# A Motion Control of Soft Gait Assistive Suit by Gait Phase Detection Using Pressure Information
## 4 Gait Assistive Control Method
### Control of Pressure Input Time  
In the conventional control method in [18], the pressure input time of the PARMs is given as a constant value. However, when the walking speed is changed, the input time should be also changed appropriately. In this paper, the gait cycle time is calculated from the assist timing, and the proper pressure input time is obtained from the calculation result. Since the assist timing in each gait cycle is detected as shown in Figure 12, the gait cycle time \(T_{c}^{i}\) (s) can be obtained as:  
\[T_{c}^{i}= \begin{cases}T_{cal}&(T_{cal}\leq T_{c}^{max})\\ T_{c}^{max}&(T_{cal}>T_{c}^{max})\end{cases} \ s.t.\ T_{cal}=(k_{a}^{i+1}-k_{a}^{i}) \Delta T \tag{9}\]  
where the superscript \(i\) is the number of the gait cycle, and \(\Delta T\) (s) is the sampling interval time. \(T_{c}^{max}\) (s) is the upper limit of \(T_{c}^{i}\). The average gait cycle time \(T_{c\text{ {arg}}}^{i}\) (s) is calculated from the last \(n\) data of \(T_{c}^{i}\) as:  
Figure 12: Sample data of the pressure and the pressure derivative for checking Equation (8). The horizontal axis shows time (s), and walking phases are distinguished by color areas. Red areas represent the stance phase, green areas represent the pre-swing phase, and yellow areas represent the swing phase. The (**upper**) figure shows the pressure in the RCB PARM. The (**lower**) figure shows the pressure derivative in the RCB PARM, and blue solid lines and yellow dashed lines are calculated value and the threshold 0 kPa/s. Red circle markers are assist timing \(k_{a}\). \[T_{c\;ave}^{i}=\frac{1}{n}\sum_{k=i-n+1}^{i}T_{c}^{k} \tag{10}\]  
\(T_{c\;ave}^{i}\) is updated in every gait cycle, and it is used for calculating the pressure input time of the PARMs. The pressure input time of the PARMs is given as shown in Figure 13. The horizontal axis shows gait percentage, and the vertical axis shows reference pressure in each PARM. ON (100%) and OFF (0%) mean the pressurized state and the depressurized state. Walking phases are distinguished by color areas. Red areas represent the stance phase, green areas represent the pre-swing phase, and yellow areas represent the swing phase. The pressurized timing and duration are given from the EMG activation time in each muscle near the PARM [37]. The pressurized timing and duration are represented as the percentage in one walking cycle, and these times are defined as the functions of the average gait cycle time \(T_{c\;ave}^{i}\) which is assumed as the one walking cycle time. The pressurized time of the TF PARMs is 90-10% which is the activation time of vastus lateralis. The pressurized time of the TB PARMs is 80-10% which is the activation time of semitendinosus. The pressurized time of the CF PARMs is 55-13% which is the activation time of tibialis anterior. The detection timing is assumed as 55% in Figure 13. For example, the CF PARMs are pressurized immediately after the pre-swing phase detection. The joint motion driven by the TF and TB PARMs is discussed as follows. In Figure 13, the TB PARM is pressurized slightly earlier compared with the TF PARM in the swing phase, and depressurizing timing of these PARMs is same in the stance phase. When only the TB PARM is pressurized, its contraction force flexes the knee joint and extends the hip joint. This motion assists the landing motion. After that, the TF PARM is also pressurized, and the leg link system is driven like an antagonistic driven system. When the same pressure is supplied in the TF and TB PARMs, these PARMs can be assumed as the air springs with the same characteristic. In this case, the stretched PARM outputs larger force than the shrunk PARM, therefore the TF and TB PARMs make the leg posture as upright standing. The PARM contraction force extends the knee and hip joints. These PARM motions are mimetic motions of each target muscle. Figure 13: Pressure input time of PARMs controlled by servo valves. The horizontal axis shows gait percentage, and the vertical axis shows reference pressure in each PARM.