New DC drive system for TT -RIM drive- starts here!!! - diyAudio
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Old 12th February 2014, 04:10 PM   #1
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Default New DC drive system for TT -RIM drive- starts here!!!

I decided to open up a new thread for this project. It originally started on a different topic so in order to give it a clear cut, I have generated this new post.

The goal for this project, is to create a top quality RIM drive system based on Maxon DC motor powered up by Linear Supply and controlled by a high precision digital tachometer, that can work with many different model of TTs on the market.

Intent is to create a final assembly and release BOM, schematic so that everybody can manufacture a copy to their will.

I have had email correspondence and spoken with Maxon Motors and have chosen a couple of candidates which are top quality but yet at a reasonable price.

The elected motors are:

1) A-max 26 Ø26 mm, Precious Metal Brushes CLL, 7 Watt
2) DCX 22 L Ø22 mm, Precious Metal Brushes, sleeve bearings


They both come in a variety of different windings so there is possibility to drive them at low speed without using gear heads with reasonably low supply voltage.
I tend to favor the second a bit as it seems more efficient. The lower impedance will allow an easier drive and to get as much torque out as possible.
Also the torque on the second is much higher and might suit better different situations, where more torque constant is needed to maintain proper speed.

I have sketched an initial idea here, please feel free to comment on it.

The first idea I had was to utilize 3 motors in a triangular shape to drive the flywheel, but the problem of synchronization was brought up and it might require some big effort to perfect and figure out that might be too difficult to overcome unless it become a full time job kind of a thing.
Therefore I folded back on the one motor idea and then re-added the flywheel because I think it can bring a good improvement on both noise and speed stability.
Obviously if somebody will shed some light on the multi-motor drive system, we can always go back to a 2 / 3 motor scheme.

As far as work team is concerned, we have this group of people:

Stefano HW designer
Pyramid SW designer
Mechanical designer TBD

We had on the other post, two offers to help on the mechanical side, but it was never confirmed when asked. I am asking to the people who offered to help, to eventually confirm their commitment if they can.
Also, if anybody with capabilities and experience can step in is more than welcome. Obviously the wider the team the faster and more efficient the work will go.

Moreover, before committing to start a new HW and SW development, we would like to confirm and discuss the concept design and refine it upfront. For this we need help of people who have experience on driving system for TTs and want to get here for the community the best design.
When we get a general consensus and confident that the concept design is what we want, then we will start spinning a prototype and get all the parts needed to create the physical model.

I hope this will be of interest to many of you and that can become an interesting project. Please let's see how many people are interested and want to be in this project....please post your interest here


Cheer!
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Last edited by Stefanoo; 12th February 2014 at 04:14 PM.
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Old 12th February 2014, 05:44 PM   #2
Bibio is offline Bibio  Scotland
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don't use a brushed motor as they wear out too quick.

Motor:
Brushless
No Gearhead so this means ''direct drive''
must have bushing rather than bearing for shaft.
3mm shaft
must go to 300RPM and still keep high torque
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Old 12th February 2014, 06:06 PM   #3
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Stefano,

Do you think we should take a harder look at DC Brushless (or more accurately AC polyphase)? If the Maxon DC motors are not inexpensive, and the solution is intended to be more "ultimate", we might want to skip ahead to DC Brushless as it might be the more final solution. This may depend on costs and complexity of sinusoidal drive. You guys with the Electrical Engineering backgrounds will need to decide.

I like the diagram. I haven't looked at these motors but if they want to run at higher rpm's we might need the pulley for changing the mechanical advantage of the motor.

Jamie
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Old 12th February 2014, 08:02 PM   #4
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ok thanks guys for the participation. Let's talk about this for a second.
What would the advantage of having an AC polyphase motor be vs a brushed DC motor? more durable? If so, the DC motor by Maxon are rated 20000 hour of use, which is more than a lifetime for a TT IMHO.
Yes, this has to be a top approach, so I am not stuck with DC motors as long as we have benefits by using other types.

As far as I know speaking with Maxon, Kronos uses DC motor from them (although he didn’t tell me the exact part number, he led me to believe it is on the A-max series), also he mentioned that a new very high end TT ($100K range) will use one of the Maxon DC with ball bearing (preloaded).
The sleeves are a bit less durable than ball bearings, but they are also a bit quieter. I think the rating of the motor would be long enough for anyone to enjoy it for many years and when moment comes it can be replaced as the price for the motor is affordable.

Now if we have noise and torque benefit by going with another type of motor I am on for it. I have also asked the guy from Maxon about speed stability and he said that he is not aware of any problem regarding speed stability but he would personally implement a current feedback to compensate for constant torque.
But my question is: if the brushless is just a poly-phase AC motor, then why wouldn’t we use directly a better AC motor? I am not clear what is the difference in terms of noise would be, but as far as I am concerned the DC brushed motor has big advantage in terms of noise over all type of motors, which would make them then, a good candidate for RIM drive application.

