BACKGROUND OF THE INVENTION

There is a need for automated guidance of agricultural vehicles, not to remove the presence of a driver but to allow greater attention to be given by the driver to the cultivation operation Automatic steering also promises to improve the effectiveness of "controlled traffic", a technique where vehicles seek to use the same ^' footprint", every time to minimise compaction damage 10 the soil Under manual control, this increases the pressure on the driver to maintain precise control of the track of the vehicle

For spraying operations, high speeds are desirable to enable a ground vehicle to challenge the role of a crop-spraying aircraft Once again the driver s task is made more demanding and an "autopilot" becomes highly desirable

Many guidance methods can be considered, ranging from buried leader cables to beacons, surveying instruments or even satellite navigation All have their drawbacks The most appealing method is to follow human practice and take guidance from the crop itself, steering the vehicle by means of the view of the rows ahead

There are however many complications as the condition of the crop changes through the growing cycle Initially the plants appear as rows of small dots among scattered random dots which are weeds Later they fuse to form a clear solid line Before long, however, the lines have thickened and threaten to block the laneways Many variations of the vision algorithm are thus required to fulfil all the seasonal requirements

Vision systems can acquire data at a very large rate A full-colour

high-resolution image can require 1 4 megabytes of memory to hold it, and twenty-five such images are received from a conventional camera each second. Many vision projects have become congested by such data rates, requiring massive computing power to extract the simplest of features

Prior attempts have been made to develop a viable automatic steering system for agricultural vehicles but these have suffered from the problems identified above. Yanmar Agricultural Equipment KK (Japanese Patent Application number J03012713) describe a complex image processor that compares a recorded crop image with a virtual crop row to determine an error signal Kubota KK (Japanese Patent Application number J01319878) discloses a system that uses linear approximation to identify crop rows on the basis of colour separation. Neither of these systems is user-friendly and both are processing intensive. Examples may be found in the Engineering literature of proposals for automatic guidance of agricultural vehicles. One such proposal is that of Gerrish et al reported in Transactions of the American Society of Agricultural Engineers. 1986. volume 95, section 5, pages 540-554. The Gerrish approach depends on edge identification techniques. Another approach is the proposal of Reid and Searcy reported in Transactions of the American Society of Agricultural Engineers 1988, Volume 31 , section 6, pages 1624-1632. The Reid and Searcy approach relies on a Bayes classifier algorithm to set threshold levels. Both of these approaches are processing intensive and failed to result in a practical solution to vehicle guidance.

OBJECT OF THE INVENTION It is an object of the present invention to provide a vehicle guidance system, particularly for agricultural vehicles

It is a further object to provide an automatic guidance system for agricultural vehicles that is user-friendly, economic and reliable.

Further objects will be evident from the following disclosure.

DISCLOSURE OF THE INVENTION

In one form although it need not be the only or indeed the broadest form the invention resides in a method of guiding a vehicle including the steps of acquiring and storing a digitised image of a scene containing at least one row of objects selecting a portion of the image analysing the portion of the image to identify said objects, performing a regression calculation on the objects to locate a position of the row in terms of a lateral displacement value compared to a previously identified row converting the lateral displacement to a steering correction signal, and applying the steering correction signal to steering means of the vehicle to effect vehicle guidance The image is acquired by means which preferably employs a video camera to generate a vision signal and an interface card which stores an image of that signal in the memory of the processor means, which is preferably a computer The signals acquired represent an image of the scene ahead of the vehicle and may with advantage be of relatively low resolution thereby reducing the volume of data to be processed

Samples are selected for numerical processing from a small portion of the image Such a portion is bounded by a parallelogram of which two sides are horizontal The other sides are located such that the last estimate of the position of a row is centred between them and they are inclined to be parallel to the image of that row

If the image is stationary with respect to the vehicle the new imaged row will be centred in the parallelogram Any change of direction or orientation will cause a displacement of the image A regression calculation is performed to locate the position of the new row, in terms of lateral displacement and change of inclination

