Soft Limits (Software Joint Limits)

A software-defined motion boundary set more conservatively than the mechanical limit, stopping the robot before it reaches the hard stop.

Soft limits are motion boundaries set by control software, positioned inside the joint's mechanical limits (hard stops, limit blocks) so the robot is stopped by software before it ever reaches the mechanical stop. At the joint level, URDF lets you write soft_lower_limit / soft_upper_limit in a safety_controller tag; ros2_control's documentation explains that once a joint reaches this boundary, a safety controller starts constraining its position, with k_position and k_velocity determining how quickly the allowed velocity and torque get tightened. At the Cartesian level, many arm controllers support defining planes or regions the end-effector may not cross, commonly called a virtual wall. In reinforcement-learning locomotion, legged_gym uses soft_dof_pos_limit to proportionally shrink the URDF's joint range and gives a negative reward for exceeding it, teaching the policy to stay away from the limits.

ExampleA joint with a mechanical range of ±170° might have its soft limit set to ±165°; during teleoperation, if the operator pushes the controller all the way, the joint stops near 165° rather than slamming into its mechanical stop.

Also called

Software Limits, Virtual Wall, Workspace Limits

Related

Joint Limits · Torque Limiting (Saturation) · Protective Stop · Workspace · Unified Robot Description Format · Functional Safety

Sources

ros2_control joint_limits.hpp：SoftJointLimits（来自 URDF safety_controller） (Chinese)
legged_gym legged_robot_config.py：soft_dof_pos_limit