BACKGROUND OF THE INVENTION 
     The present invention concerns a process and device for spraying a coating product utilized, particularly, in plants where delivery of the coating product is performed from a reservoir which contains a piston. In conventional coating plants, the rate of delivery of the coating product is sometimes controlled as a function of the speed of the conveyor which transports the articles to be coated such as automotive vehicle bodies. A calculator determines, at given time intervals of the order of several seconds and as a function of the speed of the conveyor, a desired value for the rate of flow of paint which must be delivered to a sprayer during the subsequent time interval. In the case of supplying a sprayer from a piston-type reservoir, the speed of displacement of the piston is controlled as a function of the speed of displacement of the articles. In practice, the precision of the speed of conveyance of the vehicle bodies is as significant for the surface state obtained as is the precision of the delivery of coating product furnished to the sprayer. 
     Now, it occurs that the measurement of the speed of the conveyor is imprecise or that the conveyor speed varies in an uncontrolled manner, which produces errors in the supply of product furnished to the sprayer and, as a result, irregularities in the coating which can produce defects in appearance. In addition, it occurs that the conveyor must, for safety reasons, be capable of being halted in the event of emergencies during a coating process. The response times are such that a poorly controlled quantity of paint is projected onto an article during the emergency stopping, which leads, on the one hand, to a waste of the product and, on the other hand, to the formation of an added thickness of paint which is prejudicial to the surface state. 
     Equivalent problems appear in plants where the articles to be coated are stationary during the spraying phases and where the sprayers are movable along or around the articles, for example in the case where a sprayer is carried by a multiaxial robot. 
     The invention solves the totality of these problems. 
     SUMMARY OF THE INVENTION 
     The invention provides a process for spraying a coating product from at least one sprayer supplied with a coating product from a piston-type reservoir, characterized in that the position of the piston in the reservoir is controlled by the relative position of the sprayer with respect to the articles to be coated. 
     The invention also provides a device capable of carrying out the process and more precisely a device for spraying the coating product onto articles transported by a conveyor, the device comprising a sprayer supplied with a coating product from a reservoir of the piston type, characterized in that the device comprises means for controlling the position of the reservoir piston on the basis of the position of the sprayer with respect to the articles. 
     As a result of the process and device according to the invention, a coating plant is not disturbed by variations in the speed of the conveyor of the plant and/or by emergency stops. 
     The invention is applicable to the case where the articles to be coated move in front of the sprayer, which is itself possibly movable, or where the articles are stationary during the coating phases and the sprayer is movable, for example carried by the arm of a multiaxial robot. In the first case, the sprayer can be carried by a lateral machine or a roof machine. 
     The invention will be better understood and other advantages thereof will appear more clearly from the description which follows of two embodiments of a plant for spraying a coating product according to the principals of the invention, given solely by way of example and described with reference to the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic plan view of a coating product spraying plant according to the invention. 
     FIG. 2 is a graph showing the position of a reservoir piston as a function of the position of car bodies along the axis X-X&#39; of FIG. 1. 
     FIG. 3 is a schematic plan view of another coating product spraying plant. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The plant shown in FIG. 1 includes a conveyor (not shown) for displacing articles, such as automotive vehicle bodies 1, in translation in a booth 2 along an axis X-X&#39;. It comprises essentially several lateral machines 3a-3f and a roof machine 4. Each of these machines carries at least one sprayer 5a-5i, respectively, supplied with a coating product from a respective piston-type reservoir 6a-6i. The reservoir 6a of the machine 2a includes a piston 7a controlled by an electric stepping motor 8a, piston 7a being operative to push coating product out of reservoir 6a toward sprayer 5a. Motor 8a is controlled by a conventional numerical machine control 9a connected to an automatic controller (not shown). 
     The position of pistons 7a in reservoir 6a is determined by displacement of piston 7a on a path parallel to its longitudinal axis. The position of body 1 along axis X-X&#39; is measured by a conventional sensor 10 placed along the path of travel of bodies 1 and whose output is connected to the machine control. According to the invention, the position of piston 7a is determined by the numerical machine control 9a as a function of the position of body 1 along axis X-X&#39;. More specifically, a pulse is supplied by the sensor 10 to the machine control system for each centimeter of displacement of body 1 or of the conveyor. With an average conveyor speed of 4 meters per minute, a pulse is supplied every 150 ms. After each conveyor displacement pulse received, the automatic control device interpolates the average assigned, or desired, value for the position of piston 7a which is transmitted to the numerical control 9a and to the motor 7a. 
     The operation of all of the other machines is the same. 
     The function having for its independent variable the position of body 1 along axis X-X&#39; and for its dependent variable the position of the piston is a monotonically increasing function. In addition, if body 1 should move backwards along axis X-X&#39;, i.e. the direction of its movement reverses, the piston is halted and maintained in position until body 1 returns to the position where it was located before the direction of its movement was reversed. Possibly, the connection between the piston and its drive rod is of the type disclosed in French Application 9408930 filed by Applicant and the entirety of which is incorporated herein by reference. 
     Examples of curves representative of the position of the piston with respect to advancing movement of body 1 along axis X-X&#39;, represented by the sensor pulses, are shown in FIG. 2. Curve A is used when a linear relation is to exist between the sensor pulses and the displacement of the piston. The scale coefficient is constant and is of the order of 0.0375 because the piston has a total travel path of the order of 150 mm during the time when body 1 covers a path length of 4 meters. Curve B has a variable scale coefficient which is determined experimentally as a function of the desired thickness of the coating product and of the dimension of the surfaces to be painted perpendicular to their displacement on the conveyor. Thus, the position of the piston at the middle of body 1 varies rapidly because the entire height of the vehicle body is to be painted while at the beginning and end of the coating operation of one vehicle body, the body is painted just to the top of the fenders; i.e. to the middle of its maximum height. 
     The plant shown in FIG. 3 differs from that of FIG. 1 in that the conveyor is of the stop-and-go type and in that sprayer 105a is mounted to undergo movements parallel to the axis X-X&#39;, here mounted on a multiaxial robot 112a. Elements similar to those of FIG. 1 carry identical reference numerals increased by 100. Body 101 being fixed in booth 102 during the coating product spraying operations, the relative position of the sprayer with respect to body 101 is known by a measurement of the position of sprayer 105a in booth 102, i.e. the position of the end of the arm of robot 112a. Sensor 10 of the embodiment of FIG. 1 can advantageously be replaced by a position coder 110a or an assembly of position coders placed in proximity to the joints of multiaxial robot 112a. As previously, control of the position of piston 107a in reservoir 106a is possible with the aid of a numerical machine control device 109a which controls an electric stepping motor 108a. 
     This application relates to subject matter disclosed in French Application number 94 14047, filed on Nov. 18, 1994, the disclosure of which is incorporated herein by reference. 
     While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention. 
     The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.
In a process and device for spraying a coating product onto an article from a sprayer to which coating product is supplied from a reservoir equipped with a piston which is moved along a path in order to transport the coating product to the sprayer, movement of the piston is controlled as a function of movement of the sprayer relative to the article.