GB2237699A

GB2237699A - Object tracking using CCD

Info

Publication number: GB2237699A
Application number: GB8924829A
Authority: GB
Inventors: Christopher John Morcom
Original assignee: English Electric Valve Co Ltd
Current assignee: Teledyne UK Ltd
Priority date: 1989-11-03
Filing date: 1989-11-03
Publication date: 1991-05-08
Also published as: GB8924829D0

Abstract

Coded bursts of light (4) emanating from a target are focused onto the imaging portion (1) of a CCD imager which 15 initially operated in a capture mode wherein photo-generated charge pockets (7) are serially transferred into the store section (2) of the CCD. Each burst consists of an Identification Code Burst (ICB) followed by a tracking pulse (TP). The store section contents are read out to processing circuitry which looks for the presence of an ICB and, on finding one, operates the CCD in a tracking mode in which only that photo-generated charge generated during the tracking pulse interval is transferred to the store section, charge generated at other times being dumped. The target may either have a beacon, when the code may uniquely identify the target, or a passive reflector, when the ICB may be omitted if it is the sole target. <IMAGE>

Description

Object Tracking This invention relates to arrangements for tracking objects, more particularly to arrangements for tracking objects using an optical imaging system.

Such arrangements are known. One type of prior art tracking arrangement has a CCD image sensor in which frames are repetitively transferred from an image section to a store section, the information being transferred representing the average light intensity falling on each respective pixel of the image area during the preceding frame interval. Information is then read out of the read out area and processed to effect tracking of the target.

Such prior art arrangements suffer a number of disadvantages. These include: poor discrimination in high levels of ambient illumination: poor discrimination against other bright objects in the field of view: difficulty in identifying and tracking a number of objects simultaneously.

According to the invention there is provided a target tracking system comprising: a sensor responsive to radiant energy emanating from a target to be tracked: the system being responsive only to that energy which is received by the sensor during a period of time corresponding to at least part of the duration of emanation of energy from the target.

By employing the invention, it is possible to provide improved discrimination against high levels of illumination and against other bright objects in the field of view. It is also possible to track a number of objects simultaneously.

In the preferred embodiments of the invention the radiant energy comprises light and the sensor comprises a CCD imager. This light may comprise visible, infra-red, or ultra-violet light. However, the invention is not limited to light and may also be performed utilising other types of radiation e.g. radiation from other parts of the electromagnetic spectrum, or sound waves. In such cases, the sensor would be one responsive to the particular type of radiant energy being radiated.

Particular embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which Figure 1 illustrates examples of bursts of radiation emanating from a target.

Figure 2 illustrates flow of signals through a CCD imager.

Figure 3 is a timing diagram illustrating the operation of the tracking system.

In a tracking system utilising optical imaging, objects to be tracked are either provided with a flashing beacon which emits bursts of illumination, or are provided with a passive reflector which reflects pulses emitted by the tracker itself.

Where a number of different targets are to be tracked, or it is required to ascertain the identity of a target, the illumination burst preferably consists of a burst of the form shown in Figure la.

Where only targets having passive reflectors are to be tracked, the illumination burst may have a simplified form as shown in Figure lb.

Operation of the tracking system using a coded illuminate burst of the type shown in Figure la will now be described with reference to Figures la, 2 and 3. It will be assumed that only a single target is operating within the field of view of the image sensor, and that the focused image of the beacon occupies a small number of image sensor elements.

The burst shown in Figure la consists of a first portion and a second portion. The first portion comprises an identification code burst ICE. The ICB consists of a coded burst which uniquely identifies the target. In Figure la, the ICB consists of a number of short impulses, each of duration ti, the different logical values of the code being represented by different periods of time T1 and To between successive impulses. By way of example, each impulse may have a duration t. of 20 ms, a logical "1" being represented by a time period T1 of 60,uS and a logical "0" by a time period To of 120us.

