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PicoScale Radial Runout Correction

The PicoScale can be used as a sensor in control loops to stabilize positions. Here, we demonstrate this capability by correcting a rotating pin for radial runout using SmarAct linear translation stages and the PicoScale as high precision position sensor. The position data are fed back to the compensation stages and very efficient compensation is realized.


In order to demonstrate the possibilities of an interaction between SmarAct stages and the PicoScale laser interferometer, an XY-stage made from SLC-1730-S linear positioners was mounted on top of an SR-4513-S rotary stage. On top of the stage, a polished cylindrical aluminum pin (6 mm diameter) was clamped with a set screw into a holder. Two PicoScale sensor heads were placed in a 90 degree angle and aligned to the surface of the pin. A photograph of the setup is shown in figure 1.

The pin was not centered on the rotary axis on purpose. Therefore, a radial runout of the pin surface is expected. The aim of the demonstration is to detect this runout simultaneously with the two sensor heads, and use the interferometer signal to control the XY-stage in such a way that the remaining runout is minimized.

Figure 1. Demonstrator Setup


Each axis of the XY-φ-Stage features nanometer resolution sensors. The stage is rotated in open-loop mode with a Piezo step frequency of 1kHz . The displacement signals of the two PicoScale sensor heads are continuously taken and plotted. Thus, the radial runout of the pin as well as eventual surface irregularities are shown in the PicoScale signal. Once the feedback loop in the GUI is turned on, the PicoScale displacement signals are transformed into the rotating coordinate system of the XY-stage. This is achieved by taking into account the angular value measured by the encoder in the rotary stage. Now the two linear positioners receive closed-loop movement commands according to the current displacement in their respective directions.
Now, the displacement measured by the PicoScale is almost reduced to zero, because the runout is corrected. The extent of the correction is shown by the position data measured by the encoders of the linear stages. Its amplitude is of course equal to the runout measured by the PicoScale before the feedback loop was turned on. A screenshot of the GUI of the demonstrator is shown in figure 4.

Figure 2. Screenshot of the Runout Correction control GUI. The circle graph on the right shows the trajectory of a point on the cylinder surface. The aberration measured by the PicoScale (red curve) is exaggerated 10 times to make it visible. After turning on the correction, the red dots ”snap” to the blue curve, which is just a circle with the pin diameter of 6mm.


Figure 3 shows the data taken by the PicoScale and the integrated encoders of the XY-stage. The behaviour is shown to be just as outlined in the previous section. The amplitude of the initial runout of the pin is around 85µm . After the correction, there remains a residual runout of less than one micron. This residual runout is caused by several factors:

  • The delay in the correction algorithm introduced by the LabVIEW® loop and the inevitable delay by the USB connection.
  • Deviations of the pin surface from an ideal cylinder.
  • Errors of the steel bearing of the rotator.

However, 99% of the runout is eliminated by the XY-stage.

Figure 3. Displacements of the pin surface measured by the PicoScale sensor heads and the XY-stage. The left part of the graph shows the behaviour when the feedback is off. The right part shows what happens after the feedback is turned on. The period length of the rotation appears shorter with correction, which is due to the time taken by the LabVIEW® feedback loop. A slower loop means less samples are taken per rotation.


SmarAct has built a demonstrator that shows how a PicoScale interferometer and a Piezo stage can work together to compensate for irregularities in mechanical setups. The PicoScale sensor heads are able to detect displacements caused by those irregularities in real time with sub-nanometer scale precision. The values are passed on to the controller of the stage, which uses them to command the movements necessary to correct the runout.