Reconfiguration of transport mobile robot queues in bionic assembly system.

Lazinica, A. ; Katalinic, B. ; Majstorovic, V. 等

Abstract: Changing manufacturing environment characterized by aggressive competition on a global scale and rapid changes in process technology requires creating production systems that are themselves easily upgradeable and into which new technologies and new functions can be readily integrated. To answer these novel requirements, Bionic Assembly System is presented. Bionic Assembly System is based on concepts of autonomy, co-operation and intelligence of its units. The system proposes use of autonomous mobile robots in production environment instead of using AGV's. Mobile robots are giving flexibility to the system and increase dynamics of the whole process. The most important ability of the system is possibility of constant reconfiguration of transport mobile robot queues in a self-organizing way.

Keywords: multi-robot system, assembly, reconfiguration

1. INTRODUCTION

To be able to respond for a customers demand and stay competitive in the 21st Century, manufacturing companies must possess a new kind of manufacturing system that is capable of quick responding to global market; a system which is designed to be easily upgraded with new technology, easily adaptable for new kind of products and whose production capacity is adjustable. Today's systems, even called flexible manufacturing systems, do not have such characteristics. Today's global world market requires a change in existing manufacturing systems. Cost-effective, reconfigurable manufacturing systems, whose components are reconfigurable machines and reconfigurable controllers, as well as methodologies for their systematic design and diagnosis, are the cornerstones of 21st Century manufacturing systems [1]. To fulfill this need a concept of Bionic Assembly System (BAS), was proposed by Katalinic [2]. The concept of the system (Figure 1) was developed on a real industrial demand to significantly reduce the production costs of electrical motors in mass production. Bionic Assembly System is based on concepts of autonomy, co-operation and intelligence.

2. TRANSPORT MOBILE ROBOTS

2.1. Decentralized System

As the number of mobile robots in a system increases, planning and control of the system becomes increasingly complex. The methods to handle such complexity include a centralized control method and a decentralized control method. More specifically, in a centralized control method all planning and decision-making functions are handled by a single control centre. Each mobile robot contains only sensors for localization and obstacle avoidance, actuators for movements and manipulation, and communication facility for communicating with the control centre. All the movements of mobile robots in the system are controlled from this centre and conflicts among multiple robots are easily solved. This method has been widely adopted in manufacturing industry and warehouses where multiple mobile robots are used to transfer parts and clean warehouses. One major disadvantage of the system is that whole system will stop functioning immediately if the control centre fails. That is a reason of applying a decentralized control method in Bionic Assembly System and one of the system's key advantages. Communication behavior is important design issue for coordination of transport mobile robots in the system. The communication may take place directly via explicit communication facility or indirect (pseudo communication method) through one robot sensing a change in other robots or its environment [7]. Communication between several robots can be done using wireless LAN, Bluetooth or a radio system. We can equip the individual mobile robots with a proper communication system so that each mobile robot individually senses the obstacles and passes on the information to other robot in the system. Inter-robot communication becomes necessary since competition for resources should be avoided and sharing experience could improve system performance. In a decentralized control system, co-ordination of multiple mobile robots is needed to achieve cooperation behavior. Transport mobile robot might use broadcasting to announce its location or some other information to the whole system, or might use unicasting to communicate directly with another robot (Fig. 2).

[FIGURE 2 OMITTED]

2.2 Working scenario

At the beginning of each assembly step transport mobile robot is contacting all assembly station with a question which station could perform next assembly step. Assembly stations which can perform that are sending the answer with following information contained:

* time needed to perform assembly step (not every station has the same operating speed),

* its position in environment (needed to calculate transport time from actual transport robot position to the station),

* time of waiting (there is a queue of transport mobile robots waiting for assembly in front of every assembly station).

On the basis of these three values (Fig. 3) transport mobile robot decides which assembly station to choose, i.e. where to go.

[FIGURE 3 OMITTED]

[T.sub.T] = [T.sub.O] + [T.sub.TR] + [T.sub.W], (1)

where

[T.sub.T]--total time, [T.sub.O]--operating time, [T.sub.TR]--transport time, [T.sub.W]--waiting time.

For every assembly station transport mobile robot is calculating this total time and searches for the minimum one:

min([3.summation over (i=1)] [t.sub.i]) (2)

If assembly station does not send any message to the transport mobile robot, it knows that this station is malfunctioning (error) and in that case:

[T.sub.O] = [infinity]. (3)

Robots are always waiting in a queue in front of assembly station. This queue is formed so that robots with highest priorities are always in front of the others (Fig. 4). This figure shows three assembly stations and nine transport mobile robots. Robots that are first in the queue are currently in assembly process, i.e. on them is performing assembly step. Other robots are waiting in a queue. At each time step, every transport mobile robots is communicating with every assembly station asking three time values mentioned before.

Every assembly station knows a "situation" in front of it, i.e. knows how many transport mobile robots are waiting in a queue and structure of the queue (priority level of each robot).

Information about time values are stored in vectors [T.sup.j.sub.i].

For example:

[T.sup.1.sub.O] (1) contains operating time of robot 1 on assembly station 1.

[T.sup.2.sub.W] (4) contains how much time transport robot 4 must wait in front of assembly station 2 in order to start performing assembly step. When assembly step on one transport mobile robot is finished, this robot is going to search for other assembly station for next assembly step or the product is assembled and it goes to the storage. At this time all robots in queue in front of this assembly station are moving for one place in front and whole matrix of waiting time is changing. [T.sup.3.sub.T] (6) contains how much time robot 6 needs to come to its place in front of assembly station 3. Robot's priority is considered, since robot is going behind the last robot of its own priority. Based on this values transport mobile robots are deciding whether to stay in the queue or it is more worth full (in time matter) to go in front of other station. If transport mobile robot decides to go in front of other station, it contacts the assembly station to send him position vector [S.sup.j.sub.i] which contains list of current robots in its queue. Based on this value transport mobile robot is searching for the last robot in queue which has its priority level and goes behind it.

[FIGURE 4 OMITTED]

4. CONCLUSION

Existing manufacturing systems can not cope with globalisation of industry and highly demanding customer orders. As companies move toward more flexible production lines for smaller batch sizes and shorter product cycles, more advanced systems are needed. The main disadvantage of existing systems is their inflexibility. Reason for that is use of AGV's. AGV can not interact with environment; can not cope with unexpected obstacles in its way. With rapid development of autonomous mobile robots technology, it becomes possible to incorporate them in production environment. Mobile robots are giving a new dimension of flexibility to the system--dynamics to the whole process. System is capable of quickly responding to customer demands, can adapt to any changes in working environment and can incorporate new parts of the system without stopping the production process. With incorporation of priority levels, different kind of products could be easily assembled. With use of mobile robots, reconfiguration of the whole assembly process is possible at any time. Transport mobile robots are just selecting on which assembly station to go according to spend less time in the whole process. In this way self-organisation of the system is realized. Next step is to develop simulation of reconfiguration of mobile robot queues and in that way develop algorithms and controllers which could be used on real, physical robots.

5. REFERENCES

Arkin, R. C. & Murphy, R. R.: "Autonomous navigation in a manufacturing environment", IEEE Transactions on robotics and automation, Vol 6, No. 4, 1990.

Katalinic, B.: "Design of scheduling strategies for complex flexible assembly system for the mass production of electrical motors", Proceedings of International Workshop on Emergent Synthesis--IWES 99, (Editor: K. Ueda), Kobe, Japan, December 6-7, 1999.

Koren, Y. & Ulsoy, A. G. : "Reconfigurable Manufacturing Systems", Engineering Research Center for Reconfigurable Machining Systems (ERC/RMS), Report # 1, The University of Michigan, Ann Arbor, 1997.

Stone, P. & Veloso, M.: "A survey of multiagent and multirobot systems", chapter in "Robot Teams, from Diversity to Polymorphism", Balch, T. & Parker, L. (Eds.), ISBN 1-56881-155-1, AK Peters Ltd., 2002

Table 3. Vector of time values for assembly station 1

 1 2 3 4 5 6 7 8 9
1