Patent Publication Number: US-2007113391-A1

Title: Modular assembly line with a pallet with a frame adjustable for a specific workpiece and methods of adjusting and position finding

Description:
BACKGROUND OF THE INVENTION  
      The invention relates to a method of adjusting a modular assembly line, the assembly line comprising at least one positioning unit, the positioning unit comprising a fixing member of a lower support plate and a fixing member of an upper support plate, which are arranged movably relative to each other in such a way that the fixing member of the lower support plate is transferable to a first and a second distance relative to the fixing member of the upper support plate; an actuator arranged in connection with the positioning unit, operating a tool used in the working step taking place in the positioning unit; and conveyor means, which comprise pallets conveying workpieces, and a conveyor, the conveyor means conveying workpieces to the positioning unit and away from it.  
      Further, the invention relates to a positioning unit of a modular assembly line.  
      Still further, the invention relates to a computer program, which is loadable into the internal memory of a digital computer, and to a computer program product.  
      Electronic devices, such as mobile phones, are most often assembled on automatic or semiautomatic assembly lines. It is typical that the batch sizes ordered by clients vary a lot. Therefore, the intention is to design an assembly line to be modular in such a way that several different products can be assembled on it by changing, as a rule, only product-specific parts and settings. Thus, the product change time, i.e. the time it takes to change one product assembled on an assembly line to another product, constitutes a significant cost factor. The product change time significantly affects the utilization degree of the assembly line, and in this respect, the competitiveness of the assembling company.  
      An assembly line most often comprises several assembly devices, and conveyor means that convey workpieces from one assembly device to another. An assembly device comprises a positioning unit in which the workpieces conveyed by the conveyor means are lifted out of the conveyor means for the time of the working step performed by the assembly device. Briefly, the positioning unit functions in such a way that the workpieces usually enter the positioning unit as pallets, brought by a conveyor. A pallet is stopped at the point of the positioning unit, and the workpieces on the pallet are lifted out of the pallet and against an upper support plate shaped for the specific workpiece. The lifting is carried out with a lower support plate made according to the workpiece. The workpieces are positioned between the lower and upper support plates. Subsequently, an actuator belonging to the assembly device performs the working step. After the working step, the workpieces are lowered back to the pallet, supported by the lower support plates. The pallet moves to the following working step on the conveyor.  
      The product change of an assembly line is presently carried out as follows:  
      1. The lower support plate is adjusted in such a way relative to the stopped pallet that the pieces can be lifted up from the pallet.  
      2. The upper support plate is adjusted in such a way relative to the lower support plate that the workpiece is correctly positioned between the support plates.  
      3. The lifting height of the lower support plate is adjusted in such a way that the workpiece is appropriately pressed between the support plates.  
      4. The perpendicularity of the upper support plate and the lower support plate is adjusted by machining the fixing plate of the lower support plate.  
      5. The coordinate systems of the actuator and the workpiece are fitted together by teaching the workpiece points to the actuator.  
      There are some problems in the above-described product change. Firstly, the change of the workpiece-specific support plates and tools as well as the mechanical and software-based fine adjustment of the devices of the assembly line consume a great deal of resources and time: depending on the size of the line, they take from one person-workweek to several person-workweeks. The product change time is long, because both the upper and lower support plates are adjusted. Further, the adjustment work and the resulting location of the workpiece in the coordinate system of the actuator vary according to the skill and aptitude of the person carrying out the adjustment. Adjusting is based on experimentation and the experience of the person carrying out the adjustment. Since the location of the workpiece most probably changes in the product change in an unexpected manner, the actuator must be taught in connection with each product change separately. Furthermore, if the lower support plate has to be machined to correct the perpendicularity, the whole adjustment must be started from the beginning after the machining.  
     BRIEF DESCRIPTION OF THE INVENTION  
      An object of the present invention is to provide a novel and an improved method of adjusting a modular assembly line, and a positioning unit of a modular assembly device, in which above-described problems are avoided.  
      The method according to the invention is characterized by manufacturing the fixing member of the lower support plate in such a way that the location of a workpiece-specific lower support plate is unambiguously determined in the fixing member in question; manufacturing the fixing member of the upper support plate in such a way that the location of a workpiece-specific upper support plate is unambiguously determined in the fixing member in question; aligning the fixing member of the lower support plate with the fixing member of the upper support plate and fixing said fixing members permanently to their places aligned in this manner; aligning the conveyor means with said fixing members of the upper and lower support plates; and fixing the coordinate system of the actuator and the coordinate system of the positioning unit to each other.  
