We next present a formula for calculating θ from the cosine theorem as Eq. (7), and an equation for obtaining the foot reaction force N by moment balancing as Eq. (8).
\(\cos\theta=\frac{q^2+r^2-p^2}{2qr}\) (7)
\(N=\frac{F\sin\theta\cdot r}{R}\) (8)
Calculation based on these equations revealed the necessity for strokes of 43.6 mm in the pushing direction and 98.2 mm in the pull direction.
The thrust of the cylinder is obtainable as (pressure receiving area) × (internal pressure of the cylinder). Originally, we should have chosen a reciprocating cylinder. However, because this study covers ankle spasm of cerebral infarction patients, portability must be achieved through size and weight reduction. For that reason, we chose a single acting cylinder for dorsiflexion operation.
In the selected cylinder (M24D 150.0 Airpel; Airpot Corp.), the thrust at applied pressure of 0.10 MPa is the theoretical (calculated) value of 40 N, confirming that sufficient thrust is generated for experiments. Results of calculations show that 39.4 N was obtained at 0.1 MPa. The thrust obtained during the experiment was 40.1 N, which was a satisfactory value. The calculation formula accuracy was confirmed.
3.3 Design for control system
To realize the rehabilitation operation of a physical therapist by control design, an ankle rehabilitation device was designed with the control system to switch from position control to force control. In this control, training for the target angle is repeated for a healthy person or users with weak contracture of the ankle by position control. Even if position control is performed by contracture, for patients who do not reach the target angle, position control is switched to the force control. A flowchart is shown in Figure 5.