EPFL
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ME412_EMEM_2020
Real area of contact & analog electronics

Real area of contact & analog electronics

Contact Mechanics VIA Analog Electronics

Aim of this measurement

In this class, we will attempt to reproduce the `real area of contact' measurement of Bowden and Tabor using contact resistivity. The principle of the measurement is to use two conductive bodies of a known geometry and force them into contact while simultaneously monitoring the applied load and measuring (somehow) the junction resistance. As you might know, resistivity is a bulk property, and scales inversely with the size of the resistive junction. The junction resistance is inversely related to the area of contact under certain limits, and thus can be used to measure the real (as opposed to projected) area of contact.

In the spirit of this course, this module will rely extensively on analog electronic devices that we make in the lab. This will of course require us all to be on the same page with respect to analog electronics, which will be the focus of the first two meetings of this module.

Analog Electronics

We will first learn a some of the basics about how analog electronics work. For us to work with this type of equipment in the lab, we will make extensive use of the DLL equipment on the bench. This includes a function generator, an oscilloscope, multimeters and a power supply, as well as the breadboard (and ultimately some of the functionality) of the National Instruments Elvis II + education board, which can replicate the functionality of most of the benchtop instrumentation to some extent.

In the spirit of the class, the lab exercises will be quite open ended - you'll find that the exercise doesn't necessarily have a unique solution, and leaves it up to you to come up with an approach that sufficiently addresses the exercise.

Meeting locations for on- and off-campus students:

off campus: Please note that we have moved to a Piazza forum: link here.

Zoom link for 9a.m. discussion meeting: here

On campus: Class will be held off campus until further notice.

Remote access to the experimental computer is available via NoMachine. Instructions on how to connect in general can be found here. Note that a successful connection requires the use of the EPFL VPN.

The host IP address is: 128.179.135.127

The user name is: EMSI_LAB.

The password is: emsilab

Lab exercises #1:

Propose and conduct an experiment to verify that a resistor satisfies Ohm's law
Make a voltage divider, and explain why the output voltage takes on the value that you measure
Drive a resistive load with your voltage divider, and continue to monitor the output voltage. Does this value change when you attach the load? Why or why not?
What happens when you change the value of the resistive load you are driving?
When it becomes comparable or less than the resistors used to make the divider?
When it is much larger than these resistive values?

Lab exercises #2:

Make a low-pass filter using a resistor and a capacitor. Measure the transient response of this filter at the output when you apply a constant voltage to the filter's input. What is the time constant for the filter? (hint - an easy way to do this repetitively is to apply a suitably low frequency square wave to the input, although you can make the measurement with a single trigger event).
Make a high-pass filter using another resistor and capacitor pair (they can have the same values as the components you used in the low-pass filter). Measure its transient response, and measure the time constant for your filter.
Propose and conduct an experiment to measure the 3 dB frequency for each of these filters (hint: the function generator has a sweep feature). Does the value agree with your expectation based on the capacitance and resistance values you used? For the low-pass filter, measure the phase of the input and output voltages: 1. at about 1/2 of the 3dB frequency. 2. At the 3dB frequency and 3. about 1.5 times the 3 dB frequency.
Bonus: make a bandpass filter, and measure it's frequency response. What is the phase behavior like as you increase frequency?

