Mechanism evaluation of agricultural power assist suit under development

3.1. DOF achievement rate

The DOF achievement rate is a rate that expresses ease of motion, and it is defined according to the movable range, which can be achieved by the anatomical basic position of a human body [3]. The DOF rate $n$ of each joint is defined by the following Eqs. (1-2) assuming that the range of human joint motion is represented by $\theta$, the range of motion of the joint motion for the agricultural PAS by $\varphi$, and the deviation angle of the rotating axis of the human joint and the agricultural PAS joint by $\psi$:

For example, in the case of a joint with three degrees of freedom-flexion/extension, abduction/adduction, and external rotation/internal rotation-representing the ratio of the range of joint motion as $r$, it is obtained as follows Eq. (3):

And representing the DOF achievement rate of the entire agricultural PAS as $N$, the DOF achievement rate of each joint as $n$, and the ratio between the ranges of joint motions as $R$, $N$ is defined by the following Eq. (4):

The subscripts are as follows, $s$: shoulder, $e$: elbow, $r$: rear, $h$: hip, $k$: knee, $a$: ankle.

3.2. Defining the ranges of joint motions and evaluation based on DOF achievement rate

The ranges of joint motions for the average physique of Japanese adult male, which was the manufacturing standard of the agricultural PAS, the range of joint motions for the design model, and part of the range of joint motions when wearing the agricultural PAS are each listed in Table 1. Informed consent was obtained for the measurement of the ranges of joint motions.

Using data from Table 1, the DOF achievement rates when wearing the PAS were calculated by solving Eqs. (1-4). Fig 3 shows the design model describing the joint rotation axes. Fig. 3(a) shows the joint model of humans. Fig 3(b) shows the joint model of PAS, including passive joints, ratcheted passive joints, active joints, and the assist mechanisms. Fig. 4 shows the results of calculating the DOF achievement rates, $n$ for the design model, each joint when the PAS is worn, and $N$ for the whole body. The DOF achievement rates, $N_M$ for design model and $N_W$ for the whole body wearing the PAS, are shown as values approximating 0.83 and 0.76, respectively, indicating that motion of 70 % or more of body motions are possible. The DOF achievement rate $n$ for each joint differed at the ankles and shoulders between the design model and the PAS when it is worn. The values when it is worn were lower than those of the design model. At other joints, the difference was within 5 %. At the ankle, the distance between the bottoms of the foot is fixed; thus, differences in the center of rotation of the joint and narrowing of its range of motion occurred. At the shoulder joint, the directions of motions in which the range of motion when wearing the PAS is particularly reduced from that of the design model are adduction and flexion. This occurs because interference with the shoulder joint that covers the top of the clavicle prevents the wearer from raising his arms. In order to broaden the range of motion, it is necessary to form clearance with the clavicle by holding the shoulder joint in an arch-shape. Further, regarding flexion, an increase in the angle of flexion elevates the shoulder girdle, so under present conditions, it is necessary to improve the range of motion, which is 0°. For the back, the value was low for both the design model and the PAS when it is worn. The back frame has to support the upper body part of the PAS, so later flexion and rotation joints are not provided. Flexion and extension can be realized by the rotating joint of the hip joint.

Since it is an exoskeleton, the ranges of motion of the design model of the agricultural PAS differ from those of agricultural PAS when it is worn. Problems when it is worn, which are not revealed by the model alone, are clarified by the calculation of the DOF achievement rate.

Fig. 3Design model

Fig. 4Comparison of DOF achievement rate

4.1. Work for harvesting radish

The work posture according to evaluation of posture by the OWAS method is often a half-sitting posture, with the back bent forward and with both upper limbs lower than the shoulders. The operation of the lower limb joints while harvesting radishes was a repetitious constant pattern of motions. Next, the motion of the lower limbs was accompanied by constant motion of the upper limbs, which cyclically repeated identical motions.

4.2. Work for harvesting cucumber

Cucumbers are generally grown on fence-shaped racks and harvesting of ripened cucumber is done whenever necessary. The working posture according to posture evaluation by the OWAS method is often one where the lower limbs in a half-sitting posture with both legs bent, the back bent forward, and both the upper limbs are held at a level lower than the shoulders. Unlike the case when harvesting radishes, the motion pattern is not repetitive. The reason is that the location of the harvested crop is not regular was it is in the case of the radishes. Cucumbers are at various heights from the top to the bottom of the racks, so the workers frequently raise or lower their bodies to find the cucumbers amidst the leaves and branches, or to peer into the leaves and branches.

Further, the upper limbs move irregularly because it is necessary for the workers to push aside the leaves and branches in order to search for cucumbers and pick them.

4.3. Work for pruning fruit tree

Fruit trees are generally cultivated on flat shelves at locations slightly higher than the height of the workers. Therefore, the branches and fruits are above the workers’ heads so their arms are both raised and they are facing upwards. The work posture according to the posture evaluation by the OWAS method is often one where the workers are standing straight up with both arms extended, the backs are extended, and both arms are higher than the shoulders.

Pruning work is usually done with the shoulder joints flexed at around 90° and the elbow joints flexed at around 60° and performing small motions within a range of 30° according to the height of the branches and fruit. Since the workers elevate their arms for long periods of time, fatigue accumulates from the shoulders to the upper arms. The lower limbs remain almost straight, with the knee and waist joints occasionally flexing slightly when the worker moves. As this shows, when pruning fruit trees, workers often stand straight with their arms held above their shoulders, so this PAS is expected to lighten the work by supporting the upper limbs.

Fig. 5Farm work

a) Radish

b) Cucumber

c) Fruit

4.4. Evaluation based on adaptability

Adaptability is defined as the extension of the DOF achievement rate, and is defined according to the range of motion of each joint manifested in the object of agriculture work motions and the range of joint motion of when the agricultural PAS is worn. It is defined by the following Eq. (5) when the range of joint motions of the body of a worker performing normal agricultural work is represented by ${\theta }_{adp}$, and the range of motion of joints during agricultural work by a worker wearing the agricultural PAS as ${\varphi }_{adp}$:

The values of $n_{adp}$ for each joint and $N_{adp}$ for the whole body are calculated in the same way as the DOF achievement rate. The DOF achievement rate was obtained according to the maximum range of motion of the joints, but the operation of joints during work rarely achieves the maximum range of motion, and adaptability is a value higher than the DOF achievement rate. Fig. 6 shows the results of calculating adaptability.

It is possible to harvest radishes with well-balance motions, and a high whole body adaptability score of 0.92 was achieved. The whole body scores for harvesting cucumber and pruning fruit tree were relatively high scores of 0.84 and 0.83, respectively. However, in both cases, the values for the shoulder joints were low, and in cucumber harvesting, the score for the back was particularly low. For this reason, it is presumed to be necessary to improve adaptability for shoulder joints, or in other words, improve the range of joint motions of the shoulder joints. Next, the adaptability of the back in harvesting cucumber must be improved.

Evaluation based on adaptability revealed differences in the applicability of agricultural PAS to different types of agricultural work, permitting the identification of joints where there are problems.

Fig. 6Comparison of adaptability