Piezo vibration amplification - Page 1
Forum section: electronics / projects

author: geo_leeman
author_rank: Regular Contributor
timestamp: September 10, 2017, 02:27:16 pm
content: Hey all,I'm working up a sensor that measures the resonant frequency of a bit of wire using a piezo element (see attached photos). The wire is "plucked" by the electromagnet below it and freely resonates. The idea is that as mass accumulates on the wire (freezing water) the resonant frequency will be reduced. This type of sensing methodology has been used in the past, but I'm afraid I'm overcomplicating the signal conditioning here. All I'm interested in is the frequency response to the plucking, so greatly amplifying the signal to the point of a square wave, then counting crossings seems more sensible that full FFT, etc.That being said, I've considered and played with instrumentation amps, JFETs, and JFET based op-amps like the TL071 and a circuit likehttps://electronics.stackexchange.com/questions/153911/single-supply-op-amp-audio-amplifier. Any thoughts on the best/cheapest approach here (disposable sensors)? I know I'm overthinking/complicating the situation as it's not really precision instrumentation and I'm fighting the urge to properly DC offset, etc. For what it's worth - I hooked it up to an INA128 setup on a dual supply and got nice results with gains of 1-5k. This circuit will be signal supply in it's final state though.Any thoughts appreciated!

author: Kleinstein
author_rank: Super Contributor
timestamp: September 10, 2017, 03:11:01 pm
content: For the amplifier the OP should be good enough. One will need a DC path for both the OP and the INA. Just for getting the frequency there is no need to have a very stable amplification. The amplifier does not need the coupling capacitor at the input - most piezos can have a DC voltage superimposed. Depending on the circuit following, one might get away without the capacitor at the output too -though it might help to get a stable DC level. It might be a good idea to not saturate the amplifier, but use a comparator input of the µC that does the "counting". This helps to avoid shifting of the DC level.It depends on the required resolution for the frequency measurement, if just counting the pulses is good enough. One might at least use a kind of reciprocal counting (measuring the time for a number of periods).For very high resolution one would use an ADC to sample the signal (without saturation) and from that signal calculate the frequency. An FFT is usually not the best option - best resolution would be from a least square fit. However the plastic holder is likely not stable enough to warrant that effort.With the magnet for a non resonant excitation there will be a mixture of several vibration modes. The next higher modes is at something like 6.x times the lowest mode - for the simple comparator like detection this can be a problem for the initial phase. So one might need to wait a little before taking data. The magnetization of the wire can also effect the frequency - not very much, but there is an effect. This could be a problem with changing temperature.For the mechanics, it would be good if there is low (mechanical) damping from the clamping and piezo. A little more mass and better clamping if the wire might help with this. In principle the peizo should also be able to excite the vibration, though it might need resonant excitation.

author: geo_leeman
author_rank: Regular Contributor
timestamp: September 10, 2017, 08:07:55 pm
content: Thanks for the thoughts! It would be nice to drive the wire with the piezo and that's something I'll have to try.The wire resonates at 45-55 Hz depending on exactly how I cut it. Given that, I designed and built up the attached circuit. The performance is not stellar - the gain could be higher, but I seem to be getting more high frequency content than expected. If I did the math right here, I should be low passing at ~160 Hz. Any ideas?

author: StillTrying
author_rank: Super Contributor
timestamp: September 10, 2017, 08:18:54 pm
content: In the 2nd photo the magnet looks in a very good position to pick up the after-pluck wire vibrations itself electric guitar pickup style.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 10, 2017, 10:08:59 pm
content: The higher frequency part could be the second (or even higher) modes. These should be around 6.5 times the base frequency. Usually this signal will decay faster than the lowest mode. Otherwise filtering would be an option. The cross over frequency can be lower than 160 Hz. Even just 50 Hz might be ok.One step would be a capacitor (e.g. 4.7 nF) in parallel to the 150 K resistor at the OP. Amplification by a factor of 1000 looks like a lot to me. 1 µF and 150 Ohms gives a lower frequency limit of around 1 kHz - thus way to high. So maybe change the resistor from 150 ohms to 2.2 K and / or increase the capacitance.

author: Zero999
author_rank: Super Contributor
timestamp: September 10, 2017, 10:56:29 pm
content: Quote from: geo_leeman on September 10, 2017, 08:07:55 pmThe TL071 is not specified to work off a single 5V supply. The total minimum supply voltage, recommended by the data sheet, is 10V (+5V -5V) and no characteristics are given below 7V.http://www.ti.com/lit/ds/symlink/tl072a.pdfSwap it for an op-amp which is designed for operation from 5V, such as the MCP601, MCP6022, LMC6482, LT1678 etc.

