An Unpowered Knee Exoskeleton for Walking Assistance and Energy Capture

Abstract

In order to reduce the energy consumption of human daily movement without providing additional power, we considered the biomechanical behavior of the knee during external impedance interactions. Based on the theory of human sports biomechanics, combined with the requirements of human-machine coupling motion consistency and coordination, an unpowered exoskeleton-assisted device for the knee joint is proposed in this paper. The effectiveness of this assisted device was verified using gait experiments and distributed plantar pressure tests with three modes: "not wearing exoskeleton" (No exo.), "wearing exoskeleton with assistance " (Exo. On), and "wearing exoskeleton without assistance" (Exo. Off). The experimental results indicate that (1) This device can effectively enhance the function of the knee, increasing the range of knee movement by 3.72% (p < 0.001). (2) In the early stages of the lower limb swing, this device reduces the activity of muscles in relation to the knee flexion, such as the rectus femoris, vastus lateralis, and soleus muscles. (3) For the first time, it was found that the movement length of the plantar pressure center was reduced by 6.57% (p = 0.027). This basic principle can be applied to assist the in-depth development of wearable devices.

Keywords: energy capture; energy compensation mechanism; knee joint; unpowered exoskeleton; walking assistance.

Figure 1

Typical gait phases.

Figure 2

Schematic diagram of gravity balance state. The lower limbs perform (a) positive work, (b) no work and (c) negative work respectively.

Figure 3

Fitting curve of the knee average power.

Figure 4

Power changes during assisted walking. Two conditions of joint power are described: an expected curve (red) and actual curve (blue). The area formed between the two curves is indicated using red (above the X-axis) and blue (below the X-axis) slashes respectively. When parameter $S_1$ is a negative number, $S_2$ is (a) positive, (b) 0 and (c) negative. When parameter $S_1$ is equal to 0, $S_2$ is (d) positive, (e) 0 and (f) negative. When parameter $S_1$ is a positive number, $S_2$ is (g) positive, (h) 0 and (i) negative.

Figure 5

The human exoskeleton system. (a) State diagram of wearing a lower-limb exoskeleton under normal walking conditions (b) Geometric relationship of lower limb parameters (c) Force analysis diagram of scroll spring.

Figure 6

Structure of knee-joint exoskeleton. (a) The resin prototype. (b) The aluminum alloy prototype. (c) Man–machine coupling of 3D digital prototype. (d) and (e) provide the details of the device. (f) The device is on the electronic scale.

Figure 7

The flow chart of the experimental process and data analysis.

Figure 8

Experimental equipment and marking points. (a) Motion capture experiment. (b) Distributed plantar pressure test. (c) Marker points for full body.

Figure 9

Experimental scene. (a) Dynamic capture and (b) Distributed plantar pressure test in three states. (c) Six tested muscle positions.

Figure 10

Human body reference plane and axis system.

Figure 11

Variation in hip and knee joint power of the participant during the walking task. (a) The change in knee power for three conditions during the gait cycle. (b) The variation range in hip and knee power for three conditions.

Figure 12

Participant’s hip–knee angle in the walking task. The hip–knee angle during the walking task for (a) “not wearing exoskeleton” (No exo.), (b) “wearing exoskeleton with assistance“ (Exo. on), and (c) “wearing exoskeleton without assistance” (Exo. off). (d) The change curve for the knee joint angle in a complete gait cycle. (e) Change in knee angle for 40~74% gait cycle (knee flexion to the maximum angle).

Figure 13

Variation in joint moment of participant during the walking task. (a) The change in knee moment for three conditions during the gait cycle. (b) The variation range of the hip and knee moment for three conditions.

Figure 14

Comparison of the moving length of the plantar pressure center for the walking task.

Figure 15

Variation in plantar force, areas and pressure in walking task. (a–d) Describe the changes in plantar forces, contact areas, average pressure, and maximum pressure, respectively. The time is normalized.

Figure 16

Muscle activation. These curves represent two different conditions: “No exo.” (red) and “Exo. on” (cyan). During the gait cycle (heel to heel contact), the active state of six tested muscles was analyzed, including (a) the rectus femoris, (b) vastus lateralis, (c) semitendinosus, (d) tibialis anterior, (e) peroneus longus, and (f) soleus muscle.