Method And Apparatus For Applying Coating Material To A Surface

Abstract

Coating apparatus comprises a coating device for applying coating material to the surfaces to be coated, means for moving the coating device so that the coating material is applied in strokes extending across the direction of movement of the articles, a television camera arranged to form an image of the surface to be coated over the length of the coating strokes so that the image is scanned in the same direction as the coating strokes, synchronizing means for indicating when the part of the scanning path is synchronized with the position on the surface on which the coating device is operative at any instant, detector means responsive to electrical signals from the television camera to detect throughout the scanning sequence whether the image seen corresponds to surface to be coated or not, and control means responsive to the synchronizing means and detector means to control the operation of the coating device in dependence on the image seen by the television camera.

Description

BACKGROUND OF THE INVENTION

The invention relates to the coating of surfaces of a succession of objects, for example the spray coating of vehicle bodies.

In the mass production of automobiles, the automobile bodies are normally carried in spaced relationship on a conveyor and are coated by passing them in succession through a spraying apparatus including spray guns suitably mounted to direct paint sprays onto the sides and tops of the bodies. The guns are mounted so that, as the bodies are moved through the apparatus, the guns traverse the bodies transversely to the direction of travel of the bodies. It is desirable to switch on and off the guns when they reach the limits of the surface to be coated during each traversing movement as well as at the beginning and end of the surface in the direction of travel. Apparatus for carrying out such coating is known for use with articles of known shape and size travelling in known sequence along the conveyor. It has also been known to pre-programme the apparatus for a selection of object shapes and sizes so that it is only necessary to detect the type of object arriving at the coating station in order to choose the correct programme.

It is an object of the present invention to enable coating of a succession of objects even if they arrive at a coating station in random sequence and even if the sizes and shapes of the surface to be coated are not known before the objects reach the coating apparatus.

SUMMARY OF THE INVENTION

The present invention provides apparatus for coating the surfaces of a succession of objects which are moved relative to a coating station, which apparatus comprises a coating device for applying coating material to the surfaces to be coated, means for moving the coating device so that the coating material is applied in strokes extending across the direction of movement of the articles, optical-electrical transducer means arranged to view the surface to be coated and scan in the same direction as the coating strokes so that electrical signals are formed representing a succession of regions of the image viewed along the scanning path, distinguishing means for distinguishing the region along the scanning path which corresponds to the electrical signal at any instant, detector means responsive to the said electrical signals to detect whether the region viewed by the optical electrical transducer at each part of the scanning path is an area of the surface to be coated and thereby indicate when the edges of the area to be coated have been reached, position indicating means for indicating the position of the coating device at any instant, and control means responsive to the output of the distinguishing means, position indicating means and detector means, for controlling the operation of the coating device so that coating material is applied only on the required area of surface to be coated.

To achieve alignment of the apparatus simply, it is preferred that the field of view of the optical-electrical transducer is as long as the length of the coating strokes and is wider in the direction of movement of the objects than the width of one coating stroke, means being provided to select a window of limited width in the field of view. Operation is also more convenient if means is provided for adjusting the position of the window in the field of view in a direction along the direction of travel of the objects, and for adjusting the width of the window.

Of the optical-electrical transducers available, a television camera is particularly suitable. In this case, the said distinguishing means may include counter means for counting horizontal synchronizing pulses for the horizontal scanning lines, the counter means being reset by each vertical synchronizing pulse.

The said position indicating means may include an electrical digital indicator device coupled to the means for moving the coating device so as to produce a succession of pulses for each stroke of the coating device. In this case, the control means conveniently includes a comparator for comparing the output of said counter means with the number of pulses from the electrical digital device so as to indicate when the horizontal scanning lines of the television camera coincide with the region covered by the coating device.

The detector means may include a gating circuit to which the video signal is fed.