Lab exercises, week 3

Transistors can burn out when too much current passes through them. Verify the npn vs. pnp configuration using the diode sensor on the hand-held multi-meters for the two types of transistors we have in the lab.
Use an npn transistor to make an emitter-follower using ~ 1kOhm on the emitter leg to ground, and between 10 and 15 V on the collector; use 100 Ohm on the base leg. What happens when you drive the base at 5 V peak-to-peak?
- Now connect the 1kOhm on the emitter leg to -15V, leaving everything else as it was. What changes about the output signal?
Using the same emitter-follower layout as in the prior exercise, Swap out the 1 kOhm to ground on the emitter leg for 3.3kOhm, and replace the 100 Ohm on the base ith a 33 kOhm resistor. Drive the base with a sinusoidal signal with an amplitude less than 1 V peak-to-peak, and monitor the output of the follower with the scope.
- Measure the input impedance of the follower. Put together a sketch of the equivalent circuit you're using to do this measurement.
- Measure the output impedance of the follower by attaching a 1kOhm resistor to ground. This forms a voltage divider with the follower circuit; make sure you understand how.
Build the following emitter amplifier:
- Measure its voltage gain. How does the measured gain compare to what you expect based on the construction of the amplifier?
- What do you notice about the phase of the output signal compared to the input signal's phase?
- What is the low-frequency f - 3dB point?
- Bonus: add a 15 µF capacitor to ground on the emitter leg parallel to the 750 Ohm resistor. What happens to the output?
Build a transistor switch to power on an LED. Protect the LED with a 100 Ohm resistor in the collector leg.

Lab exercises week 4

**LTSPicers only ** 1. Build the same amplifier as you evaluated last week. Observe its response to a 10kHz triangle wave input. Now, place a 15 muF cap to ground parallel to the emitter resistor. What is this doing to the emitter's impedance? At high-f, what does the RE || 15 muF look like to the emitter output? How does the output waveform change? (note, you'll have to use an extremely low amplitude to correctly record this signal).

2. Construct a current source using a transistor and a base reference at -12.7 V to a -15 V supply. Verify using a resistor as a load & Ohm's law that the current source works as expected.

3. Using the current source you just built, construct a differential op-amp by sourcing the current through the shared emitter leads of the paired transistors. Use a CA3096 transistor array for this purpose (these are available in the other lab room). For LTSpicers, use this component also.

4. Construct the second gain stage from an emitter amplifier using a pnp transistor (pins 10-12 or 13-15), configured as illustrated in the lecture notes ***note the pinouts are different for our chip***. First, measure the gain of this stage, then measure the gain of the diff-amp and em-amp in series. You might need to carefully control the function generator's DC-offset here to avoid clipping...

5. Build the last stage - the push-pull - using the remaining transisotrs on the CA3096 chip. This will ensure that you can output a bi-polar signal. Again, begin by debugging this stage independently of the amplifier stages; since we aren't yet introducing feedback, do you notice anything strange when you drive this stage with a sinusoidal input at an amplitude of a few V? Why does this happen? Now, connect this to the output stage of the pnp emitter amplifier to complete the discrete op-amp.

6. Given some time, apply some feedback by connecting a V-divider from the output to ground, with 10k in the upper leg and 1k in the lower leg. If you tie the output of this V-divider to the negative input of the diff-amp, you'll find that you have about 11 times gain. Do you?

Lab exercises week 5:

1. Plug in & power (with the correct polarity) the op-amp (LF411 in lab; AD711 in LTSpice). Monitor the output of the amplifier with an oscilloscope probe. If you short the inverting and non-inverting inputs, what do you observe on the oscilloscope? Do you see oscillations, or a steady signal? Is the signal anywhere near the power-supply voltage?

2. Try to compensate the offset voltage of the amplifier's inputs using the guidance provided in the op-amp's datasheet (available on the wiki). Can you successfully zero out the offset, such that shorting the inputs to ground yields a stable, zero-voltage output? If not, what do you see? If so, what happens when you remove the short from the input leads? Can you explain the observed outcome?

3. Use a 10k trimpot to form a V-divider between your positive and negative supply voltages, and connect this to the non-inverting input without feedback. Ground the inverting input. Try to zero the output of the amplifier. What do you observe as you vary the trimpot's setting? Can you obtain a stable output within the operating range (between the supply voltages) of the op-amp?

4. Construct an op-amp follwer. Try to measure the input impedance of the op-amp by connecting a 1MOhm resistor in series with the non-inverting input by driving the resistor with a 1 kHz sine wave. Do you measure 10 MOhms? How does this value compare with the value on the data sheet?

