GB2202415A

GB2202415A - Object identification system

Info

Publication number: GB2202415A
Application number: GB08706340A
Authority: GB
Inventors: John Matthew Oswald
Original assignee: Ferranti PLC
Current assignee: Ferranti International PLC
Priority date: 1987-03-17
Filing date: 1987-03-17
Publication date: 1988-09-21
Also published as: GB8706340D0

Abstract

A reflector (13) is carried on an object to be identified and is operable to reflect incident electromagnetic radiation. Modulating means (14) connected to the reflector (13) modulate the reflectivity of the reflector continuously in a predetermined manner which uniquely identifies the reflector. A receiver (12) receives radiation reflected by the reflector (13) and includes detector means (15) to decode the reflected radiation to determine the identity of the reflector and hence of the object on which it is carried. The radiation may be optical, and the code may be a binary sequence or sequence of audio codes. <IMAGE>

Description

OBJECT IDENTIFICATION SYSTEM A number of systems exist for identifying objects, mainly using optical techniques and varying from very expensive and complex systems to simpler systems which will locate but not identify objects. The most complex system is~probably the secondary radar system used by civil aircraft, where a transponder carried by the aircraft is triggered by received radiation from an air. traffic control radar, for example, and transmits an unique identification code. Whilst suitable to expensive and complex vehicles such as aircraft this system is not suitable, for example, for identifying motor vehicles or railway waggons.

Simpler techniques of a generally similar nature have been used for road and rail vehicles. However, these require an energy source to power the transponder and, whilst such a source may be available on a motor vehicle, it is probably not the case with rail vehicles. This problem has been overcome in short range systems by putting sufficient energy into the interrogating beam to provide power for the transponder. This involves very high radio-frequency power levels which may be considered dangerous to personnel.

Another technique which may be used is closed-circuit television with subsequent image processing to extract identification information. However, this technique is particularly vulnerable to atmospheric conditions or to dirt on the identification "label" being viewed.

Optical bar code readers have also been used to identify road and rail vehicles. These too are vulnerable to ambient conditions.

It is an object of the invention to provide an object identification system which does not suffer from the above disadvantage and which is simple and cheap to operate.

According to the present invention there is provided an object identification system which includes reflector means arranged to be carried on an object to be identified and operable to reflect electromagnetic radiation of a known frequency incident thereon, modulating means connected to the reflector means and continuously operable to modulate the reflectivity of the reflector to such incident radiation in a predetermined manner which uniquely identifies the ref lector, and receiver means operable to receive radiation reflected by said reflector means and including detector means operable to decode the reflected radiation so as to determine the identity of the reflector and hence the object on which it is carried.

Also according to the present invention the object identification system set out in the preceding paragraph may include transmitter means operable to transmit electromagnetic radiation at said known frequency.

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic block diagram of a system according to the invention; Figure 2 is a schematic block diagram of the reflector and modulating means according to a first embodiment of the invention; Figure 3 is a schematic block diagram of a transmitter and receiver for use with the label of Figure 2; Figure 4 is a schematic block diagram of part of the receiver of Figure 3; Figure 5 is a schematic block diagram of a transmitter and receiver according to a second embodiment of the invention; Figure 6 is a schematic block diagram of part of a label for use with the system of Figure 5, and Figure 7 is a schematic block diagram of a system according to a third embodiment of the invention.

Referring now to Figure 1, this shows the basic features of the system. A source 10 of electromagnetic radiation illuminates a "label" 11 and some of the radiation is reflected towards a receiver 12. The label 11 consists of a reflector 13, shown here for simplicity as a dipole antenna, and modulating means 14 which is operable continuously to modulate the reflectivity of the reflector 13. The receiver 12 includes some form of decoder 15 which identifies the modulation pattern imposed by the modulating means 14 and thus identifies the label.

The transmitter 10 may be a natural source of radiation, particularly if optical radiation is used. The actual form of the reflector 13 will depend upon the frequency of-the radiation.

The complexity of the system will depend largely upon its function. The system may be very basic if, for example, there is only one label within the detection range of the system at any time and if reflections from other objects do not prevent the receiver from resolving the signal reflected from the label. In many situations, however, these conditions may not exist and additional techniques have to be used to ensure satisfactory performance. Several techniques may be used, either singly or in combination. It is possible, for example, to make the transmitted beam directional, if the location of a label to be interrogated is known. Range gating techniques, common in radar systems, may be used if a number of labels are illuminated at different distances.

The manner in which the reflectivity of the reflector is varied will, in general, form a repeatable and identifyable pattern. This pattern may be in the form of a serial sequence of binary digits with a '1' for example being represented by a high reflectivity conditions and a 'O' by low reflectivity.

This sequence could be modulated onto a higher frequency carrier transmitted by the transmitter. Alternatively a sideband detection system may be used to reduce the effect at the receiver of direct signals from the transmitter.

