[Question] Inverse Pendulum v5 init state all 0 but Pendulum falls down

[ruiming-wu]

Question

Hi everyone,

I've checked the official document of setting initial states and noise range, and I've create the environment accordingly. But it's pretty weird that the pendulum will always fall down without any action and any noise, and it will always fall down within the same steps and the same direction. I wonder why is that happened? following is my code, feel free to run it directly :)

`import numpy as np
import gymnasium as gym
import matplotlib.pyplot as plt

if name == "main":

env = gym.make("InvertedPendulum-v5", reset_noise_scale=0.0, render_mode=None)
obs, _ = env.reset(options={"qpos": np.zeros(2), "qvel": np.zeros(2)})
print("Initial observation:", obs)

obs_list, reward_list = [], []
t = 0
done = False

while not done and t < 200:
    action = np.zeros(1)
    obs_list.append(obs)
    obs, reward, terminated, truncated, info = env.step(action)
    reward_list.append(reward)
    t += 1
    if terminated or truncated:
        print(f"Episode finished at step {t}")
        done = True

env.close()

obs_arr = np.array(obs_list)
reward_arr = np.array(reward_list)
print("Final state:", obs_arr[-1])
print("Total reward:", reward_arr.sum())
print("Episode length:", len(obs_arr))

plt.figure(figsize=(10, 5))
plt.subplot(2, 1, 1)
plt.plot(obs_arr[:, 1], label="theta")
plt.plot(obs_arr[:, 3], label="theta_dot")
plt.title("Pendulum Angle and Angular Velocity")
plt.xlabel("Step")
plt.legend()
plt.grid(True)

plt.subplot(2, 1, 2)
plt.plot(reward_arr, label="reward")
plt.title("Reward per Step")
plt.xlabel("Step")
plt.legend()
plt.grid(True)

plt.tight_layout()
plt.show()`

[ruiming-wu]

a small update, my way to reset the init states is wrong, following is corrected:

obs, _ = env.reset()
env.unwrapped.set_state(np.array([0.0000000000, -1.66666e-3]), np.array([0.000000000, 0.000000000]))
obs = np.concatenate([env.unwrapped.data.qpos, env.unwrapped.data.qvel]).ravel()
print("Initial observation:", obs)

Interesting thing happened, with an initial config that all states are 0, my environment only lasts 30 steps given a dtype=float64 of 0.0 action, but the above initial configuration with theta set to -1.66666e-3 surprisingly lasts 100 steps, I'm trying to figure out what's wrong, I've checked the inverted_pendulum_v5.xml file and it should be fine