Patent Publication Number: US-2022227438-A1

Title: Robot for tightening a series of bolt nuts on a vehicle chassis and manufacturing process

Description:
RELATED APPLICATIONS 
     The present application claims priority to European Patent Application No. 21305060.2, filed on Jan. 19, 2021, and entitled “ROBOT FOR TIGHTENING A SERIES OF BOLT NUTS ON A VEHICLE CHASSIS AND MANUFACTURING PROCESS,” which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The invention relates to a robot for tightening a series of bolt nuts on a vehicle chassis and to a manufacturing process of a vehicle using such robot. 
     The invention can be applied in the manufacturing process of heavy-duty vehicles, such as trucks, buses and construction equipment. Although the invention will be described with respect to the manufacturing process of a truck, the invention is not restricted to this particular vehicle, but may also be used in other vehicles such as passenger cars, airplanes, military vehicles and so on. 
     BACKGROUND 
     In known manner, bolts are widely used in the manufacturing processes of vehicles to attach brackets or the like to the vehicle chassis. In the example of a truck, the chassis is constituted of two longitudinal beams, provided with a plurality of holes for receiving the bolts. 
     Today, operators are needed to manually tighten the bolts nuts. In the future, this operation could be done by a robot. However, it is known that a reaction torque can be observed at the end of the tightening process. Indeed, the assembly is mechanically stressed during the tightening operation, and is subject to elastic deformation. At the end of the tightening operation, the components of the assembly tend to return to their initial shape, which creates a reaction torque. Therefore, in order to automatize this operation, the robot must be designed so that its wrist (terminal axis) is capable of withstanding the reaction torque over a long period of time (reliability, durability). Today, M14 Hexagonal Bolts represent 80% of the fasteners that are used on the chassis trucks. For the record, this kind of bolts requires a tightening torque that is about 175 N·m. Accordingly, the reaction torque value must not exceed the maximum value tolerated by the robot terminal axis. 
     Another challenge with regard to the automation of the bolt nut tightening process is to be able to tighten a series of bolts in succession by means of the robot. Indeed, a series of tests showed that the repetitive tightening on a chassis with a succession of bolts is a problem. Today, only three bolts can be tightened successively, not more. 
     The problem is mainly linked by the dispersion between the axis of the holes, which is about +/−0.4 mm, while the trajectory precision of the robot is about +/−0.2 mm. Accordingly, there are some situations in which the nut tightening tool axis is not perfectly aligned with the axis of the bolt. In such situation, the tightening tool cannot achieve the tightening process. 
     An objective would be to be able to tighten 15 bolts in a row without failure and stop. To do that, an obvious solution would be to equip the robot with a vision system that would enable to precisely locate the bolts nuts to be tighten and realign the tightening tool when necessary. However, this solution is very costly and the corresponding software would be hard to implement. This also raises the question of the reliability of the system. 
     In the Patents Literature, US 2018 0 333 811 A1 discloses a screw guide device that guides a screw to a screw hole, the screw being fastened by a screw fastening device to the screw hole formed in a work piece to be fastened. The screw guide device comprises a guide device body provided at a support that supports the work piece to be fastened and a guide member that is provided in the guide device body and that is openable and closable in a radial direction orthogonal to a central axis of a shaft section of the screw. The guide member forms a screw insert through which the shaft section is inserted when the guide member is in a closed state. 
     EP 3 251 798 A1 discloses a powered nut runner comprising a socket which is rotatably arranged in the powered nut runner and which is configured to accommodate a nut, a motor configured to exert a torque on the socket for powered fastening of a nut accommodated in the socket to a bolt, nut reservoir connecting means for connecting the powered nut runner to a nut reservoir, and a feed channel extending from the socket. The powered nut runner is configured to enable a nut stored in the nut reservoir to become automatically transported from the nut reservoir into the socket via the feed channel, when the powered nut runner is connected to the nut reservoir. 
     SUMMARY 
     An object of the invention is to provide a robot capable of tightening a series of at least fifteen bolt nuts on a vehicle chassis, without failure or stop. Another object of the invention is to ensure the durability and reliability of the robot. 
     Today, the nut tightening operations that are made by a robot are not very frequent in the vehicles manufacturing industry. It mainly exists mainly for low tightening torques, e.g. inferior to 45 N·m. Concerning medium tightening torque, e.g. 100 N·m, it exists some solutions that are used to assemble the engine. However, there is today no solution involving robots for bolts that need high tightening torque, such as 175 N·m, to tighten bolt nuts on the chassis. 
     The object is achieved by a robot. 
