Cooperation of Man and Robot Assembly — an Evaluation of an Industrial Flexible Assembly System

Cooperation of Man and Robot Assembly an Evaluation of an Industrial Flexible Assembly System Peter Holmstedt, Lena Martensson. Anders Arnstrom (2) Department of Manufacturing System. Royal Institute of Technology, Stockholm, Sweden Received on January 7,1997

Abstract In 1986 a totally new concept for flexible automatic assembly cells was launched within the research team “Assembly group”, IVF/KTH, Department of Manufacturing Systems. A prototype, Mark 11, was built and tested in the laboratory, and the industrial version, Mark II F, was installed in a company in 1991, where it carried out the automatic assembly of air motors for handheld tools.

After five operational years, the system has been evaluated from a technical and human factors point of view. The paper will discuss the whole development process, from the initial conceptual idea spawned by the university research group, to the experiences gathered by management and personnel of the system. Keywords: Automation, assembly machine, human

1. Introduction

Smaller volumes of products are normally assembled manually, particularly if they consist of many variants. Common motives for investing in automatic assembly are shorter life cycles, increased number of customer oriented variants, demands on shorter lead-times and low capital cost of WIP. It is therefore important to find new automatic assembly solutions for situations in which traditionally manual assembly is dominant. 2. The Mark II Drinciple (1984)

The Swedish Institute of Production Engineering Research, IVF and The Royal Institute of Technology, KTH, in Stockholm - Sweden, have worked with flexible automatic assembly (FAA) since 1980. The following requirements were identified at an early stage as keyfactors for a successful FAA-system(1): Automatic assembly operations, Automatic materials handling and feeding, Automatic resetting and error recovery, Flexibility for new (never assembled) products In 1984, a system was built, called Mark I, that fulfilled these requirements. It was a “robot in a line system” (7). Three major drawbacks were identified: 0 One robot in line can handle only a small number of details, Extra handling was needed between parts manufacturing and assembly, Time loss in gripper exchange and in handling was to high. The solution to these drawbacks was the Mark II system which exploited the three key features of sub-batch

Annals of the ClRP Vol. 46/1/1997

principle, vision and reduced number of gripper changes: (see figure 1) Feeding unit

t Fig 1.The flows in the sub-batch principle of Mark I1 For explanation, please see text below

The sub-batch principle: One robot could be used to assemble any number of components, assuming one part-type was assembled at a time on a train of a !ixed number of fixtures. The part-types are fed from another train where part-type A is stored in “traincarrier l ” , parttype B in train-carrier 2,etc. This system concept allows one robot to assemble so many part-types as there is room for in the feeding system. (see fig. 1) Vision: In order to avoid manual handling between parts manufacturing and assembly vision was used. Components were put in one layer only. Reduced number of gripper changes: If, for instance, 10 fixtures could be used in the fixture train. gripper changing time will be reduced by 90%.

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3. A ioint venture Droiect: lndustrv - Universitv (198586) Atlas Copco Tools A 6 (ACT) is developing, marketing and producing handheld tools for tightening, drilling, grinding etc. A strategic component of the tools are the airmotors. ACT showed a deep interest in both Mark I and Mark II, where one of the applications was the airmotor. The beginning of the ~ O ‘ S was , a tough period for ACT. One of the major problems was the excessively long lead times. A new production concept was developed based on “total flow“, customer order control and a FAA-system integrated in the assembly area. Because of a continuously increasing number of product variants, the assembly area was identified as a bottle neck in the production system (5).ACT therefore in 1985 decided to join the “assembly system research team” at KTH and IVF in Stockholm, in order to increase the assembly systems competence within the company. The joint venture project was specified as follows: An ACT-employee and payed researcher (one of the authors), was placed in the research group at KTH full time from 1986 until 1989. An ACT factory, the Tierp works, was linked to the research group. An ACT reference board on top management level was linked to the project. An ACT research budget was allocated to the project. after decisions from the reference board.

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4.Princi~le- Laboratory prototvPe Industrial application (1986-90) The realization of the Mark Il-principle into a laboratory prototype, was developed through a configuration of three interacting system units: The feeding unit, the fixture unit and the assembly unit. The system principle is the same in all cases but the application itself is adapted to the special needs and environment required by each case. The laboratory prototype is in itself a special application of the Mark IIprinciple, with the aims to test, evaluate and explain the system principle in a pedagogic way. The hardware configuration in the laboratory is given in table 1: Function Assembly and recovery Feeding unit Feed one part type at a time Fixture unit Present one fixture at a time to the robot Table 1 System units of Mark I1

Principle unit Assembly unit

Lab-realization ASEA Irb 1000 Vision system Flat boards in boxes, on a roller conveyer Single polymere fixtures on a chain conv. subbatch = 10

The evaluation of the lab-system indicated that (4): the system principle worked out as expected, a relative large floor space was needed, resetting time was short but not eliminated, complex control- and programming environment The next step was to find a suitable application for ACT, based on the Mark Il-principle, the laboratory evaluation

