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I have an extruder that is producing a molten plastic filament that cools after a brief while (2-3 seconds). The plastic stream needs to be a consistent diameter, and one method of doing this is to have a pinchwheel "pulling" the filament faster or slower based on the filament diameter.

My thought was to have a fixed-position toothed wheel pulling, and a spring-loaded idler lightly holding tension against the toothed wheel. The idler would have a long lever protruding from it (so a small change in diameter will create a large swing on the lever), with a magnet mounted on the end of the lever. A Hall effect sensor would be used to detect that the lever is in the correct location, and a microcontroller can be used to adjust puller RPM to keep the lever in the correct location.

The pinchwheel & idler configuration is a known working setup, my unknown is the interaction between the magnet and Hall effect sensor. Ideally the Hall effect sensor would act like a variable resistor or voltage divider, output voltage dependent on the strength of the magnetic field, but I'm concerned it may work on flux instead.

If I were to get one of the TO92-shaped 3-wire sensors (a passive one, I'll be looking for what's available locally rather than a specific model), what kind of behavior am I to expect here?

Alternative ideas welcome. Diameter of the filament should remain constant to 1.70mm +- 0.1mm, to give an idea of the resolution required. I'd love to get down to 0.05mm which I hope this solution can do.

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    \$\begingroup\$ Interesting. I'm also working on closed loop control of plastic extruders in 3D printers after the process you are discussing. I'm exploring the idea of observing delivered plastic volume as the object builds, because assumption errors caused by variations in plastic filament (which you might produce, for example) in an open loop system will accumulate as the model builds -- and no open loop recipe works well over volume fill % choices. You are working at one end of this process. I'm working on the other end. Be interesting to see solutions suggested here. \$\endgroup\$ – jonk Mar 31 '14 at 21:30
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I've used the Allegro 3503 ratiometric linear sensor. It's discontinued, but I suspect its similar to the 1301/1302 sensors. It is very linear when positioned between two opposing magnets. This particular config (push-push mode) is shown on page 27 of the guide that you can download at http://www.allegromicro.com/en/Design-Center/Technical-Documents/Hall-Effect-Sensor-IC-Publications/Hall-Effect-IC-Application-Guide.aspx, but there are some other interesting modes. Give that ref a good read. You are talking mm here, and not cm. I think if you have a small 1 mm range, you can probably get 0.5 mm resolution, but I can't say I've pushed it that far. Strong neodymium magnets would certainly help.

They're even easier to use than you describe. The output voltage simply changes as a function of flux density.

As a design alternative, you might consider an optical position sensitive detector from Hamamatsu. You'd just need a 1-D sensor. They're good to about 50 microns over two millimeters.

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