author: geo_leeman
author_rank: Regular Contributor
timestamp: September 11, 2017, 03:11:36 am
content: Quote from: Hero999 on September 10, 2017, 10:56:29 pmGood point - I setup the breadboard on +/- 7 VDC to continue experimenting, but will likely grab the LMC6482 on my next order.Quote from: Kleinstein on September 10, 2017, 10:08:59 pmThe gain of 1000 is high, but even with that, I'm getting about 0.3 V peak-to-peak. The vibration amplitude is very small as to not shake off any of the super cooled water trying to freeze to the wire. I will try to bump the filters down some, but it looks like the gain may need to go up more - possibly a second stage? At that point, an instrumentation amp may be cheaper than increasing the BOM count with lots of passives.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: September 11, 2017, 05:17:22 am
content: if the elcetromagnet is up to being driven at your wire resonance freq then make a continuous oscillator with feedback from the piezo to the electromagnet driverequires a bias current to linearize the forceand agc to control amplitude - very standard in oscillator circuits - typically a couple of diodes, op amps and a jfetbut the payoff is a single frequency continuous signal

author: polli
author_rank: Contributor
timestamp: September 11, 2017, 05:30:15 am
content: Forgive me if it has already been said or if it doesn't make sense, but if you are open to counting the zero crossings with a microcontroller, you won't need amplification. any microcontroller with 10bit ADC should work.You get the sample location of the zero crossing, then you solve for the precise intersample position by using any simple (or complex, depending on your accuracy requirements) interpolation function described here:http://yehar.com/blog/wp-content/uploads/2009/08/deip.pdf

author: Zero999
author_rank: Super Contributor
timestamp: September 11, 2017, 08:41:34 am
content: Quote from: geo_leeman on September 11, 2017, 03:11:36 amIf you have a dual 5V supply, i.e. both +5V and -5V, it should work fine. If you only have 0V and 5V, as per your previous schematic, it might not work at all, or it may work but not meet the specifications and be unreliable. +7V and -7V should be no problem. A single 7V supply will probably work, but may not be optimal.

author: geo_leeman
author_rank: Regular Contributor
timestamp: September 11, 2017, 01:49:16 pm
content: Quote from: f5r5e5d on September 11, 2017, 05:17:22 amSince the resonant frequency of the wire will change, I'd need some sort of PLL setup or similar then I'd think. Also it would certainly require more power.Quote from: polli on September 11, 2017, 05:30:15 amAmplification is required no matter what since the signal is a few mV at most from the piezo raw - I don't see a way around that as a 10-bit ADC might see 1 bit change at most. Great article on the interpolation though, very good stuff!Quote from: Hero999 on September 11, 2017, 08:41:34 amI still didn't get stellar results, so I must be mis-understanding the filtering or need some better signal conditioning pre/post amplification.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: September 11, 2017, 03:06:35 pm
content: Quoteno, you're missing the fact that I'm describing an oscillator that includes the wire as the resonator/phase shift element - the loop oscillates at the wire resonance frequencythe extra op amp oscillator parts don't use much powera magnet could eliminate the need for a bias current in the electromagnet drivefor some measures of resolution vs power a continuous oscillator should be the best solutionas mentioned you can do it all in te electromagnet/wire interaction without the piezo - look at the EBow guitar sustainer as a proof of concept

author: Kleinstein
author_rank: Super Contributor
timestamp: September 11, 2017, 06:13:02 pm
content: The original shown amplifier has a lower frequency cross over of about 1 kHz. So at 50 Hz it would be only a gain of about 50 left - but higher gain for higher frequency. So one would definitely need to adjust the capacitor - more like 50 µF to ground. Even a dual OP (e.g. MCP602, LMV358) is usually much cheaper than any INA. With an comparator input at the µC an amplitude of some 100 mV is sufficient.The output voltage can also depends a lot on how the wire is mechanically coupled to the piezo disk. Just clamping it with the plastic part does not look promising: it would add quite some damping to the oscillation from the plastic and may not couple the force to the piezo the right way. I would consider soldering the wire to the edge of the disk - the normal piezo disks (used as beeper) are quite sensitive to bending the disk, not so much to direct pressure over the whole disk.Using the resonator as part of an oscillator can be low noise, and relatively low power. A resonant excitation is also very effective and can thus use low power.The excite an ring down method could adjust the power used by changing the interval between measurements. It too could use resonant excitation, once the frequency is approximately known. It might be more accurate as the excitation would add possible phase shifts and interaction with the resonator. E.g. a permanent magnet would tend to lower the frequency.The accuracy of the measurement is likely anyway limited by temperature effects. So one should plan for a stable thermometer and maybe a wire with a low TC in the elastic constants (E-invar). With both methods (oscillator or ring down) a low mechanical damping helps to get an accurate frequency measurement. The simple wire fixed at one end might not be the best solution, as there is quite some effect from how the wire is held.How much is the expected frequency change ?

author: geo_leeman
author_rank: Regular Contributor
timestamp: September 12, 2017, 01:58:20 am
content: Probably a couple of Hz at most with ~0.1Hz resolution or better being ideal. Here's results from a similar sensor in the literature during it's accumulation and loss of supercooled water.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 12, 2017, 01:06:34 pm
content: Only 0.1 Hz resolution at 50 Hz would be rather poor. At least the resolution can be considerably better. Even with still a rather high damping one should be able to measure a decay over at least 100 cycle and thus 2 seconds. So even a slow running µC could give a resolution in the 0.1 mHz range. However signal noise, temperature and parasitic effects may limit the accuracy or useful resolution. The simple plastics part around the piezo could cause quite some trouble, as it is rather light and may cause quite some damping.For the low requirements just a comparator input should be good enough. To a void the higher modes a resonant excitation might be a good idea.