GB2471839A - Laser designator and guidance system - Google Patents

Abstract

A laser designator is disclosed comprising a signal generator 2 and a laser 3 for generating a modulated optical signal beam 21. The beam is directed to a remote object 22 which reflects the signal. A portion 23 of the light reflected by the remote object is focused onto a light sensitive detector 6. A cross correlator 8 calculates the cross correlation between the received, delayed signal, and the original modulation signal. The cross correlation has a peak corresponding to the time of fight of the optical signal, and hence the distance to the remote object 22. Whilst the laser designator is operating, a portion of the light reflected by the remote object 22 is also captured by a receiving lens 11 of a guidance system 12. This focuses the received light onto a multiple region light sensitive detector 13. The outputs of each quadrant of the detector 13 are passed to separate cross correlation units 15, and peak detectors 16. The amplitude of the peak is determined for each quadrant, and can be used by a guidance and control system 18, typically to guide a vehicle towards the remote object 22.

Description

GUIDING APPARATUS AND METHOD

This invention relates to guiding apparatus and a guiding method that may be used to guide or direct a guided object.

The cost of road traffic accidents, both in terms of economics and human misery is vast. For example, in 1999 the US Federal Highway Administration reported 6.3M Road Traffic Accidents (RTA) in the USA which left 3.2M people injured and 41,345 dead. The total economic cost was estimated to be $l5OBn.

Similarly, the Economic Commission for Europe reported that in 1997, European RTA injured 6,118,844 people and killed 164,677. The direct costs of Medical treatment, emergency services, damage to property & lost is economic output were estimated to be �36Bn, whilst the total economic cost to the EEC was estimated at �l5OBn.

Therefore, much research has focussed on finding ways to avoid collisions and RTA by the provision of better driver information and the active and early warning of danger and this has lead to a variety of approaches.

The simplest collision avoidance solutions rely on measuring the distance from the vehicle to the nearest vehicle in front and providing a warning light or sound to the driver if he is driving too close, given his current speed.

One simple approach to measuring distance is to use a laser rangefinder (LRF). These devices work on the basis of measuring the time of flight of a laser pulse to a remote object and back and calculating the distance from the known velocity of light.

As well as collecting data about the environment of the vehicle it is also necessary to guide the vehicle based on that information. Vehicle information systems need to be able to cope with dense traffic, and in such dense traffic flows it may be necessary to follow other vehicles.

One approach to following other vehicles is simply to process images recorded by a camera and follow a specific image. However, in many cases there are many similar vehicles on the roads and it may be difficult to carry out the image processing accurately enough.

The need for guidance systems is not restricted to traffic management systems and there is a need for low power guidance systems in a number of applications. In particular, there is a need for guidance systems that guide a io guided object towards a remote object where the remote object is designated by a designator separate from the guidance system.

According to the invention, there is provided a guidance system according to claim 1.

By providing a laser beam pointing at the remote object to be followed, the problem of guiding and the problem of processing the image to identify the object to be followed can be separated. Further, the designation of the remote object to be followed need not be made by the guided object but the designation can be from a further vehicle, other guided object or indeed fixed station. This means that it is possible for vehicles to be instructed to follow other vehicles; without loss of generality regarding the type of vehicle.

The inventors have realised that a problem with a system using laser beam pointers is that it is undesirable to use bright laser beams. There therefore needs to be a system that can carry out this task with low power laser beams.

Such laser beams can be generated using lower cost laser components and greatly reduce problems of practical use. In many applications, bright laser beams may not be appropriate or indeed safe.

By using the method of the invention, relatively low powers may be used in the laser designators.

The laser guidance system may further comprise a guidance and control system for controlling a guided object carrying the guidance system to move towards the remote object designated by the laser designator. This may be achieved by controlling the guided object to move such that the plurality of peak magnitude signals are equalised.

For a better understanding of the invention embodiments will now be described, purely by way of example, with reference to the accompanying drawings, in which: io Figure 1 shows a system according to an exemplary embodiment of the present invention.

The drawing is purely schematic and not to scale.

Referring to Figure 1, the operation of the designator (1) will be described first.

The designator emits a collimated, modulated beam of light.

In the embodiment, within the designator a signal generator (2) generates a modulation signal (20) with the property that its autocorrelation function approximates to a delta function. One example of such a signal is a maximal length sequence (MLS) or the pseudo-random binary noise signal (PRBS).

This modulation signal is applied to a laser (3) to generate an optical signal (21) which is focussed by the transmitting optical system (4) into a collimated beam. This is directed to to a remote object (22) which reflects the signal.

Thus, by directing the optical signal (21) towards the required remote object (22) the guidance system (12) guides a guided object such as a vehicle in which it is mounted towards the designated object.

