Controller for Closed-Loop Feedback of Nanopositioners

Table of Contents

Introduction
Instrumentation
Data Acquisition
     RMS Noise Measurement
     Repeatability Measurement
     Position Measurement
     Linearity and Repeatability of Movement
Conclusion

Introduction

In some applications, an external sensor such as an interferometer can be beneficial in maintaining positional stability or offsetting drift. This article shows the ability of nPoint controllers to join with quadrature sensor signals for closed-loop feedback.

Incorporating an external sensor to add this capability serves as a substitute for a common internal capacitive or strain sensor within the piezo stage. Hence, closed-loop capability can be achieved with only external sensors, without the necessity for sensors in the piezo stage.

Instrumentation

In this demonstration as shown in Figure 1, a NPXY100-100 stage and a LC.403 controller with three-channel encoder interface from nPoint was used along with Renishaw RLE20 laser interferometers with REE interpolators for data acquisition. One of the interferometers served as the sensor for closed-loop feedback, while the other is used for position monitoring.

The REE interpolator for the closed-loop sensor can be immediately plugged into the encoder interface in the LC.403. A 2 m cable would be usually provided to plug the REE interpolator in to the encoder interface. Customers can request for longer cables if needed.

NPXY100-100

Figure 1. NPXY100-100 set-up with 2 interferometer heads (a). The encoder interface inside the LC.403 (b). The encoder interface has three sub D-15 connectors.

Data Acquisition

When the encoder is used for closed-loop feedback, users can continue to access the capabilities of the nPoint controllers through the nPoint GUI, nPControl.

Users can perform open and closed-loop step response; modify the control gains; use the integrated function generator and program notch filters. The encoder sensor data is shown in the nPoint GUI. Figure 2 illustrates a closed-loop and an open-loop step response.

Open-loop step response.

(a)

closed-loop step response.

(b)

Figure 2. Open-loop (a) and closed-loop (b) step response.

RMS Noise Measurement

RMS noise is measured on the position of the NPXY100-100 when in closed-loop with the laser interferometer. The measured value is approximately 0.015 nm.

Figure 3 shows the peak-to-peak position noise presented in the nPoint GUI. The scale on the y-axis of the graph is 0.5 nm.

RMS Noise Measurement

Figure 3. Peak-to-peak position noise displayed in the nPoint GUI.

Repeatability Measurement

The stage is directed to perform 50 µm steps in either direction from the ‘center’ position. The stage is in closed-loop using head 1 as the sensor, while the data is recorded by interferometer head 2 for repeatability measurement as shown in Figure 4.

Repeatability Measurement

Figure 4. Repeatability measurement using the interferometer head 2.

Position Measurement

The stage is directed to move via the built-in function generator of the LC.403.

Head 2 measures and shows the position of the stage in two diverse cases: 1 Hz, 100 µm triangle wave and 100 Hz, 0.7 µm sine wave (see Figure 5). For the 100 Hz sine wave, the resulting movement is less owing to bandwidth restrictions.

Position Measurement

Figure 5. Hz, 100 µm triangle-wave and 100 Hz, 0.7µm sine-wave motion.

Linearity and Repeatability of Movement

The stage is directed to move 100 µm and its position is captured by head 2. Figure 6 illustrates the linearity and hysteresis beyond 100 µm. The deviation for the linearity is within 10 nm over 100 µm and for the hysteresis within 5 nm over 100 µm.

Linearity and Repeatability of Movement

Figure 6. Linearity deviation and hysteresis of motion as the stage is commanded to move through 100 µm.

Conclusion

The data obviously shows the possibility of using external sensors as direct closed-loop feedback for nPoint piezo stages.

nPoint | Nanopositioning and Motion Control.

This information has been sourced, reviewed and adapted from materials provided by nPoint | Nanopositioning and Motion Control.

For more information on this source, please visit nPoint | Nanopositioning and Motion Control.

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