JPL DARTS Lab Q&A - Recent questions without answers

Are there examples of integrating DARTS with MONTE for simulating autonomous navigation?

Mon, 14 Aug 2023 16:02:17 +0000

I would like to see examples of DARTS+MONTE integration for simulating autonomous navigation to a body (e.g., a moon, a planet, or a small-body asteroid).

Is there a method for capturing a high/low watermark value for a variable and the associated time stamp?

Wed, 08 Feb 2023 18:35:51 +0000

I'm looking for a way to capture a high/low variable value with the associated timestamp, and without triggering an event?

How do I model a spacecraft bus with two solar arrays connected by a flexible hinge

Thu, 08 Sep 2022 15:25:26 +0000

I have a spacecraft that I model as a rectangular bus with two large solar panels. I want to allow the joint to be flexible and have been looking at using the SpringDamperMotor model.

I essentially want to have a flexible 'BALL' joint, meaning the joint can flex up/down and left/right, along with twist.

Is there an example of this? I think the SpringDamperMotor model is a single-axis motor (all the examples are 'PIN' joints and robot arms), is there an example of a more complicated flexible joint?

How do I get plugins for my Unreal Engine project despite me running Linux?

Sat, 07 May 2022 01:06:41 +0000

Since we get plugins through the Epic Games store, we can't get plugins directly while using linux.

Instead, use your PC or Mac to get Epic Games Launcher and Unreal Engine. Buy various plugins and install them to either the engine or a project. Sometimes these plugins might add functionality or they may add art assets.

In the case that they add functionality, they will install themselves directly into the engine or your plugins folder.

If they install to the Engine, you need to navigate to the Engine plugins folder and find the folder with your plugins name and copy it into the linux Engine.
If they install to the project itself, in your project directory look for a folder called "plugins" and copy that folder into your linux unreal project.

In the case that they add art assets, they might either install itself into the engine or project, if so look above for how to deal with it.

If it imports art assets into your project (or makes its own), just copy that project into linux, open the project up using the linux unreal engine and migrate the files into the Unreal Engine project you're working in (i.e. UnrealDarts)

Warning! Migrating assets between projects may mess up your file hierarchy, so after you get your wanted assets be sure to move them (in the Unreal Engine) into their proper place. After moving it be sure to right-click the top-level Contents folder and "Fix up redirectors".

Also, every month Epic Games releases a bunch of free plugins. These plugins range from sound effects to art assets and programmed feature packs (like an inventory system). Might be something to check every month as these plugins can be very high quality.

I need a frame that is always located at the CM but is fixed to the body

Thu, 21 Apr 2022 00:20:30 +0000

I need a frame that is always located at the center of mass of the main body but which is also fixed relative the body frame. I thought simObj.mbody().cmFrame() would do this, but it seems like it's fixed relative to the pcr frame. Will "CapsuleBase_obj103_cm_frame" or "cegraph_cm_frame" give me what I'm looking for? I can also create one, but it's not clear to me how to create one that has both of these properties (i.e. always at the cm and rotates with the body).

If the answer is CapsuleBase_obj103_cm_frame, then will the object number of this frame always be the same?

Does dstore truncate names from cfg file longer than 63 characters

Mon, 14 Sep 2020 16:53:09 +0000

what would cause error "The "fdsimGncTest.cc" rule is not defined for the "DCFswModels" module?

Thu, 06 Aug 2020 14:44:30 +0000

I am trying to add model fdsimGncTest to module. I have added .mdl file and assembly and param .py files. The error happens on "ymk all"

should quantity be defined for integer variable in model?

Mon, 13 Jul 2020 21:06:12 +0000

for parameters/flowins/flowouts defined in .mdl file should integer variables have quantity defined?

How can I set up ornamental geometry for sensor FOV visualization?

Wed, 24 Jun 2020 21:08:37 +0000

In the slides for the DARTS Lab 2019, unit J - DspaceVizualization, slide 41, there is an image showing the use of ornamental geometry to represent a sensor field of view. I would like to use this feature for a lidar sensor in the standard viewport (not in a notebook). Could someone provide an example of how to set up this visualization?

