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Flexible Automation for the Assembly in Motion
Flexible Automation for the Assembly in Motion 1
G. Reinhart1 (1), J. Werner1 Institute for Machine Tools and Industrial Management, Faculty of Mechanical Engineering, Technische Universität München, Munich, Germany
Abstract Automation in flow assembly lines is hindered by the need for clocked lines when using automated systems. Geometrical inaccuracies and vibrations of the conveyor systems complicate the use of automation as well. The assembly in motion, meaning a robot system which is synchronized in all degrees of freedom to the moving conveyor belt, is a promising attempt to solve these difficulties. Main advantages of synchronized assembly are the avoidance of buffers and the reduction of the throughput time. The automated assembly in motion is realized by using mechatronic components and innovative feedback algorithms which are currently under development. Keywords: Assembly in Motion, Automation, Flexibility
quality of the assembly process is difficult to be assured. However, the manual flow assembly line is characterized by the idea of lean production, e.g continuous flow production and high intrasystem flexibility. Therefore it is highly economic. The automated assembly in flow assembly lines is limited by the need for synchronization of product and assembly machine which is practically only possible at a halt. Therefore, the product has to be locked out of the continuous flow line in order to be slowed down. Then the products are queued in front of the automated assembly station. Inside the station the automated process takes place. Usually the robot program is primarily based on taught points and movements. After completing the task the product is accelerated and locked again into the continuous flow line. In addition to this organizational lack of flexibility the automated assembly is characterized by low adaptability with respect to changes in the quantity of products and the switching of work contents between stations. Nevertheless, while assembling large and heavy parts the automated assembly is very efficient and therefore necessary.
1 INTRODUCTION Within the last years hybrid assembly systems consisting of manual and automated work stations have become well known. These systems are characterized by high flexibility and moderate costs [1]. Most of them are implemented for small or medium-sized products. For assembly lines of larger products there is still the requirement to divide the assembly into automated and manual assembly systems. This is due to the need for clocked lines when using automated robot stations in contrary to flow lines for manual work places. The combination which is an automated assembly in motion as shown in Figure 1, however, still needs further development. The assembly in motion or synchronized assembly has been known for over 30 years. Up to the present many researchers have developed different approaches within industrial and research projects. Despite different technical approaches it is not yet possible to implement an economic robot system which has the described feature and achieves the necessary assembly accuracy. In this paper an innovative concept of synchronization is presented that enables the mounting of wheels on a conveyed car body. The motivation is described in Section 2. Section 3 summarizes the state of the art in order to define the requirements in Section 4. The method of synchronization is explained in Section 5. In Section 6 the implementation of flexibility in automated assembly systems is addressed. The Section 7 describes the realization of the prototype. 2 MOTIVATION The synchronized assembly in motion is characterized by multiple advantages compared to either the clocked automation in flow lines or the manual assembly. In manual flow assembly lines the parts are carried by one or more workers to the production line. Then they are assembled while synchronized to the flow of the conveyor belt by the workers. This operation sequence lacks ergonomic design of the work places while handling large and heavy parts. Furthermore, a stable and high level of
Annals of the CIRP Vol. 56/1/2007
Figure 1: Prototype of the assembly in motion
-25-
doi:10.1016/j.cirp.2007.05.008
The synchronized assembly in motion combines the high and reproducible level of quality of the automated assembly and the flexibility of manual flow lines. Besides the possible reduction of assembly workers the greatest advantage is the avoidance of the queues in front of and after the assembly station. Given that these buffers represent up to 75 % of the needed space a great potential in ratio is opened. The flow lines can be shortened in length and less restriction are imposed for layout planning. In comparison, automatic stations are usually centralized or combined in order to minimize the effort of queuing the products. Whenever this is not possible automation is often rejected even though a ratio could be achieved. While integrating the assembly in motion cell into the flow line an adjustment or switching of work contents between consecutive assembling stations could be realized. Further advantages are the possible reduction of complex and costly feeding machines. Using the synchronized assembly the implemented sensors could be used to reduce the effort in positioning and orienting of the fed items [2].
conveyor. As shown in Figure 2 these deviations to the reference position can be observed in all degrees of freedom (dof), here shown only the three translative dof (x-movement in flow direction, y-movement to the side, zmovement in height). The amount of the deviation depends on the kind of conveyor system used. However, all conveyor systems show similar behavior. The deviations in Figure 2 were measured on a power and free conveyor during an emergency stop. Similar behavior is shown at a halt of the conveyor caused by queuing. In this special situation the assembling is allowed and therefore all occurring differences in the reference position have to be compensated.
