MXPA98008638A - Device for assembling motor vehicle bodies or subassembles thereof, which have an integrated deformation detection system, and assembly method that makes use of this device - Google Patents

Abstract

The present invention relates to a device for assembling a motor vehicle body or sub-assemblies thereof, comprising: an assembly station, provided with a programmable assembly means, a conveyor means, for transporting at least a portion of the structure to be assembled to an assembly station as well as to take the structure after it has been assembled outside the assembly station, at least a pair of locating and locking gates placed on two opposite sides of the assembly station , provided with locating devices to retain the component elements of the structure to be assembled in the proper assembled position, while the assembling operation is carried out, the gate can be moved towards or away from each other, between a separate non-operative condition, in which the location and immobilization devices are disengage from the structure to be assembled, and a closed operating condition, in which the locating and immobilization devices are capable of coupling the structure to be assembled, the device is characterized by the combination of the following characteristics: a) each positioning and locking gate is an independent unit, which has a base portion that can be moved in the assembly station and is adapted to be rigidly immobilized to a fixed structure of the station in the operating condition mentioned before the gate, and a active portion, which transports the positioning and immobilization devices, which extends from the base portion and do not connect directly to the fixed structure of the station in the operative condition of the gate, so that they remain free to undergo microdeflections to along the transverse direction in longitudinal relation of the station n, due to free variations of the transverse dimension of the structure to be assembled with respect to a predetermined theoretical dimension, b) between the active portion of each positioning and immobilization gate in the fixed structure there are detector means adapted to detect the microdisplacement of the active portion with respect to the fixed structure due to micro-deflection, at a predetermined distance from the immobilization region of the base portion to the fixed structure, c) the device further comprises means for processing the signals coming from the detector means and to calculate the displacement of the various regions of the active portion of each positioning and locking gate, as a function of the distance from the immobilization region, as well as to calculate in consequence the forces exchanged between each gate and the structure that is going to assemble

