Strategies in performing a manual assembly task

A study of naïve participants' strategies in manual assembly is reported. Four groups of ten participants assembled hacksaws under varying conditions of the number of aids given to participants. The aids were provision of an assembly jig, instructions on use of the jig and components set up in an ergonomically-designed workplace. Assembly took place under four conditions: (a) with no jig or instructions (b) with a jig and with no instructions on its use (c) with a jig and with instructions and (d) with a jig, instructions and also an ergonomically designed workplace in which all parts were placed within the zone of convenient reach. Video recording was used to measure performance times and strategies in assembling the hacksaw. The 40 participants used a total of 32 models of liaison sequence and 152 patterns of assembly sequence. Participants used many different strategies in their early learning and generally settled down to a single pattern after the early trials. The common strategy of participants was to pick and assemble the longer and heavier components, followed by small and lighter components.Relevance to industryParticipants showed many different patterns of assembly, even for a simple product. The data indicate a need for the industrial engineer to determine the ergonomically best layout of components for assembly and demonstrate the best assembly sequence to the operator.     

A comparison of tactile,auditory, and visual feedback in a pointing task using a mouse-type device

A mouse was modified to add tactile feedback via a solenoid-driven pin projecting through a hole in the left mouse button. An experiment is described using a target selection task under five different sensory feedback conditions (‘normal’, auditory, colour, tactile, and combined). No differences were found in overall response times, error rates, or bandwidths; however, significant differences were found in the final positioning times (from the cursor entering the target to selecting the target). For the latter, tactile feedback was the quickest, normal feedback was the slowest. An examination of the spatial distributions in responses showed a peaked, narrow distribution for the normal condition, and a flat, wide distribution for the tactile (and combined) conditions. It is argued that tactile feedback allows subjects to use a wider area of the target and to select targets more quickly once the cursor is inside the target. Design considerations for human-computer interfaces are discussed.     

Mechanical feedback analysis of a ferrofluid-based module with 2D dynamic traveling waves for tactile display application

With the advances of human-machine systems, tactile displays have become one of the important features for modern products. Tactile feedback can increase working efficiency and help humans to explore new environments or objects by the sense of touch. This study used a 3 × 3 electromagnet array and a ferrofluid bladder to build a tactile display module, which can create smooth and continuous real-time 2-D dynamic traveling waves. The interactions of magnetic fields between electromagnets in the array were used to control the directions of the magnetic lines of force to create different graph patterns. Our user test showed that the overall tactile perception rate was 74% for the 2-D dynamic graph patterns generated using the ferrofluid-based tactile display module.     

Maintaining and modifying pace through tactile and multimodal feedback

Older adults are recommended to remain physically active to reduce the risk of chronic diseases and to maintain psychological well-being. At the same time, research also suggests that levels of fitness can be raised among this group. This paper describes the development and evaluation of a mobile technology, which enables older adults to monitor and modify their walking habits, with the long-term aim of sustaining appropriate levels of physical activity. An empirical study was conducted with twenty older adults to determine the feasibility of the proposed solution, with results indicating that tactile signals could be perceived while in motion and could support participants in walking at a range of paces. However, the effects were difficult to discern due to limitations of the hardware. In response, a novel low-cost prototype was developed to amplify vibrations, and effectiveness of redundant auditory information was investigated with the goal of enhancing the perception of the cues. A second study was conducted to determine the impact of multimodal feedback on walking behavior. Findings revealed that participants were able to maintain a desired level of pace more consistently when redundant auditory information was presented alongside the tactile feedback. When the visual channel is not available, these results suggest that tactile cues presented via a mobile device should be augmented with auditory feedback. Our research also suggests that mobile devices could be made more effective for alternative applications if they are designed to allow for stronger tactile feedback.     

Basic complexity criteria and their impact on manual assembly quality in actual production

Increasing design and assembly complexity are challenges facing the automotive industry today because increasing number of car variants and build options can result in immense difficulties and lead to costly assembly errors and quality losses. In order to remain on the market these conditions must nevertheless be managed by companies in hard competition with other manufacturers.The objective of this study was to analyze the impact of newly developed basic complexity criteria (CXB) on assembly quality and associated costs for corrective measures in manual assembly of cars. Data on error rate and action costs of assembly tasks of different complexity level was collected and analyzed. The inter-relationship between different complexity criteria was analyzed to see whether any criteria had a greater impact than others.The results showed that the action costs/car increased with increasing complexity level and that several complexity criteria together resulted in increased action costs. Some criteria tended to have a greater impact than others but need more research. The results further suggest that if high complexity issues are identified and replaced by low complexity solutions the assembly related action costs in manual assembly are likely to decrease.Relevance to industryBy reduction of basic assembly complexity already in early planning stages in product development significant reduction of costly assembly related action costs in manual assembly can probably be made.     

