Setback

[Andrew Davie]

I have had a bit of a setback; I suspect that my motor or perhaps my disc hub are close to failure. My nice straight horizontal frame edges are now wobbling all over the place, almost sinusoidal, and my PID is pretty much completely unable to maintain lock. When I turn the PID off I still get the wobbly edges, and it's clear that the disc is undergoing rapid speed loss at high frequency. It's not smooth, in other words. I replaced the drive belt, did a lot of playing with tension, etc. My hearing is poor, but I can clearly catch a throbbing sound at the same frequency as the disturbances in the image. So, tomorrow I will scout for a new motor and hope that is it. It's cost me many hours of time - I actually dismounted my Nipkow to clean it and check its alignment and flatness. All seems OK, but was much worse when I put it back on the hub. Maybe i've done something bad there. Dunno. I decided to forge on regardless, even though the picture is now shocking.

[Andrew Davie]

A follow-up to this - I bought a new motor and although it spins just fine when I give it 12V - it does not spin at all when I power it through my "motor control" board. I re-did the resistor at the Arduino side (that is, on the base pin of the TIP122) by piggy-backing a 220 ohm onto the 1K - giving me a 180 ohm (confirmed 179.6 by measuring). The thinking there is 5V from the Arduino less the 1.4V drop across the TIP122 darlington and I want maximum of 20mA. With 180 ohms. 3.6/180 = 0.02 - i.e, 20mA. Measuring the PWM input signal from the Arduino, at the base of the TIP122, well actually just BEFORE the resistor, I see a nice 5V square wave input. However, I see absolutely nothing - no signal at all - at either the collector or emitter. The other side of the circuit has 13V input and ground connected to the emitter. I also see nothing at all on this. This leads me to suspect my earlier problems were the TIP122 in its death throes, and it's now (apparently) dead.

[Klaas Robers]

Andrew, solder the transistor out and check the base-emitter diode (it should measure as a diode) and the base- collector diode. Do this with a multi tester. First try this with a normal diode (1N4001) to get used to what the tester indicates.

When this is OK, test in the same way the collector- emitter. If this is a short, the transistor is dead, it died the death of heat. When a transistor gets too hot for a too long time, the emitter diffundates through the thin base and shorts to the collector.

In all these cases:
- use your oscilloscope,
- look at the output of the Arduino,
- watch the square wave PWM signal,
- look at the base of the transistor,
- there should be a similar signal,
- however about 0.7 volts high,
- or 1.4 volt if this is a Darlington.

So: MEASURE and THINK. Follow your signal. Where it is gone, there is the problem.

[Andrew Davie]

]
In all these cases:
- use your oscilloscope,
- look at the output of the Arduino,
- watch the square wave PWM signal,
- look at the base of the transistor,
- there should be a similar signal,
- however about 0.7 volts high,
- or 1.4 volt if this is a Darlington.

So: MEASURE and THINK. Follow your signal. Where it is gone, there is the problem.

Trying to think
Anyway I had already measured the signal at the arduino pin, and at the base of the transistor. So here are those two...

Signal from the Arduino

motorarduino.jpg (232.88 KiB) Viewed 6273 times

Above: Arduino signal - 5V as expected.

Signal at base of darlington TIP122

motorbase.jpg (201.85 KiB) Viewed 6273 times

Above: Base of TIP122 (darlington) - 1.4V as expected.


I will build a new board, and remove and test the original TIP122 for confirmation of your tests above.
Thanks for the response!

[Andrew Davie]

Well, that's been an interesting day or two. I'm back up and running, but with a few questions to sort out.
Firstly, I replaced the TIP122 and started seeing sensible signals on the collector. So the TIP122 had been fried. I didn't understand how, but nonetheless, that was a good sign that I now had a square wave on that pin. Secondly, I replaced the motor - with a 12V 8000rpm one from Jaycar. So just a half hour ago, it just started working again. I didn't have a "hub" for the motor shaft, so I printed up a quick 3D one and that seems to be working fine.
Now this time I paid careful attention to the heat of the TIP122, and it starts to get pretty hot pretty damn quickly. So I need to understand what's up with that. Why is it overheating? Obviously that was the reason the original blew up - or rather, went into metldown. I have 5V coming in from the Arduino in the form of a PWM, and I have 13V driving the motor. The TIP122 is being driven at 20mA on the base, so I would have to assume the new motor is drawing a lot of current and that's overheating the TIP122? Help, please? Diagnosis and solution...?
Now the REALLY nice thing is that my sinusoidal glitching on the frame boundary and the really weird "jump/offset" are now gone - so that was something to do with either the motor (likely) or the TIP122 overheating (unlikely, as it was always the same spot on the picture). Anyway, it's gone. What's NOT gone is the occasional overflow/lag that is, I believe, caused by semi-random delays in reading from the MicroSD card. I have tried to cater for this in the code, but my understanding is that there can occasionally be 400ms delays and that's a killer when you only have 500 bytes or so for the streaming frame buffer. This is a problem I'm going to have to work on - perhaps by buying an ultrafast MicroSD card - but perhaps it's something else.
In any case, I have a good picture at the moment - despite the jumping - and I'm feeling more confident about moving forward. Of course I have to understand/fix the TIP122 overheating issue, but at least I have a reasonable picture once again. I was fearful my disc hub had a bearing failure and have even scouted an old VCR in anticipation of using its hub as a replacement. So that's good... no issue with the disc.
I did a short video of "back to life" just to show where it's at. This may be the slowest "build a televisor" effort in the history of the world, but I guess it's not a race.


youtu.be/VCrkyjDSL6Y

[Klaas Robers]

Yes, I have seen "hic-upsvertical " in your video. To see that better you might disconnect the synchronisation and see by hand control that the disc runs a speed. If hic-ups occur you see a sudden vertical movement of the sync bar. That shows clearly that the sync, and thus the video is not played continuously. I had this too when making the first NBTV-CD, because I didn't understand how CDs were recorded. Days later I found out and could make CDs with uninterrupted video and sync.

[Andrew Davie]

I figured a hack/fix for the "hiccuping" and the result is pretty good. I still get my ~250ms delay in reading data from the SD card, but i basically ignore it - and let the video signal play bad data rather than halting. This works much better, and although there is the occasional visible glitch, it's very transient and much less offensive to the eye than the frame jumping. Of course I'd like to have it perfect, but this is the best I have been able to get so far.


youtu.be/jSg2ByYSJ-M