     By the provision of a sleeve-shaped centering member that is fixed around the tightening tool, it is possible, on contact with a nut, to reposition the nut tightening tool automatically in the axis of the nut. Consequently, if in the approach phase, the nut tightening tool is not perfectly in the axis of the bolt (because of the dispersion), then the centering member abuts against the nut. Thanks to the chamfer present at the end of the centering member, the continuation of the approach movement causes, by “wedge effect”, an offset movement of the tightening tool towards the axis of the bolt. This causes the tightening tool to realign with the central axis of the nut. The tightening process can then be achieved. 
     Advantageous, but not compulsory aspects of the invention are depicted in the dependent claims. 
     The invention also concerns a process which relates to a manufacturing process of a vehicle, using a robot as defined in the claims to tighten a succession of bolt nuts on a vehicle chassis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples. 
       In the drawings: 
         FIG. 1  represents a perspective view of a test stand, comprising a truck chassis beam resting on supports and a robot of the invention equipped with a tool for tightening the nuts of the bolts received in holes of the beam; 
         FIG. 2  is an enlarged view according to the rectangle II drawn on  FIG. 1 ; 
         FIG. 3  is a detailed view of the rectangle III drawn on  FIG. 2 ; 
         FIG. 4  is a detailed view of a truck&#39;s chassis, representing the nuts of the bolts allowing to fasten a plurality of brackets; 
         FIG. 5  is an enlarged view of the circle V drawn on  FIG. 4 ; 
         FIG. 6  is a perspective view of the sleeve-shaped centering member; 
         FIG. 7  is a cut view according to plane VII of  FIG. 6 . 
         FIGS. 8 and 9  represent possible failure situations that could happen with robots of prior art; 
         FIG. 10  represents a first step of the bolt nut tightening process, corresponding to the approach phase between a bolt nut and the nut tightening tool, and in which it can be seen that the nut tightening tool is not coaxially aligned with the central axis of the nut; 
         FIG. 11  is an enlarged view of  FIG. 10 ; 
         FIG. 12  is a view similar to that of  FIG. 10 , representing a second step, in which it can be seen that the nut tightening tool has been automatically realigned coaxially with the nut; 
         FIGS. 13 and 14  are views similar to  FIGS. 10 and 12 , representing the movement of the nut tightening tool until it reaches the end configuration in which the nut can be tightened; 
         FIG. 15  is a section view of a nut; 
         FIG. 16  is a perspective view of an alternative embodiment of the invention in which a force sensor is arranged at the junction between robot arm and tightening tool; and 
         FIG. 17  shows another embodiment for the centering member, having a slightly different geometry. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION 
       FIG. 1  shows a robot  2  for tightening bolt nuts  16  on a vehicle chassis  6 . The robot  2  comprises a poly-articulated arm  8  provided with an End Of Arm Tooling (EOAT)  10  that is a nut tightening tool. 
     In the example, a support  40  is provided to attach the nut tightening tool  10  to the robot arm  8 . Accordingly, the support  40  makes the interface between the robot arm  8  and the nut tightening tool  10 . 
     Basically, the robot  2  itself (without the EOAT  10 ) is known as such (It can be purchased on the market, for example to the company ABB). Similarly, the nut tightening tool  10  itself is also known per se. 
     The vehicle chassis  6  includes in practice two longitudinal steel frames (also known as truck&#39;s side beams), among which one only is represented on the figures. Each beam  6  includes a plurality of holes inside which are received bolts  12 . Each bolt  12  includes a threaded rod  14  and a nut  16 . Rod  14  includes a screw head  140  and a non-threaded portion  142  arranged at one (longitudinal) end opposite to the screw head  140 . The threaded portion of rod  14  extends longitudinally between the head  140  and the end portion  142 . The non-threaded portion  142  has a non-circular section, for example a square section. 
     The nut  16  includes a hexagonal section. More precisely, the section of the nut  16  is a regular hexagon that is both equilateral and equiangular. It is also bicentric, meaning that it is both cyclic (has a circumscribed circle) and tangential (has an inscribed circle). 
     As shown on  FIGS. 10 and 11 , each nut  16  includes a radial outer face  16 A of hexagonal section, an axial end face  16 B and a chamfer  16 C connecting the end face  16 B to the radial hexagonal face  16 A. 
     In the example, bolts  12  are “M14” hexagon head bolts, 14 being the diameter (in millimeters) of the threaded section. Also, the torque to be applied for tightening such bolt nuts is about 175 N·m. Also, bolts  12  are used to fasten a plurality of brackets  20  on the chassis  6 . These brackets  20  can serve as a support for battery packs, in particular for Hybrid Electric Vehicles (HEV&#39;s) or Full Electric Vehicles (EV&#39;s). However, in variant, these brackets  20  can obviously be used for another purpose, such as supporting the fuel tank, the EATS or the like. 
