Which detection mechanisms can a filament sensor use?

Question

Using the official Creality runout sensor, I encountered a few times that the filament entangled around something and got stuck, the printer continuing for several hours (as it didn't run out), printing mid-air aithout filament. Then I got interested in the types of filament sensors that exist such as a runout and jam sensor as those mentioned below:

The BTT filament sensor

It is supposedly able to detect both runout and jamming, which I assume uses the concept of a rotary encoder:

DIY 'super simple filament jam sensor'

From printables.com, which I assume uses simple physics, although it's unclear how this works exactly.

DIY Optical Filament Sensor

This amazing-looking optical filament sensor from thingiverse.com uses a rotary encoder.

Question

I guess that a rotary encoder would be best for this purpose because it can detect actual moving filament and potentially even exactly at which speed it is moving. However, how would it account for filament retraction or paused prints (waiting for filament color change)? Is this usually done in firmware or external control software such as OctoPrint?

I would like to understand: What different types of detection mechanisms for filament jam detection exist, and how do they work exactly? Also, which is considered most reliable and why?

Answer 1

I am going down the same rabbit hole now and would summarize filament detection types into four categories: runout, extruder issue, reel jam, and nozzle jam. In fact, that's what BTT advertises for their sensor so nothing controversial here.

The mechanisms used to detect these issues are based on movement or force. Since detecting force itself is not always easy or intuitive, it is also common to convert force into motion. The sensors involved include:

1. Micro-switches that detect physical presence either by the filament or something acting on the filament
2. Load cells that detect force directly
3. Optical encoders that toggles a signal proportional to the speed of a roller acting on a filament (just like the scroll wheel of your mouse) They can also be use as a contactless switch similar to a micro-switches usually with the goal of lower resistance on the filament
4. Hall effect sensors that act as a contactless switch

When a filament ends, it is physically not there anymore which is why a micro-switch is an easy solution. But when a filament jams, the effect is a change in tension of the filament that exerts a force on different parts of the extruder path. From the extruder's perspective, when there is a reel jam it has to pull harder, when there is a nozzle jam it has to push harder. If the extruder is damaged, it is not able to push or pull the filament. In all cases, the filament doesn't move at all.

In this answer, the filament sensor tries to detect a reel jam by converting the tension in the filament into a mechanical motion that activates the micro-switch. That design is optimizing both runout and reel jam into a single switch action. Other sensors can be used in place of the micro-switch given enough R&D.

Optical encoders (aka smart runout sensor) use the movement of filament to toggle a signal. Like a heart beat, a beating signal means filament is moving while a still signal is a potential sign of fault. The printer firmware must support this type of input for encoders to function. Optical encoders require the firmware to keep track of when the extruder is commanded to move and check that the encoder feedback matches those moves. If the feedback doesn't match the command and it exceeds some threshold, then the firmware treats it as an error. This "threshold" is what people refer to as tuning a smart runout sensor. For Marlin, the FILAMENT_RUNOUT_DISTANCE_MM field sets how much mismatch is tolerated before an error occurs. Note that this is tied to the design of the encoder and it's resolution in tracking filament movement.

One other consideration for sensors is contact vs contactless. Anything that contacts the filament adds resistance that the extruder has to overcome. It might only be a significant issue when building larger systems where the reel is far away and the total resistance from sensor and tubing starts to add up. Contactless is certainly a good idea, but is also more complex to design. For example, optical contactless that directly detects filaments can have issues with transparent material.

In summary, I think optical encoders are currently a good comprehensive solution to filament detection. The main trade-off due to it's resolution is dealing with false positive triggers and potential grinding of the filament that would worsen a fault condition if the detection takes too long. The traditional switch tends to detect immediately a real condition and lets the firmware react faster but detecting multiple types of faults require more complex mechanics.

Answer 2

It's a great question, but I don't think jam detection is particularly common on hobbyist printers.

I know the Prusa MK3.9/MK4 and XL printers intend to add jam detection using the load cell that's built into the Nextruder, where it can detect the force between the extruder gear and the nozzle. It was promised in their promotional material, but is not implemented yet. (Today it uses the load cell to perform mesh bed levelling.) Jam detection using the load cell would have to be implemented in firmware.

Vector3D build a hotend tester using the same principles, and I feel his video nicely explains how the load cell would detect skipping and jamming: The Next Generation of Hot End Testing - YouTube.

I don't have any experience of using rotary encoders on a printer, so I cannot comment on how the data from the encoder is used to alert the user of a potential problem. I would hazard a guess that it's an OctoPrint plugin which require you to use serial printing (so OctoPrint knows how much filament is being extruded/retracted moment to moment). A quick search shows there is at least one such plugin: gmatocha/Filament-Watch-Octoprint-Plugin: Octoprint Plugin that monitors filament extrusion with a rotary encoder, the README lists the required parts and may be a good starting point if you want to build one yourself.