Description

DEVICE FOR ASSEMBLING MOTOR VEHICLE BODIES Q SUBASSAMB EFFL OF THEM, WHICH HAVE AN INTEGRATED DEFORMATION DETECTION SYSTEM. AND METHOD OF MAKING USE OF THIS DEVICE BACKGROUND OF THE INVENTION The present invention relates to devices for assembling motor vehicle bodies or sub-assemblies thereof. The invention is applicable to devices for assembling vehicles of any type, such as cars, light or heavy trucks, or the like. In particular, the invention relates to devices for assembling structures made of sheet metal pressed by electrical spot welding. However, the invention is also applicable to any other assembly system, such as by adhesive, as well as to any other material such as plastic or composite materials. In more detail, the invention relates to devices for assembling motor vehicle bodies or sub-assemblies thereof, of the known type comprising: an assembly station, provided with a programmable assembly means; REF: 28333 a conveyor means, for transporting at least a portion of the structure to be assembled to an assembly station as well as for taking the structure after it has been assembled in the assembly station, at least one pair of locating and fixing gates placed on two opposite sides of the assembly station, provided with locating and fixing devices to retain the component elements of the structure to be assembled in the proper assembled position, while carrying out the assembly operation, the frame can move towards one another approaching or moving away, between a separate non-operative condition, in which the locating and fixing devices are uncoupled from the structure to be assembled, and a closed operating condition, in which the locating and fixing devices are capable of coupling the structure to be assembled. Assembly devices of the type indicated above are well known and have been used for a long time. For example, the applicant has produced and sold a flexible welding system identified under the trademark "ROBOGATE" for years, the basic concept of which has been originally shown in U.S. Patent No. 4,256,957, and the German Parallel Patent Number 28 10 822 and the which has given rise over time to numerous improvements and subsequent variants which have also formed the subject of the respective patents of the applicant. The ROBOGATE system has actually determined a milestone in the welding technique of motor vehicle bodies which was generally used until the end of the seventies and was replaced by the apparatus previously used in many vehicle manufacturers throughout the world. world. In a basic version of the system, it comprises two or more pairs of locating gates which are rapidly exchanged at the welding station and are adapted to operate on the respective types of bodies. The system is capable of operating in the body, even very different from one another, so that the same line can be used to produce different models. An additional advantage of the ROBOGATE system is that it can be adapted by a relatively simple and fast operation, and therefore with very small investments, to the production of a new body model. Another advantage is based on ensuring a uniform quality of all models of the same type on which the system operates. The present invention can be applied both to a flexible station that uses many pairs of locating gates as well as to a "rigid" station comprising a single pair of locating gates which are for operating in a single-body model. In the case of the known devices described above, the assembly means is constituted by programmable robots provided with heads for electric spot welding. However, as already indicated, the present invention can be applied whatever the technology used to connect the various parts of the structure together. An additional feature of the known device described above is based between the structure to be welded reaches the weld expression after it has been loose assembled. Devices of the same type are also known, in which only a portion of the structure to be welded is caused to enter the welding station, the remaining parts which are necessary to form the complete structure to be welded they are fed separately to the welding station and are connected here to each other. The present invention can also be applied identically to devices of this type. Following the patents mentioned above in relation to the basic version of the ROBOGATE system, the applicant has submitted additional packages in relation to various improvements and variants to this system. A recent improvement has