Experimental Vibration Analysis of a WM280 Lathe

Have you allowed for the vibration of the hard disc and fan in the computer?

I have now! I've switched to a RaspberryPi3B, which is vibration free - no disc or fan!

The breadboarded device at right is an MPU6050 6-axis Accelerometer and Gyroscope Module. It connects to the Raspberry's GPIO pins. Quite easy to use. There are two outputs,

• binary frames sent serial at 115200 baud which can be converted to USB with an adaptor or connected directly to the RX pin on an Arduino or Raspberry UART. This data is integrated on the module by a digital motion processor, ie the two sensors are corrected relative to each other by black magic.
• The module also allows raw sensor data to be read directly over I2C.

The module's DMP continuously sends acceleration, spin rate and attitude data, all in x,y & z dimensions. And temperature. Conceptually the module's accelerometer can detect lathe vibration in three planes, to answer:

1. Is lathe vibration cause for concern, and in which planes is it vibrating?
2. At what frequencies is the lathe vibrating?

No problem getting the Arduino to work, but I got into trouble by writing Python3 on the Raspberry to decode the binary stream. After several tantrums and false clues, the answer turned out to be easy. Python's integers aren't directly compatible with signed 16 bit shorts. Fortunately Python's ctypes module supports the binary sent by the MPU6050.

That sorted out, I did a few trial recordings expecting great results. Nope! Either my lathe doesn't vibrate, or more likely it's not vibrating enough for the module to detect it. Another cause for concern is the relatively low sample rate. More research needed, but looks like I've bought the wrong accelerometer.

There is a Plan B. Anticipating trouble I ordered a Keyes 801S Vibration Sensor at the same time. Although much simpler than the MPU6050, I'm hoping it will be more sensitive. Not betting the farm on it though - I couldn't find a detailed specification.

The advantage of hosting on a RaspberryPi is they run headless, on a battery if necessary, can store large logfiles, could be used to do the number crunching, and the logs (or graphics) can be accessed over wifi from Apple, Windows or Linux.

In the prototype the unboxed Raspberry and sensor are powered up resting on the lathe's headstock and then the Python program is started from a remote terminal. Logging to file is started and stopped at the lathe by grounding the green wire. Each start opens a new logfile, so many recordings can be done per session. If the sensor can be made to produce meaningful data, the computer and sensor would be boxed up and provided with push-button controls.

Dave

My brain is cooked too, but that,s from sitting too long in the sun on this beautiful day.

Will leave the Maths for later, much later.!

The vibes will only worry me when I see the chuck spinning down along the bed !

Dave, (SOD)

I have an Instrustar IS205A usb, datalogger scope and spectrum analyser which does a fair job on frequency analysis but no way can I get a printout. Current cost is about £65 from China via ebay.

Also have a Hantek 6022BE similar usb scope but not sure it does FFT; cost around £40. Best to search both on ebay for full specs.

Sorry I didn't get back to you on PM re. vibration lathe analysis but I just bought a Raspberry PI and I think I have fried my brain. What the learning curve is I daren't guess. Over the years I opted away from an electronics career to be a noise and vibration consultant - my workshop is testament to the deep allure of swarf and oil.

Basically for a manual lathe f(0) is mandrel rpm dictated by balance quality riding on background slush (noise) generated by lubicant swirl and sundry resonances plus motor speed and electrical noise. All other frequency peaks relate to gear meshes and bearing noise don't forget beat frequencies including relative speeds and rolling element speeds. Hewlett Packard used to do a handy booklet on this.

Hope this helps, Les

Interim report, I haven't given up yet.

I got no results out of the MPU6050 based accelerometer module I was experimenting with. After reading the small print I find it was a poor choice for this application because it's designed to ignore vibration! They're intended for quadcopters and RC models where vibration interferes with direction sensing so it's filtered out by the module. I thought it was possible to read the sensors directly via an SPI interface, but it turns out to be connected to the DMP chip, not the sensors. No response to an SPI probe, so either the module is faulty or the interface does something else. I've decided to follow Leslie's pointer and get an Instrustar IS205A data-logger for my Birthday. (I am 21 again this month.)

I bought this sensor, which is the more sophisticated of two very similar. It has digital and analogue outputs. This also is unsuitable! It's a species of knock detector, giving a measure of how big the bump was. Not very sensitive, and, given a wallop, the sensor takes several milliseconds to settle. Hopeless for detecting delicate repetitive movement. I guess the gold ended drum contains a weight on a spring as used in car alarms.

Now I'm waiting for delivery of a cheap MPU6050 unit that doesn't filter it's output. Judging by the wait it's on a slow boat from China! Andrew Johnson found a useful Article describing a similar endeavour to mine. It identifies a particular sensor that should definitely do the job. Only one small problem - they're $70 each plus post and packing from the US! (If you have to ask you can't afford it.) I hope the cheap MPU6050 unit works!

