WO1996016341A1

WO1996016341A1 - A position determining system and a method pertaining thereto

Info

Publication number: WO1996016341A1
Application number: PCT/AU1995/000780
Authority: WO
Inventors: Christopher Probert
Original assignee: Christopher Probert
Priority date: 1994-11-23
Filing date: 1995-11-23
Publication date: 1996-05-30
Also published as: AUPM960294A0

Abstract

A position determining system for an object including a master beacon (10) provided on the object and at least two slave beacons (12), wherein for each slave beacon (12), means is provided for calculating a distance to the slave beacon (12) from the master beacon (10) by transmitting a signal from the master beacon (10) to the slave beacon (12) and back to the master beacon (10) a plurality of times, and means is provided for calculating the position of the object from the distances to the slave beacons (12).

Description

TTTLE

A POSITION DETERMINING SYSTEM AND A METHOD PERTAINING THERETO

DESCRIPTION

The present invention relates to a position determining system and a method pertaining thereto. More particularly, the present invention relates to a position determining system and a method pertaining thereto arranged to be used over relatively short distances.

FIELD OF INVENTION Position determining systems have a wide variety of applications. Once such application is in the field of crop spraying. Some kind of position determining system or navigation system is useful to farmers when crop spraying to avoid misses and overlaps.

A solution employing an electronic positioning system would be desirable. However, in the past, electronic position deteπtiining systems have been very expensive over the short ranges travelled within a paddock. This cost is associated with the difficulty and expense of measuring the time taken by radio pulses, light, infrared, laser, microwaves and the like to travel short distances.

SUMMARY OF INVENTION In accordance with a first aspect of the present invention there is provided a position determining system for an object including a master beacon provided on the object and at least two slave beacons, wherein for each slave beacon, means is provided by calculating a distance to the slave beacon from the master beacon by transmitting a signal from the master beacon to the slave beacon and back to the master beacon a plurality of times, and means is provided for calculating the position of the object from the distances to the slave beacons. In accordance with a second aspect of the present invention, there is provided a method of determining the position of an object, including the steps of: (1) providing a master beacon on the object and at least two slave beacons remote from the object;

(2) for each slave beacon, either

(i) measuring how many times a signal can be transmitted from the master beacon to the slave beacon and back to the master beacon within a predetermined time interval; or (ii) measuring a time interval required for a signal to be transmitted from the master beacon to the slave beacon and back to the master beacon a predetermined number of times; (3) calculating from the measurements distances to the slave beacons from the object; and (4) calculating from the distances the position of the object.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows a block diagram of a master beacon for a position determining system in accordance with a first aspect of the present invention;

Figure 2 shows a block diagram of a slave beacon for a position determining system in accordance with a first aspect of the present invention; Figure 3 shows the position determining system of the present invention in use;

Figures 4 a, b and c are a schematic representation of methods for radio detection in accordance with the present invention. Figure 5 shows a further embodiment of master beacon; Figure 6 shows a schematic representation of a particular type of radio signal; Figure 7 shows a block diagram of a spread spectrum transmission;

Figure 8 shows a schematic representation of a signal showing a time difference there between; and

Figure 9 is a schematic representation of a phase difference vernier diagram. DESCRIPTION OF THE INVENTION

Shown in the Figures 1, 2 and 3 is a position determining system comprising a master beacon 10 and at least two slave beacons 12. The master beacon 10 comprises a receiver 14, a transmitter 16, a microprocessor 18, a display 20, a clock 22, a counter 24, a one- shot gate 26 and a preset delay block 27.

The microprocessor 18 controls the display 20, the clock 22 and the counter 24. The microprocessor 18 resets the counter 24 and resets and activates the clock 22 to commence a measurement cycle. The clock 22 activates and enables the one-shot gate 26. The one-shot gate 26 activates the transmitter 16. If the receiver 14 and the transmitter 16 operate on the same frequency, the one-shot gate 26 also disables the receiver 14 whilst the transmitter 16 is active as shown in Figure 3. The transmitter 16 transmits a short radio signal to a slave beacon 12 which retransmits a reply to the master beacon 10. The preset delay block 27 serves two functions. It eliminates confusion when the beacons are operated in close proximity and it reduces the transpond rate. If the transpond rate is too high, the system may generate illegal spurious radio signals.

The receiver 14 receives incoming radio signals from one of the slave beacons 12. At this point, the radio signal is delayed for a predetermined period by the preset delay block 27. The receiver 14 then increments the counter 24 and activates the one-shot gate 26. Activation of the one-shot gate 26 again activates the transmitter 16 and, if the receiver 14 and the transmitter 16 operate on the same frequency, disables the receiver 14.