Thank you

Jennifer

Why are bodyFrameCompositeSpatialInertia values stale when passed onto a variable?

Mon, 25 Nov 2019 13:43:45 +0000

For example, let's say 'base' comprises a subcomponent 'fuel tank'. I'm updating attribute 'mass' in base and fuel tank that changes the total mass of vehicle base.

base = sim.mbody().body('Base', 0, True)
vehicleMass = base.bodyFrameCompositeSpatialInertia(sim.mbody())
vehicleMass.mass()

The behavior I'm seeing is that 'vehicleMass.mass()' is stale as I've updated the subcomponent masses 'fuel tank' and 'base'.
This behavior is observed in phasing wherein the monitored value is 'vehicleMass.mass()'. Since this is observed, I use an alternative monitoring function to monitor base.bodyFrameCompositeSpatialInertia(sim.mbody()).mass() each time the sim is sim.step() '

It is also observed that .spatialInertia values do not behave this way, in contrast with .bodyFrameCompositeSpatialInertia.

Can you help users understand the design behind bodyFrameCompositeSpatialInertia and how it differs from spatialInertia? Why are values in bodyFrameCompositeSpatialInertia stale when passing onto a variable whereas spatialInertia is dynamically updated?

Thank you,

Patricia @ COMPASS JSC

I'm having trouble running a Jupyter/IPython notebook. What are some things to try?

Fri, 26 Jul 2019 03:43:17 +0000

What projects have not been completed at the end of Elliot Trapp's internship (6/20/19)?

Fri, 14 Jun 2019 22:47:35 +0000

FacadeLogScene
* If you feed an extremely large log file to movie-maker, the tool will crash due to running out of memory.

BlenderScene
* BlenderScene needs to render in two passes so that it can handle really large and really small scales. A Blender GUI project has already been created which does this is in '/home/atbe/dev/users/etrapp/blender_scale_change'. This needs to be converted to a python script that BlenderScene would then autocode.
* Scenes rendered in Blender seem too dark. Not sure why, but if this is a problem it should be fixed.

SOA
* Investigate Eigen3 vector and matrix classes/API. Investigate SOA vector and matrix classes/API. Develop a mapping from Eigen3 to SOA that minimizes refactoring downstream. Present findings.

DScene
* Add a way to dump what extra_parameters have been used during a simulation.
* Change DScene/swig/ExtraParameterSpecs.i ToVect() to only take in homogenous Python lists, no conversion to boost::any. Change ToDict() to only take homogenous Python dicts.

TutorialNotebooks
* Enable code highlighting in Jupyter Notebooks: https://stackoverflow.com/questions/51460002/highlight-part-of-the-code-in-a-jupyter-cell
* Get TutorialNotebooks/python/NotebookServer running on goldstein: https://jupyter-notebook.readthedocs.io/en/stable/extending/handlers.html
* Convert .py data in tutorials_gui to a proper relational database

* Finish implementing UpdateNotebookContent.fixDeadLinks()
* Improve the auto-generated headers
    - The links should appear over the mast image, on the right side. Should be possible with CSS styling. See J. Cameron for style guidance.

What is the best example of a sim with multiple, concurrent FSMs?

Wed, 13 Mar 2019 00:15:56 +0000

I'm looking for a good example of a simulation that runs multiple, concurrent FSMs that shows how they're attached to the simulation manager, configured, and used. Is there a good test for this?

How can we get Saturns Rings to render in animations?

Thu, 07 Mar 2019 03:00:41 +0000

If we're working with the Saturnian system, how can we get Saturn's rings to render in visualizations? Do we need to create a specific planar object with a ring texturemap manually, or is it somehow baked in as an option?

What exactly is the following state parameter doing under the hood: initTimeOffset?