      Further, the positioning unit according to the invention is characterized in that it comprises: 
          a frame, a fixing member of a lower support plate arranged in the frame, a fixing member of an upper support plate arranged in the frame and movably in direction X relative to the fixing member of the lower support plate in such a way that the fixing member of the lower support plate is transferable to a first and a second distance relative to the fixing member of the upper support plate;     fixing parts arranged in the fixing member of the lower support plate for fixing a product-specific lower support plate unambiguously to the fixing member;     fixing parts arranged in the fixing member of the upper support plate for fixing a product-specific upper support plate unambiguously in the fixing member, which fixing members of the lower support plate and upper support plate are arranged in the frame immovably in the directions perpendicular to direction X.        

      Further, the computer program according to the invention is characterized in that said computer program comprises a software code for teaching product-specific parameters to an intelligent actuator of a positioning unit in the coordinate system of the positioning unit provided by the method according to claim  1 .  
      Still further, the computer program product according to the invention is characterized in that it comprises a computer program according to claim  12 .  
      An essential idea of the invention is that the location of the workpiece, i.e. the location of the lower and upper support plates, is always the same and known in the coordinate system of the actuator.  
      An advantage of the invention is that the task of the assembly device, i.e. the workpiece handled or the measure taken therein, can be changed without adjusting the positioning unit, whereby the product change time can be made significantly shorter.  
      The essential idea of a preferred embodiment of the invention is that each workpiece located on the pallet and thus to be handled at a time has in the positioning unit an upper support plate of its own, which can change its position in the direction of movement X of the lower support plate irrespective of the other upper support plates of the same positioning unit. An advantage is that lower and upper support plates of each workpiece are pressed against each other throughout, even if all lower and upper support pairs were not completely perpendicular to each other. 
    
    
     BRIEF DESCRIPTION OF THE FIGS  
      The invention will be explained in greater detail in the attached drawings, of which  
       FIG. 1  shows schematically a modular assembly line comprising positioning units according to the invention;  
       FIG. 2  shows schematically a positioning unit according to the invention;  
       FIG. 3  shows schematically a positioning unit according to  FIG. 2 , to which product-specific upper and lower support plates are fixed;  
       FIG. 4  shows schematically a method according to the invention in the alignment step of the lower and upper support plates and the conveyor means; and  
       FIG. 5  shows schematically a side view of an upper support plate used in the positioning unit according to the invention. 
    
    
      For the sake of clarity, the invention is shown simplified in the figures. Similar parts are denoted with the same reference numerals.  
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  shows schematically a modular assembly line comprising positioning units according to the invention. The assembly line  1  comprises three assembly devices  2   a ,  2   b ,  2   c  altogether. Each assembly device has an intelligent actuator  3   a ,  3   b ,  3   c  for carrying out a given task. An intelligent actuator—hereinafter ‘actuator’—refers in this description to an actuator for which a coordinate system can be determined and to which parameters and points bound to this coordinate system can be taught, by means of which parameters and points a tool, e.g. gripper, operated by the actuator  3   a ,  3   b ,  3   c , is aligned with the workpiece during the task. The actuator  3   a ,  3   b ,  3   c  is, for example, a robot, a manipulator or the like.  
      The assembly device  2   a ,  2   b ,  2   c  further comprises a positioning unit  4  that is preferably similar in each assembly device  2   a ,  2   b ,  2   c . The positioning unit  4  comprises a lower support plate and an upper support plate, which are described in more detail later.  
      Further, the assembly line  1  comprises workpiece conveyor means, which convey workpieces  6  from one positioning unit  4  to another. The main parts of the conveyor means are a pallet  5  and a conveyor  7 . The workpieces  6 , which are cover parts of a mobile phone, for example, are arranged on pallets  5 . The pallet  5  has one or more workpieces  6 —the pallet of  FIG. 1  has two workpieces—arranged in such a way that each workpiece  6  is in a determined position and at a determined point on the pallet  5 . The conveyor  7  is arranged to move pallets  5  to the positioning units  4  and out of them. The direction of movement of the pallets  5  is shown by arrow F in the figure. The type of the conveyor  7  is, for example, a narrow-belt conveyor, a two-belt conveyor or the like. It is essential in the conveyor  7  that an unoccupied space remains under the pallet  5 , from which the workpieces can be lifted up by means of a lower support plate.  
