During extended periods of synchronised printing without intervening tool changes, alignment between extruder and gear steppers can gradually drift. Even with precise calibration, factors such as under extrusion during purging, printing at high speed near upper flow limits, friction and drag and the inertia of heavy spools can conspire to cause slippage in either the extruder or more commonly—the downstream gear stepper. Over time, this mismatch accumulates and can lead to print artefacts caused by missed steps or suboptimal extrusion rates. Although not always apparent on well-tuned systems, it's a legitimate and demonstrable issue.
Proportional sync-feedback sensors address this using linear Hall Effect sensors (or similar), providing real-time analog positional telemetry to Happy-Hare allowing it to continuously adjust and fine tune the following gear stepper's Rotational Distance (RD).
This case has been based around the excellent PSF v1.0 / 1.1 Proportional Sync-Feedback sensor kit from Kashine6 (Discord: @jacksky6) and is intended to be used with the Happy-Hare MMU ecosystem and new Flowguard / Sync-feedback controller facilities (Happy-Hare 3.4.2 or later - currently in open beta). 14.5 mm of buffer range and works with 20 mm or 25 mm springs from the kit (20 mm prefered).
This design has been completely reworked from the ground up, incorporating improvements informed by field testing of MK I and the earlier adaptation of Tshine's original dual micro-switch based Sync-feedback Sensor by Kashine6 to support Proportional Sync-Feedback sensors.
The primary goals have been to further reduce the overall form factor, optimise sensor–magnet alignment and constrain shuttle lateral movement to maximise Hall Affect ADC signal fidelity, particularly for top‑mounted MMUs and central Bowden entry ports. The assembly now measures just 63 mm in length — 9.2 mm shorter than Kashine6’s version and 3 mm shorter than MK I revision. The ECAS04 captive retention design has also been refined to make it easier to install them, hopefully without splitting the base. Printable ECAS04 clips have also been included to colour coordinate ;-)
Note
Step files (rather than STLs) are included for printing the sensor with Multi-colour tick marks or Single-colour with recessed tick marks. Also consider lowering your 1st layer speed as the coloured elements are reasonably narrow to ensure tick marks are crisp and clean. Depending on MMU / printer orientation, you may also consider inverting all parts (mirror along x axis in your slicer) to relocate the connector to the bottom of the unit to optimise wiring for your particular mmu setup.
Assembly is reasonably straight forward:
- Straighten the end of the PTFE tube you plan to use between the MMU and “Stumpy” PSF sensor. Avoid PTFE with an
ID > 2.5 mmif you want to position it close to the MMU if it has to bend to accomodate the full range of gates / lanes. Kinks can jam the shuttle which must move freely for any selected gate. - Check the body
4 mmPTFE path with the PTFE tube to make sure its smooth and doesn't bind. Use a 4mm drill to gently remove any residual print artefacts around the 4 mm - 2 mm step down chamfer - Be careful not to damage the chamfer though. - Install the
ECAS04fittings without the optional rubber boot. You still need the hard plastic clasp cover on the base though. - Press the
D4 × 15 mmmagnet into the PSF shuttle until it sits flush with the end opposite theECAS04fitting. Magnet orientation doesn’t matter; Happy-Hare will calibrate it. - Insert the
2.0 – 2.5 mm IDPTFE feeder tube from the left, feeding it through the20 mmspring (or25 mmif this is all you have) and shuttle until it bottoms out in the body recess. Adjust so it neither binds nor pulls out when held against the opposite stop. Install the colour coordinatedECAS04clip with the 90° top facing the shuttle so it’s retained and prevented from rotating by the nubs provided. - Insert the
PSF 1.0 / v1.1PCB module into the provided slot. - Slide the lid / cover on from the back over the shuttle and onto the base, aligning the parts and snapping them gently onto the locating nubs. Do not force it. Secure with a single
M2 × 6 mm SHCS. - Confirm the shuttle moves freely across its full travel before installing.
| Item | Specification | Quantity |
|---|---|---|
| PSF v1.0/1.1+ Board | Inexpensive kit from Aliexpress that includes all the required parts below | 1 |
| Spring | 0.4 mm × 6 mm × 20 mm, spring steel ( If longer, e,g 25mm, cut down and shorten it, tiding up the cut end with needle nose pliers | 1 |
| Magnet | D4 mm × 15 mm N35 | 1 |
| ECAS04 Bowden connector & clips | 2 | |
| M2×6 mm SHCS screw | 1 | |
| PTFE feeder tube | Short PTFE feeder tube installed between the “Stumpy” PSF Sensor and the MMU (2.5 mm ID recommended) - 3 mm ID PTFE tube is too soft and will deform or jam if its bent too much. |
"Stumpy" PSF needs to be used inconjunction with Happy-Hare Flowguard integrated support for Proportional Feedback Sensors (Release 3.4.2 or later).
Please refer to https://github.com/moggieuk/Happy-Hare/wiki/Synchronized-Gear-Extruder2 for proportional sensor setup, configuration, and usage instructions.
Note
You will need to manually switch across to the Happy-Hare FLOWGUARD branch before configuring the sensor until the beta concludes and is merged with the main Happy-Hare release.
cd ~/Happy-Hare
./install.sh -b flowguard
You can switch back at any time by running ./install.sh -b main but will need to manually undo some of the new Flowguard configuration options & renamed Happy-Hare parameters (It's not difficult, just a reminder).
MMU_PARAMETERS.CFG
The "Stumpy" PSF Sync-Feedback Sensor has 14.5mm of buffer / sensor range.
sync_feedback_buffer_range: 14.5 # Travel in "buffer" between compression/tension or one sensor and end (see above)
sync_feedback_buffer_maxrange: 14.5 # Absolute maximum end-to-end travel (mm) provided by buffer (see above)In MMU_HARDWARE.CFG set sync_feedback_analog_pin: mmu:<ADC GPIO> to a valid analog-capable GPIO port on your MCU you have connected the "Stumpy" PSF sensor to. Kashine6 provides a breakdown on their GITHUB of valid ADC GPIO ports for many popular MMU MCU's. Note that while many boards offer multiple ADC-capable GPIO's, only one port has been fully verified and documented for each board.
Restart Klipper, load a gate with filament and run MMU_CALIBRATE_PSENSOR to determine the minimum and maximum ADC values returned by your PSF v1.0 / v1.1 Sensor module. Use these values to configure the corresponding parameters in MMU_HARDWARE.CFG. Your sensor may produce different—or inverted—readings compared to the example below. This is expected, and Happy-Hare can accommodate it.
sync_feedback_analog_pin: mmu:<Your analog-capable GPIO> # The ADC pin where the proportional filament pressure sensor is installed
sync_feedback_analog_max_compression: 0.9964 # Raw sensor reading at max filament compression (buffer squeezed)
sync_feedback_analog_max_tension: 0.0046 # Raw sensor reading at max filament tension (buffer expanded)
sync_feedback_analog_neutral_point: 0.5005 # Sensor neutral point (tunable to apply positive tension on the filament to unburden the Bowden and extruder)