3D Computer-aided human factor engineering analysis of a grafting robot

Transcription

1 3D Computer-aided human factor engineering analysis of a grafting robot Yi-Chich Chiu 1), Suming Chen 2), Gang-Jhy Wu 1), Yi-Hao Lin 1) 1) Dept. of Biomechatronic Engineering, National Ilan University, 1, Sec. 1, Shen-Lung Rd., I-Lan 26041, Taiwan, yichiu@niu.edu.tw 2) Dept. of Bio-Industial mechatronics Engineering, National Taiwan University, 1, Sec. 4, Roosevelt Road, Taipei 106, Taiwan, schen@ntu.edu.tw Abstract The objective of this research is to conduct human factor engineering analysis on a grafting robot design using computer-aided 3D simulation technology. A prototype tubingtype grafting robot for fruit and vegetables (the grafting robot) is the first subject for a series of case studies. To facilitate the incorporation of human models into operating environment of the grafting robot, CAD computer software package I-DEAS was applied to establish the models of grafting robots and imported into another software package, JACK, for ergonomic analysis. Six human models (95 th percentile, 50 th percentile, 5 th percentile, male and female) were employed to simulate the operating conditions and working postures in real operational conditions. The lower back and upper limb stresses of robot operators were analyzed using the Lower Back Analysis (LBA) and Rapid Upper Limb Assessment (RULA) functions of JACK. The results show that: with current grafting robot design, the overall operational conditions for the operators are ergonomically reasonable, with no risk of over-stressed. However, the grafting robot could be designed with a more ample leg-space for operators to further reduce the lower back and upper limbs stresses. The optimum operation layout design for the operators to sustain minimum lower back and upper limb stresses would be: to set the grafting operation 23.2 cm away from the operator, with a height of 85 cm, and a clearance of 45 cm between rootstock and scion. Keyword: Grafting robot, Ergonomic Analysis, Lower Back Analysis, Rapid Upper Limb Assessment 1. Introduction Ergonomic engineering is now well recognized and getting more and more attention in Taiwan. This can be demonstrated by the founding of National Ergonomic Engineering Society, ROC in 1993 (Li, 2000). The major purpose of ergonomic engineering is to improve

2 daily goods and living/working environments for the human beings, whereas the human factors, such as operating limitations and user-friendliness, are incorporated into mechanical design for higher customer satisfaction. Conventional agricultural machinery design focuses on functionality, followed by security, stability, endurance and user-friendliness. Of which, user-friendliness is considered last and is often underestimated. Modern agricultural machinery design, however, put more emphases on human factor considerations, as with design for all other machines/products. Swift evaluation and modification of design to match all design criteria, which may also changed quickly, has become important. With computer-aided design and virtual design/simulation techniques, the design cycle can be cut down effectively to minimize the design and development cost. Grafting means that the crops to be cultivated (i.e. the scions) are grafted onto the rootstocks of strong vitality plants, where scions can grow up, blossom and bear fruit due to soil-based nutrient. As a well-proven seedling technology, grafting features strong resistance to diseases from soil and good adaptive capability to unfavorable growing environment, as well as improved product quality and higher output. Therefore, seedling-grafting has become a popular trend in Taiwan for cultivation of fruits and vegetables in greenhouses or in the fields. The crops using grafted seedlings include watermelon, cucumber, muskmelon, tomato, papaya, passion fruit, and eggplant, the number and categories keep rising. For this reason, many grafting farms are set up and dedicated to offer professional grafting and seedling services to the farmers. However, since grafting is a time-consuming job even for skilled workers, there is an important trend that grafting robots with growing market potentials are developed in lieu of manual grafting. With economic assistance from the Agricultural Commission, Executive Yuan, Chen et al. (2003) developed a tubing-type fruits/vegetables grafting robot (the robot). The robot utilizes rubber tubes as grafting material, and caters for the needs of seedling grafting with little difference of stem size between rootstock and scion. When compared to traditional grafting clamps, the rubber tubes feature cost-effectiveness, good accessibility and water retention for grafting worn. This can reduce the grafting cost, as well as improve the survivability of grafted seedlings. With the growth-up of seedlings, rubber tubes will automatically fall from the seedling due to embrittling effect, which save the picking and recycling labor cost. Thus, this type of grafting robot has tremendous market potential in the near future. The grafting robot developed in this research is operated by one person, which forms a man-machine system. An operating space will take shape if a machine comprises several