5. Rin is so large that you'll actually see the input impedance is dominated by the input capacitance. Measure the f3dB point, and infer what Cin is.

6. Build a non-inverting amplifier with a 10k jin the feedback loop, and a 1k to ground. Drive the input with a sinusoid, and observe the magnitude of the output swing by varying the input magnitude. What sets the limit of the output swing at 1kHz? Now, increase the driving frequency by a lot (hint ~ 1 MHz) - do you observe a change in the amplifier's response? How did the response change, and what value from the data sheet can account for the observed change?

7. Build an inverting amplifier using the same resistor values as ex. 6, and measure it's input impedance by inserting a 1k between the function generator and the 1k leg of the amplifier. What is the input impedance? What sets the input impedance value of the inverting amplifier?

8. Time permitting, use the inverting amplifier with a 10k - 100k feedback loop to drive a 390 ohm input into a push-pull. Be sure to tie the output to ground through a 1k resistor. Drive with a 1k sine wave, and monitor the output with the scope. Do you see the classical cross-over distortion of the push-pull? Now, take the output of the push-pull, and tie it to the 100k of your feedback loop, and continue to monitor the output. What changes? Now, probe the output of the op-amp directly. What does this waveform look like? What is the op-amp doing when we include the push-pull in the feedback loop?

Lab exercises week 6:

1. Build a comparator circuit using the LF411;

2. Separately, build a comparator circuit using the LM311.

3. Compare the comparators by driving them with a sinusoidal input through a 1k resistor on the input. What happens: for small amplitudes, e.g. 100 mV? For 1V amplitudes at high frequencies?

4. Convert your LM311 circuit to a Schmitt trigger circuit, using 300k and 1k to make the positive feedback gain loop and introduce hysteresis. Measure the hysteresis. Observe the voltage at the non-inverting input - can you explain this behavior?

5. Use the LM311 comparator to build a relaxation oscillator - Use a 100k and a 10k to make the positive feedback loop; use a 100k and a 0.01 microF capacitor to make the negative feedback loop. What frequency do you measure? Is the oscillation stable?

6. Wien bridge oscillator circuit - build the Wien bridge oscillator using the LF411 as described in the notes, and compare it's output with the output from the lab function generator using the FFT of the lab's scope.

Lab exercises week 7

1.Devise an experiment using LTSpice that demonstrates the enhanced performance of the instrumentation amp (LT1167) compared with the standard op-amp (AD711) for a differential signal at high common-mode voltage, with comparable gain. For a guide, consider the standard output levels of a strain-gauge signal driven with 10V supply, varying up to 20 mV at full scale.

2.Using the points for discussion on the instruments required to perform the Tabor measurement, in particular the challenges mentioned in the Keithley low-level measurement handbook, develop a schematic that includes the non-trivial elements of the measurement – e.g. thermoelectric effect, etc. Implement these circuit elements in LTSpice.

3.What are the strategies advocated by the Keithley handbook for making low-R measurements? Will these require any special circuitry to achieve? If so, what performance will you require for your current source?

4.On the basis of the requirements identified in exercise 3, design and test a current source that can complete your measurement, that will drive the current to ground through the load, capable of sourcing up to 10 mA. Using a resistive load, measure the voltage drop over the junction when the current passes through it.

Lab exercises week 8:

You are now working on your LTSpice simulations for the crossed silver wire measurements. We will provide exercises from now on that make progress toward this measurement. In parallel, we will provide instructions for the remote experiment closer to the end of the class.

1.What non-idealities of op-amp behavior will play a role in our measurement of the junction resistance? Is there anything we can do to address these shortcomings of the LF411, for instance? Are there better op-amps to use for our application? Why are these better op-amps for our measurement?

2.Return to the 3rd exercise from last week. In light of non-ideal behavior of op-amps, what processes or steps can we take to ensure our voltage amplifier will work for our measurement?

