E1 · The G1 at rest
3 minRun the block and check the last three lines.
Done when: python3 kit/sensores.py ends with soma dos contatos / peso do robô (327 N): 1.00.
Lesson 5 · 15 min
Feeling its own body: IMU, encoders, force and touch
A standing humanoid is always falling a little and correcting itself. To do that, it needs to know how it is tilted, where each joint is, and how much weight is on each foot. In this lesson you read the sensors the G1 MJCF declares, add the ones that are missing without editing the file, and check the reading against the robot's weight.
Start the lesson
In this lesson you will
- 1Proprioception × perception: what each sensor answers
- 2Why the accelerometer at rest shows 9.8
- 3Add a sensor with MjSpec, without touching the Menagerie file
- 4Ankle force × ground force: the difference is the foot
Goal: read the IMU and the force on each foot of the standing G1 and check that the sum of the ground contacts matches the robot's weight.
01What the humanoid feels
The first four answer "how is my body"; the last one, "what is around me". Click each one.
Click each block in the diagram to see what it does.
Sensors that the MJCF does not declare do not exist in the simulation until you add them.
02Four readings to get right
Click each one.
1. 9.8 at rest
The accelerometer measures the force holding the sensor up, not the motion: at rest on the floor, it reads 9.8 m/s² upward. In free fall, it would read zero.
This is how the IMU knows which way is "down" and, from that, the tilt.
2. Add
sensores.py opens the file with mujoco.MjSpec.from_file, adds an orientation sensor (framequat) on the pelvis IMU and a force sensor on each foot, and compiles.
The Menagerie file stays untouched: you change the robot in your own script, versioned together with your code.
3. 158 × 164 N
The force sensor at the foot site measures what passes from the foot to the leg; the contact measures what the floor pushes back. The difference, about 6 N, is the weight of the foot itself (0.61 kg in the model).
On a real robot, the load cell sits at the ankle for the same reason.
4. Sum = weight
At rest, the sum of the contact forces must equal the weight: 33.3 kg × 9.81 = 327 N. The 1.00 ratio shows the robot is supported only by its feet and with no push.
If it comes out below 1, something is holding the robot up (another part touching the floor, an external force).
Line that checks out
soma dos contatos / peso do robô (327 N): 1.00
03Read and add sensors
The excerpt shows how the script adds the three sensors; the block runs for 1 s with the G1 at rest.
Excerpt from kit/sensores.py
spec = mujoco.MjSpec.from_file(str(fth.caminho(MODELO)))
spec.add_sensor(name="orientacao_pelve", type=mujoco.mjtSensor.mjSENS_FRAMEQUAT,
objtype=mujoco.mjtObj.mjOBJ_SITE, objname="imu_in_pelvis")
for lado, sitio in (("esq", "left_foot"), ("dir", "right_foot")):
spec.add_sensor(name=f"forca_pe_{lado}", type=mujoco.mjtSensor.mjSENS_FORCE,
objtype=mujoco.mjtObj.mjOBJ_SITE, objname=sitio)
m = spec.compile()Terminal · G1 at rest for 1 s
F=${FTH:-$HOME/fth}; cd $F/anatomia && . $F/.venv/bin/activate
python3 kit/sensores.py --segundos 1Example from the repository
G1 parado 1 s · 7 sensores (4 do arquivo + 3 acrescentados) imu-torso-angular-velocity [0. 0. 0.] imu-torso-linear-acceleration [0. 0. 9.8] imu-pelvis-angular-velocity [0. 0. 0.] imu-pelvis-linear-acceleration [0. 0. 9.8] IMU (pelve): inclinação lateral 0.0° · frente/trás -0.1° sensor de força no tornozelo: pé esquerdo 158 N · pé direito 157 N contatos com o chão: pé esquerdo 164 N · pé direito 163 N soma dos contatos / peso do robô (327 N): 1.00
Gyroscopes at zero, accelerometers at 9.8 upward, tilt near zero, and the weight split between the two feet.
Reading [0. 0. 9.8] and concluding that the robot is moving up is the classic mistake. The accelerometer measures specific force: at rest, it shows the floor's reaction against gravity. To see real motion, compare with the gyroscope and with the pelvis height.
04Practice
Read, check, and add a sensor of your own.
E2 · Longer at rest
3 minRun with --segundos 3. If the robot were slipping or sinking, the ratio would change.
Done when: python3 kit/sensores.py --segundos 3 | tail -1 → soma dos contatos / peso do robô (327 N): 1.00.
E3 · A sensor of your own
10 minCopy kit/sensores.py to kit/meus_sensores.py (in the same folder, because it imports fth.py) and add a framequat on the imu_in_torso site, with the venv active.
Done when: cd ~/fth/anatomia && python3 kit/meus_sensores.py | head -1 → G1 parado 1 s · 8 sensores (4 do arquivo + 4 acrescentados).
05Lesson quiz
The explanation appears after you answer.
Question 1 of 3
The pelvis accelerometer of the resting G1 reads [0, 0, 9.8]. What does that mean?skill
Correct
An accelerometer measures specific force. At rest, it reads gravity pointing up; in free fall, it would read zero. The IMU gets its direction of "down" from it.Not that one
An accelerometer measures specific force. At rest, it reads gravity pointing up; in free fall, it would read zero. The IMU gets its direction of "down" from it.Review: Four readings to get rightQuestion 2 of 3
The foot force sensor read 158 N and the ground contact read 164 N. Why the difference?
Correct
The foot site measures what passes from the foot to the leg. The foot (0.61 kg in the model) weighs about 6 N, and that part only shows up in the contact.Not that one
The foot site measures what passes from the foot to the leg. The foot (0.61 kg in the model) weighs about 6 N, and that part only shows up in the contact.Review: Four readings to get rightQuestion 3 of 3
Why add sensors through MjSpec instead of editing Menagerie's g1.xml?
Correct
Editing someone else's file breaks on the next update and disappears from your Git history. Building the model in the script makes it clear what you changed.Not that one
Editing someone else's file breaks on the next update and disappears from your Git history. Building the model in the script makes it clear what you changed.Review: Read and add sensorsQuiz complete
You got {a} of {n} right. Get them all right and the lesson is marked as complete.Pick an option. The explanation shows up after you answer.
06In three sentences
BodyIMU, encoders, and force tell you how the body is doing; camera and LiDAR tell you what's around it.
APIA missing sensor goes in through MjSpec, without editing Menagerie.
CheckAt rest, the sum of the contacts is the weight: 327 N on the G1.