Preferably the output of the detector gating circuit and the output of the comparator circuit are fed to a further gate which is arranged to permit operation of the coating device only when the detector circuit indicates that the video signal is of an acceptable level and the comparator indicates synchronism between the horizontal scanning lines of the television camera and the position of the coating device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side of a coating station for spraying vehicle bodies;

FIG. 2 is a view on the line 2--2 in FIG. 1; and

FIG. 3 is a block diagram of electrical control apparatus for use with the coating apparatus of FIGS. 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In this example, a succession of vehicle bodies 11 are conveyed continuously on a conveyor 12 past a coating station 13 at which paint is sprayed onto the vehicle bodies. In order to coat the sides of the vehicle two spray guns 14 and 15 are provided on opposite sides of the conveyor path and they are movable in unison up and down vertical supports 16 and 17 so as to apply vertical painting strokes as the vehicle bodies move by. The guns 14 and 15 reciprocate up and down a fixed path by devices 10, which may for example be pneumatic means, with limit switches 8 and 9 at the ends of the path. The fixed path starts below the bottom edge of the vehicle bodies and stops above the level of the highest vehicle body to be coated. As the guns reciprocate, the supply of paint and compressed air to the guns 14 and 15 is controlled by use of a television camera 18 so that paint is only applied to the surface areas of the bodies to be coated and the paint supply to the guns is switched off as soon as the guns reach the edges of the vehicle body. A further downwardly directed spray gun 19 is horizontally reciprocated on a support 20 extending across the conveyor path so as to coat the roofs of the vehicle bodies. A downwardly directed television camera 21 controls the supply of paint to the gun 19 in just the same way as the camera 18 controls the supply to the guns 14 and 15. The way in which the camera 18 controls the supply to one of the guns 14 and 15 will now be described in detail with reference to FIG. 3, it being understood that the operation of the camera 21 is generally the same. The camera 18 is mounted in a stationary position at one side of the conveyor path so as to view the sides of successive vehicle bodies as they arrive at the coating station. The vertical field of view of the television camera is such that it covers the entire height of the vehicle bodies and extends just beyond the limits of the spray gun path. The horizontal field of view is greater than the vertical field of view since in a conventional television picture frame the ratio of width to height is 4:3. Consequently the horizontal field of view extends a substantial distance along the length of the vehicle body and covers a much greater width than the stroke width of the spray gun (that is the width of each painting stroke on the vehicle body). The camera is used to form an image of the area covered by the movement of the spray gun (that will include the surface to be coated) and control the supply to the gun at each instant in dependence on what the camera sees. Consequently it is necessary to be able to subdivide the camera field of view and identify separate regions of the field of view so that these separate regions can be correlated with the area covered by the spray gun at any instant. The video signal derived from a subdivided region of the field of view can then be detected to see if in that part of the field of view the camera is viewing, surface is to be coated or not, and thereby switch on or off the paint supply. Firstly, subdivision in the vertical direction will be considered. The spray gun is coupled to a digitising device 30 shown in FIG. 3 which provides a pulse for each unit of movement of the spray gun. It is arranged to provide a total of 100 pulses for the full movement of the gun from one end of its path to the other. In this way, the signal on output lines 31 from the digitising device 30 indicates the vertical position of the spray gun at any instant, to an accuracy of 1 percent. The height of the picture frame in the television camera is naturally divided up by the horizontal scanning lines which scan from top to bottom of the picture. Using a 625 line system with interlaced horizontal scanning, each field of horizontal lines consists of 312 lines. A horizontal synchronizing pulse is formed for each line and these pulses are fed from the camera on line 33 to a divide by three unit 32. This groups the horizontal pulses in 104 groups and by arranging for the vertical field of view of the camera to be 4 percent greater than the path of the spray gun, the spray gun path is covered by 100 groups, each group of three pulses corresponding to the same vertical unit of distance as one pulse from the digitising device 30. The output of the unit 32 is fed to a register comprising two divide by 10 units 34 and 35. The outputs of the units 34 and 35 on lines 36 count the groups of pulses from the unit 32 and a digital comparator 37 is used to compare the number of groups of pulses on lines 36 with the number of pulses on lines 31. Both numbers are in binary form. Whenever the numbers on lines 36 and 31 are the same, it indicates that the camera and spray gun are at that instant in synchronism in the vertical direction and the video signal from the camera is relevant to the region which the spray gun is facing. The comparator 37 then provides a strobe pulse on line 38. When 100 groups of pulses have been counted by units 34 and 35, a pulse is fed on line 39 to a flip-flop 40 which in turn controls a gate 41 connected in the output line 38 of the comparator 37. This pulse on line 39 has the effect of closing the gate 41 after 100 groups of pulses have been counted so that no strobe pulse can be provided for the groups of pulses between 100 and 104. This renders inoperative the excess 4 percent of the field of view of the camera. At the end of each picture frame, a vertical synchronizing pulse is fed from the camera on line 42 to each of the units 32, 34, 35 and 40 so as to clear the counted number from the units and reset unit 40 so as to open the gate 41 in preparation for counting the next 100 groups of pulses. It will therefore be seen that vertical field of view of the camera is divided up into 100 numbered strips and the field of operation of the spray gun is similarly divided up into 100 numbered strips. The numbers for the two quantities are compared and a strobe pulse formed on line 38 whenever they are the same. Owing to the speed of scanning in a television camera, the spray gun cannot be moved along its path in the time taken to scan one picture frame of the camera. Consequently it is arranged that the camera scans 200 frames during one movement of the spray gun along its path. Consequently, for each pulse from the digitising device 30, the camera will scan 2 complete picture frames and consequently the comparator 37 will provide two strobe pulses for each strip in the vertical direction.

