Machado, J. A. Tenreiro2014-02-072014-02-0720120924-090X1573-269Xhttp://hdl.handle.net/10400.22/3817Discrete time control systems require sample- and-hold circuits to perform the conversion from digital to analog. Fractional-Order Holds (FROHs) are an interpolation between the classical zero and first order holds and can be tuned to produce better system performance. However, the model of the FROH is somewhat hermetic and the design of the system becomes unnecessarily complicated. This paper addresses the modelling of the FROHs using the concepts of Fractional Calculus (FC). For this purpose, two simple fractional-order approximations are proposed whose parameters are estimated by a genetic algorithm. The results are simple to interpret, demonstrating that FC is a useful tool for the analysis of these devices.engFractional calculusFractional-order holdsGenetic algorithmsControljournal article10.1007/s11071-012-0495-y