DoF	Degree of freedom
EHA	Electro-hydraulic actuator
SMC	Sliding mode controller
ACL	Anterior cruciate ligament
PCL	Posterior cruciate ligament
ICR	Instantaneous center of rotation
SEA	Series elastic actuator
HA	Hydraulic actuator
ROM	Range of motion
MCU	Microcontroller unit
SSE	Sum of square error
RMSE	Root mean square error
BLDC	Brushless DC
I/O	Input/output
CAN	Controller area network
SPI	Serial peripheral interface
FSR	Force-sensing resistor
PID	Proportional-integral-differential
PCHIP	Piecewise cubic Hermite interpolation
Nomenclature
${\theta }_p$	Relative angle between the ground link and the point $P$ within the coupler $(°)$
$x$	Position $\left(\mathrm{mm}\right)$
$p_n$	Coefficients of polynomial with respect to ${\theta }_p$ $(-)$
$q_n$	Coefficients of polynomial with respect to $x$ $(-)$
$D_m$	Ideal volumetric displacement of the motor $\left({\mathrm{mm}}^3/\mathrm{rev}\right)$
${\dot{\theta }}_m$	Motor shaft speed $\left(\mathrm{rev}/\mathrm{s}\right)$
$Q_1$	Return flow from motor in pump, supplied flow in cylinder $\left({\mathrm{mm}}^3/\mathrm{s}\right)$
$Q_2$	Forward flow to motor in pump, return flow in cylinder $\left({\mathrm{mm}}^3/\mathrm{s}\right)$
$C_{im}$	Internal or cross-port leakage coefficient $\left({\mathrm{mm}}^3/\mathrm{s}/\mathrm{bar}\right)$
$C_{em}$	External leakage coefficient $\left({\mathrm{mm}}^3/\mathrm{s}/\mathrm{bar}\right)$
$P_1$	Pressure in the return chamber $\left(\mathrm{bar}\right)$
$P_2$	Pressure in the forward chamber $\left(\mathrm{bar}\right)$
$Q_L$	Load flow $\left({\mathrm{mm}}^3/\mathrm{s}\right)$
$P_L$	Pressure difference $\left(\mathrm{bar}\right)$
$V_1^0, V_2^0$	Two chambers of initial condition $\left({\mathrm{mm}}^3\right)$
${\beta }_e$	Bulk modulus $\left(\mathrm{bar}\right)$
$P_r$	Reference pressure $\left(\mathrm{bar}\right)$
$C_{{im}^{\prime }}$	Coefficient of the internal leakage $\left({\mathrm{mm}}^3/\mathrm{s}/\mathrm{bar}\right)$
$C_{em{1}^{\prime }}$	Coefficient of the external leakage from the return chamber $\left({\mathrm{mm}}^3/\mathrm{s}/\mathrm{bar}\right)$
$C_{{em2}^{\prime }}$	Coefficient of the external leakage from the forward chamber $\left({\mathrm{mm}}^3/\mathrm{s}/\mathrm{bar}\right)$
$A_1, A_2$	Area of each chamber $\left({\mathrm{mm}}^2\right)$
$V_t$	Total hydraulic actuator volume $\left({\mathrm{mm}}^3\right)$
$\overline{A}$	Average of cross-sectional area of chamber $\left({\mathrm{mm}}^2\right)$
$m$	Mass of the load $\left(\mathrm{kg}\right)$
$B$	Damping coefficient $\left(\mathrm{N}/\left(\mathrm{m}/\mathrm{s}\right)\right)$
$K$	Spring coefficient $\left(\mathrm{N}/\mathrm{m}\right)$
$F_L$	Disturbance $\left(\mathrm{N}\right)$
$u$	Motor shaft speed, control input $\left(\mathrm{rev}/\min \right)$
$\widetilde{s_r}$	Scaling factor $(-)$
$s$	Sliding surface $(-)$
$\lambda$	Strictly positive constant $(-)$
$x_d$	Desired position $\left(\mathrm{mm}\right)$
$e$	Tracking error $\left(\mathrm{mm}\right)$
$\eta$	Design parameter $(-)$
$\Phi$	Boundary layer thickness $(-)$