E1 · Ramp with watchdog
5 minRun the ramp block and find in the output the moment the watchdog acts.
Done when: python3 kit/rampa.py; echo "saída: $?" → juntas que pediram acima do limite: 0, watchdog: amortecimento ativado and saída: 0.
Lesson 4 · 20 min
Send low-level commands without breaking anything
Sending a target to 29 motors is easy; the hard part is what happens when the target is far away, out of range, or stops arriving. In this lesson you command the suspended G1 with three protections (MJCF limit, ramp, and watchdog) and see, in the same script, the step ask for more torque than the motor has.
Start the lesson
In this lesson you will
- 1What goes in a MotorCmd_: mode, q, dq, tau, kp, kd
- 2Ramp × step: the torque each one asks for
- 3Watchdog: a stale command becomes damping
- 4CRC and mode_machine, which the real robot checks
Goal: command the G1's 29 joints with a limit, a ramp, and a watchdog, and see the step command ask for torque above the limit.
01Three protections between the policy and the motor
Click each block.
Click each block in the diagram to see what it does.
The kit's watchdog is yours, in your controller. What the robot's firmware does with no command is not documented in the SDK: don't count on it. And it runs in the same process as the sender: if the process dies, what holds the robot is the physical stop and the gantry.
02What goes in a command
Click each part.
1. MotorCmd_
mode (1 enables, 0 disables), q (target), dq (target velocity), tau (added torque), kp and kd. There is no Ki and no global "position" or "torque" mode.
Stiff = high kp; torque only = kp and kd zero with tau filled in; damping = kp zero and a small kd.
Gains from the official example
Legs 60/60/60/100/40/40, waist 60/40/40, arms 40; kd 1 (knee 2). A starting point from the example, not a "robot default".
2. Header
mode_pr = 0 (PR: pitch/roll in series), mode_machine copied from the first LowState_, and crc = CRC().Crc(cmd) before publishing.
The Python simulator does not check the CRC and sends mode_machine = 0; the kit fills in both anyway, like the official example, so the code works the same on the robot.
3. 2 ms loop
The official G1 example sends every 2 ms (control_dt_ = 0.002). The policy can be slower (50 Hz): the fast loop resends the last target with the limits.
This is the loop where the watchdog lives: it sees when the target gets old.
03Ramp, limits and watchdog on the suspended G1
6 s of command: a 3 s ramp to the working pose (knees and elbows bent), hold, and at t = 4.5 s the "policy" freezes on purpose.
Terminal · simulator + ramp
F=${FTH:-$HOME/fth}; cd $F/operar && . $F/.venv/bin/activate
python3 kit/sim.py --s 9 --suspenso &
python3 kit/rampa.py; echo "saída: $?"
waitExcerpt · kit/rampa.py
idade = t_ciclo - comp["carimbo"]
if not amortecendo and idade > IDADE_MAX:
amortecendo = True
print(f"política sem resposta desde t = {a.trava_em:.1f} s")
print(f"watchdog: comando velho há mais de {IDADE_MAX * 1000:.0f} ms → amortecimento ativado (kp=0, kd={KD_AMORTECER:g})")
cmd.mode_pr, cmd.mode_machine = 0, modo
for i in range(op.N):
m_ = cmd.motor_cmd[i]
m_.mode, m_.dq, m_.tau = 1, 0.0, 0.0
if amortecendo:
m_.q, m_.kp, m_.kd = 0.0, 0.0, KD_AMORTECER
else:
m_.q, m_.kp, m_.kd = comp["alvo"][i], op.KP[i], op.KD[i]Terminal · the common mistake: step
F=${FTH:-$HOME/fth}; cd $F/operar && . $F/.venv/bin/activate
python3 kit/sim.py --s 9 --suspenso &
python3 kit/rampa.py --sem-rampa; echo "saída: $?"