3 STATE OF THE ART The existing approaches to an automated assembly in motion can be differentiated by the principle of synchronization between the conveyor system and the robot. There are approaches using mechanical, guided or controlled synchronizing principles. The mechanical synchronization is the simplest principle of synchronization because it is based on either a force fit or an interlocking joint. Disadvantages are the required mechanical adaptations of the conveyor system and the low accuracies [3]. More promising are the guided and the controlled synchronization. Today the guided conveyor tracking is implemented in most robot control systems. Using this method the conveyor speed is measured and the difference of the conveyor to the Tool-Center-Point (tcp) of the robot is calculated within the interpolation cycle. Then the movement of the robot is corrected by the robot control. According to Dirndorfer [3] this system cannot achieve a maximum accuracy in synchronization of less than one millimeter due to vibrations and the following error of the robot. The controlled or feedback systems use either hydraulic or electronic algorithms for synchronization. All these methods have in common a constant measurement of the deviation between the robot and the conveyor system. Hence, the robot speed can be adjusted using a feedback algorithm. So far, all applied synchronized robot systems lack in achieving the accuracy of a stationary robot. One reason is the insufficient speed of control because of long processing times. Furthermore, the synchronization is enabled only in flow direction of the conveyor systems. All other movements and therefore deviations perpendicular to the flow course are not considered. Economic application in industry is additionally hindered by the complexity and the costs of the systems. 4
Figure 2: Deviations to the reference position during stop The technical requirements, based on measurements from 2002, have been discussed by Zäh et al. [4]. Further measurements of the acceleration behavior have shown that the majority of the detected frequencies is in the range of 1.5-3 Hz, see Figure 3. Additional disturbance frequencies at about 7.7 Hz, 10 Hz and above have also been detected. Because of the fairly low amplitudes these frequencies result in only a small deviation in position. Nevertheless, the robot system has to be able to compensate all disturbances to an accuracy of 0.2 mm in order to realize the same accuracy as a stationary industrial robot.
REQUIREMENTS
4.1 Technical Requirements The major problem of the assembly in motion is the unintentional but occurring variation in the line speed of the conveyor systems. Hence, there is a difference between the reference position and the actual position of the system. This deviation reaches its maximum during a stop or halt of the conveyor because of very high negative accelerations and due to the limited stiffness of the
Figure 3: Spectrum analysis of frequencies
-26-
as the robot contacts the conveyed object, the guidance is performed by a compliant force-torque sensor system. The realized force fit enables the measurement of all occurring deviations between the robot and the product’s point of assembly. Using a compliant force-torque sensor the deviations caused by higher frequencies can be compensated.
4.2 Economic requirements Economic design is a fundamental precondition for implementing highly automated systems in industry. In general economic design of production systems relies on a variety of aspects. Examples are high availability, efficiency and the implementation of an intrasystem flexibility or even the reconfigurability of the entire system and of the applied modules [5], [6]. Reconfigurability can be seen as a multidimensional flexibility and the ability of the technical system to overcome the borders of flexibility [7], [8]. Furthermore, automated assembly systems have to be easily adaptable to different kinds of influencing factors within the assembly in a simple and fast way [9].