A measure o quality of fit is also obtained Provided the fit is good, the coordinates of the parallelogram are changed before the next image

is acquired

The deviation of the coordinates of the parallelogram from a central datum is used to initiate a steering action This may be performed by an hydraulic actuator A feedback signal is preferably taken from a transducer mounted on the steering mechanism

The method is preferably extended to include several parallelograms each centred on an image of a different row of objects, so that interruption of a single row does not disrupt the steering control effected by the method In a further form the invention resides in a vehicle guidance system comprising image acquisition means for acquiring an image of a scene containing at least one row of objects processor means for processing said image to estimate at least one row position and determine steering correction signals, input means in signal connection with said processor means for inputting parameters affecting processing in said processor means, and steering control means responsive to the steering correction signals from said processor means to control the direction of steering of the vehicle, wherein the processor means determines the steering correction signals by calculating the deviation of the at least one estimated row position from a previously estimated row position

In preference the processor means is programmed with an algorithm to perform the steps of selecting a portion of the image analysing the portion of the image to identify objects, performing a regression calculation on the objects to locate a position of the row in terms of a lateral displacement value compared to a previously identified row converting the lateral displacement value to steering correction signals

In preference the scene contains more than one row and the processor means is programmed with an algorithm wherein the step of

selecting a portion of the image includes selecting a number of portions, each portion containing only one row and the step of performing a regression calculation includes performing a regression calculation on the objects in each portion and wherein the step of converting the lateral displacement to a steering direction correction includes determining a lateral displacement value of a vanishing point of the regression lines of the more than one rows

BRIEF DETAILS OF THE DRAWINGS To assist in understanding the invention preferred embodiments will now be described with reference to the following figures in which

FIG 1 is a schematic block diagram of a vehicle guidance system,

FIG 2 illustrates aspects of the image analysis algorithm,

FIG 3 is a schematic block diagram of a variation on the system of

FIG 1 FIG 4 illustrates a display of the system of FIG 1 or FIG 2, and

FIG 5 shows a practical result of the implementation of the invention

DETAILED DESCRIPTION OF THE DRAWINGS In the drawings like reference numerals refer to like parts Referring to FIG 1 there is shown a block diagram of the components of one embodiment of a vision guidance system for a vehicle The invention will be described in terms of application to automatic steering of an agricultural vehicle following crop rows It will be appreciated that the description applies equally to other equivalent situations The system comprises a video camera 1 that records an image of the crop rows ahead of the vehicle A video interface card 2 captures single frames in the form of signals from the camera 1 and supplies them to a processor 3 The processor 3 stores the image signals for further processing The inventors have found that a commercially available CCD (charge coupled device) video camera and commercially available video card with software are an appropriate and economic combination

An input device such as a keypad 4 allows a user to input

Ό parameters that effect the operation of the processor. The operation of the system is indicated on display means 5.

A full colour image is captured in the on-board memory of video interface card 2 and can be merged "live" as a window forming part of a VGA display on display means 5 The image can be scaled horizontally and vertically with no use of processing time in processor 3. Lines and other graphics can be superimposed on the screen image (discussed later), so that the performance of the analysis system becomes very clear to see An advantage of the interface is the provision of colour. A field with a newly shooting crop may be littered with light-coloured detritus which makes discerning the crop rows difficult if brightness alone is used. Even the use of a green filter over the lens of the camera 1 makes little improvement With colour it is possible to use the chrominance signal rather than luminance to capture an image based on the "greenness" of each point The spatial resolution of chrominance is not as sharp as that of luminance but resolution is not of the greatest importance. The chrominance signal can be measured on a red/green axis or on a blue yellow axis Coupled with the light/dark luminance axis a three dimensional analysis space is defined. The red/green, blue/yellow and light/dark parameters can be set by the user with keypad 4.

