Abstract of Special Research Report (SRR-No.21)

National Institute of Occupational Safety and Health, Japan

Development of Comprehensive Safety Control Measures for Production and Construction Systems (Second Report; Development of Hazard Evaluation Methods for Large Scale Industrial Systems and Safety Control Measures for Construction Robots)



: As large scale computerized industrial systems such as chemical plants, factory automations and automated building construction systems has been used in many industrial fields, a comprehensive safety measure for these systems has become great concern for industrial safety.
    The specific research on "Development of comprehensive safety control measures for production and construction systems" is planned from 1997 to 2001 for this reason.
    This research aims mainly at establishing systematic hazard evaluation methods, safety control measures and safety validation methods for many automated and computerized industrial systems.
    Following research subjects are planned in this specific research.
  (1) Survey of actual conditions and specifications for industrial systems.
  (2) Establishment of hazard evaluation methods for chemical plants.
  (3) Establishment of hazard evaluation methods for large scale construction systems.
  (4) Development of a human-error prediction estimator.
  (5) Development of safety control system for construction robots.
  (6) Development of safety control system for factory automations.
    This "Second Report" deals with research subjects about (3),(4) and (5), as the "First Report" published in 1999 described results of research subject (1).
    Chapter 2 and Chapter 3 are related to the research subject (3). The propose of Chapter 2 is to simulate the collapse of scaffolds due to wind and to evaluate the reliability of the scaffolding system in large scale construction systems. The new layout design of wall ties is proposed in this study when mesh sheets or solid sheets are used.
    The propose of Chapter 3 is to propose new non-linear optimization algorithms for structural reliability analyses in large scale construction systems. The proposed algorithms have not only efficiency but also superior generality, robustness and calculating capability.
    Chapter 4 is related to the research subject (4). The propose of Chapter 4 is to develop a human error prediction estimator for factory logistic systems, automated building construction systems, etc. The function, structure and operation procedure for this estimator are described.
    Chapter 5 and Chapter 6 are related to the research subject (5). Chapter 5 is the report of the committee for safety control measures of construction robots held in 1999. Present safety technologies and future research subjects are described in this report.
    The propose of Chapter 6 is to develop new safety control systems for construction robots. Safety systems for man-machine cooperation are proposed in this study. Inherent safe actuations and a hierarchical safety control are also proposed.

Hazard Evaluation for Collapse of Scaffolds due to Wind

Katsutoshi OHDO, Yasumichi HINO and Yoshinori YONEYAMA

: Temporary scaffolds are typically covered with plastic sheets in Japan to prevent construction equipment from falling from the scaffolds. However, wind loads which act on the scaffolds are significant due to these sheets. This seriously impacts safety because wind loads have a large effect on the stability of the scaffolds due to the scaffolds' inherent instability under horizontal loads. In fact, the scaffolds often collapse under strong winds during construction and many people have been injured and killed.
    The first design code for scaffolds under wind loads was provided about twenty years ago to prevent these fatal accidents, but despite the introduction of the new code several accidents have happened. In the design code, calculation methods of wind loads which act on the scaffolds were described, but the collapse mechanism of the scaffolds due to wind has not been examined in detail. When scaffolds collapse under wind loads, they frequently collapse progressively. This progressive collapse often leads to the local failure of a member, causing suddenly large-scale accidents. In the design code for scaffolds, only the strength of each member is checked, but progressive collapse scenarios are not considered. Therefore, to prevent such large-scale collapse accidents, it is necessary to examine the reliability of the scaffold structural system in light of these local failures.
    In this study, progressive collapse of these scaffolds under winds was simulated and the reliability of the scaffolding system was evaluated. The reliability of these structural systems was analyzed by modeling the scaffolds as series and parallel systems. The scaffolding system is comprised of complex frame structures and has a very large number of failure modes. To avoid enumerating numerous failure modes, an optimization method was employed in the analysis and the probability of collapse was analyzed only for the dominant failure modes. In determining the reliability, the limited state equations involved non-normal variables, e.g., non-normal wind speed. For incorporating the nonnormal random variables into reliability analyses, the first-order reliability method (FORM) was used in the analysis. In this study, the collapse mechanisms of typical scaffolds are examined.
    The results show that there may be a high risk in the integrity of the scaffolds, even if the safety of the scaffolds was implied by the design code. It is proposed that the ties need to be installed within every 2 stories and 1 bay (or 1 story and 2 bays) when mesh sheets or solid sheets are used.

