Integration of Quadrature Sensor with nPoint Controllers for Closed-Loop Feedback

By AZoNano Staff Writers

Topics Covered

Introduction
Instrumentation
Data Acquisition
     RMS Noise Measurement
     Repeatability Measurement
     Position Measurement
     Linearity and Repeatability of Movement
Conclusion
About nPoint | nanopositioning and motion control

Introduction

In certain applications, an external sensor like an interferometer can be useful in offsetting drift or maintaining positional stability. This article demonstrates the ability of nPoint controllers to couple with quadrature sensor signals for closed-loop feedback.

Integrating an external sensor to add this capability serves in place of a typical internal capacitive or strain sensor within the piezo stage. Hence, closed-loop capability can be maintained with only external sensors, without the requirement for sensors in the piezo stage.

Instrumentation

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

The REE interpolator for the closed-loop sensor is directly plugged in to the encoder interface in the LC.403. A 2m cable would be normally provided to plug the REE interpolator in to the encoder interface. Customers can order for longer cables if required.

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 employed for closed-loop feedback, users can still access the capabilities of the nPoint controllers through the nPoint GUI, nPControl. Users can carry out open and closed-loop step response; alter the control gains; use the built-in function generator; and program notch filters. The encoder sensor data is displayed in the nPoint GUI. Figure 2 illustrates an open-loop and a closed-loop step response.

(a)

(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 roughly 0.015nm.

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

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

Repeatability Measurement

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

Figure 4. Repeatability measurement using the interferometer head 2.

Position Measurement

The stage is directed to move through the integrated function generator of the LC.403. Head 2 measures and displays the position of the stage in two different cases: 1 Hz, 100µm triangle wave and 100 Hz, 0.7µm sine wave (Figure 5). For the 100Hz sine wave, the resulting movement is less owing to bandwidth limitations.

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

Linearity and Repeatability of Movement

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

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

Conclusion

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

About nPoint | nanopositioning and motion control

nPoint, Inc. specializes in piezo actuator driven flexure stages available in one, two or three axes of motion. Our ultra-precision motion control scanners and controllers provide users with the highest level of performance available in the marketplace.

With design and production facilities located in Middleton, WI and distribution channels throughout the world, nPoint provides each of our customers the same outstanding service regardless of the application.

Our product lines include standard products consisting of upgrade kits for your AFM and stand alone research instruments, a growing list of OEM applications and custom design for your unique research application. Let nPoint be your complete nanopositioning solution provider.

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.

Date Added: Feb 25, 2014 | Updated: Feb 27, 2014
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