Persistence of deformation and behavior at power-off
Aliases: bistable shape · shape memory · power-off collapse · held deformation
What it is
After a power cut, does the bump still stand, or does it go flat? That decides whether the next touch meets “the last state” or “a dead, level plate.” Persistence of deformation and power-off behavior asks whether geometry can keep acting as representation with the power gone. It is not about how fast the shape moves, and not about long-term pin wear. It is whether that shape has a memory of its own.
Why it happens
Actively held deformation runs on continuous energy: pumps, electromagnets, heat. Cut power and restoring forces (springs, leak, cooling) send the surface to a mechanical default, usually flat or the factory form. Bistable mechanisms, ratchets, and shape memory locked at a given temperature write the shape into the material, so it survives the cut. The two choices define idle differently. Active hold: idle is the unpowered default, which may have nothing to do with last digital state. Bistable: idle is the last lock, which may have nothing to do with the state software will recall at boot. Power-on therefore requires a reconciliation: trust the material and rewrite software, or trust software and rewrite the material. Skip it, and the geometry in the hand is a stale or blank representation.
Studying it
After a deformation completes, cut power, wait various durations, restore power, and watch where shape and software each went.
Independent variables: hold method (continuous energy / bistable / leaky pneumatics), off duration, boot policy (follow material / follow software / flatten first). Dependent measures: height change while off, whether people act on stale geometry after boot, whether reconciliation is noticed.
Include both “someone pressed the surface while it was off” and “no one touched it.” Demos that never cut power never show persistence. Leak curves need hours, not ten seconds.
Where it stops holding
Always-plugged installations (kiosks, desk hubs) can treat active hold as persistence; power loss is the exception. Battery devices, emergency lamps, and in-car controls that can still be touched after ignition-off meet power-off as a daily case. Heat unlocks shape memory early; cold slows pneumatics and changes leak; lab room-temperature hold times are not field life. If the default shape happens to be the most common state (fully flat = no notification), collapsing on power-off can even be a feature. A safety-critical “locked” that holds shape only by continuous energy becomes an automatic unlock at power loss—often an unacceptable semantics.
Applying it
- Write down which geometry the surface must occupy with no power: fully flat, last shape, or mechanically locked safe.
- Prefer bistable or mechanical locks on battery and automotive; do not use a bump that needs continuous draw to mean “still in effect.”
- Boot must reconcile, and people must notice (a brief flatten-and-scan, or an explicit “shape replayed from software”).
- Verify: cut power at the target temperature; measure height after 1 minute, 1 hour, and overnight; restore power and ask, without a display, what state it is in. If material and software disagree and no one notices, change the hold method or force reconciliation. Run “pressed while off” as its own path.