Onshape Importer loads assembly incorrectly

[pkoprov]

5.1.0

Operating System

Windows 11

GPU Information

• Model: RTX4060
• Driver Version: 591.74

Onshape importer incorrectly imports the robot

Detailed Description

When importing a simple robot, with only 2 wheels, they appear disconnecetd and frame is somehow inisde of a tree of one of the wheels. The wheels are mated through revolute joint to their brackets in Onshape. Brackets are mated to the frame via fixed joint.

Steps to Reproduce

1. Import from Onshape

2. press play

This is how it looks in Onshape

And this is how it looks in Isaac Sim

First - Why is Frame inside a Wheel_Assy_RH?

Second - why wheel assys are not connected via Fixed joint?

[michalin]

Hi @pkoprov, thank you for posting your question. This seems like an unexpected behavior. We are looking into it and will get back to you.

[rgasoto]

Hello! Would be good to know what is on the frame subassembly. The recommended course of action is ensuring the entire fixed subassembly be placed on a group mate to ensure the importer will interpret it as a rigid structure. Another thing to check is the order the mate is made. In Onshape it does not matter much but sometimes it may cause some confusion in the converter to decide what component is should be combined as a single rigid body.

The Onshape importer will try to make some assumptions but sometimes the topology may not be correctly interpreted.

[pkoprov]

Renato,

Here is the link to the Onshape file I have in case you want to look at it: Onshape

The Frame looks like this:

In terms of mating - the PKIS_Scout assembly has only 2 mates - both for the wheels.
Did you mean the order they are selected in the mate?

[pkoprov]

@rgasoto

I made the first object in both fastened mates to be the Chassis inside Frame. Now the hierarchy loads correctly:

But wheels are still not fastened to the Frame or Chassis inside the Frame. The only joints are revolute joints for the wheels inside wheels assemblies

[PeterNV]

Hi,

To properly fasten wheels to the frame:

1. Create a Group mate in Onshape for all components that should be rigidly attached
2. Include the Frame/Chassis and the wheel mounting brackets in this group
3. Keep the revolute joints separate for the actual wheel rotation

Expected Structure:

PKIS_Scout (root) 
├── Frame (rigid body with Chassis) 
├── Wheel_Assy_RH
│      ├── Bracket (fixed to Frame via group mate) 
│      └── Wheel (revolute joint) 
└── Wheel_Assy_LH 
     ├── Bracket (fixed to Frame via group mate) 
     └── Wheel (revolute joint)

The key is ensuring the Onshape importer correctly interprets which components form rigid bodies versus articulated joints through proper mate ordering and group mates.

[pkoprov]

Peter,
I will try again tomorrow and let you know, but there is an issue in the workflow. In real settings, the robot assembly in CAD consists of subassemblies. Every one of them will have its own joints and rigid groups. Are you implying that the robot assembly in Onshape should be flat?

[PeterNV]

I think your issue is missing the Bracket so make sure to create a Group mate in Onshape and include the Frame/Chassis and the wheel mounting brackets in this group.

[pkoprov]

Peter,

Here is my frame in Onshape. It consists of 1 sub-assembly and multiple parts. They are all connected by the Group “Frame”.

Here is my Wheel assembly in Onshape. It consists of Motor subassembly JGA-25-371 and other parts.

Here is what is included in the Group Wheel hub

This group is connected by a fastened mate to a shaft inside JGA25-371.

Bracket is connected via fastened mate to the JGA25-371 body

The main assembly has these wheel assemblies connected to the frame assembly via fastened mates.

When I import this to Isaac sim this is what I see:

It looks like all the joins and the structure are OK, but it is not. Wheel assembly Xforms include only parts that are grouped as Wheel hub and a shaft from the motor assembly. These are the components that are mated by the Fastened mate in Onshape.

The motor assemblies themselves are somehow included in the Frame Xform and are dislocated from their original positions. Why?