The transmit, receive, increment counter and transmit again cycle continues until the clock 22 reaches a preset time. Once the preset time has elapsed, the clock 22 disables the one- shot gate 26, preventing the one-shot gate 26 from activating the transmitter 16. Further, the clock 22 signals the microprocessor 18 that the preset time interval has lapsed. The microprocessor 18 then reads a value from the counter 24 which corresponds to the number of times the radio signal was transmitted from the master beacon 10 to a slave beacon 12 and back again. From the value read from the counter 24, the microprocessor 18 can calculate the distance from the master beacon 10 to the slave beacon 12. As shown in Figure 2 each slave beacon 12 comprises a receiver 28, a transmitter 30, a pulse catcher 32, a preset delay block 33 and a one-shot gate 34. The receiver 28 receives the signals transmitted from the master beacon 10. The received signal is then input to the pulse catcher 32. The pulse catcher 32 determines whether the received signal is intended for the slave beacon 12 or another slave beacon 12. If the pulse catcher 32 identifies the received signal as being intended for the first mentioned slave beacon 12, the preset delay is processed by the preset delay block 33, then the one-shot gate 34 is activated. The preset delay block 33 operates in a similar manner to the preset delay block 27. The one- shot gate 34 enables the transmitter 30 and disables the receiver 28 whilst the transmitter 30 is active. The transmitter 30 transmits a reply signal to the master beacon 10. The reply signal may be the received signal or some other predetermined signal. The slave beacons 12 may be isolated from one another, so that the master beacon 10 can contact each slave beacon 12 independently, by using different frequencies for each slave beacon 12, or different pulse identification codes for each slave beacon 12, or any other method including exclusion time arrangements. It is important that only one of the slave beacons 12 will respond to the signals transmitted from the master beacon 10. In use, the position determining system is arranged as shown in Figure 3. The master beacon 10 is located on an object which is desired to be tracked, such as a crop sprayer or some other vehicle. The slave beacons 12 are provided at stationary locations remote from each other and from the anticipated path of the object.

