[Question] PD Gains being set to odd values for default Humanoid28 file

[AliBharwani]

I'm trying to follow the tutorial on modifying humanoid_env.py to work for a different robot, in my case Humanoid28.

I downloaded the USD file and the stiffness values are all in the range of 100-600, with damping values being one tenth of them for each drive. When observing the Humanoid28 in IsaacSim, by clicking under "Joints" I can see each of the joints has the proper stiffness / damping values from the USD file itself.

However, when looking at the output of _log_articulation_joint_info in articulation.py, I see some totally different, strange values:

Simulation parameters for joints in /World/envs/env_.*/Robot:
+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
|                                                              Simulation Joint Information (Prim path: /World/envs/env_.*/Robot)                                                              |
+-------+------------------+--------------+--------------------+----------------------+----------+-------------------------------------------+------------------------+------------------------+
| Index | Name             |  Stiffness   |      Damping       |       Armature       | Friction |              Position Limits              |    Velocity Limits     |     Effort Limits      |
+-------+------------------+--------------+--------------------+----------------------+----------+-------------------------------------------+------------------------+------------------------+
|   0   | abdomen_x        | 34377.46875  | 3437.746826171875  | 0.02500000037252903  |   0.0    | [-1.0471975803375244, 1.0471975803375244] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   1   | abdomen_y        | 34377.46875  | 3437.746826171875  | 0.02500000037252903  |   0.0    | [-1.0471975803375244, 1.5707963705062866] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   2   | abdomen_z        | 34377.46875  | 3437.746826171875  | 0.02500000037252903  |   0.0    | [-0.8726646304130554, 0.8726646304130554] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   3   | neck_x           | 2864.7890625 | 286.4789123535156  | 0.017000000923871994 |   0.0    | [-0.8726646304130554, 0.8726646304130554] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   4   | neck_y           | 2864.7890625 | 286.4789123535156  | 0.017000000923871994 |   0.0    | [-0.6981316804885864, 1.0471975803375244] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   5   | neck_z           | 2864.7890625 | 286.4789123535156  | 0.017000000923871994 |   0.0    | [-0.7853981852531433, 0.7853981852531433] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   6   | right_shoulder_x | 11459.15625  | 1145.9156494140625 | 0.019999999552965164 |   0.0    | [-3.1415927410125732, 0.7853981852531433] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   7   | right_shoulder_y | 11459.15625  | 1145.9156494140625 | 0.019999999552965164 |   0.0    | [-3.1415927410125732, 1.0471975803375244] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   8   | right_shoulder_z | 11459.15625  | 1145.9156494140625 | 0.019999999552965164 |   0.0    | [-1.5707963705062866, 1.5707963705062866] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   9   | left_shoulder_x  | 11459.15625  | 1145.9156494140625 | 0.019999999552965164 |   0.0    | [-0.7853981852531433, 3.1415927410125732] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   10  | left_shoulder_y  | 11459.15625  | 1145.9156494140625 | 0.019999999552965164 |   0.0    | [-3.1415927410125732, 1.0471975803375244] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   11  | left_shoulder_z  | 11459.15625  | 1145.9156494140625 | 0.019999999552965164 |   0.0    | [-1.5707963705062866, 1.5707963705062866] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   12  | right_hip_x      | 17188.734375 | 1718.8734130859375 | 0.019999999552965164 |   0.0    | [-1.0471975803375244, 0.2617993950843811] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   13  | right_hip_y      | 17188.734375 | 1718.8734130859375 | 0.019999999552965164 |   0.0    | [-2.4434609413146973, 1.0471975803375244] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   14  | right_hip_z      | 17188.734375 | 1718.8734130859375 | 0.019999999552965164 |   0.0    | [-1.0471975803375244, 0.6108652353286743] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   15  | left_hip_x       | 17188.734375 | 1718.8734130859375 | 0.019999999552965164 |   0.0    | [-0.2617993950843811, 1.0471975803375244] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   16  | left_hip_y       | 17188.734375 | 1718.8734130859375 | 0.019999999552965164 |   0.0    | [-2.4434609413146973, 1.0471975803375244] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   17  | left_hip_z       | 17188.734375 | 1718.8734130859375 | 0.019999999552965164 |   0.0    | [-0.6108652353286743, 1.0471975803375244] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   18  | right_elbow      |     0.0      |        0.0         | 0.014999999664723873 |   0.0    |         [-2.7925267219543457, 0.0]        | 5.939047009418473e+36  |          0.0           |
|   19  | left_elbow       |     0.0      |        0.0         | 0.014999999664723873 |   0.0    |         [-2.7925267219543457, 0.0]        | 5.939047009418473e+36  |          0.0           |
|   20  | right_knee       |     0.0      |        0.0         | 0.019999999552965164 |   0.0    |         [0.0, 2.7925267219543457]         | 5.939047009418473e+36  |          0.0           |
|   21  | left_knee        |     0.0      |        0.0         | 0.019999999552965164 |   0.0    |         [0.0, 2.7925267219543457]         | 5.939047009418473e+36  |          0.0           |
|   22  | right_ankle_x    | 11459.15625  | 1145.9156494140625 | 0.009999999776482582 |   0.0    | [-0.5235987901687622, 0.5235987901687622] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   23  | right_ankle_y    | 11459.15625  | 1145.9156494140625 | 0.009999999776482582 |   0.0    | [-0.9599310755729675, 0.9599310755729675] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   24  | right_ankle_z    | 11459.15625  | 1145.9156494140625 | 0.009999999776482582 |   0.0    | [-0.6981316804885864, 0.6981316804885864] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   25  | left_ankle_x     | 11459.15625  | 1145.9156494140625 | 0.009999999776482582 |   0.0    | [-0.5235987901687622, 0.5235987901687622] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   26  | left_ankle_y     | 11459.15625  | 1145.9156494140625 | 0.009999999776482582 |   0.0    | [-0.9599310755729675, 0.9599310755729675] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
|   27  | left_ankle_z     | 11459.15625  | 1145.9156494140625 | 0.009999999776482582 |   0.0    | [-0.6981316804885864, 0.6981316804885864] | 3.4028234663852886e+38 | 3.4028234663852886e+38 |
+-------+------------------+--------------+--------------------+----------------------+----------+-------------------------------------------+------------------------+------------------------+

Where are these values coming from? I think they're the ones that are being applied in the simulation, even though the "joints" for Humanoid28 have the correct values from the USD (see screenshot):

I did some more logging and checked the values coming in from the root_physx_view (on line 1298 in articulation.py), and it seems to already be these very high / strange PD gains. I don't understand where these values are coming from when the USD seems to specify totally different values.

[RandomOakForest]

Thanks for posting this. The discrepancy you notice may be related to the different units used in the Isaac Sim UI. See this doc for more details. I will move this post to our Discussions. Thanks.

[RandomOakForest]

Following up, let us know if we can be of further help. Thanks.

[Mayankm96]

Isaac Sim uses the units degrees and meters for angles and distances respectively. Meanwhile, PhysX internally uses radians for angles.

Since the stiffness and damping units depend on the unit chosen, what you set in the UI is related to degrees. The printed values are those from PhysX which are in radians.

So a stiffness of 600 "Nm/deg" corresponds to 34377.6 "Nm/rad". Hence what you see is the correct conversion.

We understand this is confusing that the UI and internal workings use different units but right now there isn't an easy fix for this from IsaacLab side.

@RandomOakForest --> We should assign this issue to IsaacSim/Kit teams to make this consistent. Noone really in robotics uses degrees 😓