Influence of three principles of pacing on the temporal organisation of work during cyclic assembly and disassembly tasks

A study was conducted to investigate the influence of different approaches to arranging the pace and temporal organisation of repetitive assembly and disassembly tasks on both average performance and its variability and to compare assembly and disassembly times derived with psychophysical methods to a more traditional methods-time measurement (MTM) approach. The conditions studied were a traditional assembly line arrangement, where assemblies were started at a pace of 110 MTM (repeated on two occasions), a batch condition, where subjects were required to complete 36 assemblies within the total amount of time allowed at 110, MTM and a psychophysical condition, where subjects were allowed to choose their pace (repeated on two occasions). Overall, the results suggest that the mean time spent working in each cycle (the ‘on-time’) remained fairly constant across conditions, while the idle ‘off-time’ in between on-times was shorter and of less varied duration in the more autonomous batch and psychophysical conditions. During the second psychophysical (self-paced) condition, subjects completed a significantly higher number of assemblies than during the 110 MTM line condition. The higher pace was achieved through reduction in mean off-times and the potential implications for musculoskeletal risk are discussed.

Statement of Relevance: Higher levels of autonomy over work pace, which intuitively would be beneficial from an ergonomics standpoint, actually led to subjects selecting to organise work such that off-times (idle times) were reduced. In contrast, active ‘on’ times were not affected much by autonomy. These results point to a reason that piecework would be associated with increased risk for musculoskeletal disorders.     

Maximum acceptable effort for connector assembly in automotive manufacturing

The manual assembly of connectors has job risk factors associated with musculoskeletal disorders including force, posture and repetitiveness. A variety of laboratory simulated connector assembly tasks, based on pilot work conducted at an automotive manufacturing plant, were studied using adapted psychophysical methods. The maximum acceptable frequency of connector assembly for six grip types was determined for various combinations of force and distances. In studies with a three-day acclimation and trials scheduled for 8-h days, 4-h trial lengths are sufficient. Distance did not influence acceptable frequency for hand/arm motions between 7 and 16 mm. There might be differences in maximum acceptable frequency for grip type, and force might affect acceptable frequency. Force × Cycle Rate (FCR) or time-weighted average percent maximum acceptable effort (TWA-%MAE) for these short duration tasks can be used for guidance.     

Modelling and control of an assembly/disassembly mechatronics line served by mobile robot with manipulator

The aim of this paper is to reverse an assembly line using a mobile platform equipped with a manipulator. By reversibility we mean that the line is able to perform disassembly. For this purpose, an assembly/disassembly line balancing (A/DLB) and a synchronised hybrid Petri nets (SHPN) model will be used to model and control an assembly/disassembly mechatronics line (A/DML), with a fixed number of workstations, served by a wheeled mobile robot (WMR) equipped with a robotic manipulator (RM). The SHPN model is a hybrid type, where A/DML is the discrete part, and WMR with RM is the continuous part. Moreover, the model operates in synchronised mode with signals from sensors. Disassembly starts after the assembly process and after the assembled piece fails the quality test, in order to recover the parts. The WMR with RM is used only during disassembly, to transport the parts from the disassembling locations to the storage locations. Using these models and a LabView platform, a real-time control structure has been designed and implemented, allowing automated assembly and disassembly, where the latter is assisted by a mobile platform equipped with a manipulator.     

A psychophysical study of high-frequency arm lifting

Ergonomics research on worker lifting in industry, and the many tools and methods that have resulted from it, have most often concentrated on the maximum amount of weight that a worker is capable and willing to lift in a given situation. In most psychophysical research on lifting, the frequency is one of a number of controlled variables along with container size, lift range, etc. Most of the relatively few studies that have investigated frequency as the response variable have used relatively heavy loads. In the study reported here, the focus was on the lifting of light weights and the subject acceptance of maximum frequency of lift for a two-handed lifting task. The lift range was set at approximately knuckle to shoulder height and was intended to simulate industrial jobs where the worker is tasked with either loading or unloading relatively light weight items to or from a processing line operation. Twelve college-age male subjects were used. Two conditions of weight, 0.7 kg (1.5 lb.) and 4.45 kg (10 lb.) were used and the subject adjusted his frequency of lift by communicating with the researcher, who adjusted a metronome to pace the task. The subjects were instructed to work at as fast a rate as they could for an hour period without becoming overheated, overly tired, out of breath or in pain. Measurements of oxygen consumption and heart rate were taken to supplement the psychophysical measure of lift frequency. Two replications of each weight condition were performed. At the conclusion of the metronome-paced sessions, an additional session for each weight condition was performed where the subject was instructed to lift as fast and consistently as they could with no external cuing device. The mean frequencies of lift identified in the experiment were 31.21 lifts per minute and 23.50 lifts per minute for the 0.7 kg and 4.5 kg lift weights respectively. The two weight conditions were significantly different from each other in their effects on subject metabolic energy expenditure with the subjects tending to work significantly harder physiologically at the heavier weight.     

