Compact Drum Monitor

[holtzman]

I am thinking about making a smaller monitor, something portable and easy to use...
This way I came to idea of a 1500 rpm, double-spiral 32 line drum tv. The light commutation can be made by means of a striped cylinder driven by stepper motor.
Drum diameter is planned to be 21 cm (approx. 9"). So the whole unit is going to be like a bit oversized toaster. The image size should be about 41X26mm, and I am not sure if any magnification will be applied.
One of the problems is that I have no free time to actually build it

[Harry Dalek]

I am thinking about making a smaller monitor, something portable and easy to use...
This way I came to idea of a 1500 rpm, double-spiral 32 line drum tv. The light commutation can be made by means of a striped cylinder driven by stepper motor.
Drum diameter is planned to be 21 cm (approx. 9"). So the whole unit is going to be like a bit oversized toaster. The image size should be about 41X26mm, and I am not sure if any magnification will be applied.
One of the problems is that I have no free time to actually build it

Hi Holzman

Good luck on the project I like the plan for the 2 drums .

I think your the only one on this forum anyway to get a working 2 motor mechanical tv

[holtzman]

Here is some progress with my compact drum monitor. I would be happy to hear your thoughts about the phonic wheel design.
If I only make it, it will be a monitor with 3 motors because phonic wheel is basically a synchronous motor

[gary]

Nice work, I certainly hope you can find the time to implement something like this.

As a simplification, you might want to consider replacing the cylinder with a partitioned light box, with the light (say LEDs) in each partition "commutated" according to the appropriate zone on the drum.

A little more far out, but certainly not outside the realms of do-ability, is to replace the bolts in the phonic wheel with neodymium magnets and actually use it as the motive force (i.e. turn it into a synchronous motor).

A combination of the two ideas would reduce the number of motors to 1 home made one.

[DrZarkov]

I like that concept, exsp. the oldfashioned way of synchronisation. But I would simplify the scanner: It would be enough to get a spinning mirror with only 4 or six sides (which is much easer to make/get), or a Mihaly/Traub scanner, which is easier to make and adjust than a polygon mirror.

[holtzman]

Thanks everybody for your input!

I like that concept, exsp. the oldfashioned way of synchronisation. But I would simplify the scanner: It would be enough to get a spinning mirror with only 4 or six sides (which is much easer to make/get), or a Mihaly/Traub scanner, which is easier to make and adjust than a polygon mirror.

It's not a mirror drum. I meant just an aperture drum. The small drum inside is not a mirror drum too, it is a clear plastic drum with black stripes.
I wish I had time to build a mirror drum... But by now it's far too complicated.

[Harry Dalek]

HI do the 16 bolts on the phonic wheel control the main drum and the transparent striped one controlled via the main drum via a revolution or half revolution .

Its very interesting its hard always changing things because this idea or that is easier ,i am very much one that finds it hard to stick to one plan .

BTW what did you use to do the plans very well done .

[holtzman]

Yes Harry you got it exactly right. I plan to place an optical interrupter somewhere in the centre of the bolts disc.

This is what i have already tried - to drive a stepper by pulses from the LDR optofork. I connected only 1 set of stepper coils to optofork circuit and after kick-start it rotated synchronously. There was a flywheel on its shaft. Now maybe if I add connection to the second coils' set via capacitors, the motor will rotate even better, without kick-starting?

The program I use for planning is a 3d engineering program.

[Harry Dalek]

Yes Harry you got it exactly right. I plan to place an optical interrupter somewhere in the centre of the bolts disc.

This is what i have already tried - to drive a stepper by pulses from the LDR optofork. I connected only 1 set of stepper coils to optofork circuit and after kick-start it rotated synchronously. There was a flywheel on its shaft. Now maybe if I add connection to the second coils' set via capacitors, the motor will rotate even better, without kick-starting?

The program I use for planning is a 3d engineering program.

Hi Holtzman
Its Taken me a while to work out any 2 Motor mechanical tv one has to control the other other wise you have no hope ...it is sort of logical a nipkow mirror drum or any one motor tv the rotation line controls the start of the next line /where as you need a pulse to tell that next motor what to do .


Opto fork is some thing i have not tried yet ,i was thinking of a magnetic version bit like the phonic wheel but it looks like the optofork need to go shopping a play around with them.

come to think about it i think some old vcrs have the optical one to stop the tape on rewind pretty sure i have seen them on my pulled apart top loading beta vcrs.

Keep the info coming very interesting project

[gary]

maybe if I add connection to the second coils' set via capacitors, the motor will rotate even better, without kick-starting?

Yes a capacitor to create a 90 degrees phase lag on the second winding will give to a self starting synchronous motor, how much cogging you will get without the input being a nice sine wave remains to be seen, but of course you can make it a sine wave with appropriate circuitry. I have driven Nipkow disks at 750 RPM using this method. The only thing to look out for are resonances as the input frequency sweeps up to the final value. Also, depending on how high the final speed is to be (determined by how many slots in the drum I suppose) you may need to drive the stepper with a voltage around 10 times it's rated value with the current being limited by a high wattage resistor or car lamp etc - this increases maximum rate of change of the current in the winding, which in turn increases the torque at higher speeds.

RPM = (4*f*60)/n, where n is the number of steps per revolution of the motor

[Jean-luc]

Hi Holtzman

Congratulations, it's a very nice and interesting project ,good using of 3D software,and the plans are very clear.
Two questions:
-How will you do for the phase adjustment of the drum without possibility of to turn the coils ?The probability to obtain an entire image is 1/16 at the first try.
-how will you draw the two turns spiral on the cylinder?
For one turn it's very easy, only an oblique straight line is necessary,but for two, I expect some junction problems.

[holtzman]

Hi Jean-luc,

in my 60 lines tv I use 2 buttons, one accelerating the main motor, another slowing it down. This way it's possible, firstly, to put the disc into sync, and secondly, to adjust the phase. The advantage is that there is nothing mechanical to build. And even if there is no automatic sync, it's possible to force the tv into sync.
Originally, phonic wheels acted mainly as a brake, so the sync was catched automatically. In my 60 line tv the sync motor acts as an additional power, so it's nessesary sometimes to push the disc forward in order to allow the sync system to catch.
Though in drum project I want to try a phonic wheel, I'll preserve this 2 buttons way of sync because I'm too lasy to make an adjustable coils unit.

[holtzman]

Jean-luc:
"how will you draw the two turns spiral on the cylinder?
For one turn it's very easy, only an oblique straight line is necessary,but for two, I expect some junction problems."
Very clever remark, I am not sure yet how. There should be no problem to print out the holes pattern and to transfer it by a needle. I had printing problems with Nipkows, but there was specific problem with not accurate width-length ratio in plotter. Here this is not an obstacle because hole pattern is not round.

[M3DVQ]

It may be easier to form the drum first and mark out the spiral afterwards. Then you don't have to worry about wrapping the surface at an exact diameter.

Off the top of my head how about some sort of jig which scribes a spiral on the drum. Something like the image below

also knowing with a suitable gear ratio you can mark out your radial angles at the same time. Turn the handle n times, mark off the spiral with a perpendicular line, and repeat

You'd want to use anti-backlash gears I would think, the sort of thing you can find in old CD rom drives or tape mechanisms etc.

[holtzman]

Thanks everybody for your ideas!
In fact, there is no reason not to try a 2 slit drums system instead of a drum with 2 turn aperture spiral plus commutator.
I hope to be able to control the stepper accurately enough for making a raster this way. And the slits can be easily formed by black adhesive tape on clear PVC drum.