Vibration measurements of Lenco motor

[christosd]

Hi all, 

I've often wondered why vibration measurements haven't been used more often in Hifi. I suppose it is due to the high cost of sensors, measurement and analysis equipment. 

I have often  threatened those around me that I'll one day get something together. I haven't done any nice displays but I'll go through the basic setup. Excuse the mess but I did just chuck this together. I used relatively cheap MEMS triaxial accelerometers. It is a great design where miniature etched semiconductor forks respond to movements and give an output proportional to the Acceleration. They requires a simple dc supply, are internally regulated and give a ratio-metric output of the acceleration from dc to about 2kHz (from memory). It ends up being about just ok for electric motor measurement and gives about 30 mV per m/s2. They have slightly higher noise floor than piezoelectric seanors (worth 1000 bucks each) but if a good understanding of the noise floor limits are exercised MEMS seanors are fun and cheap! 

THE SENSOR 

 

Although the sensor can be had for under 20bucks, the majority of the work is in the breakout boxes and power supply. 

 

BREAKOUT BOX AND POWER SUPPLY. 

 

What essentially this sensor does is give a voltage that instantly represents Acceleration at any time. Even the DC Acceleration due to gravity. A vibrating body essentially has an oscillating Acceleration. This sensor measures this instantaneous Acceleration as a continuous waveform but we need a system to acquire and analyse the signal. As this is a low budget exercise, I'm using a National Instruments compact daq backplane with 2x4 voltage input modules that I have borrowed. It is old hat, doesn't have any anti aliasing but does the job if you over sample and ensure nyquist is way above the expected signal content from the sensor. 

NI cDAQ

 

This has a usb interface to a laptop with a cracked screen (low Lenco-Owner budget) running a simple version of an analysis software called Dasylab. I think this one might have accidentally fallen off the internet onto my computer quite some time ago. 

Dasylab software. 

 

This software is completely configurable in a modular way to provide for acquisition scaling, analysis and display. This screen shot shows an overview of the software I wrote to record 2 triaxial accelerometers, and to look at the isolation factors of Hifi equipment racks to help @Grizzly design some of the best racks around.

The problem I had was that my Lenco motor was transmitting vibrations to the Lenco top plate because the underside of my Lenco plate had some damping material that had sagged and was touching the motor. The mechanical short circuiting of the motor isolator spings caused a lift in noise floor on the deck that I noticed in between some cranked high volume listening last week.... which wasn't ideal.

Rather than just fix the sagging  damping material issue,  I wondered what the motor source vibration was. Was it rotor imbalanceor or was it electrically induced vibration? The only way to seperate these is to look at the frequency of the vibration. A vibration at motor speed (25Hz for Lenco 3pole motor) could be bent shaft or imbalance. 

Vibration at 100Hz is electrical and usually happens when the rotor of an electrical motor isn't centred properly in the magnetic field from the stator windings. 

This is a picture of the bench test setup. 

 

The motor was temporarily wired up to run on the bench, (actually on a foam mount) to give better acceleration response and to give more repeatable response. The baseline spectrum (Acceleration in m/s2 vs Frequency) is as follows. 

This plot shows that the vibration between the cursors around 25Hz is much lower than the 100Hz vibration. It is therefore not a rotor balance, bent shaft, or bearing looseness issue. 

I decided to try adding 2 gms of blue tack to the rotor to see if the theory is right. 

 

The plot of the induced imbalance is as follows. 

As you can see, the vibration of the component between the cursors has tripled (from around 0.06m/s2 rms to 0.2 m/s2 rms) This tells me that I could improve the balance of the unit through the use of approximately 0.6gms at the right place but the issue is dominated by 100Hz vibration more than a magnitude higher at 0.6m/s2 so I won't bother with balancing. 

Adjustment of the nose screw of the Lenco motors can change the rotor position and the amount of "motor hum" and there is a procedure over at Lenco Heaven for doing this using just your ear. I thought I would do this with this method. This is what happens when you wind the screw in a little too far. 

The peak at 100Hz increased dramatically and multiple harmonics of 2xline frequency came up at 200, 300, 400 Hz etc. It just became audible buzzy when the 200Hz peak climbed up actually so I tweaked the nose screw to minimise all of the vibrations. 

Next, I'm going to use the measurement system to make sure that the motor isolation springs are doing their job with the motor installed again. Any other ideas are welcome. 

[frankn]

That’s an idler drive so any vibration in the motor will be transmitted won’t it?  I would have thought that whether it matters depends on the damping effect of the idler wheel material & platter construction. 
The springs isolate the motor from the plinth don’t they, would adding any absorption material within the coils of the springs be beneficial? 
Would there be any benefit from adding damping to the underneath of the platter?

can you substitute a better motor?

Edited July 3, 2021 by frankn
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[audiofeline]

Looks like an excellent way to specifically identify and isolate what is contributing to the noise.  Reading your description I'm surprised the approach hasn't been taken more often.  I look forward to further updates.

 

[christosd]

1 hour ago, frankn said:

That’s an idler drive so any vibration in the motor will be transmitted won’t it?  I would have thought that whether it matters depends on the damping effect of the idler wheel material & platter construction. 
The springs isolate the motor from the plinth don’t they, would adding any absorption material within the coils of the springs be beneficial? 
Would there be any benefit from adding damping to the underneath of the platter?

can you substitute a better motor?

It's not just an idler drive, it's a beautiful idler drive design! 

