If the LBB is rotating, there will

If the LBB is rotating, there will introduce another type of friction, namely, the viscous friction. The coulomb friction and viscous friction together tend to slow down the motion and stabilize the LBB. In conclusion, the friction does not harm but even improve the balance performance in the sense that the PGCU designed to balance a certain nominal LBB with a balance torque τn can also balance any LBBs whose required balance torques are within [τn − τf , τn + τf ]. Admittedly, the friction will consume energy when the exoskeleton moves dynamically. This problem cannot be solved by the pure passive mechanism. An actuated exoskeleton may solve this problem, which is out of scope for the paper.