New NBTV camera project

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Postby aussie_bloke » Thu Mar 21, 2013 9:56 pm

Okay everyone here's the latest news on my NBTV camera's sync circuitry.

In the past month I have been on and off progressively constructing all of the circuits needed for synchronization for the camera which turn out to be a good few!

Firstly as seen in previous post, I've made the sync encoder which I got running real good which makes some nice square wave pulses so that's that part done.

Then I had to make the rest of the sync circuit which involved a 400Hz clock circuit and a 4046 chip and the rest of the components, it's about 90% complete.

Then I had to make the video/sync mixer circuit, using what I had at hand I had to compact it all onto a small circuit board which I done successfully. As it runs off 10V I used a 317 chip and resistors to set the voltage due to not having a 7810 at hand. I forgot to buy 33uf caps from Jaycar so had to chain three 10uf caps in parallel.

Then I had to build the motor speed control circuit, there appears to be 2 parts to it, one part controlled by a 317 regulator circuit and the other controlled by a circuit involving a BUZ71 mosfet. As my motor requires 2.67A to run, I had to modify the 317 circuit so going by a given schematic for high power regulation, I added a TIP31 high power transistor across the 317 to pass high current and that worked well for the motor so that's that part done! I mounted the TIP31 and BUZ71 transistors onto a heat sink to dissipate the heat, I will also have to add heat sinks to the regulators for each circuit!

So I am getting there with the circuitry, am surprised how much is involved to synchronize the camera!!! The circuitry is about I guess 75-85% done, I need to interconnect the boards and find suitable way to lay them out on the camera. Anyhow here below is a block diagram layout of the camera followed by pics of the circuits I've built:
Attachments
layout.jpg
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motorspeedct.jpg
motorspeedct.jpg (76.07 KiB) Viewed 4016 times
motorspeed.jpg
motorspeed.jpg (85.48 KiB) Viewed 4016 times
encodersync.jpg
encodersync.jpg (95.12 KiB) Viewed 4016 times
syncvidmix.jpg
syncvidmix.jpg (118.37 KiB) Viewed 4016 times
circuits.jpg
circuits.jpg (102.33 KiB) Viewed 4016 times
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Postby Harry Dalek » Sat Mar 23, 2013 12:26 pm

Good work Troy you'll have a camera that can record images to wav files view live off the camera viewfinder and pc screen that thanks to Gary and hes Bigscreen software .

I like neat circuit work i do all my circuits will copper wire tracks under the circuit board nice to do way with as many wires from the top as ..Down with rats nest wiring i say.
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Postby aussie_bloke » Wed Mar 27, 2013 3:40 pm

G'day all.

I have made further progress with the NBTV camera since the last post, mainly with the motor side of things. Have got the motor now rigged up to drive the VCR head disc spindle, have put a sewing machine rotary switch as a pulley on the spindle and a pulley slightly bigger on the motor. I have fired it up and it is working but I have ran into a big problem.

The problem is the motor seems to need to draw more current to drive the spindle than it does when running on its own and on initial tests this caused the 3 ohm 20W resistor to smoke after 30 seconds!!! Then I rigged up my high current speed control circuit and within seconds the TIP31 transistor smoked up and went BOOM!!!

I also notice too that the motor struggles to rotate the spindle at the start but gradually as the spindle increases in momentum it will spin more freely which I guess the motor will run off lesser power.

So to solve this problem should I simply incorporate a super heavy duty BTJ transistor across the 317 chip instead of the heavy duty TIP31 or should I put a much bigger gear on the spindle? And is there other things I need to solve to get around this issue?

Any advice would be much appreciated.
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Postby Harry Dalek » Wed Mar 27, 2013 4:17 pm

aussie_bloke wrote:G'day all.

I have made further progress with the NBTV camera since the last post, mainly with the motor side of things. Have got the motor now rigged up to drive the VCR head disc spindle, have put a sewing machine rotary switch as a pulley on the spindle and a pulley slightly bigger on the motor. I have fired it up and it is working but I have ran into a big problem.

The problem is the motor seems to need to draw more current to drive the spindle than it does when running on its own and on initial tests this caused the 3 ohm 20W resistor to smoke after 30 seconds!!! Then I rigged up my high current speed control circuit and within seconds the TIP31 transistor smoked up and went BOOM!!!




I also notice too that the motor struggles to rotate the spindle at the start but gradually as the spindle increases in momentum it will spin more freely which I guess the motor will run off lesser power.

So to solve this problem should I simply incorporate a super heavy duty BTJ transistor across the 317 chip instead of the heavy duty TIP31 or should I put a much bigger gear on the spindle? And is there other things I need to solve to get around this issue?

Any advice would be much appreciated.