3 units. So, there is a need to consider the suitable ways the operating units are arranged to make operators work easily and comfortably, with no sacrifice of high accuracy, good safety and efficiency. In view of the operators physiological conditions during grafting operations, seated working can relieve the weight of legs, reduce power consumption and minimize the load of circulatory system. These advantages, instead, result in higher working performance and lower worker fatigue. However, long-lasting seated working may also affect digestive and respiratory systems, or even lead to abdominal muscular relaxation and cyrtosis. In particular, the diseases of vertebra and back muscle will yield greater pressure against relaxation. In such case, it is worthy to discuss the comfort of operators under vertebra bending and stress, as well as the space required for hand working during the processes of grafting. The space required for hand working must also consider accessible working range of operators in order to maintain and improve the working performance. Aimed to evaluate and improved the human factor related design of the grafting robot developed, this research conducted ergonomic analysis on the grafting robots via computeraided 3-D virtual simulation technology. The main issue investigated is the comfort of operators when executing grafting operations with various grafting robots designs. 2. Literature review 2-1. Applications of anthropometry database Since World War, many advanced countries spared no efforts to set up anthropometry databases covering the public, armed forces, aircrew, labor force and migrants, due to increasing importance of anthropometry data in everyday life and work. Up to now, there are over 90 large-sized anthropometry databases, of which the U.S. and some European countries have a big share, and Asian countries account for about 10 (a half shared by Japan). Conversely, some medium-/small-sized anthropometry databases in Taiwan cannot meet the requirements of applications (Chen, 1997). Under the sponsorship of Occupational Safety and Health Research Institute of Labor Force Commission (OSHRI), Executive Yuan Taiwan, National Tsing Hua University started in 1994 to implement an anthropometry investigation and surveying plan for labor force, in cooperation with ergonomic researchers from nine domestic universities. According to the population structure in Taiwan, approximately 1200 workers were sampled and measured with regard to 266 static indicators and 42 dynamic activities (Chen, 1997). The programs of Anthropometry Database, set up by OSHRI, can be downloaded to PCs. Anthropometry

4 comprises linearity (including height, width and depth), curves (including periphery and curvature) and articulation angle according to its different shapes. The human body is divided into four sections (head and neck, trunk, upper limb and lower limb). Using data of Anthropometry from OSHRI, Wang and Wang (1997) proposed the concept of working space, which covers stand working, seated working, visual placement, visual display terminal (VDT) and minimum space requirements at special locations. The seated working space includes 7 options: 1. height of work-surface, 2. height of seat, 3. moving space of thighs, 4. horizontal working area, 5. vertical working area, 6. height of conveyor belt, 7. seating space. The design size of workstation is referred to the estimations of relevant dimension of domestic population. Given the fact of some changing sizes in the applications, attentions shall be paid to actual situations such as height of operators and specific jobs, thereby minimizing occupational injury and improving the working performance Review of ergonomic research To enable a better understanding of human capability and environmental limitations, ergonomic engineering is applied to design of tools, instruments, systems and working methods as well as environment. As one of technologies most commonly applied and closely related to occupational safety and health, it is aimed at improving the safety and health, performance and comfort of the employees. Lower back pain is a major occupational injury in many countries. According to the investigation of OSHRI, 48.9 % domestic workers suffer from lower back pain (Chen et al., 1995). The investigations of Li et al. (1997) for semi-conductor industries in Hsin-chu Science Park show that, 38.4 % employees in chip manufacturing suffered from lower back discomfort. It can thus be seen that lower back pain is a common occurrence in both hightech and conventional industries. The height of work-surface has immediate impact upon upper limbs such as shoulders, back and head. At a higher work-surface, head and back must straighten up, and shoulders shall be raised, leading to a nervous state of arm muscle and aching pain of shoulders, neck and arms. At a lower work-surface, back must be bent, and head shall incline forwards, leading to discomfort of back and neck. So, work-surface will exert impact upon working performance and health. In recent years, seated work-surface is recommended to be lowered down for relaxation of upper limbs You et al., 1998). In general, the optimum height of working seat is popliteal height. In the excess of popliteal height, the lower part of thigh will be subjected to pressure, leading to interrupted blood circulation of lower limbs. In the presence of extremely low seat, 1.there is a sharp

5 angle between trunk and thigh, resulting in excessive vertebral bending; 2.it makes standing and seating difficult; 3.a bigger space for lower limb is required (Pheasant, 1986). If the height of seat is adjustable, it should be between popliteal height of 5 th percentile female and 95 th percentile male, plus 2 cm tread. If only the popliteal height of male is applied, a footrest shall be provided for female staff. According to data base, the averaged popliteal height of 5 th percentile female is 35.0 cm, and that of 95 th percentile male is 43.7 cm. It is thus estimated that the height of adjustable industrial seat should be 37.0 to45.7 cm, and that of nonadjustable industrial seat is 45.7 cm (Wang and Wang, 1997). Among human body parts, the musculoskeletal system subjected to and suffered from occupational injury most frequently. Musculoskeletal disorder is popular symptom in the work force. If the types of injury can be accurately diagnosed within short notice, it is possible to implement educational training and improve ergonomic engineering environment rapidly for a lower shutdown loss due to occupational injury (Deakin, et al., 1994). To help understand musculoskeletal disorder and work-related potential risk factors, many countries made efforts to study musculoskeletal injury record and assessment methods, of which questionnaire is one of the efficient data collection methods (Kuorinka, et al., 1987). Self-assessment questionnaire methods for repetitive occupational injury include: symptom questionnaire, simplified human body checklist, Nordic musculoskeletal questionnaire (NMQ) (Deakin, et al., 1994) and musculoskeletal questionnaire. For common workplaces, musculoskeletal questionnaire recommended by Chiu and Chen (2001) is a musculoskeletal investigation method for self-assessment. In addition to questionnaire, musculoskeletal investigation technology commonly used is aimed at providing a better understanding of work-related injury. These technologies are divided into four groups according to different positions (Chiu and Chen, 2001): 1. Upper limb checklist: Detailed checklist for upper extremity cumulative risk analysis trauma disorder, Rapid upper limb assessment (RULA), Strain index, and Utah shoulder moment model works. 2. Lower back/waist checklist: Lift/manual material handling walk-through checklist, Liberty mutual psychophysical approach to design carrying tasks, Manual handling operations regulations. 3. Whole body checklist: Musculoskeletal disorders (MSDs), Baseline risk identification ergonomic factors (BRIEF), Ovako working posture analysis system (OWAS). 4. Others: Job/task physical exertion checklist.

