Uninova - Institute for the Development of New Tecnologies

CRI - Center for Intelligent Robotics

Robotic Pilot Assembly System

Overview of the Robotic Pilot Assembly System

This System was conceived as a generic infrastructure for a range of products and aimed at supporting three kinds of activities: training, research and demonstration. Main design constraints and adopted solutions are presented. This Pilot Unit is a significative platform to support cooperation activities in various international projects.

Partial view of the cell.

INDEX

1. INTRODUCTION

The Flexible Manufacturing and Assembly System -- NOVAFLEX -- installed at the Center for Intelligent Robotics (CRI) of UNINOVA was conceived as a demonstration unit able to handle a set of typical activities of a Computer Integrated Manufacturing (CIM) system.

Main Components of the NOVAFLEX System

Besides the machining and assembly subsystems, the Pilot Unit includes a storage component, an input section for raw materials, a delivery section for finished products and a transportation subsystem that links all the other components.
As one of the basic design goals, the system was required not to be restricted to a particular type of product. The objective was to build a relatively generic infrastructure, that could be adaptable to a range of products with minimal setup effort. The requirement was for a flexible infrastructure, with a representative set of manufacturing resources, and not a special purpose system.
This Pilot Unit will support three kinds of activities:

  • training,
  • demonstration,
  • research.
    • Another constraint to the final architecture resulted from the need to integrate some already existing equipments, namely the CNC machines.
    In fact this was accepted as a challenge typically found in real manufacturing systems. The evolution of an existing manufacturing system has to take into account the existing machinery.
    Therefore the various aspects of integration and interoperability between components had to be considered.
    As a pre-requisite to design the system's architecture, it was necessary to analyze and understand the main requirements from each class of potential users taking into account the three kinds of intended activities.

    Training

    In the present framework of transformation of the Portuguese Industry, in which deep changes are imposed by the need to be competitive in a context of open market, a key factor is the training of technical people that will perform such transformation.
    The training needs are not only for engineers but also for medium level technicians, who are, in fact, the system's operators responsible for its regular operation.
    CRI intends to use this Pilot Unit to support training / retraining actions for both levels of technical people. On the other side, taking profit of the good relationships between CRI and Faculdade de Cincias e Tecnologia -- Universidade Nova de Lisboa, this unit will also support the regular engineering curricula, specially in terms of the final project (5th year). In the same spirit, the system will also provide an important support for the post-graduation students (Master and Ph.D. thesis).

    Demonstration

    A second line of activities intended for this Pilot Unit is the demonstration and analysis of feasibility of solutions. It is not easy to convince people from industry about new solutions just based on "paper" projects. Simulation-based approaches, although important as a design step, are not sufficient to understand and evaluate all behavior of a planned system. The realization of a physical demonstration system is therefore a very important tool to help in the discussion of the solution and also a catalyzer for gathering precise definitions of requirements and to refine solutions.
    This unit can therefore support the development of demonstrating solutions jointly developed by research people and enterprises.

    Research

    Finally, being CRI a research center, one important goal for the Pilot Unit is naturally the support to the various ongoing and planned R&D activities. One important design requirement was therefore, the need to support various research areas, like systems integration, cell control and scheduling, planning, monitoring, diagnosis and error recovery, sensorial perception, etc..
    Another very important aspect is the possibility of different groups being simultaneously using different subsystems of the Unit for separate experiments. As a matter of fact, this situation is expected to be the most common practice during the systems life time.
    These requirements led to an architecture in which NOVAFLEX can be operated either as an integrated FMS/FAS system or as a set of isolated subsystems (machining, assembly, transportation and storage, etc.). This last aspect has particular consequences on the design of the control architecture.
    Therefore, the need to support these different research areas implied the design of a flexible architecture, from the topology to the control points of view. An easy reconfiguration of its operating mode is an important requirement to support concurrent research activities.
    Besides the constraints derived from the mentioned goals there were also constraints imposed by the configuration of the physical space and by the available budget. Space constraints obviously influenced the topologic design, specially at the level of materials flow. The system was designed to be installed in two adjacent rooms, with a total area of approximately 60 m2. In order to facilitate the materials flow, it was necessary to open a connecting "gate" between the two rooms. This gate was not planned in the original building. Budget constraints had, naturally, important consequences, either at the components level, and at the topologic level.

