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Homing & Fiducial

Establish a connection with the top camera, then set up the datum board. The center dot on the datum board is the homing fiducial — the key calibration point that fine-tunes the machine's XY position to compensate for minor limit-switch variation.

The homing fiducial is the 1mm dot in the center of the Opulo logo on the datum board.

Configuring the Top Camera​

  1. Navigate to Machine Setup → Heads → ReferenceHead H1 → Cameras → OpenPnpCaptureCamera Top → Device Settings tab.
  2. Select LumenPnP Top from the Device dropdown.
  3. Set Resolution/Format to 1280x720, 10fps — a low frame rate is crucial to avoid bandwidth issues.
  4. Click Apply, then File → Save Configuration.

Preparing for Homing​

  • Machine Setup → Heads → ReferenceHead H1 — Homing Method set to None
  • Machine Setup → Nozzle Tips → ReferenceNozzleTip N045 → Auto Recalibration: Manual
  • Machine Setup → Nozzle Tips → ReferenceNozzleTip N24 → Auto Recalibration: Manual
Critical: level the nozzles before homing — now and every time. OpenPnP does not remember previous nozzle positions. If nozzles are not level before homing, they may collide with the machine, causing misalignment or serious hardware damage.

Click Home to perform a rough homing sequence. The nozzles move to X/Y/Z zero (front-left corner) and park.

How to Level the Nozzles​

Do this every time the machine has been powered off, and after any crash.

  1. Power the machine on but don't home it.
  2. Loosen the nozzle holder on the rotation axis just enough that the tip can be moved by hand.
  3. Slide a piece of paper onto the datum board under the head, then lower Z by hand until the first nozzle just pinches the paper.
  4. Without moving Z, bring the second nozzle down to the same pinch. If it's higher or lower, adjust the holder.
  5. Tighten, raise Z, and re-check — tightening usually shifts the tip a little, so verify after.

Two nozzles at visibly different heights is the warning sign. If one tip is clearly lower than the other, stop and fix it before running the homing sequence.

Setting the Homing Fiducial Location​

  1. Navigate to Machine Setup → Heads → ReferenceHead H1.
  2. In the Configuration tab, change Homing Method to ResetToFiducialLocation.
  3. Click Apply and save.
Do NOT click homing again until you have fine-tuned the homing fiducial location in the next section.
  1. Click Position Camera Over Location — the top camera moves above the datum board.
  2. Ensure LED lights are on (Machine Controls → Actuators → LED → On).
  3. In the top camera Device Settings, toggle Auto off for Exposure to enter manual mode.
  4. Use the jog controls to center the reticle over the homing fiducial (center of the Opulo logo).
  5. In Machine Setup → Heads → ReferenceHead H1, click Capture Camera Location for the Homing Fiducial.
  6. In the Calibration Rig section, click Capture Camera Location next to Primary Fiducial.
  7. Click Apply and save.

Center the reticle as precisely as you can. This position becomes the machine's XY datum, so an error here shows up as a uniform offset on every board — it won't look like a calibration problem, it'll look like your board origin is wrong.

Fine-tune Camera Exposure​

  1. In the top camera Device Settings, right-click the feed and select Show Image Info to enable the brightness histogram.
  2. Adjust the exposure slider.
Keep all other camera settings at default and only adjust the exposure. Aim for a sharp peak on the right and a larger cluster to the left. The histogram is only an example — focus on a distinct right peak and a noticeable left cluster, not an exact match.
Not seeing anything in the top camera feed? Ensure the lens cap is removed.

Record your exposure value in a text file — OpenPnP may not retain camera settings after restart. Each boot, toggle Auto Exposure on then off, then type your recorded value. Turn off the histogram when done and save.

Exposure values are worth keeping per lighting condition. If you work with the room lights on in the day and off at night, you may end up with two values that both work; write both down rather than re-tuning each time.

Test Fiducial Homing​

Click Home and watch the machine home via limit switches, then move the top camera over the homing fiducial. When scanning, the fiducial will have a red-colored hue.

If you get the error FIDUCIAL-HOME no matches found, OpenPnP can't find the fiducial — adjust the Homing Fiducial Pipeline.

FIDUCIAL-HOME: No Matches Found​

This is the most common error on a first setup. OpenPnP is running the fiducial-detection pipeline over the camera image and the pipeline isn't returning a candidate. Work through these in order — the first one fixes most cases.

The camera isn't actually over the fiducial. Click Position Camera Over Location again and look at the feed. If the datum board isn't in frame at all, the captured homing fiducial location is wrong. Re-capture it.

The fiducial isn't lit. The dot is a dark mark on a light board, and the detection pipeline needs contrast. Turn the ring LEDs on and re-run. Working in a dark room with no LEDs is the second most common cause.

Exposure is wrong. If the board is blown out to white, or so dark the dot merges into the background, the pipeline has nothing to threshold. Re-check the histogram before touching the pipeline.

The pipeline's size limits don't match the scale you're at. In Machine Setup → Heads → ReferenceHead H1 → Camera → Part Detection, open the Homing Fiducial pipeline. The first stage filters detected circles by diameter in pixels. At 1280×720 with the stock lens, the 1 mm dot lands in a fairly narrow pixel range — if you changed resolution or moved the camera height, that range is no longer valid. Widen the min/max diameter before changing anything else.

The lens is dirty or the reticle is confusing the detector. Clean the lens, then re-run.

Checking It Worked​

A correct homing cycle looks like this: the machine homes on limit switches, moves the top camera over the datum board, pauses briefly while it scans, then parks. If it parks without the pause, it didn't find the fiducial and silently fell back.

After a successful homing fiducial capture, re-homing twice in a row should park the head in the same position both times. If it drifts between runs, the captured location wasn't centred well — go back and re-capture.

Next: Continue to the MM/Pixel Calibration.