ergonomic evaluation of virtual assembly tasks

ERGONOMIC EVALUATION OF VIRTUAL ASSEMBLY TASKS

Menelaos Pappas, Vassiliki Karabatsou, Dimitris Mavrikios, George Chryssolouris

University of Patras, Greece [email protected], [email protected],

[email protected], [email protected]

This paper investigates the use of Virtual Reality and Human Simulation technologies for the ergonomic evaluation of manual assembly tasks. Within this concept, a virtual environment has been developed for the realistic representation of assembly stations into which digital humans can be imported and used to evaluate assembly executions in terms of ergonomics aspects. The industrial need lies on the reduction of process time and on the ergonomic optimization of the assembly workstations. The use of this environment enables the identification of critical steps during an assembly execution, which provides stress to humans resulting in increased assembly times and over wearied operators. A real life assembly task of a commercial refrigerator has been simulated and presented in order to demonstrate environment capabilities.

1. INTRODUCTION In modern manufacturing industries, minimization of both product development cycle times and costs, are strategic objectives (Chryssolouris, 2005). In designing process workstations, such as assembly ones, several physical prototypes and ramp-ups need to be built for the verification of human related factors. In complex manual tasks, the human involvement is very critical as it influences the feasibility, the cycle time, the working comfort and the safety of an operation. In manufacturing, in assembly and in related work, where human operators are involved, the flexibility that a human brings with it provides difficulties in modelling their behavior. In this case, the interaction between humans and the products in all phases of the product life cycle, such as design, production, operation and maintenance, must be studied. The easiness of assembling a product has to be taken into account at the early stages of the design, where no physical models of the final product are available. Moreover, ergonomic problems demand empirical data on human capabilities and have to be examined at the early design stages (Chryssolouris et al., 2003; Chryssolouris et al., 2000).

Virtual Reality (VR) and Human Simulation offer a new tool that needs to be properly exploited in order for the Design for Manufacturing /Maintenance

2 Digital Enterprise Technology

/Assembly / Disassembly techniques, to include the human behavior, and the human dimensions, which up to now, have been difficult to incorporate (Lu et al., 1999).

The use of such techniques provides a fast and flexible way of creating realistic virtual representations of complete assembly workspaces by integrating the human presence and intervention into the form of digital mannequins as well as by supporting the optimization of the human-product-process relationship. They have been explored during the last few years for industrial processes verification. As a result, a number of VR and Digital Human based systems oriented to assembly processes simulation have been presented in the scientific literature (Gomes de Sa and Zachmann, 1999; Rajan et al., 1999; Feyen et al., 2000; Ottosson, 2002; Sundin and Medbo, 2003; Chryssolouris et al., 2004).

In this paper, a virtual environment has been developed by simulating a real life assembly task of a refrigerator’s assembly line. Videos for the assembly execution by real operators have been used in oder to identify the critical body postures. Ergonomic evaluation of these postures is then performed with the use of digital mannequins. In this way, ergonomic parameters are calculated for each critical posture and any uncomfortable or stressed positions are identified. 2. THE VIRTUAL ASSEMBLY ENVIRONMENT 2.1 Implementation environment The virtual assembly environment has been developed with the use of the Division Mockup 2000i2 platform for the development of Virtual Reality based applications (PTC site, 2006). Division Safework has also been used as the mannequin tool. Six default mannequins (5th, 50th and 95th percentile male and female) have been provided for the creation of any human being, by using the Human Builder option. The advanced anthropometry capabilities of the Human Builder option enable the direct control of 104 anthropometric variables as well as the somatotype of the Division Safework mannequin. The system currently “runs” on an SGI Onyx2 Infinite Reality2 RK Workstation. All the virtual parts of the environment have been designed with the use of the PTC’s ProEngineer Wildfire 2. 2.2 Pilot application scenario The pilot application scenario, selected to be an assembly station simulation of a commercial refrigerator assembly line, has been developed in order to demonstrate the VR environment capabilities. The increased stress and fatigue of workers related to the tasks performed in this station are the main factors for the investigation of their correctness from the ergonomic point of view. Therefore, the primary objective of this work is a human-centered evaluation of the assembly task and its further improvement, in case it is found ergonomically inadequate.

