E1 · The G1 tree
5 minRun the "list the joints" block and mark the 6 joints of one leg in the output.
Done when: the last line is RESUMO: 29 juntas articuladas + 1 base flutuante · nu=29 · bate com nu: sim and saída: 0.
Lesson 3 · 15 min
Degrees of freedom, joints, and limits
"23 to 43 degrees of freedom" is marketing; the joint tree in the file is the body your controller will command. In this lesson you list the G1's 29 joints with limits in degrees and maximum torque, understand why the leg has six, and check that the number matches the actuators.
Start the lesson
In this lesson you will
- 1The body tree: pelvis, legs, waist, and arms
- 2Why the leg has 6 joints and in what order
- 3Limits are in radians in the file, in degrees in your head
- 4Degrees of freedom in the file × on the manufacturer's spec sheet
Goal: list the G1's joints with limits in degrees and maximum torque, and check that the number of articulated joints matches nu.
01The 29 joints of the G1 from Menagerie
Each block is a branch of the tree that starts at the pelvis. Click to see the joints.
Click each block in the diagram to see what it does.
12 in the legs + 3 in the waist + 14 in the arms = 29 = nu.
02How to read a line of the tree
Click each step.
1. Name
left_hip_roll_joint: left leg, hip, roll rotation (tilting sideways). Pitch bends forward and backward; yaw turns around the vertical axis.
The indentation in the output shows what hangs from what: the ankle depends on the knee, which depends on the hip.
2. Limit
The file stores range="-0.087267 2.8798" in radians; the script converts it: -5° to 165° at the knee.
The hip roll is asymmetric and mirrored: left from -30° to 170°, right from -170° to 30°. The leg opens far outward and only a little inward, where the other leg is.
3. Maximum torque
The joint's actuatorfrcrange attribute caps what the motor can do: 139 N·m at the knee, 88 at the hip pitch, 50 at the ankle, 25 at the shoulder, 5 at the wrist pitch and yaw.
This is the model's limit. Resellers quote 90 N·m (base) and 120 N·m (EDU) for the real G1's knee (reported): one more point where the file and the robot can differ.
4. Check
The total number of joints (njnt=30) includes the free base. The number that matters to the controller is the number of articulated joints, which must match the number of actuators.
If they don't match, there is a passive joint (with no motor) or a motor outside a joint: both change your controller.
Last line of the script
RESUMO: 29 juntas articuladas + 1 base flutuante · nu=29 · bate com nu: sim
03The G1 tree
juntas.py reads everything from the MJCF and prints one line per joint.
Terminal · list the joints
F=${FTH:-$HOME/fth}; cd $F/anatomia && . $F/.venv/bin/activate
python3 kit/juntas.py unitree_g1/scene.xml; echo "saída: $?"Example from the repository
unitree_g1/scene.xml · 29 juntas articuladas
junta mín° máx° torque máx (N·m)
left_hip_pitch_joint -145 165 88
left_hip_roll_joint -30 170 139
left_hip_yaw_joint -158 158 88
left_knee_joint -5 165 139
left_ankle_pitch_joint -50 30 50
left_ankle_roll_joint -15 15 50
right_hip_pitch_joint -145 165 88
right_hip_roll_joint -170 30 139
right_hip_yaw_joint -158 158 88
right_knee_joint -5 165 139
right_ankle_pitch_joint -50 30 50
right_ankle_roll_joint -15 15 50
waist_yaw_joint -150 150 88
waist_roll_joint -30 30 50
waist_pitch_joint -30 30 50
left_shoulder_pitch_joint -177 153 25
left_shoulder_roll_joint -91 129 25
left_shoulder_yaw_joint -150 150 25
left_elbow_joint -60 120 25
left_wrist_roll_joint -113 113 25
left_wrist_pitch_joint -93 93 5
left_wrist_yaw_joint -93 93 5
right_shoulder_pitch_joint -177 153 25
right_shoulder_roll_joint -129 91 25
right_shoulder_yaw_joint -150 150 25
right_elbow_joint -60 120 25
right_wrist_roll_joint -113 113 25
right_wrist_pitch_joint -93 93 5
right_wrist_yaw_joint -93 93 5
RESUMO: 29 juntas articuladas + 1 base flutuante · nu=29 · bate com nu: sim
saída: 0abrir.py from lesson 2 prints juntas: 30 (1 base flutuante + 29 articuladas). If you use njnt=30 as the number of motors, you build a command vector with 30 slots and MuJoCo rejects it: ctrl has nu=29. Count only the articulated joints.
04Practice
Read the tree and draw conclusions from it.
E2 · The H1 tree
5 minRun the same script on the H1 and compare: how many joints does its leg have, and what is the highest declared torque?
Done when: python3 kit/juntas.py unitree_h1/scene.xml | tail -1 → RESUMO: 19 juntas articuladas + 1 base flutuante · nu=19 · bate com nu: sim.
E3 · The tightest joint
5 minFind the leg joint with the smallest range of motion on the G1 and explain why it is small.
Done when: python3 kit/juntas.py | grep ankle_roll shows -15 15 on both lines, and you wrote a sentence about why.
05Lesson quiz
The explanation appears after you answer.
Question 1 of 3
The Menagerie G1 has njnt=30 and nu=29. Where does the difference come from?skill
Correct
The free base is a joint (of type free) and has no motor. The 29 articulated joints have one actuator each.Not that one
The free base is a joint (of type free) and has no motor. The 29 articulated joints have one actuator each.Review: How to read a line of the treeQuestion 2 of 3
Why does a humanoid's leg usually have 6 joints?
Correct
Position (3) and orientation (3) of the foot call for six degrees of freedom. Three at the hip, one at the knee, and two at the ankle is the G1's arrangement and that of many others.Not that one
Position (3) and orientation (3) of the foot call for six degrees of freedom. Three at the hip, one at the knee, and two at the ankle is the G1's arrangement and that of many others.Review: The 29 joints of the G1 from MenagerieQuestion 3 of 3
Unitree's spec sheet says 1 waist joint; the Menagerie model has 3. How do you handle that?
Correct
The G1 has variants from 23 to 43 degrees of freedom. The model represents one of them; your controller has to match the version of the robot you'll use.Not that one
The G1 has variants from 23 to 43 degrees of freedom. The model represents one of them; your controller has to match the version of the robot you'll use.Review: The 29 joints of the G1 from MenagerieQuiz 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
TreeFree pelvis, 6 per leg, 3 at the waist, 7 per arm on the Menagerie G1.
LimitsRange in radians and torque in actuatorfrcrange, read from the file.
CheckArticulated joints = nu; the base doesn't count.