Please let me know your thoughts.
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Old 12th February 2014, 08:25 PM   #5
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I like the idea of having the idler wheel or "flywheel" in your picture on the outer rim. It remind me of the D&R turntable from the vintage era. Most idler drive tables typically have the idler wheel underneath the platter and driving the inner rim or the underbelly of the platter like Lenco. Having a motor/wheel island independent of the plinth is a good idea. Flexible and can be use to drive other passive platters. The belt drive motor island is popular and it would nice to see some aftermarket idler island for consumers who wants a different flavor, provided the speed is adjustable, of course. The VPI style using two motors belt-driving a flywheel is NOT what I am talking. Anyway, if these motor pods become popular we don't have to resort to using vintage turntables all the time as Garrard 301 or Thorens TD-124 prices are off the roof.


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Old 12th February 2014, 08:49 PM   #6
sreten is offline sreten  United Kingdom
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Hi,

FWIW if you build a negative impedance power supply equal to
the DC motors winding resistance, then effectively the speed
will directly relate to to the applied voltage and back emf.

Flywheels stuck on top of motors tend to be more for show
than effect, unless they really have more inertia than the
motors armature, most don't. That is for belt drives.

Best place for a big flywheel is under the motor.

For direct idler drives they are utterly pointless, they
will have no effect compared to the platters inertia.

rgds, sreten.
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Old 12th February 2014, 08:56 PM   #7
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Quote:
Originally Posted by directdriver View Post
I like the idea of having the idler wheel or "flywheel" in your picture on the outer rim. It remind me of the D&R turntable from the vintage era. Most idler drive tables typically have the idler wheel underneath the platter and driving the inner rim or the underbelly of the platter like Lenco. Having a motor/wheel island independent of the plinth is a good idea. Flexible and can be use to drive other passive platters. The belt drive motor island is popular and it would nice to see some aftermarket idler island for consumers who wants a different flavor, provided the speed is adjustable, of course. The VPI style using two motors belt-driving a flywheel is NOT what I am talking. Anyway, if these motor pods become popular we don't have to resort to using vintage turntables all the time as Garrard 301 or Thorens TD-124 prices are off the roof.


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Thank you so much, I am glad you like my idea. You are an expert on TT design and if you like the idea it means a lot to me.
Sreten also had a good observation, what do you think about that?

Yes, the basic idea was to have an independent pad that would adapt to many tables (which means the wheel will have to come out a little more to allow users with TT on a frame to be able to utilize it), that way people won't need to buy vintage table anymore unless they really want to have a piece of vintage on their collection

The flywheel, in my original idea, wasn't only for added inertia, which of course the platter will have more then the flywheel, but I feel it will still add some and if flywheel is reasonably heavy the effect will be even more meaningful, but also and especially it will dampen any residual noise coming from the motor away from the stylus and the heavier the more dampening it will have.

Please you guys with a high experience on TT design feel free to comment and adjust my initial idea if needed so that perhaps we can move forward and start the mechanical design of the pod.
also, is a DC motor then a good solution for everybody?
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Old 12th February 2014, 09:23 PM   #8
bfg4wd is offline bfg4wd  United States
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Default new thread

Stefanoo,
Found this new thread, still willing to help produce parts for the prototype, I do not know how much help I will be in design though.

As for BLDC motors, or poly-phase AC motors, my experience has been with industrial servo systems 5KW and above. It is hard to tell how loud the motor is, but the noise from the inverter can be quite loud mostly the carrier frequency for the PWM. When a servo system is tuned with the speed response to high it will ring very loud.

I did find this: https://estore.ti.com/drv8312-69m-kit.aspx
I did not find any specs for the motor though.
I was going to purchase it to see how loud the motor itself is.

Thanks,
Ted
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Old 12th February 2014, 09:29 PM   #9
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Ted,

thank you very much for your support. Then perfect you and Jamie can help with production of prototype mechanical parts and mechanical drawing from when we will release.
AS far as electrical noise, DC motor is perfect if driven by a Linear very low noise supply. No PWM as you said it will be way too noisy.
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Old 12th February 2014, 10:16 PM   #10
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Stefano,

As I understand it the choice between DC and Brushless DC (AC Polyphase) is primarily about speed control. A DC motor or AC Synchronous motors are essentially dumb devices, they do not know how fast they are going or where they are located. Speed encoders, position encoders, torque measurement etc. can all be added externally and controlled for, but even then some of that control is very indirect. You are trying to do something with voltage to effect torque to effect speed in a DC motor, this is a very loose hold on speed and you are trusting inertia and other things to smooth out the dynamic changes in speed.

A DC Brushless motor typically includes some "intelligence" in the motor itself. Circuitry is provided where your controller can read a signal from the motor regarding speed. The motor is essentially designed to give controllers more to work with. Now, read back through Pyramid's post though where he says the Hall effect still isn't good enough for very precise speed control and we can do even better by using some of the features of the BLDC motor for speed control.

An AC Synchronous motor and a BLDC are nothing like each other and I don't understand your suggestion to just do AC instead of BLDC. That's like saying direct drive is just like rim drive, so why don't we just do direct drive. Totally different animals.

Due to costs, complexity and other considerations, DC may be the best choice, but I think it is really just about what people are familiar with and not based on any engineering fundamentals. That's fine, but lets be accurate in the reason for the choice. If you want DC, let's do DC.

Jamie
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