6 2-3. Applications of JACK ergonomic analysis software JACK ergonomic analysis software is a 3-D simulation software jointly developed by Center for Human Modeling and Simulation, University of Pennsylvania and Ransom Science Co. (JACK, 2001). JACK is applied to simulate human motion control, working modes, vision control, searching modes, detailed hand behavior, collision detection and risk prevention in ambient environment. Scope of applications of JACK includes simulating the operation of space aircrafts, control of submarines and piloting simulation of jet aircrafts. The pilot products may be firstly simulated for improvement and then put into batch production, thus saving most of test cost required in manufacturing process. Alternatively, Ergonomic simulation is helpful to avoid any risk against actual persons, and facilitates the modification and quick preview of new products (JACK, 2001). An extremely high pressure will occur at intervertebral discs between 4 th Lumbar and 5 th Lumbar (L4/L5), or between 5 th Lumbar and 1 st Sacrum (S1), which becomes a lifting load limit for assessment purpose. Lower back analysis (LBA) from Jack selects the stress of L4/L5, and lower back stress under any posture and load condition. In accordance with National Institute of Occupational Safety and Health America (NIOSH), LBA provides the following three criteria for risk identification (NIOSH, 1981): (1) Less than 3400N: acceptable load, represented by green. (2) More than or equal to 3400N and less than 6400N: danger load, represented by yellow. (3) More than or equal to 6400N: extremely danger load, represented by red. As for case (1), the assessment results show that most of people will not be hurt in such operating conditions. As for case (2), it shows that some individuals are exposed to a certain risk hazard, thus further improvement is recommended. As for case (3), it shows that most of people are exposed to such hazards, and operating conditions are recommended for improvement. The research shows that this classification method features a higher level of suitability. However, it is worthy to notice that, these classifications belong to recommended assessment, i.e., case (1) does not guarantee the absence of any danger hazard. The RULA in JACK s was designed according to RULA checklist jointly developed by McAtamney and Corlett (1993). It is well-suited to assess upper limb injuries due to its quick response and convenience. As RULA focusing on observation and analysis, the human body is divided into groups A and B, of which group A comprises upper arms, forearms and wrist, and group B comprises neck, trunk and legs. After observing the working posture, it is

7 required to offer scores for maximum operating angles of various positions, and assess the action level according to the physical exertion state and strength. Action level is divided into four grades as listed in Table 1. It indicates that the working posture is acceptable if total check score is between 1~2, or required for further investigation and improvement (if necessary) if the score is between 3~4, or required for future investigation and improvement if the score is between 5~6. Immediate investigation and improvement is recommended if the score is 7. Thus, built-in RULA for JACK is used to assess the working posture of grafting robot operators in this research. Table 1. Action level of RULA working posture Action level Score Proposed 1 1~2 Acceptable 2 3~4 Further investigation and improvement (if necessary) 3 5~6 Future investigation and improvement 4 7 Immediate investigation and improvement By integrating Taimanese static and dynamic anthropometry database, Jack digital human modeling software, and motion analyzer, Lin (2000) established assessment tasks in a virtual simulation environment, where Jack 3D digital human model can provide the data of human height and operations in real working environment. This system is preliminarily applied to set-up procedures of semi-conductor chip factories (carrying chip boxes, loading and unloading), and assess if operators are exposed to possible hazards in order to improve the procedures and minimize the job-related injuries. After setting-up a computer workstation simulation assessment system, Yang (2002) strived to explore the suitability and advantages/disadvantages of computer components (screen switch, printer switch, host switch, switchboard and mouse) currently available for wheel chair users using RULA provided by Jack. It is found that, existing computer desktops place some restrictions upon wheel chair users with regard to operating space and range, whereas unfavorable working posture is observed when operating peripheral equipments. Leskinen and Haijanen (1996) employed Jack to analyze two lifting tasks, one is lifting 5 kg clamps, and the other is lifting 12 kg boxes, with a lifting height from 70 to 170 cm. The

