Measurements On the Slip-Ring - Electrical Noise

1. Effect of the Slip-Ring on the signal when turned ON - Geophone measurement
2. Measure of the noise induced by the Slip-Ring using voltage amplifiers - Geophone

The noise induced by the slip-ring is measured when using geophones:

Section 1:
- A geophone located at the sample location is measured with its signal going directly to the ADC and going through the slip-ring
- The voltage amplifiers where saturating due to high frequency noise
Section 2:
- A Low Pass Filter is added before the voltage amplifiers
- Using a Oscilloscope, high frequency noise at 40kHz generated by the slip-ring has been identified
- With the additional low pass filter at the input of the voltage amplifiers, the slip-ring don’t add any measurable noise to the signal

1 Effect of the Slip-Ring on the signal when turned ON - Geophone measurement

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

1.1 Experimental Setup

Goal: The goal is to determine if some noise is added to a signal passing through the slip-ring.

Setup: Two measurements are made with the control systems of all the stages turned OFF.

One geophone is located on the marble while the other is located at the sample location (figure 1).

The two signals from the geophones are amplified with voltage amplifiers with the following settings:

Gain: 60dB
AC/DC switch: AC
Low pass filter at the output set at 1kHz

Figure 1: Experimental Setup

Measurements: Two measurements are done:

Measurement File	Description
`mat/meas_018.mat`	Signal goes through the Slip-ring (as shown on the figure above)
`mat/meas_019.mat`	Signal from the top geophone does not goes through the Slip-ring

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

Column number	Description
1	Geophone - Marble
2	Geophone - Sample
3	Time

1.2 Load data

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

meas_sr = load('mat/data_018.mat', 'data'); meas_sr = meas_sr.data;
meas_di = load('mat/data_019.mat', 'data'); meas_di = meas_di.data;

1.3 Analysis - Time Domain

First, we compare the time domain signals for the two experiments (figure 2).

figure;
hold on;
plot(meas_di(:, 3), meas_di(:, 2), 'DisplayName', 'Geophone - Direct');
plot(meas_sr(:, 3), meas_sr(:, 2), 'DisplayName', 'Geophone - Slip-Ring');
hold off;
xlabel('Time [s]'); ylabel('Voltage [V]');
xlim([0, 50]);
legend('location', 'northeast');

Figure 2: Effect of the Slip-Ring on the measured signal of the geophone at the sample location - Time domain

1.4 Analysis - Frequency Domain

We then compute the Power Spectral Density of the two signals and we compare them (figure 3).

dt = meas_di(2, 3) - meas_di(1, 3);
Fs = 1/dt;

win = hanning(ceil(5*Fs));

[px_di, f] = pwelch(meas_di(:, 2), win, [], [], Fs);
[px_sr, ~] = pwelch(meas_sr(:, 2), win, [], [], Fs);

figure;
hold on;
plot(f, sqrt(px_sr), 'DisplayName', 'Slip-Ring');
plot(f, sqrt(px_di), 'DisplayName', 'Wire');
hold off;
set(gca, 'xscale', 'log');
set(gca, 'yscale', 'log');
xlabel('Frequency [Hz]'); ylabel('Amplitude Spectral Density $\left[\frac{V}{\sqrt{Hz}}\right]$')
xlim([1, 500]);
legend('Location', 'southwest');

Figure 3: Effect of the Slip-Ring on the measured signal of the geophone at the sample location - Frequency domain

1.5 Conclusion

The voltage amplifiers are saturating during the measurements (as shown by the LED on figure 1)
This saturation is mainly due to high frequency noise => a LPF will be added at the input of the voltage amplifiers in the further measurements
The measurements will be redone

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

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

2.1 First Measurement without LPF

2.1.1 Measurement Description

Goal:

Determine if the noise induced by the slip-ring is a limiting factor when measuring the signal coming from a geophone

Setup:

The geophone is located at the sample location
The two Voltage amplifiers have the same following settings:
- Gain: 60dB
- AC/DC option: AC
- Low pass filter at the output set to 1kHz
The signal from the geophone is split into two using a T-BNC:
- One part goes directly to the voltage amplifier and then to the ADC
- The other part goes to the slip-ring=>voltage amplifier=>ADC

