A magnetic recording disk drive with rotational vibration (RV)
cancellation uses the position error signal (PES) and the signal from a
RV sensor to determine when to enable and disable RV cancellation. An RV
feedforward compensation signal to be summed with the VCM control signal
is "switchable", meaning that it can be enabled or disabled by the disk
drive servo control processor. The determination to enable or disable is
made from a comparison of the PES with a threshold PES, which may be an
estimate of the off-track position of the head calculated from the RV
sensor signal. The estimated off-track is compared to the absolute value
of the actual or measured PES. If the estimated off-track is smaller than
the actual PES, then the state of the RV cancellation is switched, i.e.,
if it is enabled it is disabled and if it is disabled it is enabled.