Voron ERCF Filament Stress Sensor

Operation

Filament Stress Sensor detects filament Tension (Stretch) and/or Compression (Pressure) and signals Happy Hare software and ERCF v2 accordingly.  Two discrete signals are output, one for Compression and another for Tension.  These signals are connected to BTT MMP ERCF v2 control board on an endstop discrete input pin. If the filament is under neither Tension nor Compression, neither signal is output (OPEN).  Happy Hare ERCF V2 software uses these two signals to modify the gear ratio between the Extruder and ERCF filament gears such that the filament is maintained stress free after initial adjustments.  This allows the ERCF filament Gear drive to off load some of the force needed to feed filament into the hot end. ECRF Sync must be active for this feature to work.

Theory of Operation

The Filament Stress Sensor uses two 3144 Hall Effect transistors to sense the presence of a magnet and turn on a ground sensing signal if one is present.  The small magnet is mounted on a shuttle that is connected to an upstream PTFE tub.  The shuttle is mounted in a housing that is in turn connected to a downstream PTFE tube, the other end of which is connected to the CW2 Extruder.  The Shuttle is free to move about 10mm inside the housing. One Hall Effect sensor is located at one end of the Shuttle's range of movement and the other Hall Effect sensor is mount at the opposite end of travel.  The sensors are separated far enough that there is a dead band between the two extremes in which neither signal is output.

If Happy Hare (HH) senses Tension, it increases the speed of the ERCF Gear Stepper slightly until Tension is no longer sensed.  Conversely if HH senses Filament Compression, it slows the ERCF Gear Stepper speed until Compression no longer exists.  In practice the speed of the ERCF Gear Stepper and the CW2 Extruder Stepper become synced after one or two adjustments with little, if any, further adjustment needed.

The Filament Stress Sensor was inspired by Annex Engineering's Belay Sensor.  The general approach herein is very similar.  This sensor however uses Hall Effect transistors rather than micro switches, measures both Tension and Compression, and is slightly more compact.  This design is limited to Voron ERCF V2 Muli-Media devices whereas Annex's is more general.

Annex Engineering sensor can be found here:

https://github.com/Annex-Engineering/Belay

Assembly Instructions:

1. Download and print the .stl files.  A Fusion 360 Model is provided as well.
2. Insert a M6 collet into the large hole in the Shuttle,
3. Insert a 4MM OD x 3MM ID PTFE tube into the collet and through the Shuttle.  The PTFE tube should extend 8MM past the end of the Shuttle.  The other end of this tube will be connected to the ECRF Selector ECAS,
4. Install the Collet Clip to fix the PTFE tube in place,
5. Insert a 3x2mm magnet into the hole provided in the Shuttle. This magnet will activate either transistor when it is in front of one or the other.  Make sure the top of the magnet is flush with the top of the Shuttle.  Make sure the top of the magnet is the magnet's North Poll. If in doubt, suspend the magnet from a thread of at least 1ft in length.  The magnet will align itself with the earth's magnetic pools.  Mark the North poll of the magnet and ensure that it is the exposed surface once installed,
6. Insert a ECAS PTFE tube connector in the round end hole in the Housing,  
7. Insert the Shuttle into the Housing and ensure it can move freely throughout its range of travel,
8. Form the Hall Effect wires at a right angle as close to the transistor body as possible.  The larger flat body of the transistor must face down in the Housing cutouts.  The wires must point upward and fit through the holes in the Hall Effect Lid.  The outer two wires must be bent outward at a 25° angle.  This is to provide more space for terminations in the following steps.
9. Insert the wires in the holes provided in the Hall Effect Lid,
10. Straighten the Hall Effect wires such that they are perpendicular to the Hall Effect Lid,
11. Bend the Left Most wire on the Transistor that is closest to the Housing end that will attach to the ERCF Selector and solder to the corresponding leg of the rear transistor.
12. In a similar manner bend the middle wire of the aft transistor forward and solder to the middle wire of the forward transistor.
13. Insert the Cover over the Hall Effect Lid, passing the remaining 4 wires through corresponding holes in the Cover.
14. Install Heat Sets in the Housing body and in the Plug,
15. Install the Hall Effect assembly created above into the housing using 2 M2.5x8 screws,
16. Wire 5Vdc, Ground, and Signals to available ERCF control ports by soldering wires to the appropriate wires protruding from the Cover and terminating the other end per the BTT MMB manual. One set of wires (5Vdc, GND, & Signal) are terminated in one connector.  The remaining signal wire is terminated in a second connector.  It is not necessary to terminate 5Vdc and GND on the second connector.  The wires are labeled on the cover ( V-5Vdc, G-Ground, T-Tension, and C-Compression),
17. Configure the following Klipper parameters:
    1. mmu_parameters.cfg:
       1. sync_feedback_enable: 1  
       2. sync_multiplier_high: 1.05  
       3. sync_multipler_low: 0.95 
    2. mmu_hardware.cfg:
       1. sync_feedback_tension_pin: !xxxx
       2. sync_feedback_compression_pin: !yyyy  where xxxx and yyyy are the port names corresponding to the signal condition. “!” indicates the signal is inverted.
18. Test correct operation of the sensor using mmu_sensors command:
    1. Push the Sensor toward the Selector.  sync_feedback_tension should show TRIGGERED.
    2. Pull the sensor away from the Selector: sync_feedback_compression should show TRIGGERED.

BOM