     In the example of the figures, a plurality of bolts, typically eleven bolts are used to fasten one bracket  20 . Obviously, each bracket  20  includes a plurality of holes for the passage of the bolt screws  14 . 
     In the embodiment of the figures, the arm  8  includes 6 axes of rotation J 1  to J 6 , which means that it is a 6-axis robot. Six axes allow the robot to move in the X, Y and Z planes (Cf.  FIG. 1 ), as well as position itself using roll, pitch, and yaw movements. 
     In reference to  FIG. 1 , the beams of the chassis extend along the X direction, while the bolts  12  extend along the Y direction. 
     Also, robot  2  is preferably a “small robot”, meaning that the robot payload is between 10 kg and 20 kg, while the robot reach is between 1.2 m and 1.65 m. As a reminder, the robot payload is the weight the robot can lift. Payload includes the weight of the EOAT and the weight of the product being picked, if appropriate. The robot reach is the distance from the center of the robot to the fullest extension of the robotic arm. This measurement determines the robot&#39;s work envelope. 
     In the example, the robot  2  is capable of following a path with an accuracy of +1-0.2 mm (relative to a control path). In this respect, and as known per se, the robot  2  further includes a robot arm trajectory control unit (Not shown) for repeating a nut tightening operation on a series of bolts. 
     Basically, there is an integrated software that enables to program the trajectory of the robot, depending on the location of the bolt nuts to be tighten. This integrated software uses the coordinates X, Y, Z of the nuts to be tighten as input data. The software may include different computer programs for different pre-established bolt configurations. 
     In the example of the manufacturing process of a truck, the longitudinal beams that constitute the chassis of the truck (or frame) delimit a succession of holes at standard locations. Also, on the same model of truck, bolts are at the same locations, which means that the coordinates of the bolt nuts to be tighten can be recorded in advance and that there is no need to reprogram the robot at each new truck. 
     Robots and associated software have largely been used in the industry during the last decades, which means that writing a computer program for automatically moving the EOAT at different locations to achieve a specific task, e.g. nut tightening, is now part of general knowledge, that is why the software integrated into the robot  2  of the invention is not further described. 
     Advantageously, the nut tightening tool  10  is of the electric type, which means that it is electrically driven. However, it could obviously be pneumatically driven in alternative. 
     Preferably, the nut tightening tool is of the low reaction torque type, which means that the reaction force that is transmitted to the robot at the end of the tightening process is null or almost null (while the tightening torque is about 175 N·m). In practice, such “low reaction tool” enables to have not more than 0.5 N·m that is transmitted to the robot arm. For the record, the maximum torque that is supported by the robot terminal axis (J 6 ) is between 6.4 N·m and 16.7 N·m, depending on the size of the robot (1.2 or 1.65 m). 
     In the example, the torque range of the nut tightening tool  10  is comprised between 140 N·m and 190 N·m. 
     The tightening tool  10  consists of an inside socket  17  comprising a gripping portion for driving the end portion  142  of the bolt screw  14  in a first direction (e.g. counterclockwise), until the screw head  140  abuts against the frame  6  and an outside socket  18  for driving the nut  16  in a second direction (e.g. clockwise) opposite to the first direction when the screw head  140  is in contact with the frame  6 . Advantageously, the gripping portion of the inside socket  17  has a shape complementary to that of the end portion  142  of the screw  14  (e.g. square-shaped), so as to allow a male/female engagement (the male part being the screw end portion  142 , the female part being the inside socket  17 ) and as to transmit the rotating movement of the inside socket  17  to the screw  14 . 
     The nut tightening tool  10  is known as such, that is why it is not further described herein (See JP2000110816A for further details). 
     The “low reaction torque” effect is achieved thanks to a gearbox system (not shown) arranged between the E-motor and the outside socket  18  (See WO 2016/169844A for more precisions). 
     The outside socket  18  includes one end orifice  19  whose shape is complementary to that of the nuts  16 , which means that it is hexagonal as well, in order to drive the nut  16  in rotation by male/female complementarity (the male part being the nut, while the outside socket is the female part). 
     Advantageously, and as shown on  FIG. 2 , the inventors are of the opinion that the distance d between the axis Y 10  of the outside socket  18  and the axis J 4  should be as small as possible in order to avoid the deformation of the support  40  during the engagement phase. 
