Stepper motor sync?

A build to model parts for a Baird lens box, the type seen on televisors, suitable for 3D printing and use on a televisor model.

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Stepper motor sync?

Postby McGee2021 » Mon Jun 19, 2017 10:33 am

I had an idea recently on vacation to replace the cogged wheel on the 3D printed televisor im making with a stepper motor, but while still keeping the universal motor to provide most of the torque. I have noticed that with some of the stepper motors i have bought, they are exceptionally high in torque while turned on. By high, i mean needing two of my arms and a foot long pole connected to the axis to act as a lever, and i still had trouble! You can by stepper motors with 2,4,6,8,10,12,14,15,16,18,20, and up to fifty poles, but after a point they become un-affordable. So the basic concept would be to have a sync signal from the video signal converted to be able to power up the stepper motor, and wait for the universal motor to come up to speed. The stepper motor would not be powered up all the way, so as not to retard the speed of the disc. So far, its an idea and a hastily explained one at best, but still needs to be tested and have some designing done.
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Re: Stepper motor sync?

Postby Klaas Robers » Mon Jun 19, 2017 7:06 pm

Strong stepper motors have the disadvantage that they don't rotate with a constant speed. But you can compensate that by making a flexible (rubber) coupling between the motor and the disc.

I use a DC motor, designed for rotating a VLP disc. These are rather large plastic discs, 30 cm diameter, 3 mm thick. They have to run 25 rev/min (PAL) or 30 rev/min (NTSC). The motor has a magnetic pick-up that gives 18 pulses per revolution. It took me about one year of thinking before I had found a way to synchronise that to the 32 sync pulses per revolution. That solution may be also a solution for synchronising a stepper motor with an arbitrary number of poles to the NBTV sync.
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Re: Stepper motor sync?

Postby rommert » Tue Jun 20, 2017 1:24 am

On my disc monitor i use a 48 steps per revolution steppermotor as a synchroniser. The stepmotor is coupled to a dc motor with a permanent magnet field. To simulate a universal motor this dc motor is connected to the powersupply via series resistors. ( from an english ex colleague
i understood that an universal motor is a series wound motor ) That kind of motor has the property that the rpm's is both dependend from the load and the supply voltage.
( this in contrast with shunt wound motors or permanent magnet motors from which the rpm's tends to be independend from the load within reasoable limits ) When the rpm's from the dc motor is adjusted reasonably close to the wanted 750 rpm by the powersupply voltage the steppermotor can either retard or accelerate the disc. It should be nice to use a 32 step per revolution stepper motor. The syncpulses from de nbtv signal could then be used as clockpulses for the steppermotor. ( I never have seen such a stepper motor ) With a 48 steps per revolution you need a clock frequency one and a half times higher. ( 600 Hz. in stead of 400Hz. )
I have supplied the recovered syncpulses from the nbtv signal to a LC circuit tuned to1200 Hz. in order to multiply the frequency by 3 (1200 Hz. )
and divided then by 2 (600 Hz. ) The high Q from the LC circuit takes care for the missing syncpulse. ( frame sync in the nbtv signal ) At start up a part of the series resistor for the dc motor is shorted by a push button which is released when the system is close enough to 750 rpm.
( I also made a rpm counter on the monitor, which is very handy for this purpose ) There was some experimentation needed to find the correct ballance between the torque from the dc motor and the steppermotor to archieve proper synchronisation
Frame synchronistion is done manual by turning the complete contraption from disc, dc motor and steppermotor by hand.
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Re: Stepper motor sync?

Postby Harry Dalek » Fri Jun 23, 2017 1:03 pm

I tried syncing a stepper motor on my mechanical SSTV but the feed back from the hall effect sensor was causing mistakes in the pulse width it would work but you have to get the mechanical detection accurate the encoder i just used little magnets lining them up you eye to the line start was not good enough ...optical system would be the way i should of went works fine for nbtv.
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