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Intelligent Assist Systems for Flexible Assembly
Intelligent Assist Systems for Flexible Assembly 1
J. Krüger1,2, R. Bernhardt2, D. Surdilovic2 Institute for Machine Tools and Factory Management, Technical University Berlin, Germany 2 Fraunhofer Institute for Production Systems and Design Technology, Berlin, Germany Submitted by G. Spur (1), Berlin, Germany
Abstract The growing number of product variants, smaller lot sizes, reduced time to market and shorter lifecycles of products have lead to increasing demands on automation equipment and concepts. As a solution, hybrid human integrated approaches are proposed. The idea is to combine human flexibility, intelligence and skills with the advantages of sophisticated technical systems. Intelligent assist systems (IAS) represent a novel class of assembly systems capable of working with human operators in two modes: workplace sharing and time sharing. This paper presents a novel collaborative robot system ("Cobot") capable of sharing the workspace with the human co-worker and collaborating with him through direct physical contact. Keywords: Co-operative Assembly, Robot, Man-machine system
1
INTRODUCTION
Considerable research and development efforts have addressed the automation of manual assembly and other material handling processes through the use of manipulation robots. However, only specific robotic applications have been established, mainly in the mass production of relatively simple parts. Functional limitations are: high costs, restriction and/or expensive adaptation to unstructured variable environments, complex programming (educated personals), insufficient availability, changeable and shrinking production etc. On the other hand conventional material handling devices (generally referred to as “assist devices” such as articulated arms, balancers, hoists, jib cranes etc.) provide a positive impact on ergonomic safety but are quite limited in terms of efficiency, precision and safety. Furthermore, conventional material handling equipment faces ergonomic and work related health problems. The lack of responsive reaction and payload inertia managing results in operator fatigue, stress, or, even worse, in several musculoskeletal disorders. According to reports of Agencies for Safety and Health at Work (www.osha.org), about 30% of European workers suffer from back-pain that are mainly caused by
high physical demands, such as the lifting and manual handling of loads, repetitive movements, awkward posture etc. This problem causes enormous social, health and economic costs, estimated to range from 2.6 to 3.8% of Gross National Product of member states. The reflection on an appropriate human involvement in assembly and disassembly [1] shows, that a more effective and integrated “holistic” approach integrating sophisticated handling and sensory systems [2] is urgently needed to solve the above-mentioned problems. A new generation of ergonomic assist devices, referred to as “intelligent assist systems” or “intelligent assist devices” “collaborative robots” (cobots), holonomic manipulators etc., use a computer control to improve movement of heavy items and to respond precisely to the operator's wishes [3, 4]. This paper presents a novel so-called “Intelligent Power Assist Device” (IPAD) which allows direct co-manipulation with a human operator (figure 1) and integrates sophisticated force-feedback and programming (skilllearning) functions, as well as compliant motion guidance (“compliant virtual walls”) and semi-autonomous functions, e.g. “homing”, “pressing” etc.
Figure 1: IPAD - a reliable approach for advanced material handling
Annals of the CIRP Vol. 55/1/2006
2
NOVEL IPAD COMBINING POWER ASSIST AND MOTION GUIDANCE FUNCTIONS
The power assist robots commonly provide the interface with the human operator via a force sensor, e.g. human enhancers [5], admittance displays etc. and significantly reduce efforts needed to manoeuvre the manipulator. However, safety is a critical problem since these systems possess high power and can potentially injure the operator. The pure motion guiding systems are intrinsically passive and safe, however, they do not provide the inertia management and force amplification that is highly necessary for manoeuvring heavy parts. Therefore, an ideal cooperating material handling system would combine the passive motion guidance [3, 6-8] with a limited power assist [4] which would be enough to compensate for frictional and acceleration/deceleration forces and yet not enough to injure the operator. The IPAD system described in this paper uses the Continuous Variable Transmission (CVT) principle based on modified differential gears that are more suitable for power transmission and control. 3
Recently, CVTs technology is becoming rather attractive because of a growing interest within the automotive industry to develop a new generation of engine transmissions that increase efficiency, fuel economy and safety. Although there are different CVT principles, the most applied concept is based on variators with adjustable working diameters of the two main pulleys. These CVTs can meet reliable torque/speed requirements, however, they are weighty and expensive for applications in material handling systems. A simple and promising CVT has been designed by the authors based on differential gear mechanisms (figure 2). The differential gears are commonly applied in automotive systems to split the input engine torque two ways, allowing each output to spin at different speeds. However, the differential possesses two degrees of freedom (DOFs), and can also be applied to adjust transmission ratios between one of the two input speeds, e.g. ring-gear ω r = q& r (figure 2), and the output, e.g. carrier ω = q& , by controlling the speed of the second input. In the considered case this is sun gear (figure 2) connected to the drive ω d = q& d . Taking into account that the carrier position is a function of the positions of sun and ring gears q = f (qd , qr ) , the relationship between angular velocities for the considered gear (figure 2) is defined by
∂f ∂f q& d + q& r = kq& d + (1 − k ) q& r ∂q d ∂q r
ω kω d ≈ +1 ωr ωr
(4)
Obviously, if the human realizes the rotation ω r , it is possible to regulate the above transmission ratio i by measuring ω r and controlling drive velocity ω d .
Figure 2: Elemental hypocyclic differential gear 4
DIFFERENTIAL GEAR BASED CVT
q& =
i=
KINEMATIC STRUCTURES OF IPAD WITH DIFFERENTIAL CVT
This section discusses the usage of differential CVT in novel power assist systems. The elemental mechanical configuration corresponding to an IPAD with a configuration, referred to as serial structure, is presented in (figure 3).
Figure 3: Basic serial configuration
(1)
where k is so-called stationary transmission ratio characterizing the differential train performance, obtained for fixed ring gear (in this case the differential becomes a hypocyclic planetary gear)
k = k q& r =0 =
ω − ωr ωd − ωr
Figure 4: Basic parallel configuration (2)
Commonly in speed reduction gears is k << 1 . The angular velocities relationship can be written as
ωd = k ω + (1 − k )ωr
(3)
Where k = 1 / k . For k << 1 and ωd << ωr , the equation (1) becomes ω ≈ kω d + ω r , i.e.
This system consists of a SCARA-like manipulator with two links. The first joint is connected to the output of a differential train. The second joint is via a teeth-belt (with the transmission ratio c1 ) connected to the ring-gear of the differential. Thus by closing an additional internal coupling chain between manipulator joints, the basic so-called “cobotic” structure is realized. The internal chain allows controlling the relationship between joints rates of change Δq1 / Δq 2 (figure 3). Indeed, the system is underactuated, since only q d1 is driven. Commonly, the friction in the
system arrests the ring-gear (and thus the joint q 2 ) reducing DOFs to one. Then, the drive system can only move the first joint, causing both links to rotate as one body around this joint. On the other hand, the differential gear regulates the transmission ratio via angular velocity, which means a mistake could cause the system to move, in spite of the operator’s wishes to do so. However, contrary to the industrial robotic systems, an erroneous motion in the new IPAD appears not to be critical. As mentioned above, the role of the drive system in (figure 3) is to regulate the transmission ratio between manipulator joints, rather than to move the system. Consequently, the power of the drive system is significantly lower than in industrial robots, and the risk of injury or environmental damage is negligible. Moreover, the possibility of the drive and differential CVT to bring/transmit a limited power to the system is essential for realizing the power assist (e.g. inertia managing) functions. The serial system in (figure 3) possesses only one drive and is mainly intended to realize path guidance functions. The system functional principle is described in the following. Similar as in conventional handling devices, the human operator moves the system via an appropriate interface on the manipulator tip, causing the rotation of the second joint. When the drive is inactive, this rotation is transmitted via teeth-belt (connected to the ring-gear) and the differential gear according to (4). Since in this case both joints are connected with a fixed transmission ratio, the system moves like a mechanism with one DOF, along a fixed path. By activating the control system, it becomes possible to control the motion transmission ratio i12 = Δq 2 / Δq1 based on measured rate of change Δq1 produced by the operator. By these means, a programmable nominal trajectory in the manipulator’s workspace can be realized. According to the principle “human moves - machine directs”, the operator thereby realizes the main motion. It is worth mentioning that the new IPAD generally does not require a force sensor (like in an admittance display). The human action is sensed using a simple position sensor in joints. Adding an additional drive and CVT results in so called parallel cobot structure (figure 4), which can realize power assist control function (compensation for payload inertia) along a virtual wall. 5
PATH GUIDANCE CONTROL
Path-guidance control consists of planning the nominal trajectory within the robot working space as well as its realization. Unlike conventional robotic systems, and especially for the IPADs, the nominal path defines only the geometrical relation, not velocity profiles and timing. This is due to the fact that the path is defined by human movement and can generally not be predicted. Since the nominal motion is not prescribed, the main problem is the computation of the position error in a given time interval. For the sake of simplicity, let us consider a serial cobot structure sketched in (figure 3). The realization of path guidance is based on the incremental system model which defines the relation between position rate of changes in Cartesian and joint spaces related by manipulator Jacobian.
⎧Δq1 ⎫ ⎧Δx ⎫ ⎨ ⎬ = J (q )⎨ ⎬ Δ y ⎩ ⎭ ⎩Δq2 ⎭
(5)
The ratio Δy over Δx defines a direction α in the motion plane (figure 5). The rates of motion change build incremental segments of a desired path, creating a virtual wall along which the cobot may move. In order to implement this wall, the control system must regulate the motion transmission ratio i = Δq 2 / Δq1 using differential CVT. Introducing Δq 2 = iΔq1 in (5) yields
⎧1⎫ ⎧Δx ⎫ ⎨ ⎬ = J (q)⎨ ⎬Δq1 ⎩i ⎭ ⎩Δy ⎭
(6)
which is the basic cobot kinematic equation. The joint coordinate q1 , corresponding to the single cobot DOF, represents the so called “master axis,” which is directly controlled by the human. Correspondingly, the joint q2 is referred to as the “slave axis” and must be controlled by the differential gear drive (figure 2).
Figure 5: Virtual wall segments The virtual wall direction can be expressed in the form
tan (α ) =
Δy J 21 Δq1 + J 22 Δq2 J 21 + J 22 i = = Δx J11 Δq1 + J12 Δq2 J11 + J12 i
(7)
where J ij ( i, j =1,2) denotes elements of the Jacobian matrix. In a defined manipulator position, different directions of motion can be achieved depending on the transmission ratio. Based on (7) we can determine the transmission ratio i required to follow a given direction α
i=
J 21 − J11 tan (α ) J12 tan (α ) − J 22
(8)
The corresponding drive angular velocity (i.e. angular rate of change Δqd ) that realizes this transmission ratio is computed based on (1-4). Finally, in each time interval the nominal drive position may be obtained by i
qd 0 = qd 0
i −1
+ Δqd
(9)
Based on a given virtual wall direction, the nominal drive motion can be determined using the model (5-9) and measurements of actual joint and drive positions. Once the nominal position is computed, we can accomplish the cobot motion in the desired direction bringing the error to zero using the appropriate control, e.g. τ cd = PID(qd 0 − qd ) . More complicated algorithms based on cobot dynamics can also be applied [9].
6
EXPERIMENTAL RESULTS
The feasibility of the control algorithms was verified by an industrial cobot prototype (figure 6) based on a rapid control implementation with dSpace and Matlab/Simulink. A typical path tracking task (figure 7) shows, how a virtual wall consisting of two linear segments can accurately be realized. A comparably high positioning accuracy with respect to the big inertia of the manipulator arms moved by low power drives (100 W) was achieved with a simple PID control.
By this, it can be shown that a close collaboration of man and automation device can efficiently be supported by a new control approach. This does not only provide basic functions for passive motion guidance and power assistance but also integrates features as homing and virtual walls, which increase efficiency and security for cooperative workplaces. This leads to Intelligent Power Assist Devices, which build a new automation platform not only for production but also for logistics or medical applications.
Figure 8: Conceptual Cobot construction for flexible automotive assembly Figure 6: Cobot prototype with differential CVT
8 REFERENCES [1] Bley, H., Reinhart, G., Seliger, G., Bernardi, M., and Korne, T., 2004, Appropriate Human Involvement in Assembly and Disassembly. Annals of the CIRP, 53: 487 - 510. [2]
Krüger, J., Nickolay, B., Heyer, P. and Seliger, G., 2005, Image based 3D Surveillance for flexible ManRobot-Cooperation. Annals of the CIRP, 54: 19 - 23.
[3]
Colgate E. Wannasuphoprasit W, Peshkin, M., “Cobots Robots for Collaboration with Human Operator”, Proceedings of the ASME Dynamic Systems and Control Division, DSC-Vol. 58, pp. 433440, 1996.
[4]
Akella, M.; Peshkin, M.; Colgate E. Wannasuphoprasit W.; 1998, “Cobots – a novel Material Handling Technology”, Proceedings of the ASME Winter Annual Meeting. Kazerooni H. „The Human Power Amplifier Technology at the University of California, Berkeley, DSC Vol. 57-2, IMECE Proceedings of the ASME Dynamic Systems and Control Division, ASME 1995.
[5] Figure 7: Results of path tracking of a virtual wall (nominal position – dashed line; real motion – solid line) Actually the cobotic principles for path guidance and power assistance are transferred to crane portal structures in order to support tasks which are essential for automotive production such as flexible synchonisation with conveyor belts (i.e. cockpit assembly – figure 8). 7
[6]
Chung, W.J.; Nakamura, Y.; Sordelan, O.J.; 1995, „Prototyping a Nonholonomic Manipulator“. Proceedings of the 1995 IEEE ICRA, Nagoya, Japan.
[7]
Book W., Charles H., Davis H., Gomes M., “The Concept and Implementation of a passive Trajectory Enhancing Robot”, Proceedings of ASME Dynamic Systems Control Division, DSC-Vol. 58, pp.633-638, 1996.
[8]
Delnondedieu Y., Trocazz J., „PADyC a Passive Arm with Dynamic Constraint: A prototype with two DOF”, proceedings IEEE Conference on medical Robotics and Computer Assisted Surgery, pp. 173-180, 1995.
[9]
Surdilovic D., Bernhardt R. : „Novel Interactive Robotic Systems”, In Proceeding of the 16th IFAC World Congress, Prague, Chech. Republic (CD, proceeding, full paper Nr. 05173) 2005.
SUMMARY
The paper describes a new cobot system with differential CVT. This system is significantly cheaper, control is easier and the system in general is more efficient than cobot systems based on spherical CVT applied in first cobot prototypes [3, 4]. Path guidance functions as well as power amplification functions can easily be realised by this system. The first experiments with a functional prototype show the reliability of the new interactive handling system.
J. Krüger1,2, R. Bernhardt2, D. Surdilovic2 Institute for Machine Tools and Factory Management, Technical University Berlin, Germany 2 Fraunhofer Institute for Production Systems and Design Technology, Berlin, Germany Submitted by G. Spur (1), Berlin, Germany
Abstract The growing number of product variants, smaller lot sizes, reduced time to market and shorter lifecycles of products have lead to increasing demands on automation equipment and concepts. As a solution, hybrid human integrated approaches are proposed. The idea is to combine human flexibility, intelligence and skills with the advantages of sophisticated technical systems. Intelligent assist systems (IAS) represent a novel class of assembly systems capable of working with human operators in two modes: workplace sharing and time sharing. This paper presents a novel collaborative robot system ("Cobot") capable of sharing the workspace with the human co-worker and collaborating with him through direct physical contact. Keywords: Co-operative Assembly, Robot, Man-machine system
1
INTRODUCTION
Considerable research and development efforts have addressed the automation of manual assembly and other material handling processes through the use of manipulation robots. However, only specific robotic applications have been established, mainly in the mass production of relatively simple parts. Functional limitations are: high costs, restriction and/or expensive adaptation to unstructured variable environments, complex programming (educated personals), insufficient availability, changeable and shrinking production etc. On the other hand conventional material handling devices (generally referred to as “assist devices” such as articulated arms, balancers, hoists, jib cranes etc.) provide a positive impact on ergonomic safety but are quite limited in terms of efficiency, precision and safety. Furthermore, conventional material handling equipment faces ergonomic and work related health problems. The lack of responsive reaction and payload inertia managing results in operator fatigue, stress, or, even worse, in several musculoskeletal disorders. According to reports of Agencies for Safety and Health at Work (www.osha.org), about 30% of European workers suffer from back-pain that are mainly caused by
high physical demands, such as the lifting and manual handling of loads, repetitive movements, awkward posture etc. This problem causes enormous social, health and economic costs, estimated to range from 2.6 to 3.8% of Gross National Product of member states. The reflection on an appropriate human involvement in assembly and disassembly [1] shows, that a more effective and integrated “holistic” approach integrating sophisticated handling and sensory systems [2] is urgently needed to solve the above-mentioned problems. A new generation of ergonomic assist devices, referred to as “intelligent assist systems” or “intelligent assist devices” “collaborative robots” (cobots), holonomic manipulators etc., use a computer control to improve movement of heavy items and to respond precisely to the operator's wishes [3, 4]. This paper presents a novel so-called “Intelligent Power Assist Device” (IPAD) which allows direct co-manipulation with a human operator (figure 1) and integrates sophisticated force-feedback and programming (skilllearning) functions, as well as compliant motion guidance (“compliant virtual walls”) and semi-autonomous functions, e.g. “homing”, “pressing” etc.
Figure 1: IPAD - a reliable approach for advanced material handling
Annals of the CIRP Vol. 55/1/2006
2
NOVEL IPAD COMBINING POWER ASSIST AND MOTION GUIDANCE FUNCTIONS
The power assist robots commonly provide the interface with the human operator via a force sensor, e.g. human enhancers [5], admittance displays etc. and significantly reduce efforts needed to manoeuvre the manipulator. However, safety is a critical problem since these systems possess high power and can potentially injure the operator. The pure motion guiding systems are intrinsically passive and safe, however, they do not provide the inertia management and force amplification that is highly necessary for manoeuvring heavy parts. Therefore, an ideal cooperating material handling system would combine the passive motion guidance [3, 6-8] with a limited power assist [4] which would be enough to compensate for frictional and acceleration/deceleration forces and yet not enough to injure the operator. The IPAD system described in this paper uses the Continuous Variable Transmission (CVT) principle based on modified differential gears that are more suitable for power transmission and control. 3
Recently, CVTs technology is becoming rather attractive because of a growing interest within the automotive industry to develop a new generation of engine transmissions that increase efficiency, fuel economy and safety. Although there are different CVT principles, the most applied concept is based on variators with adjustable working diameters of the two main pulleys. These CVTs can meet reliable torque/speed requirements, however, they are weighty and expensive for applications in material handling systems. A simple and promising CVT has been designed by the authors based on differential gear mechanisms (figure 2). The differential gears are commonly applied in automotive systems to split the input engine torque two ways, allowing each output to spin at different speeds. However, the differential possesses two degrees of freedom (DOFs), and can also be applied to adjust transmission ratios between one of the two input speeds, e.g. ring-gear ω r = q& r (figure 2), and the output, e.g. carrier ω = q& , by controlling the speed of the second input. In the considered case this is sun gear (figure 2) connected to the drive ω d = q& d . Taking into account that the carrier position is a function of the positions of sun and ring gears q = f (qd , qr ) , the relationship between angular velocities for the considered gear (figure 2) is defined by
∂f ∂f q& d + q& r = kq& d + (1 − k ) q& r ∂q d ∂q r
ω kω d ≈ +1 ωr ωr
(4)
Obviously, if the human realizes the rotation ω r , it is possible to regulate the above transmission ratio i by measuring ω r and controlling drive velocity ω d .
Figure 2: Elemental hypocyclic differential gear 4
DIFFERENTIAL GEAR BASED CVT
q& =
i=
KINEMATIC STRUCTURES OF IPAD WITH DIFFERENTIAL CVT
This section discusses the usage of differential CVT in novel power assist systems. The elemental mechanical configuration corresponding to an IPAD with a configuration, referred to as serial structure, is presented in (figure 3).
Figure 3: Basic serial configuration
(1)
where k is so-called stationary transmission ratio characterizing the differential train performance, obtained for fixed ring gear (in this case the differential becomes a hypocyclic planetary gear)
k = k q& r =0 =
ω − ωr ωd − ωr
Figure 4: Basic parallel configuration (2)
Commonly in speed reduction gears is k << 1 . The angular velocities relationship can be written as
ωd = k ω + (1 − k )ωr
(3)
Where k = 1 / k . For k << 1 and ωd << ωr , the equation (1) becomes ω ≈ kω d + ω r , i.e.
This system consists of a SCARA-like manipulator with two links. The first joint is connected to the output of a differential train. The second joint is via a teeth-belt (with the transmission ratio c1 ) connected to the ring-gear of the differential. Thus by closing an additional internal coupling chain between manipulator joints, the basic so-called “cobotic” structure is realized. The internal chain allows controlling the relationship between joints rates of change Δq1 / Δq 2 (figure 3). Indeed, the system is underactuated, since only q d1 is driven. Commonly, the friction in the
system arrests the ring-gear (and thus the joint q 2 ) reducing DOFs to one. Then, the drive system can only move the first joint, causing both links to rotate as one body around this joint. On the other hand, the differential gear regulates the transmission ratio via angular velocity, which means a mistake could cause the system to move, in spite of the operator’s wishes to do so. However, contrary to the industrial robotic systems, an erroneous motion in the new IPAD appears not to be critical. As mentioned above, the role of the drive system in (figure 3) is to regulate the transmission ratio between manipulator joints, rather than to move the system. Consequently, the power of the drive system is significantly lower than in industrial robots, and the risk of injury or environmental damage is negligible. Moreover, the possibility of the drive and differential CVT to bring/transmit a limited power to the system is essential for realizing the power assist (e.g. inertia managing) functions. The serial system in (figure 3) possesses only one drive and is mainly intended to realize path guidance functions. The system functional principle is described in the following. Similar as in conventional handling devices, the human operator moves the system via an appropriate interface on the manipulator tip, causing the rotation of the second joint. When the drive is inactive, this rotation is transmitted via teeth-belt (connected to the ring-gear) and the differential gear according to (4). Since in this case both joints are connected with a fixed transmission ratio, the system moves like a mechanism with one DOF, along a fixed path. By activating the control system, it becomes possible to control the motion transmission ratio i12 = Δq 2 / Δq1 based on measured rate of change Δq1 produced by the operator. By these means, a programmable nominal trajectory in the manipulator’s workspace can be realized. According to the principle “human moves - machine directs”, the operator thereby realizes the main motion. It is worth mentioning that the new IPAD generally does not require a force sensor (like in an admittance display). The human action is sensed using a simple position sensor in joints. Adding an additional drive and CVT results in so called parallel cobot structure (figure 4), which can realize power assist control function (compensation for payload inertia) along a virtual wall. 5
PATH GUIDANCE CONTROL
Path-guidance control consists of planning the nominal trajectory within the robot working space as well as its realization. Unlike conventional robotic systems, and especially for the IPADs, the nominal path defines only the geometrical relation, not velocity profiles and timing. This is due to the fact that the path is defined by human movement and can generally not be predicted. Since the nominal motion is not prescribed, the main problem is the computation of the position error in a given time interval. For the sake of simplicity, let us consider a serial cobot structure sketched in (figure 3). The realization of path guidance is based on the incremental system model which defines the relation between position rate of changes in Cartesian and joint spaces related by manipulator Jacobian.
⎧Δq1 ⎫ ⎧Δx ⎫ ⎨ ⎬ = J (q )⎨ ⎬ Δ y ⎩ ⎭ ⎩Δq2 ⎭
(5)
The ratio Δy over Δx defines a direction α in the motion plane (figure 5). The rates of motion change build incremental segments of a desired path, creating a virtual wall along which the cobot may move. In order to implement this wall, the control system must regulate the motion transmission ratio i = Δq 2 / Δq1 using differential CVT. Introducing Δq 2 = iΔq1 in (5) yields
⎧1⎫ ⎧Δx ⎫ ⎨ ⎬ = J (q)⎨ ⎬Δq1 ⎩i ⎭ ⎩Δy ⎭
(6)
which is the basic cobot kinematic equation. The joint coordinate q1 , corresponding to the single cobot DOF, represents the so called “master axis,” which is directly controlled by the human. Correspondingly, the joint q2 is referred to as the “slave axis” and must be controlled by the differential gear drive (figure 2).
Figure 5: Virtual wall segments The virtual wall direction can be expressed in the form
tan (α ) =
Δy J 21 Δq1 + J 22 Δq2 J 21 + J 22 i = = Δx J11 Δq1 + J12 Δq2 J11 + J12 i
(7)
where J ij ( i, j =1,2) denotes elements of the Jacobian matrix. In a defined manipulator position, different directions of motion can be achieved depending on the transmission ratio. Based on (7) we can determine the transmission ratio i required to follow a given direction α
i=
J 21 − J11 tan (α ) J12 tan (α ) − J 22
(8)
The corresponding drive angular velocity (i.e. angular rate of change Δqd ) that realizes this transmission ratio is computed based on (1-4). Finally, in each time interval the nominal drive position may be obtained by i
qd 0 = qd 0
i −1
+ Δqd
(9)
Based on a given virtual wall direction, the nominal drive motion can be determined using the model (5-9) and measurements of actual joint and drive positions. Once the nominal position is computed, we can accomplish the cobot motion in the desired direction bringing the error to zero using the appropriate control, e.g. τ cd = PID(qd 0 − qd ) . More complicated algorithms based on cobot dynamics can also be applied [9].
6
EXPERIMENTAL RESULTS
The feasibility of the control algorithms was verified by an industrial cobot prototype (figure 6) based on a rapid control implementation with dSpace and Matlab/Simulink. A typical path tracking task (figure 7) shows, how a virtual wall consisting of two linear segments can accurately be realized. A comparably high positioning accuracy with respect to the big inertia of the manipulator arms moved by low power drives (100 W) was achieved with a simple PID control.
By this, it can be shown that a close collaboration of man and automation device can efficiently be supported by a new control approach. This does not only provide basic functions for passive motion guidance and power assistance but also integrates features as homing and virtual walls, which increase efficiency and security for cooperative workplaces. This leads to Intelligent Power Assist Devices, which build a new automation platform not only for production but also for logistics or medical applications.
Figure 8: Conceptual Cobot construction for flexible automotive assembly Figure 6: Cobot prototype with differential CVT
8 REFERENCES [1] Bley, H., Reinhart, G., Seliger, G., Bernardi, M., and Korne, T., 2004, Appropriate Human Involvement in Assembly and Disassembly. Annals of the CIRP, 53: 487 - 510. [2]
Krüger, J., Nickolay, B., Heyer, P. and Seliger, G., 2005, Image based 3D Surveillance for flexible ManRobot-Cooperation. Annals of the CIRP, 54: 19 - 23.
[3]
Colgate E. Wannasuphoprasit W, Peshkin, M., “Cobots Robots for Collaboration with Human Operator”, Proceedings of the ASME Dynamic Systems and Control Division, DSC-Vol. 58, pp. 433440, 1996.
[4]
Akella, M.; Peshkin, M.; Colgate E. Wannasuphoprasit W.; 1998, “Cobots – a novel Material Handling Technology”, Proceedings of the ASME Winter Annual Meeting. Kazerooni H. „The Human Power Amplifier Technology at the University of California, Berkeley, DSC Vol. 57-2, IMECE Proceedings of the ASME Dynamic Systems and Control Division, ASME 1995.
[5] Figure 7: Results of path tracking of a virtual wall (nominal position – dashed line; real motion – solid line) Actually the cobotic principles for path guidance and power assistance are transferred to crane portal structures in order to support tasks which are essential for automotive production such as flexible synchonisation with conveyor belts (i.e. cockpit assembly – figure 8). 7
[6]
Chung, W.J.; Nakamura, Y.; Sordelan, O.J.; 1995, „Prototyping a Nonholonomic Manipulator“. Proceedings of the 1995 IEEE ICRA, Nagoya, Japan.
[7]
Book W., Charles H., Davis H., Gomes M., “The Concept and Implementation of a passive Trajectory Enhancing Robot”, Proceedings of ASME Dynamic Systems Control Division, DSC-Vol. 58, pp.633-638, 1996.
[8]
Delnondedieu Y., Trocazz J., „PADyC a Passive Arm with Dynamic Constraint: A prototype with two DOF”, proceedings IEEE Conference on medical Robotics and Computer Assisted Surgery, pp. 173-180, 1995.
[9]
Surdilovic D., Bernhardt R. : „Novel Interactive Robotic Systems”, In Proceeding of the 16th IFAC World Congress, Prague, Chech. Republic (CD, proceeding, full paper Nr. 05173) 2005.
SUMMARY
The paper describes a new cobot system with differential CVT. This system is significantly cheaper, control is easier and the system in general is more efficient than cobot systems based on spherical CVT applied in first cobot prototypes [3, 4]. Path guidance functions as well as power amplification functions can easily be realised by this system. The first experiments with a functional prototype show the reliability of the new interactive handling system.