Oh No smoke is not good ! Bang or boom and smoke is worse :(

Do you have a smaller motor or something else at least to replace and test your motor circuits , then you know its not that .

Perhaps a way you need a bypass switch direct to dc till its up and running then switch back to the the circuit....if start up is causing the problem and its ok once its running it might be a way.
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Postby Steve Anderson » Wed Mar 27, 2013 4:26 pm

Now, on the assumption you have no data on this motor aside (perhaps) its nominal voltage of 12V (sure about that?), what is its no-load current? i.e. driving nothing at 12V.

If there's some way you can measure its RPM at no-load speed that will help in assessing the correct gearing/reduction ratio required.

There is some inertia to overcome at start-up once driving the disc, but for only a few seconds, perhaps a soft-start circuit is required.

Most DC motors in this category run at a nominal several thousand RPM, so you could well be looking for a reduction ratio of between 5:1 and 10:1..

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Postby aussie_bloke » Sat Mar 30, 2013 10:53 am

Just a quick update on the motor situation.

I have found when the motor is connected to a physical load (i.e disc spindle) the motor will draw more current to drive it. So with just a 3 ohm 20 watt resistor connected in series, the motor running without a load is drawing 1.9A and has a voltage drop of roughly 9V but when connected to the physical load the current increases to 4.1A and the voltage drops to about 1.8V and the load resistor takes the full power load with about 13 volts drop across it so that poor resistor is copping 53W of power when it can only handle 20W so no wonder it was smoking :lol: .

Anyways I then bravely and stupidly decided to remove the load resistance and the motor went full speed and then the power supply blew its 10A fuse so no wonder the TIP31 transistor smoked and blew up :lol: as I didn't have the load resistor connected.

So I then replaced the TIP31 transistor I tried again and it worked but the transistor was overheating despite being connected to a heat sink as the plastic washer was melting so I then decided to put in an even higher powered BTJ transistor (a D2498) to take the current load and that worked good!

So now having the coarse speed control working, it's working not too badly, as the load resistor is taking most of the voltage drop I am getting low voltage across the motor so I get a variable voltage adjustment from 0.5V to 0.9V. I will of course have to chain more load resistors in parallel to handle up to 53W of power load

Now question, seeing this motor is running on a very low voltage but higher current, will there be any issues in running the fine tuning speed circuit with it that involves 200k pot and BUZ71 MOSFET transistor?

Anyways I will upload pics of my progress in the next post later this arvo/evening, right now I've gotta get off this computer and go outside and enjoy the sunshine and then go to the garage and do some project stuff :wink: .
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Postby Steve Anderson » Sat Mar 30, 2013 2:35 pm

This doesn't make sense to me, I'm not convinced this is a 12V motor, for the amount of current and power required it seems quite physically small. If it's marked clearly with 12V, well...

It could be faulty, some shorted turns in the rotor or stator. If it's been overloaded in the past and got really hot that is a possibility, where did you get it? New?

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Postby aussie_bloke » Sat Mar 30, 2013 6:14 pm

Well basically the voltage reading across the entire motor circuit (load resistor + motor) is 13.5-14V, but the voltage drop across the load resistor reads about 12V and the voltage drop across the motor reads 1.8V. But when there's no load attached and the current is at 1.9A the voltage drop across the motor increases to 9V. Those are the readings so if they're not right then I guess something's wrong with the motor but the motor runs quite okay so I'm confused...
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Postby Steve Anderson » Sat Mar 30, 2013 10:04 pm

Have you any reduction drive between the motor and disc? i.e. reduction gears or a belt drive as I mentioned before? If not what you're seeing may explained by this.

At 750RPM motors like this are effectively stalled.

I've looked back through all the photos in this thread, there's a few posted on Tue Oct 23 with a belt-drive reduction, but it doesn't appear to be the same motor...

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Postby aussie_bloke » Sat Mar 30, 2013 10:17 pm

Okay here below are some pics showing my latest progress with the camera on the motor side of things.

Also today I decided to do a test of the encoder circuit only with the motor driving the spindle, I had the IR LED/phototransistor positioned where I thought would be best for pickup. Anyways firing it up, I am getting the pulses but there is a bit of instability because the encoder disc is slightly off centre and the disc probably is slightly off centre too and is a bit warped. I have made a video on this to show you what I mean which is down below, I would like to know will this instability due to slight inaccuracy cause any major sync problems with the video output?
Attachments
encodertest.avi
Encoder test video
(8.63 MiB) Downloaded 255 times
1.jpg
Motor with pulley glued to wood mount
1.jpg (71.62 KiB) Viewed 3912 times
2.jpg
Motor mounted to camera and belt connected to spindle pulley and motor pulley
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3.jpg
Motor mounted to camera and belt connected to spindle pulley and motor pulley
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4.jpg
Motor mounted to camera and belt connected to spindle pulley and motor pulley
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5.jpg
Motor speed control circuit with high powered D2498 transistor to handle high current
5.jpg (87.38 KiB) Viewed 3912 times
6.jpg
Voltage drop across motor and current flow while motor is not connected to disc spindle
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7.jpg
Voltage drop across motor and current flow while motor is connected to disc spindle
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8.jpg
IR LED/phototransistor positioned in front of the encoder disc
8.jpg (61.97 KiB) Viewed 3912 times
9.jpg
Signal out from encoder circuit
9.jpg (52.74 KiB) Viewed 3912 times
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Postby gary » Sat Mar 30, 2013 10:29 pm