Improvement of a Non-Linear Optimization Algorithm for Structural Reliability Analyses

Tetsuya SASAKI

: Recently, the structures are becoming more complex and performance requirements are becoming more ambitious. On the other hand, the requirements for safety and cost reduction are also becoming more demanding. To balance these inconsistent requirements, the need for a structural reliability analysis, which employs probabilistic information of loads, material properties and geometry of components, has been growing. In general, a structural reliability analysis requires the failure probability of the structure which is defined by the multi-dimensional integral of the joint probability density function of basic probabilistic variables. However, it is usually difficult to directly compute this multi-dimensional integral because of the arbitrary nature of the integration domain and typically high dimension of the problem. To overcome these difficulties, indirect methods such as the first-order reliability method (FORM) or the second-order reliability method (SORM) has been proposed as well as the Monte Carlo simulation methods.
    An important step of FORM/SORM is to find a design point, the point on the limit-state surface of minimum distance to the origin in the standard normal space, and the total computation time of FORM/SORM mainly depends on this procedure. To find a design point is also useful for the Monte Carlo simulation method because once a design point is found, the failure probability can be efficiently computed using the importance sampling technique weighted around the design point. Basically, any constrained non-linear optimization algorithms may be applied to find a design point and a variety of algorithms have been applied or proposed to solve structural reliability problems. However, recent development of the finite element reliability analysis requires more efficient algorithms because it takes much more time to calculate the inexplicit limit-state function by the finite element method.
    In this paper, some modifications are made to an existing constrained non-linear optimization algorithm in order to improve the efficiency to find a design point without losing generality, robustness and capacity. Through numerical examples typically appearing in structural reliability analyses, the proposed algorithm is compared with existing algorithms which have been already shown to be suitable for structural reliability analyses. The comparison reveals that the proposed algorithm has not only efficiency but also superior generality, robustness and capacity.

Development of a Human-error Prediction Estimator

Jian LU, Shigeo UMEZAKI and Kiyoshi FUKAYA

: As for recent labor accidents, it is said that, while the accidents which originate from equipment defects decrease relatively, the accidents which originate from human factor such as the human-error of the worker increase. However, even as for the accidents that seem to originate from human error, it is not rare that the equipment defects are found still to be the basic reason, after some detailed investigation.
    In this research, for large-scale production and construction systems, such as automatic building construction systems, and logistic lines in Factory Automation (FA), a real scale simulation system is developed. Being capable of extracting equipment defects that may trigger the human-error of workers, this system is called as "Human-error Prediction Estimator". With the latest Virtual Factory (VF) technology, this system can be used to verify beforehand the prevention measures against human-error and the defect in the layout, and to verify the easiness of the maintenance work and the effectiveness of safety methods of equipment.
    In this paper, the necessity, the function, the structure and the operation procedure of the estimator are described. In addition, as the estimator is considered also to be applicable to integrated support system for safety design which is to be developed according to our future research plan, the related consideration is also examined for constructing the system.

Report of the Investigation Meetings on the Safety Control Technology for Construction Robots and Research Subjects

Hiroyasu IKEDA, Shigeo UMEZAKI, Tetsuya SASAKI, Shoken SHIMIZU, Hajime TOMITA, Jian LU, Katsutoshi OHDO and Seiji TAKANASHI