phase 1 positioning by conveyor tracking (~10 mm)
5 A NEW METHOD OF SYNCHRONIZATION The applied method of synchronization is based on two basic principles. First there is the measurement and guidance of the robot tcp according to the specific assembly point of the conveyed object. This is in opposite to all known approaches which guide the robot by measuring the deviations between conveyor base and the robot base. In these approaches it is assumed that there is no resilience, reduced stiffness or tolerances within these systems or the conveyed product. Given that these tolerances can not be denied, the proposed new method of synchronization tolerates all intrasystem tolerances of the robot, the conveyor system and the product. The second principle is the differentiation of the occurring frequencies into lower and therefore controllable frequencies and higher frequencies which have to be tolerated. This differentiation is needed due to the limited reaction time of robots caused by an implemented reference value filter, which is required to obey for example the maximum joint acceleration. Robots with higher payloads usually have a set filter of about 180200 ms. Hence, a maximum control frequency of about 5 Hz can be achieved. The limit between these different kinds of frequencies is neither stable nor accurate. Further developments in the field of control technique will shift this limit towards higher frequencies. Nevertheless, there will always be frequencies which can not be controlled, e.g. because of the resonance or the inertia of the system. In order to realize the second principle and compensate all movements caused by the different frequencies a combination of a highly accurate and fast guiding algorithm as well as a compliant construction for example of the used gripper system needs to be implemented. The guidance of the robot is performed similar to the predictive guidance by Clarke [10] where the computation of the robot movement is continuously compared to the programmed reference movement. For that an algorithm has been developed that uses all seven axes: six of the robot and 1 linear axis, to guide the robot to realize the assembly process. Furthermore, a smoothing of the values considering the maximum acceleration of the robot described by Lange [11] has been implemented. Predicted deviations are compensated by an adaptive feed forward control according to Lange und Hirzinger [12]. For that the standardized feedback functionality of the robot control which is implemented in most robot controls has been used. The applied assembly process is divided into three different phases as shown in Figure 4. Within the first phase a rough positioning of the robot using the conveyor tracking is realized. The second phase starts a soon as the point of assembly is recognized by the implemented vision system. From this moment the vision system guides the robot. Using this non-contact guidance an accuracy of about 1 mm and 1° can be achieved. As soon
phase 2 positioning by image processing (~1 mm, ~1°)
non-contact
phase 3 positioning by compliant force-torque sensor (~0.2 mm, ~0.5°) force fit point of contact
Figure 4: Phases of assembly process The prediction of the path in phases 2 and 3 is realized by using multiple scans of the applied sensor systems. By doing so, the reference values can be calculated in advance. Useful for this calculation are for example the speed of the conveyor or the pendular-like movement of the product around the mounting point of the power and free conveyor. Due to the assembly process an overall accuracy of about 0.2 mm and 0.5° can be achieved. 6 FLEXIBLE DESIGN The constructive design of the automated assembly in motion is based on the changeable means of production. This approach as proposed by Zäh [7] describes a number of principles and guidelines to achieve changeability of which modularity, universality and standardization have been realized in this system. The assembly station is build up of process modules. Those can easily be exchanged due to their autonomy. Modularity of the software is enabled by the differentiation within the method of synchronization [11]. The robot control is divided into a positional control of the robot and a computation of the desired path. The adaptation of the position control of the specific robot can be performed during installation. The computation of the path is done according to the assembly process. In this way it is not only possible to adapt the assembly process but also to exchange the robot without adjusting the program. Standardization in the context of this application means the use of commercially available parts as much as possible. Furthermore, all components are standardized by means of mechanical interfaces e.g. the gripper system or software interfaces e.g. the robot sensor interface, Ethernet or bus systems. This enables compatibility by fast switching of single components or modules. The principle of universality is implemented within the algorithm of synchronization. This algorithm is neither specified for the assembly process nor for the used robot. In addition most of the robot path is computed according to the different sensors and has not to be programmed or taught. The sensor systems are mostly non contact sensors and therefore non-specific regarding the product or process. Thereby a system was built, which is not based on a fixed backbone or platform but instead is characterized by the ability to completely exchange the used components. Due to the complexity of the system and the focus on the synchronization algorithm not all proposed guidelines to changeability could be implemented so far. Nevertheless,
-27-
necessary. The described robot system realizes this kind of synchronization in all six degrees of freedom. The algorithm is based on the separation of controlling and tolerating of the different occurring frequencies and deviations. Using different sensor systems an overlying of the computed path and even the prediction of the robot movement could be implemented. The developed robot system can be used in all kinds of flow assembly lines. Possible scenarios for the conveying belt synchronous assembly are all assembly processes where large and heavy items are mounted like the cockpit, the seats or the retractable roof. Major advantages of the synchronized assembly are the avoidance of buffers in front of and after automated assembly cells, the avoidance of special conveyor systems for automation and of ergonomically critical manual workstations.
