2. Make a Bouncing Block#
In this tutorial, we will be making a bouncing block using OpenSim Creator:
![_images/tut2_constraints-added.png](_images/tut2_constraints-added.png)
Fig. 2.1 The model thatās made in this tutorial. It is a three-body system with a spring attached between the head to the foot. Contact geometries are used to make the model bounce on the floor, and constraints are used to prevent it from rolling around (download model
).#
In Make a Pendulum, we created a pendulum: one of the most basic physical systems that can be modelled. This tutorial reinforces concepts from that tutorial by building a slightly more complex model that contains collisions, forces, and constraints. This tutorial builds the model incrementally (i.e. from the ground up), so that we can explore each model-making decision one step at a time.
2.1. Prerequisites#
This tutorial assumes that you have already completed Make a Pendulum. Because of that, the content here will skip over some steps.
If you feel lost at any point, there should be partial solutions available along the way. Keep an eye out for the download model
links.
2.2. Topics Covered by this Tutorial#
Creating a slightly more advanced (than Make a Pendulum) model that also contains collisions, forces, and constraints.
Editing the collision surfaces/forces (orientations, radii, etc.).
How forces are used in OpenSim models. An example of how they can be used.
Why constraints are sometimes necessary in OpenSim models.
2.3. Create the Foot#
Because of how OpenSim computes a modelās spatial layout, the most straightforward way to develop a new OpenSim model is to start at whichever body will be directly attached to ground (e.g. foot
) followed by adding bodies that are, in turn, attached to that (e.g. knee
).
Starting from the āmiddleā (e.g. knee
), or a āleafā, of a modelās topology graph is more challenging because the position and orientation of those entities may change when they are attached to bodies that are (topographically) closer to ground.
Note
The body that is attached to ground does not need to be the body that is spatially closest to ground in the model. It just needs to be whichever body you think should be positioned relative to ground. In this tutorial, we are starting at the foot
, which happens to also be the body thatās closest to the ground. Conversely, OpenSim Creator contains an example model, bouncing_block.osim
, which is similar to the model we will build here, but built from the head
towards the foot
.
The reason that this tutorial builds this model from the foot
upwards is so that we can experiment with collisions earlier in the tutorial. If we built the model from the head
, we would have to wait until the knee
, foot
, and foot_collision
were added before we could experiment with collisions.
The first thing we need to add to our model is the foot
body. As explained in Make a Pendulum, all bodies (which are frames) need to be attached to other frames in the model and, ultimately, attached to ground. In our model, the foot
will be an (initially) freely-moving element in the scene, so we will directly attach it to the ground with a FreeJoint
.
Using similar steps to what was taught in Make a Pendulum:
Create a new model.
Add a body called
foot
into the model. It should have a mass of1 kg
(the default) and be joined toground
with aFreeJoint
calledground_to_foot
. Attach aSphere
geometry to it.Click the sphere in the 3D viewport and use the properties panel to change its
Appearance
property to a red color.
You can then raise foot
above the ground slightly by altering the ground_to_foot
jointās ty
coordinate:
Use the navigator panel to select the
ground_to_foot
joint within the modelāsjointset
Expand
ground_to_foot
in the navigator panel and select thety
coordinateUse the properties panel to change
ty
āsdefault_value
property from0.0
to0.5
This should produce a model with a red sphere (foot
) that is raised above the ground:
![_images/tut2_added-foot.png](_images/tut2_added-foot.png)
Fig. 2.2 The model after adding the foot
body and changing ground_to_foot
ās ty
to 0.5
(download model
)#
Note
Why do we change the ty
coordinate, rather than changing (e.g.) the translation
property of foot_offset
?
In Make a Pendulum we moved bodies around by altering the translation
property of offset frames. Here, we are changing the ty
coordinate. This exploits the fact that FreeJoint
s have alterable translation coordinates.
Both of these approaches for moving bodies around in the model have equivalent side-effects. However, coordinates have the added benefit of being user-editable. The official OpenSim GUI contains a coordinates
panel that lets users easily change coordinates. This enables users to (e.g.) later change ty
to make the model start higher off the ground.
