Looking for an off-the-shelf solution for film tension for a scanner

I'm building a motion picture film scanner. It will have a simple transport from the feed to the takeup reels that looks similar to this design being used by the Kinograph project: https://www.youtube.com/watch?v=6DtXLO2K-Uw

However, rather than using spring loaded tension hubs as kinograph does, I'd like to use a single roller on a linear actuator, that adjusts position dynamically based on the tension on the film itself. Something like what the MWA Spinner scanner uses: https://www.youtube.com/watch?v=NugH6zdXVmw

I'm doing this because my scanner needs be able to work with multiple gauges of film, and each has different tension requirements. In order for the kinograph design to work with multiple gauges, you'd need to physically swap out the springs with each format change. I don't want to have to do that. Also, film with lots of splices in it sometimes requires less tension, so I want to be able to control that from software as needed.

In this video of the MWA scanner, it's a roller on the lower left side of the scanner deck that's got about a foot of travel. I don't know what the mechanism is that they're using, but I'm guessing it's more sophisticated than a simple spring - some kind of load cell to read the tension and then something to adjust the roller position up and down as needed

Question is - can I buy something like this pre-made? Mine is a one-off scanner so I'm willing to spend a little money on it since it'll save a ton of time in trying to engineer it myself (my background is in film, and while I've built a scanner or two, they've mostly been modifications of existing machines so I didn't have to do this kind of mechanical engineering from scratch).

Thanks!

friolator: However, rather than using spring loaded tension hubs as kinograph does, I'd like to use a single roller on a linear actuator, that adjusts position dynamically based on the tension on the film itself.

I'm always intrigued by the enthusiasm to replace working simplicity with complexity :)

Why not just have interchangeable tension rollers?

In any case this sounds like a question for a Forum that deal with film processing. Finding "something like this pre-made" does not sound like an Arduino question.

...R

I'm asking in the "Motors Mechanics and Power" forum, for suggestions on things I can use. The scanner is controlled by an arduino.

As for why, just because a spring-loaded mechanism works, doesn't mean it's the best solution for my application. How does coming here to criticize my post help anyone? I mean, other than upping your post count?

friolator: How does coming here to criticize my post help anyone?

I did not criticize your post. I just expressed my opinion. Has light-hearted banter been outlawed?

...R

"Why not just have interchangeable tension rollers?"

Asks a question I answered in the original post, detailing why I'm looking to go this route.

"In any case this sounds like a question for a Forum that deal with film processing."

...Is dismissive, essentially saying "you're looking in the wrong place for this" (when in fact, there are few, if any "forums for film processing," film processing isn't what I'm asking about, and the question is absolutely appropriate here, as I'm integrating this into an arduino-based system).

Sorry if I'm coming across as touchy here, but I'm tired of going to forums to ask for advice and getting non-answers that tell me my question is wrong or second guessing why it's being asked.

Making a tensioner by using a DC solenoid with a PWM for control is the easy part. Now you need a feedback mechanism to allow the control to function.

Paul

Paul_KD7HB: Making a tensioner by using a DC solenoid with a PWM for control is the easy part. Now you need a feedback mechanism to allow the control to function.

Thanks. This is essentially what I was thinking if I have to roll my own.

I was just hoping I could save some time and buy something that already does this. The basic principle is similar to what you might find in many industrial applications, particularly web presses, where there's a continuous roll of paper that's printed on at high speed. Tensions are super important in that application, but there are other machines that work with materials that are the same width as motion picture film, in similar ways - such as plastics.

friolator: Thanks. This is essentially what I was thinking if I have to roll my own.

I was just hoping I could save some time and buy something that already does this. The basic principle is similar to what you might find in many industrial applications, particularly web presses, where there's a continuous roll of paper that's printed on at high speed. Tensions are super important in that application, but there are other machines that work with materials that are the same width as motion picture film, in similar ways - such as plastics.

While trying to make money for college, I worked in a pulp/paper mill and a composition roofing mill. Both machines had the same problem you are discussing. The solution in both was to have a roller on the sheet that held a loop of paper. When the loop moved up, the speed of the paper roll was increased. If the loop moved down, the speed was reduced. If the paper in the loop broke, the supply stopped the paper.

You have a much different problem in that you don't want a weighted roller on the film loop. So your solution will be nothing like an industrial solution.

Paul

Sounds like a job for a servo of some sort to adjust the other end of the spring... Need all the mechanical details to figure out how feasible this is.

Paul_KD7HB: You have a much different problem in that you don't want a weighted roller on the film loop. So your solution will be nothing like an industrial solution.

How so? Most film scanners use dancer arms, which are spring-loaded rollers that move as the tension changes. if it goes beyond a threshold, all the motors shut off and the film is unthreaded (too much or not enough tension). The speed of the transport isn't changed, but the roller on the dancer arm moves with the tension change to keep the film taut.

The MWA scanner I linked to in my original post actually uses something like you describe: the roller on the bottom left moves up and down to keep the film at the desired tension.

I had assumed the roller would be on the emulsion side and would cause wear.

If not the case, then place two IR LED/sensor pairs so one or the other will be obscured and indicate the speed needs to be adjusted.

Paul

the roller itself is more like a capstan. The film doesn't slide across it, the roller turns with the film. So it doesn't matter if the emulsion or the base side is touching it, there's no damage or slippage. In fact, all the rollers in this scanner are like this, rather than using film-gauge specific rollers, it will use particle transfer rollers, simultaneously cleaning the film while transporting it.

I was thinking of using a load cell to determine the actual tension and reacting based on that. This would allow me to dial in a specific tension (in some cases, such as with old film or film with many weak splices, you need less tension).