Hi everyone,
I’d like to make a few projects that revolve around two motors, and two sensors. One to provide speed and the other tension. But my skills are really low! And I’d love to find someone that could help, and I could repay somehow.
The main project is a movie film cleaner. I run a business scanning a lot of old super 8 films, and I’ve found a particular type is pretty susceptible to growing mould. It needs to go from one reel to another, through a bath, and then into a drying chamber.
I need to regulate the speed at which the film travels at, I was thinking about a rotary encoder mounted on a roller the film travels over.
I also need to regulate the tension on the film, which I was going to use a dancer arm like what a large web rolling machine or an old reel to reel audio tape deck uses. The sprung dancer arm would have either another encoder, a potentiometer, or maybe a tilt tensor.
I was thinking the take up reel speed would be controlled by the roller with encoder, and then the supply reel speed would be regulated by the dancer arm measuring tension, feeding out when needed to keep the arm in a mid position.
My understanding is I need a PID loop, but I’m really not sure where to start, what system like an arduino, pi or PIC I could use to control it.
If anyone could point me in the right direction, it would be hugely appreciated!
Thank you
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I’m not sure you would need a PID - it’s likely that a simple acceleration/deceleration function would suffice. However, the hardware design isn’t affected by whether or not you use a PID so the decision could be left until after you have a prototype running. I can’t see why the take-up spool would need an encoder - take-up speed would be driven from the encoder of the film roller. For both reels you could use either a simple DC motor or a stepper: controllers for both are readily available.
The ‘sprung dancer’ could be implemented with angled arms swinging across two interrupters. The logic to implement this in the MCU would be simple, and the two arms could easily be made adjustable to facilitate setting different configurations - I would guess that the unspooling tension could differ considerably depending on the mouldiness of the film surface. Gravity is an excellent source for a constant tension.
Any of the MCUs you mention would be suitable, even the simplest Arduino, as the processing load is minimal. You might make the choice based on the programming environment that best suits you.
Hi Andrew
Digging back into the old grey matter I seem to remember the system used by the very good Revox reel to reel tape recorders. The tape transport motors were I think brushed types as when fast winding and rewinding a small reverse voltage was applied to the supply reel to provide the required tension.When in recording mode pulses were supplied by a flywheel on the capstan with little notches and the speed regulated using these much like a modern system. When in this mode the take up and supply spool had this lower voltage applied just enough to provide the correct tension. The system had 3 motors, one on each reel and one driving the capstan.
This might give you a couple of ideas. I think it is much what you are trying to do.
Cheers Bob
Thanks all,
I think the DC motors will be the way to go, at the moment on my prototype I have some cheap DC geared motors which can drive the take up successfully.
On one of the film scanners I have, it uses a similar system to that 3 motor reel to reel. It has two large AC torque motors which can hold the back tension, a DC motor on the capstan, and a couple load cells on some rollers.
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Hi Andrew
You won’t be able to use a geared motor the supply reel to supply tension. The gearbox will prevent that or you will have a lot of tension even if you don’t destroy the gear box.
You don’t have to apply a reverse voltage across the supply reel motor for tension. You just need to arrange a resistor across the motor. The motor becomes a generator and the resistor load will supply tension. The degree of tension can be regulated with resistor value.
This resistor method would be almost self regulating. As the diameter of the supply spool decreases the tension will decrease but the spool will spin faster so generating more current so increasing the tension and so on in ever decreasing circles. I thing you can see what I mean here.
The actual resistor value required will have to be determined by experiment I think. Also keep an eye on the wattage rating. It could be a bit higher than you might think.
Cheers Bob
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