formed the subject of the European patent application EP-A-0 642 878, which has already been mentioned before. In this document, a welding station is described which has a structure much less complicated, lighter and more flexible with respect to the previous modalities of the ROBOGATE system. In most conventional solutions, the locating gates mentioned above are slidably guided, in order to make them interchangeable rapidly in the welding station with the gates provided for a different type of body to be welded on top guided guides. parallel to the direction of the conveyor line at the welding station, which requires providing these heavy fixed support frames. These racks, in addition to contributing greatly to the cost and volume of the welding station, also make it difficult for welding robots to find the necessary space in order to operate on the body to be welded. In the solution described in the European patent application number EP-A-0 642 878, each locating gate can be moved independently of the other gates since it is provided with a self-driven lower carriage guided on a rail provided along the floor of the welding station. Therefore, the locating gates no longer require that longitudinal upper guides extending across the entire length be provided between their working position in the welding station and their standby position separate from the welding station, Thus, it is no longer necessary to provide the heavy and bulky support frame, which usually constituted gantry-like structures connected to one another longitudinally, which was necessary in the classic ROBOGATE plants. Therefore, the space available for the welding robots is greatly increased which also allows, if desired, the number of robots to be increased in order to obtain an increase in the number of welding points made within a predetermined time The robot can have much easier access to all regions of the body that will be welded in order to carry out the proper welding. In addition, since each location gate can be moved independently of the other gates, the separation between each gate and the subsequent gate along the line is free and variable, which provides greater flexibility in the design and installation of the plant. . Finally, the installation of the plant itself is of a lower cost compared to the more conventional plants. This known system, which has been sold by the applicant under the trademark "OPEN-ROBOGATE" has been described in the European patent application EP-A-0 642 878, which has been mentioned before with reference to a first embodiment, which is related to the case in which all the components of the structure to be welded are transported to the welding station from the conveyor line, in a condition already preassembled or in some other way with the various elements supported separately from each other, but in positions close to the final assembly position. The additional European patent application number 97830456.6 has proposed a new version of the "OPEN-ROBOGATE" system which the conveyor line increasingly feeds only a portion of the structure to be welded (typically, the floor panel) to the station of welding, while the sides of the body are formed directly in the positioning gates, which then help to bring them into engagement with the floor panel when the gates move in their closed operative position. Obviously, in this second version, so-called "toy-tabbing" toy stations, which are provided in most conventional solutions upstream of the welding station, in order to assemble loose bodies before welding, are eliminated. In addition, the conveyor line becomes simpler and less expensive with respect also to the embodiment shown in the European patent application EP-A-0 642 878, since the complete structure of the body is formed at the beginning, from its components only in the welding station and not upstream of it. The present invention provides a further improvement of the "OPEN-ROBOGATE" system both with reference to the variant shown in the European patent application EP-A-0 642 878 and in the variant formed by the subject of the European patent application number 97830456.6 . Furthermore, the invention is applicable both to a station of the type indicated above which is flexible, i.e., comprising many pairs of interchangeable positioning gates to each other in order to adapt themselves to various body types that are going away. to be welded, as well as to a rigid station, comprising a single pair of positioning gates dedicated to a single body model.

