Quadrature Sensor Integrated with a Controller for Closed-Loop Feedback

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 useful in maintaining positional stability or offsetting drift. This article shows the capability of nPoint controllers to combine with quadrature sensor signals for closed-loop feedback.

Incorporating an external sensor in order to add this capability serves for a typical internal capacitive or strain sensor in the piezo stage. Therefore, closed-loop capability can be maintained with just external sensors, without the necessity for sensors in the piezo stage.

Instrumentation

In this article, the a LC.403 controller with 3 channel encoder interface and a NPXY100-100 stage from nPoint, and Renishaw RLE20 laser interferometers with REE interpolators were employed for data acquisition (Figure 1). One of the interferometers was employed as the sensor for closed-loop feedback, whereas the other one was used for monitoring the position.

The REE interpolator for the closed-loop sensor is plugged in directly to the encoder interface in the LC.403. Normally, a 2 m cable would be supplied to connect the REE interpolator to the encoder interface. Customers can also order 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

If the encoder is used for closed-loop feedback, users can still access the the nPoint controllers’ capabilities through the nPoint GUI, nPControl.

Users can perform open and closed-loop step response, program notch filters, alter the control gains and use the built-in function generator. The encoder sensor data is shown in the nPoint GUI. An illustration of an open-loop and closed-loop step response is displayed in Figure 2.

Open-loop step response

(a)

closed-loop step response

(b)

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

RMS Noise Measurement

While in closed-loop with the laser interferometer, an RMS noise measurement can be performed on the position of the NPXY100-100. The measured value is approximately 0.015 nm.

The peak-to-peak position noise displayed in the nPoint GUI is illustrated in Figure 3. 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. It is in closed-loop utilizing head 1 as the sensor, whereas the interferometer head 2 records the data for repeatability measurement (Figure 4).

Repeatability measurement

Figure 4. Repeatability measurement using the interferometer head 2.

Position Measurement

The stage is directed to move through the built-in function generator of the LC.403. The position of the stage is measured and displayed by head 2 in two different cases – 1 Hz, 100 µm triangle wave and 100 Hz, 0.7 µm sine wave (Figure 5). The resulting movement is less for the 100 Hz sine wave because of bandwidth limitations.

Position Measurement

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

Linearity and Repeatability of Movement

The stage is directed to move 100 µm, while head 2 records the position of the stage. The linearity and hysteresis over 100 µm is shown in Figure 6. 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 shown above clearly reveals the possibility of employing external sensors as direct closed-loop feedback for nPoint piezo stages.

nPoint

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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