectively; Figure 3 is an enlarged representation of the antenna of the receiver of Figure 1; and Figure 4 is a schematic representation of apparatus, comprising a transmitter and a receiver, in accordance with a second embodiment of the present invention.

Reference is first made to Figure 1 of the drawings, which illustrates apparatus in accordance with a first embodiment of the present invention including a transmitter 10 and a receiver 12. The apparatus may be utilised as a "child alarm", whereby an adult, carrying the receiver 12, is alerted that a child, carrying the transmitter 10, has wandered outwith a predetermined range.

The transmitter 10, the operation of which will be described below, includes a power cell 14, an on/off switch 16, a timer 18, an encoder 20, a radio frequency (r.f) modulator 22 and an antenna 24. The receiver 12 includes an antenna 26, an r.f. demodulator and amplifier 28, a range detector 30, a decoder 32, an AND gate 34, a timer 36, an alarm circuit 38 and an alarm buzzer 40.

In operation, the timer 18 produces signals at regular intervals, the signals being uniquely encoded by the encoder 20 before being converted to an r.f. output and transmitted from the antenna 24. The transmitted signal is in the form of a pulse width modulated (PWM) wave of constant amplitude (see Figure 2a).

At the receiver 12, the incoming signal is demodulated (Figure 2b) and amplified within a range that does not reach the saturation voltage (Figure 2c). The amplified, demodulated signal is then split, part of the signal passing to the range detector 30 where the voltage of the signal is correlated to the distance between the transmitter and receiver 10, 12 (curve A, Figure 2d). The graph of Figure 2d illustrates the relationship between signal voltage and distance, which it will be noted is closer to a linear relationship than a conventional current/distance curve (curve B) . Readings obtained in this manner thus provide higher resolution, and greater accuracy.

In the case of a child alarm, a reading indicating that the transmitter is within a predetermined range of the receiver, say 15 metres, produces a negative output from the range detector 30, while an indication that the transmitter and receiver are more than 15 metres apart results in a positive output.

The other part of the amplified demodulated signals passes to the decoder 32 which compares the encoded signal received from the transmitter 10 with a receiver code: if there is identity the decoder 32 produces a positive output, though if the signal does not match the receiver code, indicating that the signal was produced by a source other than the matched transmitter 10, a negative output results.

The output signals from the range detector 30 and the decoder 32 pass to the AND gate 34, the output of which controls resetting of the timer 36 which, in turn, controls the enabling of the alarm circuit 38 and the operation of the buzzer 40. A negative output from the gate 34, indicating that the transmitter is within range or that a spurious signal has been received, causes the timer 36 to be reset. A positive output however, does not reset the timer, which has a period set to encompass the output of at least one, and preferably two or more, signals from the transmitter 10. If the timer is permitted to run for its full period the alarm circuit 38 is enabled and the buzzer 40 sounded. The provision of the timer 36 thus minimises the possibility of false alarms, as a single spurious signal, for example a reflected signal, will not result in the alarm being sounded.This form of timer arrangement is known as a "watchdog" timer.

Reference is now made to Figure 3 of the drawings which illustrates the receiver antenna 26 in greater detail. The antenna 26 is formed of a length of conductive material laid onto a printed circuit board (PCB) 42 in the same manner as conventional circuit board tracks. The antenna defines a zig-zag or saw-tooth pattern comprising a plurality of straight portions 44 angled (ear) at 450 to one another. The length L of this line is dependant upon the frequency of the signal to be received, as is the length of each portion 44 and the total number of portions 44 in the antenna.

This configuration of receiver antenna 26 is insensitive to the orientation of the transmitter antenna 24 such that where, for example, the transmitter 10 is worn on a child's wrist, movement of the child's arm will not result in variation of the strength of the signal received by the antenna 26, which might otherwise result in inaccurate distance measurements and spurious alarm activations.

Reference is now made to Figure 4 of the drawings, which illustrates a receiver 50 forming part of apparatus in accordance with another embodiment of the invention.

The apparatus may be utilised as a "child tracker", whereby an adult, carrying the receiver 50, may determine the location of a child carrying an appropriate transmitter.