Andrew's information confirms analysing vibration data is difficult. Rather than target the whole machine, the project concentrates on motor bearings and was able to detect problems early. If I get this to work, might be better to develop a kind of electronic stethoscope so the operator can 'listen' to suspect machines one part at a time, as a way of simplifying the data.

Dave

Can't find my circa 2016 post on the vibration analyzer/balancer I built up ....Anyway, used software from Miklos T Koncz , a Hungarian. Built up the accelerometer sensors using ( now a bit old..) Free scale MMA7260 3 axis analogue output devices, into the laptop sound card, with an optical rotation position sensors.

Works very well indeed - used it to balance spindles etc, up to 25K rpm.

Might have a go and instrument the lathe...

Accelerometer modules

Sensor selection control box

Balancing a 20Krpm spindle on a sensitive drill press.

Red arrow is the balancing 'weight' - a small strip off adhesive aluminium tape. White circle is rpm and position center.

Accelerometer super-glued to support arm to measure lateral acceleration.

Doh! That's impressive! Balancing a spindle is a nice application and I like the graphical display too. I'm going to have to lift my game...

Off searching for Miklos T Koncz now.

Ta,

Dave

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Upon which topic ... These are like the accelerometers we mostly used in the Lab, in the 70s and 80s

**LINK**

https://www.bksv.com/en/products/transducers/vibration/Vibration-transducers/accelerometers/4371

The cables and amplifiers were also ‘not inexpensive’

Three of those, mounted on a 1” cube, at each of several locations, was typical.

MichaelG.

Yep, thats the technology of the day, back then - used many variants of those when we did the MIL-STD-810 environmental qualification tests on avionic boxes - used to close the vibration loop on the electromagnetic shakers.

Those sensors were expensive for sure! And the Charge amplifiers likewise.

The one in the photos still works, in my draw, with its charge amp..

Dave, seems Miklos has disappeared. Cannot find any trace of him or his doings anymore. Maybe I can help you out with his kit if you are interested....PM me..

Joe

.

Fond memories, Joe

MichaelG.

.

Edit: ___ seeing your note to Dave, I’ve just spotted this:

http://users.atw.hu/aerotarget/Balancing/Manual.pdf

Edited By Michael Gilligan on 18/06/2020 08:10:54

Michael,

Yes, I found that as well, but his web site is gone, and he does not answer his email anymore. The manual gives interesting insight, but how to obtain his software - it was inexpensive.

Must remember we are speaking of a dynamic balancing tool here - not at all what Dave is chasing - this tool does not do vibration analysis in the true term - just enough to do balancing.

Vibration analysis with a view to machine reliability analysis is a life's work and a handful of dissertations all on its own!

You may know the SA Rooivalk Helicopter...I had a 'small' team ( 9 people..!) working on developing a real time vib. analysis strap-on system to predicate lifetime and service interval requirements on its rotor gearbox. That was interesting - digging out the few hundred hertz gear-teeth vibrations from the wapping of the rotor blades from a few rpm to maybe 500rpm, the turbine reduction box noise, etc....There were more than a handful of TMS 320 series DSP's,...And a few maths geeks far smarter than I was on the subject!

.

Understood, Joe

... I was just intruding on the ‘overheard‘ conversation.

My own experience was in the “Environmental Engineering“ test facility at BAe Dynamics [a.k.a. Army Weapons] in Stevenage.

MichaelG.

Many thanks to Michael for finding the Miklos Kunze paper; the closest I got yesterday was a DVD advert I suspect was Gay Porn!

The paper's another good read with plenty of discussion of pros, cons, whys and wherefores. Koncz complements Andrew's paper, which centred on AnalogDigital MEMS technology and uses LTSpice to display waveforms as a demonstrator, rather than exploiting a sound card. It shows how digital signal analysis can detect bearing wear with data from three ADXL1002 accelerometers ($75 each). Miklos' sensors are a loudspeaker and repurposed relay coil, much cheaper, and he tackles turbine balance. I'm sort of in the middle; can lathe vibration be detected and causes identified? Is it worth doing? I don't know yet.

Struggling with the maths is a major challenge! I'm OK with:

But integrals are beyond me! (Though I think Eq4 below means I could use the easier right-hand side.)

Delighted to hear Joe had a couple of maths geeks on the Rooivalk Helicopter. It means I'm not the only one swamped by sums!