8 analysis methods for lifting posture include: 1. calculate the articulation moment by Jack tool, especially for intervertebral discs at lower back L5/S1; 2. simulate 50 th percentile male model using the recommended weigh limit (RWL) after revision of NIOSH in It is found that, when lifting 5 kg clamps in squatting posture, the lower back moment (other than the standing posture) has a good negative correlation with RWL. Using RWL as an important reference data, Haijanen et al. (1996) designed the lifting clamps for a new production line according to: 1. expert recommendations; 2. posture of JACK. It is found that, Jack provides a bigger simulated RWL, owing to the fact that Jack enables quasi-optimum lifting tasks. 3. Materials and methods As mentioned before, the primary task of this research is to conduct ergonomic analysis on a prototype grafting robots using 3-D simulation technology. Ergonomic analysis software, Jack, was applied to develop computer-simulated human models and integrated with real working environments in grafting operations for various case studies usages. The LBA and RULA functions of JACK facilitated the analyzing and evaluation of the lower back stress and upper limb injury possibility. According to the simulation analysis results, optimum equipment design, layout, and operational criteria were expected to be developed to improve the working environments and gain maximum comfortableness for the robot operators. The physical models of grafting robot components were built up by CAD software, I- DEAS, in the first. These component models were then imported into JACK were they were assembled and incorporated with the human models and the working environments to construct models of man-machine systems. Next, the tasks of the grafting operators were carried out in simulated working conditions, while the comfortableness and the body stress under various working postures were analyzed to avoid any over-stress condition, and to insure the comfortableness of the operators Introduction of the grafting robots Figure 1 shows the important components and configuration of the subject of this research, a tubing-type grafting robot. The grafting processes while using such robot are: the operator grips a rootstock by left hand in a seating posture and places it into the rootstock chucking and fetching unit, then step on the rootstock pedal and causes the gripper to grasp the rootstock. Rootstock seat which includes the rootstock gripper then rotates to the rootstock cutting unit, where the rootstock was cut to the desired shape, then

9 to the grafting unit. Meanwhile, the operator grips a scion seedling by right hand and places it to the scion gripper, stepped on the scion pedal to grip the scion. Then, scion seat, which includes the scion gripper, rotates to the scion cutting unit, where the scion was cut to the desired shape, then to the grafting section. A tube of proper length was then cut and supplied by the tube supplying and cutting unit. The scion seat then moves downwards, which allows the inserting of the scion into the tube. Then rootstock gripping seat moved upwards, rootstock was inserted into the tube. Due to the flexibility of the tube, the scion and rootstock enclosed and jointed within the tube. Finally, the grafted seedling was sent out from the rootstock seat, and then taken out manually. Feeding wheel of tube Scion cutting unit Tube supplying and cutting unit Scion chucking and fetching unit Rootstock cutting unit Pneumatic source Grafting unit Rootstock chucking and fetching unit Control panel Figure 1. Main components and configuration of the tubing-type grafting robot 3-2. Development of Computer Models Computer modelling of the grafting robot Figure 2 shows the procedures of developing the grafting robot model. Firstly, the geometric models of the robot components (including the machine body, the grippers of rootstock and scion, the cutting units of rootstock and scion, the grafting unit, the cutting section of tube and seedling, etc.) were prepared by the Master modeler module of I-DEAS (Figure3). These component models were transferred into IGES format and imported into JACK, wherein they were assembled and attached with additional mechanic constrains or

10 boundary conditions such as linear or rotary articulations. Figure 4 shows a model of grafting robot completed in Jack environment. Enter I-DEAS CAD environment Build the element models of the grafting robot Export the element model as IGES files Complete the integrated model of the grafting robot Assemble the element models Import the element models into the software JACK Figure 2. Procedures of grafting robot model development Figure 3. Grafting robot modeling in I-DEAS graphic environment

11 Figure 4. Modeling of grafting robot in JACK environment Digital human models development Figure 5 shows the procedures of human model development. As illustrated in Figure 6, JACK provides a typical human model library, which contains typical anthropological data of male, female and child (code names Jack, Jill and Little Jack) with various percentile ranges. A human model has 69 limb sections (including 17 at vertebrae, 16 at hands), 68 articulations, and 135 degrees of freedom. The library was developed according to an anthropometry database which was built up based on measuring 132 ergonomic items of 9000 servicemen of the U.S. military force. Specific ergonomic dimensions, deviated from the typical ones, can be specified with advanced scaling functions. As shown in Figure 7, the dimensions of main body parts can be specified through a scaling interface (JACK, 2001). Users can adjust the length of forearm and upper arm first, and JACK will automatically revise the dimensions and properties of other human parts accordingly to maintain proper nominal human body ratio. User defined human models can be added to the library with names and definitions facilitate the future usage.