A problem which has been encountered in the past with welding stations of the type described above is based on the fact that the assembled structure has a width, i.e., a dimension along a transverse direction along the longitudinal direction of the welding station, which does not always correspond to a predetermined theoretical direction. Even the most modern and sophisticated modalities do not avoid variations with respect to the predetermined width which, at least, is in the order of a few millimeters. With ROBOGATE stations of the conventional type, in which the positioning gate is guided by means of rigid support frames, when the structure to be welded is fixed between the positioning gates, it is driven to assume a theoretical configuration. Therefore, when the welding is completed and the positioning gates are opened leaving the welded structure free, the latter can be subjected to a kind of relaxation which leads to a condition that does not correspond to the desired theoretical. If one wishes to improve the accuracy of the system by accurately studying the deformation to which the welded structure is subjected, complicated detection systems should be provided using straight calipers, or subsequently an off-line measurement evaluation which is costly and involves of problematic tests.

OBJECT OF THE INVENTION The object of the present invention is to provide an assembly device which is provided with an integrated system for detecting the width of the assembled structure, which is simple, cheap and efficient to use.

BRIEF DESCRIPTION OF THE INVENTION In order to obtain the objective indicated above, the invention provides a device for assembling motor vehicle bodies or sub-assemblies thereof of the type indicated at the beginning of the present description, characterized by the combination of the following characteristics: a) each gate positioning and fixing is an independent unit, which has a base portion that can be moved in the assembly station and is adapted to be rigidly fixed to a fixed structure of the station in the operating condition mentioned before the gate, and a active portion, which carries the fixing and fixing devices mentioned above, which extend from the base portion and do not connect directly to the fixed structure of the station in the operative condition of the gate, so that they remain free to experiment microdeflections along the transverse direction in relation to the longi direction tudinal of the station, due to variations of the transverse dimension of the structure to be assembled with respect to a predetermined theoretical dimension, b) between the active portion of each positioning and fixing gate in the fixed structure are interposed detector means adapted to detect microdisplacement of the active portion with respect to the fixed structure due to micro-deflection, at a predetermined distance from the area where the base portion of the gate is fixed to the fixed structure, 10 c) the device further comprises means for f processing the signals coming from the detector means and for calculating the displacement of the various areas of the active portion of each positioning and fixing gate, as a function of the distance of the fixed area, as well as to calculate, if desired, the force exchanged between each gate of the structure to be assembled. The characteristic mentioned before (a) is known ^ per se and is typical of assembly stations "OPEN-ROBOGATE" of the type which has formed the object of the European patent application EP-A-0 642 878 and 97830456.6. The studies and tests carried out by the applicant have shown that the positioning gates used in stations of this type, being free to undergo micro-deflections (which of any In this way, due to the coupling of the gates on the structure to be welded, they can advantageously be used to detect, in a very simple and fast way, the forces to which the structure is subjected. structure that is to be assembled when it is coupled by the placement gates. Thus, for example, by placing a needle detector on the fixed structure of the station adjacent to the lower portion of each positioning gate, immediately above the fixing region where the gate is fixed to the fixed structure, it is possible to detect the micro-displacement (typically of an order of less than 0.1 mm) which takes place due to the coupling of the gate on the structure to be welded. This detection allows the displacement of the free portion of the positioning gate, at various distances from the fixing region, to be calculated with great precision. The forces exchanged with the structure to be welded along the transverse direction can be determined accordingly based on the linear relationship. Therefore, it is possible to verify the phenomenon in a simple and precise manner so that the station can be adjusted until an accuracy with respect to the width of the assembled structure is achieved much greater than currently possible. It is also possible in this way to detect any force large enough to cause a decoupling of the base portions of the positioning gates of the fixed structure when they are immobilized, when the gates are closed in the structure to be welded. As already indicated, providing a simple detector at any point between the fixed structure and a region of the positioning gate adjacent thereto will allow the length and direction of the micro-deflection to which the structure of the structure is subjected to be detected. gate, so that you can detect the value of the force directed outward or inward to which the structure to be welded will be subjected, according to the structure to be assembled is larger or narrower of the theoretical dimension. The detection of the micro-deflection of the positioning gates can be used to calculate the value of the force to which the structure is subjected with a very high precision, without the need for costly and problematic detection systems using straight calipers or the like. The detection system can be used to determine the forces acting on the structure in the various stages of operation of the station, such as during the initial stage when placing the various components of the structure to be welded, when they are immobilized the various components, and during the subsequent welding step and subsequently when the positioning gates are uncoupled and open at the end of the welding operation. By means of the system according to the invention, it is possible to identify the main sources of any force which is detected during the assembly operation. The calculation of the forces along the transverse direction (outward or inward), to which the structure is subjected during assembly represents an essential factor in predicting the probability of the reverse flexion phenomenon of the structure after decoupling. of the placement gates. The system according to the invention allows the phenomenon to be controlled and studied easily and precisely, so that the assembly operations and the preceding sub-assembly and stamping stations can be adjusted in the best way. The device according to the invention is an assembly device which has an integrated system for detecting the width of the assembled structure that allows to study the phenomenon of any deformation to which the structure is subjected during the assembly operation, in order to return to the station capable of operating with very high precision, for example with errors in the transverse dimension of the welded body less than 1 mm. The invention also provides a method for assembling motor vehicle bodies or sub-assemblies thereof using the device described above. In a preferred embodiment of the device according to the invention, the detector means mentioned above comprises first and second detector means placed respectively on the front end and the rear end of each positioning and locking gate, so that the detection is carried out at the four corners left front, right front, left rear and right rear of the assembly station. According to a further preferred feature of the invention, display means are associated with the processing means capable of displaying a plan view image of the assembly station where the values are displayed of a parameter that is chosen from: the displacement detected by the detector means, the calculated displacement in the upper part of each gate or the resultant force exchanged between each gate and the body, respectively in the four corners left front, right front, left rear and right rear of the station assembled, the image also shows the average values calculated respectively between the values between the front left and right front corners, between the values between the left rear and right rear corners, between the values between the right front and right rear corners and between the values between the corners left front and left back row. Furthermore, preferably, with each displayed value there is an associated color according to a code which associates determined colors at certain intervals of variation of the parameter. In this way, users can quickly verify the proper operation of the system or can carry out, if necessary, adjustment operations if they have abnormal situations, such as an unwanted opening of two gates in the area where they are rigidly fastened , due to an excessive transverse dimension of the body.