Measurements: Two measurements are done:

Measurement File	Description
`data_012`	Slip-Ring OFF
`data_013`	Slip-Ring ON

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

Column number	Description
1	Measure of the geophone at the sample position with a direct wire
2	Measure of the geophone at the sample position going through the slip-ring
3	Time

2.1.2 Load data

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

sr_off = load('mat/data_012.mat', 'data'); sr_off = sr_off.data;
sr_on  = load('mat/data_013.mat', 'data'); sr_on  = sr_on.data;

2.1.3 Time Domain

We compare the signal when the Slip-Ring is OFF (figure 4) and when it is ON (figure 5).

Figure 4: Comparison of the time domain signals when the slip-ring is OFF

Figure 5: Comparison of the time domain signals when the slip-ring is ON

2.1.4 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.

% Direct measure
[pxdoff, ~] = pwelch(sr_off(:, 1), win, [], [], Fs);
[pxdon,  ~] = pwelch(sr_on(:, 1),  win, [], [], Fs);

% Slip-Ring measure
[pxsroff, f] = pwelch(sr_off(:, 2), win, [], [], Fs);
[pxsron,  ~] = pwelch(sr_on(:, 2),  win, [], [], Fs);

Finally, we compare the Amplitude Spectral Density of the signals (figure 6);

figure;
hold on;
plot(f, sqrt(pxdoff), 'DisplayName', 'Direct - OFF');
plot(f, sqrt(pxsroff), 'DisplayName', 'Slip-Ring - OFF');
plot(f, sqrt(pxdon),  'DisplayName', 'Direct - ON');
plot(f, sqrt(pxsron),  'DisplayName', 'Slip-Ring - ON');
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]);

Figure 6: Comparison of the Amplitude Spectral Sensity

Figure 7: Comparison of the Amplitude Spectral Sensity - Zoom

2.1.5 Conclusion

The fact that the Slip-Ring is turned ON adds some noise to the signals
The signal going through the Slip-Ring is less noisy than the one going directly to the ADC
This could be due to better electromagnetic isolation in the slip-ring

Questions:

Can the sharp peak on figure 7 be due to the Aliasing?

2.2 Measurement using an oscilloscope

2.2.1 Measurement Setup

We are now measuring the same signals than in the previous section, but with an oscilloscope instead of with the Speedgoat ADC.

2.2.2 Observations

Then the Slip-Ring is ON (figure 8), we observe a signal at 40kHz with a peak-to-peak amplitude of 200mV for the direct measure and 100mV for the signal going through the Slip-Ring.

Then the Slip-Ring is OFF, we don’t observe this noise at 40kHz anymore (figure 9).

Figure 8: Signals measured by the oscilloscope - Slip-Ring ON - Yellow: Direct measure - Blue: Through Slip-Ring

Figure 9: Signals measured by the oscilloscope - Slip-Ring OFF - Yellow: Direct measure - Blue: Through Slip-Ring

2.2.3 Conclusion

By looking at the signals using an oscilloscope, there is a lot of high frequency noise when turning on the Slip-Ring
This can eventually saturate the voltage amplifiers (seen by a led indicating saturation)
The choice is to add a Low pass filter before the voltage amplifiers to not saturate them and filter the noise.

2.3 New measurements with a LPF before the Voltage Amplifiers

2.3.1 Setup description

Goal: The goal is to see if we can remove high frequency noise from the signals before the voltage amplifiers in order to not saturate them.

Setup: We are measuring the signal from a geophone put at the sample position. Using a BNC slitter, one part is going directly to the Low pass filter, voltage amplifier and ADC (first column), the other part is going through the slip ring before the low pass filter and the voltage amplifier (second column).

The two voltage amplifiers have the same following settings:

Gain: 60dB
AC/DC option: DC
Low pass filter at the output set to 1kHz

The low pass filter is a first order low pass filter RC circuit. It is added before the Voltage Amplifiers and has the following values:

\begin{aligned} R &= 1k\Omega \\ C &= 1\mu F \end{aligned}

And the cut-off frequency is \(f_c = \frac{1}{RC} = 160Hz\).