further optimization will enhance the changeability of the entire system. 7 REALIZATION In order to validate the described concept a robot based prototype has been realized within this research project. The implemented assembly process is the mounting of the wheels to a car body as shown in Figure 1. This particular process is characterized by a tolerance of 0.13 mm. The entire process from detecting the car to gripping the wheels, synchronizing the robot and finally assembling the wheel has been realized in order to realistically reproduce the process. The prototype is based on an industrial robot which is mounted on a linear unit and carries up to 180 kg. Due to the linear axis the length of synchronized work is up to 7 m. The car body is conveyed by a power and free conveyor system with a speed of 8 m/min. The conveyor system has been adapted so that a movement of 0.5 m in height can also be realized. This adaptation is necessary in order to simulate the behavior of different kinds of conveyor systems. Special interest has been paid to the gripper system, shown in Figure 5. As described previously a vision system and a compliant force torque sensor have been implemented. The vision system, consisting of a camera and a laser is positioned in the middle of the gripper system in order to prevent occlusion and to realize near process measurements. compliant force torque sensor
camera and laser system
automatic screwers
gripper
9 [1]
REFERENCES
Lien, T. K., Rasch, F. O., 2001, Hybrid Automaticmanual Assembly Systems. In: Annals of the CIRP 50/1: 21-24. [2] Santochi, M., Dini, G., 1998, Sensor Technology in Assembly Systems. In: Annals of the CIRP 47/2: 503-524. [3] Dirndorfer, A., 1993, Robotersysteme zur förderbandsynchronen Montage, Dissertation (1993), Springer, Berlin. [4] Zäh, M. F., Werner, J., Lange, F., 2006, System to realize a conveying belt synchronous assembly, 17th International DAAAM Symposium "Intelligent Manufacturing & Automation: Focus in Mechatronics and Robotics", Technische Universität Wien: 449450. [5] Koren, Y., Heisel, U., Jovane, F., Moriwaki, T., Pritschow, G., Ulsoy, G., Brussel, H. V., 1999, Reconfigurable Manufacturing Systems. In: Annals of the CIRP 48/2: 527-540. [6] Feldmann, K., Slama, S., 2001, Highly flexible Assembly – Scope and Justification. In: Annals of the CIRP 50/2: 489-498. [7] Zäh, M. F., Werner, J., Prasch, M., 2006, Changeable Means of Production. In: Westkämper, E. (Ed.): First CIRP Seminar International Seminar on Assembly Systems - ISAS, Stuttgart: 33-38. [8] Schuh, G., Lösch, F., Gottschalk, S., Harre, J., Kampker, A., 2004, Gestaltung von Betriebsmitteln für die Serienproduktion, ZWF - Zeitschrift für Wirtschaftlichen Fabrikbetrieb, 99: 212-217. [9] Arai, T., Aiyama, Y., Maeda, Y., Ota, J., 2000, Agile Assembly System by "Plug & Produce". In: Annals of the CIRP 49/1: 1-4. [10] Clarke, D. W., Mothadi, C., Tuff, P. S., 1987, Generalized Predictive Control - part I. The basic algorithm, Automatica: 137-148. [11] Lange, F., Hirzinger, G., Frommberger, M., 2006, Impedance-based Smoothing for Visual Servoing along Edges. In: Joint Conference on Robotics ISR2006, Munich. [12] Lange, F., Hirzinger, G., 1996, Learning Force Control with Position Controlled Robots, IEEE Internation Conference on Robotics and Automation, Minneapolis / USA.