The transmitter may take a similar form to the transmitter 10 described above. However, the receiver 50 is somewhat different from the receiver 12, having both an omnidirectional antenna 52 and a directional loop antenna 54, the output signals from both being directed to an amplifier transistor 56, the signal from the loop antenna 54 being utilised to control the transistor gate, as will be described. The output signal from the transistor 56 passes through a demodulator 58, before being split. One part of the signal passes through a further amplifier 60 and a decoder 62, while the other part passes through a comparator 64. The output signals then pass to an AND gate 66, where the presence of two positive outputs, in a similar manner to the first-described embodiment, causes the gate AND 66 to provide a positive signal to enable an LED power circuit 68 to light an LED 70.

As will now be described, the receiver 50, and in particular the loop antenna 54, may be moved to detect the angular location of the transmitter relative to the user.

When an incoming signal from the transmitter is detected by the loop antenna 54 the phase of the antenna output signal, in comparison to the phase of the output signal from the omnidirectional antenna 52, will be dependant on the angle of the loop antenna 54 to the transmitting antenna. If the loop antenna 54 is turned 900 from the transmitting antenna, the phase of the loop antenna output signal will be moved by 900 relative to the omnidirectional antenna output signal. These two signals may be combined, utilising the phase shift to cancel or limit the omnidirectional antenna output signal. Conveniently, as in this embodiment, this may be achieved by utilising the loop antenna output to open the gate of the amplifier transistor 56 such that the phase difference controls the magnitude of the omnidirectional antenna output signal passing through the transistor 56.If the phase angle of the loop antenna signal is "in-phase", the resulting output signal from the transistor 56 will exhibit some gain. If the phase angle is "off" then the amplifier 56 will be switched on at a time when the incoming signal is at a low level, and the resulting output signal will be significantly lower and distorted. The resulting signal is then demodulated.

In this embodiment the demodulated signal is divided, one portion passing to the comparator 64, where the signal level is measured to determine the orientation of the loop antenna 54 relative to the transmitting antenna. The comparator circuit will provide a positive output when a high level signal is detected, indicating that the loop antenna 54 is "facing" the transmitting antenna.

In a somewhat similar manner to the first embodiment, the other portion of the signal is amplified and decoded, to ensure that the signal received is from the appropriate transmitter. The outputs from the comparator 64 and the decoder 62 are passed to the AND gate 66, which will thus only provide a positive output when an appropriately coded high level signal has been received. The output from the gate 66 enables the LED power circuit 68, to illuminate the LED 70.

To ensure that the LED 70 is not illuminated when the loop antenna is at 1800 to the transmitting antenna, an RF shield 72 is provided within the receiver 50, to one side of the antenna 54.

The illustrated embodiment may also be utilised as a non-directional monitor, by disconnecting the loop antenna 54 and injecting a stale signal into the amplifier 56 from a stable signal source 74 (in other embodiments it may simply be sufficient to disconnect the loop antenna 54).

The LED 70 will thus be illuminated in response to incoming signals from the transmitter while the transmitter remains within range (typically 200 metres). If desired, the receiver 50 may be configured such that intensity of the LED illumination varies with the intensity of the signal received from the transmitter. Also the LED may flash with varying intensity, and then be illuminated continuously when the received signal reaches a predetermined level, indicating that the transmitter is close by, for example less than 10 metres away.

This second embodiment may be provided separately, or may be combined with the first described embodiment to provide, for example, a child alarm which has two modes of operation. In the first mode, the alarm provides a warning signal when a child carrying the transmitter wanders out of a designated "safe" range. If the accompanying adult cannot immediately locate the child, the receiver is switched to the direction finding second mode. This allows the adult, by rotating the receiver, to determine the direction they must follow to locate the child.

It will be clear to those of skill in the art that the above-described embodiments are merely exemplary of the various aspects of the present invention and that various modifications and improvements may be made thereto without departing from the scope of the invention. The configurations of the transmitters and receivers may be subject to variation and it will be recognised that the various circuit components described may well be replaceable with other components or arrangements, for example the AND gates 34, 66 and amplifier transistors 56 described above may be replaced with one of a variety of components which would perform the same function, as well known to those of skill in the art. In addition, the means of alerting the user may be varied as appropriate and, for example, in the second embodiment, the LED could be replaced by a sounder.

