admittance control
Admittance control is the mirror image of impedance control: instead of taking a desired position and producing a yielding force, it takes a measured force and produces a commanded motion. The robot carries a force sensor that feels how hard the world is pushing on it; the controller reads that push and decides how far and how fast to move in response, as if the robot were a virtual spring-and-damper being pressed by hand. So when you lean on the robot, it senses your shove and glides away by an amount you have programmed — heavy pushes giving bigger motions, light touches giving small ones.
Why have two flavours that aim for the same soft, contact-friendly feel? It comes down to the robot's hardware. A naturally stiff, geared, position-controlled industrial arm cannot easily be made to gently 'give' on its own, but it is excellent at going precisely where commanded — so you bolt a force sensor to its wrist, read the contact force, and turn it into a position command: that is admittance control. A lightweight, backdrivable arm that already moves freely is better suited to the opposite approach (impedance control). Admittance control thus lets a rigid, accurate machine fake the softness it cannot achieve mechanically.
The trade-off is that this scheme leans entirely on its force sensor and on closing the loop fast enough; in very stiff contact it can become twitchy or unstable, because a tiny extra push reads as a large force and triggers a large move. Tuning the virtual mass, stiffness, and damping is the art of keeping it smooth and safe.
A worker grabs the tool on a heavy industrial arm and tugs it toward a fixture. The wrist sensor feels the pull, and admittance control lets the stiff arm float along with the hand, as if it were weightless.
Feel the force, command the motion — softness faked by a stiff machine.
Rule of thumb: admittance control suits stiff, accurate arms with a wrist force sensor; impedance control suits light, backdrivable arms that can yield on their own.