In this chapter, we want to use some simple methods to prove the efficacy of our mechanism. First, we simulate our walking assist device with human’s gait in CAD. In CAD program, we set our assist device to fit in with the degrees in gait analysis as shown in Figure 3.1. Figure 3.1 shows the gait cycle and the simulation. In the first picture, the gait is at double support phase and the right hip joint rotates 30° counterclockwise. As for the hip joint in left leg, it rotates 10°clockwise and rotation doesn’t happen in knee joint. The second picture shows the start of the single support phase. Right leg is at single support phase with a rotation 20° counterclockwise with respect to the hip joint. And the rotation of knee is 20°clockwise. The left leg at this phase is at the swing phase with rotation of hip joint and knee joint for 10°counterclockwise and 35°clockwise. And then the third gait, after the right leg contacts the ground, the center of gravity on the human tilts forward and the rotation of right hip joint goes back to the neutral. So it doesn’t have any rotation in the right leg at this phase. At this phase, the rotations of hip joint and knee joint on the left leg are 10° counterclockwise and 50°clockwise. Finally, the fourth gait which are similar to the first gait with smaller degrees and the position of the right leg is exchanged with the left leg. The right leg is at the stand phase and left leg is at the swing phase in first four pictures, being opposite to the last four pictures. And the degrees are 10° and 20° on right and left legs in the fourth picture respectively. As a result, we don’t depict the last four pictures in detail. We can complete the whole comparison between gait cycle and our pictures of CAD. The pictures of CAD can be suitable for the gait.
According to the discussion above, the mechanism can be realized on actual walking.
Figure 3.1 Simulation of assist device (a) The gait cycle of human. (b) The gait cycle of the walking assist device.
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Next, we use finite element method to make sure our mechanism whether it has problems on its structure. According to the result above, the right leg and left leg exchange with each other and the fourth picture is similar to the first picture. Consequently, we only analyze the limit of mechanism in the first three pictures.
Before setting the boundary condition, we want to discuss the forces applied on the structures. Our purpose is to enhance the elderly’s walking ability so that they don’t need to use the crutches any more. With the aid of our mechanism, they can take care of themselves. Since our mechanism doesn’t cover the ankle and foot, the walking assist device doesn’t support the weight of human. That is, the weight of human is supported by themselves on both legs and it means that we can deem the assist is mounted on human body. However, when the gait is at the single support phase, the center of gravity change, so the weight of leg on swing phase may change the center of gravity. We are not sure how much weight is applied on the device during the center of gravity changing. Finally, the weight of the swing leg may focus on the standing leg. So we set two conditions to simulate the mechanism at the single support phase. First one is no weight applied on the device which only supports the weight of itself. Second one is whole human’s weight applied on the device. As the assumptions above if the device can support the whole weight, we can claim that our walking assist device is strong enough for all the conditions.
Figure 3.2(a) shows our boundary conditions at double support phase. We make the place where the device is worn on the human body be fixed support, including the waist, calf, and thigh. We don’t set the weight of human body here due to the double support phase. In Figure 3.2(b), it is the result of the first gait in Figure 3.1. We can observe that the maximum stress whose value is 3.6 MPa occurs at the place where the fixed bar mounted on.
Figure 3.2 Boundary condition and result of first gait (a) The boundary conditions at double support phase. (b) The result and the place where the maximum stress occurs at double support phase.
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Before talking about the gait at the single support phase, we need to discuss the weight of human body segments first. We need to know how much weight should be applied on the device. Table 3.1 shows the normalized mass of human body segments. The weight of our wearer is about 70 kg in total, so they are 7 kg to both thighs and 3 kg to both calves. These values are consequently used in our boundary conditions.
Table 3-1 Normalized mass of body segments (standard human) [31]
Segment Segment mass/ Total body mass
Hand 0.006
And then we discuss the simulation at the single stand phase of the gait cycle in Figure 3.1. The right leg is at the stand phase and the left leg is at the swing phase at the second gait in Figure 3.1. Therefore, the boundary conditions in second gait are set to be the same as the two boundary conditions discussed above. In the first case, we don’t apply the weight on the device as shown in the Figure 3.3(a). And in the second case, we apply the weight on. The weights of both thighs and both calves are applied on specific parts in our device as shown in Figure 3.3(b). In Figure 3.3(b), the whole weight of thigh (70N) is separated into four parts since the weight needs to be averagely distributed in the device.
The same situation can be set on the calf portion in the third gait. And then Figure 3.4(a) and Figure 3.4(b) show the results of the first boundary condition and the second boundary condition respectively. We can observe that the maximum stress whose values are 16.6 MPa and 91 MPa for the two cases occur at the places where the fixed bars are mounted on.
Finally we talk about the simulation of the third gait in Figure 3.1. In this gait, we set the boundary condition the same as the second pictures and the condition can show in Figure 3.5(a) and (c). And then we can get the results with or without weight applied respectively as shown in Figure 3.5(b) and (d). In Figure 3.5(b) and (d), the maximum stresses whose values are 9.2 MPa and 92 MPa occur at the places where the fixed bars are mounted on.
In result, we can know that the yield strength of the material must exceed 92 MPa so as to be strong enough no matter how much weight is applied on the device.
Figure 3.3 Boundary conditions of second gait (a) The boundary condition without weight applied on right leg single support phase. (b) The boundary condition with weight applied on right leg single support phase.
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Figure 3.4 Results of second gait (a) The result and the place where the maximum stress occurs without weight applied on right leg single support phase (b) The result and the place where the maximum stress occurs with weight applied on right leg single support phase.
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Figure 3.5 Boundary conditions and results of third gait (a) The boundary condition without weight applied on right leg single support phase. (b) The result and the place where the maximum stress occurs without weight applied on right leg single support phase. (c) The boundary condition with weight applied on right leg single support phase. (d) The result and the place where the maximum stress occurs with weight applied on right leg single support phase.
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