Alternatively, the image may be captured as an array in main memory of processor 3, where the software is able to access it for processing At some cost in overall speed, part of the image can be intermittently copied to a display memory so that it can be seen on the display means 5 and the effectiveness of the algorithm assessed.

The processor 3 can conveniently be a commercially available personal computer such as a 386 or 486. The processing algorithm may be implemented in the C programming language. The main output of the processor 3 are signals to control solenoid valves 6 to control hydraulic pressure from the reservoir 7 to steering means 8 A sensor 9 provides feedback on the position of the steering

means to the processor 3 The steering means 8 comprises a double ended hydraulic ram that adjusts the steering direction of the wheels of the vehicle The steering means 8 acts independently of the steering wheel of the vehicle and without affecting its operation. Movement of the wheels due to the operation of the steering means 8 does not move the vehicle's steering wheel. The system is over-ridden by manual operation of the steering wheel

The processor 3 calculates steering corrections based on an analysis of the image captured by the video interface card 2 from the camera 1 The task is to identify a row of crops and locate its displacement from a datum position There will certainly not be a well- defined object with shape which could be analysed by outline methods, even if time permitted, nor is there a well defined edge that can be identified by edge analysis techniques. In the early stages of growth, the crop takes the form of a spotty row of variously-sized blobs At its best, it is a linearly-connected domain with a highly irregular outline If a window can be established within which members of only a single crop row will be present, however, an averaging technique can be used. The analysis method makes heavy use of information learned from previous frames. With knowledge of the location of a row, a window can be set for the next frame where movement of the vehicle should not have carried it as far as an adjoining row The new frame will yield a new window for searching the following frame and so on. The task therefore becomes one of making the best estimate of a line through a row of objects within the frame.

Within the frame a count is made of the pixels which exceed a threshold and are therefore deemed to be objects. The proportion of bright to total pixels is therefore known. The 'correct' value of this proportion is a property of the object. For crops it will be related to the plant's stage of development, varying from. say. 0 1 or less when the plants are newly emerged to 0 5 or more as the canopy closes. (Above

this value the farmer would not wish to enter the crop with a vehicle unless harvesting)

If the proportion of bright pixels is seen to fall below the target value, the threshold is according lowered by one count for the following frame The target value is held in the computer as an adjustable parameter It is increased or decreased by the farmer during start-of-field set-up using the keypad 4 The display means 5 shows the actual scene overlaid by the image quantised by the threshold The user can adjust the threshold until the width of the white pixels matches the thickness of the crop

Until the threshold is changed again, the threshold will adapt to variations in picture brightness to preserve the proportion of bright cells in the window Because of the 'fuzzy' boundaries of the crop rows, there is a deal of latitude in the setting before performance deteriorates Regression is conventionally used to fit the best straight line to a sequence of points, usually pairs of measurement samples or readings from which statistics are to be drawn The regression line minimises a quadratic cost function, the sum of the weights of the points times the squares of their distances from the line In the present case, however, the measurements are brightness values for a two dimensional array of points and evaluation of the cost involves a double summation The cost function is

C=I m(x,y) (x-xfit-sfit y) ² The best-fit line is defined by the offset and slope parameters xfit and sfit which minimise C, xfit and sfit can be solved from the simultaneous equations by

dC dC

--o, -- dxfit dsfit

A routine within the program accepts initial values of xfit and sfit which define the window for summation This window is both laterally

displaced and sheared to be centred on the last perceived line fit. The routine computes values fitmean and fitsiope which are the corrections to be made to xfit ana sfit to achieve a minimum. It also computes a variable quality from which a decision can be made about the validity of the result. The weights m(x. y) are given by the function picbit (x, y) which accesses the vision data The computation is arranged to yield totals and moments in an efficient manner, where m is the total, mx is the total horizontal moment about the (sheared) window centre-line, mxx is the second moment and my, mxy and myy have similar appropriate meanings The results are given by fitmean = ( mx -myy - mxymy)l(m -myy - my ² ) and fitsiope = {m -mxy - mx -my)l{m-myy - my ² )

The moment of inertia about the fitted line can be computed. If the fit is good, the result should be small. If the crop is scattered, however, the moment of inertia will be larger As a test, this moment of inertia is compared against myy, the moment of inertia about a horizontal axis. The ratio gives a measure of the quality of fit and the information is only acted upon if the quality is sufficiently high - typically greater than 4.0. Often a row may fade out half way down the field. For this reason, the computation is performed not just for one row but for two or three.