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and the ACT factory environment. How to eliminate the assembly area as a bottle neck remained a key issue (5). In 1988 ACT decided to develope and integrate a FAA system for assembly of the airmotors in the assembly area, together with the research group and based on the Mark II results (2). The reasons for the investment and the answers to the bottle neck key-questions were: A possibility to shorten the lead times Increased flexibility Automation because of a prediction for lack of work force on the 90’s. The hardware configuration for the ACT-application “Mark Il-Fis given in table 2: Principle unit Assembly unit

Factory-realization Adept Scara robot Vision system

Feeding unit

Flat boards in boxes, in an automatic paternoster feeder

Fixture unit

Single active metal fixtures on an indexing table. subbatch = 8

Function Fast assembly and recovery. Open controller. Feed one part type at a time without setup. Reduced floor space needs. Present one fixture at a time to the robot

Table 2 System units of Mark IIF The Mark Il-F system was built up at the system supplier’s premises, for programming and testing (2).This development phase resulted in being far longer than expected, mainly due to three factors: More complicated programming than expected, Modified assembly principles and methods, Mechanical break down of the paternoster feeder. The test results before installation indicated that (figure 5): Resetting time = 0 Number of initial variants in the system = 43 Manual operations needed were to fill the paternosterieeder, to order variant and number of airmotors and to pick up finished airmotors. Some processes were not stable enough.

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Fig 2 The Mark I/-F application

Fixture unit

5.lnstallation in an industrial environment (1990-91) During the development of MARK I1 F it was assumed that the system should be operated by the assembly workers who should order their airmotors directly from the computer terminal of MARK II F. The system would run unmanned. It turned out that the programming of the system took much longer than expected. MARK II F was truly a complex system, tailor made for the company in it's application. It was the first system of its kind, the different subsystems had never been designed to be put together into this particular system. Different kinds of disturbances occurred which made it clear to management that one operator per shift was needed to supervise the system. To this end two operators were given one week of training in how to run the robot, how to enter data for ordering the airmotors and how to load the paternoster feeder. Furthermore the production engineer was trained in handling the system and in the basic concepts of the programme. The adjustment of the 'programme was taken care of by the programmer of the supplier company. To the assembly workers this turned out differently than intended. Instead of dealing directly with the equipment, they now give their order of airmotors to the operator of the system. Simultaneously with the implementation another big change occurred within the company. A sister company was closing down and the production should be taken over by the Tierp-plant. The production engineer was occupied with this change for most of his time, and although he was the only one in the company knowledgeable in the programming of MARK I I F, he did not have the time to ensure the system's smooth operational status was achieved, see also (9).

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6. Production a continuous evaluation (1991-96) At an early stage in the development of MARK I1 F it was agreed that a human factors evaluation should be made. The assumption was that the introduction of a flexible automatic assembly system would increase the qualitative content of assembly work. In June 1990, one year before installation a questionnaire was handed out to the personnel of the assembly department, (54 persons). The topic was their work content and their expectations on MARK II F. In the results of the questionnaire it was shown that the assembly workers were very pleased with their work content (8).To assemble the product of the company, handheld tools, nine different groups were given the full responsibility for the assembly. Their tasks were planning, airmotor assembly, subassembly of other parts, final assembly, testing of every tool and packaging. This way of organizing the assembly work gave the workers a comprehensive understanding of the assembly process. Job rotation took place according to the wishes and needs of each individual worker, no particular scheme was set up for this. Consequently the risk for musculo-skeletal disorders was minimized.

The expectations on the robotic system were very high. Assembly and adjustment of airmotors was considered to be the least attractive assembly task due to difficulties in estimating the correct function of the airmotor. MARK II F was installed at the company in June 1991 and in December 1992 a second questionnaire was filled in by all assembly workers, at the time 71 persons. Among the results could be mentioned that for those 43 individuals who took part in both questionnaires the cognitive activity was judged more positively in the second questionnaire which can be explained by other factors than the introduction of MARK II F. An IS0 9000 certification for the company has led to a more elaborate testing of all tools produced giving more cognitive activity. The tasks of the operators of the system were planning, material supply, monitoring, intervention and handling of minor disturbances, and manual adjustment of the airmotors. The latter task had not been planned when designing the. robotic system and was therefore not programmed into the system. This led to dissatisfaction for the operators as this manual task took too much of their time. These operators are pleased to have learnt something about computer support in the production. They are however not pleased with the training they have been given, particularly with the lack of knowledge about the programme. Not understanding why the system behaves as it does is annoying to the operators. As a consequence many errors occur which the operators do not know how to handle. They have to ask the external programmer for assistance. Not having the technical competence within the company is a costly disadvantage and results a less optimal usage of the system. From a technical point of view, ACT had the following experience(6): Changed assemby principles and methods increased the cycle time and decreased process stability. Instable assembly processes required security functions in the software which increased the cycle time Multifunctional grippers and the fixation of the vision camera to the robot, were not stable enough and caused vibrations. In 1995 the assembly department delivered 3 000 tools per week, 1500 of those had their airmotors assembled by the robot. There is a general positive attitude within the company towards the introduction of automated equipment. It is however the experiences of management that it is much more difficult to automate assembly than manufacturing due to the customer order control, small batch sizes and a high number of product variants. The implications of this is a continuous resetting between different variants, which requires stable and reliable assembly processes. When it comes to introduction of automated assembly systems in the company, management have made the following experiences (3): specify the technical and human demands on the equipment together with the supplier in order to get a joint frame of reference; be careful and conscientious about the testing of the hardware and the software, which should be carried out by the supplier before delivery:

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a resource of production engineering must be available within the company not only during the installation phase but also for a long period to follow: engage a project manager all through the project. If there is no project manager the project looses in "status", which may prolong the installation process: involve not only production engineers, but also operators in the project and in the installation work. It is not unusual at the introduction of new technology that the training of the operators is given too little attention during the project planning phase. The balance between level of technology and activities for training of the operators is critical. Investing a lot of money in a highly sophisticated equipment does not pay off unless the staff gets a proper training. Ideally the operators should take part in the design of the equipment which is a learning phase just as valuable to the end-user as to the designer. Involvement of the users increases the possibilities for a successful implementation of the equipment and for the operators to take on a greater responsibility.

7. Conclusion Introduction of new technology, like the MARK II Fassembly system, must be handled in "the right way", from both a strategical and operational point of view. From a strategical point of view, our conclusions from this project are: Technical comoetence needed: Make sure that you have the competence needed. If not, you run the risk of being dependent upon subsuppliers you can't control, like the programming competence in the MARK II F-project. Focus on the oroiect: Make sure that you focus on the project from the very beginning until you have a stable production and with the same project leader. Also use a co-project leader so that you have a "redundant management system". Adapt to the manufacturina strateqv: Make sure that the new technology fit's into the companies manufacturing strategy. When, for example, you choose the control environment for your assembly system, make sure that the control system is "open enough" and easy to program and/or similar to present control systems. Mixed- and steowise automation: Make sure and plan for a stepwise introduction of the automation technology and an acceptable mix between manual and automated operations. From a operational point of view, our conclusions from this project are: Robust assemblv processes: Make sure that the assembly processes intended for automation, are 100% stable before introducing the system to the factory floor. If not, the operators loose faith and you loose cycle time because of two reasons: System stop and also increased cycle times when trying to compensate for instable processes in the software. Robust svstem hardware desian: Make sure you identify weak points in the system hardware design at an early stage: Grippers must be reliable, for example

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don't combine to many functions in the same gripper, the fixation of the vision camera must be related to the accelerations and "speed- forces" on the camera itself in order to avoid vibrations etc. Trainina of oDerators: Make sure that the operators will be trained properly, before and after the system introduction, so that they can take active part in all phases of the system introduction. Svstem care: Make sure that the operators and the production engineers "care for the system" together. Training is one way, a deep involvement through teams during the whole process, is an other way. We are sure that these aspects, even though they stem from one installation only, are valid in a generalised way. There is more behind designing and implementing a new assembly system than meets the eye. 8. References

Arnstrom A., Grondahl P., 1987, A high speed small batch automatic assembly system with . flexible feeding and automatic set-up, proceedings 8th Int. Conf. Assembly Automation in Copenhagen Denmark, 237-248 Arnstrom A., Holmstedt P., Erlandsson A., 1990, Mark II for airmotor Assembly - An Industrial Case Study, 11th Int. Conf. on Assembly Automation, Nov. 11-14, Dearborn, Michigan, USA Arnstrom, A., MBrtensson, L. , 1993, The Assembly System MARK II F - Evaluation of the System in Operation, Report Department of Manufacturing Systems, Royal Institute of Technology, Stockholm, Sweden. In Swedish. Grondahl P., Holmstedt P., etc., 1990, Evaluation of the Flexible Automatic Assembly System Mark II Technology, Capacity and Economics, Report IVFlKTH Department of Manufacturing Systems, Royal Institute of Technology, Stockholm, Sweden, TRlTA LVE-9004. In Swedish Holmstedt P., 1989, Flexible Automated Assembly according to the Mark Il-principle, in an industrial perspective, Licentiate Thesis, at Department of ManufacturingSystems, Royal Institute of Technology, Stockholm, Sweden, TRITA-LVE Holmstedt P., 1994, Evaluation of Mark 11-F, an evaluation report for the Swedish National Board for Technical Development - NUTEK. In Swedish. Makino H., Arai T., 1994, New Developments in Assembly Systems, ClRP Annals 1994, pp 50151 1, Hallwag Ltd, Berne MBrtensson, L., 1995, Requirements on Work Organisation - Doctoral Thesis at Department of EnvironmentalTechnology and Work Science, Royal Institute of Technology, Stockholm, Sweden, ISRN KTH/IMA/R-95/2--SE, ISSN 11042656. Wiendahl H-P., Thies J.M., Zengtrager K., 1996, Construction and start-up of complex assembly systems, ClRP Annals 1996. Hallwag Ltd, Berne