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Measurements On the Slip-Ring

Table of Contents

We determine if the slip-ring add some noise to the signal when it is turning:

1 Effect of the rotation of the Slip-Ring - Noise

All the files (data and Matlab scripts) are accessible here.

1.1 Measurement Description

Setup: Random Signal is generated by one SpeedGoat DAC.

The signal going out of the DAC is split into two:

  • one BNC cable is directly connected to one ADC of the SpeedGoat
  • one BNC cable goes two times in the Slip-Ring (from bottom to top and then from top to bottom) and then is connected to one ADC of the SpeedGoat

All the stages are turned OFF except the Slip-Ring.

Goal: The goal is to determine if the signal is altered when the spindle is rotating.

Measurements:

Data File Description
mat/data_001.mat Slip-ring not turning but ON
mat/data_002.mat Slip-ring turning at 1rpm

For each measurement, the measured signals are:

Variable Description
t Time vector
x1 Direct signal
x2 Signal going through the Slip-Ring

1.2 Load data

We load the data of the z axis of two geophones. 

sr_off = load('mat/data_001.mat', 't', 'x1', 'x2');
sr_on  = load('mat/data_002.mat', 't', 'x1', 'x2');

1.3 Analysis

Let’s first look at the signal produced by the DAC (figure 1). 

figure;
hold on;
plot(sr_on.t,  sr_on.x1);
hold off;
xlabel('Time [s]'); ylabel('Voltage [V]');
xlim([0 10]);

random_signal.png

Figure 1: Random signal produced by the DAC

We now look at the difference between the signal directly measured by the ADC and the signal that goes through the slip-ring (figure 2). 

figure;
hold on;
plot(sr_on.t,  sr_on.x1  -  sr_on.x2,  'DisplayName', 'Slip-Ring - $\omega = 1rpm$');
plot(sr_off.t, sr_off.x1 - sr_off.x2,'DisplayName', 'Slip-Ring off');
hold off;
xlabel('Time [s]'); ylabel('Voltage [V]');
xlim([0 10]);
legend('Location', 'northeast');

slipring_comp_signals.png

Figure 2: Alteration of the signal when the slip-ring is turning

dt = sr_on.t(2) - sr_on.t(1);
Fs = 1/dt; % [Hz]

win = hanning(ceil(1*Fs));

[pxx_on,  f] = pwelch(sr_on.x1  - sr_on.x2,  win, [], [], Fs);
[pxx_off, ~] = pwelch(sr_off.x1 - sr_off.x2, win, [], [], Fs);

psd_noise.png

Figure 3: ASD of the measured noise

1.4 Conclusion

  • The measurement is mostly limited by the resolution of the Speedgoat DAC (16bits over \(\pm 10 V\))
  • In section 2, the same measurement is done but voltage amplifiers are added to amplify the noise

2 Measure of the noise induced by the Slip-Ring using voltage amplifiers - Noise

All the files (data and Matlab scripts) are accessible here.

2.1 Measurement Description

Goal:

  • Determine the noise induced by the slip-ring when turned ON and when rotating

Setup:

  • 0V is generated by one Speedgoat DAC
  • Using a T, one part goes directly to one Speedgoat ADC
  • The other part goes to the slip-ring 2 times and then to one voltage amplifier before going to the ADC
  • The parameters of the Voltage Amplifier are:
    • gain of 80dB
    • AC/DC option to AC (it adds an high pass filter at 1.5Hz at the input of the voltage amplifier)
    • Output Low pass filter set at 1kHz
  • Every stage of the station is OFF

First column: Direct measure Second column: Slip-ring measure

Measurements:

Data File Description
mat/data_008.mat Slip-Ring OFF
mat/data_009.mat Slip-Ring ON
mat/data_010.mat Slip-Ring ON and omega=6rpm
mat/data_011.mat Slip-Ring ON and omega=60rpm

Each of the measurement mat file contains one data array with 3 columns:

Column number Description
1 Signal going directly to the ADC
2 Signal going through the Slip-Ring
3 Time

VID_20190503_160831.gif

Figure 4: Slip-Ring rotating at 6rpm

VID_20190503_161401.gif

Figure 5: Slip-Ring rotating at 60rpm

2.2 Load data

We load the data of the z axis of two geophones. 

sr_off = load('mat/data_008.mat', 'data'); sr_off = sr_off.data;
sr_on  = load('mat/data_009.mat', 'data'); sr_on  = sr_on.data;
sr_6r  = load('mat/data_010.mat', 'data'); sr_6r  = sr_6r.data;
sr_60r = load('mat/data_011.mat', 'data'); sr_60r = sr_60r.data;

2.3 Time Domain

We plot the time domain data for the direct measurement (figure 6) and for the signal going through the slip-ring (figure 7);

sr_direct_time.png

Figure 6: Direct measurement

sr_slipring_time.png

Figure 7: Measurement of the signal going through the Slip-Ring

2.4 Frequency Domain

We first compute some parameters that will be used for the PSD computation. 

dt = sr_off(2, 3)-sr_off(1, 3);

Fs = 1/dt; % [Hz]

win = hanning(ceil(10*Fs));

Then we compute the Power Spectral Density using pwelch function. 

[pxdir, f] = pwelch(sr_off(:, 1), win, [], [], Fs);
[pxoff, ~] = pwelch(sr_off(:, 2), win, [], [], Fs);
[pxon,  ~] = pwelch(sr_on(:, 2),  win, [], [], Fs);
[px6r,  ~] = pwelch(sr_6r(:, 2),  win, [], [], Fs);
[px60r, ~] = pwelch(sr_60r(:, 2), win, [], [], Fs);

And we plot the ASD of the measured signals (figure 8); 

figure;
hold on;
plot(f, sqrt(pxoff), 'DisplayName', 'OFF');
plot(f, sqrt(pxon),  'DisplayName', 'ON');
plot(f, sqrt(px6r),  'DisplayName', '6rpm');
plot(f, sqrt(px60r), 'DisplayName', '60rpm');
plot(f, sqrt(pxdir), 'k-', 'DisplayName', 'Direct');
hold off;
set(gca, 'xscale', 'log');
set(gca, 'yscale', 'log');
xlabel('Frequency [Hz]'); ylabel('ASD of the measured Voltage $\left[\frac{V}{\sqrt{Hz}}\right]$')
legend('Location', 'northeast');
xlim([0.1, 500]);

sr_psd_compare.png

Figure 8: Comparison of the ASD of the measured signals when the slip-ring is ON, OFF and turning

Questions:

  • Why is there some sharp peaks? Can this be due to aliasing?
  • It is possible that the amplifiers were saturating during the measurements. This saturation could be due to high frequency noise.

2.5 Conclusion

  • The measurements are re-done using an additional low pass filter at the input of the voltage amplifier

3 Measure of the noise induced by the Slip-Ring rotation - LPF added

All the files (data and Matlab scripts) are accessible here.

3.1 Measurement description

Setup: Voltage amplifier:

  • 60db
  • AC
  • 1kHz

Additionnal LPF at 1kHz

Goal:

Measurements:

Three measurements are done:

Measurement File Description
mat/data_030.mat All off
mat/data_031.mat Slip-Ring on
mat/data_032.mat Slip-Ring 6rpm
mat/data_033.mat Slip-Ring 60rpm

Each of the measurement mat file contains one data array with 3 columns:

Column number Description
1 Direct Measure
2 Slip-Ring
3 Time

3.2 Load data

We load the data of the z axis of two geophones. 

sr_of = load('mat/data_030.mat', 'data'); sr_of = sr_of.data;
sr_on = load('mat/data_031.mat', 'data'); sr_on = sr_on.data;
sr_6r = load('mat/data_032.mat', 'data'); sr_6r = sr_6r.data;
sr_60 = load('mat/data_033.mat', 'data'); sr_60 = sr_60.data;