waitExample from the repository
simulador: G1 29 motores · DDS domínio 1 em lo · 9 s · suspenso: sim estado recebido: 29 juntas · pose inicial lida · mode_machine = 0 limites: 29/29 alvos já dentro da faixa do MJCF (margem 0.05 rad), 0 recortados política sem resposta desde t = 4.5 s watchdog: comando velho há mais de 100 ms → amortecimento ativado (kp=0, kd=2) rampa de 3 s · todas as juntas pediram menos de 50 % do torque máximo juntas que pediram acima do limite: 0 ciclo de envio: 2 ms · watchdog: amortecimento ativado saída: 0 simulador: fim em t = 9.0 s · robô suspenso (pés fora do chão)
With a ramp, every joint stays below half the ceiling. The policy freezes, the watchdog sees the command get old and damps.
simulador: G1 29 motores · DDS domínio 1 em lo · 9 s · suspenso: sim estado recebido: 29 juntas · pose inicial lida · mode_machine = 0 limites: 29/29 alvos já dentro da faixa do MJCF (margem 0.05 rad), 0 recortados política sem resposta desde t = 4.5 s watchdog: comando velho há mais de 100 ms → amortecimento ativado (kp=0, kd=2) degrau · junta mais perto do limite: left_wrist_pitch (9 de 5 N·m, ≈ 170 %) juntas que pediram acima do limite: 4 ciclo de envio: 2 ms · watchdog: amortecimento ativado saída: 1 simulador: fim em t = 9.0 s · robô suspenso (pés fora do chão)
The same target all at once: four joints ask for more than the ceiling, and the wrist asks for almost double what it has. On the real robot this means a jolt and a saturated motor; the script exits with code 1.
04Practice
The three protections, measured.
E2 · Watch the step fail
5 minRun the common-mistake block and say which joint went furthest past the limit and why that one.
Done when: python3 kit/rampa.py --sem-rampa; echo "saída: $?" → juntas que pediram acima do limite: 4 and saída: 1.
E3 · Read the limit in the MJCF
5 minFind in the simulator's MJCF the torque ceiling of the wrist that blew past it in the step.
Done when: grep -c 'ctrlrange="-5 5"' ~/fth/unitree_mujoco/unitree_robots/g1/g1_29dof.xml → 4 (pitch and yaw of both wrists, 5 N·m cap).
05Lesson quiz
The explanation appears after your answer.
Question 1 of 4
Which field does NOT exist in a G1 MotorCmd_?skill
Correct
The command has mode, q, dq, tau, kp and kd. There is no integral term: the PID with Ki from the old course does not match the SDK.Not this one
The command has mode, q, dq, tau, kp and kd. There is no integral term: the PID with Ki from the old course does not match the SDK.Review: What goes in a commandQuestion 2 of 4
The policy stopped sending targets. What does the kit's watchdog do?
Correct
A stale command is a sign that something got stuck. Damping is the safe mode; coming back on its own would hide the failure.Not this one
A stale command is a sign that something got stuck. Damping is the safe mode; coming back on its own would hide the failure.Review: Three protections between the policy and the motorQuestion 3 of 4
Why did the step ask for more torque than the ramp, with the same target?
Correct
In the PD, torque ≈ kp·(q* − q). With a ramp the error stays small the whole time; with a step, the entire error shows up in the first cycle.Not this one
In the PD, torque ≈ kp·(q* − q). With a ramp the error stays small the whole time; with a step, the entire error shows up in the first cycle.Review: Ramp, limits and watchdog on the suspended G1Question 4 of 4
The simulator does not check the CRC. Why does the kit fill it in anyway?
Correct
The official example computes CRC().Crc(cmd) and copies the mode_machine. The simulator rehearsal is worth more when the code is the same as on the real robot.Not this one
The official example computes CRC().Crc(cmd) and copies the mode_machine. The simulator rehearsal is worth more when the code is the same as on the real robot.Review: What goes in a commandQuiz complete
You got {a} of {n} right. Get them all right and the lesson is marked as completed.Choose an option. The explanation appears after your attempt.
06In three sentences
LimitTarget clipped to the MJCF range; torque capped at the ctrlrange.
RampFrom the current point to the target over seconds, never all at once.
WatchdogA stale command turns into damping and does not turn back on on its own.