Claims

1. Apparatus for use in indicating the orientation of a receiver relative to a transmitter, the apparatus including a transmitter a receiver, the receiver having; an omnidirectional antenna and a directional antenna; means for combining signals received by the antennae such that the output of the combining means is determined by the relative orientation of the directional antenna to the transmitter; and means for providing an output indicative of said orientation.

2. The apparatus of claim 1 wherein the output of the combining means is a function of the phase difference between the antennae output.

3. The apparatus of claim 1 or claim 2 wherein the directional antenna is a loop antenna.

4. The apparatus of claim 1, 2 or 3 wherein the combining means is in the form of amplifier transistor, and the output signal from the directional antenna is used to control the . . . . . . . . of the transistor to control the passage of the omnidirectional antenna output signal through the transistor.

5. The apparatus of any of the preceding claims wherein means for provided for switching the input from the combining means from the directional antenna to a stabile signal generator to permit the receiver to operate as a signal monitor.

6. The apparatus of any of the preceding claims wherein the receiver includes comparator means for determining, based on the output of the combining means whether or not the directional antenna is aligned with the transmitting antenna.

7. The apparatus of claim 6 wherein an output from the comparator means indicating alignment of the orientation indicating means is enabled to provide an appropriate output.

8. The apparatus of any of the preceding claims wherein the receiver includes a shield for at least the directional antenna.

9. The apparatus of any of the preceding claims wherein the orientation indicating means provides an output related to the distance between the receiver and transmitter.

10. The apparatus of any of the preceding claims wherein the receiver has means for correlating the strength of signal received from a transmitter to the distance therebetween, and means for providing an output indicative of said difference.

11. Apparatus for use in measuring the distance between a transmitter and a receiver, including a receiver having means for correlating the strength of signal received from a transmitter to the distance therebetween, and means for providing an output indicative of said distance.

12. The apparatus of claim 10 or claim 11 wherein the output provided by the indicating means is in the form of a visual indication of distance.

13. The apparatus of claim 10 or claim 11 wherein the output of the distance indicating means is an alarm signal for providing an indication that the distance exceeds or is within the predetermined distance.

14. The apparatus of any of the preceding claims wherein the transmitter produces an intermittent output.

15. The apparatus of any of the preceding claims wherein the transmitter and receiver are provided as unique pair, the transmitter producing an encoded signal and this being the only signal recognised by the receiver.

16. The apparatus of any of the preceding claims wherein the receiver includes a timer which is reset each time a signal of a predetermined strength is received, failure to reset the timer within a predetermined time period resulting in activation of alarm means.

17. The apparatus of claim 16 wherein a plurality of signals are produced by the transmitter within a timed period.

18. The apparatus of any of the preceding claims wherein the transmitter produces a constant amplitude output signal such that as the distance between the transmitter and receiver increases the amplitude of the signal reaching the receiver decreases.

19. The apparatus of claim 18 wherein the receiver includes means for demodulating and amplifying the received signal, the voltage of the demodulation and amplified signal being correlated to a corresponding distance.

20. The apparatus of any of the preceding claims in which the receive includes an antenna which is insensitive to the orientation of the transmitter antenna.

21. The apparatus of claim 20 in which said receiver antenna is linear and includes a plurality of angled portions.

22. The apparatus of claim 21 wherein said receiver antenna defines a zig-zag or ........... pattern.

23. The apparatus of claim 22 wherein the angle between each adjacent antenna portion is approximately 450.

24. The method of determining the distance between a transmitter and the receiver, the method comprising: a constant amplitude radial signal from one location; receiving said signal at a second location; demodulating and amplifying said signal; correlating the voltage of said demodulated and amplified signal to the distance between said locations.

25. A receiver antenna comprising a plurality of angled portions, whereby the strength of signal received by the antenna is substantially unaffected by the orientation of the transmitter antenna.