Figure 5: Gripper system of the assembly in motion The compliant force torque sensor is mounted on top of the gripper system close to the robot flange. Due to the limited payload of the sensor three additional springs were positioned around it to parallelize the forces. The applied compliance compensates deviations between the robot and the assembly part up to 3 mm and 3° in all dof. The gripping of the wheel takes place on the tread so that the hole of the axis is free for the vision system. Furthermore, five automatic screwers have been implemented to enable the mounting of the wheel. First tests of the components have shown the principal behavior of the system. Nevertheless, further tests have to determine the maximum accuracy of the applied system. 8 SUMMARY The assembly in motion is one of the key factors in enabling economic automation in flow assembly lines. Due to the limited stiffness of conveyor and robot systems synchronization of the robot tcp and the assembly point is
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G. Reinhart1 (1), J. Werner1 Institute for Machine Tools and Industrial Management, Faculty of Mechanical Engineering, Technische Universität München, Munich, Germany
Abstract Automation in flow assembly lines is hindered by the need for clocked lines when using automated systems. Geometrical inaccuracies and vibrations of the conveyor systems complicate the use of automation as well. The assembly in motion, meaning a robot system which is synchronized in all degrees of freedom to the moving conveyor belt, is a promising attempt to solve these difficulties. Main advantages of synchronized assembly are the avoidance of buffers and the reduction of the throughput time. The automated assembly in motion is realized by using mechatronic components and innovative feedback algorithms which are currently under development. Keywords: Assembly in Motion, Automation, Flexibility
quality of the assembly process is difficult to be assured. However, the manual flow assembly line is characterized by the idea of lean production, e.g continuous flow production and high intrasystem flexibility. Therefore it is highly economic. The automated assembly in flow assembly lines is limited by the need for synchronization of product and assembly machine which is practically only possible at a halt. Therefore, the product has to be locked out of the continuous flow line in order to be slowed down. Then the products are queued in front of the automated assembly station. Inside the station the automated process takes place. Usually the robot program is primarily based on taught points and movements. After completing the task the product is accelerated and locked again into the continuous flow line. In addition to this organizational lack of flexibility the automated assembly is characterized by low adaptability with respect to changes in the quantity of products and the switching of work contents between stations. Nevertheless, while assembling large and heavy parts the automated assembly is very efficient and therefore necessary.
1 INTRODUCTION Within the last years hybrid assembly systems consisting of manual and automated work stations have become well known. These systems are characterized by high flexibility and moderate costs [1]. Most of them are implemented for small or medium-sized products. For assembly lines of larger products there is still the requirement to divide the assembly into automated and manual assembly systems. This is due to the need for clocked lines when using automated robot stations in contrary to flow lines for manual work places. The combination which is an automated assembly in motion as shown in Figure 1, however, still needs further development. The assembly in motion or synchronized assembly has been known for over 30 years. Up to the present many researchers have developed different approaches within industrial and research projects. Despite different technical approaches it is not yet possible to implement an economic robot system which has the described feature and achieves the necessary assembly accuracy. In this paper an innovative concept of synchronization is presented that enables the mounting of wheels on a conveyed car body. The motivation is described in Section 2. Section 3 summarizes the state of the art in order to define the requirements in Section 4. The method of synchronization is explained in Section 5. In Section 6 the implementation of flexibility in automated assembly systems is addressed. The Section 7 describes the realization of the prototype. 2 MOTIVATION The synchronized assembly in motion is characterized by multiple advantages compared to either the clocked automation in flow lines or the manual assembly. In manual flow assembly lines the parts are carried by one or more workers to the production line. Then they are assembled while synchronized to the flow of the conveyor belt by the workers. This operation sequence lacks ergonomic design of the work places while handling large and heavy parts. Furthermore, a stable and high level of
Annals of the CIRP Vol. 56/1/2007
Figure 1: Prototype of the assembly in motion
-25-
doi:10.1016/j.cirp.2007.05.008
The synchronized assembly in motion combines the high and reproducible level of quality of the automated assembly and the flexibility of manual flow lines. Besides the possible reduction of assembly workers the greatest advantage is the avoidance of the queues in front of and after the assembly station. Given that these buffers represent up to 75 % of the needed space a great potential in ratio is opened. The flow lines can be shortened in length and less restriction are imposed for layout planning. In comparison, automatic stations are usually centralized or combined in order to minimize the effort of queuing the products. Whenever this is not possible automation is often rejected even though a ratio could be achieved. While integrating the assembly in motion cell into the flow line an adjustment or switching of work contents between consecutive assembling stations could be realized. Further advantages are the possible reduction of complex and costly feeding machines. Using the synchronized assembly the implemented sensors could be used to reduce the effort in positioning and orienting of the fed items [2].
conveyor. As shown in Figure 2 these deviations to the reference position can be observed in all degrees of freedom (dof), here shown only the three translative dof (x-movement in flow direction, y-movement to the side, zmovement in height). The amount of the deviation depends on the kind of conveyor system used. However, all conveyor systems show similar behavior. The deviations in Figure 2 were measured on a power and free conveyor during an emergency stop. Similar behavior is shown at a halt of the conveyor caused by queuing. In this special situation the assembling is allowed and therefore all occurring differences in the reference position have to be compensated.