ErgoSAM: A preproduction risk identification tool

Simulating work in a workstation with computer manikins is for many companies too expensive to acquire and master. An alternative method to detect high musculoskeletal loads early in the planning process is ErgoSAM. This article describes the users' and potential users' view of ErgoSAM and the suggestions of possible and desired improvements. Some improvements were introduced in a new version of ErgoSAM and validated at Volvo Car Corporation, Sweden. The new version demonstrates improved capacity to predict the occurrence of high loads on the operator when performing an assembly task, described in the MTM method SAM and in an assembly environment. © 2005 Wiley Periodicals, Inc. Hum Factors Man 15: 309–325, 2005.     

Maximum frequency acceptable to female workers for one-handed lifts in the horizontal plane

Abstract

A laboratory study was conducted to determine the maximum frequencies acceptable to female workers for one-handed lifts in the horizontal plane. A psychophysical method was used to determine maximum acceptable frequency for an 8-hour workday. Ten female college students were required to lift continuously three different loads to two different reach distances (38 and 63 cm) on a 91 cm high work table. The lifting task was paced by a repeating timer which the subject controlled according to her subjective feelings of fatigue. Heart rate and RPE were measured during the last 5min of the experiment to determine the physiological level of functioning and perceived exertion. Psychophysically determined maximum acceptable frequencies were compared with the standards based on methods-time measurement (MTM) analysis.

Statistical analysis showed that both the weight of the load and reach distance had a significant effect on maximum frequency acceptable to the subjects. No single value for percentage of maximum frequency can be used to establish permissible one-handed lift limits in women; rather, this value depends upon the weight of the object and distance of lift. The average maximum acceptable frequency was 51% of the maximum frequency that the subjects could maintain for a period of 4min. The subjects selected workloads which resulted in a mean heart rate of 101 beats/min. The subjects rated the perceived exertion ranging from ‘fairly light’ to ‘somewhat hard’. Performance based on MTM analysis ranged from 11% below to 32% above the maximum workload acceptable to the subjects. The non-significant heart rate differences found among the six load-distance combinations lend strong support for the use of psychophysical methodology in future studies of fatigue criteria. The study also supports the previous findings that separate physiological fatigue criteria are needed for tasks involving arm work and whole body exertion.     

The impact of poor assembly ergonomics on product quality: A cost–benefit analysis in car manufacturing

The study aimed at analyzing the relationship between assembly ergonomics, assemblability (“ease of assembly”), and product quality and at quantifying these relationships in economic terms. This was in order to better to support the development of more ergonomic product and assembly solutions, particularly at early stages of the car development process. The assembly of 24,443 cars was studied for 8 weeks in an assembly plant and for another 16 weeks as factory‐complete vehicles. The results show increased risks for quality errors of 3.0 and 3.7 times and total action costs that were 8.7 times and 8.2 times higher for high and medium physical load assemblies compared to low physical load assemblies for 55 tasks assessed. © 2009 Wiley Periodicals, Inc.     

An integrated approach to selective-disassembly sequence planning

De-manufacturing (DM) is defined as a process to disassemble certain parts or components from a product. The parts or components are selected for recycling, reuse, maintenance or disposal. Selective-disassembly as the disassembly of the selected parts is a key process in DM. Allowing a partial and non-procedural disassembly sequence in DM, selective-disassembly aims to minimize the number of removals regardless of assembly indenture levels. It is necessary for selective-disassembly to have an effective and optimal sequence planning in order to reduce tremendous time and cost involved in product DM. The Wave propagation (WP) method, a dominant approach to selective-disassembly sequence planning, focuses on topological disassemblability of parts. It is inefficient to achieve the aim because of two missed considerations: tool accessibility to a fastener in non-procedural and partial disassembly, and batch removability to directly access a part for separation or replacement.This paper presents an integrated approach to selective-disassembly sequence planning. The two examples presented here demonstrate that the approach is efficient and practical for DM. The implemented approach can efficiently generate a feasible and near-optimal sequence plan for selective-disassembly, with ensuring both batch disassembly of components and tool accessibility to fasteners.     

