IDEA pressure guage in print head ?

[ian 32]

I was cooking around with the idea.

Would it not be super to have a pressure guage in the print head.. when the extruder is pushing too much material into the print head, the sensor will pickup the pressure and order the extruder motor to slow down.

On the other side if the pressure sensor sees that there is little pressure, then a signal is sent to the extruder motor.. faster please.

Also if the pressure sensor sees there is no pressure, even after the extruder motor has been ordered to print faster, the sensor will tell the printer that there is no more filament and will stop and shut down the maschine.

The sensor results could also be automatically emailed to the user... ie... report no more filemanet, maschine off, please attention !

what do you guys think ?

Its just a rough idea but might be really good for the ultimaker?

Best wishes.

Ian

[destroyer2012 0]

I have been thinking this way for a while as well. The problem I see is, how are we going to fit a pressure sensor in the head without making it leak? The best place to measure pressure is in the zone where plastic is liquid. Pressure is generally measured by the strain of some elastic material like a spring. Where will we fit said tiny spring in the head and still have a relatively short (read: correctly sized) hot zone?

The pressure is likely to be quite high so maybe the best way to go would be a strain gauge on the outside of the head. The most workable thing I can think of is measuring the strain along the length of the bowden tube. While not a direct measurement of the pressure that's important, at least it gives us some handle on the pressure applied to the system, and if the bowden tube is screwed directly into the nozzle (not allowing for other flexible components) this might be the best sense we can get of the pressure in the hot zone. But this measurement might be liable to noise from different head positions causing the bowden tube to be differentially stressed.

If there's any friction at all along the path of the filament then the pressure in the bowden tube becomes more isolated (mathematically) from the pressure in the hot zone. It'd be like a spring with a damper and than another spring attached, and we are measuring the second spring hoping to get insight on the first.

This all sounds nice but I think you don't really need to measure the pressure as long as you build the system so it's pretty rigid. Our biggest enemy is low pressure, since that results in oozing and slow flow. So if we just drive the system at max pressure all the time we shouldn't have to worry since volume in equals volume out, assuming a calibrated extruder. The only times when I have had problems with building pressure is when I was overextruding. Once I calibrated my E steps/mm and measured my filament, assuming I started at the right layer height everything was fine. Thus we should be wary of overengineering the problem.

[joergen 2]

Playing the devil's advocate, do we need a pressure sensor in the head? can't we derive almost all necessary parameters that influence the pressure in the head from the known input variables (material, diameter, e steps, xye speeds, temp, nozzle diameter, specific heat capacity (from the PiD auto-tune)), and therefore the max extrusion (mm^3) per sec?

[destroyer2012 0]

No because nozzle height from the print surface affects the pressure inside the head. THus for example if you start a bit too close to the bed then the pressure builds up during that layer and subsequent layers that all happen on top of each other so when you travel to a smaller area or you start doing an overhang you will see some blobbing. A similar effect occurs when making solid infills if you happen to be even slightly over-extruding.

The counter argument being, just set your nozzle height perfectly or input nozzle height into the equation, and I do think this might be the better solution. However, with a heated bed it is much harder to set the nozzle height (at least for me it is) because the bed expands upwards as it heats. Does this mean we should rig up a complex pressure sensor just because I suck at setting z=0? Maybe an auto bed leveler would be a better solution.

What are some things you can do with a pressure sensor that you cannot do currently? Detect when the filament is about to be stripped and stop extrusion. Sense when extrusion is not happening and pause the print. Ensure consistant extrusion. Do these things justify the complexity? Idunno

[ascended 0]

A liquid pressure sensor or gas pressure sensor could be used if it's housed correctly. The biggest issue is going to be in housing it in such a way that it's away from the heat and raw plastic (as that would foul the sensor).

The biggest gain i see in the pressure sensor is when you want to back the head off. Rather than dwindling the amount of plastic in the head and guess at volume - you can use the pressure sensor to slowly drop the pressure down so the print continues but the volume is low enough that when you back it off it doesnt drool all over your work

I'm sure that with the right control software/slicer, you could control the head pressure based on the feature you are printing, and do some look ahead too.

As with all things, more data/control points (and using them correctly) results in a better final product. Be it a weather forecast, engine monitoring or 3d printing

[rewolff 0]

I was thinking: Why not measure the force the bowden tube is exerting on the feeder mechanism? That means it doesn't need to move along with the head.

When the force becomes to big, one of the following is going on:

* you're moving too close to the bed.

* You're moving too fast.

* your nozzle is clogged.

* you have a plug in your print-head.

[phut 0]

My thoughts were much the same - to measure the forces somewhere at the extruder - but that only works well if you're after the forward forces only; the pressure gauge would obviously work better in the cases where you're doing retractions because of the play in the bowden assembly.

A simplex way of measuring pressure at the extruder would be to change the motor out to an encoded PM motor and look at the control currents - which as long as you have minimised the friction with ball bearings at the pinch point and geartrain would be a fair indication of pressure.

Of course changing the extrusion motor to PM also brings along a host of its own problems.