A portion (23) of the light reflected by the remote object is collected by the receiving lens optical system (5) and focussed onto a light sensitive detector (6) creating an electrical signal which is input to an analogue to digital converter (7). The cross correlator unit (8) calculates the cross correlation function between the received, delayed signal and the original modulation signal. This correlation function has a cross-correlation peak at a time delay corresponding to the time of flight of the optical signal and hence the distance to the far object. A peak detector (9) detects this cross correlation peak and timing, calculation and display electronics (10) compute the range to the far object from the measured time delay.

Whilst the laser designator is operating, a portion of the light reflected by the remote object is also captured by the receiving lens (11) of the guidance system (12) mounted on the laser guided vehicle. This focuses the received light onto a multiple region light sensitive detector (13). The outputs of each io quadrant of the detector are passed through separate signal processing chains comprising analogue to digital converters (14), cross correlation units (15) and peak detectors (16).

The multiple region light sensitive detector (13) may have an appropriate is number of light sensitive regions. Each of these correspond to a region in the view seen by the guided object. Typically, the receiving lens may be mounted facing forward on the guided object so that there is a direct correlation between the light sensitive regions and directions along a forward-back axis of the guided object. The regions may relate to different directions from the forward-back axis. Thus, when the guided object is heading directly towards the remote object, all regions may receive the same intensity of light. When the guided object starts to drift from this direction, one or more of the regions will receive an increased signal and one or more a reduced signal. This change is detected and used in the guidance system as will now be explained.

In some applications, it may be sufficient to have only a left region and a right region, for detecting whether the signal is from the left or the right of the forward-back axis. However, it is relatively straightforward to provide a system with four regions, such as four quadrants, which allows the laser guided vehicle to receive information about the location of the remote object both left to right and up and down. Thus, the four quadrants may be oriented to receive light from above the forward-back axis, below the forward-back axis, to the left of the forward -back axis and to the right of the forward-back axis.

The analogue to digital converters (14) convert the received optical signal from each quadrant into a digital signal which is then cross correlated with the output of a signal generator (17) which has been configured to generate the same modulation signal as the signal generator (2) in the laser designator. As the laser designator and guidance system are distinct and separate entities, there will be no specific phase relationship between modulation generated by the designator and that generated by the laser guided vehicle. However, because the signals have the same waveform the output of the cross io correlation unit in the laser guided vehicle will be a cross correlation peak. The amplitude of this peak is determined by the peak detector (16) for each quadrant and will be proportional to the amount of the reflected beam signal that has been focussed on each quadrant and hence can be used to derive the error in aiming of the guided vehicle with respect to the target. The is amplitude measured is output as a peak signal by each peak detector (16).

Note that the correlation measurement in the guidance system is slightly different to that in the laser designator. The system in the laser designator may be used to measure range by detecting the delay in the cross-correlation peak. In contrast, in the guidance system, since the exact time the modulation sequence was emitted by the designator is not known, the exact time corresponding to the peak in correlation cannot be used to calculate the range. Instead, the relative sizes of the peaks in correlation measured by the four different detectors, i.e. the peak signals, give an indication of the direction towards the remote object indicated by the designator (1).

These four peak signals are input to the guidance and control system (18) which act to control the motion of the guided vehicle. In this embodiment, a simple control loop aims towards the state where the magnitude of the peak signal from each of the quadrants is the same, as far as possible, indicating that the guided vehicle's direction is aligned with the target. This control approach is well known.

The primary benefit of this approach is that by carrying out the cross-correlation operation in the guidance system, the signal to noise ratio of the detected quadrant signal amplitude is improved allowing the overall optical power emitted by the designator to be significantly reduced with respect to a short pulse designation system.

If the modulation signal is a maximal length sequence then the cross-correlation operation can be efficiently computed using a Hadamard transform.

Alternatively if the modulation signal is an oversampled maximal length sequence, the cross correlation peak can be efficiently found using the coarse/fine approach described in European patent EP1252535 which describes the use of similar modulation sequences for the purposes of range is finding.

In a further embodiment, the system takes account of the fact that there may be multiple designators operating at the same time. As will be appreciated, this is distinctly possible in busy environments. Therefore, information may be embedded within the modulation signal to allow guided vehicles to distinguish between different designators and hence choose the correct goal. Suitable techniques for embedding such information into modulation sequences of this type are described in European patent application 05794517.2 to Instro Precision Limited.

In particular, one of a number of different modulation sequences can be selected. The signals can be provided to have low cross-correlation between different signals with high autocorrelation to allow faint signals to be detected.

One way this can be achieved is to add variable numbers of bits to existing sequences, which may be maximum length. By using variable numbers of bits the repeat lengths of different sequences are different, so after several repeats of the maximum length sequence any random corrrelations in the first cycle will shift to being out of phase.

Alternatively, an input modulation sequence can be phase-shifted from time to time in a pseudo-random but repeatable way to generate different modulation sequences.