Sat, 09 Feb 2019 20:40:30 +0000

Trying to adjust the planet relative longitude coordinate when the simulation start time does not correspond to the epoch of the frame used in providing the entry state.

What DARTS or SPICE function calculates when an Earth satellite is in Umbra, and what percentage of Penumbra?

Tue, 30 Oct 2018 16:58:03 +0000

Does DARTS or SPICE have a function for estimating solar eclipse percentage and intervals for satellites in Earth orbit?

Tue, 30 Oct 2018 16:41:17 +0000

Using SPICE for orbital dynamics, given a satellite's position and/or orbital elements, what functions are available for estimating:

(1) Sun vector

(2) An Earth satellite's orbital plane inclination wrt solar plane, and

(3) Percentage eclipse (satellite)

How do you disable input validation of parameters?

Wed, 17 Oct 2018 14:58:56 +0000

Once input validation is setup for a parameter class, is they a way to disable the input validation checks from running? In other words, how do you turn off input validation once a parameter class is setup to use it?

PropulsionSystemAssembly question: see more information.

Fri, 31 Aug 2018 00:42:43 +0000

Withhin the PropulsionSytemAssembly, is a fuel tank "parent body" designation ignored if a body graph is provided that contains the fuel tank?

Is there an option to set burn time for a fixed and throttled thrusters?

Fri, 31 Aug 2018 00:34:46 +0000

Why is PYTHON_BIN defined in the site configuration

Wed, 01 Aug 2018 18:02:01 +0000

We noticed that an environment variable is defined, PYTHON_BIN, that points to the system installed python. This appears to be used in the build process but it's not clear why the user environment defined python is not being used. Why is there a separate hard-coded python used in the make process?

How do i create a camera that follows an object does not rotate with the object?

Wed, 20 Jun 2018 14:58:52 +0000

I have created an angle that follows an object through a simulation, but the object begins tumbling. I have already tried using viewAroundFrame and setupChaseView. This causes the viewport visualization to begin spinning while the object appears fixed. I would like the visualization to be fixed, while the object is seem spinning.

Where is a good example for using the DshellCommon.params.BaseParam class TargetAdditionalFrameParam()?

Fri, 09 Feb 2018 22:10:45 +0000

A common interface type with MD/Nav is vectors expressed in an EME2000 frame, centered at a Target planetary body. The Target PCI and PCR frames should be loaded from SPICE for a given Epoch, and PCI = PCR at sim time = 0.0 for typical sim. Having a way to configure the simulation with an EME2000 frame of shared origin with PCI and PCR would be very useful. Is there an example of this (or something similar) somewhere in the sim framework?

What is the correct way to generate random numbers in a Dshell model?

Tue, 06 Feb 2018 20:15:31 +0000

A common need for Dshell models is some element of randomness. Ideally, this randomness should have a few properties:

The model can be seeded for deterministic results.
If no seed is given, the randomness is nondeterministic. (is this feature really necessary?)
Generating random values within the model has no side effects outside the model. That is, the randomness in other models is not affected.
Need to work with rollback and zero-crossing capability which requires that the sim be able to go back in time duing a hop to the exact previous state.
Checkpointing and playback should be possible. By this I mean that at a given time it shouldn't matter if you got there from feeding the model some intermediate state or if you started from the initial state. The numerics should be the same.

It seems to me that this is easy to get wrong if you aren't careful. Rather than implementing this ad hoc, is there some API, best practice and/or standard example to follow when implementing a model with randomness?

Is there a way to compute body inertia based on geometry and density?

Sun, 07 Jan 2018 19:36:16 +0000

For bodies with primitive shapes, we should be able to compute their inertia and mass properties using (uniform) density and geometry properties.

Set jointAxes of an instantiated BodyParam

Tue, 05 Dec 2017 22:10:00 +0000

How do I set the 'jointAxes' of a BodyParam object that has already been created? I can set the 'jointType', but the 'jointAxes' stays False.