    2. GENERAL ARCHITECTURE

    2.1. Global Overview

    The NOVAFLEX has 5 subsystems:
    1. FMS subsystem,
    2. Multi-Robot FAS subsystem,
    3. Automatic Warehouse subsystem,
    4. Transportation subsystem and
    5. Sensorial subsystem.
    Each subsystem can be operated autonomously or as part of an integrated system. Subsystems 4 and 5 are mainly complementary to the first 3 modules. The transportation subsystem itself can work in separate sectors, to support the isolated operation of any of the other subsystems.
    The transportation medium is a pallet-based conveyor belt. Each pallet can be adapted to transport different kinds of parts and products.

    2.2. FMS subsystem

    The FMS subsystem includes a small scale milling machine (DENFORD StarMill) and a turn (DENFORD StarTurn).

    DENFORD StarMill

    DENFORD StarTurn

    A 6 DOF Robot (ABB IRB2000) serves these machines, assuring the "link" between them and the conveyor system.
    The NC equipment was already existing before the design of this system and was integrated with the robot to form a flexible machining cell. The robot is installed on top of a controllable axis, allowing two different operating positions in order to serve the two machines.

    View of machining subsystem's IRB200 robot moving chariot

    The materials to be machined are transported to the robot working area by conveyor-belts. These materials can come either from the automatic warehouse or from the entry point for raw materials. After processing, parts are sent to the expedition point, to the automatic warehouse or to the assembly system.

    Machining Subsystem

    2.3. Multi-Robot FAS subsystem

    The main characteristics of the assembly subsystem are the possibility of performing:

    Multirobot Assembly Subsystem

    The assembly subsystem is therefore composed by two robotized cells. They can be operated in an isolated or in an integrated way.

    The available robots are:

    2.3.1. Assembly Cell 1

    This cell is centered on the SCARA robot (BOSCH SR840) which includes an automatic tool exchange mechanism.

    General View of Scara BOSH SR840

    Some of the most important features of this robot are presented below:
    The control system for the SR 840 is a CNC control with several ranges of programming possibilities.
    This robot allows movement on straight lines with optional location in space, and with programmable path velocity, through linear interpolation; it also allows movement in an orbit with optional location in space, and with programmable path velocity through circular interpolation; PTP movements are also possible.
    The robot can be programmed using a PASCAL like language - BAPS. With this language, the user can have guarded movements, which seems to be an important feature of this robot.

    BOSH SR840 Scara changing tools

    A magazine of tools, with various grippers, is available to the robot through its tool exchange system. This system is the BOSCH Exchange System GWS 20.

    BOSH Tool Exchange System GWS 20

    The magazine holds the grippers currently not in use. A proximity switch is integrated with the magazine for presence checks and code interrogation.
    Available grippers are:
    1. SCHUNK Pneumatic parallel-gripper
    2. SCHUNK Pneumatic centric-gripper
    3. SCHUNK Pneumatic parallel-gripper
    4. SCHUNK Pneumatic parallel-gripper

    View of BOSH RS840 Scara and gripper

    Mounted on the robot wrist, there is a force/torque sensor (SCHUNK FTS 30). The controller of this sensor provides the Fx, Fy and Fz force components and the Mx, My and Mz momentous; it also assures the interface to the computational architecture.
    Assembly operations can be performed on top of fixtures installed in selected pallets. Pallets can be stopped within the robot working area resorting to a positioning device with a precision of 0.1 mm.
    Parts feeding is also assured by pallets, whose stop locations are controllable by the computational system.
    The implemented architecture supports a "dynamic" buffer of up to ten pallets.
    Finished products and subassemblies processed in this cell may be sent to the warehouse, to another cell or to the delivery section.

