我正在嘗試使用以下代碼為 acrobot 創建一個簡單的重力補償控制器：from pydrake.all import *file_name = "acrobot.sdf"  # from drake/multibody/benchmarks/acrobot/acrobot.sdfplant = MultibodyPlant()parser = Parser(plant=plant)robot = parser.AddModelFromFile(file_name)plant.AddForceElement(UniformGravityFieldElement([0.0, 0.0, -9.81]))plant.Finalize()nq = plant.num_positions()nv = plant.num_velocities()nx = nq + nvnu = plant.num_actuators()assert (nx, nu) == (4, 1)plant_ctx = plant.CreateDefaultContext()x_plant = plant.GetMutablePositionsAndVelocities(plant_ctx)x_plant[:] = [0.1, 0.2,              0.3, 0.4]tau_g = plant.CalcGravityGeneralizedForces(plant_ctx)print tau_g不幸的是，tau_g始終為零。似乎重力矢量不適用于機器人。如何解決這個問題？