Wrong mass computation of assemblies with standard components

[dettmann]

Hi,

when I create an assembly from own parts and parts coming from onshape's standard component catalogue (e.g. a hex nut), I noticed that the mass computation is not correct. This is because the massProperties (in line 143 of onshape_to_robot.py) show that all standard components have zero mass. The weired thing is that within onshape the masses are set correctly and also the computation of an assembly with standard components is correct. So, is this a bug somewhere in onshape-to-robot or directly in the onshape API?

I appreciate any thoughts, thanks.

[csamid]

Late to the party, but this error occurs because Onshape's standard content is not considered an Onshape part :(
However, there is a work around. Export the standard content as a STEP file and then import that STEP file into the document :)

[Niklas-Mulsow]

Hi together,

as i understand that right, this would mean we have always to replace every screw, washer, nut, etc. by hand via the step ex- and import? Since it is part of our business to create simulation models for robotics, this greatly diminishes the use of onshape for us. Can you please verify, if there is a better option available, for example to do the replacement automatically with a sricpt.

Best
Niklas

[csamid]

Hi Niklas,

It might be possible to automate this process with Onshape's Model Translation API I can look into implementing this if you'd like. If so, can you provide me with a simplified model to test on. Finally, on average how many unique standard content parts are used per robot model?

Best
Samid

[Niklas-Mulsow]

Hi Samid,

thanks for you support. Normally it will be around 30 to 150 standard parts. I'll try to create a model to test on this week.

Best
Niklas

[csamid]

Hi Niklas,

Good news. I think I have a working solution. I created a very simple model, so no rush on your part. I'll create a pull request later this week.

Best
Samid

[Niklas-Mulsow]

Hello, sounds good, Thank you very much.

Best, Niklas

[Niklas-Mulsow]

Hey,

i got a message from the onshaoe support to that issue: Please have a look on it, it should be fixable:

The issue is with the logic in the api call.  They are calling for mass properties of each part.  Part metadata responses show whether the part is Standard Content or not.  What the author of the scipt is missing is when this call is used for standard content parts, they need to pass the document id that the standard content is in in the linkedDoucmentId field.  Then you will get the mass.
 
I did a specific test of this and saw 0 mass without it and XYZ of mass with it.  Response example:
 
Without linkedDocumentId
{
  "bodies": {
    "JHD": {
      "principalInertia": [
        0,
        0,
        0
      ],
      "principalAxes": [
        {
          "btType": "BTVector3d-389",
          "z": 0,
          "x": 1,
          "y": 0
        },
        {
          "btType": "BTVector3d-389",
          "z": 0,
          "x": 0,
          "y": 1
        },
        {
          "btType": "BTVector3d-389",
          "z": 1,
          "x": 0,
          "y": 0
        }
      ],
      "mass": [
        0,
        -1.1873558866243225e-10,
        1.1873558866243225e-10
      ],
      "volume": [
        0.0000012938125651453984,
        0.0000012936935647496897,
        0.0000012939315655411076
      ],
      "periphery": [
        0.0009838007856934833,
        0.0009837897816683328,
        0.0009838117897186338
      ],
      "centroid": [
        0,
        0,
        0,
        -0.000001,
        -0.000001,
        -0.000001,
        0.000001,
        0.000001,
        0.000001
      ],
      "inertia": [
        0,
        0,
        0,
        0,
        0,
        0,
        0,
        0,
        0,
        -1.2938125651453983e-12,
        -1.2938125651453983e-12,
        -1.2938125651453983e-12,
        -1.2938125651453983e-12,
        -1.2938125651453983e-12,
        -1.2938125651453983e-12,
        -1.2938125651453983e-12,
        -1.2938125651453983e-12,
        -1.2938125651453983e-12,
        1.2938125651453983e-12,
        1.2938125651453983e-12,
        1.2938125651453983e-12,
        1.2938125651453983e-12,
        1.2938125651453983e-12,
        1.2938125651453983e-12,
        1.2938125651453983e-12,
        1.2938125651453983e-12,
        1.2938125651453983e-12
      ],
      "hasMass": false,
      "massMissingCount": 1
    }
  },
  "microversionId": "93a5e600bbcd93bdb7bd044d"
}
 
With linkedDocumentId
 
{
  "bodies": {
    "JHD": {
      "principalInertia": [
        6.443038617919366e-8,
        0.0000014109841454990842,
        0.0000014135436698420994
      ],
      "principalAxes": [
        {
          "z": 1,
          "x": 0,
          "y": 0
        },
        {
          "z": 0,
          "x": 1,
          "y": 0
        },
        {
          "z": 0,
          "x": 0,
          "y": 1
        }
      ],
      "mass": [
        0.010389314898117548,
        0.010389312652981377,
        0.010389317143253722
      ],
      "volume": [
        0.0000012938125651453982,
        0.0000012936935647496895,
        0.0000012939315655411074
      ],
      "periphery": [
        0.0009838007856934833,
        0.0009837897816683328,
        0.0009838117897186338
      ],
      "centroid": [
        1.2536974258291559e-18,
        7.135062707114396e-18,
        -0.017776927663227745,
        -0.0000010001804666733743,
        -0.0000010002912075783782,
        -0.01777793742517876,
        0.000001000180466675841,
        0.000001000291207592651,
        -0.017775917901280722
      ],
      "inertia": [
        0.0000014109841454990842,
        0,
        0,
        0,
        0.0000014135436698420994,
        0,
        0,
        0,
        6.443038617919366e-8,
        0.0000014109825156227593,
        -1.297141190798301e-12,
        -1.3184106310348743e-12,
        -1.297141190798301e-12,
        0.0000014135420432070719,
        -1.3334637074334925e-12,
        -1.3184106310348743e-12,
        -1.3334637074334925e-12,
        6.442896833561772e-8,
        0.0000014109857753752806,
        1.297141190836796e-12,
        1.318410630643441e-12,
        1.297141190836796e-12,
        0.0000014135452964769984,
        1.333463705853549e-12,
        1.318410630643441e-12,
        1.333463705853549e-12,
        6.443180402276963e-8
      ],
      "hasMass": true,
      "massMissingCount": 0
    }
  },
  "microversionId": "93a5e600bbcd93bdb7bd044d"
}

Best to inform the author they need to check if the part metadata comes back as SC however you can pass that same id for all the parts if you want and it would solve this.
Lou Gallo
PD/UX/Support/Community
Onshape, Inc.

[csamid]

Oh great, this is a much better than the solution I came up with. Will try to implement this instead.

Thanks,
Samid

[Niklas-Mulsow]

Perfecto,

thank you a lot. Let me hear, Then I can test it ;=).

Best Niklas

[Gregwar]

Hello,

Shouldn't linkedDoucmentId parameter should be passed also for non standard documents? drom API documentation:

The id of the document through which the above document should be accessed; only applicable when accessing a version of the document. This allows a user who has access to document a to see data from document b, as long as document b has been linked to document a by a user who has permission to both.
https://cad.onshape.com/glassworks/explorer/#/Part/getMassProperties

So I think the same problem might arise with non standard content (if documents with non standard contents are linked)

[Gregwar]

(For example if I link a document from the community, the same issue might arise with the same possible solution)

[csamid]

Oh yea, that could be a problem. Should I include this in the #110 pr? I would just modify the part_mass_properties method to include linkDoucmentId parameter.