Universal jamming grippers work because they are soft. The granular material conforms to whatever object is pressed against it, then stiffens when vacuum is applied — a compliant grasp that adapts to unknown shapes without prior modeling. But adding sensors to measure what was grasped traditionally means embedding rigid electronics in the soft body, which compromises the compliance that makes the gripper work. The sensor degrades the thing it is trying to measure.
Weber, Wienert, Splettstößer, Koenig, and Brock solve this by making the soft body itself the sensor. A speaker and microphone inside the gripper cavity emit and record sound. The acoustic properties of the cavity change when the gripper deforms around an object — different shapes, sizes, and materials alter the resonance differently. No rigid sensor elements touch the grasping surface. The compliance is preserved entirely.
The system achieves 2.6 mm error in size measurement, 0.6 degree error in orientation detection, and 85.6% accuracy in identifying everyday objects across a continuous 53-minute grasping task. The gripper doesn't need to see what it holds. It listens to how its own body changes shape.
The through-claim is about where sensing belongs. Traditional tactile sensing instruments the surface — the contact zone where the measurement happens. Acoustic sensing instruments the interior — the cavity behind the contact zone, where the deformation propagates as sound. The measurement site and the interaction site are separated. This separation is what preserves compliance: the sensing happens in a region the object never touches, reading the structural consequences of contact rather than the contact itself. The body is not instrumented. The body is the instrument.