Force/Pressure Sensor

I would like to add some kind of pressure sensor to the heater barrel so I can measure the downward force applied by the pinch roller driver. One pressure sensor I have seen mentioned is called the Flexiforce force sensor. It looks like they just use some kind of pressure/resistance sensitive ink. However it looks be a proprietary formulation.

I was wondering if anyone had any suggestions on some kind of force sensitive ink or other material that I could place under the base of the barrel to get a pressure/force measurement.

Greetings Damon, et al,

I have used the flexiforce sensors (and also similar sensors made by interlink, [www.interlinkelectronics.com] ) They both have a non-linear response (resistance roughly inversely related to applied force), and some hysteresis effects, but they produce a huge signal, compared to strain gages. The most common of these are disk shaped, although interlink makes (or at least made, not sure now) square pads and a long, narrow "belt" shaped device.

What I'm not clear on is where you plan to put it -- between the barrel and whatever holds the barrel? If it's up high enough, it might stay cool enough not to be damaged. Wouldn't it need to be a ring shaped sensor. I don't think either companies devices would take significantly elevated temperatures. I don't know offhand of any suitable materials, but some of the conductive polymers -- or even conventional polymers loaded with carbon black or nanotubes -- might be the basis for something. A means to measure the pressure in the melted-plastic zone would be a great thing for controlling the extrusion process, but melted plastic (and worse, re-solidified plastic) are not compatible with the pressure sensors I've read about -- let alone can afford.
The closest I've seen are sensors for use in automobile engines, either under the spark plug, or (for research) ported into the cylinders themselves. Not cheap. An oil-pressure sensor (if one could be found to take the heat) might serve.

The QTC devices sound interesting, but I haven't tried them, and don't know if they can take elevated temperatures. See [www.peratech.com]
Has anybody reading this used those?

Anybody else have ideas on how we could sense the melt pressure?

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Larry Pfeffer,

My blog about building repstrap Cerberus:
[repstrap-cerberus.blogspot.com]

The plan would be to place the force sensor between the heater barrel and the support. I am not trying to directly measure the pressure of the melted plastic. Just the downward force applied by the filament drive.

You are right that I need a hole or at least a U shaped sensor because of the body of the barrel. It may be possible to cut a hole in the flexiforce sensor, but at $16 a sensor I am not too keen on buying a whole bunch to test that idea.

With proper heat sinking I do not think temperature would be a problem. Also the flexiforce sensor has a high temperature version that handles up to 200F. As mentioned earlier, the top of my barrel was at about 200F. I think by replacing some of the brass with stainless, I can get that down a little more.

Maybe it would be possible to make a capacitive sensor, but I do not want to spend all my building and testing sensors.

The problem with measuring the force on the barrel is that is not representative of the the nozzle pressure. Most of the force is used pushing the very viscous cold plastic at the top of the heater barrel rather than forcing the much less viscous plastic out of the nozzle aperture.

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[www.hydraraptor.blogspot.com]

Anyone considered using piezo speakers?

nophead, I agree that measuring the force applied to the barrel is not representative of the pressure at the tip of the barrel. However getting an idea of the forces involved pushing the filament through the barrel could provide useful information. Especially related to slipping of the pinchroller and how much force is required for a given temperature and extrusion rate.

Having the force sensor is not critical to have, but if it were something relatively easy to add I would try to add it to my design just be able to collect more data points.

annodomini2, I have considered piezoelectrics, but I am not sure how well they work for absolute force measurements. Still I do have a few piezo discs laying around and I may take the time to do a few tests.

I published some figures at the end of of this post: [hydraraptor.blogspot.com]

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[www.hydraraptor.blogspot.com]

Just took a look at the QTC sensors. They have them at [www.maplin.co.uk]. It looks like they could work quite well for force sensing and the price is pretty cheap.
I did come up with one idea for creating a pressure sensor. I bought some graphite powder like is used for lubricating locks and silicone sealant. From a few papers I have seen, about a 10 percent mix will produce a pressure sensitive resistor in the Kohms range.
If that idea does not work, I will order the QTC sensors

Just an update on the silicone/graphite test. The results were not successful. If there was any conductivity it was very intermittent.

After doing a little more research, it looks like carbon black is the material I should be using. I should be able to find this in an artists supply. The particle size should be in the 100nm range which is what carbon black is in. The graphite powder used for lubricating locks is definitely much larger.I could see individual flakes.

When I find some carbon block, I will try the test again.