2.4. Add Contact Surfaces & Forces#
If you simulate the model at this point, foot
will just fall through the floor. The reason this happens is because the chequered floor and red sphere geometry are only decorative: the foot
body is effectively a 0D point in space that has nothing to collide with.
In order to give foot
a āsizeā that can ācollideā with stuff, we need to explicitly add ContactGeometry
into the model at locations where we logically expect collisions to take place. In this case, we will add ContactGeometry
at the same location as the decorations.
To attach a collidable floor (a ContactHalfSpace
) to the ground of the model:
Open the
Add
menu from the main menu, or by right-clicking an empty part of the 3D sceneOpen the
Contact Geometry
sub-menu and clickContactHalfSpace
In the popup, create a
ContactHalfSpace
with the following properties:
![_images/tut2_floor-properties.png](_images/tut2_floor-properties.png)
Fig. 2.3 Properties for the floor_contact
component (a ContactHalfSpace
) attached to the ground. Careful that orientation
is set to -90
(degrees) in Z. +90
would behave differently, because a ContactHalfSpace
only collides on one side.#
To attach a collidable sphere (a ContactSphere
) to foot
:
Open the
Add
menu from the main menu, or by right-clicking an empty part of the 3D sceneOpen the
Contact Geometry
sub-menu and clickContactSphere
In the popup, create a
ContactSphere
with the following properties:
![_images/tut2_footcontact-properties.png](_images/tut2_footcontact-properties.png)
Fig. 2.4 Properties for the foot_contact
component (a ContactSphere
). The radius
is set to match the Sphere
decoration used on the foot
. The ContactSphere
is attached to foot
so that collisions it encounters affect foot
. Note: You can edit the radius
property of foot_contact
if you canāt see it in the scene. It may be hidden inside the foot
sphere (they have the same radius).#
With those two contact geometries added, the model now contains enough contact geometry to model the collisions we are interested in. However, if you try to simulate this model you will find that foot
still just falls through the floor š. Whatās going on?
In OpenSim, contact geometries only express a geometry that may participate in contact (collision) detection. They do not express the force that is generated whenever contact occurs. We need to separately add a suitable contact force (in this case, a HuntCrossleyForce
) into the model. The force handles what physically happens to the model (i.e. which forces are applied to model) whenever a collision occurs.
To add a contact force (HuntCrossleyForce
) to the model:
Open the
Add
menu from the main menu, or by right-clicking an empty part of the 3D sceneOpen the
Force
sub-menu and clickHuntCrossleyForce
In the popup, just click the
add
button (donāt edit anything)If it isnāt selected, select the force in the navigator panel (
forceset/HuntCrossleyForce
)Open the forceās context menu by right-clicking it in the navigator panel, or click by clicking the lightning (āactionsā) icon
Click
Add Contact Geometry
buttonUse that action to add
floor_contact
andfoot_contact
to the force. They should end up being listed inHuntCrossleyForce
ās properties
![_images/tut2_assigncontactgeometry.png](_images/tut2_assigncontactgeometry.png)
Fig. 2.5 The model after assigning floor_contact
and foot_contact
to HuntCrossleyForce
.#
With the contact force added, simulating this model should show foot
hit floor
, bounce a little, then stop. You can change the HuntCrossleyForce
ās properties to change how stiff the contact force is, how much energy is dissipated by the contact, etc.
![_images/tut2_collision-forces-added.png](_images/tut2_collision-forces-added.png)
Fig. 2.6 The model after adding floor_contact
, foot_contact
and a HuntCrossleyForce
. Simulating (Ctrl+R
) the model should show the foot
sphere fall through the scene until it collides with the surface. It should then bounce a little and stop (download model
).#
2.5. Attach the Knee & Head to the Foot#
The next step is to add a ākneeā and āheadā to our foot
. This mostly involves the body-addition steps that were previously explained in Make a Pendulum.