Measurements: Two measurements are done:

Measurement File	Description
`mat/data_016.mat`	Signal from the geophone at the sample location - Slip-Ring OFF
`mat/data_017.mat`	Signal from the geophone at the sample location - Slip-Ring ON

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

Column number	Description
1	Direct measurement
2	Signal going through the slip-ring
3	Time

2.3.2 Load data

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

sr_lpf_off = load('mat/data_016.mat', 'data'); sr_lpf_off = sr_lpf_off.data;
sr_lpf_on  = load('mat/data_017.mat', 'data'); sr_lpf_on  = sr_lpf_on.data;

2.3.3 Time Domain

We compare the signal when the Slip-Ring is OFF (figure 10) and when it is ON (figure 11).

Figure 10: Comparison of the time domain signals when the slip-ring is OFF

Figure 11: Comparison of the time domain signals when the slip-ring is ON

2.3.4 Frequency Domain

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

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

Fs = 1/dt; % [Hz]

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

Then we compute the Power Spectral Density using pwelch function.

% Direct measure
[pxd_lpf_off, ~] = pwelch(sr_lpf_off(:, 1), win, [], [], Fs);
[pxd_lpf_on,  ~] = pwelch(sr_lpf_on(:, 1),  win, [], [], Fs);

% Slip-Ring measure
[pxsr_lpf_off, f] = pwelch(sr_lpf_off(:, 2), win, [], [], Fs);
[pxsr_lpf_on,  ~] = pwelch(sr_lpf_on(:, 2),  win, [], [], Fs);

Finally, we compare the Amplitude Spectral Density of the signals (figure 12);

figure;
hold on;
plot(f, sqrt(pxd_lpf_off),  'DisplayName', 'Direct - OFF');
plot(f, sqrt(pxsr_lpf_off), 'DisplayName', 'Slip-Ring - OFF');
plot(f, sqrt(pxd_lpf_on),   'DisplayName', 'Direct - ON');
plot(f, sqrt(pxsr_lpf_on),  'DisplayName', 'Slip-Ring - ON');
hold off;
xlim([0.1, 500]);
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', 'southwest');

Figure 12: Comparison of the Amplitude Spectral Sensity

Figure 13: Comparison of the Amplitude Spectral Sensity - Zoom

2.3.5 Conclusion

Using the LPF, we don’t see any additional noise coming from the slip-ring when it is turned ON
However, we should use a smaller value of the capacitor to have a cut-off frequency at \(1kHz\)
We here observe a signal at \(50Hz\) and its harmonics

2.4 Measurement of the noise induced by the slip-ring with additional LPF at 1kHz

2.4.1 Measurement description

Setup: Voltage amplifier:

60db
AC
1kHz

Additionnal LPF at 1kHz

geophone at the sample location slit into 2 BNC:

first one (column one): direct wire
second one (second column): slip-ring wire

Additionnal LPF is added before the voltage amplifiers

Goal:

Measurements:

Three measurements are done:

Measurement File	Description
`mat/data_035.mat`	All off
`mat/data_036.mat`	Slip-Ring ON

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

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

2.4.2 Load data

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

sr_lpf_1khz_of = load('mat/data_035.mat', 'data'); sr_lpf_1khz_of = sr_lpf_1khz_of.data;
sr_lpf_1khz_on = load('mat/data_036.mat', 'data'); sr_lpf_1khz_on = sr_lpf_1khz_on.data;

2.4.3 Pre-processing

There is a sign difference between the signal going directly to the ADC and the signal going through the slip-ring. We add a minus sign on the signal going through the slip-ring.

We also subtract the mean value as the voltage amplifiers were on the DC option.

sr_lpf_1khz_of(:, 1) =   sr_lpf_1khz_of(:, 1)-mean(sr_lpf_1khz_of(:, 1));
sr_lpf_1khz_of(:, 2) = -(sr_lpf_1khz_of(:, 2)-mean(sr_lpf_1khz_of(:, 2)));

sr_lpf_1khz_on(:, 1) =   sr_lpf_1khz_on(:, 1)-mean(sr_lpf_1khz_on(:, 1));
sr_lpf_1khz_on(:, 2) = -(sr_lpf_1khz_on(:, 2)-mean(sr_lpf_1khz_on(:, 2)));

2.4.4 Time Domain

We compare the signal when the Slip-Ring is OFF (figure 14) and when it is ON (figure 15).