Assembly failures and action cost in relation to complexity level and assembly ergonomics in manual assembly (part 2)

Earlier studies have demonstrated strong relationships between manual assembly at high physical load levels and increased amounts of quality defects compared to assembly at low physical load levels. A recent Swedish interview study of engineers in design and manufacturing engineering indicated that assembly complexity factors are of additional importance for the assembly quality. The objective of this study was therefore to examine the significance of high and low complexity criteria and the relationships between assembly ergonomics and assembly complexity and quality failures by analyzing manual assembly tasks in car manufacturing. In total, 47 000 cars were analyzed and the results showed several significant correlations between assembly ergonomics and assembly complexity, assembly time, failures and action costs. The action costs for high complexity tasks were 22.4 times increased per task per car compared to low complexity tasks.     

Applying a fusion of wearable sensors and a cognitive inspired architecture to real-time ergonomics analysis of manual assembly tasks

High value manufacturing systems still require ergonomically intensive manual activities. Examples include the aerospace industry where the fitting of pipes and wiring into confined spaces in aircraft wings is still a manual operation. In these environments, workers are subjected to ergonomically awkward forces and postures for long periods of time. This leads to musculoskeletal injuries that severely limit the output of a shopfloor leading to loss of productivity. The use of tools such as wearable sensors could provide a way to track the ergonomics of workers in real time. However, an information processing architecture is required in order to ensure that data is processed in real time and in a manner that meaningful action points are retrieved for use by workers.In this work, based on the Adaptive Control of Thought—Rational (ACT-R) cognitive framework, we propose a Cognitive Architecture for Wearable Sensors (CAWES); a wearable sensor system and cognitive architecture that is capable of taking data streams from multiple wearable sensors on a worker’s body and fusing them to enable digitisation, tracking and analysis of human ergonomics in real time on a shopfloor. Furthermore, through tactile feedback, the architecture is able to inform workers in real time when ergonomics rules are broken. The architecture is validated through the use of an aerospace case study undertaken in laboratory conditions. The results from the validation are encouraging and in the future, further tests will be performed in an actual working environment.     

Modelling of shoulder and torso perception of effort in manual transfer tasks

The aim of the present study was to develop statistical models of perceived effort at the shoulder and torso levels associated with manual load transfer tasks. The motions were directed from a home location toward one of twenty-two target shelves distributed in the right hemisphere. A total of 2149 ratings were obtained from 31 subjects for effort perception at the selected joints, using a ten-point modified Borg scale. Regression models, developed for the perception associated with each body part, included target locations (azimuth, height and distance), posture constraints (standing or sitting), task types (one or two handed transfer conditions), and demographic and anthropometric measures (stature, body weight, gender, and age) as parameters. The models provide a prediction of effort perception with adjusted r-square coefficients of 0.41 and 0.50 for the shoulder and torso, respectively. The results indicate that space and posture interact in a complex way to affect the rating of perceived effort, and are in agreement with the hypothesis that the ‘sense of effort’ is primarily associated with the efference copy of the descending motor command. Since a level of effort is not associated with a unique pattern of motor command, it is proposed that effort perception is likely to result from a summation of the components of the motor command. The models can be applied to optimize the spatial organization of the work environment in an attempt to reduce the risk of musculoskeletal injury.     

An experimental study on augmented reality assisted manual assembly with occluded components

Augmented Reality (AR) technology has increasingly been applied to facilitate manufacturing tasks such as training, maintenance, and safety management, in which manual assembly frequently occurs. Previously, studies confirmed the values of AR-based assembly systems, but few explored support of spatially restricted assembly where components are visually occluded from operator. In this regard, this research aims to develop new AR functions that assist manual assembly in such situations. The focus is on validating the effectiveness of various assistive information in AR, including assembly interface, operator’s hand movement, and operator held components. Subjective and objective measures are used to evaluate assembly experiments of different degrees of difficulty. Analyses of the experimental data reveal whether or not and how effectively each information offers guidance in occluded condition. In particular, a time reduction in more difficult assembly is realized by showing operator hand movement in AR. The hand model initially offers an operator a visual clue for quickly and roughly locating the assembly interface in a large unseen area, prior to precisely localizing in a smaller region guided by tactile sensing. However, the effectiveness of incorporating the held component is not evident, as positional deviation between real and virtual objects may reduce human’s hand-eye coordination. These findings not only provide preliminary design guidelines of AR assembly functions for occluded components but also demonstrates a novel yet practical application of AR technology in smart manufacturing.     