3 STATE OF THE ART The existing approaches to an automated assembly in motion can be differentiated by the principle of synchronization between the conveyor system and the robot. There are approaches using mechanical, guided or controlled synchronizing principles. The mechanical synchronization is the simplest principle of synchronization because it is based on either a force fit or an interlocking joint. Disadvantages are the required mechanical adaptations of the conveyor system and the low accuracies [3]. More promising are the guided and the controlled synchronization. Today the guided conveyor tracking is implemented in most robot control systems. Using this method the conveyor speed is measured and the difference of the conveyor to the Tool-Center-Point (tcp) of the robot is calculated within the interpolation cycle. Then the movement of the robot is corrected by the robot control. According to Dirndorfer [3] this system cannot achieve a maximum accuracy in synchronization of less than one millimeter due to vibrations and the following error of the robot. The controlled or feedback systems use either hydraulic or electronic algorithms for synchronization. All these methods have in common a constant measurement of the deviation between the robot and the conveyor system. Hence, the robot speed can be adjusted using a feedback algorithm. So far, all applied synchronized robot systems lack in achieving the accuracy of a stationary robot. One reason is the insufficient speed of control because of long processing times. Furthermore, the synchronization is enabled only in flow direction of the conveyor systems. All other movements and therefore deviations perpendicular to the flow course are not considered. Economic application in industry is additionally hindered by the complexity and the costs of the systems. 4
Figure 2: Deviations to the reference position during stop The technical requirements, based on measurements from 2002, have been discussed by Zäh et al. [4]. Further measurements of the acceleration behavior have shown that the majority of the detected frequencies is in the range of 1.5-3 Hz, see Figure 3. Additional disturbance frequencies at about 7.7 Hz, 10 Hz and above have also been detected. Because of the fairly low amplitudes these frequencies result in only a small deviation in position. Nevertheless, the robot system has to be able to compensate all disturbances to an accuracy of 0.2 mm in order to realize the same accuracy as a stationary industrial robot.
REQUIREMENTS
4.1 Technical Requirements The major problem of the assembly in motion is the unintentional but occurring variation in the line speed of the conveyor systems. Hence, there is a difference between the reference position and the actual position of the system. This deviation reaches its maximum during a stop or halt of the conveyor because of very high negative accelerations and due to the limited stiffness of the
Figure 3: Spectrum analysis of frequencies
-26-
as the robot contacts the conveyed object, the guidance is performed by a compliant force-torque sensor system. The realized force fit enables the measurement of all occurring deviations between the robot and the product’s point of assembly. Using a compliant force-torque sensor the deviations caused by higher frequencies can be compensated.
4.2 Economic requirements Economic design is a fundamental precondition for implementing highly automated systems in industry. In general economic design of production systems relies on a variety of aspects. Examples are high availability, efficiency and the implementation of an intrasystem flexibility or even the reconfigurability of the entire system and of the applied modules [5], [6]. Reconfigurability can be seen as a multidimensional flexibility and the ability of the technical system to overcome the borders of flexibility [7], [8]. Furthermore, automated assembly systems have to be easily adaptable to different kinds of influencing factors within the assembly in a simple and fast way [9].