To determine the position of the object, the master beacon 10 firstly calculates the distance to each slave unit 12. This is achieved by the master beacon 10 contacting each slave unit 12 in turn using the method described above to obtain a value from the counter 24 for each slave beacon 12. The distance from the master beacon 10 to each slave unit 12 can be calculated from the values read from the counter 24, since the distance from the master beacon 10 to the slave beacon 12, after deducting the time delays caused by the preset delay blocks 27 and 33, will be inversely proportional to the value read from the counter 24 at the end of the predetermined time interval. Once the distance to each of the slave units 12 has been calculated, the position of the object can be calculated by the microprocessor 18 by triangulation, since the position of the slave beacons 12 are fixed and known. The calculated position can then be compared with a desired position, and any variance therebetween can be shown to the user on the display 20. The position of the master beacon 10 and the slave beacons 12 may be described in either Cartesian co¬ ordinates or polar co-ordinates. Where the position is measured in polar co-ordinates, the measurement of the angle between the master beacon 10 and at least one of the fixed slave beacon 12 can be done with a rotating laser or radar. The desired position is calculated by storing a previous path, for example the previous lap of a paddock. The desired position is then one implement width from the previous path. It is further envisaged that a "pause" mode may be provided. Such a pause mode would store the current position and the previous path. Thereafter, movements of the master beacon 10 would not form part of the current path. The display 20 would show the difference between the present position and the position of the master beacon 10 when the pause mode was activated. When the pause mode is deactivated, the operation of the master beacon 10 returns to normal, as described above. It is envisaged the pause mode would be useful to allow farmers to refuel and return to the previous position before continuing to spray without losing any tracking information. An alternative method of calculating the distance involves the counter 24 stopping after a preset number of return signals, instead of the clock 22 stopping after a preset time. The value of the clock 22 when the counter 24 reaches the preset number could then be used to calculate the distance from the master beacon 10 to the slave beacon 12. The value of the clock 22 would be directly proportional to the distance. Shown in Figures 4 a, b and c are schematic representations of methods which may be employed to detect radio signals by the receiver 14 and the receiver 28. Figure 4a illustrates a leading edge above the noise technique, where a graph 50 is a graphic representation of the amplitude (in decibels) versus time (in seconds) of a radio frequency signal. The graph 50 displays a rising edge 52, a plateau 54 and a falling edge 56 that define the incoming radio signal in the time domain. The graph 50 also displays a noise signal 58, with an intersection 60 between the rising edge 52 and the noise signal 58. The rising edge 52 of the incoming radio signal triggers the receiver 14 or the receiver 28 (as appropriate) at a triggering amplitude 62. As the amplitude at which the plateau 54 occurs can vary, the distance from the intersection 60 and the triggering amplitude 62 to the plateau 62 varies along the time axis. This variation could cause spurious results. Figure 4b illustrates a more accurate method for detecting the radio signal. Figure 4b is similar to Figure 4a and like numbers denote like parts. The method illustrated has a triggering point 64 positioned on the falling edge 56. The triggering point 64 is immune the changing signal and noise levels. However, spurious, illegal emissions can be generated if the slope of the falling edge 56 is too steep, that is if the fall time is excessively fast. Another method for detecting the incoming radio signal is illustrated in Figure 4c. Figure 4c is similar to Figure 4a and like numbers denote like parts. In Figure 4c there is shown a rapid, small shift 66 in the AM carrier signal frequency. The shift 66 can be detected by an FM detector such that the receiver 14 or the receiver 28 (as appropriate) is triggered. This third method has the advantage of being both immune to signal and noise level variations and can be performed quickly without generating spurious, illegal emissions. Both of the abovementioned methods for distance calculation involve a technique of continuously transponding a radio signal between the master beacon 10 and the slave beacons 12 to measure the distance to the master beacon 10. As an alternative to continuous transponding, two non-continuous methods are also proposed, namely a phase angle measurement technique and a spread spectrum technique. The phase angle measurement technique involves the master beacon 10 generating a radio signal that is continuously modulated by a sinewave whose quarter wavelength is longer than the required maximum range. This radio signal is transmitted to the slave beacon 12 which receives the signal and retransmits the radio signal without changing the modulation frequency or the phase angle. This transmission to and retransmission from the slave beacon 12 can be done by full duplex on a different frequency or by half duplex on the same frequency carrier. The signal that is received at the master beacon 10 is compared with the original, continuous modulation radio signal. The only difference between the original and the received radio signal will be a phase difference due to the distance travelled by the received radio signal. The phase difference can be measured using techniques such as counting the time between the two wave fronts using a high resolution phase discriminator. One way of providing the required resolution is to use the vernier principle. Referring to the accompanying drawing Figure 9 "Phase Difference Vernier" and the previously mentioned drawings, a precision reference frequency (the vernier frequency), very close to the master frequency 74 is compared with the received signal 76 for co-incidence which marks the start of measurement. The number of cycles required to reach co-incidence between the vernier frequency and the master frequency from the start point will be an accurate measure of the phase change between wave 74 and 76 and thus the distance travelled by the signal.

For example, wave form 100 is generated by squaring the transmitted wave 74, wave form 101 is generated by squaring the received wave 76 and square wave 102, the vernier wave, is generated by dividing wave form 100 by a vernier factor close to one. Dividing the transmit wave 76 by 1.0001 will yield a vernier period of .9999 times one transmit wavelength and a resolution of one part in 10,000. In Figure 9, the vernier period is 24/25th of one transmit wavelength.

If 102 is measured from the time of co-incidence between itself and 101 (squared receive wave), 102 will be in co-incidence with 100, after a number of cycles equal to the phase time difference divided by the time difference between the transmit and vernier wavelengths. In other words, the phase difference between the transmitted signal 74 and the received signal 76 is measured by vernier. The advantages of this method are that resolution can be set by choosing the vernier factor and relatively simple, low speed, low cost devices will provide the required accuracy. For example, a Gilbert Cell Mixer can be used. This is a balanced digital mixer chip device which accepts a signal frequency at each of its two input ports and delivers a resulting frequency at its output port. If the two inputs are equal in frequency and their phase relationship is held constant, the output is calculated by the formula:-

Output frequency = (Input frequency A) + (Input frequency B x cos of the phase angle difference).