      The assembly process of the assembly line  1  is controlled with a control system  8  comprising a digital computer. In the internal memory of the computer, there are loaded a computer program or programs including for example product-specific parameters of a software code for controlling the intelligent actuators  3   a ,  3   b ,  3   c , and a software code needed in controlling the conveyor  7 .  
       FIG. 2  shows schematically a positioning unit according to the invention, and  FIG. 3  shows schematically the positioning unit according to  FIG. 2 , having product-specific upper and lower support plates fixed to it. The positioning unit  4  is universal, in other words it has changeable product-specific parts. By changing the product-specific parts, the positioning unit  4  can be used in handling different workpieces.  
      The positioning unit comprises a frame  10 , which, in turn, comprises fixing members  11  of the lower support plate and fixing members  12  of the upper support plate  12 . The fixing members  11  of the lower support plate are arranged movably in direction X relative to the frame  10  but substantially immovably in the directions perpendicular relative to this direction. The fixing members  11  of the lower support plate move guided by guide tracks, for example, and the force providing the movement is generated for instance electrically, hydraulically or pneumatically.  
      The fixing members  11 ,  12  of the lower support plate and the upper support plate are aligned in directions perpendicular to direction X, and fixed to their places aligned in this way. The fixing is implemented unambiguously in such a way that after the fixing the location of the fixing members  11  of the lower support plate is not adjustable or changeable relative to the fixing member  12  of the upper support plate. The fixing is implemented for instance by welding, riveting, a screw joint or the like reliable fixing method. The length of the movement in direction X is also arranged unambiguously, in other words the length of the movement is not adjustable after the arrangement.  
      The fixing member  11  of the lower support plate, as also the fixing member  12  of the upper support plate, comprises fixing parts  33  with which product-specific support plates  13   a ,  13   b  and  14   a ,  14   b  are fixed.  
      In  FIG. 3 , two lower support plates  13   a ,  13   b  and corresponding two upper support plates  14   a ,  14   b  are detachably fixed to the fixing members  11 ,  12  of the lower and upper support plates. There are two pairs of lower and upper support plates, because each pallet  5  to be handled in the positioning unit  4  comprises two workpieces. Thus, all workpieces of the pallet  5  can be handled with one single movement of the fixing members  11  of the lower support plate.  
      The lower and upper support plates  13   a ,  13   b  and  14   a ,  14   b  are designed in such a way that they keep the workpiece  6  pressed between them in place when the actuator  3   a ,  3   b ,  3   c  performs the task directed at the workpiece in question. The piece-specific lower and upper support plates  13   a ,  13   b  and  14   a ,  14   b  represent the prior art known per se, and therefore they are not described in greater detail here.  
      The fixing parts  33  of the fixing members  11 , 12  of the lower and upper support plates and the lower and upper support plates  13   a ,  13   b  and  14   a ,  14   b  themselves are manufactured in such a way that each support plate  13   a ,  13   b  and  14   a ,  14   b  is unambiguously fixed to the corresponding fixing member  11 ,  12 . In other words, the location of the support plate  13   a ,  13   b  and  14   a ,  14   b  in the fixing member  11 ,  12  is not adjustable. The fixing can be implemented in such a way, for example, that the location of the support plates  13   a ,  13   b  and  14   a ,  14   b  is aligned with cotter bolts, the actual fixing being carried out with screws, wedges, magnets or the like openable joints.  
      The lower and upper support plates  13   a ,  13   b  and  14   a ,  14   b  are workpiece-specific components. When the product to be assembled on the assembly line  1  changes or when the task of the assembly device  2   a ,  2   b ,  2   c  changes, the lower and upper support plates  13   a ,  13   b  and  14   a ,  14   b  that have been used are replaced with the lower and upper support plates  13   a ,  13   b  and  14   a ,  14   b  needed in the handling of the new workpiece or task.  
       FIG. 4  shows schematically a method according to the invention in the alignment step of the lower and upper support plates and the conveyor means. The alignment is carried out as follows.  