Improving assembly precedence constraint generation by utilizing motion planning and part interaction clusters

In this paper, we present a technique that combines motion planning and part interaction clusters to improve generation of assembly precedence constraints. In particular, this technique automatically finds, and clusters, parts that can mutually affect each other’s accessibility, and hence may impose assembly constraints. This enables the generation of accurate precedence constraints without needing to examine all possible assembly sequences. Given an assembly model, our technique generates potential disassembly layers: spatial clustering is used to generate part sets. Next, motion planning based on rapidly-exploring random trees (RRT) with multiple trees is used to evaluate the interaction between these part sets. Specifically, motion planning is used to determine which part sets can be removed from the assembly. These sets are added to the first disassembly layer and removed from the assembly. Part sets that can be removed from the simplified assembly are then added to the second layer. If the process gets stuck, parts in the parent set are regrouped, and the process continues until all disassembly layers are found. The resulting structure reveals precedence relationships among part sets, which can be used to generate feasible assembly sequences for each part set and the whole assembly. We present theoretical results related to the algorithms developed in the paper. Computational results from tests on a variety of assemblies are presented to illustrate our approach.     

复杂装配体多层次装配路径规划研究

针对传统装配路径规划方法应用于复杂装配体时出现的组合爆炸问题,提出了一种基于人机交互的改进A-Star(A*)算法多层次装配路径规划方法。算法在传统启发式路径规划算法的基础上引入了干涉威胁概率、平滑度代价、权重系数参数,实现了算法不同侧重方向的最优路径寻找。算法首先根据基于人机交互的路径规划方法结合操作者的装配经验将复杂装配体划分为多个装配层次段,其次对各层次段中的装配零部件运用改进A*算法求解最优拆卸路径,并最终根据路径反演原则生成整个复杂装配体最优装配路径。结合算法仿真对比与KUKA工业机器人路径规划实例验证,得出上述方法较传统启发式路径规划方法提升了路径规划效率,满足了工业生产中复杂装配体自动路径规划需求。     

Worker selection of safe speed and idle condition in simulated monitoring of two industrial robots

Industrial robots often operate at high speed, with unpredictable motion patterns and erratic idle times. Serious injuries and deaths have occurred due to operator misperception of these robot design and performance characteristics. The main objective of the research project was to study human perceptual aspects of hazardous robotics workstations. Two laboratory experiments were designed to investigate workers' perceptions of two industrial robots with different physical configurations and performance capabilities. Twenty-four subjects participated in the study. All subjects were chosen from local industries, and had had considerable exposure to robots and other automated equipment in their working experience. Experiment 1 investigated the maximum speed of robot arm motions that workers, who were experienced with operation of industrial robots, judged to be 'safe' for monitoring tasks. It was found that the selection of safe speed depends on the size of the robot and the speed with which the robot begins its operation. Speeds of less than 51 cm/s and 63 cm/s for large and small robots, respectively, were perceived as safe, i.e., ones that did not result in workers feeling uneasy or endangered when working in close proximity to the robot and monitoring its actions. Experiment 2 investigated the minimum value of robot idle time (inactivity) perceived by industrial workers as system malfunction, and an indication of the 'safe-to-approach' condition. It was found that idle times of 41 s and 28 s or less for the small and large robots, respectively, were perceived by workers to be a result of system malfunction. About 20% of the workers waited only 10 s or less before deciding that the robot had stopped because of system malfunction. The idle times were affected by the subjects' prior exposure to a simulated robot accident. Further interpretations of the results and suggestions for operational limitations of robot systems are discussed.     

