# Verification of Industrial Worker Walking Efficiency with Wearable Hip Exoskeleton
## 2 Materials and Methods
### Ultralight Wearable Hip Exoskeleton Device  
The walking-assistive device used in this study was an ultralight wearable exoskeleton for hip joint assistance weighing 1.4 kg and developed to support the walking mobility of industrial workers.  
A hip exoskeleton device called \"We Innovate Mobility\" (WIM) uses a single motor while walking to assist the movement of both hip joints simultaneously. When a rotational force is generated by the motor drive, torque is applied to both thigh supports (or adaptive frames). A flexion auxiliary torque is generated on the leg moving forward for the swing; conversely, an extension auxiliary torque is generated on the trailing leg. The drive strategy is based on human gait mechanics [20] and is a form-factor design in which the main body equipped with a single motor is in front of the pelvis, as shown in Figure 1.  
Table 1 presents the WIM dimensions and hardware specifications. The main body volume when the thigh frame is folded is \(23.8\times 10.0\times 5\) cm\({}^{3}\). This space consists of a single electric motor, battery, and controller. The battery capacity is 3.35 Ah, and it can be used continuously for approximately 2 h with a peak-assist torque of 4 Nm. The main body comes in one size, whereas the waist belt and thigh attachments are available in small and medium sizes, and they can be worn with a waist circumference ranging from 26 to 36 inches (66.04-91.44 cm). The stroke length of the adaptive frame ranges from 160 to 350 mm. This ensures compatibility with various heights and leg lengths. Exoskeletons that are wearable on top of clothing have a structural difference between the rotational drive and joint rotation centers. If this mismatch is not addressed with an adjustable flexible mechanism, the device can be ob  
Figure 1: Wearable hip exoskeleton (WIM) used in the study.  
This complements the shortcomings of existing wearable exoskeletons that place dual actuators on the sides of the hip joint and improves convenience in that users do not have difficulty moving, even in a narrow space. In addition, the WIM's compact size improves wearability by applying a soft waist belt, and the foldable storage reduces the device volume at least five times compared to that of existing rigid-type frame exoskeletons [15].  
The hip exoskeleton WIM has 11 degrees of freedom (DoF) mechanically; these consist of 3 DoF in the hip, 2 DoF in the adaptive frame, and 6 DoF in the thigh connector. Only 1 DoF is an active joint at the hip. The thigh frames on the sagittal plane are driven in both the widening and narrowing directions. The assistance torque control is performed using current sensing without the torque sensor, and it is based on backdrivable actuators [21]. Backdrivable actuators are flexible drives with minimal drive friction and resistance, enabling them to move smoothly even when the device's torque is off. The generation of assistance torque is based on the difference in hip angles sensed in a sine waveform during walking. Using this information, a symmetrical torque in the form of gait assistance is generated for both hips.