phase 1 positioning by conveyor tracking (~10 mm)
5 A NEW METHOD OF SYNCHRONIZATION The applied method of synchronization is based on two basic principles. First there is the measurement and guidance of the robot tcp according to the specific assembly point of the conveyed object. This is in opposite to all known approaches which guide the robot by measuring the deviations between conveyor base and the robot base. In these approaches it is assumed that there is no resilience, reduced stiffness or tolerances within these systems or the conveyed product. Given that these tolerances can not be denied, the proposed new method of synchronization tolerates all intrasystem tolerances of the robot, the conveyor system and the product. The second principle is the differentiation of the occurring frequencies into lower and therefore controllable frequencies and higher frequencies which have to be tolerated. This differentiation is needed due to the limited reaction time of robots caused by an implemented reference value filter, which is required to obey for example the maximum joint acceleration. Robots with higher payloads usually have a set filter of about 180200 ms. Hence, a maximum control frequency of about 5 Hz can be achieved. The limit between these different kinds of frequencies is neither stable nor accurate. Further developments in the field of control technique will shift this limit towards higher frequencies. Nevertheless, there will always be frequencies which can not be controlled, e.g. because of the resonance or the inertia of the system. In order to realize the second principle and compensate all movements caused by the different frequencies a combination of a highly accurate and fast guiding algorithm as well as a compliant construction for example of the used gripper system needs to be implemented. The guidance of the robot is performed similar to the predictive guidance by Clarke [10] where the computation of the robot movement is continuously compared to the programmed reference movement. For that an algorithm has been developed that uses all seven axes: six of the robot and 1 linear axis, to guide the robot to realize the assembly process. Furthermore, a smoothing of the values considering the maximum acceleration of the robot described by Lange [11] has been implemented. Predicted deviations are compensated by an adaptive feed forward control according to Lange und Hirzinger [12]. For that the standardized feedback functionality of the robot control which is implemented in most robot controls has been used. The applied assembly process is divided into three different phases as shown in Figure 4. Within the first phase a rough positioning of the robot using the conveyor tracking is realized. The second phase starts a soon as the point of assembly is recognized by the implemented vision system. From this moment the vision system guides the robot. Using this non-contact guidance an accuracy of about 1 mm and 1° can be achieved. As soon
phase 2 positioning by image processing (~1 mm, ~1°)
non-contact
phase 3 positioning by compliant force-torque sensor (~0.2 mm, ~0.5°) force fit point of contact
Figure 4: Phases of assembly process The prediction of the path in phases 2 and 3 is realized by using multiple scans of the applied sensor systems. By doing so, the reference values can be calculated in advance. Useful for this calculation are for example the speed of the conveyor or the pendular-like movement of the product around the mounting point of the power and free conveyor. Due to the assembly process an overall accuracy of about 0.2 mm and 0.5° can be achieved. 6 FLEXIBLE DESIGN The constructive design of the automated assembly in motion is based on the changeable means of production. This approach as proposed by Zäh [7] describes a number of principles and guidelines to achieve changeability of which modularity, universality and standardization have been realized in this system. The assembly station is build up of process modules. Those can easily be exchanged due to their autonomy. Modularity of the software is enabled by the differentiation within the method of synchronization [11]. The robot control is divided into a positional control of the robot and a computation of the desired path. The adaptation of the position control of the specific robot can be performed during installation. The computation of the path is done according to the assembly process. In this way it is not only possible to adapt the assembly process but also to exchange the robot without adjusting the program. Standardization in the context of this application means the use of commercially available parts as much as possible. Furthermore, all components are standardized by means of mechanical interfaces e.g. the gripper system or software interfaces e.g. the robot sensor interface, Ethernet or bus systems. This enables compatibility by fast switching of single components or modules. The principle of universality is implemented within the algorithm of synchronization. This algorithm is neither specified for the assembly process nor for the used robot. In addition most of the robot path is computed according to the different sensors and has not to be programmed or taught. The sensor systems are mostly non contact sensors and therefore non-specific regarding the product or process. Thereby a system was built, which is not based on a fixed backbone or platform but instead is characterized by the ability to completely exchange the used components. Due to the complexity of the system and the focus on the synchronization algorithm not all proposed guidelines to changeability could be implemented so far. Nevertheless,
-27-
necessary. The described robot system realizes this kind of synchronization in all six degrees of freedom. The algorithm is based on the separation of controlling and tolerating of the different occurring frequencies and deviations. Using different sensor systems an overlying of the computed path and even the prediction of the robot movement could be implemented. The developed robot system can be used in all kinds of flow assembly lines. Possible scenarios for the conveying belt synchronous assembly are all assembly processes where large and heavy items are mounted like the cockpit, the seats or the retractable roof. Major advantages of the synchronized assembly are the avoidance of buffers in front of and after automated assembly cells, the avoidance of special conveyor systems for automation and of ergonomically critical manual workstations.
