DIY Turntable Control Calculator

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flyinglemur

Hi Guys,

I just thought I'd knock up a quick calculator for a turntable motor control system. You can go as deep as you want into topologies (From simple LM317-style regulators up to full-blown Buck/Boost/Flyback circuits) and if you need assistance on that, please email me at : liam(at)liammartin.com.

This would be a closed loop (ie. with feedback) system, so it'll try to keep the speed as constant as possible with respect to load.
The overall accuracy will depend primarily on the % of allowed variation around the reference voltage...although most switching regulators are accurate to within a few percent these days.

However, this is simply a calculator for the 'handy-to-know' ratios - contained herein are :

Platter vs Drive wheel RPM
Motor RPM + Drive wheel vs Platter RPM
DC-DC Controller Feedback Resistances based on the above.

http://liammartin.com/Misc/TTCalc/Turntable_Driver_Calc.xls

If you spot a hole in the calculations, please let me know!
I hope this is of help.

Liam

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Mark Kelly

Your platter speed to pulley speed ratio is wrong.

The correct ratio for rubber belts is (platter diameter + belt thickness) / (pulley diameter + belt thickness).

For non rubber belts the belt thickness term on top and bottom must be multiplied by twice the Poisson's ratio of the material. For rubber and other incommpressible materials the Poisson's ratio is 0.5, hence the short version above.

flyinglemur

Cheers for that! I will make amends ASAP. It's a couple of MM, if that...so can it be declared negligible?

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flyinglemur

Updated to reflect Mark Kelly's suggestions, however I will assume a non-compressible material - if you want to go into compressible belt media, so be it - and I will pop it in the calculator if enough people ask for it! Hope it's OK!

L

cbdb

The correct ratio for rubber belts is (platter diameter + belt thickness) / (pulley diameter + belt thickness).

Maybe I am missing something, but this dosnt make sense to me. Draw a picture with 2 pulleys and a very thick belt. The only thing that matters is the inside surface of the belt (the surface touching the pulleys). The thickness dosnt matter.

flyinglemur

Maybe I am missing something, but this dosnt make sense to me. Draw a picture with 2 pulleys and a very thick belt. The only thing that matters is the inside surface of the belt (the surface touching the pulleys). The thickness dosnt matter.

I thought this too.

Jacques Merde

I thought this too.

The effective radius of each pulley or platter actually occurs at somewhere near the belt centerline.

dnewma04

Maybe I am missing something, but this dosnt make sense to me. Draw a picture with 2 pulleys and a very thick belt. The only thing that matters is the inside surface of the belt (the surface touching the pulleys). The thickness dosnt matter.

You are missing something, Mr. Kelly is dead on with his explanation.

Nanook

well, belt drive is not the only drive system out there....

after doing some thinking (which is not always my way...), I've given up on belt drives when designing a drive system, I'm sticking with either idler drive or rim drive. I've looked at various systems and ran my choice and basic design past a mechanical engineer whose opinion I trust.

As Mark Kelly points out, the modulus of elasticity is quite high for rubber (regardless of thickness but at smaller thicknesses the forces required to deform it is smaller) if using a rubber belt at room temperature. This implies that in small enough diameters to allow use in belt drive systems the modulus of elasticity does need to be considered. I think this is one of the reasons why many have reported the benefits of changing belt materials--- belts do vibrate and can introduce "smearing" of the sound. Even some rather expensive tables have benefitted from a change in belt materials or in motor or drive type upgrades (like the Teres Verus upgrade)

Perhaps a different belt material could be used. I have used high strength nylon upholstery thread to very good effect. Difficult to get the correct length and tension required to drive a platter, but stable once done. I assume a good quality silk thread would also provide the same, and perhaps unwaxed dental floss. Something that may be considered is what some have used. 1/4" cassette tape, or 1/2" video tape or mylar (as per the Redpoint and Teres designs) with suitable pulleys.

Take care all...

flyinglemur

This is true - but one can disregard the belt diameter and use the same formulae for calculating rim-drive ratios, I believe.

Nanook

I was referring to the modulus of elasticity...

using a rim drive or idler negates the issue of the modulus of elasticity to being essentially meaningless (but not rigourously meaningless), particularly if the diameter of the drive wheel or idler is large compared to any rubber that it may use...

and really the sheet is not needed.

(motor RPM)=((diameter of driven pulley or platter)/(diameter of drive pulley)) *record speed

this does not take anything but the ratio of diameters into consideration.

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Mark Kelly

The modulus of elasticity of the belt material has almost nothing to do with the belt drive calculation and in any case the modulus of elasticity of rubber is quite low, it's the bulk modulus which is very high (and that's a different thing altogether).

For idlers, on the other hand, the modulus of elasticity of the idler rubber is quite important.

Nanook , the formula you use is wrong for both belt and idler.

For belt, the thickness of the belt must be included.

For idler, the effective Hertzian contact depths for the both the pulley / idler and idler / platter contacts must be taken into account. These cannot be calculated without knowing the relative contact forces. This is complicated for a design like the Garrard by the fact that the two forces are not symmetrically disposed across the idler.

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