Measurements On the Slip-Ring - Electrical Noise

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:

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

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;

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

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]);

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

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).

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:

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

Measurements: Two measurements are done:

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

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;

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

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');

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:

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

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;

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)));

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

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');

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).

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);