15mm rod Phantom depth sensor fast moves focus assist focus bar lidar magic arm rigging sensor plane shift

LiDAR / Focus Assist Rigging — Preventing Shift on Fast Moves¶

CONFIRMED ANY RIG WITH FOCUS MOTOR

Summary¶

A physical LiDAR sensor (used for SFX focus distance measurement, e.g., aligned to the camera's sensor plane) can shift position during fast robot moves if it is mounted via a magic arm. A magic arm that is too long, attached to an insufficient anchor, or not clamped tightly enough will flex under acceleration and cause the LiDAR's aim to drift. Community solutions focus on getting the LiDAR closer to the 15 mm rod system and building a rigid bridge or double-support mount.

The MRMC Focus Assist Depth Sensor is normally clamped directly to a 15 mm rod and is designed for this environment. Third-party LiDAR units (e.g., for Phantom sensor-plane focus) may require custom rigging.

Symptoms¶

LiDAR shifts aim during fast moves, causing incorrect focus distance readings.
Magic arm is found in a slightly different position after a fast run compared to before.
Focus distance output is consistent at slow speeds but drifts at high acceleration.

Community Solutions¶

Mount to the Focus Bar (Not a Magic Arm)¶

The most reliable approach is to clamp the LiDAR directly to the focus bar (the 15 mm rod that the focus motor rides on). This keeps the LiDAR rigidly co-located with the focus system and minimises relative movement.

If the LiDAR body is too large or the connector exits at an inconvenient angle, use a dedicated rod clamp or cheese plate as an intermediary — but keep the connection path short and direct.

Build a Bridge Between Two Rods¶

If the LiDAR needs to be at a specific position that a single rod cannot accommodate:

Build a bridge plate or crossbar spanning from the focus bar to the opposite rod on the 15 mm rig.
Mount the LiDAR to this bridge — it is now supported at two points instead of one.
This greatly reduces flex compared to a single-rod cantilevered magic arm.

Use a Second Focus Bar¶

For rigs that allow it, adding a second focus bar on the opposite side of the lens gives a symmetrical two-point mount for the bridge above.

3D Print a Lightweight Custom Mount¶

For lightweight LiDAR units, a custom 3D-printed bracket can be designed to: - Mount directly to an existing rod clamp. - Position the LiDAR at exactly the correct alignment to the sensor plane. - Be lighter than a generic magic arm, reducing resonance.

MRMC Focus Assist Depth Sensor¶

The MRMC Focus Assist Depth Sensor is designed to be clamped to a single 15 mm rod and is typically positioned alongside the lens. It is the reference design for depth-sensor focus integration with Flair. If using this sensor, check that the rod clamp is fully tightened and the sensor body is not flexing relative to the lens.

See Focus Assist Depth Sensor QSG for mounting guidance.

Notes¶

The alignment to the sensor plane (not the front of the lens) is critical for the focus distance reading to be accurate. Mark or measure the sensor plane position on the camera body before rigging the LiDAR.
For Phantom cameras: the sensor plane position can be verified against the camera's official dimensional drawings.