Does that encoder include a missing pulse segment? If so that would probably be the source of the pulse instability and is normal. There will also be a little bit of jitter with the trigger point due to minor variations in pulse width, this shouldn't be a problem.

BTW scale is always a bit of a problem with posted pictures but the wiring I see inside the motor seems surprisingly thick - could this motor be a brushed RC type? In any case did you ever measure the DC resistance of a winding?
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Postby Steve Anderson » Sat Mar 30, 2013 10:35 pm

In my best Mythbusters voice..."There's your problem..." No reduction drive, both pulleys are are of (as good as dammit) equal sizes...you will need a reduction drive....

As I said before, these motors are designed to run at several thousand RPM, 750RPM is effectively a stall (locked shaft) to them. It's a bit like trying to move your manual car off from stationary in 4th gear.

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I only just noticed the recent (changed motor) photos, sometimes photos can take ages to download by which time I've read the text and moved on...
Last edited by Steve Anderson on Sat Mar 30, 2013 11:24 pm, edited 1 time in total.
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Postby Steve Anderson » Sat Mar 30, 2013 11:10 pm

Where your no-load tests run with the motor in this condition shown below? Even better without the pulley. Listen to it, its speed most likely produce produce quite a high-pitched sound, with load it will be a bit lower in frequency. If the noise (which could be substantial) is a problem, you'll have to find a replacement motor...

At 750RPM, which could only be around 10% of it's design speed, each rotor winding has not enough inductance to limit the current rise before being switched out of circuit via the commutator. The currents you're seeing are probably only limited by the winding resistance.

The general sort of applications for these motors are model (electric) R/C aircraft, hobbyist robots, R/C racing cars and R/C boats. In all those applications there will be some form reduction drive (perhaps with the exception of model aircraft). Others spring to mind too, few though use direct drive. Few have any concerns about noise.

In addition these motors are controlled via PWM rater than a varying DC voltage...it's far more efficient which preserves battery life, and low weight and bulk (no heatsinks).

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Postby aussie_bloke » Mon Apr 01, 2013 3:24 pm

Steve Anderson wrote:In my best Mythbusters voice..."There's your problem..." No reduction drive, both pulleys are are of (as good as dammit) equal sizes...you will need a reduction drive....

As I said before, these motors are designed to run at several thousand RPM, 750RPM is effectively a stall (locked shaft) to them. It's a bit like trying to move your manual car off from stationary in 4th gear.

Steve A.

I only just noticed the recent (changed motor) photos, sometimes photos can take ages to download by which time I've read the text and moved on...


Thanks for the info Steve. I had to make do with what I had at hand and that was the best I could come up with for pulleys, the rest I have don't fit the motor spindle nor the disc spindle.

Answering Gary's question yep that motor is from a RC car, I thought with a heavier spindle a faster motor would be better to drive it. It has a resistance of about 4.5 ohms so it would draw about 2.67A when run from 12V going by the I=V/R formula.

Anyways I guess I will revert back to my lower powered motor and try it out my luck with it. Just realizing my lower powered 9V motor has a much smaller pulley already attached and you suggesting a reduction drive, I will try it out and see how it goes. Just not sure if I can get 750 RPM from it though. I have other motors I can try too, will see how I go.
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Postby Steve Anderson » Mon Apr 01, 2013 3:31 pm

That motor will certainly drive the disc at 750RPM, but is simply needs that reduction drive, no ifs or buts.

Driving the disc with a reduction drive I would guess around 1-2A once up to speed. But that is a very rough guess.

Measuring a DC brushed motors resistance has little bearing on its running current consumption as that is mostly defined by the inductance and the switching frequency which is determined by the commutation speed and load.

When someone designs a DC brushed motor they attempt to get the DC resistance as low as possible, it is this that determines what makes the motor get hot. Only resistance (in this context) can generate heat, leaving aside friction in bearings and brushes.

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Last edited by Steve Anderson on Mon Apr 01, 2013 3:54 pm, edited 1 time in total.
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