The duration of each strobe pulse will be the same and will be equal approximately to the duration of three horizontal synchronizing pulses from the camera as the number fed to the comparator 37 on lines 36 will change for every three synchronizing pulses on line 33. The strobe pulses are fed to a gate 43 which is opened by each strobe pulse for the duration only of the strobe pulse.

The synchronization of the camera signals with the position of the spray gun in a horizontal direction will now be described. As the horizontal field of view of the camera is much wider than the stroke width of the spray gun, it is necessary to limit the signals from the camera to a narrow window of similar width to the stroke width of the spray gun and located at a point in the field of view corresponding to the location of the spray gun in a horizontal direction. To achieve this, horizontal synchronizing pulses, which define the position of the left hand end of the field of view, are fed from the camera along line 33 to a monostable 44 which stretches the duration of the pulses up to the required position in the field of view for the left hand edge of the window. The output of the monostable 44 is fed to a further monostable 45 which generates pulses, herein called window pulses, which dictate the position and width of the window in the field of view. The time of occurrence in each horizontal scanning line of the leading edge of the window pulse dictates the position of the window in the field of view and the duration of the window pulse determines the width of the window. The pulse stretching achieved by the monostable 44 is adjustable so that the window can be adjusted easily in order to synchronize with the spray gun after the camera and spray gun are set up in position. The width of the window can similarly be adjusted to suit the spray gun arrangement by adjustment of the monostable 45. The video signal from the camera representing the image seen throughout the scanning sequence is fed on line 46 to a gate 47 controlled by the window pulses from the monostable 45. In this way, the video signal for much of the horizontal field of view is blanked out and the video signal is only allowed through the gate 47 for the duration of each window pulse and that as explained above is chosen to correspond with the vertical band along which the spray gun is operating.

In order to decide if paint should or should not be supplied to the spray gun at any instant, the video signal passed by the gate 47 is passed to a detector comprising high and low level detector units 48 and 49 coupled to a NOR gate 50. The detector units 48 and 49 check the contrast or illumination level indicated by the video signal, and if the signal has a level corresponding to the contrast level of the object to be coated, then a pulse, called an object pulse, is passed by the NOR gate 50. The object pulse is then compared in time with the strobe pulses by gate 43 so that when an object and strobe pulse occur simultaneously, gate 43 provides an action pulse to trip an electronic switch to start the coating discharge unit. As shown in FIG. 3, the output of gate 43 is fed to a flip-flop 51 which in turn controls a thyristor switch device 52. The paint discharge control unit 53 which may be a pneumatic valve, or solenoid valve is operated by an A.C. supply 54 through the thyristor switch 52. A zero volt detector 55 is coupled between the A.C. supply and the switch 52 in order to ensure that switching on occurs at voltage zeros.

