Track following servo for higher density disk files

Abstract

Described is a novel technique and associated arrangement for determining the noise-free value of a system parameter (e.g., head position in a disk drive) which is time variable and (usually) has a noise component as detected. The technique involves processing the (as detected) noise-including value and passing it, plus a 'Time-derivative version' thereof, through 'Second Order/Summing' filter means. For instance, the technique is described as particularly useful with the 'track-following servo' (part of the transducer positioning means) in a high density disk file (where track density is higher than usual) to secure superior head-displacement error values which are more noise-free. Thus, (see FIG. 4), a pair of first values V.sub.a, V.sub.b are secured by detecting motor current sense voltage V.sub.i, integrating it with respect to time and passing the result through a pair of novel 'second order filter' means--one band-pass (F.sub.1), the other low-pass (F.sub.2); while also securing a third value V.sub.c by detecting position error (signal voltage, including noise components) in the usual manner and passing it through a 'second order low-pass' filter F.sub.3 ; then algebraically summing all filter outputs to secure a relatively noise-free position error signal V.sub.xo [V.sub.xo =V.sub.a +V.sub.b +V.sub.c ]. Thus, the summing filter array gates the position-error voltage but attenuates the (mechanical) noise component (cf. filters F.sub.2, F.sub.1 centered at w.sub.o --e.g., 600 Hz is satisfactory here; and having a prescribed 'Quality factor' Q). Such 'second order' filters are novel in themselves, and particularly as so applied (most especially as so-used in such a servo system).