Firmware Finito

(that means finished !)

Now that the hardware is behaving as it should, I decided to get back to the firmware and see about giving it some necessary cleanup.  There were features to add and unnecessary/debug code to clean out.

The main feature I needed to add, and the one that I was hesitant to tackle, was the acceleration feature.  I knew I needed to add an option to spin up the motor gradually rather than the full-on power up I have been using.  The main idea of course is to compensate for the inertia of the platter and belt system, the heavier the platter, the more important this feature is.  In fact, on my Ariston RD11 they designed the motor pulley with a slip clutch to deal with the inertia problem (you will find this on many Thorens turntables as well).

The way the acceleration needed to work is that when the user started up the turntable, the Bi-Onda controller would have to start spinning at a much lower frequency than 60Hz and slowly, or quickly, ramp up to the final frequency set by the user for their turntable.  I concentrated on acceleration and felt that deceleration was not as big an issue and decided to omit a deceleration routine.

My problem was trying to figure out how I would shoehorn this feature into the existing software without having to re-write the existing code or change the overall structure.  While writing a post on a blog for assistance, the ideas became clear to me and I was able to figure out a relatively simple way to implement it that would only require modification of one function subroutine and would work with any action that started up the turntable (calibration modes, 33 and 45 rpm etc.).  That also reminds me that another reason to implement acceleration is the feature that allows the controller to increase the motor speed to reach 45 RPM, this higher drive frequency can cause difficulty in these motors to start turning from 0 to the high frequency of the 45 RPM mode.  Accelerating from a lower speed usually works well in dealing with this issue.

I made the acceleration feature flexible to anyone's needs.  In calibration mode you are able to turn acceleration on or off and you can set your own parameters for: starting frequency, frequency step in Hz and the delay time between steps.  These are all stored in eeprom and only need to be set once.  So you can pretty much set up your own acceleration ramp (linear).

So now I will need to build an enclosure for this unit and put it in operation.

SoundBound

Dang! It's Working!

The basic electrical tests and scoping show that the new PCB layout is working fine.  Time to hook it up to an AC motor for the real deal.  First of all I wanted to take a look at the stock configuration of my Ariston RD11 turntable, so here is a picture of the 'underbelly' and we can see the phasing cap (yellow) that feeds the second coil of the AC motor.  I took the oscilloscope to the motor to see what the two phases looked like as per stock.

Close up of phase capacitor

Ariston RD11 showing AC motor

Here in the picture you can see that the phase shift is fairly decent by the cap but the waveforms themselves are not the best looking sine waves ever seen.  This is off the 120VAC line voltage.  You can see that the capacitor does affect the quality of the waveforms to some degree, both being affected.

AC at motor coils without TT controller

Time to remove the motor and connect it to the Bi-Onda controller.  I first calibrated both sine waves at the tranformer outputs to 119VAC rms as measured on my multimeter.  I then connected each channel to a coil as per the Ariston configuration: AC direct coil to sine A and the cap shifted second coil to sine B outputs.  When I started up the controller and ran the motor, beauty, turning perfectly.  The motor runs nicely.  I then scoped the outputs to the motor like I did before and here is the photo, the waveforms are very nice and clean sine waves.  Me happy.

AC at motor driven by the BiOnda Controller - clean!

I did check the output and it looks like the voltage dropped about 9VAC with the motor connected and running.  Since the power supply is unregulated, I expected some voltage drop and am happy with that and a small drop should not affect the overall performance.  Now this of course is a no load condition, no platter is being turned so I will have to check it again when the TT is assembled and running the platter.  One very nice thing that I did notice, is an improved torque.  With the motor in stock condition off of the AC I noted how easily I could stop the motor from turning by gripping it with a particular amount of pinch of the spindle.  When driven by the motor controller I noticed that it was still possible, but I had to exert more pressure, thus increased torque.  Nice, not sure I was going to have more torque, but looks like I do.

I also tried changing the sine frequency to alter the RPMs and the motor responded nicely.  I did notice however in 45RPM mode the motor had a problem in starting it's rotation.  As soon as I gave it a bit of a spin it would run nicely.  This likely means that I should add an acceleration routine to the controller software to help the motor spin up, especially for the 45 RPM mode.  I also noted that the motor was spinning backwards, no big problem, just reverse the hookup of sine A & B outputs, but I will look into the software too, could be I incorrectly calculated the direction of phase shift of sine B, which would easily be corrected by changing the subtraction to addition or vice versa, which ever is currently in the code.  Will test that in the next couple of days.