The general objective is to transfer the assembly task in a controlled environment, and to experiment with the current set-up in order to quantify its ergonomically related factors. This should minimize the pre-set “point factors”, in our case being the time and the worker’s fatigue.

Ergonomic Evaluation of Virtual Assembly Tasks 3

The developed virtual environment is shown in Figure 1. The task to be analyzed is the assembly of the side glasses of the refrigerator. Figure 2 shows a snapshot of the real assembly performance.

Figure 1 – The Virtual Assembly Environment

Figure 2 – Real assembly performance

Based on the available video of the real assembly performance, this assembly task was divided into five key-frames/postures during the simulation. These key-frames are:

• Key-Frame 1: The performer takes an appropriate body posture in order to grasp the side glass from the glasses bench and pull it towards his body. He is leaning slightly to the front while his hands are grasping the glass from the front side. His legs are fairly straight and close to the bench (Figure 3a).

• Key-Frame 2: The performer takes an appropriate body posture in order to pull off the glass. The spine is slightly curved forward while the legs and feet are in a bended position allowing better support and balance. The left hand is in an elbow-upward position grasping the glass from the top while the right hand is straight and grasping the glass from the bottom (Figure 3b).

• Key-Frame 3: The performer moves from the glass bench to the refrigerator. He is holding the glass while moving towards the refrigerator and rotating counter-clockwise. The various parts of the body take several positions with the these presented being the most stressing ones. The spine is fairly straight while the legs are slightly bended and the hands are bended while holding the glass (Figure 3c).


• Key-Frame 4: The performer takes an appropriate body posture in order to insert the lower corner of the glass into the refrigerator’s side panel recesses. His spine and legs are straight and his arms bended so as for him to be allowed to grasp the glass in its middle from both sides (Figure 3d).

• Key-Frame 5: The performer slides the glass through the recesses. He slightly bends forward while his left hand pushes the glass. His right hand is also pushing the glass from the inner side. His right leg is placed in front of the left thus, providing more leverage and balance (Figure 3e).

a

b

c

d

e

Figure 3 – Key-frames of the side-glass real assembly process

After having defined all critical postures during this assembly task, the next step

was to simulate these postures in the virtual environment by using digital humans and getting ergonomic assessment 3. ERGONOMIC ANALYSIS 3.1 Digital Human description The Division Safework Tool was used for the performance of digital human simulation. This tool provides digital human models of different anthropometrics by using several databases. The one used for this simulation had all the characteristics of the real performer, as regards his body dimensions, and represented 50% of the global population in terms of a somatotype.

The digital human was positioned in the same five critical postures that the performer takes while assembling the refrigerator’s side glasses. In Figure 4, the digital human is performing the five key-frames of the assembly task. Posture and


carry analysis was performed for the ergonomic evaluation of those postures required for the specific assembly task.

a

b

c

d

e

Figure 4 – Key-frames of the side-glass virtual assembly process

3.2 Posture analysis Posture analysis was performed for the five body postures of the digital performer with the use of the GENICOM database for the angles limitations, provided by Safework (Figure 5).

Figure 5 – Safework posture analysis dialogue boxes (input and output)

Several body parts can be selected in the input box, for a specific posture, and a

posture score is calculated by the system and is presented in a graphical mode. The color of the bar represents the comfort score of the current angle, whilst the percentage value indicates how close the current angle is to the optimum comfort value (10).


In this study, the following body parts and their respective degrees of freedom (DoF) were selected based on the nature of the performed motions (Figure 6):

• neck in flexion and extension • thigh (left and right) in abduction and adduction • thigh (left and right) in flexion and extension • leg (left and right) in flexion and extension • arm (left and right) in flexion and extension • arm ( left and right) in abduction and adduction • forearm (left and right) in flexion and extension • centre of prehension -wrist (left and right) in flexion and extension.

Figure 6 – Mannequin’s body parts and their degrees of freedom

For each posture and each studied body part, a posture score was calculated based on the joint angles’ values, in each of the five postures (key-frames). Posture analysis results of key-frame 4 are presented as an example in Table 1. Part of the graphical representation is shown in Figure 7, where the overall comfort score 42,3% is given.

