Turntable Speed Regulation

[Babooino]

hello trolls,


i am in the process of building a turntable to my lathe. i need now some tips on a speed regulation circuit.
i am working with a dc motor and a pwm driver based on ne555. i can go easily from 16 to 78 without loosing torque.
my platter is around 12kg aluminum (with a car flywheel added to it) and spinning smooth but with a little variation.
my initial idea is to put some kind of motor with a encoder feeding the pin 2 (trigger) of the ne555 chip. would that work? any tips?

cheers!

[Fela Borbone]

Hello!
The DC motors I tryed to control with PWM and uC(several sizes and voltages)needed frecuencies included in audible area, to be effective.That leads to an stroky behaviour. Pure PWM control can be softened whit a "buffer" transistor, first transistor operated with PWM and the result in colector filtered(using capacitors). This "filtered" voltage can be used to drive the final power transistor in a more "linear" (not by saturation)manner. That means bigger heat shink.
As triggering, maybe an infrared sensor and windowed disc or alike?
Other option is the schematics Flo had published somewhere in this forum,an op-amp based comparator for speed control.An interesant choice for this pourpose.
But you may need a device to generate a voltage proportional to de actual speed of the plate if its not build-in in the motor. Motors are also generators if motion is forced to its shaft.In a past project I used a brushles motor shaft as the platter shaft.tBrushless motors(like the ones in HDD)run smooth an produces usable voltage after rectifing and fitering.Filter capacitor choice is tricky, if too big, the voltage changes too slow.This voltage may be amplified to fit your needs.
I will work more on this in my next project, is also about speed control for a platter.
Hope to see pics of your platter in the future, sounds fantastic!
Good luck!

[Babooino]

i saw flo's circuit. i am more inclined to use an opamp design. someone told me today that i could do that regulation in the psu instead of the driver. clues?

pics soon!

[Babooino]

other option i have is to use an ashland hysteresis motor with a frequency inverter and belt drive the platter.
it would be goo as i already have a stash of these motors from my old ampexes.
thoughts

[Fela Borbone]

Hi!
If the power supply is a PC type switched one, Is usually operated with error amplifiers, I mean, comparing the voltage supplyed with a reference voltage,and altering the PWM rate to adjust output. the way the spsu senses its output can be altered to sense a device providing voltage proportional to the platter speed (tachometer or alike) so the pwm control is slave to the platter speed.
Most PC spsu uses the TL494 chip.in its datasheet you can find a better explanation than mine
http://www.ti.com/lit/ds/symlink/tl494.pdf

If you´ve got quality hystesis motors, is a good option, if the torque is rigth IIRC there's a thread here about controlling them

[Babooino]

Hi!
If the power supply is a PC type switched one, Is usually operated with error amplifiers, I mean, comparing the voltage supplyed with a reference voltage,and altering the PWM rate to adjust output. the way the spsu senses its output can be altered to sense a device providing voltage proportional to the platter speed (tachometer or alike) so the pwm control is slave to the platter speed.
Most PC spsu uses the TL494 chip.in its datasheet you can find a better explanation than mine
http://www.ti.com/lit/ds/symlink/tl494.pdf

If you´ve got quality hystesis motors, is a good option, if the torque is rigth IIRC there's a thread here about controlling them

i searched the controller thread you mentiined but no success
would you point me a link?

[Fela Borbone]

Hello!
The link shows the TL494 datasheet form Texas Instruments website (www.ti.com), in my computer the link works.For shure there must be similar projects somewhere in the net that deserves a look.Controlling speed is not the pourpose this chip was designed for, but the voltage supplied, and so the speed of the DC motor connected to it, will be proportional to the "voltage sensing" input. This input can be USUALLY easy located because is USUALLY the only wired connection that links primary transformer circuit with secondary."USUALLY" in capitals means that not all spsu are exactly the same but the most old good PC power sources are.You don't need a lot of money for this experiments, If one unit don't work well, just try another.

Also for using PC power supply for other uses than computer, sometimes it needs to have load to work(sometimes not), and the "power on" cable should be wired to do so.

[Babooino]

thanks! actually i meant the link to hysteresis controller link. couldn'T find it anywhere....

[Fela Borbone]

Hello!
http://www.lathetrolls.com/viewtopic.php?f=1&t=995&hilit=rek+o+kut+50hz
This is for syncrhonous AC.
About the psu DC motor speed regulation, I was plannig do something like this in the near future, so I was curious and did a search...I didn't found anything usefull .I'll do some experiments maybe next week

[Babooino]

great, thanks Fela! after experimenting a bit i came to the conclusion that you have to turn on the motor HOURS before the cutting. After i did that, it stabilizes and the rpm is rock stable! Now i will try to minimize the rumble....but there isn't too much though.

pics and samples to come.