Film-Tech Forum ARCHIVE: Frame Counting

This is topic Frame Counting in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Brian Tristam Williams (Member # 1235) on 08-13-2002, 02:22 AM:

Ok, I asked this question in another thread, but it hasn't been addressed.

How does one implement a frame-counter on a Christie P35GPS? I need to count frames per second, so I need a signal from the projector that tells me when each frame (or sprocket-hole) passes. I'll use this signal to drive a microcontroller counter of my own design.

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"One man can make a difference."

Posted by Paul G. Thompson (Member # 655) on 08-13-2002, 04:50 AM:

One way to do it is with a scanning beam shooting through the sprocket perfs of running film similar to an exciter lens and a solar cell. Then divide that by 4, and you will have exactly the number of frames that have passed.

(Naturally, presuming you are using 4 perf per frame film)

I am sure some electrical engineers can clue you in on a circuit that will wait for 4 pulses in before it gives one pulse out.

Posted by William Hooper (Member # 71) on 08-13-2002, 06:31 AM:

Aftermarket sports car places used to stock add-on tachometers. Many of them had magnets that are stuck on the front of the crankshaft & some sensor which picks it up each whirlaround. If I had to start from scratch, I'd start from there.

Posted by Frank Angel (Member # 248) on 08-13-2002, 07:01 AM:

I have a Reel-End Alarm unit that uses a small sensor module; it contains an IR LED and IR sensor. You mount the module next to the upper magazine reel shaft and put a piece of sensing foil on the shaft. As the shaft turns, the foil is counted by the sensor module and the unit calculates the remaining time on the reel based on reel and hub size. The module will pulse any contrasting, reflective surface that moves past it. This probably would be perfect for what you need.

When I get to the office I can give you the manufacture and part number.

BTW, This Reel-end Alarm unit has a digital remaining time display for both projectors and when 3 minutes are left on the outgoing reel, a warning is sounded. What is so nice about it, as opposed to the old mechanical bell warning, is that it has an RS32 output so that you can put other readout modules in a remote locations as needed, such as in the rewind room. Very neat unit.

Posted by Rick Sanjurjo (Member # 1365) on 08-13-2002, 08:14 AM:

Brian

All the above are good suggestions. There are a myriad of ways of getting an input but the three basic methods would be a magnetic sensor, an optical sensor and a shaft encoder. The exact method would depend on what you have available to you as far as parts and tools. I have done it three different ways on the Simplex.

1. Drilled holes and installed cylindrical magnets on a pulley so that one magnet when by for every frame and counted then with a Hall effect sensor. Allergo makes a large selection of sensors.
2. Made a steel pulley to replaced one of the aluminum pulleys and then used a proximity sensor from an old failsafe to count the teeth.
3. Printed a circle with alternating dark and light bands on a piece of Mylar, mounted the Mylar to a piece of Plexiglas and then mounted the Plexiglas to a pulley and used and optical sensor to read the dark bands as they went by. This is a home made optical encoder.
4. Mounted an optical encoder off one of the pulleys and used it to count.

One thing I might try that would require very little mechanical modifications would be to mount a light sensor behind the shutter and see if you can count the shutter going by. It will probably require a litle bit of signal conditioning but it will be easy to install on the projector.

Good luck

Rick

Posted by Brian Tristam Williams (Member # 1235) on 08-13-2002, 10:11 AM:

Hey, that last one was a brilliant idea. Light sensor. Hmmm... Lemme think about that one.

Perhaps I could bolt a hall-effect sensor to the shutter...

Posted by Rick Sanjurjo (Member # 1365) on 08-13-2002, 10:08 PM:

A hall effect sensor near the shutter would only work if you mount a magnet on the other side of the shutter so that the magnetic flux seen by the hall effect changes as the shutter passes in front. An old proximity sensor might be better as it doesn't need a magnet the presence of the metal on the shutter will cause a change in it's output.

Rick

Posted by Randy Stankey (Member # 64) on 08-13-2002, 10:35 PM:

How about an optical sensor that counts the flickers of light coming out of the aperture as the shutter spins by?

Posted by Kenneth Wuepper (Member # 1174) on 08-14-2002, 02:58 PM:

Hello, Brian,

What is your purpose in counting the frames? If you are trying to determine FPS then you can put a common tachometer on the motor shaft. That is the way we set the FPS using a variable speed motor on a standard projector.

You can determine the proper shaft rotation by dividing 24 into the nominal speed of the driving motor at sound speed. 1750/24= revolutions per frame. motor rpm/frames per second = revolutions per frame.

The determination of actual frames through the aperture is quite another matter.

KEN

Posted by Bill Gabel (Member # 1105) on 08-14-2002, 03:18 PM:

What about a shaft encoder?

I have optical shaft encoders on my projectors.
That takes a bi phase impulse to my mag and digital
reproducers and frame/footage counter.

Posted by Brian Tristam Williams (Member # 1235) on 08-15-2002, 10:33 AM:

Good question, Kenneth.

The purpose is to keep a log of the projector speed, on a computer, to see if the speed varies at all, due, perhaps, to power frequency fluctuations or whatever. We have a DTS sync problem we haven't solved yet.