My Pitch Computer Prototype..

[JayDC]

SUPER ELEGANT right now, lol..


So it controls a presto 6n. TT speed, lathe motion, and head. I'm using a motor that provides a hall effect feedback for the turntable, this info is processed by a P.I.D. control loop, and the drive is timed via the PID feedback, Real Time Clock, and a Quartz Crystal. This means it's stable as fuck, and will fight drag.

The whole system right now uses 2 20mhz Stamps, one for the lathe and one for the tt drive. As of yet I only have one pitch programmed, and have not started work on the variable pitch aspect. I'm waiting for some chips to arrive so i can learn more about AD/DA converter chips. Once they arrive, i will start work on the variable pitch.

The head is controlled via a servo, and the lathe is controlled via a stepper motor and a h-bridge. Once i clean up the lathe I'll post some pictures, need to find a better camera too..

Lots of testing tweaking to continue..

The final look is planned to be 505mm x 3u.. it's not quite a 19" rack mount, but is the same size as the presto.. I'm making mine vertical, to be place on the left side of the unit. Anodized Aluminum panel, aluminum buttons and knobs, not the knobs pictured..

sorry the pics suck, i used a web cam..

[opcode66]

Nice work man. Keep at it!

[cymbalism]

looking sharp man

[Aussie0zborn]

Neat.

[leo gonzalez]

Cool!

but what do you mean by this?

I'm using a motor that provides a hall effect feedback for the turntable.

sorry the pics suck, i used a web cam..

[d]

Hey JayDC

Nice work. Just a little question here.
What kind of driver are you using for stepper motor? Microstepping, halfstepping? I have tried and it was bad for me. Too much noise and too much pronounced steps.

Anyway You have done a very nice control unit.

[JayDC]

d:

I'm using a L293D Quad Half-H Driver to drive a nema 17 stepper with a 0.9° step angle.


Leo:

The tt motor has a built in hall effect, this means that the motor output's a small current that is proportional to the speed of the motor shaft. This allows the tt motor to be in a close feedback loop with the controller. This value is then used in a P.I.D control loop I coded.

"A proportional–integral–derivative controller (PID controller) is a generic control loop feedback mechanism (controller) widely used in industrial control systems – a PID is the most commonly used feedback controller. A PID controller calculates an "error" value as the difference between a measured process variable and a desired setpoint. The controller attempts to minimize the error by adjusting the process control inputs."

[d]

Jay, I have tried this but it was too much of a stepping. Not smooth as it should be. First I thought of a linistepper for this purpose but got dc motor with tacho generator and it works very nice. Btw I used half stepping and linistepper driver can do microstepping.
Of course as I understand it is better for you to control stepper than any other.

[JayDC]

well it seems on the one pitch i'm using right now, it's ok.. if I do run into the problems ur stating, I can easily switch to a nema 17 motor w/ a hall effect, like the one I'm using for the tt motor. Just didn't want to go that route unless it is needed, since it raises the overall cost.

I got one acetate left for testing on, so I may trying coding a couple more pitches tonight and see wats wat..

[d]

hmmm... nice then... When I tested was quite some time ago, so do not remember all the cons about it, but I switched to another type of motor.

Can you discribe more about TT motor? What exact model of motor are you using and hall sensor. I am into thoughts right now about it for my lathe. Seems like my motor is too weak for a new TT i have machined.

[JayDC]

The motor i have chosen is 4000rpm w/ 60oz-in torque @ 55watts.. This was spec'd around the premotec motor that flo used on his 6n. I would have used the same premotec motor, but they are very hard to get at a reasonable price in the usa..

the motor i am using is slightly more powerful then the premotec

[d]

So you use it as belt drive motor?

Then maybe you have a link on simple how to control it with hall sensor?

[JayDC]

yes it is belt drive.

I am not sure wat u mean..

All the hall effect does is give a measurement variable to the microcontroller. I coded a P.I.D algorithm to find an error between the setpoint, and the hall effect. This basically means, if you tell te controller to spin the motor at 3501rpm, and the actual measured speed is 3488, then te controller adjusts it's control loop to fix the difference.