12 Define human size according to the anthropologic data library Initiate Build Human Define gender, height and weight Define human body dimension of interest Initiate Advanced Setting Define work posture according to the workposture library Choose Human Model Choose Seated_straight in postures Define operational posture of hands according to the hand-posture library Open Shape Hand Choose Precision_grip Complete the human model development Figure 5. Procedures of human model development

13 Figure 6. Human model specification interface of Jack Figure 7. Human model advanced scaling interface of Jack

14 The anthropometric data in this paper was sourced from a research report of IOSH, Labor Force Commission (IOSH86-H124), Executive Yuan, ROC (Wang and Wang, 1997). According to the ratio of gender and ages in HR statistic data of Statistics Department of Executive Yuan, random samplings were implemented in the factories in Hsin-Chu county, Taiwan. The investigation involved 735 valid male samples and 465 female samples ranging from 18 to 65 years old. For each and every sample, 266 static human part dimensions and 42 dynamic limb movement angles were measured. This paper utilized 7 human part dimensions, including height, bideltoid breadth, distance from elbow lower point to acromion (shoulder-elbow), space from elbow lower edge to seating surface (elbow-rest), space from finger tip to rear point of elbow (elbow-fingertip), space from paropia to seating surface (eyerest), and space from upper edge of knee to ground (sit knee height), as illustrated in Table 2 and Figure 8. Table 2 lists the above mentioned anthropometric data of Taiwanese labor force. In the Table, the th percentile represents the height percentile, i.e., 95 th percentile portrays taller persons, 50 th percentile portrays averaged height, and 5 th percentile represents relative shorter ones.

15 Table 2. Anthropometric data of Taiwanese labor force (age18-65) Name of dimensions Gender Average Std. dev. 5 th %ile 50 th %ile 95 th %ile (1) Height, cm Male Female Weight, kg Male Female (2) Bideltoid breadth, cm Male Female (3) Shoulder-elbow, cm Male Female (4) Elbow-rest, cm Male Female (5) Elbow-fingertip, cm Male Female (6) Eye-rest, cm Male Female (7) Sit knee height, cm Male Female

16 (2) (1) (3) (5) (6) (4) (7) Figure 8. Dimension definitions of human model Working postures in different working environments were defined through the human posture specification interface of Jack, in which many typical postures, includes: driving, seated straight, stand relaxed, walking double support or walking single support, crawl, kneel one knee or kneel two knees, squat, stand working, running and lay on side, etc., can be selected (Figure 9). Seated-straight posture was selected and used in this research. The hand working postures were defined through the hand posture specification interface of Jack (Figure 10), which included the optional postures such as maximum grip, fist, control grip, reset and point, for both left and right hands. New finger postures can also defined and added by the users or loaded from other databases.

17 Figure 9. Human posture specification interface in Jack Figure 10. Hand posture specification interface in Jack

18 Integration of man-machine models As shown in Figure 11, workers operate grafting robot in a seated posture, thus seatedstraight posture in Jack was selected as the working posture. Since workers grips seedling manually to put into the grafting robot, precision-grip posture in Jack was selected as the hand posture. After completion of setting-up, the seated human model was shifted and fastened onto the center of working chair, with the feet placed on the pedal, as shown in Figure 11. The behavior of arms, foot and seedlings were defined according to the grafting process. Then, the seedling weight, initial speed, and time of start-up and stop were defined to build up the motion path of the arms, feet and the seedling. The sequence of grafting processes was defined by means of shifting or locking. Figure 11. Integrated man-machine model Verification of integrated man-machine model Model verification was conducted to insure that the computer models and the following simulation analysis can accurately represent correspondent objects and conditions in the real world. The models and procedures simulations were first validated and modified through visualized movement comparison with Jack s animation functions (shown in Figure 12). Then the actual operation time was compared with the simulated one to further examine the validity of the integrated man-machine models. The complete simulated grafting processes include: a worker grips a rootstock by left hand and puts it into the rootstock gripper then steps on the rootstock pedal by left foot for cutting. Meanwhile, the worker grips a scion seedling by right hand and places it into the

19 scion gripper then steps on the scion pedal by right foot for scion cutting and following grafting. Finally, the grated seedling is taken out from the rootstock unit. Visualized examination of simulated animation movements of grafting processes gave satisfactory results. The postures, the start-up and stop time, and the duration of each and every procedure were defined and simulated according to the actual grafting processes as shown in Figure 13. Figure 12 shows items and time sequences of all pre-defined movements for relevant human and machine parts, and the materials that were involved in the grafting processes. Listed at left part of Figure 12 were the simulated animation items, which include the model of grafting robo, left hand, right hand, gripping/placing, behavior of rootstock and scion, while the time sequence of correspondent movements were displayed at the right part. Movements with operation time overlapped were highlighted by black boxes as shown in the figure. Operation time simulation was categorized into time for rootstock processing, scion processing, grafting and time for complete operation cycle. The rootstock processing time indicates the time from gripping of rootstock to completion of rootstock cutting; the scion processing time indicates the time from gripping of scion to completion of scion cutting; the grafting time indicates the time from start-up of grafting to extraction of grafted seedling. Simulated processing time was compared with measured time in actual operation and shown in Figure 14. It is found that, the difference between the simulated and the real processing time for rootstock processing is 0.9%, for scion processing is 1.4%, for grafting is 2.1%, and for the complete operation cycle is 6.1%, with maximum difference less than 1 second. Computer-aided simulation analysis appears to be feasible according to the comparison results.