BRIEF DESCRIPTION OF THE DRAWINGS Additional features and advantages of the invention will become apparent from the description that follows with reference to the accompanying drawings, given solely by way of non-limiting examples, in which: Figure 1 is a diagrammatic view of a welding station according to the prior art, in a plant transverse to the longitudinal direction of the station, figure 2 is an enlarged scale view of a welding station according to the invention, figure 3 shows a detail of figure 2, Figures 4, 5 show two diagrams which show the operation principle of the device according to the invention, and figure 6 is a diagrammatic view of the image displayed on a video terminal associated with the processing means provided in the device according to the invention.

DESCRIPCipw ™ ÜTAT.T.AG > A t raí * Mnr > AT.tt > An PPtrat * tnA Figure 1 shows an assembly station for welding motor vehicle bodies, of the type produced and sold by the applicant under the trademark "OPEN-ROBOGATE". According to a technique known per se, the device comprises the welding station 1 including a fixed structure 2 provided with means 3 for receiving, positioning and immobilizing the lower portion of the motor vehicle body 4 which is fed to the station 1 by means of a conveyor line 5. The construction details of these parts are not described or illustrated here, since they can be made in any known manner and are not found, taken alone, within the scope of the invention. The invention is applicable both to the case in which the body 4 is assembled loosely upstream of the welding station and transported therefrom by means of the line 5, as in the case in which the line 5 carries only the portion bottom of the body 4 to the welding station, while the remaining portions (essentially the sides of the body and the upper connecting transverse members) are added to the floor panel at the welding station. Also according to a known technique, when the body 4 reaches the welding station, it is placed and immobilized by means of a pair of positioning and locking gates 6, 7. Each of the positioning and locking gates 6, 7 is provided with positioning and immobilization devices shown diagrammatically and designated with the number 8 in Figure 2, adapted to couple the body 4 when the gates 6, 7 are transported from an inoperative open condition (shown with a dashed line in Figure 1) ) to an operational closed condition (shown by a solid line in Figures 1, 2). Once the body 4 has been immobilized in the proper position for assembly, it can be welded by means of welding means, such as programmable robots 9 provided with electric spot welding heads. When the welding is complete, the positioning and locking devices 8 are opened and the positioning gates 6, 7 return to their non-operating open position, to allow the welded body 4 to leave the welding station while a new structure is going on. to be welded enters the interior of the station. The invention is applicable both in the case in which the welding station 1 comprises a single pair of positioning gates 6, 7 provided for a predetermined body model, and for the case in which the station is provided with many pairs of 6, 7 positioning gates quickly interchangeable with each other in the position of a job and adapted for different types of motor vehicle bodies. In this case, when the positioning gates 6, 7 are in their non-operative open condition (shown by dotted lines in Figure 1) they are capable of moving longitudinally to allow a new pair of positioning gates (not shown in the drawings) ) enter the welding station. For this end, also according to a technique known per se, each gate 6, 7 is provided with a lower self-driven carriage 10 guided on rails 11 provided along the floor of the station. When the positioning gates 6, 7 are in their closed operative position, they are rigidly connected and immobilized to the fixed structure 2 by means of immobilization devices (not shown in the drawings). As shown in Figures 1, 2, each of the gates 6, 7 has its base portion 12 which is immobilized to the fixed structure 2. More precisely, in the immobilized condition, the base portion 12 of each gate 6, 7 is pressed transversely against the fixed structure 2 into two vertical supports 13, 14 which are vertically aligned with each other (shown only diagrammatically in the figure). 2) . As is clearly evident from the foregoing description, each positioning gate 6, 7 has its active portion 15 extending upwardly from the base portion 12, which is free of any direct connection to the fixed structure 2 of the soldering station. According to the invention, with each of the gates 6, 7 of positioning slightly above the upper support 13 there is an associated needle detecting device S. The device S, whose structure can be of any known type, has a needle 16 (figure 3) adapted to cooperate with a rod 17 rigidly connected to the respective positioning gate, so as to be able to detect the displacements of the gate 7 with respect to the fixed structure 2 of the station along a direction A (FIG. 3) transverse in relation to the longitudinal direction of the welding station. The detector S emits a signal which is fed to the processing means 18 (figure 3) of any type adapted to process and display this signal, as well as to carry out calculations which will be described in the following. As already discussed in the foregoing, the width of the body 4 to be welded, that is, the dimension along the transverse direction A, may be greater or smaller than the theoretical dimension. As a result of this, when the positioning gates 6, 7 are brought to their closed operative position and the positioning and locking devices 8 couple the body 4, the micro-deflections (which in any case are not greater than 1 mm / 1000 newtons ) and the upper free portions 15 of the two