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Run-Out Measurement


In high precision engineering the precise knowledge of the movement of a rotating target is of crucial interest. Radial run-out and wobble of a rotating spindle may have significant influence on the quality of a workpiece, and thus needs to be reduced. However, any correction requires an error signal, and thus the run-out and wobble need to be measured accurately.
Subsequently, by applying adequate control loops or lookup-tables, the rotating target may be kept on the desired trajectory.


The setup of this demonstrator is shown. A polished cylinder (stainless steel) was mounted on a SmarAct xy-stage as well as a rotary stage. The xy-stage can either be used to set a specific eccentricity to prove the capability of measuring large run-outs of up to 1 mm, or to correct for it. Two line focusing sensor heads were assembled at a specific height with a 90° orientation to measure the eccentricity, and a third sensor head was mounted above the second one. The latter pair was used to infer the wobble of the cylinder. 

Measurement procedure

The cylinder was rotated by small increments and at each angle the relative displacement of the target with respect to each sensor head was recorded. In the figure the values for the x- and y-direction are shown. The 90° phase shifted signal is (in first order) interpreted as the eccentricity of the sample. The wobble is calculated from the position data of sensor heads 2 and 3. Consequently, a numerical fit is applied to the data that incorporates the wobble of the pin. Thus the effect that each sensor head records the shape of an ellipse instead of a circle is taken into account.



The residues from the pin are due to the non-perfect surface of the pin which is overlaid with bearing errors.
The figure shows these residues of sensor head 1, for all recorded angles between 0 and 360°.
The residues are within ±1.5 µm.
The line focusing heads allowed to track eccentric movements of up to 1 mm while being insensitive to wobble. Thus, the PICOSCALE can be used in high precision engineering or wherever radial run-out and wobble are crucial parameters.