20 Grafting robot model Left hand Right hand Left hand Right hand Seedling fetch/settle Rootstock Scion Figure 12. Grafting processes animation setup

21 Figure 13. Detailed process defining interface in Jack

22 Rootstock Scion Actual operation Animation simulation Grafting One cycle Time, sec Figure 14. Comparison of processing time between animated simulation results, and actual measurement Verification of LBA and RULA applications Experiments were conducted to insure that the LBA and RULA functions of Jack can fulfill the expected tasks, and the researchers in this paper applied these functions properly. The low back and upper limb stresses while operating the grafting robot with three operating postures, extending arms and bending (bending posture), usual operating posture (usual posture), and close in operating posture (close in posture), were simulated and analyzed with Jack s LBA and RULA functions and compared with real operators carried out the works with the correspondent postures. One male and one female operator carried out the complete grafting operation 20 times. Table 3 lists the investigated anthropometric data of the male and female operators. For each and every time, the physical feeling of the operator was observed and compared with the LBA and RULA analyzed scores. Figure 15 and Figure 16 show example of the LBA and RULA analysis results of Jack, respectively. As shown in Table 4, the relative uncomfortable bending posture resulted in higher LBA score and RULA class (3). On the other hand, normal posture and close in posture resulted in

23 lower LBA score and RULA class (2 or 1). These results coincided with what were expected and insure that the ergonomic problems of the grafting robot operators can be analyzed and evaluated with LBA and RULA functions of Jack. Table 3. Anthropometric values for verification of LBA and RULA Name of Dimensions Male Female (1). Height, cm (2). Weight, kg (3). Bideltoid breadth, cm (4). Shoulder-elbow, cm (5). Elbow-rest, cm (6). Elbow-fingertip, cm (7). Eye-rest, cm (8). Sit knee height, cm Table 4. Comparison of comfort of actual persons and JACK functionality Operation posture Gender Comfortable Simulated value Arm Waist LBA, N RULA Bend down Normal Burrowed Male No No Female No No Male Yes Yes Female Yes Yes Male Yes Yes Female Yes Yes

24 Figure 15. LBA function menu of JACK Figure 16. RULA function menu of JACK 3-3. Computer-aided ergonomic simulation and analysis

25 The recommended horizontal operating range for domestic workers in seated posture covers normal grip and maximum grip as shown in Figure 17 (You et al., 1998). Considered the operations of grafting, only the reachable or accessible range of arms was taken into account. When operated within the range from normal grip to maximum grip, body or arm bending is expected, rather than the extending posture of arms. Thus, LBA of Jack was applied to assess the lower back stress of the operators, and RULA was used to assess optimum arm postures. Finally, optimum operating range was decided through integration of LBA and RULA analysis results. Seven human part dimensions for 6 human models (Table 2, 95 th percentile, 50 th percentile, 5 th percentile male and female) were defined with the help of human scaling functions of Jack. This paper endeavors to investigate the proper operating distance, the proper height of seedling placement, and the optimum distance between the rootstocks and scions. Maximum operational range Normal operational range Edge of table Shoulder bredth Distance to table edge Upper value G Measured for 5 th %ile Taiwanese femal,cm Lower value G Measured for 95 th %ile Taiwanese male,cm Figure 17. Recommended horizontal operating ranges Analysis of varying operating distance The operating distance means the horizontal distance from the operators to the seedling collectors, i.e. the gripping range (Figure 16). The upper limit of operating distance is 59.7 cm, which is the maximum grip of female as shown in Figure 16. A female will find it difficult to operate beyond this distance. The lower limit of operating distance is 53.7 cm, i.e.

26 the distance of male s knee touching the grafting robot in seated posture. The operating distance after touching the grafting robot cannot be calculated from anthropometric data. In this paper, the operating distance was calculated with Collision detection function of Jack. Simulation started with preset male s knee and grafting robot parameter values until male s knee touched the grafting robot. In a Jacks simulation environment, the load to hand for LBA was set as 50 g (the weight of grafted seedling), the height of chair was set as 45.7 cm, nonadjustable. Seven operating distances were defined and evaluated, which are: exceeding recommended upper limit (normal grip of 5 th percentile female) 10 cm, recommended upper limit, mean of the recommended upper and lower limit (normal grip of 95 th percentile male), recommended lower limit and 10 cm less than recommended lower limit. The correspondent values are 69.7, 59.7, 56.7, 53.7, 43.7, 34.8 and 23.2 cm, respectively. Since the operating distance of rootstock placement was the same as that of the scion placement, only 10-times rootstock placement were taken for LBA and RULA assessment and analysis Analysis of proper seedling placement height For higher seedling placement position, operators must lift their arms, results in muscular fatigue of shoulder. For lower seedling position, the operators must bend or bow, leading to fatigue of neck and lower back. The effects of various seedling placement height, incorporated with an adjustable and a nonadjustable chairs, were analyzed. A nonadjustable chair with a fixed recommended height of 45.7 cm, was used to simulate the operations of 6 human models incorporated with different seedling placement heights. Then, an adjustable chairs, which set to three optional heights, 10 cm less than the recommended chair height, 10 cm more than the recommended chair height, and recommended chair height, were used to simulate the operations of male and female models with different seedling placement heights. The rootstock placement height (85 cm) of current designed is 2cm less than the scion placement height (87 cm) as required by the grafting process. The preliminary test shows that: the lower back stress is similar for rootstock or scion placements in grafting processes. Thus, only rootstock processing is analyzed in this paper. The relative height, Δh, was defined as the vertical distance between the rootstock placement position and the operator chair (Δhrootstock placement height chair height). Usually, a smaller Δh indicates an increasing demand of body bending and a higher possibility of back injury or aching pain; a bigger Δh indicates an increasing demand of extending arm upwards and a higher possibility of arm aching pain.