      First, the fixing member  11  of the lower support plate of the positioning unit  4  is aligned with the fixing member  12  of the upper support plate by means of auxiliary tools  15   a ,  15   b . The auxiliary tool  15   a ,  15   b  is in the embodiment of  FIG. 4  an elongated, rigid bar having a round cross-section. The embodiment of the method shown in  FIG. 4  utilizes two auxiliary tools  15   a ,  15   b , but the number of auxiliary tools may naturally be something else. In addition, it is to be noted that the cross-section of the auxiliary tool does not necessarily have to be round. Two first alignment surfaces  16   a ,  16   b  are arranged in the fixing member  11  of the lower support plate, and correspondingly, two second alignment surfaces  17   a ,  17   b  are arranged in the fixing member  12  of the upper support plate. The first and the second alignment surfaces  16   a ,  16   b  and  17   a ,  17   b  are in this embodiment round holes piercing the fixing member. The alignment surfaces  16   a ,  16   b  and  17   a ,  17   b  are arranged in such a way that when the auxiliary tools  15   a ,  15   b  can be arranged simultaneously on the alignment surfaces  16   a ,  17   a  of both fixing members  11 ,  12 , the fixing members  11 ,  12  are aligned with each other. In addition, the fixing members  11 ,  12  are then parallel with each other. After this, the fixing members  11 ,  12  are fixed permanently to these places, after which their locations relative to each other cannot be adjusted any longer. In this connection, also the lifting height of the fixing member  11  of the lower support plate is adjusted, and it is fixed in such a way that it cannot be adjusted after this.  
      Next, the conveyor means are aligned with the positioning unit  4 . This is carried out in such a way that the auxiliary pallet  19  used in the alignment is stopped in the positioning unit  4 . The auxiliary pallet  19  comprises two third alignment surfaces  18   a ,  18   b . In the presented embodiment, these surfaces are round holes piercing the auxiliary pallet  19 . The holes of the third alignment surfaces  18   a ,  18   b  are positioned in the auxiliary pallet  19  in such a way that the conveyor means are aligned correctly relative to the positioning unit when the auxiliary tool  15   a ,  15   b  can be arranged simultaneously through these holes and the holes formed by the first and second auxiliary surfaces  16   a ,  16   b  and  17   a ,  17   b . At the same time, the conveyor means are parallel relative to the fixing members of the lower and upper support plates. The positioning unit  4  is fixed to the location thus aligned in such a way relative to the conveyor means that its location cannot be changed. The positioning unit  4  can be fixed for instance with screws to a table board known per se, not shown in the figures.  
      The auxiliary pallet  19  may be an ordinary pallet  5  used in conveying workpieces and comprising the required third alignment surfaces  18   a ,  18   b , or it may be a special tool used merely for adjustment.  
      In aligning conveyor means, it may not be necessary to use the same auxiliary tools as when aligning the fixing members  11 ,  12 . Instead, auxiliary tools for conveyor means can be used that are arranged on the third auxiliary surfaces  18   a ,  18   b  and that are intended to be used only for aligning conveyor means with the fixing members  11 ,  12  of either the lower or the upper support plate or both.  
      As the third step, the coordinate system of the actuator  3  is fixed to the coordinate system of the positioning unit  4 . This is carried out in such a way, for example, that the actuator  3  is provided with a teaching tool  21 , which is moved from one point of the workpiece pressed between the support plates to another. The teaching tool  21  preferably comprises two pins  35   a ,  35   b  arranged at a distance from each other, whereby not only the location of the point but also the angle of the teaching tool  21  is found out. The points to be taught may also be positioned in the fixing plate  12  of the upper support plate, for instance. The movements of the teaching tool  21  are stored in the memory of the control system  8  controlling the actuator.  
      The above steps are carried out in the first assembly of the positioning unit  4 , i.e. when the universal positioning unit  4  is manufactured. The steps need not be performed in a case of product or task change, in other words when product-specific lower and upper support plates  13   a ,  13   b  and  14   a ,  14   b  are fixed to the positioning unit. In a case of product or task change, the only thing to do is to fix the required lower and upper support plates to the fixing members. Separate adjustment is not performed, because the location of the support plates is unambiguous and the location of the workpiece in the coordinate system of the actuator is always the same.  
      The advantage that can be achieved with the invention is represented by the fact that in a prior art product change the product change time is about 16 hours. Utilizing the invention results in a product change time of about 15 minutes. The invention reduces the product change to an extremely simple event: detaching the old product-specific upper and lower support plates from the positioning unit and replacing them with product-specific upper and lower support plates required by the new workpiece. No adjustment is required.  
