Ripple-free Control Response by Using a Fractional Order Hold Device

Abstract: In digital control, discrete-time signal reconstruction is usualy carried out by the zero-order hold (ZOH), although other options exist which exhibit interesting properties. A remarkable alternative is the fractional-order hold (FROH), which provokes no intersample ripple under steady state and, if properly tuned, can often place the discretized plant model z-domain zeros in more convenient locations than the ZOH. This zero placing technique, in turn, gives rise to closed-loop performance improvements for many cases. However, since it is formulated in the z-domain, it cannot anticipate continuous-time closed-loop behaviour. Besides, going through zero placement is burdensome, especially for high-order models. This work presents a new and simpler method which directly links the tuning of the FROH with some desired closed-loop features, bypassing zero placement. It is based on the frequency domain, which makes it possible to capture the continuous-time behaviour. The simulations performed have shown clear improvements over zero placement, in terms of time-domain performance figures, intersample behaviour, and the energy consumed in step responses