The second portion of the tracking pulse burst consists of a single impulse or tracking pulse Tp which occurs a fixed time t after the last impulse of the p Identification Code burst, Tp occurring at time to By way of example, this fixed time tp may be toms.

The first phase of operation is target identification. This involves identifying the target to be tracked from within the scene. In Figure 2, illumination 4 from a target falls on the image section 1 of a CCD image sensor, a shield 3 being provided to shield the store section 2 from incident illumination. Initially the CCD image sensor is clocked so as to continuously transfer lines of charge collected in the image section of the CCD sensor into the line readout section 8 in the CCD store section 2. The ICB from the target within the field of view generates a pattern of charge 7 within a column 5 of the image section of the CCD, which pattern of charge moves in the direction indicated by 6 and whose spatial characteristics represent the code emitted by the target.

When this charge pattern is read out of the sensor, its spatial pattern is detected and decoded using conventional image processing techniques to identify the target and ensure that it is the correct target to be tracked. The system will also note the precise time t at which the O tracking pulse Tp is received.

In Figure 3, line A depicts the pattern of illumination emanating from the target which is incident on the sensor. The code burst ICB is followed by a tracking pulse Tp which occurs at time to in each cycle.

The illumination bursts repeat with a cycle time tf shown for convenience as the time between successive occurances of successive tracking pulses Tp.

Lines B, C & D indicate various clock pulse patterns used to drive the CCD imager. I /d is the CCD image clock pulse sequence. S ffi is the CCD stove clock pulse sequence. R is the CCD line read-out section clock pulse sequence.

Line E depicts a gate pulse which selects either the charge dumping mode or the charge collecting mode of the image section of the CCD imager. When the gate pulse is high during time toll, any photogenerated charge developed in the imager is dumped. When the gate pulse is low, photogenerated charge is allowed to accumulate. During time t12 charge simply accumulates, and during time tl3 charge is transferred to the store section 2 of the imager. Line F indicates the various operational modes of the CCD imager, line G indicates the modes of operation of the overall system.

As shown in Figure 3, once the unique ICB of the wanted target has been identified, the system will switch to the tracking mode. In the tracking mode, the CCD sensor is operated so that, for most of the time any photogenerated charge generated within the image section of the sensor is dumped and not transferred to the read-out section. This may be achieved using the reverse clocking technique which is the subject of our GB Patent GB2140651B, by pulsing the anti-blooming gate electrode of the sensor, or by any other convenient technique.

Whichever technique is used, shortly before the time t at 0 which the next tracking pulse Tp is expected to be received, the sensor is switched from charge dumping to charge collecting mode in which charge is collected and integrated on the image section of the CCD. This mode is denoted by t12 in Figure 3 and is timed such that the tracking pulse arrives during the charge collecting mode.

At the end of the charge collecting mode, the collected pattern of charge is transferred to the CCD store section 2 during the frame transfer period tl3.

Another period of charge dumping commences once the frame has been transferred. During this charge dumping period, data is sequentially read out of the CCD store section 2 via the line readout register 8. The tracking pulse appear as a bright spot in the captured image and standard techniques are then employed to track the motion of the target.

Because the overall charge collection period need only be long enough to capture the tracking pulse, the level of spurious charge generated by the scene illumination is kept to a minimum, allowing successful tracking to be achieved even with high levels of ambient illumination.

It can be seen that, if there are several targets in the field of view, the ICB and the corresponding pattern of charge it generates as the CCD sensor is clocked, can provide discrimination between targets. In addition, the use of a pulsed code allows the system to discriminate against random bursts of illumination or bright points of light within the field of view, e.g. stars.

The pulsed code is usable either with targets having an active beacon which emits flashes of light, or with targets having a passive reflector, in which case the tracking system itself emits flashes of light which are reflected back to the tracking system. Each beacon can be given its own individual code, while the bursts of light emitted by the tracking system for reflection by the passive reflector can have a different code from those allocated to the beacons. This allows targets having beacons to be distinguished from each other, and targets having passive reflectors to be distinguished from those having beacons.