To add the knee
to the model:
Add a body called
knee
into the model. It should have a mass of1 kg
and be joined tofoot
with aPinJoint
calledfoot_to_knee
. Attach aSphere
geometry to it.Use the properties panel to change the
translation
property of thefoot_offset
(/jointset/foot_to_knee/foot_offset
) from(0, 0, 0)
to(0, 0.5, 0)
. This is so that thefoot
is offset from the origin of thefoot_to_knee
andknee
is co-located with it (i.e. it swings at the knee).Click on the new
knee_geom_1
sphere and use the properties panel to change theAppearance
property such that the sphere is red.
To add the head
to the model:
Add a body called
head
into the model. It should have a mass of1 kg
and be joined toknee
with aPinJoint
calledknee_to_head
. Attach aBrick
geometry to it.Use the properties panel to change the
translation
property of theknee_offset
(/jointset/knee_to_head/knee_offset
) from(0, 0, 0)
to(0, 0.5, 0)
. This is so that theknee
is offset from the origin ofknee_to_head
andhead
is co-located with it (i.e. it swings at the head).Click on the new
head_geom_1
cube and use the properties panel to change theAppearance
property such that the cube is red
These steps should create all the necessary bodies in the system, but it will look a little bit unusual (the ālinksā are missing):
![_images/tut2_bodies-added.png](_images/tut2_bodies-added.png)
Fig. 2.7 The model after adding the knee
and head
bodies. The bodies are joined to each over with PinJoint
s that pivot on the knee and head respectively. The model looks unusual because there are no visual ālinksā between the bodies and because the model isnāt angled yet (download model
).#
Much like at the end of Make a Pendulum, we can make the model look better by adding extra geometry between the bodies. This can be achieved by attaching decorative geometry to offset frames placed between the blocks.
To add a decorative link between the foot
and knee
:
Right-click the
foot
body (/bodyset/foot
) in the navigator panel to open its context menu.Open the
Add
menu and clickOffset Frame
Select the created offset frame (
/bodyset/foot/foot_offsetframe
)Use the properties panel to change the offset frameās
translation
property to(0.0, 0.25, 0.0)
. This makes the offset frame sit betweenfoot
andknee
.Right-click the offset frame in the navigator, or click the lightning icon (āActionsā) in the properies panel to open the offset frameās context menu
Open the
Add
menu and clickGeometry
. Attach aBrick
geometry to the offset frameSelect the brick through the navigator (
/bodyset/foot/foot_offsetframe/foot_offsetframe_geom_1
), or by clicking it in the 3D viewportUse the properties panel to edit the brickās
half_widths
property to something like(0.025, 0.25, 0.025)
. This creates a thin ārodā betweenfoot
andknee
![_images/tut2_after-adding-first-decorative-link.png](_images/tut2_after-adding-first-decorative-link.png)
Fig. 2.8 The model after adding the first decorative link. (download model
)#
To add a decorative link between the knee
and head
:
Right-click the
knee
body (/bodyset/knee
) in the navigator panel to open its context menuOpen the
Add
menu and clickOffset Frame
Select the created offset frame (
/bodyset/knee/knee_offsetframe
)Use the properties panel to change the offset frameās
translation
property to(0.0, 0.25, 0.0)
. This makes the offset frame sit betweenknee
andhead
.Right-click the offset frame in the navigator, or click the lightning icon (āActionsā) in the properies panel to open the offset frameās context menu
Open the
Add
menu and clickGeometry
. Attach aBrick
geometry to the offset frameSelect the brick through the navigator (
/bodyset/knee/knee_offsetframe/knee_offsetframe_geom_1
), or by clicking it in the 3D viewportUse the properties panel to edit the brickās
half_widths
property to something like(0.025, 0.25, 0.025)
. This creates a thin ārodā betweenknee
andhead
These steps add decorative features to the model that make it easier to see whatās going on. After doing them, you should have something that looks like this:
![_images/tut2_decorations-added.png](_images/tut2_decorations-added.png)
Fig. 2.9 The model after adding decorative bricks between the foot
and the knee
and between the knee
and the head
(download model
).#
If you try simulating this model, you will find that it falls vertically and remains mostly motionless. The reason why that happens is because all of the bodies in the model (foot
, knee
, and head
) are vertically aligned along Y.
