Workflow diagram of the SAPM. xz, αx, αy mean the displacement in the z-direction, and rotational angles around the x- and y-directions of the US probe, respectively; dj|j=1,2,3 are the moving distances measured by the measurement and buffer units j; Fj|j=1,2,3 are forces applied to the measurement and buffer units j; Fj−0|j=1,2,3 are predetermined forces for the measurement and buffer units j; φj|j=1,2,3 are the rotatinal angles of the stepper motors j in the adjustment units j; Fz indicates the force exerted on the US probe in the z-direction; Mx and My are the torques applied on the US probe around the x- and y-directions, respectively.