Referring now to Figure 2, this shows in schematic form the details of a 'label' suitable for use at a frequency of the order of several hundred MHz. Figure 2 shows that the actual reflector of the label in this case consists of a half-wave dipole antenna 20 shortened as necessary by the addition of capacitive and inductive components. A pair of variable capacitance diodes 21 are connected across the centre of the dipole, and the assembly is adjusted such that the dipole is resonant at the required operating frequency when the diode voltages are zero but is detuned when the diodes are reverse biased.

Biasing of the diodes is performed by a batteryoperated signal generator. As shown in the drawing, this uses a crystal oscillator 22 and a frequency divider 23 to drive an address counter 24 and PROM 25.

The label may be very small since the circuitry may be provided on a single semiconductor chip and the dipole antenna need only be, say, 100sam long. The antenna may be made in printed form on a card and the only other component required is a small battery of the type used in watches and calculators.

The power consumption is very low since no radio-frequency power is generated and the load on the signal generator is merely the capacitance of the two variable-capacitance diodes. Hence a battery of the type mentioned may operate for several years, even with the device operating continuously.

If sideband detection is used then this involves the transmission of a continuous carrier wave by the transmitter, say at 459MHz. The label operates to vary the reflectivity of its reflector at some lower frequency or sub-carrier.

Identification data may be modulated onto this sub-carrier.

The label of Figure 2 will thus use the crystal oscillator 22 to generate the sub-carrier frequency of say, 32768 Hz, which is used to modulate the bias of the variable-capacitance diodes 21.

The PROM 25 is programmed with the label identification code which may, for example, be a 32-bit binary word. This is read out by the address generator 24 at a rate determined by the output of the frequency divider 23, say 2048Hz, and is modulated onto the sub-carrier by differential phase modulation. This is a technique which may vary the phase of each code element, for example the phase may be changed to indicate a '1' and left unchanged to indicate a 'O'. An element frequency of 2048 Hz means that the 32-bit word occupies 1/64 seconds, and if a synchronising blank period of the same length follows, then the word is repeated 32 times per second. A 32-bit word enables a very large number of different codes to be provided even allowing for "start" and error correction bits.

Figure 3 is a block schematic diagram of a transmitter and receiver suitable for use with a 'label' of the type just described. The transmitter 10 shown in Figure 3 basically consists of a voltage-controlled oscillator 30 generating a carrier wave signal at the desired frequency. Frequency control is provided by dividing-down the carrier frequency in a divider 31, comparing the output of this at 32 with a reference frequency generated by a crystal oscillator 33, and using any error signal to control the output frequency of the oscillator 30.

The output of oscillator 30 is applied to the antenna 34 by way of a circulator 35.

The signal from the oscillator 30 also passes through a frequency translator 36 to be described later, to the mixer 37 of the receiver 12. The receiver has an input from antenna 34 by way of circulator 35. The output of the mixer 37 passes through a tuned amplifier 38, which selects one of the sidebands of the reflected signal, and to a phase detector 39 which decodes the received signal to provide the identification code of the label from which the signal has been detected.

Figure 4 shows detail of the frequency translator 36 of Figure 3. The function of this is to provide for the mixer 37 a signal which changes the radio frequency carrier-plussidebands signals applied to the mixer into separate low-frequency carrier and sideband components, one of which is selected by the tuned amplifier 38.

The frequency translator includes a crystal oscillator 40 the output of which is divided-down by divider 41 to provide inputs to a 90" phase shifter 42 and a multiplier 43. The output of phase-shifter 42 is applied to a second multiplier 44. The output of oscillator 30 is applied to multiplier 44 and also through a 900 phase-shifter 45 to multiplier 43. The outputs of combiners 43 and 44 are added by adder 46 and provide the signal for application to the mixer 37.

The operation of the transmitter and receiver of Figures 3 and 4 is best explained using particular values of frequency, in agreement with those used to explain the operation of Figure 2. As already suggested, the transmitter carrier frequency may be 459MHz. Again as already suggested, the label will reflect this 459MHz carrier modulated by the 32768Hz sub-carrier. The reflected signal will thus comprise the carrier frequency, an upper sideband at 459.032768MHz and a lower sideband at 458.967232MHz.

The frequency translator of Figure 4 receives the 459MHz signal from the transmitter oscillator 30 and the crystal oscillator 40 generates a frequency which is divided-down by divider 41 to 21845Hz. The output of the frequency translator is therefore 459.021845Hz. This is applied to the mixer 37.

The output of the mixer thus comprises a carrier component at 21845Hz, an upper sideband component at 10923Hz and a lower sideband component at 54613Hz. The tuned amplifier is arranged to pass and amplify only the upper sideband component at 10923Hz, and the other components are rejected.

The e decoding and error correction techniques used are conventional. The phase detector 39 operates by comparing the 10923Hz signal which it receives from the tuned amplifier 38 with the same signal delayed by ten cycles.