27 The operating distance of this paper was set as the mean value of upper and lower limit (56.7 cm), the chair height is set as the recommended height of nonadjustable chair, or a height 10 cm more and less than the recommended one, i.e., 45.7, 35.7 and 55.7 cm, respectively. And, the placement height of rootstock was set as 115, 100, 85, 70, 55 and 40 cm. Ten seedling placements cycles under different seedling placement height and chair height are analyzed with LBA and RULA functions in Jack Analysis of proper distance between rootstock and scion The horizontal distance between rootstock and scion placement position will affect the fatigue degree of arms as discussed previously. The current distance between rootstock and scion positions was 25 cm. In addition to this current designed values, 15, 35, 45, 55 and 65 cm were analyzed with LBA and RULA functions for 10 grafting operation cycles. Other parameters setting included 50 g load on hand, 56.7 cm operating distance, and 45.7 cm height of nonadjustable chair Analysis of optimized combination The optimum operational criteria, while considering aforementioned three factors altogether, would be evaluated and recommended according to previous simulation and analysis results. 4. Results and discussion 4-1. Analysis of changes of operating distance 6 human modes are used to simulate the lower back stress and arm operations under different operating distances, with the results shown in Figure 18 and listed in Table 5. Figure 18 shows the lower back stress under different operating distances. It is clearly seen that, the curve of lower back stress has a positive correlation with the operating distance, indicating both a bigger operating distance and a higher lower back stress. In principle, the design of grafting robot lacks of a leg space, namely, the legs cannot extend into the lower side of machines. This paper is intended for individual discussion and analysis of original design and revised design with added leg space. According to original design, the operating distance of male is required to be more than 53.7 cm, otherwise, the knee will touch the grafting robot. It is found that the human model has a lowest lower back stress if the operating distance is 53.7 cm. This indicates that the human model withstands a lowest lower back stress and operates smoothly the grafting robot under such an operating distance.

28 In the presence of a leg space under the machine, the operating distance may be less than 53.7 cm, with the lower back stress simulated in Figure 18. It is found that, if the operating distance is less than 34.8 cm, the lower back stress is lowest and differs little. The operating distance of 34.8 and 23.2 cm represents respectively normal grip distance of male and female, indicating that the lower back stress is the lowest when the operating distance is within normal grip range. Thus, the operators may handle easily without bending, as demonstrated by minimized lower back stress. Table 5 lists RULA under different operating distances. It is found that, if the operating distance exceeds 69.7 cm, 3 human models have action levels of grade 3, indicating that most individuals feel discomfort of upper arm. If the operating distance is less than 59.7 cm, most of action levels are represented by grade 2, and a little by grade 1, indicating that risk exposure for upper arm is acceptable under such an operating distance. The analytical results of LBA and RULA and lower back in original design show that the optimum operating distance is 53.7 cm. If a leg space is provided under the machine, an optimum operating distance can be selected when normal grip range of female is 23.2 cm. According to the analytical results by simulating a leg space under grafting robot, it is recommended that grafting robots should be provided with a leg space in order to minimize any lower back pain arising from long-lasting operation of grafting robot. LBA, N male 95th %ile male 50th %ile male 5th %ile female 95th %ile female 50th %ile female 5th %ile Operational distance, cm Figure 18. Lower back stress under different operating distances

29 Table 5. RULA analysis sheet under different operating distances Operational Male RULA Female RULA distance, cm 95 th %ile 50 th %ile 5 th %ile 95 th %ile 50 th %ile 5 th %ile Analysis of changing seedling placement height Figure 19 shows lower back stress occurred under different seedling placement heights if the height of nonadjustable chair is 45.7 cm. It is found that, the lower back stress of human model begins to differ from each other if Δh exceeds 54.3 cm. Of which, the lower back stress of both smaller height and mean height of female will increase, but that of both bigger height and mean height of male will decrease. So, the excess of 54.3 cm for Δh is removed to avoid considerable difference. If Δh is lower than 24.3 cm, the lower back stress of 6 heights will increase rapidly by 2~3 times, so Δh less than 24.3 cm poses a serious injury to lower back. If Δh is between 24.3~54.3 cm, the lower back stress of 6 heights is very stable and differs little, namely, placement height of rootstock is between 70~100 cm. Table 6 lists RULA values under different placement heights. It is seen that, if Δh is more than 84.3 cm or less than 9.3 cm, action level is up to grade 3 or even 4, indicating that most of upper arms are exposed to a highest level of risk hazard. If Δh is between 24.3~69.3, action level is generally up to grade 2, indicating that risk exposure for upper arm is acceptable. In sum, lower back stress is more satisfactory if Δh is between 24.3~54.3 cm. It can also be found from the curve of lower back stress that, lower back stress is lower if Δh is 39.3 cm. With a reference to analytical results of RULA, it is found that risk exposure for upper arm is lower if Δh is 39.3 cm. Thus, Δh=39.3 cm is an optimum height, namely, the best placement height of rootstock is 85 cm.

