A Physical experimental study of variable-order fractional integrator and differentiator H. Sheng, H.G. Sun, C. Coopmans, Y.Q. Chen and G.W. Bohannan Eur. Phys. J. Special Topics, 193 (2011) 93-104 Published online: 04 April 2011 DOI: 10.1140/epjst/e2011-01384-4