So, I think we're often taught to have the highest inertia possible, with high mass platters, and weight concentrated around the perimeter.
But does this make sense with DC motors that have optical encoders that send corrections to the motors?
Examples would be the Linn Radikal, Mober and the Teres motors.
Wouldn't the inertia, make it more difficult for the motor to make a correction?
Would love peoples thoughts here and maybe an explanation of why the math wouldn't work out that way (if you applied a small change in force in an effort to correct the RPM variation)
But does this make sense with DC motors that have optical encoders that send corrections to the motors?
Examples would be the Linn Radikal, Mober and the Teres motors.
Wouldn't the inertia, make it more difficult for the motor to make a correction?
Would love peoples thoughts here and maybe an explanation of why the math wouldn't work out that way (if you applied a small change in force in an effort to correct the RPM variation)
THe motor might be being corrected on a fractional second by second basis, but the very nature of the flywheel effect of the heavy weight platter will kill any short term fluctuation in speed because it's a flywheel.
For instance if the mtoor was a really bad motor which was fluctuating all over the place all on its own, the flywheel (along with the damping effect of the belt connection) would still iron out the bulk of that fluctuation.
For instance if the mtoor was a really bad motor which was fluctuating all over the place all on its own, the flywheel (along with the damping effect of the belt connection) would still iron out the bulk of that fluctuation.
Hi, Yes, I believe it would be a waste of money using extreme motor control, like in the systems you mentioned. if you are going down the heavy periphery platter route. On my T/T, (which is a stepped down 18VDC battery driven unit) my speed control & wow are very, very low using this method without any feedback control.
Cheers
Cheers
That's exactly what I'm wondering. Moreover, I'm wondering if a platter with less weight would actually respond better to feedback control.
Take the Linn/Radikal system as an example. Or the Mober/Linn system. While the Linn platter is not "heavy" compared to the heavy behemoth approaches, would it be able to make more accurate corrections if it weren't fighting the inertia of the peripheral weight in the platter.
Take the Linn/Radikal system as an example. Or the Mober/Linn system. While the Linn platter is not "heavy" compared to the heavy behemoth approaches, would it be able to make more accurate corrections if it weren't fighting the inertia of the peripheral weight in the platter.
The speed control itself will be a source of error, since it relies on some sort of sensor to determine the speed it thinks is happening - but all sensors have error and noise (whereas inertia is effectively perfect). With a lot of inertia the noise from the control sensor can be reduced in bandwidth. Once the correct speed is reached all the control is doing is trying to add wow&flutter (from the noise of the sensor), and the inertia fights back reducing this - in other words inertia can make up for inperfections in the sensing used.
Also along the way the losses and friction are also overcome by the control loop (at longer timescales) and by the inertia (at shorter timescales).
However control loops need to be stable and that may mean the loop has to be tuned to the amount of inertia present.
Changing between speeds is not helped by more inertia of course, since the response/settling time of the loop will be affected by the inertia and the max acceleration/deceleration is limited by the ratio of power available to inertia.
Also along the way the losses and friction are also overcome by the control loop (at longer timescales) and by the inertia (at shorter timescales).
However control loops need to be stable and that may mean the loop has to be tuned to the amount of inertia present.
Changing between speeds is not helped by more inertia of course, since the response/settling time of the loop will be affected by the inertia and the max acceleration/deceleration is limited by the ratio of power available to inertia.
Another crucial element is the torque of the motor in relation to the platter mass... more so with a belt driven system like this.
A high torque motor will at some level start to overcome the "benefit" of that particular platter (flywheel) mass , but a very low torque motor will struggle to bring the flywheel up to chosen speed.
What you need is a three bears choice of both
A motor with a high enough torque to bring the platter up to speed in a timely manner without destroying the belt each time, hopefully without help, but "weak" enough to just keep the platter at that chosen speed.
A high torque motor will at some level start to overcome the "benefit" of that particular platter (flywheel) mass , but a very low torque motor will struggle to bring the flywheel up to chosen speed.