30 The following paragraphs present an analysis of lower back stress of male and female when the height of adjustable chair is 35.7, 45.7and 55.7 cm, respectively. Figure 20 shows the lower back stress under different chair heights and placement heights. It is found that, the curve of lower back stress for the same human model is similar to each other under different chair heights. This means that chair height hasn t immediate influence upon lower back stress, and the factors to be considered include not only chair height or seedling placement height, but also the relative height of seedling and chair (Δh). Consequently, an optimum height is 85 cm (Δh39.3 cm) if the seedling placement height is simulated and analyzed based on the recommended height of nonadjustable chair cm. In the case of adjustable chair, it is recommended that the relative height (Δh) of seedling placement location and chair should be 39.3 cm, which guarantees the lowest and balanced lower back stress male 95th %ile male 50th %ile male 5th %ile female 95th %ile female 50th %ile female 5th %ile L4/L5 Force, N Δh, cm Figure 19. Lower back stress under different seedling placement heights Table 6. RULA analysis sheet for different seedling placement heights h, cm Male RULA Female RULA 95th %ile 50th %ile 5th %ile 95th %ile 50th %ile 5th %ile

31 L4/L5 Force, N male 95% È ª 35.7 cm male 95% È ª 45.7 cm male 95% È ª 55.7 cm female 5% È ª 35.7 cm female 5% È ª 45.7 cm female 5% È ª 55.7 cm G h, cm Figure 20. Lower back stress under different chair heights and seedling placement heights 4-3. Analysis of changing distance between rootstock and scion The lower back stress under different distances between rootstock and scion is shown in figure 21, where the stress curve shall be of U type. If the distance between rootstock and scion is too close or too far away, the lower back stress of 6 human models will increase considerably. So, the distance between rootstock and scion would have significant influence upon the lower back stress. The lower back stress of 6 human models will rapidly increase if the distance between rootstock and scion is more than 45 cm, or increase slowly in the case of distance less than 45 cm, showing a lowest level of lower back stress at distance of 45 cm.

32 Table 7 lists RULA values under different distances of rootstock and scion. It is found that, when the distance between rootstock and scion is less than 15 cm, the action level is up to grade 3, indicating that this distance isn t suitable to all human models. Moreover, the action level is often up to grade 2 when the distance between rootstock and scion is between 25~65 cm, indicating that risk exposure for most of upper arms is acceptable. According to analytical results of LBA and RULA, all human models have lowest lower back stress and optimum operations of upper arms, if the distance between rootstock and scion is 45 cm. It is recommended that the distance between grafting robot s rootstock and scion should be 45 cm, thus creating a comfortable state for the operators. LBA, N male 95th %ile male 50th %ile male 5th %ile female 95th %ile female 50th %ile female 5th %ile Distance between rootstock and scion unit, cm Figure 21. Lower back stress under different distances between rootstock and scion unit Table 7. RULA analysis sheet for different distances of rootstock and scion Distance, Male RULA Female RULA cm 95 th percentile50 th percentile5 th percentile 95 th percentile50 th percentile5 th percentile

33 Analysis of optimized combination According to the analysis of varying operating distance, an optimum distance is 53.7 cm, or 23.2 cm in the presence of leg space. Furthermore, an optimum height is 85 cm (Δh39.3 cm) according to the analysis of changing seedling placement heights. And, an optimum distance is 45 cm from analysis of the changing distance between rootstock and scion. Based on these three optimum conditions, hand load of LBA is set as 50 g (weight of grafted seedling), operating distance set as 53.7 cm, 23.2 cm, chair height set as 45.7 cm--height of nonadjustable industrial chair, distance and height of seedling placement set as 45 cm and 85 cm respectively, under Jack simulation environment. Then, 10-times seedling placement postures are assessed and analyzed in terms of LBA and RULA. The optimum simulation parameters of JACK are listed in Table 8, where model A indicates a change operating distance, model B indicates a changing seedling placement location/height, model C indicates a changing seedling placement location/distance, model D indicates optimum combinations without leg space, and model E indicates optimum combinations with leg space. The lower back analysis sheets for various optimum conditions and combinations are listed in Table 9, where it is found that model D, E have a lower LBA than model A, B, C, indicating that optimum combinations contribute to the decrease of lower back stress. Besides, model E can minimize efficiently the lower back stress with optimum combinations of leg space. RULA values of various optimum conditions and combinations are listed in Table 10, where RULA is often up to grade 2, indicating that risk exposure of upper arms is acceptable in such case. Based on the analytical results of LBA and RULA, it is recommended that optimum combinations of leg space should be improved for the grafting robots, namely, the operating distance is 23.2 cm, seedling placement height is 85 cm, distance between rootstock and scion is 45 cm. Table 8. Optimum simulation parameters of JACK Mode Simulated optimum parameter values