Decision: coin linear resonant actuator (LRA), 10 mm diameter, about 3.5 mm tall, resonant near 175 to 235 Hz, driven closed loop by the DRV2605L. Reference part Precision Microdrives C10-100 (175 Hz, about 1.5 G, 37 ms to rated amplitude, 177 ms natural stop, about 35 to 50 ms with braking). Second source Vybronics VG1040003D (about 2.0 G, about 10 ms to half amplitude).
Finalized order, 2026-09: the Vybronics VG0832022D 8 mm coin LRA is the default motor across the bands, with the VG1040003D 10 mm reserved for the waist and back zones that want more delivered acceleration. The C10-100 stays as the reference part the psychophysics numbers were modeled on.
Why LRA: the Pacinian channel is most sensitive from 200 to 300 Hz with the peak near 250 Hz, and an LRA parks all its energy there. ERM motors couple frequency and amplitude, so "stronger" also means "different pitch," which blurs Law 2, and they rise and brake slowly, which blurs Law 3. Voice coils and piezos are cleaner but too costly or need high voltage on the skin.
Alternatives rejected: 10 mm pancake ERM (cheap, no driver needed, poor control); voice coil tactors (lab grade, expensive); piezo (tens of volts on skin).
Measurement plan: delivered acceleration at the skin through the actual band liner, rise time to 50 percent and to rated, brake time with DRV2605L braking on. These replace the modeled motor stage in the latency budget.