      Preferably, the tool designer determines as early as at the designing stage of the upper and lower support plates the parameters required by the actuator  3 , in other words where the workpiece  6  is positioned relative to the positioning unit  4  and the tool used by the actuator  3 . The support plates  13   a ,  13   b  and  14   a ,  14   b  can be positioned extremely accurately, whereby the accuracy of the operation of the positioning unit  4  depends only on the accuracy of manufacture of the upper and lower support plates  13   a ,  13   b  and  14   a ,  14   b . The actuator  3  is provided with information on three points with which the coordinate system can be determined. Further, the actuator can be provided with information on those paths of movement that it must follow in the working step, or other information relating to the working step or workpiece.  
      In connection with the product change, the required product-specific upper and lower support plates  13   a ,  13   b  and  14   a ,  14   b  and the computer program code to be executed in a processor of the control system  8  are provided. The computer program code can be transferred to the control system  8  from an external memory means, such as a CD-ROM. It can also be transferred via a telecommunications network, for instance by connecting the control unit  8  via a wireless access network to the Internet. It is also feasible to use a hardware implementation or a combination of hardware and software solutions.  
       FIG. 5  shows schematically a side view of an upper support plate used in the positioning unit according to the invention. The upper support plate  14  comprises a frame  22 , by which the upper support plate  14  is fixed to the fixing members  12  of the upper support plate. It is to be noted that the fixing members are not shown in  FIG. 5 . Further, the upper support plate  14  comprises a counterpart  23 , which is provided with product-specific surfaces  31  shaped according to the workpiece to be handled and the requirements of the task.  
      The frame  22  and counterpart  23  of the upper support plate are attached to each other with fixing members  24 , which in the embodiment of  FIG. 5  comprise an Allen bolt  25 , a retaining plate  26  and an elastic part  29 . The threaded part  32  of the bolt  25  is arranged through the fixing hole  28  of the counterpart in such a way that the retaining plate  26  is under the bolt head, the elastic part  29  being between the retaining plate  26  and the counterpart  23 . The threaded part  32  is locked in a threaded hole  27  formed in the frame  22 .  
      The diameter of the fixing hole  28  of the counterpart is dimensioned to be somewhat greater than the diameter of the bolt  25 . Further, a protrusion  30  of a shape of a circular cone is formed in the counterpart  23 , around the bolthole  28 , and the frame  22  is provided with a countersink of a substantially similar shape.  
      The bolt  25  is fixed to the threaded hole  27  at such a depth that the elastic part  29  is somewhat pressed, but yet in such a way that the elastic part  29  can be compressed even further, in other words it can be compressed in the longitudinal direction of the bolt  25  so that it even becomes shorter. Owing to the flexibility of the elastic part  29 , the counterpart  23  can be inclined to some extent to either side relative to the frame  22 . Now, when the lower support plate with its workpieces is lifted against the counterpart  23  of the upper support plate, which lower support plate is not, for some reason, parallel with the counterpart  23 , the position of the counterpart  23  adapts to the position of the lower support plate. In other words, the lower and upper support plates  13 ,  14  need not be exactly parallel in order for the whole workpiece to be pressed evenly between them. Owing to the conical protrusion  30  and the countersink, the counterpart  23  always returns to the same position and location relative to the frame  22  after the lowering of the lower support plate  13 . Preferably, each workpiece  6  located on the pallet  5  and thus to be lifted up from the pallet at a time has an upper support plate  14  of its own, which can change its position to conform to the position of its lower support plate pair, irrespective of the upper support plates fixed to the fixing member of the upper support plate.  
      The structure of the upper support plate  14  may also be such that there is no frame  22 , but the counterpart  23  of the upper support plate is fixed directly to the fixing members  12  of the upper support plate. The elastic part  29  may also be a spring or the like reversibly compressible member. In the same way, the movement of the counterpart  23  relative to the frame  22  can be controlled not only with cone structures described here but also with guide tracks or the like.  
      The drawings and the related description are only intended to illustrate the idea of the invention. The details of the invention may vary within the scope of the claims. Thus, on the assembly line  1 , the positioning units  4  of the assembly devices  2   a ,  2   b ,  2   c  may be different from each other.