     A sleeve-shaped centering member  22  (also known as the “cone”), typically in steel CK45, is arranged around the nut tightening tool  10 . The centering member  22  defines a central axis Y 22 . Possibly, any other material could be used for manufacturing the centering member  22 , such as Copper or an alloy. This can further help in reducing the number of nuts that are damaged/marked during the tightening process. 
     This centering member  22  comprises a frustoconical surface  24 , which makes it possible, on contact with a nut  16 , to reposition the nut tightening tool  10  automatically in the axis Y 16  of the nut  16 . 
     Advantageously, the angle A 24  of the surface  24  relative to the axis of the centering member  22  is comprised between 30° and 60°, preferably equals to 30° or 45°. Inventors think that an angle of 30° could be of particular relevance. 
     Basically, and as shown on  FIG. 7 , the frustoconical surface  24  is a chamfer extending between one end surface  26  of the centering member perpendicular to a central axis Y 22  of the centering member  22  and an inner radial surface  28  parallel to the central axis Y 22  of the centering member  22 . 
     In one embodiment, the inner radial surface  28  is smooth (i.e. of circular section). In one alternative embodiment represented on  FIG. 17 , the section of the inner surface  28  could be different, e.g. to match with the section of the nut  16 . Accordingly, in this alternative embodiment, the inner surface  28  includes some recesses, typically, six recesses, to fit the nut edges. Inventors are of the opinion that this may help to improve the engagement of the centering member  22  around the nut  16 . 
     Preferably, and as shown on  FIG. 3 , a clamping bolt  30  is provided for clamping the centering member around the nut tightening tool, said clamping screw  30  passing through two tabs  32  (Cf.  FIGS. 6 and 7 ) facing each other and separated one from the other by an axial slot  34 . In this respect, tabs  32  each include a through hole for receiving the threaded section of bolt  30 . Obviously, in a variant not shown, any other means could be used to fasten the centering member  22  around the tightening tool  10 . 
     In the example, the centering member  22  further includes an internal radial shoulder  36 , which connects the inner radial surface  28  with another inner radial surface  2   a  This means that there are two different diameters D 1  and D 2  on the inside of the centering member  22 . D 1  is the diameter of the radial surface  28  and D 2  is the diameter of the radial surface  29 . The diameter D 2  is strictly greater than the diameter D 1 . 
     Advantageously, the diameter D 2  is roughly equal to the outer diameter of the outside socket  18  and the diameter D 1  is roughly equal to that of the circumscribed circle of the hexagonal section of the nut  16 . Obviously, a clearance exists between centering member  22  and socket  18  to allow mounting the centering member  22  around the outside socket  18 . Also, there is also a clearance between the nut and the outside socket  18  to allow positioning the socket  18  around the nut  16 . 
     Advantageously, the centering member  22  projects axially beyond the nut tightening tool  10 . Basically, in the example of the figures, the axial end of the outside socket  18  abuts against the radial shoulder  36  of the centering member  22 . 
     It is now referred to  FIG. 8  that shows a problem that may be encountered with prior art robots. More specifically,  FIG. 8  shows that two bolts that are spaced one from the other by a distance d. Manufacturing tolerances for such distance are +/−0.4 mm, while the precision of the robot are not less than +/−0.2 mm. Therefore, there are some situations, such as the one of  FIG. 8 , in which the nut tightening tool axis Y 10  is not perfectly aligned with the axis Y 16  of the bolt  16 . In such situation, the outside socket  18  of tool  10  cannot be engaged around the nut  16  and the nut tightening tool  10  cannot achieve the tightening process.  FIG. 9  shows that the same problem is encountered when the bolt  12  is engaged crosswise inside the hole delimited in the beam  6 . 
     Referring now to  FIGS. 10 and 11 , it can be seen that such problems can be overcome using the centering member  22  of the invention. Indeed, and as shown on  FIG. 10 , when the central axis Y 18  of the outside socket  18  does not match with the central axis Y 16  of the nut  16 , the frustoconical surface  24  of the centering member  22  axially abuts against the nut  16 . Thanks to the inclined nature of the contact surface  24  present at the end of the centering member  22 , the continuation of the approach movement along Y causes, by “wedge effect”, an offset movement xl of the tightening tool  10  towards the axis of the nut  16 . 
     As a matter of facts, and as shown on  FIG. 11 , when the centering member  22  abuts against the nut  16 , the nut  16  exerts on the frustoconical surface  24  a reaction effort RN which is substantially perpendicular to the surface  24 . This reaction effort RN, combined with the axial effort RY exerted by the EOAT  10 , generates a resulting effort RF that is oriented so as to push the centering member  22 , and therefore the EOAT  10 , radially away from the central axis Y 16  of the nut  16 . This causes the outside socket  18  to reach the configuration of  FIG. 12  in which it is coaxially aligned with the nut  16 . 