3.Return to the 4th exercise from last week. Use your current source to drive current through a 1mOhm resistor. Measure the voltage over this resistor using an op-amp of your choice, and see whether the signal is amplified appropriately according the prescribed feedback gain.

4.Now, set up a noise simulation for this measurement. I recommend this source for guidance on LTSpice simulation of noise:link here

Lab exercises, week 9

1.Identify a method for eliminating line noise (the 50 or 60 Hz noise from power-supply pickup) that is recommended by Keithley. Can you propose a circuit to carry out this technique? Hint: it might have multiple components, such as an integrator and a zero-crossing detector.

2.Explain in detail why the lock-in technique will not work as well as the delta measurement method for our measurement. This should be included in your report in the methods section.

3.Conceive a way to implement either the delta mode measurement or the offset compensation method for your current source.

4.Using simulated thermoelectric potentials, verify that with the current source implemented you’ve implemented can be used to recover a 1 mOhm resistance via Ohm’s law.

5.Determine a means of introducing line noise into your LTSpice model. Can you also include white noise in the line noise to make it more realistic? Post your LTSpice models for the line noise on Piazza.

Bonus: can you design a lock-in or phase-sensitive detection scheme for your load cell? There is a design proposed on the Analog website that could prove useful. This part is not available in LTSpice; perhaps you can find a zero-drift alternative, which implements a chopper-stabilization method similar to phase-sensitive detection?

Exercises: week 10

1. Recover the key Hertz contact scaling results using the geometry and elastic properties of Silver, for bars that are 6.35 mm in diameter.

2. Estimate the estimated junction resistance for these bars at an applied load of 10g, 100g, 300 g and 3 kg. Use the value of silver bulk conductivity provided in the Tabor manuscript, or the value we measure in the second video.

3. Install NoMachine software on your computer, and initiate a connection with the lab's experimental computer as a trial run. Message me on Piazza to notify me when you're ready to try the connection.

4. Prepare a detailed experimental protocol to carry out the measurement using the equipment presented in today's lecture. The sooner you submit this to me via email, the better. This is an individual assignment, as each student must carry out a measurement.

Videos, course notes & readings

Videos:

Day 1: Please refer to our Switch channel, located here. I request that you please review the introductory remarks video, the Wien bridge oscillator demo, the Hertz theory video, and finally the introduction to resistors and simple circuit laws video, from 8 a.m. until 10 a.m. tomorrow morning.

Day 2: Lecture on switchtube is here

Day 3: lecture on switchtube is here

Day 4: lecture, part 1 here; part 2 here

Day 5: lecture on switchtube is here.

Day 6: lecture is in two parts: part 1 is here, part 2 is here will be posted shortly to our Switchtube channel

Day 7: lecture on switchtube here

Day 8: lecture on switchtube here shortly.

Day 9: lecture posted to switchtube here.

Day 10: lecture - no audio :( - posted to switchtube here. Second recording available here. Demonstration video here.

Day 11: No lecture video; just introduction to remote experiment here.

Day 13: Lecture video on grading for final reports: here

Day 14: Videos for today: an overview of experimental methods, Instability in Dynamic Fracture, Mechanical measurements in life sciences, Large-scale experiments in engineering mechanics with industrial applications

Videos prepared by your classmates: Lerville-Rouyer, Pierson and Schouwey, A presentation of results from Hugo, Antoine and David (switch.ch), M aelle, Arnaud and Mengbo present their work.

pdf notes 2020:

Day 1: Hertz theory and introduction to analog electronics. Notes 2019, Notes 2018, Reading (linked) The reading is from The Art of Electronics (AoE), chapter 1. These readings remain relevant for the entire module.

Day 2: Notes

Day 3: Notes

Day 4: Notes

Day 5: Notes

Day 6: Notes

Day 7: Notes

Day 8: Notes

Day 9: Notes; readings: one, two, three

Day 10: Notes; readings below in manuals, etc.

Day 12: LTSpice experimental introductory video

Day 13: Slides on the grading rubrick.