You are right about the transmission to the platter through the idler, however there is a very high mechanical impedance to the platter (afforded by its sheer mass) so the vibration doesn't transmit to the platter very effectively (thankfully). I've never had hum before. 

Also, the idler in a Lenco is mounted on a long flexible arm with an isolated pivot so transmission to the plate is minimised. Most modded lencos have a damping material added to the idler arm too. 

I have a damped plate, heavier bearing, large plinth etc, all of which add mechanical impedance to the platter system. 

My problem (now solved) was that some of the damping material under plate was heated up by the warm Lenco motor such that it sagged and touched the motor! 

 

Someone on Lenco heaven tried to add rubber damping to motor springs once and it increased hum.

Adding damping in theory  reduces motor vibs but adds a transmission path via the velocity damper to the plate. 

 

All this accelerometer stuff is just an interesting  exploration and probably not necessary for good sound but any ideas for what measurements people want to see are welcome. Maybe a top plate measurement in situ with different motor springs or foam chunks in the springs to lightly dampen them? 

I'm a bit worried the sensors won't be able measure low enough once we get to the top plate. We'll see! 

[frankn]

Yes, fun stuff but hard to see whether it affects the sound. The damping properties of the entire system is the key. Can you mount a miniature sensor on the cartridge cantilever???    Probably better to use a laser measurement  - another home diy project for the shed!!
Measuring on the top of the plinth, arm mount or platter(be interesting to see that if it could be done) when in motion would give you an idea if any of those “spikes” get through the system. 

[christosd]

I could measure the platter in service but I'd have to run around the table with the equipment very quickly

 

Seriously, I do have a long enough cable that could get 4 to 5 rotations of cable twist whilst measuring near the centre of the platter ... Should be long enough time to get a few measurement averages. 

 

The cantilever vibration - we can all measure by analysing the phono stage output voltage signal!

 

[Warren Jones]

Keep in mind groove modulation of the micro detail cut into an LP is less than 1micron, a human hair is about 50microns. So any micro vibration will modulate the audio signal.

 

I think you are on the right path to improve the drive. Damping only reduces the amplitude of vibration over time it does not remove it. So balancing the motor must be a good thing.

 

There is more than 1 path for motor vibration to effect the platter, one is via the idler wheel and the other via the motor mounting and plinth

 

Here is a a test setup for you.

 

Mount the motor in the TT and turn the motor on with the Idler wheel disconnected from the platter. Blue Tak the platter still and place the sensor and the stylus on a stationary LP If you have a scope monitor the output of the phono-pre this will give you an indication of the level of vibration transmitted from the motor to the platter via the plinth. It will also allow you check the sensor. IME a phono cartridge is more sensitive than most accelerometers.

[Warren Jones]

3 minutes ago, christosd said:

I could measure the platter in service but I'd have to run around the table with the equipment very quickly

 

Seriously, I do have a long enough cable that could get 4 to 5 rotations of cable twist whilst measuring near the centre of the platter ... Should be long enough time to get a few measurement averages. 

 

The cantilever vibration - we can all measure by analysing the phono stage output voltage signal!

 

 

 

That will not work as the cable will vibrate the sensor. You could make a fixture that sits on the spindle that does not rotate, like the rumble thingymagig that Thorens used for rumble mearurements

[frankn]

Agreed, measuring the cartridge output is probably the best. 

[christosd]

The Mems sesnor will be unaffect by the cable but the rumble sensor sounds cool. 

Your previous post about groove height / displacement, Warren, reminds me that I really should change the measurement units from Acceleration into velocity by integrating the signal  with respect to time/frequency. For example. An Acceleration of 1m/s2 at a low frequency has a much larger displacement that the associated displacement of a 1m/s2 Acceleration at a higher frequency. Velocity sits in the middle and is geared better to the how the cartridge produces a signal. Same analysis, just different units really. 

Edited July 3, 2021 by christosd

[Warren Jones]

I forgot to add, measuring the cartridge output has the added advantage of the RIAA eq.  most TT noise is low frequency and will be boosted by the RIAA so it becomes easier to measure, but it's also real world measurement.

[Warren Jones]

It's unlikely the sensor will be unaffected by the cable, if the cable moves relative to the sensor this will tug on the sensor. if the sensor is sensitive it will affect the measurement. But it's uncertain which axis will be affected.

[christosd]

Or just give up all of this and get a Holbo linear tracker with air bearing platter. Heard one last weekend and loved it. 

[Warren Jones]

Where is fun that....

[christosd]

It's just us Nerds up early on a Sunday. 

[frankn]

In my life before retirement we used mems sensors for seismic monitoring but I remember that they weren’t linear across all frequencies, but especially low frequency bands. Is your sensor properly calibrated across the bandwidths you are trying to measure?  Actually now I think back it’s the ultra-low frequencies (0-20 hz) so likely not an issue. 

Will not be an issue if all measurements are simply before & after at specific frequencies and not referenced to absolute values. 

Edited July 4, 2021 by frankn
Detail

[frankn]

It’s better joining in this than listening to Simon Birmingham waffle on Insiders/ABC. 

[christosd]

Ha ha Simon B can be a drip.. This Mems is. Flat as a tack down to DC. I'll look up specs when I get back.

The next nerdy topic for me this morning is why does my back hurt at the same part of the Dorset Vale road climb to Mt Bold on my road bike? 

Edited July 4, 2021 by christosd

[frankn]

Attach you mems!  
most likely to be related your flexibility and overall fitness assuming the bike fit is correct.