To make the simulation more interesting, we are going to angle the whole model and also change the initial joint angle of foot_to_knee
to give the knee a ākinkā. To do this:
Angle the whole model by selecting
ground_to_foot
āsrz
coordinate (/jointset/ground_to_foot/rz
). Use the properties panel to change the coordinateāsdefault_value
property to0.698
.Give the knee a ākinkā by selecting
foot_to_knee
āsrz
coordinate (/jointset/foot_to_knee/rz
). Use the properties panel to change the coordinateāsdefault_value
property to-1.396
.
These steps should put the model into a more interesting arrangement:
![_images/tut2_angles-added.png](_images/tut2_angles-added.png)
Fig. 2.10 The model after altering the ground_to_foot
ās and foot_to_knee
ās rz
values. Altering those values puts the model into a more interesting arrangement (download model
).#
2.6. Add a Spring between foot
and head
#
We have now added all of the bodies and joints that make up the model. However, the only forces acting on the model are gravity and the foot collision. Consequently, a simulation of the model wonāt be very impressive. The model will fall a little, then foot
will collide with floor
, then the rest of the (non-colliding) model will roll around and clip through the floor.
The reason this model is unexciting is because there are no forces between the modelās bodies. We have attached three bodies (foot
, knee
, and head
) with two PinJoint
s and let them drop through space. The joints merely enforce constraints between the bodies. So the model acts like a passive hinged device that flops around.
We can add forces to this model to make it more interesting. Specifically, we will add a PointToPointSpring
between the foot
and head
, which will make the modelās head ābounceā away from the foot whenever it gets too close (e.g. when the model hits the floor).
To add a PointToPointSpring
between foot
and head
:
Open the
Add
menu from the main menu, or by right-clicking an empty part of the 3D sceneOpen the
Force
sub-menu and clickPointToPointSpring
In the popup menu, give the spring the following properties:
![_images/tut2_add-spring-popup.png](_images/tut2_add-spring-popup.png)
Fig. 2.11 Properties for a PointToPointSpring
between the foot
and the head
in the model. The springās stiffness
and rest_length
properties are chosen to try and make the model equilibrate towards the foot
being separated from the head
slightly (after a few bounces).#
If you simulate the model after adding the spring, you should see that the model hits the floor, collides, bounces a little bit, and then starts to roll:
![_images/tut2_spring-added.png](_images/tut2_spring-added.png)
Fig. 2.12 The model after adding a PointToPointSpring
between the foot
and the head
. The spring prevents the head
from clipping through the foot
and makes the simulation more interesting-looking. However, when simulating, the model bounces around a little bit and begins to roll around. This is because the model isnāt constrained along the vertical axis (download model
).#
2.7. Constrain foot
and head
to stay along Y#
The model is now logically completeāin the sense that it contains all of the mechanical components we wantābut it isnāt particularly stable. If you simulate the model, you will find that it bounces a little bit and then starts to roll around on its foot, rather than continuing to bounce up and down.
The reason this happens is because the model isnāt perfectly balanced. It is slightly heavier on knee-side, which causes the whole model to start leaning and, ultimately, roll around. One way to prevent this from happening is to add constraints into the model that prevent it from rolling.
One way to think of constraints is that they are extra ārulesā that the model must obey. When the model is assembled and simulated, the simulator has to ensure that each state of the simulation obeys the these constraints. Constraints are commonly used to simplify models in some (acceptable) way.
OpenSim has support for a few different constraints, such as:
Enforcing a constant distance between two frames in the model (
ConstantDistanceConstraint
)Enforcing that a frame āfollows alongā some other frame. E.g. that the frame is only allowed to be some distance along the Y axis of some other frame (
PointOnLineConstraint
)Enforcing that two frames must be at the same location and orientation (
WeldConstraint
)
For our model, we want to enforce that the foot
and head
are constrained to always be along the Y
axis (i.e. X = 0 && Z = 0
). This enforces that both foot
and head
fall and bounce vertically, which means that the foot_to_head_spring
is always bounced straight-on.