3.3 Time Domain

We plot the time domain data for the direct measurement (figure 9) and for the signal going through the slip-ring (figure 11);

sr_direct_1khz_time.png

Figure 9: Direct measurement

xlim([0, 0.2]); ylim([-2e-3, 2e-3]);

sr_direct_1khz_time_zoom.png

Figure 10: Direct measurement - Zoom

sr_slipring_1khz_time.png

Figure 11: Measurement of the signal going through the Slip-Ring

3.4 Frequency Domain - Direct Signal

We first compute some parameters that will be used for the PSD computation. 

dt = sr_of(2, 3)-sr_of(1, 3);

Fs = 1/dt; % [Hz]

win = hanning(ceil(10*Fs));

Then we compute the Power Spectral Density using pwelch function. 

[px_d_of, f] = pwelch(sr_of(:, 1), win, [], [], Fs);
[px_d_on, ~] = pwelch(sr_on(:, 1), win, [], [], Fs);
[px_d_6r, ~] = pwelch(sr_6r(:, 1), win, [], [], Fs);
[px_d_60, ~] = pwelch(sr_60(:, 1), win, [], [], Fs);

figure;
hold on;
plot(f, sqrt(px_d_of), 'DisplayName', 'OFF');
plot(f, sqrt(px_d_on), 'DisplayName', 'ON');
plot(f, sqrt(px_d_6r), 'DisplayName', '6rpm');
plot(f, sqrt(px_d_60), 'DisplayName', '60rpm');
hold off;
set(gca, 'xscale', 'log');
set(gca, 'yscale', 'log');
xlabel('Frequency [Hz]'); ylabel('ASD of the measured Voltage $\left[\frac{V}{\sqrt{Hz}}\right]$')
legend('Location', 'northeast');
xlim([0.1, 5000]);

sr_psd_1khz_direct.png

Figure 12: Amplitude Spectral Density of the signal going directly to the ADC

3.5 Frequency Domain - Slip-Ring Signal

[px_sr_of, f] = pwelch(sr_of(:, 2), win, [], [], Fs);
[px_sr_on, ~] = pwelch(sr_on(:, 2), win, [], [], Fs);
[px_sr_6r, ~] = pwelch(sr_6r(:, 2), win, [], [], Fs);
[px_sr_60, ~] = pwelch(sr_60(:, 2), win, [], [], Fs);

figure;
hold on;
plot(f, sqrt(px_sr_of), 'DisplayName', 'OFF');
plot(f, sqrt(px_sr_on), 'DisplayName', 'ON');
plot(f, sqrt(px_sr_6r), 'DisplayName', '6rpm');
plot(f, sqrt(px_sr_60), 'DisplayName', '60rpm');
plot(f, sqrt(px_d_of), '-k', 'DisplayName', 'Direct');
hold off;
set(gca, 'xscale', 'log');
set(gca, 'yscale', 'log');
xlabel('Frequency [Hz]'); ylabel('ASD of the measured Voltage $\left[\frac{V}{\sqrt{Hz}}\right]$')
legend('Location', 'northeast');
xlim([0.1, 5000]);

sr_psd_1khz_slipring.png

Figure 13: Amplitude Spectral Density of the signal going through the slip-ring

3.6 Conclusion

  • We observe peaks at 12Hz and its harmonics for the signal going through the slip-ring when it is turning at 60rpm.
  • Apart from that, the noise of the signal is the same when the slip-ring is off/on and turning
  • The noise of the signal going through the slip-ring is much higher that the direct signal from the DAC to the ADC
  • A peak is obverse at 11.5Hz on the direct signal as soon as the slip-ring is turned ON. Can this be due to high frequency noise and Aliasing? As there is no LPF to filter the noise on the direct signal, this effect could be more visible on the direct signal.

Author: Dehaeze Thomas

Created: 2020-11-12 jeu. 10:29