An example of a physical system whose mechanical accuracy can be improved by feedback control is a motor-driven spindle. Such a system is being used as a test-bed to study measurement and actuation systems as well as control algorithms. The specific apparatus reported utilizes an eddy-current probe for runout error measurement, a piezo-electric crystal to move the spindle to reduce the error, and a minicom-puter using a FORTRAN program for the feedback controller. The spindle runout before correction is in the 2.5 itm (100 gin.) range; with the error correction system in place, this error is reduced to less than 0.25 um (10 pin.)-an order of magnitude improvement. While the corrected runout figures for this spindle are still above those of a precision air-bearing spindle, the technique presents new possibilities for precision spindle performance including correction for wear, thermal deformations, and unbalanced loads.