To constrain foot
to only be allowed to roll along the Y axis, follow these steps:
Open the
Add
menu from the main menu, or by right-clicking an empty part of the 3D sceneOpen the
Constraint
sub-menu and clickPointOnLineConstraint
In the popup menu, give the constraint the following properties:
![_images/tut2_foot-Y-lock-properties.png](_images/tut2_foot-Y-lock-properties.png)
Fig. 2.13 Properties for foot_Y_lock
. This constraint prevents the foot from being able to roll along the floor to a different X/Z coordinate in the scene by enforcing foot
to follow the (0.0, 1.0, 0.0)
line from ground
.#
After adding that constraint, you should find that foot
no longer rolls around the scene, but head
still freely swings around foot
as much as it can.
To constrain head
, such that is only follows along Y in ground
, follow these steps:
Open the
Add
menu from the main menu, or by right-clicking an empty part of the 3D sceneOpen the
Constraint
sub-menu and clickPointOnLineConstraint
In the popup menu, give the constraint the following properties:
![_images/tut2_head-y-lock-properties.png](_images/tut2_head-y-lock-properties.png)
Fig. 2.14 Properties for head_Y_lock
. This constraint prevents head
from being able to swing around the foot
wildly, which ensures that it always bounces towards foot
straight-on.#
With both of those constraints in place, the model now bounces up and down without rolling around. This is very useful for figuring out what tuning parameters (e.g. head
weight, foot_to_head_spring
stiffness) lead to an optimal bounce without having to also handle balancing.
![_images/tut2_constraints-added.png](_images/tut2_constraints-added.png)
Fig. 2.15 The final model after adding PointOnLineConstraint
s that make the head
and foot
bodies stay along the Y axis, rather than having the freedom to roll around (download model
).#
Note
Wait a second, did we just cheat š¤? A ārealā model wouldnāt have these invisible constraints, right?
Yes - technically. But using constraints in this way is a design choice. Design choices are dictated by realities and needs, rather than idealism. Do you need an ideal model, or will a simplified model adequately answer your question?
Take this model as an example. If your main objective is to figure out which angles, masses, and spring strains lead to a perfectly balanced model (e.g. because ideally optimizing the modelās balance is what you need), then you probably donāt want to use constraints because they are interrupting the main objective of your model.
Conversely, if your main objective is to roughly tune the modelās spring and body masses to get the right amount of vertical ābounceā, and you know that the modelās overall balance isnāt relevant (e.g. because you know the device is going to be bounced down a tube or linear rail), then you probably do want to use constraints because they will make it easier to focus on the main objective of your model.
Choosing the right constraints is even more important with complex models. Sure, you could try to perfectly balance a perfectly-modelled human on a perfectly-designed bicycle, and ensure that the various muscle controls etc. keep the bike balanced, but getting that right will probably take a very long time (assuming itās even possible, given the number of parameters involved). Conversely, you could model a rougher human model on a simpler bicycle model that is constrained to only roll along a 2D plane. That would take significantly less time to model and might be āgood enoughā to answer your research question.
The art of modelling is in figuring out which simplifications are suitable for your problem. Thereās a reason why physicists tend to model everything as a sphere - and frequently get away with it š.
2.8. (Optional) Extra Exercises#
Now that you have a working model, you can experiment a little bit by doing these extra exercises.
Experiment with the body masses and spring parameters. What happens if
head
is heavier? How does the springstiffness
affect how the model bounces? Can the floorās contact forces be modified to reduce how much of the drop force is dissipated each bounce? Can you make it bounce for longer?Try opening your model in the official OpenSim GUI. Save your model to an
.osim
and open it in the official OpenSim GUI. This should let you edit coordinates, plot things, etc. The official GUI has features that OpenSim Creator does not have. The benefit of using open file types (.osim
) is that you can use multiple tools with your model file.
2.9. Next Steps#
This tutorial was similar to Make a Pendulum, but it introduced some of the more practical parts of designing a model. Things like adding collision geometry, adding forces, and deciding on constraints. These are all important parts of the model-building process that come up repeatably when designing OpenSim models.
As you experienced in this tutorial, a time-consuming step of building an OpenSim model can be initially adding and placing the bodies/joints. The next tutorial, Use the Mesh Importer, introduces an alternative approach for performing these first steps.