     The inventors are of the opinion that the more the angle A 24  of the chamfer  24  is wide, the more is the reaction force RF. In order to avoid the damage of the nut during this phase, a smaller angle (30°) would be probably better. 
     Also, and as represented on  FIG. 15 , the inventors have thought of increasing the angle A 16  of the nut chamfer  16 C from 15-30° to 60° in order to further decrease the damage observed of the nut  16  during the engagement phase (Cf.  FIG. 15 ). 
     In parallel, a software is used by the robot to follow the centering member  22  in the displacement during this phase without forcing. This software, which is known as “SoftMove” (Property of company ABB), allows the robot to be compliant or floating in order to adjust to external forces or variations in work objects. With “SoftMove”, the robot is compliant, which facilitates accuracy and reliability. The “SoftMove” enables to lower the stiffness of the robot in a specified Cartesian direction while mainly maintaining the original behavior in other directions: Robot can be “free floating” in a specified direction and can have a spring function in a specified direction. Stiffness and damping parameters can be used to control the compliance. This software also enables gravity compensation: The stiffness can be lowered in a vertical direction. 
     The basic SoftMove program, which does not consider the addition of the centering member  22 , includes the following steps:
         1. Robot: Approach movement-&gt;the socket is at the edge of the screw dog point expected position   2. Robot: Activation of the tightening tool continuous rotation (speed 20 rpm, no torque control)   3. Robot: Activation of the “SoftMove” computer program (60% Stiffness Z axis, 15% stiffness XY axis)   4. Robot: Linear movement to the final engage position (linear speed 10 mm/s)   5. Robot: Wait robot reach 0 speed and check the reached position. Nut is now in contact of the bracket.   6. Robot: IF reached position is ok (socket totally engaged in the nut)
           -&gt;Activation of the tightening program (300 rpm rundown and 175 Nm tightening torque)   
            ELSE IF reached position is not ok (socket not engaged in the nut)
           -&gt;Move back linear movement of 15 mm-&gt;restart at Step  4 .   
           7. Robot: Wait tightening done   8. Robot: Move back linear movement (50 mm)   9. Robot: Move to next bolt.       

     In order to avoid as much as possible damaging the nuts  16 , and to have a better precision of the engagement force between the centering member  22  and the nut  16  than that of the SoftMove system, a force control sensor  100  could be added between the robot and the tool support  10  (Cf.  FIG. 16 ). 
     For the phase of the tool engagement, in which the centering member  22  contacts with the nut  16 , some parameters have been set on the electric tightening tool power focus. Two phases are needed for the tightening strategy program. One phase for the tool engagement on the nut and a second phase for the tightening operations. The parameters selected are set for the speed, torque, time and angle. 
     Advantageously, no camera is needed. In other words, the robot  2  is devoid of camera. 
     The outside socket  18  can then be moved in direction of nut  16  to reach successively the configurations of  FIGS. 13 and 14 . Arrows A 1  on  FIGS. 13 and 14  represent the direction of movement of the sleeve  18 . 
     Once configuration of  FIG. 14  has been reached, the tightening process per se can be achieved. In details, the gripping portion of inside socket  17 , i.e. the extremity of inside socket  17 , engages around the end portion  142  of screw  14 . Then, the inside socket is driven in rotation (See arrow R 1 ), leading the screw  14  to rotate and move longitudinally to the left in the configuration of  FIG. 14  (See arrow A 2 ) because of the threaded engagement with the nut  16  which is, itself, kept immobile in translation as abutting against the bracket  20 . The movement of screw  14  in its longitudinal direction continues until the screw head  140  abuts against the frame  6 . Afterwards, inside socket  17  stops rotating and outside socket  18  starts rotating in the opposite direction R 2 , leading the nut  16  to rotate in the same direction. In other words, the outside sleeve  18  transmits a torque to the nut  16 , which forces it to rotate, as depicted by arrow R 2 . This enables to clamp the brackets  20  between the nut  16  and the beam  6 . In the order, the bracket  20  (more generally the component to be fastened) is clamped between the beam  6  and the nut  16 . 
     Obviously, even if the robot  2  is represented on  FIG. 1  in a test environment, in which is the beam  6  rests on fixed supports, the robot of the invention is designed for being integrated on the line of production of vehicles, e.g. trucks. In other words, this robot  2  is dedicated to be used in the manufacturing process of a vehicle. 
     It is to be understood that the present invention is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims.