Bionic Assembly System: A New Concept of Self Organizing Multi-Robot System


Bionic assembly system: A new concept of
self- organizing multirobot system

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 (BAS) is presented. BAS is based on concepts of autonomy, cooperation 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. In this paper concept of the system is presented with focus on transport mobile robots, which are a backbone of the system .
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Keywords: multirobot system; assembly process; mobile robot; navigation; simulation.

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 FMS, 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 . This paper is a further development of Bionic Assembly System (BAS) . BAS solves following problems of existing manufacturing systems:

1 Lack of flexibility to deal with the diversification of products and their uses. Existing manufacturing systems cannot cope with changes in product design and manufacturing plans nor keep in pace with the dynamics of the market.

2. Lack of flexibility to deal with replacement of equipment and the increasing complexity of the system itself. Manufacturing equipment cannot be easily replaced nor can simultaneous use of new and old machinery be easily integrated.

3 Planning execution loop. As complexity of manufacturing systems increase time between planning and executing becomes too long. That is, we have a big chance that new plan lead only to applicable solution. In such cases it is questionable if planning/scheduling/re-scheduling pay off.

Main elements of a proposed system are autonomous mobile robots. They have to function autonomously, have to adapt themselves and act in strong correlation between each other and their environment (shop-floor). Design of task-oriented behavior of single robot and cooperation between different robot classes under production constraints is a main task to be solved
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1 Concept of bionic assembly system

Two types of systems are dominating in production: machining and assembly systems. Modern machining systems produces the parts which are quite independent from the final product build up from those parts. That is the main reason why the machining systems are more universal and have higher level of automation than the assembly systems, leaving assembly as a most expensive phase in the production. One of the priority research tasks in the development of future assembly systems is finding a more flexible, efficient, robust system solution that allow much more re-use rate of assembly units. To fulfil this need a concept of BAS was proposed. The concept of the system was developed on a real industrial demand to significantly reduce the production costs of electrical motors in mass production. BAS is based on concepts of autonomy, cooperation and intelligence. BAS is composed out of two subsystems (Figure 1): Core subsystem and Supplementary subsystem of BAS, these subsystems are divided by system border. The core subsystem is central part of assembly system. It includes: all

assembly stations, mobile robots and assembly pallets, repair station, quality control station. The dominating activity in the core subsystem is assembly, quality control and repair. The supplementary subsystem is surrounding the core subsystem. The main activity in this subsystem is the storage of parts and components. The main activity on the system border between the core and supplementary subsystems is the supply of the assembly stations with the parts and components, which have to be assembled.

3 Mobile robot types in bionic assembly system Industry applications of autonomous mobile robots is the area that has received little attention when compared with research performing in the context of other areas (surveillance, humanoid, soccer, hazardous environment, etc.). The preponderance of the current research in mobility surrounding Flexible Manufacturing Systems (FMS) involves the use of Automated Guided Vehicles (AGV’s) . These vehicles simplify the problem of navigation by restricting their paths to striping the floor in some manner or by using buried cables. A major issue is just how ‘flexible’ such systems are. The state-of-the-art of mobile robot technology and predictions of future development are giving a clear view that mobile robots are going to be essential part of every manufacturing process in not so far future (Asl et al., 2001). Robots now can intelligently go from place to place and collect parts and take them to the appropriate work cell which opens up a new different way of structuring the manufacturing environment. The main advantage of mobile robots is their flexibility. Keeping in mind these facts, BAS is completely structured of mobile robots. To realize BAS three mobile robot type are introduce

Transport Place of assembly, carrier of assembling product. Autonomous mobile
Robot with exchangeable palette.
Supply Delivering of assembly parts to assembly stations. Autonomous mobile
Robots adapt to transport assembly parts in quantities.
Energy changing batteries of the transport robot at the actual
Position and transporting of these to the charging station.
Autonomous mobile robots adapt to transport and exchange batteries.

Such robots should be able to function autonomously and smoothly in order to cope up
With unstructured and highly complex working environment of BAS. They have to deal
With following sources of uncertainty and dynamic events:
• Incoming parts variation: variable availability and arrival rates and poses of
in coming parts.
• Variable quality of incoming parts: faulty parts must be removed before they
either cause errors in system or become assembled into products.
• Mixed batch of parts to be assembled: the assembly station assembling a range
of variants of electrical motors types.
• Variable availability of resources: resources are tools and machines with which
the assembly station needs to directly coordinate.
• Response to dynamical events in self organizing assembly system: for example,
opening alternative assembly ways to bypass machines break ups or allowing
more assembly stations and transport agents in core subsystem to increase
production capacity. Supply class provides assembly class units with needed
parts, energy class exchanges empty batteries. Booth actions take place at
actual position of assembly unit supporting self – organised assembly structure.
The behavior of autonomous mobile robots and a system as whole is inspired by
biological life. Biological organisms are capable of adapting to environmental changes
and sustain life through functions such as self-recognition, self-growth, self-recovery and
evolution. They accomplish self-organisation capability through communication and
evolve intelligence through learning. All these characteristics of biological organisms
serve as an example for the biologically inspired manufacturing systems that is, for
design of autonomous mobile robot’s behaviour in such systems. Table 2 gives
similarities that is, relations between BAS and biological life. Table uses several terms
that are defined as follows -
• unit – basic component which performs a task
• task – specific operation to be accomplished
• source – basic system in need of accomplishing a task
• performance – unit movement to source for task completion.

One of the most important aspects of BAS is the use of priorities . That means that to every transport robot priority is assigned (importance of finishing assembly
process in matter of time in relation to other transport mobile robots) and the robot with
Bionic assembly system: new concept of self-organising multirobot system 21
higher priority has always an advantage over one of lower priority. Priority is expressed
in levels and so:
• Priority level 1: it means that the transport mobile robot is carrying a
product with highest priority. It has advantage in relation to robots
that have lower priority level.
• Priority level 2: this level expresses high priority. It has advantage in
relation to all robots that have lower priority level, but has to leave
all higher priority robots to go in front of it.
• Priority level 3: low priority. All robots have advantage in front of
transport mobile robots having this level of priority.
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 . This
figure shows three assembly stations and nine transport mobile robots.

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
centralised control method and a decentralised control method. More specifically, in
a centralised control method all planning and decision-making functions are handled by a
single control centre. Each mobile robot contains only sensors for localisation 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 decentralised control method in BAS and one of the system’s key
advantages .

There are a great number of robots functioning independently in the factory
environment. All robots have own controller and are equally important, that is, there are
no highly ranked robots or controllers which are giving orders. The transport mobile
robot will have general knowledge of the plant layout and will determine its position
with a global positioning system. The local environment around the vehicle will be
detected using sensors mounted on the robot. This enables the robot to avoid dynamic
and unexpected static obstacles. In any unforeseen situation, the robot is able to plan a
new path or find a solution without waiting for commands from the control centre. The
function of the control centre is only limited to the broadcasting of traffic flow
information received from all robots and the allocation of tasks in the system.
Inter-robot communication becomes necessary since competition for resources
should be avoided and sharing experience could improve system performance .

Coordination of multiple mobile robots needs to address three main issues

• how to appropriately divide the functionality of the system into
multiple robots
• how to realise the dynamic configuration of the system and
• how to achieve cooperation behaviour.

4 Transport mobile robots working scenario
The function of transport mobile robot is to carry a palette on which the product is to be
assembled from one assembly station to another. Since there is not just one assembly
station which can perform same assembly step (some parts of a product are same for all
products), transport mobile robot should decide on each assembly step to which
assembly station it should go. 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 transport mobile robot decides which
assembly station to choose, that is, where to go.
Three time values which impact the decision of transport mobile
robot to which assembly station to go
Basically, the robot is calculating the total time summing the all three values:
T O TR W T =T +T +T (1)
where TT – total time;
To – operating time;
TTR – transport time;
TW – waiting time.

Transport mobile robot has to be capable of avoiding static (assembly stations, machine
parts…) and dynamic (other transport robots, supply robots and energy maintenance
robots) obstacles. At this moment we have developed the controller which navigates
the transport mobile robot from the beginning to the end of the cycle. The controller is
written in C++ and vector field histogram method has been used for navigation. Firstly
the navigation was done by using the artificial potential fields method, but this method
has a lot of lacks, so the vector field histogram method seemed as a better solution.

When the transport mobile robot has the type of the product it has to go to the loading
station to get one palette on which the product should be assembled. Then it has to go
from one assembly station to another in order to assembly the right combination of the
cubes. After the product is assembled it has to go to the unloading station which takes the
palette with final product. At this moment, transport mobile robot has fulfilled his
assembly mission and it is going back to the initial position (robot pool) and waits for
new order to come. Since the robot is equipped with battery sensor which measures the
state of the energy level, robot should go to the recharging station first if its battery level
is less then 15% of full energy. By this stage one work cycle of a transport mobile robot
is finished.

5 Webots software
For this simulation of simplified BAS we have decided to use Webots professional
software for simulation of mobile robots behaviour. This is the most realistic and reliable
software of this kind at a moment. The simulation system used in Webots uses virtual
time, making it possible to run simulations much faster than it would take on a real robot.
Depending on the complexity of the setup and the power of your computer, simulations
can run up to 300 times faster than the real robot when using the fast simulation mode.
The graphical user interface of Webots allows you to easily interact with the simulation
while it is running. The robot’s behaviour is written using the C++ or Java programming
language. Moreover, any Webots controller can be connected to a third party software
program, such as MatLab, LabView, etc. through a TCP/IP interface. Once tested in
simulation your robot controllers can be transferred to real robots. This is the real power
of Webots, because there is no other software that has capability of transferring the
controller programs on real robots.

6 Conclusion
Existing manufacturing systems cannot cope with globalisation of industry and highly
demanding customer orders. As companies move towards 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 cannot interact with environment; cannot 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
Bionic assembly system: new concept of self-organising multirobot system
way self-organisation of the system is realised. 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.

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Galantucci, L.M., Percoco, G. and Spina, R. (2004) ‘Assembly and disassembly planning by using
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