Contact Forces and Contact Geometry

[Omar Sheikh]

I am working with an upper limb model and I have a list of Contact geometry, with the one of important to me being the ball. Under Contact Forces I can see that there is a contact force listed, but I"m not sure how to record data on the contact force. When I run data analysis the contact force seems to not be recorded.

I appreciate your help. Thanks!

[Jiang Ping]

Hi,

You are using OpenSim API or Analysis tool in GUI?

best regards,

Jiang

[Omar Sheikh]

Hi,
https://simtk.org/forums/posting.php?mo ... =91&p=9982
You are using OpenSim API or Analysis tool in GUI?

best regards,

Jiang

I believe that I'm using the Analysis tool. I'm using OpenSim itself (but I'm not sure if that answers your question). It's just that I cannot report a force on the contact object when I try to run an analysis.

[Ayman Habib]

Hi Omar,

If you're running the AnalyzeTool from the GUI then you can add a ForceReporter and it will record for each force in the model, the net forces produced by the force. Please consult the doc. page below on how to add ForceReporter from the set of available analyses.
http://simtk-confluence.stanford.edu:80 ... lysis+Tool

Hope this helps,
-Ayman

[Omar Sheikh]

I understand how to run a Force Reporter but the ForceReporter doesn't seem to report the Contact force. That's what I'm having difficulty with

[Ayman Habib]

Hi Omar,

The followup question is whether you observe contact being enforced when you run a simulation or not. If the former then it's a reporting issue, if the latter then you need to revisit how contact is defined. Please check the Tug-of-war model included with the distribution as it includes a contact-force. Generally you need to define both ContactGeometr(ies) and ContactForce(s) for the contact to take effect.

Best regards,
-Ayman

[Omar Sheikh]

Thank you for your help. I'm seeing that I have indeed not defined a contact force. I am looking at the Tug-of-War model right now in an XML editor but I"m struggling to understand how exactly contact forces are defined.

In the model I am working on a hand is grasping a ball (which is contact geometry, rather than a contact mesh) and I want to know the force of the hand on the ball when grasping. How would I defined that contact force?

[Omar Sheikh]

I'm still not quite sure how to code for a contact force that results from the fingers making contact with the ball. Furthermore, would this force just be in the ForceSet?

[Jennifer Hicks]

Omar -

There are a few models included in the OpenSim distribution that have contact geometry defined. You can take a look at the "BouncingBlock" model and the "ToyDropLandingModel" to see how contact is defined. Please let us know after looking at these models if you have remaining questions.

Best wishes,
Jen

[Omar Sheikh]

I realized after looking at the User's guide that the contact force was indeed defined in my code.

Below is the code for the contact force.

Code: Select all

				<ElasticFoundationForce>
					<!--Flag indicating whether the force is disabled or not. Disabled means that the force is not active in subsequent dynamics realizations.-->
					<isDisabled>false</isDisabled>
					<!--Material properties.-->
					<ElasticFoundationForce::ContactParametersSet name="contact_parameters">
						<objects>
							<ElasticFoundationForce::ContactParameters>
								<!--Names of geometry objects affected by these parameters.-->
								<geometry>cont2distph cont3distph cont4distph cont5distph ball</geometry>
								<stiffness>1500000</stiffness>
								<dissipation>0.1</dissipation>
								<static_friction>25</static_friction>
								<dynamic_friction>25</dynamic_friction>
								<viscous_friction>10</viscous_friction>
							</ElasticFoundationForce::ContactParameters>
						</objects>
						<groups />
					</ElasticFoundationForce::ContactParametersSet>
					<!--Slip velocity (creep) at which peak static friction occurs.-->
					<transition_velocity>10</transition_velocity>
				</ElasticFoundationForce>

Below is how the contact geometry is defined.

Code: Select all

		<ContactGeometrySet>
			<objects>
				<ContactMesh name="cont2distph">
					<!--Body name to connect the contact geometry to-->
					<body_name>2distph</body_name>
					<!--Location of geometry center in the body frame-->
					<location>-0.005 0 -0.002</location>
					<!--Orientation of geometry in the body frame-->
					<orientation>-1.7 1.5 0</orientation>
					<!--Display Pref. 0:Hide 1:Wire 3:Flat 4:Shaded-->
					<display_preference>4</display_preference>
					<!--Display Color-->
					<color>0 1 1</color>
					<filename>fingerPadScaled0310e-4.obj</filename>
				</ContactMesh>
				<ContactMesh name="cont3distph">
					<!--Body name to connect the contact geometry to-->
					<body_name>3distph</body_name>
					<!--Location of geometry center in the body frame-->
					<location>-0.006 0 -0.002</location>
					<!--Orientation of geometry in the body frame-->
					<orientation>-1.7 1.5 0.1</orientation>
					<!--Display Pref. 0:Hide 1:Wire 3:Flat 4:Shaded-->
					<display_preference>4</display_preference>
					<!--Display Color-->
					<color>0 1 1</color>
					<filename>fingerPadScaled0310e-4.obj</filename>
				</ContactMesh>
				<ContactMesh name="cont4distph">
					<!--Body name to connect the contact geometry to-->
					<body_name>4distph</body_name>
					<!--Location of geometry center in the body frame-->
					<location>-0.005 0 -0.003</location>
					<!--Orientation of geometry in the body frame-->
					<orientation>-2.9 1.9 1.2</orientation>
					<!--Display Pref. 0:Hide 1:Wire 3:Flat 4:Shaded-->
					<display_preference>4</display_preference>
					<!--Display Color-->
					<color>0 1 1</color>
					<filename>fingerPadScaled0310e-4.obj</filename>
				</ContactMesh>
				<ContactMesh name="cont5distph">
					<!--Body name to connect the contact geometry to-->
					<body_name>5distph</body_name>
					<!--Location of geometry center in the body frame-->
					<location>-0.005 -0.001 -0.004</location>
					<!--Orientation of geometry in the body frame-->
					<orientation>-3.2 2.1 1.55</orientation>
					<!--Display Pref. 0:Hide 1:Wire 3:Flat 4:Shaded-->
					<display_preference>4</display_preference>
					<!--Display Color-->
					<color>0 1 1</color>
					<filename>fingerpadL.obj</filename>
				</ContactMesh>
				<ContactSphere name="ball">
					<!--Body name to connect the contact geometry to-->
					<body_name>secondmc</body_name>
					<!--Location of geometry center in the body frame-->
					<location>0.004 -0.05 -0.06</location>
					<!--Orientation of geometry in the body frame-->
					<orientation>0 0 0</orientation>
					<!--Display Pref. 0:Hide 1:Wire 3:Flat 4:Shaded-->
					<display_preference>4</display_preference>
					<!--Display Color-->
					<color>0.200000002980232 0.200000002980232 1</color>
					<radius>0.0375</radius>
				</ContactSphere>
			</objects>
			<groups />
		</ContactGeometrySet>

I figured out how to measure contact forces but I'm not sure what forces are being measured. For example, I believe the ForceReporter lists the force exerted by the second metacarpal on the ball.

My goal is to measure the forces exerted by each individual distal phalanges on the ball but I'm not sure how to tell OpenSim to do that. The four distal phalanges corresponding to each finger each have a fingerpad. I want to be able to include the fingerpad in the contact with the ball.