# A Motion Control of Soft Gait Assistive Suit by Gait Phase Detection Using Pressure Information
## 3 Gait Phase Detection Method
### Modification of Phase Detection Method
#### Novel Detection Method  
In the novel detection method, the pressure value is also used in addition to the pressure derivative value. The procedure of the novel detection method is shown as follows.  
1. The time step \(k_{1}\) is detected as the timing that the pressure derivative of the CB PARMs \(\dot{P}_{c}\) crosses the threshold \(\dot{P}_{th}\).
2. The measured pressure of the CB PARMs \(P_{c1}\) (kPa) at the time step \(k_{1}\) is recorded, and the previously detected \(k_{1}\) and \(P_{c1}\) are set as \(k_{2}\) and \(P_{c2}\) (kPa).
3. The difference of \(P_{c1}\) and \(P_{c2}\) are calculated as \(\Delta P_{c}=P_{c1}-P_{c2}\) (kPa). When \(\Delta P_{c}\) is less than the threshold \(\Delta P_{th}\), \(k_{1}\) is set as the assist timing \(k_{a}\).
4. Return to step 1 and repeat these steps.  
First, the time step \(k_{1}\) is detected in step i as:  
\[k_{1}=k\ \ s.t.\ \ \ (\dot{P}_{c}(K)-\dot{P}_{th})(\dot{P}_{c}[k-1]-\dot{P}_{ th})<0 \tag{7}\]  
Equation (7) is the condition that the measured pressure of the CB PARMs crosses the threshold \(\dot{P}_{th}\) at the time step \(k\). Figure 11 shows the sample data of the pressure and the pressure derivative for checking Equation (7). These are the same data as shown in Figure 8. The threshold \(\dot{P}_{th}\) was set as 0.0 kPa/s since this timing contains the phase changing timing between the pre-swing phase and the swing phase. In Figure 11, 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. Red circle markers are all of the detected time step \(k_{1}\) using Equation (7). There are still many candidates of \(k_{1}\) in one walking cycle because of the signal noise. Therefore, additional conditions using the pressure value are given as follows. In step ii,  
Figure 10: Control sequence of PARM pressure using conventional pre-swing phase detection.  
neighboring two data satisfies Equation (7) are recorded. In step iii, by using the two data, these difference \(\Delta P_{c}\) is calculated. The assist timing \(k_{a}\) is detected using \(\Delta P_{c}\) as:  
\[k_{a}=k_{1}\ \ s.t.\ \ \ \Delta P_{c}<\Delta P_{th} \tag{8}\]  
Equation (8) is the condition that the measured pressure of the CB PARMs decreases more than the threshold \(\Delta P_{th}\) at the time step \(k_{1}\). Finally, the assist timing \(k_{a}\) is obtained as shown in Figure 12. Circle markers are all the assist timing \(k_{a}\), and there is only one marker in one walking cycle. The target of Equation (8) is the changing timing from the pre-swing phase to the swing phase. There is large pressure difference between the marker of the time step \(k_{a}(=k_{1})\) and the previous marker of the time step \(k_{2}\). The condition \(\Delta P_{c}<\Delta P_{th}\) means the feature mentioned above. The threshold \(\Delta P_{th}\) was set as \(-5\) kPa in Figure 12. \(\Delta P_{th}\) is configured in each trainee appropriately by considering the detectability index \(J_{p}\). The large \(\Delta P_{th}\) will be effective to avoid the wrong detection, however, \(\|\Delta P_{th}\|\) should be lower than \(J_{p}\). Thanks to the novel detection method, the issue of the conventional detection method will be solved. Since the non-detection time is not required, the trainee can walk with a high walking rate.  
Figure 11: Sample data of the pressure and the pressure derivative for checking Equation (7). 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 detected time step \(k_{1}\).