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.     

Parametrization of manual work in automotive assembly for wearable force sensing

This research aims to model manual assembly processes by parameterizing operator force readings, specifically for engine and coolant hose connections in an automotive manufacturing line. During automotive assembly, many processes are still performed manually by the human operator due to the complexity of automating the process or product with current technology. Processes include completing hose connections and subsequent “push–pull–push” verification testing. Manual work introduces an opportunity for human error because of natural variation when completing tasks; even a slight inconsistency in operation can lead to an incomplete or missed process. These incomplete processes can pass post-production checks, such as a pressure test, but later loosen and fail, causing rework or warranty issues. To minimize human error, operator force is parameterized to provide real-time feedback to the connection status. The operator force was chosen to classify connections and to verify testing quantitatively. The parametrization was completed by partitioning the shear and normal forces using custom fixtures, with shear being the primary force type required by the process. The varying finger and hand engagement for the different connector locations were factored into the parametrization to encompass a broader range of manually completed tasks. It was found that operator forces in finger engagement for manual assembly could be effectively represented by a limited set of measurable parameters.     

On a system of understanding assembly illustrations in an assembly manual

This paper presents an assembly illustration understanding system. The system is eventually expected to be applied to a robot which specializes in automated mechanical assembly. Assembly illustrations in an assembly manual usually have two features: 1) In addition to the figures corresponding to mechanical parts, several special line-drawings referred to as auxiliary lines in this paper are often employed for the visualization of the assembly relations among mechanical parts; 2) The assembly illustrations in an assembly manual are disposed sequentially so that the subgoal of an assembly illustration will definitely appear in its succeeding illustrations as an assemblage. Both features are important clues to the analysis and understanding of assembly illustrations. By extracting the auxiliary lines, the system recognizes assembly relations among mechanical parts, and the 3D shape of the mechanical parts as well. Moreover, based on the assembly relations, it conjectures the structural details of mechanical parts such as insertion holes which are usually invisible. After that, it characterizes the appearance of the completed assemblage described by the current illustration. The system finally verifies the result by matching with the figures in a succeeding illustration in which the completed assemblage is given as a subpart.     

Comprehensive manual handling limits for lowering,pushing, pulling and carrying activities

The objective of this study was to develop a set of mathematical models for manual lowering, pushing, pulling and carrying activities that would result in establishing load capacity limits to protect the lower back against occupational low-back disorders. In order to establish safe guidelines, a three-stage process was used. First, psychophysical data was used to generate the models' discounting factors and recommended load capacities. Second, biomechanical analysis was used to refine the recommended load capacities. Third, physiological criteria were used to validate the models' discounting factors. Both task and personal factors were considered in the models' development. When compared to the results from prior psychophysical research for these activities, the developed load capacity values are lower than previously established limits. The results of this study allowed the authors to validate the hypothesis proposed and tested by Karwowski (1983) that states that the combination of physiological and biomechanical stresses should lead to the overall measure of task acceptability or the psychophysical stress. This study also found that some of the discounting factors for the task frequency parameters recommended in the prior psychophysical research should not be used as several of the high frequency factors violated physiological limits.     

Rating of acceptable loads in manual sorting of postal parcels

The psychophysical test, the rating of acceptable load (RAL) were used to assess acceptable weights for dynamic lifting in postal workers engaged in sorting parcels. The standard test (RAL_St) and a work-simulating test (RAL_W) were administered to 103 volunteers: all experienced male sorters. In the RAL_St, subjects selected the weight which would be acceptable for lifting in a box with handles from table to floor and back to the table once every 5 min for the working day. For the RAL_w, the box was without handles and the weight was chosen to be acceptable for transfer 4-6 times/min from a table to the parcel container and back to the table. Both tests were made during normal working hours at postal sorting centres. The overall means for RAL_St and RAL_w were 16·4 kg and 9·4 kg respectively (p < 0·001): both being substantially higher than the average parcel weight of 4 kg. The RAL_St and RAL_w tests proved to be repetitive and sensitive for differentiating the effects of load and task variables in actual manual materials handling. Thus they appear to be applicable to the evaluation of manual materials handling problems.     