Figure 14: Comparison of the time domain signals when the slip-ring is OFF

Figure 15: Comparison of the time domain signals when the slip-ring is ON

2.4.5 Frequency Domain

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

dt = sr_lpf_1khz_of(2, 3)-sr_lpf_1khz_of(1, 3);

Fs = 1/dt; % [Hz]

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

Then we compute the Power Spectral Density using pwelch function.

% Direct measure
[pxdi_lpf_1khz_of, f] = pwelch(sr_lpf_1khz_of(:, 1), win, [], [], Fs);
[pxdi_lpf_1khz_on, ~] = pwelch(sr_lpf_1khz_on(:, 1), win, [], [], Fs);

% Slip-Ring measure
[pxsr_lpf_1khz_of, ~] = pwelch(sr_lpf_1khz_of(:, 2), win, [], [], Fs);
[pxsr_lpf_1khz_on, ~] = pwelch(sr_lpf_1khz_on(:, 2), win, [], [], Fs);

Finally, we compare the Amplitude Spectral Density of the signals (figure 16);

figure;
hold on;
plot(f, sqrt(pxdi_lpf_1khz_of), 'DisplayName', 'Direct - OFF');
plot(f, sqrt(pxsr_lpf_1khz_of), 'DisplayName', 'Slip-Ring - OFF');
plot(f, sqrt(pxdi_lpf_1khz_on), 'DisplayName', 'Direct - ON');
plot(f, sqrt(pxsr_lpf_1khz_on), 'DisplayName', 'Slip-Ring - ON');
hold off;
xlim([1, 500]);
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', 'southwest');

Figure 16: Comparison of the Amplitude Spectral Sensity

2.4.6 Difference between the direct signal and the signal going through the slip-ring

We subtract the signal coming from the direct wire to the signal going through the slip-ring when the slip-ring is ON and when it is OFF (figure 17).

Figure 17: Difference between the signal going directly to the ADC and the signal going through the slip-ring before the ADC

Then we compute the Power Spectral Density using pwelch function (figure 18).

% Direct measure
[px_diff_lpf_1khz_of, f] = pwelch(sr_lpf_1khz_of(:, 1)-sr_lpf_1khz_of(:, 2), win, [], [], Fs);
[px_diff_lpf_1khz_on, ~] = pwelch(sr_lpf_1khz_on(:, 1)-sr_lpf_1khz_on(:, 2), win, [], [], Fs);

Figure 18: Amplitude Spectral Density of the difference between the signal going directly to the ADC and the signal going through the slip-ring before the ADC

2.4.7 Conclusion

Using the LPF, we don’t see any additional noise coming from the slip-ring when it is turned ON
The signal going through the slip-ring only differs from the direct signal by some 50Hz and its harmonics

Measurements On the Slip-Ring - Electrical Noise

Table of Contents

1 Effect of the Slip-Ring on the signal when turned ON - Geophone measurement

1.1 Experimental Setup

1.2 Load data

1.3 Analysis - Time Domain

1.4 Analysis - Frequency Domain

1.5 Conclusion

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

2.1 First Measurement without LPF

2.1.1 Measurement Description

2.1.2 Load data

2.1.3 Time Domain

2.1.4 Frequency Domain

2.1.5 Conclusion

2.2 Measurement using an oscilloscope

2.2.1 Measurement Setup

2.2.2 Observations

2.2.3 Conclusion

2.3 New measurements with a LPF before the Voltage Amplifiers

2.3.1 Setup description

2.3.2 Load data

2.3.3 Time Domain

2.3.4 Frequency Domain

2.3.5 Conclusion

2.4 Measurement of the noise induced by the slip-ring with additional LPF at 1kHz

2.4.1 Measurement description

2.4.2 Load data

2.4.3 Pre-processing

2.4.4 Time Domain

2.4.5 Frequency Domain

2.4.6 Difference between the direct signal and the signal going through the slip-ring

2.4.7 Conclusion