Investigate weird oscillation in main

[arjo129]

No description provided.

[chapulina]

Are you referring to this?

I've been looking into our collision shapes (which affect buoyancy), and I noticed that their shapes and placements are a bit unusual:

Looking at #5, it looks like these shapes have been manually picked to provide stability. In fact, I tried disabling buoyancy for just one of the fins, and that makes a huge difference for stability:

diff --git a/lrauv_ignition_plugins/worlds/buoyant_tethys.sdf b/lrauv_ignition_plugins/worlds/buoyant_tethys.sdf
index e3bc998..09a1773 100644
--- a/lrauv_ignition_plugins/worlds/buoyant_tethys.sdf
+++ b/lrauv_ignition_plugins/worlds/buoyant_tethys.sdf
@@ -29,6 +29,11 @@
     <plugin
       filename="ignition-gazebo-buoyancy-system"
       name="ignition::gazebo::systems::Buoyancy">
+      <enable>tethys::base_link</enable>
+      <!--enable>tethys::horizontal_fins</enable-->
+      <enable>tethys::vertical_fins</enable>
+      <enable>tethys::propeller</enable>
+      <enable>tethys::buoyancy_engine</enable>
       <!-- TODO: fix behaviour near surface, see https://github.com/osrf/lrauv/issues/102
         <graded_buoyancy>
         <default_density>1000</default_density>

Should we revisit the collisions now that we have more forces involved? My first suggestion would be to reduce complexity by removing all collisions except for main_body_buoyancy. I don't think we're interested in the collisions being actually used to calculate contact, right?

[arjo129]

The collisions determine the buoyancy (This is why I was advocating for a separate <buoyancy> tag in sdformat...). Removing any of the collisions will destabilize the vehicle as the weight of the fins will create a torque which will not be counteracted by the main body buoyancy (I've been here before).

[chapulina]

the weight of the fins will create a torque which will not be counteracted by the main body buoyancy

So you mean that the volume of each part has been fine-tuned so that its buoyancy offsets its weight? Or at least that this is what we're going for? If so, I'd like to write some tests and documentation to make sure we keep them this way.

[arjo129]

Yes that is correct.

…

On Sat, 30 Oct 2021, 04:34 Louise Poubel, ***@***.***> wrote: the weight of the fins will create a torque which will not be counteracted by the main body buoyancy So you mean that the volume of each part has been fine-tuned so that its buoyancy offsets its weight? Or at least that this is what we're going for? If so, I'd like to write some tests and documentation to make sure we keep them this way. — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#138 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAEEMQGWKOQVDVM746XNXMDUJMAL5ANCNFSM5GBAASBQ> . Triage notifications on the go with GitHub Mobile for iOS <https://apps.apple.com/app/apple-store/id1477376905?ct=notification-email&mt=8&pt=524675> or Android <https://play.google.com/store/apps/details?id=com.github.android&referrer=utm_campaign%3Dnotification-email%26utm_medium%3Demail%26utm_source%3Dgithub>.

[arjo129]

To add to that I just reallized one complication when writing the tests is that the base link and battery are unique in this case. The baselink should have the volume of the main hull whereas the battery link should have no volume as it contributes no buoyancy.

…

On Sat, 30 Oct 2021, 04:48 Arjo Chakravarty, ***@***.***> wrote: Yes that is correct. On Sat, 30 Oct 2021, 04:34 Louise Poubel, ***@***.***> wrote: > the weight of the fins will create a torque which will not be > counteracted by the main body buoyancy > > So you mean that the volume of each part has been fine-tuned so that its > buoyancy offsets its weight? Or at least that this is what we're going for? > If so, I'd like to write some tests and documentation to make sure we keep > them this way. > > — > You are receiving this because you authored the thread. > Reply to this email directly, view it on GitHub > <#138 (comment)>, or > unsubscribe > <https://github.com/notifications/unsubscribe-auth/AAEEMQGWKOQVDVM746XNXMDUJMAL5ANCNFSM5GBAASBQ> > . > Triage notifications on the go with GitHub Mobile for iOS > <https://apps.apple.com/app/apple-store/id1477376905?ct=notification-email&mt=8&pt=524675> > or Android > <https://play.google.com/store/apps/details?id=com.github.android&referrer=utm_campaign%3Dnotification-email%26utm_medium%3Demail%26utm_source%3Dgithub>. > >

[chapulina]

the volume of each part has been fine-tuned so that its buoyancy offsets its weight?

I started poking around and I think it may not be that simple. I believe the vehicle should be statically stable with:

• ⬆️ forces: buoyancy on all parts + neutral buoyancy engine
• ⬇️ forces: weight of all parts, including drop-weight

If that's correct, I'd expect the vehicle to be stable (completely still?) if I remove the following plugins (which I believe only apply when the vehicle is in motion):

• joint controllers
• thruster
• lift-drag
• hydrodynamics

and all that's left are

• buoyancy
• buoyancy engine
• detachable joint (holding drop weight)

I'd expect the vehicle to be stable in this configuration, but it currently sinks.

[braanan]

@chapulina @arjo129 Does this jittery oscillation also happen at depth or is it just on the surface? Is the buoyancy plugin accounting for partially submerged volumes etc? Thanks

[chapulina]

Does this jittery oscillation also happen at depth or is it just on the surface? Is the buoyancy plugin accounting for partially submerged volumes etc?

We have uniform buoyancy everywhere right now, no distinction between water and air until #38 is addressed. So the depth shouldn't make a difference here.