What you need is a three bears choice of both
A motor with a high enough torque to bring the platter up to speed in a timely manner without destroying the belt each time, hopefully without help, but "weak" enough to just keep the platter at that chosen speed.
As MikePP and Mark have stated, it's all about good design properties.
My Kenwood TT has quite respectable performance with decent specs, obviously whoever planned the design work thought things out carefully.
(Unlike some of the crap produced today)
Notice the platter weight etc. - it doesn't need some 3 inch thick solid monster platter like those behemoth designs.
And the W/F specs when tested in the shop are within the claimed amount at 0.03%.
My Kenwood TT has quite respectable performance with decent specs, obviously whoever planned the design work thought things out carefully.
(Unlike some of the crap produced today)
Notice the platter weight etc. - it doesn't need some 3 inch thick solid monster platter like those behemoth designs.
And the W/F specs when tested in the shop are within the claimed amount at 0.03%.
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What happens if you have a hysteresis motor, like the Papst outer rotor; with no cogging, and when driven by a xtal based 3 phase supply like the SG4 almost literally zero vibration.
The mass of the outer rotor design provides a high moment of inertia, and the generated power supply is accurate to a few parts per million.
As far as I can see it's only the rubber belt that stops this being an ideal solution, and if this is replaced by a non-stretch belt of Mylar or similar material it's about as good as it gets
The mass of the outer rotor design provides a high moment of inertia, and the generated power supply is accurate to a few parts per million.
As far as I can see it's only the rubber belt that stops this being an ideal solution, and if this is replaced by a non-stretch belt of Mylar or similar material it's about as good as it gets
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Indeed, the motor design, being pretty much perfected, and used in many turntables (Thanks to Technics) gives fine performance, without being so overdone and expensive like those eliteist esoteric monstrosities.
It goes to show that those expensive machines are only made to satisfy the egos of self-proclaimed purists.
And the bottom line?...... all you really need is a properly spinning platter and a cartridge to ride the grooves.
The fancy cartridges are also overpriced for the same reasons.
I have a DIY TT and I use a floppy drive 5.5 inch motor. The torque is very high indeed. Regarding the cogging I have no idea. I think depending on the production some floppy disk motor have higher cogging and some less especially the latest production with bigger rotor.
Unfortunately these 5.5 inch floppy disk are no longer available.
As a power source I used a normal 550 Watt PC feeder.
I also try a drive with 2 motor and the fly wheel, but I failed as the mechanical lathe guy did not make a good balanced and precise fly wheel so it introduced vibration to the TT. However I think the fly wheel drive unit and a platter around 5 to 6 Kg is a good and valid project is executed well.
Having made the fly unit pod I may resume in the future the project for my DIY mag floating TT. Perhaps when I have finished the present lento project so I have anyhow something to play records.
Rgds
Adelmo
I have been developing a dc drive for my LP12 and have found inertia of the platter does have an impact on how to control speed. With a lightweight platter I found controlling the speed was quite easy with a speed correction every revolution working well. 33rpm could be locked on accurately within a few revs. With the LP12 platter in place It took some developing. The motor can’t spin up the platter so quickly and I have a speed correction no more frequently than every two revolutions, so the speed has time to settle. It’s still possible to control speed with the same accuracy; within 0.02% with my setup. The impact of the heavier platter is that it takes more revs from startup to get the speed stable and accurate. With a really heavy platter it would take even more time.
Am sharing some uneducated thoughts so I could be wrong. But if we look at things when they are in motion, the correction required would not be much for platter with moderate weight as the platter would be rotating the pulley too. a small subplatter with small length belt would also probably be advantageous as the fluctuation would transmitted in less time than large belt (Without much damping and probably with less creep and slip) . And if platter weight is reduced the resonance, feedback and vibration to the whole system will be more so some compromise is good. Also read in the forum somewhere where the high torque motor was tested against others. How stylus resonances when playing affects speed (Regardless of platter weight) was also discussed in the forum somewhere. Probably by LD and others. But I dont remember which thread.
Regards