Figure 4.

Illustrated definition of the Tegotae function for the knee controller. The control variable χ_i represents the control command that increases or decreases the knee joint stiffness (Equation 9). We use χ_i to adjust the P-gain K_{knee, i} in the knee controllers for the implementation of stiffness control mechanism, in Equation (10). $N_i^V$ and $N_j^V$ represent the sums of the vertical force sensor values corresponding to the heel, metatarsal, and toe joints of the corresponding (i) and opposite (j) legs, respectively. (A) Tegotae function for corresponding-leg sensory information. The value of $N_i^V{\chi }_i$ is positive when the foot sensors on the corresponding leg detect a large vertical GRF ($N_i^V>0$) and the control command for the knee is to increase the stiffness (i.e., χ_i > 0). (B) Tegotae function for opposite-leg sensory information. The value of $N_j^V(-{\chi }_i)$ is positive when the foot sensors on the opposite leg detect a large vertical GRF ($N_j^V>0$) and the control command for the knee is to decrease the stiffness (i.e., χ_i < 0).