A comparison of risk assessment of single and combination manual handling tasks: 3. Biomechanical measures

Anecdotal evidence suggests organisations experience difficulty assessing the risk in manual handling tasks. One reason for this difficulty may be that many common tasks are a combination of lift, lower, push, pull and carry tasks. No prior reports of attempts to assess the risk in combination tasks using biomechanical measures could be found. The aim of the study was to compare the risks assessed in single manual handling tasks with those in combination tasks. Nine male and nine female students performed combination and single handling tasks. The force applied by subjects to a box was recorded and, together with kinematic data on subject posture collected via video, used in a twodimensional dynamic model to estimate the lumbar compression force and lumbar shear force. The hand force, peak lumbar compression force and peak lumbar shear force for each combination task were each compared with the same variable for the single tasks which comprised the combination, using repeated measured analysis of variance with specific contrasts. In at least one of the twelve comparisons performed for each dependent variable, the combination task value was significantly different to the single task value. It is concluded that the risk in combination manual handling tasks can not be accurately assessed by using estimates based on biomechanical measures of single tasks.     

用于虚拟拆装的力反馈方法

研究虚拟拆装系统中,为增强用户的沉浸感,会引入力反馈设备,但目前力反馈设备价格昂贵,同时力反馈算法也不成熟导致了力反馈的引入无法商用和推广。使用力反馈操纵杆作为力反馈设备,对虚拟拆装中常见的结构约束关系进行分析,并对约束关系变化产生的力反馈进行建模和实验实现。结果表明在精度要求不高的拆装场合,力反馈方法能够给操作人员带来直观、真实的力觉操纵感受,同时由于力反馈操纵杆价格低廉,适于推广使用。     

网络化测力机器人系统设计与实现

本文提出了一种两自由度的节点型测力机器人，并对原理样机的软硬件设计方案等作了简要介绍。     

Intelligent design and planning of manual assembly workstations: A neuro-fuzzy approach

The design of manual assembly workstations, as with most forms of designs, is highly iterative and interactive. The designer has to consider countless constraints and solutions for contradictory goals. In order to assist the designer in design process, it is required to develop a new intelligent methodology and system. This paper develops a neuro-fuzzy hybrid approach to intelligent design and planning of manual assembly workstations. Problems, related to workstation layout design, planning, and evaluation, are discussed in detail. A fuzzy neural network is used to predict the ranges of anatomical joint motions and to design or adjust workstations and tasks. The neuro-fuzzy computing scheme is integrated with operator's posture analysis and evaluation. For training and test purposes, experiment is carried out to simulate assembly tasks on a multi-adjustable assembly workstation equipped with a flexible PEAK motion measurement and analysis system. The trained neural network is capable of memorizing and predicting the joint angles associated with a range of workstation configurations. Thus, it can also be used for the design/layout and on-line adjustment of manual assembly workstations. Thus, the developed system provides a unified, computational intelligent framework for the design, planning and simulation of manual assembly workstations.     

An investigation of perceived exertion via whole body exertion and direct muscle force indicators during the determination of the maximum acceptable weight of lift

The objective of this study was to identify the perceived exertion mechanisms (direct muscle force and whole body exertion) associated with the decision to change the weight of lift during the determination of the maximum acceptable weight of lift (MAWL). Fifteen males lifted a box of unknown weight at a rate of 4.3 lifts/min, and adjusted the weight until their MAWL was reached. Variables such as the predicted muscle forces and heart rate were measured during the lifting exertion, as well as the predicted spinal loading in three dimensions using an EMG-assisted biomechanical model. Multiple logistic regression techniques were used to identify variables that were associated with the decision to change the weights up and down prior to a subsequent lift. Results indicated that the force in the left erector spinae, right internal oblique, and left latissimus dorsi muscles as well as heart rate were associated with decreases in the weight prior to the next lift. It appears that a combination of local factors (muscle force) and whole body exertion factors (heart rate) provide the feedback for the perceived exertion when decreasing the weight. The up-change model indicated that the forces of the right erector spinae, left internal oblique, and the right latissimus dorsi muscles were associated with the decision to increase the weight prior to the next lift. Thus, local factors provide feedback during the decision to increase the weight when starting from light weights. Collectively, these findings indicate that psychophysically determined weight limits may be more sensitive to muscular strain rather than spinal loading.