1. A system for controlling velocity of a plant, comprising: a first order state space model comprising a velocity value, v(t), where v(t) equals the sum of f(t) and b*u(t), f(t) represents the combined effects of internal dynamics and external disturbance of the plant, u(t) is a control signal, and b is a constant to an approximate value; a linear extended state observer, which is a function of a single performance parameter, for estimating the value of f(t); and a monitor adapted to cancel the effect of f(t) on the velocity value by an estimate from the linear extended state observer.