Measuring speed of objects passing through a plastic pipe

[Sandor Akarki]

I have a 2" diameter plastic pipe into which objects fall (the pipe stands, 90 degrees to the ground level). Those objects are different shape and density so their speed gaining is not consistent. They do not spin. I would like predict their speed when they end the pipe (fall to the ground). I pretty much can do the software and controller part - I would like to know how it would be the best to measure the speed of the object knowing G. I would like to have 3-4 points where I would collect position information from.
My question is what I should be using here; some sort of RF or IR or similar (to know when the object entered the line) or so?
If you have a particular P/N for the sensor in need, I do appreciate that, too.

 

[pebe]

What are the objects made from?

 

[schmitt trigger]

What are the objects made from?

How large are the objects? Do they occupy the full pipe (or at least a significant portion) of the pipe diameter?

Additionally, is the pipe transparent, translucent, or completely opaque?

 

[tinkerer73]

Personally I would have at least two emtter/receiver setups in the pipe as far apart as possible, so that as objects pass through, the microcontroller "sees" the breaks, and measures the time between them, then calculates speed.

If you can use at least two pairs so that the smallest object will break at least one of them, that would be better. You will have to have the software based on hardware interrupts that disable the redundant ones as soon as the first is broken.

 

[Sandor Akarki]

What are the objects made from?

Various. Those can be stone (magnetite - ferrous, quartz), pieces of wood mainly; possibly copper and lead chunks.

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How large are the objects? Do they occupy the full pipe (or at least a significant portion) of the pipe diameter?
Additionally, is the pipe transparent, translucent, or completely opaque?

They occupy about 1/8 of the diameter to 3/4 of the pipe diameter.
Simple building gray plastic pipe (designed for outgoing water sewers).

 

[chuckey]

Because of gravity, the items will accelerate, so you have to measure their speed at at least two places, i.e. have four position locators. I would think that pairs of photocell/photodiodes would be the simplest. The light transmission and reception path must be masked so the light beams do not interfere with each other (horizontal slots?). The particles must not overlap in position as they pass the lit areas. Because of the amorphous shapes of the particles, I do not think that you will get very accurate measurements.
Frank