    2.3.2. Assembly Cell 2

    This cell is based on a ABB IRB 2000 robot.
    Some of the most important features of this robot are presented below:
    A very interesting feature is the possibility to have power and air supply available at the grippers, because user wiring and pressurized air supply is routed inside the robot arm.
    The robot is controlled through soft keys, joystick and the robot-language ARLA.
    This robot allows the same type of movements as the robot described in Assembly Cell 1, but with more spatial flexibility due to its 6th dof.
    The robot is installed on top a movable chariot along an axis that can be controlled incrementally.
    One of the top positions of this axis is planned as the normal operating area for cell 2. The other top position "inserts" the robot in the working area of cell 1, allowing for multi-robot cooperation tasks.
    The movement of the robot chariot may be synchronized with the conveyor belt in order to allow assembly operations while a pallet is moving. There are, obviously, some limitations regarding the operations that are possible in this mode. One main limitation is the reduced precision resulting from the uncertainty about the position of the pallet, which is subject to sliding and vibration factors. Another potential problem is due to the different speeds of the robot and the conveyor. This last aspect may be attenuated by synchronizing the conveyor driving motors and the robot chariot motor.
    A magazine of tools, with various grippers, is available to the robot. The tool exchange system is the SCHUNK Pneumatic Exchange System GWS.
    The magazine held the grippers currently not in use.

    ABB IRB2000 changing tools.

    Available grippers are: PGN64, PZN64 and RH918 from SCHUNK.
    The tool's magazine moves with the robot along the axis.

    View of ABB IRB2000 robot and tool.

    Due to the large working area of the IRB 2000 robot, no special requirements were imposed regarding the materials feeding conditions.
    As in cell 1, the assembly operations are planned to be performed with the support of fixtures installed in pallets and a positioning system (precision of 0.1 mm).
    In a future stage, other feeding devices, as well as additional fixturing tables may be installed.

    2.4. Transportation Subsystem

    The transportation subsystem is, perhaps, the most "visible" part of the entire unit. It is a conveyor-based pallet transportation network that links the various subsystems.
    Each pallet is designed to support parts/products up to 10 Kg, and with a volume of up to 200x200x200 mm.

    Main pallet flows in NOVAFLEX

    As a pallet can be used for different purposes - for carrying raw materials, machined parts, subassemblies or finished products - different pallet flows have to be supported. In order to have a dynamic system, it is therefore necessary to have a pallet identification system. With such a system, it is possible to re-route - dynamically - pallets according to a global control strategy.
    The selected identification system is based on a read/write memory device, attached to each pallet. The information stored in a pallet's memory will "drive" the actions to apply to that pallet: activate a positioning device (stopper), actuate an elevator, etc. .

    The installed pallet identification system is the BOSCH ID80/E composed of Mobil Data Tag (MDT), attached to each pallet and Read/Write Units (SLS).
    The communication between a SLS and a MDT may occur when the pallet is within a range of 15 mm. The data transfer can be done with the stopped or moving pallet .
    the SLS components, controlled by a host unit, are distributed along the transportation subsystem and located in all strategic points where decisions can be made about the next route to be followed by the pallet.

    Examples of information to be stored in a MDT:
    The SLS module includes a program memory, a data memory, I/O ports and a communications interface.
    Due to its programming capability, an SLS can actuate as local controller (through its I/O ports). But it can also be linked to a host system through an interface DIN 66019. Therefore, centralized or distributed controlled strategies can be implemented and evaluated on the installed architecture.
    the topology of the transportation subsystem was constrained by the need to allow cooperation between the two robots of the two assembly cells. As a consequence, the robots had to be installed inside the rectangle defined by conveyors 5,6,7 and 9
    One aspect to be noted is the adopted solution to link the machining cell and the assembly / warehouse zone (conveyors 1 and 2). Due to the restricted gate between the two rooms, a two-layered solution, served by two elevator devices, was adopted.
    Conveyor 3 is reversible, allowing transportation of pallets from the assembly/warehouse room to the machining cell and vice-versa.
    Conveyor 4 is linked to the warehouse subsystem, becoming one of the key points of the system, leading to potential bottlenecks.
    This conveyor is used in the following flows:
    To bring pallets into the 2 assembly cells, conveyors 5, 6 and 7 will be used. Conveyor 7 also includes a positioning device (stopper) to be used during assembly operations in cell 2, which is a constraint to the flow of pallets towards cell 1. During assembly operations using robot 2, no pallet can be moved to cell 1. This situation can easily be found in the Petri net model.

    Petri Net Model of the FAS

    The control algorithm must take these constraints into account. In order to operate cell 1, the required pallets have to be transferred to the corresponding working area, before the start of assembly operations in cell 2. This constraint also justifies the dynamic buffering of 10 pallets associated to cell 1, as mentioned before. This buffer is implemented by conveyor 9 and part of 5 and 7.

