Thin devices often sacrifice travel and simulate a detent with a haptic actuator

What it is

The shell has less than a millimeter left; a snap dome will not fit; the cap barely moves. Thin devices then fire a piezo or linear actuator at the electrical threshold so the pad feels a “collapse.” Haptic-simulated detents admit that mechanical travel has already been sacrificed, and substitute a vibration in time for a drop in displacement. They do not bring travel back. They change the channel that issues the receipt.

Why it happens

A real detent is a sudden displacement of the cap relative to the finger bone. A motor simulation is an acceleration pulse of the chassis or a local patch, the skin flicked laterally or normally. The Pacinian channel is good at “there was a hit,” so that part is easy; the static deformation Merkel cells want is almost absent, so “the cap sank” is not. The pulse must also land within a few milliseconds of electrical fire; late, it becomes an echo after the effect, and people will not use it to end the action. A chassis actuator paints the event across the whole glass, so neighbors and the palm feel it too—poor spatial selectivity. Local piezo is better, still coupled through housing modes. Gloves, films, and a device on a soft pad eat or detune the pulse; a simulated receipt is more environmentally fragile than a mechanical drop.

Studying it

On the same thin panel, compare no feedback, a chassis pulse, local piezo, and a control key that still has real travel.

Independent variables: pulse delay relative to the electrical threshold, waveform (single pulse / a force profile that mimics collapse), actuator site, gloves and support surface. Dependent measures: eyes-covered confirmation, whether the pulse decides release, rate at which a neighbor is felt as “also pressed,” delay at which the pulse becomes an echo.

Deliberately delaying the pulse 30–80 ms draws the boundary between “still a receipt” and “just an effect.” Asking whether it feels high-tech turns a simulated detent into brand animation.

Where it stops holding

Stacking a strong motor on a key that still has real travel makes two events fight; people do not know which hit to trust. Users with hearing loss still have a mechanical drop; a motor that is mostly an audible click is not an equivalent substitute for them. Safety-critical confirmation should not ride only on a motor the system can mute (power saving, do-not-disturb, driver failure: the pulse vanishes, the key still fires). As the battery dies the motor weakens first, so the simulated receipt dies before the function. Side keys and camera keys that still have millimeter travel need not be forced into pure simulation.

Applying it

• Admit that thinness has closed the mechanical event channel, then design the pulse: aligned with the electrical threshold, before release, as local as possible.
• Provide a fallback when the motor is off or dead: sound, a visible state, or a ban on pulse-only confirmation for irreversible actions.
• Do not stack a simulated collapse on a key that still has a real drop.
• Verify: eyes covered, pulse on / off / delayed; see whether people still release at the pulse. After delay they bottom out or look at the screen, the pulse is not a receipt. Retest with film, gloves, and the device on a bed. Kill the motor driver on purpose and confirm that irreversible actions still have another confirmation.

Related

• Same group: C10.08.1 A tactile detent (a sudden drop in resistance at the fire point) marks that the action was received · C10.08.2 Keys with too little travel cannot give clear mechanical feedback at the moment of press · C10.08.3 If the detent and the electrical make point are out of sync, feel and effect misalign
• Adjacent: C4.11 Substitutes for Contact Feedback · C10.01 Tactile Localization of Physical Keys
• Search: haptic key · simulated detent · piezo button