Within the program, two variables represent the state of the vehicle s location These are the lateral movement of the rows, measured either at the vanishing point or at the centre of the view, and the change in the aggregate slope of the rows These variables are used to steer the vehicle.

Aspects of the image analysis algorithm are illustrated in figures 2a to 2h. Figure 2a depicts rows of newly sprouted plants as viewed from a camera mounted on an agricultural vehicle. The newly sprouted plants appear in relatively neat rows As depicted in figure 2b, steering information can be derived from straight lines fitted to the rows of plants.

This is achieved Dy selecting part of the image by applying a window as

shown in figure 2c This is done statistically in the processor 3 A regression line is fitted using a least squares procedure as shown in figure 2d

Figure 2e depicts that the moment about the regression line gives a measure to guard against errors To improve accuracy a group of three windows is used for the calculations as shown in figure 2f The three windows are upαated by corrections to all valid regression lines

Movement in the vanishing point of the three lines indicates a change in the heading of the agricultural vehicle (figure 2g) and movement of the pattern indicates lateral displacement of the vehicle

(figure 2h)

In an alternative embodiment of the invention shown in Fig 3, the steering control elements are controlled from a separate processor 10 This allows the structure of the control software to be simplified The processor 10 receives input from the keypad 4, steering sensor 9 and main processing computer 1 1 The main processing computer is conveniently a standard personal computer based on the 486 chip set The processor 10 controls the solenoid valves 6 to effect automatic steering of steering means 8 The steering control processor 10 can be turned on or off - allowing manual control to be unimpeded - and set point values can be sent from keypad 4 to set the steering target angle The steering sensor 9 may be a hall effect sensor that measures rotation of the wheels of the vehicle The solenoid valves 6 can be switched in a four-millisecond cycle to give smooth and precise control A schematic representation of the display means 5 is shown in Fig

4 In the preferred embodiment the display means is a conventional visual display unit associated with the computer 1 1 or processor 3 The screen 12 is divided into four segments A central segment 13 provides an image of the crop rows as viewed by the camera 1 The image is overlaid with regression lines 14 and windows 1 5 calculated by the algorithm A further portion 16 of the screen displays parameters describing the fit of the regression lines 14 to the image A visual indication of the steering

correction is provided below the central segment 13 A line 17 indicates the position of the vehicle wheels relative to a straight ahead mark 18 The data for the line 17 is drawn from the sensor 9 A second line 19 indicates the correction from straight ahead applied by the system The lines 17 and 19 are constantly changing in the display Set-up data 20 is provided at the bottom of the screen 12 The set-up data 20 can be adjusted at any time by entering a set-up mode and using the keypad 4

The display is central to the user-friendly nature of the system A driver of a vehicle implementing the system has immediate indication of the operations of the system

The particular benefit of this method is a relatively small number of numerical calculations which may typically only involve one hundred picture elements (pixels) per image analysed The method also allows very rapid tracking of an image change, so that control is not lost if a severe disturbance occurs

A practical implementation of the invention is shown in Fig 5 The graph shows the actual performance of a test vehicle on a 35 second test run at a speed of one metre per second Deflection from ideal direction is measured in centimetres The graph shows that after an initial 'acquisition' period the vehicle maintained direction to within 2 centimeters either side of the ideal direction

Throughout the specification the aim has been to describe the preferred embodiments of the invention without limiting the invention to any one embodiment or specific collection of features