Measurements On the Slip-Ring

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
3. Effect of the rotation of the Slip-Ring - Noise
4. Measure of the noise induced by the Slip-Ring using voltage amplifiers - Noise

Section 1
- 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

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', 'soutwest');

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 TODO New LPF at 1kHz

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

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

3.1 Measurement Description

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

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.

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

3.2 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');

3.3 Analysis

Let's first look at the signal produced by the DAC (figure 14).

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

Figure 14: 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 15).

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');

Figure 15: 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);

Figure 16: ASD of the measured noise

3.4 Conclusion

The measurement is mostly limited by the resolution of the Speedgoat DAC (16bits over \(\pm 10 V\))
In section 4, the same measurement is done but voltage amplifiers are added to amplify the noise

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

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

4.1 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

Figure 17: Slip-Ring rotating at 6rpm

Figure 18: Slip-Ring rotating at 60rpm

4.2 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;

4.3 Time Domain

We plot the time domain data for the direct measurement (figure 19) and for the signal going through the slip-ring (figure 20);

Figure 19: Direct measurement

Figure 20: Measurement of the signal going through the Slip-Ring

4.4 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 21);

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]);

Figure 21: 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.

4.5 Conclusion

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

Measurements On the Slip-Ring

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 TODO New LPF at 1kHz

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

3.1 Measurement Description

3.2 Load data

3.3 Analysis

3.4 Conclusion

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

4.1 Measurement Description

4.2 Load data

4.3 Time Domain

4.4 Frequency Domain

4.5 Conclusion