In this work an interval observer is proposed for on-line estimation of differentiation errors in some class of high order differentiators (like a high-gain differentiator from [26], or homogeneous nonlinear differentiator from [24 or super twisting differentiator [15]). The results are verified and validated on the telescopic link of a robotic arm for forestry applications in which the mentioned approaches are used to estimate the extension velocity while the interval observer gives bounds to this estimation.