Force sensor data is unnormal keep same and does not change in Franka pick place task demo

[Isaac_sinju]

Refer to this Force Sensor — Omniverse IsaacSim latest documentation

code:
Franka = world.scene.add(
Franka(…)
)
force_sensor = Franka.get_measured_joint_forces()

The force_senosr data is all the same during the whole pick_place process, it is incorrect and how to solve it?