My alogritm also takes into account a real time clock, and a quartz crystal, this is to make sure the M in rpm is based on physical time, and not the slightly inaccurate timing of the microcontroller.

if you drag ur finger on the platter the motor will try to fight you to keep the platter at 45.. Similar to how my technics work, just alot more torque.


here is wat i based on, my final code is alot more complex..
http://forums.parallax.com/showthread.php?77656-PID-Control-Intro-with-the-BASIC-Stamp

[d]

Thanks Jay, this is exactly what I wanted to know.

[flozki]

is it stable enough... with the belt?

just gained a lot of experience with belt driven video heads from 60ies, 70ies video recorders. they rotate with around 3000 rpm.
same system. one pulse from a sensor per rotation. sometimes a few more... it must be super accurate. and as soon the belt is worn out, aged you almost have no chance to get the regulation loop stable.
this is why they switched on very early already to direct drive motors. much better performance.

so would be interesting how stable you achieve to get your turntable over the belt with one hall sensor?

neumann vms80 uses regualr dc direct drive motor. around 200-400 W
and a encoder attached with 1000 pulses per revolution.
i dont know how many the technics have. 300 ? not sure but definitely not 1000.

what about using strobe impulses of the platter and feed it into microcontroller?

[JayDC]

what about using strobe impulses of the platter and feed it into microcontroller?

Was just thinking that actually.. I ordered a QTI sensor from parallax, so I can also add the platter speed to the P.I.D. Also thought, if the error between the platter and the motor got too large, the unit could instruct you to change the belt.

Direct Drive would be awesome, just not sure how to do it in a way that is easy to put in kit form though. There in not a lot of exposed shaft on the 6n.

[d]

I was thinking a little more drastic things to try... That is battery drill mechanism. As I have tried with colleagues to put it on the big wind machines in diameter of 1meter and they handled very well. Used them with PWM. They were quite stable and powerfull.
Of course I cannot tell how stable they are without testing carefully but I think that is usable. Usually these battery drills are equiped with planar reduction mechanism. It should be very good for direct drive.

I have tried couple months ago to spin my TT 5 kg in weight with flex tube connected to battery drill and it just went without any problem, so just need a good PWM for stable operation.

What do you think about that?

[flozki]

any gear will produce rumble.
then i would prefer a belt solution with a little pitch drift.
if the flywheel is big enough. our the turntable heavy enough you can reach excelent results.
but with gear. i guess you will always hear the rumble. might be good enough for some tests but not for professional cutting...

[JayDC]

Well, I guess the best way would be to use a motor that maintains high torque at low RPM, and mount the platter shaft via a collar to the motor shaft. I can get said motor, its data sheet says it will maintain constant torque at any motor speed. It's very expensive, needs a motor controller, uses brushes, but puts out 21lbs-in of torque.

Another approach would be to swing the motor like the original configuration, and use one of the idler wheels. This will never be completely silent, and I feel that a belt drive is a far better approach even though its sort of a trade off.

Scully's are belt drive, and people cut masters on them.. I'd love to upgrade my Presto to the quality of a Scully.. Neumann would be awesome, but i think I'd be better off just starting from scratch, maybe a few parts are salvageable.

A Presto 8DG would most likely be the candidate that would more easily upgrade to Neumann quality. Unfortunately, atm I only have a 6N to work with.

I'm just trying to be realistic..


@flo, I may have used the wrong term for the feedback on the motor. The spec sheet says:

A pulse output is provided to
digitally read the speed the shaft is turning. The average value
of the voltage on PG is directly proportional to speed, so this
output can be used as a true tachometer for speed feedback
with an external filter or averaging circuit which usually
consists of a resistor and capacitor (i.e. 20k and 0.1uF).

[d]

@Flozki

yes, I understand that it could produce some rumble. But this planar reducer is best of all what I see to used that is most silent. Well... Anyway I think I would try to cut a silent groove when I will be ready and will see the result... Anyway if I will achieve some kind of result I will post here.