positioning gates 6, 7 can take place. These micro-deflections can advantageously be used, since they allow the forces exchanged between the positioning gates 6, 7 and the structure 4 along the transverse direction A to be determined. This is carried out by detecting the microdisplacement of each gate 6, 7 where the detector S is located, ie, at a distance Lx along the vertical direction from the upper support 13 of the fixed structure 2. As a result, the microdisplacement of the various parts of each gate 6, 7 at various distances L along the vertical direction from the support 13 can be detected, and the resultant forces exchanged along the transverse direction A can be calculated between gates 6, 7 and the body, according to a proportional relationship. The device according to the invention is therefore provided with an integrated system for detecting the transverse dimensions of the body and the forces to which it is subjected during the assembly operation, by means of which it is possible to obtain, in an extremely easy manner and at the same time accurate, a complete picture of the phenomenon of deformations to which the body is subjected during the assembly operation. With the aid of the device according to the invention it is therefore possible to improve the adjustment of the welding system, by studying the deformations induced in the structure, determining the sources of the generated forces and carrying out the corrections accordingly, until a dimensional quality of the welded structure is obtained which is exceptional with respect to the standards currently possible. Figures 4, 5 are two diagrams which show respectively the relation between the linear displacement detected by the detector S, designated by d., And in the case of figure 4, the resultant force £ exchanged between the respective frame 6 or 7 and the motor vehicle body and, in the case of Figure 5, the displacement D calculated in the manner described above, at the upper end of the gate. As shown, both diagrams show linear correlations with a variation slope at the point P for the deformation value in which an undesired opening of the positioning gates 6, 7 takes place in the supports 13, 14, due to a dimension excessive cross-section of the body. In these diagrams four areas I, II, III, IV are identified that correspond to the four different ranges of variation of the parameters mentioned above. Area I is an ideal operating range, while area IV is an abnormal operating range, which requires immediate intervention by operators. As will become readily apparent in the following, a different color is assigned for all practical purposes, to each of these areas, so as to provide immediately a persible display of the operating conditions of the system. For example, with the areas I, II, III and IV the colors green, yellow, orange and red are associated, respectively. Figure 6 shows an example of how this result can be obtained. In this figure, the image displayed on a video terminal associated with the control device of a system according to the invention is shown diagrammatically. The image shown diagrammatically represents the welding station in plan view. In the preferred embodiment of the invention, two detector devices are provided for each gate 6, 7 which are located at the front end and at the rear end of the gate, respectively. Therefore, the detection is carried out in the four corners left front, right front, left rear and right rear, of the station. The image shown in figure 6 includes a central area and left and right areas. The left area shows the values of a predetermined parameter (in the illustrated example, this parameter is the calculated value of the displacement in the upper part of the gate, but what is shown is also applicable in the display of displacement values detected by the detectors or the values of the resultant force exchanged between the body and each gate), respectively at the front end and at the rear end of the left gate. Similarly, in the right area, the displacement values calculated at the top of the right gate, at the front end and at the rear end of the gate are displayed. In the central area, the average values indicate respectively between two values left front and right front, between the two values left rear and right rear, between the two values left front and left rear and between the two values right front and right rear. In addition, the cell in which each value is displayed has the color corresponding to the pre-established code, which associates a given color with each given interval of variation of the parameter of the four intervals I-IV which have been mentioned above. In this way, the user obtains immediate, perceivable information about the operating conditions of the system. In the case of the color code example which has been mentioned above, ideal operating conditions take place when all areas of the displayed image are green or are green or yellow. The system is in the limit of acceptable operating conditions when some of the areas are orange, whereas immediate intervention is required if one or more of the image areas displayed are red. Due to the measures mentioned above, therefore the system is able to be controlled and if necessary adjusted in an immediate and easy way. Naturally, although the principle of the invention remains the same, the details of construction and embodiments may vary widely with respect to what has been described and illustrated by way of example only, without departing from the scope of the present invention. It is noted that in relation to this date, the best method known by the applicant to carry out the aforementioned invention, is the conventional one for the manufacture of the objects to which it relates. Having described the invention as above, property is claimed as contained in the following:

Claims (8)

1. A device for assembling a motor vehicle body or sub-assemblies thereof, comprising:. an assembly station, provided with a programmable assembly means; a conveyor means, for transporting at least a portion of the structure to be assembled to an assembly station as well as for taking the structure after it has been assembled outside the assembly station, at least a pair of gates locating and immobilizing placed on two opposite sides of the assembly station, provided with locating and locking devices to retain the component elements of the structure to be assembled in the proper assembly position, while carrying out the operation of Assembled, the gate can be moved towards or away from each other, between a separate non-operative condition, in which the locating and immobilizing devices are decoupled from the structure to be assembled, and a closed operating condition, in the which locating and immobilizing devices are capable of coupling the structure that goes Assembled, the device is characterized by the combination of the following characteristics: a) each positioning and locking gate is an independent unit, having a base portion that can be moved in the assembly station and is adapted to be rigidly immobilized to a fixed structure of the station in the operating condition mentioned before the gate, and an active portion, which carries the positioning and immobilization devices, which extends from the base portion and does not connect directly to the fixed structure of the station in the operational condition of the gate, so that they remain free to experience micro-deflections along the transverse direction in relation to the longitudinal direction of the station, due to slight variations of the transverse dimension of the structure that is going to assemble with respect to a predefined theoretical dimension, b) between the portion a For each positioning and immobilization gate in the fixed structure there are detector means adapted to detect the micro-displacement of the active portion with respect to the fixed structure due to micro-deflection, at a predetermined distance from the immobilization region of the base portion to the fixed structure, c) the device further comprises means for processing the signals coming from the detector means and for calculating the displacement of the various regions of the active portion of each positioning and immobilization gate, as a function of the distance from the immobilization region, as well as to calculate in consequence the forces exchanged between each gate and the structure to be assembled.

2. The assembly device according to claim 1, characterized in that the active portion of each positioning gate extends upwardly from the base portion.

3. The assembly device according to claim 2, characterized in that the assembling means is constituted by programmable robots provided with electric spot welding heads.

4. The assembly device according to claim 1, characterized in that the detector means comprises a needle detector placed immediately above the base portion of the respective positioning gate.

5. The assembly device according to claim 1, characterized in that the detector means comprises a first detector means and a second • detector means placed respectively on the front end and the rear end of each positioning and locking gate, so that the detection is carried out in the four corners left front, right front, left rear and right rear of the assembly station . 6. The assembly device according to claim 5, characterized in that display means are associated with the processing means for displaying a diagrammatic image in plan view of the 15 assembly device, wherein the values of a parameter to be displayed are chosen from among the displacement detected by the detector means, the

F displacement calculated at the top of each gate, or even the resulting force exchanged between 20 each gate and the structure to be assembled, respectively in the corners left front, right front, left rear and right rear of the device, the image also shows the average values calculated respectively between the values in the corners 25 front left and right front, between the values between the left rear and right rear corners, 'between the values between the right front and right rear corners and between the values in the left front and left rear corners.

7. The assembly device according to claim 6, characterized in that the image displayed, with each of the values is associated with a color, according to a color code that assigns colors given at given intervals of variation of the parameter.

8. A method for assembling motor vehicle bodies or sub-assemblies thereof, characterized in that it comprises the steps of: - providing an assembly station provided with a programmable assembly means, providing a conveyor means for transporting at least a portion of the structure that is to be assembled to the assembly station and to take the structure after it is assembled out of the assembly station, to provide at least a pair of positioning and locking gates arranged on two opposite sides of the assembly station provided with the positioning and immobilization devices for retaining the component elements of the structure to be assembled in the proper assembly position, while the assembly operation is carried out, the gates can be moved towards and away from each other, between a separate non-operational condition, in which the devices The placement and immobilization are uncoupled from the structure to be assembled, and a closed operative condition in which the positioning and immobilization devices are capable of coupling the structure to be assembled, the method is characterized in that it also comprises the following stages: providing each positioning and locking gate as an independent unit with a base portion that can be moved in the assembly station and adapted to be rigidly immobilized to the fixed structure of the station in the operational condition of the gate, and a active portion that transports the positioning and immobilization devices, which extends from the base portion and does not connect directly to the fixed structure of the station in the operative condition of the gate, so that it remains free to undergo microdeflections in a transverse direction in relation to the longitudinal direction of the station, due to sliding variations of the transverse dimension of the structure to be assembled with respect to a predetermined theoretical dimension, after the gates have been removed and immobilized in closed operative position, detect the micro-displacement of the portion active of each gate with respect to the fixed structure, due to micro-deflection, at a predetermined distance from the immobilization area of the base portion to the fixed structure, - on the basis of the micro-displacement detected in this way, calculate the micro-displacement of the various regions of the active portion of each positioning and locking gate as a function of the distance from the immobilization region, and calculate accordingly the force exchanged between each gate in the structure to be assembled.