Tuesday, 24 April 2012

The Turntable Motor Controller Project - Part 3 - Design Goals

Enough of the motor stuff, let's get to the controller and what I want it to accomplish.  There are other manufacturers that have AC motor controllers out there, they seem to fall into what I feel are two product categories, the more basic model that is intended to only generate a single precise AC sine wave and still have a capacitor to generate the second shifted AC waveform and the higher end category that generates two separate AC waveforms driving each coil independently and not using the capacitor.  Products like the Linn Valhalla fall into the first category, as for the second category there are a couple out there and they don't come cheap.

With this one I am shooting for the stars and will look to make a unit that doesn't pull any punches, go big or go home.  So here is what it should have:
  1. Precise digital generation of the sine wave
  2. Dual independent sine wave generation
  3. Generation and selection of frequencies for 33.3 rpm and 45 rpm
  4. Frequency adjustment with a precision of 0.01 Hz
  5. Adjustable phase shift of the two sine waves
  6. Calibration mode to adjust output for any specific turntable/motor combination
  7. Ability to adjust frequency during play
  8. The ability to turn off or dim the lights that will be on the unit
  9. Be able to run high or low voltage motors
That should do it.  No tall order here.  Well, I have to be honest, I am cheating a bit, as I right this blog I am writing post development, so it will be easy to meet my spec list as it's already in prototype stage.  Thought I would come clean, don't want a James Fry/Oprah moment here. 

Here is what I have put together and am currently working on.  It's a micro controller based unit that is used to digitally synthesize two AC sine waves, the focus for the micro controller is that the sine waves are precise and stable and finely adjustable by digital means.  So there are no analog components used in the actual generation of the waves.  This is important to assure that there is no drift or alteration due to component drift or tolerances or heat etc.  The analog components only come in to play after the signals are generated and are for filtering and amplification of the sine waves.  The precision of the micro controller assures a stable and precise wave form.  The micro controller also serves to control the system and the user adjusts all parameters digitally with buttons via an LCD display (except for amplification calibration which is by potentiometers).

So I have prototyped a system with a micro controller unit, more on that later, a 16x2 LCD display and only four buttons to manage all the functions.  There will also be a power switch, a contrast control pot and maybe the odd switch to turn off LED's etc.  Oh, and a couple of LED's that display selected RPMs (45 or 33.3).  My analog circuits have filter stages and a cal pot that then feeds the power op amps.  If running a low power motor, it should be possible to drive it directly off the op amps but for 110V based motors I use step up transformers to bring the voltages up to the proper levels.

A first generation circuit board has already been designed ( and will already need to be modified! ) and is in prototype stage.  I will elaborate on some of my experiences with prototyping the unit in another blog, there is enough material there for it's own space. The software has been written that covers all the major functions and operations and seems to be working nicely.

Next I will be describing some of my experiences during the early design/prototyping stages.  Thanks for tuning in.


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