So with a frequency of 10kHz and a 0 degree phase angle difference, the result will be 20kHz. At 45 degrees it will be 17.071kHZ. In the position determining system, comparing the transmitted signal with the received signal for phase angle difference will yield a figure which, after processing by the microprocessor, can be related to distance. Shown in Figure 5 is the master beacon 10 with a modulator 70 and a phase shift measuring block 72. The modulator 70 controls the frequency and phase of the radio signal transmitted at the transmitter 16. The signal from the transmitter 16 is compared to the radio signal received at the receiver 14 in the phase shift measuring block 72 to measure the phase difference. As the phase difference is proportional to the distance from the master beacon 10 to the slave beacon 12, the distance can then be calculated from the phase difference. If the distance to be measured is not greater than one quarter wavelength of the modulating frequency, there will be no confusion between quadratures (quarter wave cycles). For example, a 100kHz modulating wave has a wavelength of 30 km and thus has a 1/4 wavelength 7.5 km. If the distance travelled by the signal was 31 km, the phase difference would be the same as the phase difference when the signal travels 1 km. Keeping the measure distance less than 1/4 wavelength avoids this confusion Shown in Figure 6 is a schematic representation of a transmitted radio signal 74, a received radio signal 76 and a phase difference 78 between the transmitted radio signal 74 and the received radio signal 76. The phase difference 78 is less than the 1/4 wavelength 80, and can therefore be measured to calculate the distance between the master beacon 10 and the slave beacon 12.

The second technique, the spread spectrum technique, is similar to the above technique except that a pseudo-random train of pulses is substituted for the continuous sinewave. The master beacon 10 transmits a pulse to the slave beacon 12. At the slave beacon 12, the pulse is received and transmitted without change to the modulation frequency or the phase angle. The retransmitted pulse is received at the master beacon 10 and compared with a delayed replica of the original pulse. The delay required to match exactly the retransmitted pulse and the original pulse is proportional to the distance between the master beacon 10 and the slave beacon 12. Shown in Figure 7 is a block diagram of the circuit used in this technique. The master beacon 10 includes a pseudo-random pulse generator 82 and delay comparator 84. The pseudo-random pulse generator 82 controls the frequency and phase of the radio signal transmitted at the transmitter 16. The signal from the transmitter 16 is compared to the radio signal received at the receiver 14 in the delay comparator 84 to measure the time difference. As the time difference is proportional to the distance from the master beacon 10 to the slave beacon 12, the distance can then be calculated from the time difference. Shown in Figure 8 is a transmitted radio signal 86, a received radio signal 88 with a time difference 90 between the transmitted radio signal 86 and the received radio signal 88 required to give a perfect match between the aforementioned radio signals 86 and 88.

Modifications and variations such as would be deemed apparent to a skilled addressee are deemed within the scope of the present invention.

Claims

1. A position determining system for an object characterised by including a master beacon provided on the object and at least two slave beacons, wherein for each slave beacon means is provided for calculating a distance to the slave beacon from the master beacon by transmitting a signal from the master beacon to the slave beacon and back to the master beacon a plurality of times, and means is provided for calculating the position of the object from the distances to the slave beacon.

2. A position determining system according to Claim 1, characterised in that the master beacon includes a counter, a clock, a transmitter, a one-shot gate and a receiver, wherein the counter resets and activates the clock to commence a measurement cycle, the clock activates and enables the one-shot gate and the one- shot gate activates the transmitter.

3. A position deteraiining system according to Claim 2, characterised in that the master beacon includes a delay block.

4. A position detemiining system according to Claim 2 or 3, characterised in that the receiver of the master beacon is arranged to receive incoming signals from the slave beacons and to increment the counter and activate the one-shot gate to again activate the transmitter.

5. A position determining system according to any one of Claims 2 to 4, characterised in that the cycle of transmission and receipt of signals is arranged to be continued until the clock reaches a pre-determining time or until the counter has received a preset number of signals, such that the distance of each slave beacon from the master beacon can be calculated.

6. A position determining system according to any one of the preceding claims, characterised in that each slave beacon includes a receiver, a transmitter, a pulse catcher and a one-shot gate, wherein the receiver is arranged to receive a signal from the master beacon, which received signal is then input to the pulse catcher and then the one-shot gate is activated which enables the transmitter to transmit a reply signal to the master beacon.

7. A position determining system according to Claim 6, characterised in that each slave beacon includes a delay block.

8. A position determining system according to any one of the preceding claims, characterised in that each slave beacon is independently contactable by the master beacon.

9. A method of determining the position of an object characterised by including the steps of:

(1) providing a master beacon on the object and at least two slave beacons remote from the object;

(2) for each slave beacon, either

(i) measuring how many times a signal can be transmitted from the master beacon to the slave beacon and back to the master beacon within a predetermined time interval; or (ii) measuring a time interval required for a signal to be transmitted from the master beacon to the slave beacon and back to the master beacon a predetermined number of times;

(3) calculating from the measurements distances to the slave beacons from the object; and (4) calculating from the distances the position of the object.