When following a transient trajectory, a simple PID controller will not achieve perfect tracking, nor do you want perfect tracking. Instead when designing a gait you must take into account that the dynamics of the system and controller will transform the reference trajectory into an achieved trajectory, which is what you are actually trying to control. In many situations the reference trajectory is thus not what is actually desired, and yet it is what is being commanded. I am trying to change the controller on our robots to allow us to command the actual desired trajectory, so that if the dynamics change the motion stays the same.