Here's what I'm seeing

>>> test_param = BodyParam(
...     bodyToCM=[0., 0., 0.],
...     cmInertia=[[1., 0., 0.],
...                [0., 1., 0.],
...                [0., 0., 1.]],
...     jointType='FULL6DOF_INERTIAL',
...     mass=10000,
...     prescribed=True,
...     source=None)

>>> print(test_param)
BodyParam({
negativeIntegralSense : True,
          prescribed : True,
           jointAxes : False,
         jointLimits : False,
         bodyToJoint : [0.0, 0.0, 0.0],
            bodyToCM : [0.0, 0.0, 0.0],
     bodyToJointQuat : [0.0, 0.0, 0.0, 1.0],
          inbToJoint : [0.0, 0.0, 0.0],
      inbToJointQuat : [0.0, 0.0, 0.0, 1.0],
                mass : 10000,
           jointType : 'FULL6DOF_INERTIAL',
           cmInertia : [[1.0, 0.0, 0.0], [0.0, 1.0, 0.0], [0.0, 0.0, 1.0]],
})

>>> test_param['jointType'] = 'SLIDER'
>>> test_param['jointAxes'] = ([0, 0, 1], )

>>> print(test_param)
BodyParam({
negativeIntegralSense : True,
          prescribed : True,
           jointAxes : False,
         jointLimits : False,
         bodyToJoint : [0.0, 0.0, 0.0],
            bodyToCM : [0.0, 0.0, 0.0],
     bodyToJointQuat : [0.0, 0.0, 0.0, 1.0],
          inbToJoint : [0.0, 0.0, 0.0],
      inbToJointQuat : [0.0, 0.0, 0.0, 1.0],
                mass : 10000,
           jointType : 'SLIDER',
           cmInertia : [[1.0, 0.0, 0.0], [0.0, 1.0, 0.0], [0.0, 0.0, 1.0]],
})

How do I specify the initial attitude after the config is assembled?

Tue, 29 Aug 2017 23:31:12 +0000

I want to run a 6 dof simulation where I specifiy an initial angle of attack and side slip of 0 degrees. Technically I don't believe this can be done before the config is created because the relative velocity is required and is not available until after the config is created (unless I assume certain things about the config). How can I specifiy the attitude if I know the exact quaternion I want? I assume that I need to do it immediately after this command:

sim.createAssemblies(config)

Can I do it by changing parameters after the assemblies are created (i.e. getDVar('.Dshell.Assemblies.NSYT.param.Attitude.initAngleAttack')(myAOA)) ?

And if I do it that way, do I need to rebuild the config?

Is there a way to query SPICE file data directly using the DShell Framework?

Fri, 11 Aug 2017 22:01:51 +0000

If we're setting up a SPICE frame, is there a way to query other things from SPICE files like mass, radii, etc.?

-Erik

Does a model exist to implement a non-uniform gravity field on a body using spherical harmonics?

Fri, 11 Aug 2017 20:44:43 +0000

I need to implement a 2x2 gravity model for a SPICE Body Target. Specifically, something like

C(i, 0) == -J(i).
C(2, 0) = -1.938209808166424e-04;

C(2, 1) = 2.991514234204881e-08;
S(2, 1) = -1.648106326605655e-06;
C(2, 2) = 2.017047750717614e-04;
S(2, 2) = -3.795310702217140e-06;

What gravity model should be used, and how do I implement these parameters?

What support is available for doing inverse kinematics?

Wed, 24 May 2017 04:58:41 +0000

What does all the color coding on the Dshell gui signify?

Mon, 22 May 2017 13:39:24 +0000

For models, signals etc.

How does the configuration kinematics mode work in ROAMS?

Mon, 22 May 2017 13:35:28 +0000

Is it possible to attach a body with an existing parent body to another body?