At the moment I'm exploring digital signal maths with SciPy and MatPlotLib and they probably wouldn't be the end solution. Ideally the computer should be cheap and portable so it can be used fearlessly in a workshop. So far an Arduino Nano and RaspberryPi3B both collected data successfully from a digital 6-axis accelerometer. The Raspberry has enough poke to run a TFT display or do XWindows over WiFi. The Nano isn't powerful enough to analyse the data or display it. Albeit slowly a Raspberry Pi4 could read, analyse and display results but it doesn't have a built-in Sound Card, or analogue input (to read analogue sensors). A Nucleo is an interesting possibility so I bought a F429ZI to play with: much faster than an Arduino, plus Graphics, Audio and Floating Point accelerators. Whether or not I can leverage its goodies remains to be seen!

Not sure yet if this is an actual project or I'm idly exploring possibilities for fun and interest! If the concepts can be made to work I'll pull them together into a build. Or maybe I'll have a melt-down on the way! (My clock-analyser project stuck when I realised it depended on a good GPS signal. My unit doesn't work reliably indoors and having to take clocks outside for testing is a bust. Nor am I cracking on with 3D-printing... )

Too little time, too many distractions, and insufficient discipline.

Dave

I have adopted a much more simplistic approach to as turned quality, if it is not acceptable.

Marginally change spindle speed using the VFD, load the toolholder with some form of added weight or change to a carbide boring bar which is much stiffer (and heavier).

Thank God for that! I think, post career, that all those elements are what keeps up healthy, alive, alert and the brain active. ( sorry, what were we discussing...?)

on your issue with the GPS signal..

My clock-analyser project stuck when I realised it depended on a good GPS signal. My unit doesn't work reliably indoors and having to take clocks outside for testing is a bust.

- I had a similar issue with a 'mobile' GPS stabilised reference oscillator - I made a sort of 'signal repeater' - I used an active GPS antenna, fed it 5v via two 10uh inductors using an SMA T connector - the antenna on one end of the T, the 5V on another, and then a 2nd , non active antenna, on the 3rd end of the T.

Place the active antenna outside with good sky view, and place the 2nd antenna with its business end on top of the antenna you wish to get the signal into. My antenna each had 6meter cables, so was easy to locate - Worked a charm.

Dave, if interested PM me your email and I can send you all I have on the Miklos system - lots of PDF's and photos of setups, quite interesting.

When I were a lad we used a penny balanced on it's edge on top of the headstock.

Neil

Dave,

If you are using a computer that has access to the internet you do not need GPS to have an accurate clock. With the NTP daemon running your clock will be synchronized within ms to UTC.

Adrian

Hi Adrian,

I was trying to do better than NTP, which is normally more than good enough for most purposes! NTP is accurate to within a few milliseconds, and then suffers jitter due to the computer's built in clock. NTP has to regularly correct computer clocks because they drift, and the software trusts computer time which is usually slightly wrong.

How often computer time is corrected by NTP depends on the operating system. Linux is much better than Windows. Try Time.is : the difference between NTP and what your computer's clock believes at the moment is displayed top left. My Ubuntu computer this morning is 'exact', ie -0.004 seconds (±0.035 seconds)

NTP is good but not very good. (notice ±0.035 second tolerance mentioned by time.is above)

Some GPS modules output an accurate hardware seconds tick. The tick is derived from multiple satellites (more than needed for ordinary navigation) and corrected by the module for accuracy. It's possible because each satellite is in a known position and sends atomic clock standard time signals. As the receiver knows where it is on the ground, and where all the satellites are, it can apply the corrections necessary to generate a second pulse correct to within nanoseconds of atomic UTC. GPS time is as good as it's possible to get in the home and almost everywhere else!

An expensive GPS unit designed for time applications can get within 1 nanosecond of true UTC. The affordable consumer module I'm using claims 10nS jitter. It's a challenge to maintain anything like that accuracy inside my code running on an Arduino, but I reckon I'm good to about 10 microseconds within each second. The long term accuracy is extraordinary because the Arduino's internal clock is corrected to GPS time once per second.

The idea is to measure a mechanical clock pendulum with a much better time standard. With a GPS signal, the analyser can 'see' micro-deviations within each pendulum swing as might be due to the escapement, alignment, or vibration, and also measure long-term changes. By logging humidity, temperature and barometric pressure as well as pendulum timings, it's possible to detect errors due to the environment as well as rate errors.

The idea of using an ultra-accurate clock to correct mechanical time isn't new. During the 19th Century, most astronomers were employed doing it. Their job was to compare the Observatories mechanical clock with star transit times, find the clock's rate, and calculate the necessary corrections. Every night, all night, weather permitting. Then locals would set their clocks from observatory time. Ship's chronometers were the most important customers. The observatory would also check time-pieces to determine rate errors, so that owners could correct readings. Knowing 'it runs consistently slow by 2.4 seconds per day', means the navigator can fix the error and not end up on the rocks!

Dave

Edited By SillyOldDuffer on 19/06/2020 10:20:55