Table 1 – Posture analysis results for the key-frame 4

PART SIDE DOF SCORE PART SIDE DOF SCORE Neck - fle/ext 5/10 - - - - Thigh right fle/ext 0/10 Thigh left fle/ext 0/10 Thigh right abd/add 10/10 Thigh left abd/add 7/10 Leg right fle/ext 0/10 Leg left fle/ext 0/10 Arm right fle/ext 10/10 Arm left fle/ext 5/10 Arm right abd/add 0/10 Arm left abd/add 2/10 Forearm right fle/ext 10/10 Forearm left fle/ext 2/10 Wrist right uln/rad 0/10 Wrist left uln/rad 0/10


Figure 7 – Comfort scores window for key-frame 4

A distribution of the percentage comfort score is presented in Figure 8, showing that postures 2 and 5 have low comfort scores, while posture 3 has the best score. One crucial factor that characterizes postures 2 and 5 is the fact that the performer tries to push or pull the glass by using a discomfort grasping strategy. The posture scores for the right arm and wrist are equal to zero in these postures. As for the overall comfort score, the fact that the performer not only does he have to carry the glass but also to rotate it from a sideward to an upward position, contributes negatively to the process ergonomics. This can be seen from the posture scores of the wrists as well as the right arm and left forearm which are equal to zero.


Figure 8 – Overall posture-based comfort diagram 3.2 Carry analysis Carry analysis was also performed by using the Safework’s capabilities. Snook and Ciriello 1991 general guidelines, included in the Liberty Mutual Manual Materials Handling Guidelines, were used for the analysis. The carry analysis dialogue box is shown in Figure 9. The input for this analysis is:

• The frequency of the carry task • The distance of carry • The population sample

whilst the output is the maximum allowed load weight under these circumstances.

Figure 9 – Safework carry analysis dialogue box This analysis has been performed in order for one to study if the carrying of a

refrigerator’s side-glass, weighing 16 kgs, in the existing frequency namely, one every 5 min, is ergonomically acceptable for a 50% sample of male population and a carry distance of 2.1 m. After all calculations were made, the maximum weight allowed for this data was 33kgs, which was more than the real weight and thus, carry analysis has shown acceptable results for this carry task. Acceptable weights for all male percentiles are shown in Table 2.

Table 2 – Acceptable weighs for male population

Male Percentile Acceptable weight 10 48 25 41 50 33 75 26 90 19


4. DISCUSSION This paper presents a straight-forward method for the ergonomic analysis of manual assembly tasks with the use of Virtual Reality and Human Simulation techniques. This method is quite generalized and task-independent, and it could be used by engineers and process designers during the process design phase to evaluate assembly workstations from the ergonomic point of view. In order for the concept of the method to be analysed, a pilot application was thoroughly described, which was based on a real life assembly task of a commercial refrigerator. In the frame of this application, a virtual environment was developed for the realistic representation of the assembly workstation in which digital human operators could be imported in order to evaluate assembly executions in terms of ergonomics aspects. The use of this environment enables the identification of critical points in the assembly procedure, through posture and carry analysis, which provide stress to human operators and result in increased assembly times and over wearied operators. The critical identification points (for example key-frames 2 and 5 in the studied assembly task) also indicate where re-designing of the assembly procedure is required for better ergonomics and efficiency. 5. REFERENCES 1. Chryssolouris G. Manufacturing Systems: Theory and Practice. 2nd edition. New York: Springer-

Verlag, 2005. 2. Chryssolouris G, Karabatsou V, Kapetanaki G. Virtual Reality and Human Simulation for

Manufacturing. CIRP Journal of Manufacturing Systems, 2003; 32(6). 3. Chryssolouris G, Mavrikios D, Fragos D, Karabatsou V. A virtual reality-based experimentation

environment for the verification of human-related factors in assembly processes. Robotics and Computer Integrated Manufacturing, 2000; 16(4): 267-276.

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ergonomic evaluation of virtual assembly tasks

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