Encoder Renishaw Vionic - Test Bench
-Table of Contents
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This report is also available as a pdf.
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-You can find below the document of: -
- - --We would like to characterize the encoder measurement system. -
- --In particular, we would like to measure: -
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- Power Spectral Density of the measurement noise -
- Bandwidth of the sensor -
- Linearity of the sensor -
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Figure 1: Picture of the Vionic Encoder
-1 Encoder Model
--The Encoder is characterized by its dynamics \(G_m(s)\) from the “true” displacement \(y\) to measured displacement \(y_m\). -Ideally, this dynamics is constant over a wide frequency band with very small phase drop. -
- --It is also characterized by its measurement noise \(n\) that can be described by its Power Spectral Density (PSD). -
- --The model of the encoder is shown in Figure 2. -
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Figure 2: Model of the Encoder
--We can also use a transfer function \(G_n(s)\) to shape a noise \(\tilde{n}\) with unity ASD as shown in Figure 4. -
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| Characteristics | -Manual | -Specifications | -
|---|---|---|
| Range | -Ruler length | -> 200 [um] | -
| Resolution | -2.5 [nm] | -< 50 [nm rms] | -
| Sub-Divisional Error | -\(< \pm 15\,nm\) | -- |
| Bandwidth | -To be checked | -> 5 [kHz] | -
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Figure 4: Expected interpolation errors for the Vionic Encoder
-2 Noise Measurement
- -2.1 Test Bench
--To measure the noise \(n\) of the encoder, one can rigidly fix the head and the ruler together such that no motion should be measured. -Then, the measured signal \(y_m\) corresponds to the noise \(n\). -
-2.2 Results
--First we load the data. -
-load('noise_meas_100s_20kHz.mat', 't', 'x'); -x = x - mean(x); --
-The time domain data are shown in Figure 4. -
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-The amplitude spectral density is computed and shown in Figure 5.
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Figure 5: Amplitude Spectral Density of the measured signal
--Let’s create a transfer function that approximate the measured noise of the encoder. -
-Gn_e = 1.8e-11/(1 + s/2/pi/5e3); --
-The amplitude of the transfer function and the measured ASD are shown in Figure 6. -
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Figure 6: Measured ASD of the noise and modelled one
-3 Linearity Measurement
- -3.1 Test Bench
--In order to measure the linearity, we have to compare the measured displacement with a reference sensor with a known linearity. -An interferometer or capacitive sensor should work fine. -An actuator should also be there so impose a displacement. -
- --One idea is to use the test-bench shown in Figure 7. -
- --The APA300ML is used to excite the mass in a broad bandwidth. -The motion is measured at the same time by the Vionic Encoder and by an interferometer (most likely an Attocube). -
- --As the interferometer has a very large bandwidth, we should be able to estimate the bandwidth of the encoder if it is less than the Nyquist frequency that can be around 10kHz. -
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Figure 7: Schematic of the test bench
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