Stiffness

How strongly an object or joint resists deforming — the same force causes less deformation the stiffer it is.

Stiffness is defined as the ratio of an applied force to the displacement it produces, k = F/δ (F is force, δ is displacement along the force's direction), measured in N/m; for rotation, it's torque divided by angle, in N·m/rad. Its reciprocal is called compliance. The term carries two meanings in robotics. One is structural stiffness: links, gearboxes, and belts all bend or twist under load, which directly affects how accurately the end effector reaches its target. The other is control stiffness — the 'virtual spring' a controller creates, such as the proportional gain in PD control or the stiffness matrix K in impedance control. High stiffness tracks a target accurately and resists disturbances but hits hard on impact; low stiffness is more compliant and safer for tasks involving contact. The 'stiffness' and 'damping' settings in reinforcement-learning locomotion configs refer to exactly this — the PD gains applied at each joint.

Examplelegged_gym sets stiffness = 80 N·m/rad for each joint of the ANYmal C robot: when the policy's target angle differs from the actual joint angle by 0.1 rad, the joint produces a restoring torque of about 8 N·m, before the damping term is subtracted.

Also called

Rigidity, Spring Constant

Related

Compliance · Damping · Impedance Control · Stiffness and Damping Gains · Proportional-Derivative Control · Mass-Spring-Damper System

Sources

Stiffness - Wikipedia
legged_gym: anymal_c_rough_config.py
Modern Robotics: Mechanics, Planning, and Control (Lynch & Park, free preprint)