Alternative techniques known to those skilled in the art include the use of Gray codes or sequences where the maximal length sequence starting phase or starting polynomial are carried with time in a pre-determined deterministic manner know to the laser-guided vehicle.

io The invention is applicable to all types of guided objects i.e. vehicles in the broad sense of that term, and not just to traffic management systems. For example, laser-guided systems may be used in factories to guide automatic guided vehicles. The system may be used to guide flying vehicles such as aircraft, for example to indicate an approved landing site, as well as vehicles is supporting a single human such as automatic wheelchair systems, the laser designator in this case being under the control of the user and the guidance system guiding the wheelchair to the chosen destination.

The invention may also have application to guiding objects such as rocket propelled ordnance, unmanned aircraft, and unmanned ground vehicles that may need to be guided to a target. Existing systems for guiding such craft may use high powered lasers, for example Nd:YAG lasers operating at 1064nm, and the system according to the invention may allow lower powered lasers to be used. This can reduce costs as well as making it less likely that the target will detect the light beam used to designate the target and possibly take countermeasures.

The use of the term "remote object" is not intended to imply any particular distance between the laser designator and the remote object and the distance will of course depend on the application.

Further, although the embodiments above use a laser to generate the collimated beam of light, other approaches may be used, for example using a light emitting diode and a separate collimator. Where a laser is used, a low cost semiconductor laser may be used.

Claims (12)

1. CLAIMS1. A guidance system comprising: a designator (1) for designating a remote object with a collimated beam of light, including a modulation sequence generator (2) for generating a modulation sequence and modulating the collimated beam of light; and a guidance system (12) for mounting in a guided object, the guidance system comprising: a receiving optical system (11) having a plurality of guidance system io light sensitive detectors (13) for receiving a plurality of received optical signals reflected by the remote object; a guidance system modulation sequence generator (17) for generating a modulation sequence corresponding to the modulation sequence generated by the designator modulation sequence generator (2); a guidance system cross correlation unit (15) for carrying out a cross correlation between each of the received optical signals and the modulation sequence; and a guidance system peak detector (16) arranged to detect the magnitude of the peak in each of the cross correlations corresponding to each of the guidance system light sensitive detectors (13)to generate a respective plurality of peak magnitude signals.
2. 2. A guidance system according to claim 1 further comprising a guidance and control system (18) for controlling the guided object to move towards the remote object designated by the designator (1) by controlling the vehicle to move such that the plurality of peak magnitude signals tend towards equal values.
3. 3. A guidance system according to claim 1 or 2 comprising a plurality of guidance system cross-correlation units, one for each guidance system light sensitive detector (13) and a plurality of guidance system peak detectors, one for each guidance system light sensitive detector (13).
4. 4. A guidance system according to claim 1, 2 or 3 wherein the plurality of guidance system light sensitive detectors are the four quadrants of a four-quadrant light sensitive detector and the receiving optical system(1 1) further comprises a lens arranged to focus received light on the four-quadrant light sensitive detector (13).
5. 5. A guidance system according to any preceding claim wherein the modulation sequence generators (2,17) are arranged to generate a modulation sequence including information to distinguish between a plurality io of different designators.
6. 6. A guidance system according to any preceding claim wherein the designator (1) further comprises a light sensitive detector (6) for receiving an optical signal reflected by is the remote object; a cross-correlation unit (8) for carrying out a cross correlation between the received optical signal received in the light sensitive detector and the modulation sequence; a peak detector (9) arranged to detect the time of a peak in the cross correlation; and a timing unit (10) for calculating the range to the remote object from the time of the peak detected by the peak detector (9).
7. 7. A guidance system according to any preceding claim wherein the designator comprises a laser (3) controlled by the modulation sequence generator (2) for generating the collimated beam of light.
8. 8. A method of guidance comprising: generating a modulation sequence in a designator (1); transmitting a collimated beam of light towards a remote object, the collimated beam of light being modulated by the modulation sequence; receiving a plurality of received optical signals reflected by the remote object in a plurality of light sensitive detectors (13); generating a modulation sequence in a guidance system corresponding to the modulation sequence generated in the laser designator (1); carrying out a cross correlation between each of the received optical signals and the modulation sequence; and detecting the magnitude of the peak in each of the cross correlations corresponding to each of the light sensitive detectors (13) to generate a plurality of peak magnitude signals.
9. 9. A method according to claim 8 further comprising controlling a guided io object to move towards the remote object designated by the designator (1) by controlling a guided object to move such that the plurality of peak magnitude signals tend towards equal values.
10. 10. A method according to claim 8 or 9 wherein the plurality of light is sensitive detectors are the four quadrants of a four-quadrant light sensitive detector and the receiving optical system(1 1) further comprises a lens arranged to focus received light on the four-quadrant light sensitive detector (13).
11. 11. A method according to claim 8, 9 or 10 further comprising including information in the modulation sequence to distinguish between a plurality of different laser designators.
12. 12. A method according to any of claims 8 to 11 further comprising receiving an optical signal reflected by the object in the designator (1); carrying out a cross correlation between the received optical signal received in the light sensitive detector and the modulation sequence in the laser designator (1); detecting the time of corresponding to a peak in the cross correlation; and calculating the range to the object from the time of the peak detected by the peak detector (9).