A connection is provided along line 56 between the digitising device 30 and flip-flop 51 so that when the device 51 has been opened by a pulse from gate 43, it is closed again by the next pulse from the digitising device 30. This means that the discharge of paint is separately controlled for each strip in the vertical direction corresponding to one pulse from the device 30. After coating one strip, a further action pulse must pass the gate 43 in order to achieve further application of paint. In this way, each separate strip representing 1 percent of the length of movement of the spray gun is separately assessed to see if paint should or should not be supplied. In practice, mechanical and pneumatic inertia integrates the instructions to the discharge control unit 53 so as to achieve smooth and uniform response.

In this way, paint is applied to the area of the surface to be coated but supply to the spray gun is stopped when outside that area. Very little paint is wasted owing to the maximum error of 1 percent in assessing the position of the spray gun.

It will be appreciated that as the television camera scans the objects to be coated across the entire path length of the spray gun at the coating station, accurate control is achieved over the areas coated without any pre-knowledge of the size or shape of objects arriving at the coating station, nor of their sequence of arrival. Besides detecting the upper and lower limits of the vehicle bodies to be coated, the television camera will similarly detect leading and trailing edges of the bodies in the direction of movement along the conveyor and thereby prevent spraying of paint, and reciprocation of the guns, in the spaces between adjacent bodies on the conveyor.

The television camera can also be used to detect a color indicating marker on a vehicle body as it approaches the coating station and thereby control the selection of the appropriate color of paint supply to the spray gun.

Although the above description refers to the discharge control unit 53 controlling the supply of paint to the spray gun, it will be appreciated that this will normally involve the control of supply of compressed air, as well as paint, which is used to effect the spray.

The invention is not limited to the details of the foregoing example. For example, the coating process need not necessarily involve the use of spray guns. Furthermore, although the invention is particularly suitable for coating vehicle bodies, it may also be used for coating surfaces of other objects.

Claims

I claim:

1. Apparatus for coating the surfaces of a succession of objects which are moved relative to a coating station, which apparatus comprises a coating device for applying coating material to the surfaces to be coated, means for moving the coating device so that the coating material is applied in strokes extending across the direction of movement of the articles, optical-electrical transducer means arranged to view the surface to be coated and scan in the same direction as the coating strokes so that electrical signals are formed representing a succession of regions of the image viewed along the scanning path, distinguishing means for distinguishing the region along the scanning path which corresponds to the electrical signal at any instant, detector means responsive to the said electrical signals to detect whether the region viewed by the optical electrical transducer at each part of the scanning path is an area of the surface to be coated and thereby indicate when the edges of the area to be coated have been reached, position indicating means for indicating the position of the coating device at any instant, and control means responsive to the output of the distinguishing means, position indicating means and detector means, for controlling the operation of the coating device so that coating material is applied only on the required area of surface to be coated.

2. Apparatus according to claim 1, in which the optical-electrical transducer forms an image of the surface which image is scanned to provide said electrical signals.

3. Apparatus according to claim 1 in which the field of view of the optical electrical transducer is as long as the length of the coating strokes and is wider in the direction of movement of the objects than the width of one coating stroke, means being provided to select a window of limited width in the field of view.

4. Apparatus according to claim 3, in which means is provided for adjusting the position of the window in the field of view in a direction along the direction of travel of the objects.

5. Apparatus according to claim 4, in which means is provided for adjusting the width of the window in said direction of travel.

6. Apparatus according to claim 2, in which the optical-electrical transducer is a television camera.

7. Apparatus according to claim 6 in which the said distinguishing means includes counter means for counting horizontal synchronizing pulses for the horizontal scanning lines, the counter means being reset by each vertical synchronizing pulse.

8. Apparatus according to claim 7 in which the said position indicating means includes an electrical digital indicator device coupled to the means for moving the coating device so as to produce a succession of pulses for each stroke of the coating device.

9. Apparatus according to claim 8 in which the control means includes a comparator for comparing the output of said counter means with the number of pulses from the electrical digital device so as to indicate when the horizontal scanning lines of the television camera coincide with the region covered by the coating device.

10. Apparatus according to claim 9 in which the detector means includes a gating circuit connected to receive the video signal, the output of the detector gating circuit and the output of the comparator circuit being connected to a further gate which is arranged to permit operation of the coating device only when the detector circuit indicates that the video signal is of an acceptable level and the comparator indicates synchronism between the horizontal scanning lines of the television camera and the position of the coating device.