further optimization will enhance the changeability of the entire system. 7 REALIZATION In order to validate the described concept a robot based prototype has been realized within this research project. The implemented assembly process is the mounting of the wheels to a car body as shown in Figure 1. This particular process is characterized by a tolerance of 0.13 mm. The entire process from detecting the car to gripping the wheels, synchronizing the robot and finally assembling the wheel has been realized in order to realistically reproduce the process. The prototype is based on an industrial robot which is mounted on a linear unit and carries up to 180 kg. Due to the linear axis the length of synchronized work is up to 7 m. The car body is conveyed by a power and free conveyor system with a speed of 8 m/min. The conveyor system has been adapted so that a movement of 0.5 m in height can also be realized. This adaptation is necessary in order to simulate the behavior of different kinds of conveyor systems. Special interest has been paid to the gripper system, shown in Figure 5. As described previously a vision system and a compliant force torque sensor have been implemented. The vision system, consisting of a camera and a laser is positioned in the middle of the gripper system in order to prevent occlusion and to realize near process measurements. compliant force torque sensor
camera and laser system
automatic screwers
gripper
9 [1]
REFERENCES
Lien, T. K., Rasch, F. O., 2001, Hybrid Automaticmanual Assembly Systems. In: Annals of the CIRP 50/1: 21-24. [2] Santochi, M., Dini, G., 1998, Sensor Technology in Assembly Systems. In: Annals of the CIRP 47/2: 503-524. [3] Dirndorfer, A., 1993, Robotersysteme zur förderbandsynchronen Montage, Dissertation (1993), Springer, Berlin. [4] Zäh, M. F., Werner, J., Lange, F., 2006, System to realize a conveying belt synchronous assembly, 17th International DAAAM Symposium "Intelligent Manufacturing & Automation: Focus in Mechatronics and Robotics", Technische Universität Wien: 449450. [5] Koren, Y., Heisel, U., Jovane, F., Moriwaki, T., Pritschow, G., Ulsoy, G., Brussel, H. V., 1999, Reconfigurable Manufacturing Systems. In: Annals of the CIRP 48/2: 527-540. [6] Feldmann, K., Slama, S., 2001, Highly flexible Assembly – Scope and Justification. In: Annals of the CIRP 50/2: 489-498. [7] Zäh, M. F., Werner, J., Prasch, M., 2006, Changeable Means of Production. In: Westkämper, E. (Ed.): First CIRP Seminar International Seminar on Assembly Systems - ISAS, Stuttgart: 33-38. [8] Schuh, G., Lösch, F., Gottschalk, S., Harre, J., Kampker, A., 2004, Gestaltung von Betriebsmitteln für die Serienproduktion, ZWF - Zeitschrift für Wirtschaftlichen Fabrikbetrieb, 99: 212-217. [9] Arai, T., Aiyama, Y., Maeda, Y., Ota, J., 2000, Agile Assembly System by "Plug & Produce". In: Annals of the CIRP 49/1: 1-4. [10] Clarke, D. W., Mothadi, C., Tuff, P. S., 1987, Generalized Predictive Control - part I. The basic algorithm, Automatica: 137-148. [11] Lange, F., Hirzinger, G., Frommberger, M., 2006, Impedance-based Smoothing for Visual Servoing along Edges. In: Joint Conference on Robotics ISR2006, Munich. [12] Lange, F., Hirzinger, G., 1996, Learning Force Control with Position Controlled Robots, IEEE Internation Conference on Robotics and Automation, Minneapolis / USA.
Figure 5: Gripper system of the assembly in motion The compliant force torque sensor is mounted on top of the gripper system close to the robot flange. Due to the limited payload of the sensor three additional springs were positioned around it to parallelize the forces. The applied compliance compensates deviations between the robot and the assembly part up to 3 mm and 3° in all dof. The gripping of the wheel takes place on the tread so that the hole of the axis is free for the vision system. Furthermore, five automatic screwers have been implemented to enable the mounting of the wheel. First tests of the components have shown the principal behavior of the system. Nevertheless, further tests have to determine the maximum accuracy of the applied system. 8 SUMMARY The assembly in motion is one of the key factors in enabling economic automation in flow assembly lines. Due to the limited stiffness of conveyor and robot systems synchronization of the robot tcp and the assembly point is
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