In a modification, the tracking pulse may be omitted, the FCB itself performing the function of the tracking pulse. In this case, the gate pulse controlling charge dumping is timed such that the ICB occurs during the CCD frame transfer period. This results in the temporal code i.e. the sequence of flashes) being translated into a spatial pattern of charge within the sensor.

If only passive targets are to be tracked, and only one such target is to be tracked at any one time, the pulses emitted by the tracking system may consist of single impulses of the type shown in Figure ib. In this case the CCD imager can be gated substantially simultaneously with the tracking pulse, as the time of transmission of the transmitted pulse, and hence the time of reception of the received pulse, is known accurately.

There is no target identification phase, the system operating continuously in the tracking mode.

If it is required to image the scene illumination as well as the tracked object, then this may be achieved by collecting and reading out a single image field in conventional Tv fashion in between capturing the tracking information. A field store is however required to allow continuous display of image fields captured in this way.

Claims

1. A target tracking system comprising a sensor responsive to radiant energy emanating from a target to be tracked: the system being responsive only to that energy which is received by the sensor during a period of time corresponding to at least part of the duration of emanation of energy from the target.

2. A target tracking system as claimed in claim 1 in which the said radiant energy emanates in the form of pulses.

3. A target tracking system as claimed in claim 2 in which the pulses comprise coded pulses.

4. A target tracking system as claimed in claim 3 in which the pulses comprise spaced apart bursts of pulses, each burst comprising a plurality of pulses comprising the code.

5. A target tracking system as claimed in claim 4 in which each burst comprises a tracking pulse spaced apart from the plurality of pulses comprising the code.

6. A target tracking system as claimed in claim 5 in which the system is responsive to the tracking pulse.

7. A target tracking system as claimed in claim 4 in which the system is responsive to the plurality of pulses comprising the code.

8. A target tracking system as claimed in any of claims 3 to 7 comprising a plurality of targets, in which different codes are assigned to different targets.

9. A target tracking system as claimed in any preceding claim, comprising at least one target comprising a beacon which emits the said radiant energy.

10. A target tracking system as claimed in any preceding claim, comprising at least one target comprising a passive reflector.

11. A target tracking system as claimed in claim 2 in which the target comprises a passive reflector, the said pulses comprising pulses emitted by the system and reflected by the said passive reflector, the system being responsive only during the said period of time.

12. A target tracking system as claimed in any preceding claim, in which the sensor comprises a CCD image sensor.

13. A target tracking system as claimed in claim 12 in which the said CCD image sensor comprises an image portion and a store portion.

14. A target tracking system as claimed in any of claims 1-10, or claims 12-13 as appendant to claims 1-10 in which the system is responsive only to that energy which is received by the sensor during a period of time corresponding to at least part of the duration of emanation of energy from the target, only during periods of time in which the system is tracking a target.

15. A target tracking system as claimed in claim 14 as appendant to claim 13 in which, at periods of time other than those in which the system is tracking a target, the said CCD image sensor is continuously clocked so as to transfer data from the said image portion to the said store portion.

16. A target tracking system as claimed in claim 14 as appendant to claim 13, or claim 15, in which, during intervals of time in which the system identifies the presence of a target, the CCD image sensor is operated in a tracking mode in which charge is transferred from the said image portion to the said store portion only during the said period of time corresponding to at least part of the duration of emanation of energy from the target, and in which during periods of time other than the said period of time, charge generated in the said image portion is dumped.

17. A target tracking system as claimed in claim 16 in which charge is dumped using a reverse clocking technique.

18. A target tracking system as claimed in claim 16 in which charge is dumped by pulsing an anti-blooming electrode of the said image portion.

19. A target tracking system substantially as hereinbefore described with reference to the accompanying drawings.