    2.5. Automatic Warehouse Subsystem

    This subsystem consists of an array of 50 storage slots, served by a 3 axis manipulator. Each slot can store a pallet and the parts/products it is carrying on.
    There is a conveyor link between the warehouse and the transportation subsystem.

    General view of the automatic warehouse.

    There is an SLS attached to the automatic warehouse arm. This SLS can perform read/write operations from/to the MDTs installed in the pallets. The warehouse controller has the possibility to check if a pallet is present or not, through an SLS operation. More then detecting the presence or absence of pallets in slots, the system can even determine which pallet type the slot contains. This is an important feature during initialization and is also a safety feature preventing picking a wrong pallet type.

    View of the automatic warehouse arm.

    As the warehouse is an important component of the system, several material flows from/to it can be identified:
    Machined parts
    From/To FAS1 or FAS2
    Subassemblies
    From/To FAS1 or FAS2
    Finished products
    To delivery zone
    Raw materials
    To FMS
    To FAS1 or FAS2
    From Input Zone
    Pallet-installed Gabarits
    To FAS1 or FAS2
    Empty pallets
    From FAS1, FAS2 or FMS

    2.6. Sensorial Subsystem

    Besides the pallet identification systems, a multi-sensorial perception system is also being planned.
    Due to the specifities of this area, the design and installation of such system was organized as a separate sub project, not described in this paper.

    2.7. Safety Requirements

    The safety aspects are very important in these type of systems. In a country where there is no deep tradition regarding safety conditions and procedures, it seems very important to consider it seriously in a system that is going to be used in training activities.
    Therefore the analysis and installation of safety "devices" is being planned according to the ISO standard 11161 - Industrial Automation Systems - Safety of Integrated Manufacturing Systems - Basic Requirements.
    This standard was developed to provide safety requirements and guidelines for the design, construction and installation, programming, operation, use and maintenance of integrated manufacturing systems. It describes the potential danger situations associated to such systems and provides a set of recommendations to avoid risks.

    2.8. Pneumatic Installation

    A pneumatic network was installed in order to be used by various devices, namely the robot grippers and tool exchanging mechanisms.

    3. COMPUTATIONAL ARCHITECTURE

    In order to define the computational architecture to be used in this system, it is necessary to consider the diversity and heterogeneity of existing controllers.
    In this system, several local controllers must coexist: ABB robot controllers, BOSCH robot controller, Transportation subsystem controller (BOSCH PLC CL 300), warehouse controller, platform controller, CNC Milling and Turning machines controller.
    Each controller has its own facilities. It is, therefore, necessary to define an architecture able to integrate all these controllers, assuring a coherent and effective interoperability between them.
    This part of the project is not finished yet. It is our intention to design a distributed control architecture based on autonomous agents. An infrastructure to support negotiation and other forms of cooperation between agents is being investigated.
    The client-server pardigm in a network of UNIX machines, resorting to Remote Procedure Calls, is being used.

    4. CONCLUSION AND FUTURE DEVELOPMENTS

    the installed FMS/FAS system described above is not a finished or closed project.
    The purpose was to build an open infrastructure to which new functionality can be added and evaluated.
    The NOVAFLEX is intended as a dynamic system, able to adapt to the research challenges in Intelligent Manufacturing Systems.
    A set of internal research projects are now being started to explore, evaluate and extend the installed system, namely in the areas of supervision architecture's, perception systems and dynamic scheduling.

    Another area that can benefit from the existence of this unit is the field of Systems Modeling. Experiments with OOP languages, generalized Petri Nets and EXPRESS/STEP are being developed.
    As a benchmark, a toy clock - NovaCLOCK - was designed and is being used as our first test case.
    This benchmark illustrates various kinds of operations, requiring different robots:
    The insertion of pin P by one robot requires the second robot to hold the hand H.

    Detail of clock hand assembly

    NovaCLOCK

    This page was based in the paper "Development of a FMS/FAS System - the CRI Pilot Unit" by José Barata and Luís Camarinha-Matos.
    HTML version by João Silva.

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    Please send comments to jms@dee.fct.unl.pt
    Updated: February 9, 1996