Standing working posture compared in pregnant and non-pregnant conditions

During pregnancy, an increase in body weight occurs together with changes in body weight distribution and in fit between body dimensions and workplace layout. These changes may cause alterations in working posture which may, in turn, have adverse consequences for the biomechanical load on the musculoskeletal system and so increase the risk of musculoskeletal disorders. Using photographic posture registration, the standing working posture was studied in 27 women during the last stage of pregnancy and after delivery (the experimental group). The women performed an assembly task while standing at various workplace layouts. The postural differences between the pregnant condition and the non-pregnant condition were studied and the effect of the various workplace layouts assessed. Ten non-pregnant controls were also studied twice to establish the effect of the time interval between the measuring occasions. We found that the women of the experimental group stood further from the work surface in the pregnant condition compared to the non-pregnant condition, the hips were positioned more backwards, and, in order to reach the task, they increased the flexion of the trunk, increased the anteflexion of the upper arms, and extended the arms more. At the workplace layout in which the work surface height was self-selected, the postural differences due to pregnancy were smallest or even absent, compared to the postural differences in the other workplace layouts studied. Ergonomists and workers in occupational health services should be alert to the consequences for the biomechanical load on the musculoskeletal system and the risk of development of health complaints caused by postural changes due to pregnancy. An adjustable workplace layout may prevent some problems.     

基于循环迭代的飞机虚拟拆装过程建模方法

将循环迭代法应用于飞机虚拟拆装,研究飞机的拆装过程,提出一种基于循环迭代的虚拟拆装过程建模方法。结合飞机拆装过程动态循环的特性和虚拟人拆装行为并行相似的特点,建立虚拟人拆装行为参数化模型,规范虚拟人拆装行为的统一表达;在此基础上,分析虚拟人与部件的交互关系,构造虚拟人拆装行为、拆装次序、部件状态之间的迭代函数,建立虚拟拆装过程模型,实现虚拟人拆装行为的参数化描述和部件拆装过程的表达。以分布式虚拟维修训练平台中的集中故障显示接口组件拆装为例,对该方法进行仿真验证和比对,结果表明,以虚拟人拆装行为变化表示的部件状态变化使部件的拆装更易于表达,拆装训练效率有所提高。     

A Pareto-discrete hummingbird algorithm for partial sequence-dependent disassembly line balancing problem considering tool requirements

To reduce the disassembly costs to enterprises and improve the disassembly efficiency of waste products, this study proposed a partial sequence-dependent disassembly line balancing problem (PSD-DLBP) and established a multi-objective mathematical model to simultaneously minimize the number of workstations, total disassembly time, idle balance index and the number of disassembly tools. Then, a Pareto-discrete hummingbird algorithm (PDHA) was proposed to address PSD-DLBP effectively. The PDHA includes two stages: self-searching stage and information-interacting stage. With these two stages, the exploration and exploitation abilities of PDHA can be balanced. Later, the effectiveness and superiority of the PDHA were verified by comparing it with the other four algorithms for two different-scale examples. Finally, the model and PDHA were applied to the optimization of a partial sequence-dependent disassembly line of waste laptops. The optimization results show that the partial disassembly can make the line smoother and the utilization efficiency of workstations higher than full disassembly, and PDHA is superior in solving the PSD-DLBP.     

设备拆装混合现实系统中的拆装逻辑设计

针对机械部件在拆装过程中存在设备投资大、损耗程度高、危险性大、拆装效率低等问题, 本文采用混合现实技术, 设计出一套虚拟拆装系统. 其中, 拆装逻辑是拆装系统的核心, 拆装系统的通用性取决于拆装逻辑的设计. 为了解决虚拟拆装系统通用性差的问题, 本文通过对常见机械模型的连接方式和装配关系进行分析和总结, 设计出一种基于图遍历算法的数据结构来表达各个模型的逻辑关系, 并针对特种阀门、六关节机械臂等设备的拆装进行实验. 实验结果表明: 该拆装逻辑解决了拆装顺序的可定义和通用性等问题, 能够对模型部件按工艺流程进行虚拟拆解和装配, 最终实现虚拟拆装系统的通用性.     