: In many cases, the construction industry is, unlike production industry at large, comparatively dependent on manual work based on judgments by workers as there are a few repetitive work For this reason, industrial robots used in manufacturing are not adequate to be applied construction sites. In fact, the construction robots are required to have transfer technology and high-grade intelligence, which are beyond the technological category of the current industrial robots, but the current construction robots have not yet reached such a high technological level. Particularly, as to the robots moving around in an environment in which they are working jointly with human workers unless the safety technology for assuring safety for the human workers has been established, the construction robots would not easily spread.
    Under the circumstances, our institution held an "Investigation Meeting on the Safety Control Technology for Construction Robots" for the purpose of identifying safety and technological problem; with the current automatic construction systems and construction robots. This meeting was held four times in total. The institution listened to the attended experts in robots from major construction companies and universities about the subject together with the presentation of construction robot introduction cases, and discussed problems with the automatization of construction work.
    This chapter summarizes examples of the automatization and robotization of construction work describes the current technology for such automation and robotization, and identifies the problems with such automatization discussed at the meetings.
    The construction robot introduction cases to be covered in this report encompass a wide applications, including material installation, concrete floor finishing, concrete placing, fire-resistant coating material spraying, external wall installation, material transfer, welding and interior construction On the other hand, element technology cases for the automatization and systematization of construction work encompass automatic building construction system, automatic crane and high-place work, remote control, environment (sign) recognition, handling, teaching, etc.
    As for the safety and technological problems, typical items were picked up from each field related to sensing, manipulator and transfer mechanism, control and system, and analyzed. As a result the limited applications of the current general-purpose robot technology and the immaturity of the safety related technology were brought into sharp relief. That is, it was found that the performance of assuring the safe motion of movement and manipulation had not yet been realized, and the safety control of controllers and communicators had not yet been established. It was also found that there were many assignments to be fulfilled before developing the construction robot, such as the risk assessment of special working environments different from factories and designs.
    Based on the above investigations and findings, this chapter summarizes the problems to be solved and the possibility of solutions to such problems.

Development of the Safety Control System for Construction Robots --Inherently Safe Manipulator and Hierarchized Sensing System--

Hiroyasu IKEDA, Noboru SUGIMOTO, Shoken SHIMIZU and Jian LU

: Recently, the introduction of robots into construction work, in which automatization and labor savings are in rapid promotion, has been strongly desired. In building interior construction work, for example, as it involves a lot of manual labor, simple machines are used to ease the heavy, muscular labor of human workers. A system for enabling robots to collaborate with human workers in similar construction work is under research for future realization. However, according to this system, safety is not ensured for human workers in coexistence with robots, and safety work cannot be realized by merely taking measures to prevent human workers from approaching the danger condition, which is the basis of safety assurance under the conventional safety standards.
    This study is to propose a safety robot control mechanism which enables robot arms for building interior construction work to achieve the intended work while securing the safety of human workers.
    Even if the robot happens to touch human workers, the robot should not be permitted to apply an excessive impact or pressure over the allowable limits to human bodies.
    In order to satisfy these safety conditions, the construction working robot is classified under new robot class based on stop condition and designed based on the principle of machinery safety. This safety design principle which all machines must follow explains the risk reduction process. However, the inherently safe robot can be achieved by removing the hazards so that it may not produce risk itself. The functional safety will be conducted after the execution of the approach of such inherently safe for far remaining hazards.
    This report adopted the inherently safe actuator as an inherently safe technology which could be applied to the construction working robot, and also adopted fail-safe and hierarchizing interlock of sensors for the obstacle detection as a functional safe technology.
    The hierarchizing interlock of sensors is always monitoring the surrounding area by using a radar sensor with a self-diagnosis function. When the human worker approaches the robot, this system switches the maximum speed to a lower speed. Furthermore, when the human worker touches the soft-touching bumper switch of the robot, this system makes the robot stop immediately. At this time, the radar sensor, the bumper switch and the speed monitoring function for the actuator should not fail to the danger side. Therefore, in order to ensure the stopping process of the control system, the presently conceivable highest level of safety should be applied to the fail-safe and redundant technology in combination to these devices and the function.
    The inherently safe actuator system can regulate its output according to the work. This system has a device using Magnet-Rheological (MR) fluid and a friction brake together. The MR device can control mechanical force by making use of the characteristics of the MR fluid, and can regulate the torque to be transmitted from the motor output axis to the arm axis by changing the magnitude of the current applying to the MR device. Therefore, in any motion of the robot arm which may cause collision of the robot arm with a human worker, the transmitted torque is limited to adapt the robot arm to disturbance, and in any other motion of the robot arm, a strong servo control restrains disturbance.

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