11. Apparatus according to claim 10, in which means is provided for grouping a number of horizontal scanning lines of the television camera so that each group of scanning lines is equivalent to a unit change in position of the coating device corresponding to one pulse from the electrical digital indicator device.

12. Apparatus according to claim 3 in which the location of the said window in the field of view is determined by a delay device responsive to the horizontal synchronizing pulses and arranged to provide an adjustable delay signal after the occurrence of a horizontal synchronizing pulse.

13. Apparatus according to claim 12 in which the delay device is arranged to actuate a pulse generator to generate window pulses of variable width, the window pulses being fed to a gating circuit so that the time of occurrence and width of the window pulses controls the position and width of the window in the field of view.

14. Apparatus according to claim 13, in which the gating circuit receiving the window pulses is connected to receive the video signal from the television camera as an input and provide an output to the said detector means.

15. Apparatus according to claim 1 in which more than one coating device is provided and more than one optical-electrical transducer is provided.

16. Apparatus according to claim 1 in which the coating device comprises a spray gun.

17. Apparatus for coating the surfaces of a succession of objects which are moved relative to a coating station, which apparatus comprises a coating device for applying coating material to the surfaces to be coated, means for moving the coating device so that the coating material is applied in strokes extending across the direction of movement of the articles, a television camera arranged to form an image of the surface to be coated over the length of the coating strokes so that the image is scanned in the same direction as the coating strokes, synchronizing means for indicating when the part of the scanning path is synchronized with the position on the surface on which the coating device is operative at any instant, detector means responsive to electrical signals from the television camera to detect throughout the scanning sequence whether the image seen corresponds to surface to be coated or not, and control means responsive to the synchronizing means and detector means to control the operation of the coating device in dependence on the image seen by the television camera.

18. A method of coating the surfaces of a succession of objects which comprises moving the objects in succession past a coating station at which a coating device and optical sensing device are located, moving the coating device across the direction of movement of the objects so that coating material is applied in strokes, forming an image by the optical sensing device of the surface to be coated, which image extends over the length of the coating strokes and is scanned in the same direction as the coating strokes to produce electrical signals corresponding to the image formed along the scanning path, sensing the level of the electrical signals to determine limits of the area of the surface to be coated and thereby control the emission of coating material from the coating device.

19. A method according to claim 18 including sensing the position of the coating device throughout the coating stroke and synchronizing the said electric signals with the position of the coating device.

20. A system for coating the surfaces of objects, comprising in combination:

coating means for selectively applying coating material;

support means for successively presenting the surfaces of the objects to said coating means;

traversing means for causing relative movement in one direction between said coating means and said support means whereby coating material is applied along a path in said one direction on the surfaces of the objects;

optical-electrical transducer means for distinguishing between an uncoated and a coated condition of said surface of the object along said path of the coating material being applied;

discharge control means connected to said coating means for selectively controlling discharge of coating material therefrom;

position responsive means responsive to movement of said traversing means for periodically actuating said discharge control means to closed condition a great number of times during a traversing stroke whereby the coating material path is subdivided into a corresponding great number of potential control zones; and

means responsive to said optical-electrical transducer means for overriding the effect of said position responsive means on said discharge control means in response to an uncoated condition remaining along the path of the coating material being applied.

21. In a system for coating objects, which includes conveyor means for moving objects along one path, coating means for applying coating material to the objects and located at a coating station along said path, drive means connected to said coating means for traversing said coating means back and forth along a second path transverse to said one path, and valve means for controlling flow of coating material to said coating means as said coating means is moved in one direction along said second path, the improvement comprising:

position responsive means responsive to movement of said coating means in said one direction for periodically actuating said valve means to closed condition a great number of times during movement of said coating means in said one direction whereby potentially to terminate application of coating material at each of a corresponding great number of incremental movements of said coating means;

optical-electrical transducers means focused upon the surface of an object being coated for distinguishing between coated and uncoated condition of such surface; and

means responsive to said optical-electrical transducer means for overriding the effect of said position responsive means on said valve means in response to uncoated condition remaining in said one direction along the surface being coated.