Mon, 22 May 2017 13:33:48 +0000

Normally we attach individual bodies (or even bodies that are base bodies) to other bodies in the multibody model tp change the multibody's tree topology without any problems. Since bodies can have only one parent body (to meet the tree topology requirement), how do we go about attaching a body that already has a parent to another body? An example of this is from robotics where a manipulator may pick up an articulated task object (eg. pliers) where the body that it grasps is not the base body for the task object,

Can we change the basebody for a floating-base multibody system?

Mon, 22 May 2017 13:24:56 +0000

For a non-floating-base system, there is no ambiguity and the base body is the body attached to the multibody root. However, for a floating-base system, any body in priniciple can be the base body (connected to the multiboby root via a 6dof hinge). Having built the multibody tree with a specific base body choice, is it possible to change the model to use a different body as the base body?

Is there a way to force a specific Dshell model order for debugging purposes?

Mon, 22 May 2017 13:20:41 +0000

Normally Dshell's sorter determines the model calling order based on their inter-connections and inter-model breaks and constraints. However, there are times when an alternative model order needs to used such as during comparisons with older runs that used a different model order. Is there a way to overrides the sorter's model order with a custom one specified by the user?

Is there a way to disable a module build entirely based on a site config flag?

Mon, 22 May 2017 13:14:56 +0000

Sometimes a module is entirely dependent on the availability of another tool. When this tool is unavilable, the dependent module is unusable. Is there a simpler way than instrumenting the module's Makefile.yam to disable the module from builds based on a tool availability flag?

What do the various colors and arrows in the signals column in the Dshell gui assembly's page signify?

Mon, 22 May 2017 05:59:25 +0000

What is the meaning of the different colors and symbols on that page?

Is there a way to find out whether a DartsModel instance has implemented a method override?

Mon, 22 May 2017 05:57:25 +0000

There are mulitple stub implementations of preDeriv, updateFlowOuts etc methods available in the DartsModel base class. The derived DartsModels classes normally define override implementations of the specifc methods they need. Is there are way to find out which of the methods have been overriden at run-time (without having to look at the source code)?

How does one set the masked coordinates granularity for a DartsModel?

Mon, 22 May 2017 05:53:53 +0000

How does one set single and multiple granularity values?

How does one set the flow output granularity for a DartsModel?

Mon, 22 May 2017 05:52:28 +0000

How does one set single and multiple granularity levels?

Is there a way to get the files from svn for a merged branch?

Mon, 22 May 2017 05:51:01 +0000

Once an SVN branch created by pyam is merged via a "pyam sync" or "pyam save" command, it is no longer available in its original repo location. So how does one checkout the files on this branch if the need arises?

How can one fly a sensor along a specified trajectory?

Mon, 22 May 2017 01:41:48 +0000

Normally we use the simulation to propagate the system state over time. However there are times when a specific vehicle trajectory is provided along with sensor data. How can one use the simulation model to fly the vehicle on the provided trajectory to compute the associated sensor outputs?

What is the connection between masked and prescribed hinges?

Mon, 22 May 2017 01:39:06 +0000

How can I use the eclipse IDE with the software?

Mon, 22 May 2017 01:36:21 +0000

Is there a ROS interface available?

Mon, 22 May 2017 01:35:11 +0000

Is there a ROS interface available to connect the simulation with other external software?

Is there support for URDF mechanism models?

Mon, 22 May 2017 01:33:39 +0000

Can Ndarts import/export models in OSRF's URDF format?

How are the "spatial" vector notions here related to those in the Lie group differential geometry approaches?

Mon, 22 May 2017 01:31:59 +0000

Especially the Lie group notation and conventions used in the Murray, Li, Sastry book.

Are Denavit-Hartenberg (DH) frames supported?

Mon, 22 May 2017 01:29:16 +0000

What is the precise definition of spatial velocity used in the software?

Mon, 22 May 2017 01:22:25 +0000

There are different definitions of spatial velocity in the literature. Which definition is used in the software, and how does into relate to the other ones (espcecially in the Lie group differential geometry approaches (eg. Murray et al text).

How does one use the position navigation mode within ROAMS?

Tue, 14 Mar 2017 18:17:27 +0000