Embodied · contact & passivity

The buzz is a standing wave.

A robot may touch an unknown world safely only if its port is passive: Re Z(jω) ≥ 0 at every frequency. Network theory says that is exactly |Γ| ≤ 1: the Smith chart's unit disk, in mechanical dress. Digital control holds each force for one sample period, which delays the spring by T/2, and a delayed spring is a negative damper. Leave the disk and the contact rings.