30 KiB
Measurements On the Slip-Ring - Electrical Noise
- Introduction
- Effect of the Slip-Ring on the signal when turned ON - Geophone measurement
- Measure of the noise induced by the Slip-Ring using voltage amplifiers - Geophone
Introduction ignore
The noise induced by the slip-ring is measured when using geophones:
-
Section sec:meas_slip_ring_geophone:
- 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 sec:meas_sr_geophone:
- 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
Effect of the Slip-Ring on the signal when turned ON - Geophone measurement
<<sec:meas_slip_ring_geophone>>
ZIP file containing the data and matlab files ignore
All the files (data and Matlab scripts) are accessible here.
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 fig:setup_slipring).
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
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 |
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;Analysis - Time Domain
First, we compare the time domain signals for the two experiments (figure fig:slipring_time).
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'); <<plt-matlab>>
Analysis - Frequency Domain
We then compute the Power Spectral Density of the two signals and we compare them (figure fig:slipring_asd).
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'); <<plt-matlab>>
Conclusion
- The voltage amplifiers are saturating during the measurements (as shown by the LED on figure fig:setup_slipring)
- 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
Measure of the noise induced by the Slip-Ring using voltage amplifiers - Geophone
<<sec:meas_sr_geophone>>
ZIP file containing the data and matlab files ignore
All the files (data and Matlab scripts) are accessible here.
First Measurement without LPF
Measurement Description
Goal ignore
Goal:
- Determine if the noise induced by the slip-ring is a limiting factor when measuring the signal coming from a geophone
Setup ignore
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 ignore
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 |
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;Time Domain
We compare the signal when the Slip-Ring is OFF (figure fig:sr_geophone_time_off) and when it is ON (figure fig:sr_geophone_time_on).
<<plt-matlab>>
<<plt-matlab>>
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 fig:sr_geophone_asd);
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]); <<plt-matlab>>
<<plt-matlab>>
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 fig:sr_geophone_asd_zoom be due to the Aliasing?
Measurement using an oscilloscope
Measurement Setup
We are now measuring the same signals than in the previous section, but with an oscilloscope instead of with the Speedgoat ADC.
Observations
Then the Slip-Ring is ON (figure fig:oscilloscope_sr_on), 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 fig:oscilloscope_sr_off).
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.
New measurements with a LPF before the Voltage Amplifiers
Setup description
Goal ignore
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 ignore
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 ignore
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 |
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;Time Domain
We compare the signal when the Slip-Ring is OFF (figure fig:sr_lpf_geophone_time_off) and when it is ON (figure fig:sr_lpf_geophone_time_on).
<<plt-matlab>>
<<plt-matlab>>
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 fig:sr_lpf_geophone_asd);
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'); <<plt-matlab>>
<<plt-matlab>>
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
Measurement of the noise induced by the slip-ring with additional LPF at 1kHz
Measurement description
Setup ignore
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 ignore
Goal:
Measurements ignore
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 |
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;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)));Time Domain
We compare the signal when the Slip-Ring is OFF (figure fig:sr_lpf_1khz_geophone_time_off) and when it is ON (figure fig:sr_lpf_1khz_geophone_time_on).
<<plt-matlab>>
<<plt-matlab>>
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 fig:sr_lpf_1khz_geophone_asd);
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'); <<plt-matlab>>
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 fig:diff_sr_direct).
<<plt-matlab>>
Then we compute the Power Spectral Density using pwelch function (figure fig:diff_sr_direct_psd).
% 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); <<plt-matlab>>
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