Effect of cycle to cycle task variations in mixed-model assembly lines on workers’ upper body and lower back exertions and recovery time: A simulated assembly study

Differences in cycle-to-cycle work content in mixed-model assembly lines (MMAL) may increase exposure to risk factors of work-related musculoskeletal disorders (WMSD). This study investigated the effect of conveyor type and product mix/sequence in MMALs on joint loads and recovery time. An assembly task was simulated using 9 conveyor (continuous moving, synchronous indexing, and asynchronous indexing) and product mix/sequence (single product, 3 products with fixed sequence, and 3 products with random sequence) configurations. Results show the self-paced conveyor resulted in the least frequency of reaching. However, subjects did not use the available time for recovery and worked at a faster pace than in the continuous and synchronous indexing conveyors. In conclusion, an asynchronous indexing conveyor provided workers the flexibility they needed to complete their job correctly without reaching but recovery time was not sufficient.Practitioner summaryCycle-to-cycle task variations in MMAL may increase workers’ risk of developing WMSDs. An asynchronous indexing conveyor, with a minimum work time built in, may allow workers to complete their job with less reaching outside the reach envelope while providing sufficient recovery time.     

Simultaneous remote haptic collaboration for assembling tasks

Stand-alone virtual environments (VEs) using haptic devices have proved useful for assembly/disassembly simulation of mechanical components. Nowadays, collaborative haptic virtual environments (CHVEs) are also emerging. A new peer-to-peer collaborative haptic assembly simulator (CHAS) has been developed whereby two users can simultaneously carry out assembly tasks using haptic devices. Two major challenges have been addressed: virtual scene synchronization (consistency) and the provision of a reliable and effective haptic feedback. A consistency-maintenance scheme has been designed to solve the challenge of achieving consistency. Results show that consistency is guaranteed. Furthermore, a force-smoothing algorithm has been developed which is shown to improve the quality of force feedback under adverse network conditions. A range of laboratory experiments and several real trials between Labein (Spain) and Queen’s University Belfast (Northern Ireland) have verified that CHAS can provide an adequate haptic interaction when both users perform remote assemblies (assembly of one user’s object with an object grasped by the other user). Moreover, when collisions between grasped objects occur (dependent collisions), the haptic feedback usually provides satisfactory haptic perception. Based on a qualitative study, it is shown that the haptic feedback obtained during remote assemblies with dependent collisions can continue to improve the sense of co-presence between users with regard to only visual feedback.     

Cyclic-waiting systems

The paper analyzes a queuing system where customers are accepted for service either at the time of arrival (if the server if idle) or at the times that differ from it by intervals multiple of cycle time T. Formulas are derived to find the number of customers in the system, waiting time, and the existence condition for ergodic distribution.     

基于LabVIEW估算空转电流均值

LabVIEW可作为上位机应用在EPB耐久试验台，实现创建数据库、批量处理数据等功能。数据批量处理中一个重要的环节是估算空转电流的简单算术平均值。传统求均值的方法是直接从数据的初值和终值着手，但大数据多组初值和终值浮动范围大，直接法不适用。针对这种情况，首先采用估算空转电流初值和终值的算法“点积法”，再用以阈值插值一维数组模块，根据电流初值和终值找到空转电流开始时间与结束时间，最后由已知值求得空转电流均值。实验结果表明，该算法能有效地用于数据库估算空转电流数据均值，在批量数据处理中具有可行性。     

An experimental evaluation of psychophysical criteria for repetitive lifting work

Two experiments were performed to test the reliability and validity of psychophysically determined maximum acceptable workloads for setting lifting standards. The perceived workload in a repetitive diagonal lifting task was found to be a positively accelerated function of the weight lifted and of the work pace respectively. A twofold increase in objective workload resulted in a four- to fivefold increase in perceived workload. This relation was independent of previous occupational experience of lifting work. The psychophysically assessed maximum acceptable workloads for this type of lifting task appeared to be satisfactorily reproducible when subjects had to adjust work pace or when they were left free to adjust both the weight and the work pace. However, the results raised several questions concerning the applicability of the psychophysical assessment of maximum acceptable lifting work. Slight changes in the instructions given to the subjects had a definite effect on the selection of workloads. Furthermore, the workloads selected by subjects with previous occupational experience of lifting work — i e, warehouse workers — were systematically lower than those selected by subjects without such previous experience — i e, office employees. At the same time, the warehouse workers rated perceived exertion higher than the office employees, indicating that previous occupational experience of lifting work enhanced the subjective assessment of physical effort. There were no consistent relations between the workloads found acceptable by the subjects and their physical characteristics and performance capacity.