How to detect a self collision?

[Robokan]

I have a Go1 with self collision enabled. I want to differentiate between collisions with the ground and collisions with itself in order to punish it with a negative reward or early termination during training. Ideally I would like to know the magnitude of the collisions as well. Anyone know how to do it?

[shaoxiang]

You can try using contact sensor to detect different contact collisions and measure collision forces.
The tutorial is very simple, you need to repeatedly review the examples:
https://isaac-sim.github.io/IsaacLab/source/tutorials/04_sensors/add_sensors_on_robot.html

Welcome to join QQ isaac-sim group for communication: 723139415

[Robokan]

Thanks for the link. This is what I have initially tried and wasn't able to detect any collisions.

I used this config.
self_collision: ContactSensorCfg = ContactSensorCfg( prim_path="/World/envs/env_.*/Robot/.*_calf", history_length=3, update_period=0.005, track_air_time=False, filter_prim_paths_expr=["/World/envs/env_.*/Robot/.*_calf"] )
in my code:

self_collision_forces = self.self_collision.data.force_matrix_w self_collision_net_forces = self.self_collision.data.net_forces_w_history if (torch.any(torch.abs(self_collision_forces) > 0.0)): print("self_collision_forces=",self_collision_forces) if (torch.any(torch.abs(self_collision_net_forces) > 0.0)): print("self_collision_forces=",self_collision_net_forces)
Neither print statements happen even when there are self collisions between the "calf" bodies on the Go1.

[Mayankm96]

Is the sensor made part of the interactive scene? Did you add it to the scene or ensure that you call sensor.update(dt)?

[shaoxiang]

I have also encountered situations where contact sensor data cannot be read or data is all 0. It took me two days to solve this problem.
You can refer to it: #571
Be sure to instantiate ContactSensor in _setup_scene and pass it to self.scene.sensors["contact_sensor"]

[Robokan]

I use:
self.self_collision = ContactSensor(self.cfg.self_collision)
self.scene.sensors["self_collision"] = self.self_collision
in my _setup_scene

and
self.scene.update(dt=self.physics_dt)
before I check:
self.self_collision.data.force_matrix_w

which always contains zeros.

Interestingly the force_matrix_w has a shape of (num_envs, 4, 1, 3). If there are 4 calf on the Go1 why isn't this matrix (num_env,4,4,3)?

From what I can tell looking at contact_sensor.py line: 260
self.contact_physx_view decides it has only one filter because filter_prim_paths_expr=["/World/envs/env./Robot/._calf"] in my config is just one entry even though it should resolve to 4 filtered bodies. Then on line: 296 it uses that 1 filter to create self._data.force_matrix_w. The result is the shape: (num_envs, num_bodies, num_filters, 3). If we are looking at collisions between 4 bodies and 4 filtered bodies then num_filters should be 4 not 1 shouldn't it?

Alternatively if I look at net_forces_w I do get forces when 'calf' collide with the other 'calf' but I also get collisions with the ground which is not in the filter.