The system described above is suitable for identifying road or rail vehicles passing a point such as a toll gate. A transmitter power of around 50 milliwatts is adequate for the purpose as the operating range is short. Each vehicle to be identified carries a small card containing the dipole antenna, circuitry and battery. Adequate identification will take place at vehicle speeds, of around 40mph or less.

For use at longer ranges a system operating at radar frequencies may be used. This would be suitable, for example, for identifying ships at sea. This would require each ship to carry a radar reflector whose reflectivity could be varied in a manner which would identify the ship. Such a system could be combined with a conventional radar which would locate the ship and then use a narrow beam of radiation at, say, 3GHz to illuminate the reflector carried by the ship. Figure 5 illustrates such a system and shows a radar antenna 50 having its attitude controlled by a conventional pointing system 51.

The transmitter 52 and receiver 53 share a common frequency reference 54 and the common antenna 50 is shared using a suitable switch 55 such as a TR cell. The e radar illumination would have to be pulsed rather than continuous to prevent transmitted power from reaching the receiver 53. The receiver is-followed by a range gate 56. In the particular embodiment to be described an analogue-to-digital converter 57 converts the output of the range gate 56 to digital form for application to a bank of digital filters 58 controlled by a control circuit 59.

Appropriate filter outputs pass through output gates 510 to give the identity code.

Figure 6 illustrates the circuitry for a label for use with the system of Figure 5. A sub-carrier frequency of, say, 5MHz, is generated by an oscillator 60. This sub-carrier is modulated at a much lower frequency in modulator 61 by the identity code which may, in this case, be a sequence of six audio-frequency tones chosen from a selection of, say, 32 such tones. The label therefore includes a tone generator 62 controlled by a PROM 63 containing the identity code and itself driven by an address counter 64. A crystal-controlled clock oscillator 65 is provided to drive both the tone generator 62 and the address counter 64.

In operation, the label uses a sequence of six tones, each lasting, say, half a second, to modulate the 5MHz sub-carrier which is used to vary the bias on the variable-capacitance diodes connected to the reflector antenna. The receiver includes 32 digital filters, one tuned to each of the possible tone frequencies. The filter control circuit 59 determines, from the outputs of the filters 58, when a valid sequence of six tones has been received and allows the output gates 510 to provide the identification signal.

Yet another embodiment may use optical radiation, either visible or, more probably, infra-red. This is most suitable for short range systems. Figure 7 shows the system in general outline. The transmitter 70 is conveniently a directional source of continuous radiation, though it may in certain situations be able to make use of natural light or of ambient light produced by light sources provided for some completely different purpose, such as flood lights. Each object to be identified, probably a vehicle following a clearly-defined path, would carry a corner cube reflector 71 having at least one reflecting surface of electro-optic material whose reflectivity may be modulated by a modulating circuit 72.The receiver may be a simple arrangement of a detector 73, an amplifier 74 and a phase detector 75, though more complex systems may be used to suit the techniques used by the label modulating circuit 72.

It will be realised that many conventional techniques for signal enhancement may be used with the present invention, and the above description has indicated only a few of these.

The main advantages of the invention, as has already been stated, are several. One of the most important is that a "label" may be produced at low cost and has a long life, even though arranged to operate continuously. In addition, there is no need to transmit power at high levels to power the label; the transmitted power need only be sufficient to ensure that reflected power is detectable by the receiver over the intended range, which may be quite short. Furthermore, a suitable choice of transmitted signal frequency renders the system substantially unaffected by weather conditions or atmospheric pollution.

Claims

1. An object identification system which includes reflector means arranged to be carried on an object to be identified and operable to reflect electromagnetic radiation of a known frequency incident thereon, modulating means connected to the reflector means and continuously operable to modulate the reflectivity of the reflector to such incident radiation in a predetermined manner which uniquely identifies the reflector, and receiver means operable to receive radiation reflected by said reflector means and including detector means operable to decode the reflected radiation so as to determine the identity of the reflector and hence of the object on which it is carried.

2. A system as claimed in Claim 1 which includes transmitter means operable to transmit radiation of said known frequency.

3. A system as claimed in Claim 2 in which said transmitted radiation is contained in a directional beam of radiation.

4. A system as claimed in any one of the preceding claims in which the radiation is radio frequency radiation, the reflector means comprising a dipole antenna connected to a variable capacitance device such that the dipole may be made reflective or non-reflective in accordance with a voltage applied to the variable capacitance device by the modulating means.

5. A system as claimed in any one of Claims 1 to 3 in which the radiation is optical radiation, the reflector means comprising an optical retro-reflector the optical reflectivity of which may be varied by the modulating means.

6. A system as claimed in either of Claims 4 or 5 in which the modulating means is operable to modulate the reflectivity of the reflector at a second frequency much lower than said known frequency, identification of the reflector being produced by impressing an identification code on to said second frequency.

7. A system as claimed in Claim 6 in which the identification code comprises a multiple-bit binary sequence.

8. A system as claimed in Claim 6 in which the identification code comprises a sequence of audio-frequency tones.

9. An object identification system substantially as herein described with reference to the accompanying drawings.