Framelinvel sensor and global velocity #2582

Hardwarize · 2025-04-17T07:12:59Z

Hardwarize
Apr 17, 2025

Intro

Hi!

I am a master degree student at UNED, I use MuJoCo for my research on human motion synthesis.

My setup

MuJoCo python

My question

Hi,
I'm trying to get the global velocity (velocity eith regarding global frame) for a given body.

Which one is the best way to achieve this: using mj_objectVelocity or via sensors?

Trying to look for the answer by myself I've found a situation that i can not explain and I'm not sure if I have missunderstood the way that sensors works.

The model that I've used to test the situation is an arm, with three bodies:

body0 attached to the world with a slide joint that can slide in X direction
body1 attached to body0 (no joint)
fingertip (another body) attached to body1

When I apply a force to the slide joint the entire arm move to positive X, then I measure the velocity in body0 and fingertip and both give me the same value using mj_objectVelocity for Xbody (this is expected because the are attached).

The problem is that I've also attached a sensor in fingertip, and when I read the value I see a sightly different value. Why is this? Have I misunderstood how sensor works?

I know that mj_objectVelocity give velocity in body frame and sensor, by default, in global frame, but in this situation both frames are parallel.

I've set a very small step time in simulation to make sure that this is not the problem, but not sure neither if this is relevant.

Thanks for the help

Minimal model and/or code that explain my question

import mujoco

model_xml = """
<mujoco model="reacher">
	<compiler angle="radian" inertiafromgeom="true"/>
 <visual>
  <global offwidth="800"/>
</visual>
	<default>
		<joint armature="1" damping="1" limited="true"/>
		<geom contype="0" friction="1 0.1 0.1" rgba="0.7 0.7 0 1"/>
	</default>
	<option gravity="0 0 0" integrator="RK4" timestep="0.00001"/>
	<worldbody>
   		<camera name="fixed" pos="0 0.1 0.35" xyaxes="1 0 0 0 1 0"/>
     
     	<!-- Arena -->
		<geom conaffinity="0" contype="0" name="ground" pos="0 0 0" rgba="0.9 0.9 0.9 1" size="1 1 10" type="plane"/>

		<!-- Global Axis Red: X, Green: Y, Blue: Z -->
		<geom conaffinity="0" fromto="0 0 0 0.07 0 0" name="x_axis" rgba="1 0 0 1" size=".002" type="capsule"/>
		<geom conaffinity="0" fromto="0 0 0 0 0.07 0" name="y_axis" rgba="0 1 0 1" size=".002" type="capsule"/>
		<geom conaffinity="0" fromto="0 0 0 0 0 0.07" name="z_axis" rgba="0 0 1 1" size=".002" type="capsule"/>

		<body name="body0" pos="0 0 .01">
			<geom fromto="0 0 0 0.1 0 0" name="link0" rgba="0.0 0.4 0.6 1" size=".01" type="capsule"/>
			<joint axis="1 0 0" limited="false" name="joint0_slide" pos="0 0 0" type="slide"/>
			<body name="body1" pos="0.1 0 0">
				
				<geom fromto="0 0 0 0.1 0 0" name="link1" rgba="0.0 0.4 0.6 1" size=".01" type="capsule"/>
				<body name="fingertip" pos="0.11 0 0">
					<geom contype="0" name="fingertip" pos="0 0 0" rgba="0.0 0.8 0.6 1" size=".01" type="sphere"/>
				</body>
			</body>
		</body>
	</worldbody>
	<actuator>
  		<motor gear="200.0" joint="joint0_slide"/>
	</actuator>
 
 <sensor>
  <framelinvel objname="fingertip" objtype="xbody"/>
</sensor>
</mujoco>
"""

model = mujoco.MjModel.from_xml_string(model_xml)
data = mujoco.MjData(model)

data.ctrl[0] = 1

scene_option = mujoco.MjvOption()
scene_option.frame = mujoco.mjtFrame.mjFRAME_BODY
scene_option.flags[mujoco.mjtVisFlag.mjVIS_JOINT] = True
scene_option.flags[mujoco.mjtVisFlag.mjVIS_TRANSPARENT] = True

for step in range(3000):
    mujoco.mj_step(model, data)
    

with mujoco.Renderer(model, width=800, height=400) as renderer:    
    renderer.update_scene(data, camera="fixed", scene_option=scene_option)
    media.show_image(renderer.render())

####### Velocity in body0 #######
vel_body0 = np.zeros(6)
mujoco.mj_objectVelocity(model, data, mujoco.mjtObj.mjOBJ_XBODY, 1, vel_body0, False)
print(vel_body0[3:]) 
#  [5.50222491 0.         0.        ]

####### Velocity in fingertip #######
vel_fingertip = np.zeros(6)
mujoco.mj_objectVelocity(model, data, mujoco.mjtObj.mjOBJ_XBODY, 3, vel_fingertip, False)
print(vel_fingertip[3:])
#  [5.50222491 0.         0.        ]

####### Sensor measured velocity in fingertip #######
print(data.sensordata)
# array([5.50041629, 0.        , 0.        ])

Confirmations

I searched the latest documentation thoroughly before posting.
I searched previous Issues and Discussions, I am certain this has not been raised before.

Answered by v-r-a

Apr 17, 2025

Hi,

I am a MuJoCo user. I hope this helps.

Please insert mujoco.mj_forward(model, data) before print(data.sensordata) and you will see that the values match exactly. This is because the sensor value is stale by one step of integration. Please have a look at the simulation pipeline, especially this part.

The function mj_step() stops after advancing the simulation by one time step. The new sensor values corresponding to the advanced state are yet to be computed. Do the forward dynamic calculations using mj_forward(), and everything is up to date.

View full answer

v-r-a · 2025-04-17T18:12:56Z

v-r-a
Apr 17, 2025

Hi,

I am a MuJoCo user. I hope this helps.

Please insert mujoco.mj_forward(model, data) before print(data.sensordata) and you will see that the values match exactly. This is because the sensor value is stale by one step of integration. Please have a look at the simulation pipeline, especially this part.

The function mj_step() stops after advancing the simulation by one time step. The new sensor values corresponding to the advanced state are yet to be computed. Do the forward dynamic calculations using mj_forward(), and everything is up to date.

0 replies

Hardwarize · 2025-04-17T18:17:51Z

Hardwarize
Apr 17, 2025
Author

Hi @v-r-a ,
thanks a lot, that is exactly what was happening.

0 replies

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Framelinvel sensor and global velocity #2582

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Framelinvel sensor and global velocity #2582

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Hardwarize Apr 17, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 2 comments

Uh oh!

v-r-a Apr 17, 2025

Uh oh!

Hardwarize Apr 17, 2025 Author

Hardwarize
Apr 17, 2025

v-r-a
Apr 17, 2025

Hardwarize
Apr 17, 2025
Author