HD1910 CAD Model Analysis

Learning objectives: understand the mechanical structure of the microduck-replica-cad repository, grasp the part differences between the two servo versions, and learn to export the full-assembly STEP, preparing for the automated Onshape MJCF export in Automated Onshape→MJCF Modeling.

3.1 Repository Structure Overview

The mechanical CAD drawings live separately in fanhao375/microduck-replica-cad:

microduck-replica-cad/
├── feetech/          # Feetech HD1910 assembly + all parts
│   ├── microduck_feetech.SLDASM    # full-machine assembly
│   └── ... *.sldprt                 # individual parts
├── xl330/            # original Dynamixel XL330 version
│   ├── microduck_xl330.SLDASM
│   └── ... *.sldprt
└── stl/              # ready-to-print STL files

Key distinction: parts are not interchangeable between the two servo versions. The Feetech HD1910 and the original Dynamixel differ in outline, mounting-boss orientation, and output-shaft position. When printing, you must select parts from the matching version to avoid discovering a mismatch during assembly.

3.2 The Robot Duck's Mechanical Topology

MicroDuck is a bipedal robot. Each leg roughly contains:

JointDescriptionCount (per leg)
Hipallows forward/backward swing and lateral opening (pitch/roll/yaw combination)3
Kneeallows shank flexion/extension (pitch)1
Ankleallows foot pitch and roll2

The whole machine has 14–15 servos (some replicas add degrees of freedom at the tail or head); the exact count and limits follow your CAD assembly tree.

3.3 Mapping the Kinematic Tree (Preparing for MJCF)

When exporting MJCF in Automated Onshape→MJCF Modeling, you must tell Onshape/onshape-to-robot which parts are links and which joints are actuated. This must be clarified in advance. The recommended kinematic tree (for 14 DOF):

base(torso/base)
 ├── thigh_right  (hip pitch) ── knee_right(knee) ── ankle_right(ankle)
 └── thigh_left   (hip pitch) ── knee_left (knee) ── ankle_left (ankle)

The hip roll/yaw usually sit between the torso and the thigh; if your assembly places roll/yaw after hip pitch, the joint hierarchy in the exported MJCF must match reality one-to-one, or the simulation will error out immediately.

3.4 Exporting the Full-Assembly STEP

The microduck-replica-cad repository uploads SolidWorks source files (.sldasm/.sldprt) by default and does not provide a pre-packaged STEP; you must export it manually:

  1. Open feetech/microduck_feetech.SLDASM in SolidWorks;
  2. Choose "File → Save As";
  3. Select file type STEP AP214 (recommended, best compatibility);
  4. Save as microduck_feetech.step.

No SolidWorks? The free, open-source FreeCAD can open .sldasm and export STEP, but FreeCAD's compatibility with SolidWorks assembly constraints is limited; some constraints may be lost, so verify assembly integrity before exporting.

3.5 Three Checks Before Exporting STEP

  1. Assembly hierarchy must not collapse: confirm parts are not all flattened to the root node, or the exported MJCF joint hierarchy will be scrambled;
  2. No over- or under-constrained mates: assembly constraints must be complete, or joint positions will drift after importing the STEP into Onshape;
  3. Servo mounting orientation is correct: the HD1910's mounting-boss orientation must match the thigh/shank parts — this is the most error-prone point between the two versions.

3.6 Engineering Pitfall Log

  1. Ankle joint seizure: a classic replica problem. After 3D printing, the servo horn interferes with adjacent structure, restricting rotation or seizing it. Check the horn-to-shell clearance in CAD, and if necessary correct the assembly interference in the STEP source and widen the clearance.
  2. Part walls too thin: some structural parts have insufficient wall thickness and fracture under load after printing. Thicken load-bearing regions or raise the infill ratio.
  3. Printing the wrong part: downloading both the feetech and xl330 STL sets and selecting the wrong version during printing leads to assembly failure. Print by directory and version labeling.

3.7 Chapter Summary

You should now be able to:

  • Locate the correct Feetech-version parts in microduck-replica-cad;
  • Understand MicroDuck's kinematic tree (preparing for MJCF joint definitions);
  • Export the full-assembly STEP AP214 from SolidWorks.

In the next chapter, import this STEP into Onshape and use onshape-to-robot to auto-generate an MJCF model with accurate inertia parameters — no more manual estimation.