# A Motion Control of Soft Gait Assistive Suit by Gait Phase Detection Using Pressure Information
## 3 Gait Phase Detection Method
### Pressure Configuration in Calf Back PARMs  
The proper pressure in the CB PARMs is required for measuring the pressure change, and it is configured by the walking experiments on the treadmill as shown in Figure 7. The trainee wears the gait assistive suit, and the initial pressure in the CB PARMs is set as several values, when the trainee stand upright on the flat floor and the CB PARMs don't slack. The initial pressure is adjusted by the pressure regulator 2 shown in Figure 3. While the trainee walks on the treadmill, the pressure in the CB PARMs is measured, and we will evaluate the pressure change amount. In this paper, the detectability index is defined as follows:  
\[J_{p}=\text{max }P_{(t)}-\text{min }P_{(t)}\ \ \ (0\leq t\leq T_{c}) \tag{5}\]  
where \(T_{c}\) (s) is a gait cycle time. Equation (5) calculates the maximum pressure change amount \(J_{p}\) (kPa) in the one cycle motion. When \(J_{p}\) is larger, the assist timing detection will become easier. The experimental parameters are given as follows. The trainee walks on the treadmill 40 steps of 20 cycles with its speed of 4.0 km/h in each trial. In this paper, we used the manual treadmill (All market japan Inc., YT-RR). A metronome is used for equalizing the walking rate to make the data analysis easy, and its tempo is adjusted in each trainee. The initial pressure is given as 0, 50, 100, 150, 200, 250, 300 kPa. The walking data with the initial pressure of 0 to 300 kPa are measured in randomized order. Each pressure data is measured in one trial respectively. After each trial, the trainee rests for 5 min. Finally, 20 cycle data of \(J_{p}\) in each leg is obtained, and the proper pressure will be determined. Figure 6: Deformation of CB PARM in pre-swing phase ((**left**) long length state, (**right**) short length state). The red line represents a wire whose length is constant. At the start of the pre-swing phase, the PARM is stretched and its length is long. After that, the stretched PARM contracts in a short time, and its length and pressure become short and low. For example, one healthy male trainee in his 30 s with the height of 171 cm and the weight of 72 kg wore the gait assistive suit and walked on the treadmill with its walking rate 100 beats per minute (BPM). The experimental result is shown in Figure 8, whose initial pressure in the CB PARMs is set as 200 kPa. The horizontal axis shows time (s), and walking phases are distinguished by color areas. The gait phases are classified by the synchronized recorded video data and the classification method of [37]. 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 cut-off frequency of the low-pass filter of the pressure was set as 10 Hz which assumes to be sufficiently high for measuring the human walking motion. The lower figure shows the pressure derivative. The cut-off frequency of the numerical differentiation of the pressure was also set as 10 Hz. As a result, the measured pressure was changed around the given initial pressure, and its change amount is calculated as \(J_{p}\) in each cycle. The pressure derivative signal changes large in the pre-swing phase. The heel contact occurs between the swing phase and the stance phase, however, the pressure data seems to be smooth. Therefore, the effect of the impact force will be negligibly small for phase detection. The calculated index \(J_{p}\) of the CB PARMs is shown in Figure 9. The horizontal axis shows the given initial pressure. Blue and red colors mean values of the RCB and LCB PARMs. Markers are average values, and thin bars are standard deviations. The detectability \(J_{p}\) became large when the pressure was over 100 kPa since the PARMs were sufficiently shrunk and not loose through whole walking cycle. In this case, we set the proper pressure for this trainee to 150 kPa since it showed the largest detectability of the LCB PARM.