I did play with the phase change too and was able to sense a slight change in the motor's vibrations as I went far off the 90 degree mark in either direction. In the end it felt like a phase shift of 91.4 degrees seemed to provide the least amount of vibration in my hand. 


Next, will have to improve the heatsink, I have been holding off on getting a good one together but I did notice the small aluminum plate I was using getting a bit too warm, so will have to find an appropriate one.  Then I just need to hook it up fully and spin some vinyl.  I do want to do some restoration and upgrading of the Ariston at this point too, so I may do this before putting the unit back into full operation.


Thanks for tuning in, keep checking back!
If you have any questions, feel free to leave it in the comments.
SoundBound

PCBs and Happy Times !!

So here is a sneak peek to the new PCBs.  They look good even though it is only the standard green, but once they are in an enclosure, well, you don't see them much anymore.


The switch and LED board is up top and you can see the main PCB here.  I started populating the boards a couple of days ago.  I only hooked up the power supply section, the microcontroller section and the low pass filters.  Thus leaving the power op-amps out so I could verify stage by stage.  The power supply was built up but I left out a series resistor that feeds the power to the board so that I could check it out before powering up the rest of the circuit.  The power supply looked to be working very well, with very low ripple.  Happy there.  Then I checked out the microcontroller section by powering it from it's USB connector first.  No problems here either, the LCD and buttons worked great.

New PCB layout with fatter traces and ground fill.

So I checked the sine wave outputs on the low pass filter section and happiness here too, the waves looked really nice and clean.  I replaced the single turn output amplitude adjustment pots with nice multi-turn trim pots.  This made the adjustment of precise amplitude easy and worked great.  So far so good.

Next step was to power the microcontroller with the PCB mounted supply.  I placed a pop-out wire jumper for the series resistor (this resistor is only needed in case you are going to install the zener diode voltage clamp which was not needed with the transformer I am using), I used a wire I can remove so that I can measure current draw later on. Measuring all voltages I confirmed that the board was working fine off the on-board power supply.

At this point I am very happy with the unit and next step will be plugging in the op-amps and testing the amplification and output transformer stages.  Coming soon, so check back to see the results.

Soundbound

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.

SoundBound

The Turntable Motor Controller Project - Part 2 - A Bit More About Synchronous Motors

Before describing my controller design goals let's talk a bit more about AC synchronous motors and what makes them spin.  The AC motor needs voltage delivered in alternating current but unlike a DC motor which only needs to be fed a single DC current into two wires, an AC motor needs multiple sets of wires (coils) to be fed the AC current, most AC synchronous motors have either 2 or 3 sets of coils, 2 phase and 3 phase respectively.  The thing is that these AC waveforms have to be fed into the motor out of phase from each other, ie, slightly shifted in time (technically in degrees) from each other in order for the motor to rotate.

3 Phase AC

Now in terms of pure performance, a 3 phase motor is better, it has smoother rotation if not a bit more torque.  The downside is mainly the cost of the motor and especially the complexity of driving the 3 separate AC phases.  Most turntables that use AC motors use 2 phase ones, they are cheaper and the two shifted phases are easily generated by simply adding a capacitor to the second coil which will phase shift the AC by the degrees needed for it to work (90 degrees, 3 phase motors require AC feeds each shifted by 120 degrees).  You can see the different waveforms in the pictures.  The thing about using a capacitor to introduce the phase shift is that it is cheap and simple to implement, but not likely very precise, usually close enough that the motor rotates as expected.  If the phase shift is not precisely 90 degrees the motor may be subject to increased vibration, so you want it as close as possible to 90 degrees. 

The motors used in turntables can be either low voltage (20-30V AC) or line power driven (110V) so this has to be considered when I design the controller.  The turntables I mentioned previously use line power level motors (110V here, 220V in other countries).

So I am going to be dealing with 2 phase motors for my project, they are easy to come by and are inexpensive and work well enough.  Interestingly, stepper motors are basically AC synchronous motors that if driven by AC waveforms as above will also work.  Stepper motors can be found in all kind of computer devices such as printers, scanners and some disk drives.  So it is conceivable, with a good controller, to scavenge some of these motors and use them to drive a turntable !  Nice and cheap (usually free if you hunt for discarded printers by the curbside as I have been known to do) but maybe we will see how that works out some other time.  Most of these steppers are low voltage ranging from 5-24V so that needs to be considered.

So this is not a very deep description on AC motors, but hopefully is enough to  understand where I am heading on this project.