Pressure supports only a few controllable levels
Aliases: force steps · pressure quantization · just noticeable force · force reproduction
What it is
A sensor can report eight-bit force; a hand cannot park on that many steps. Few controllable pressure levels means the force landings a person can intend and hit repeatedly are usually two or three—contact, light, hard—and beyond that it becomes guesswork. The claim is about the human actuator’s step count, not about sensor resolution, baseline wander, or whether confirming feedback is present. Those later constraints cut an already small number further.
Why it happens
Isometric force is shakier than holding a joint angle. Finger-flexor output fluctuates, and the pulp is a nonlinear spring, so “a little more” is a very different increment at different starting forces. Weber fractions make the just-noticeable force difference a fraction of current force, but telling two stimuli apart is not the same as voluntarily sitting on step n: the latter also fights drift, holds position, and has no mechanical notch. Without a physical step, people recall a target force from effort sense; working memory barely holds a force ruler with four marks. Treating continuous force as many discrete commands therefore slices a narrow channel into slots that invade each other.
Studying it
Use force reproduction and level hitting: present a standard force, then ask people to return to it with no prompt or with only a level lamp; or slice continuous force into N bins and watch hit rate as N grows.
Independent variables: number of target levels, haptic/visual level cues, whether a numeric force readout is allowed, pulse versus hold. Dependent measures: fraction landing in the correct bin, adjacent-bin confusions, sliding out of the bin after a 2–3 s hold.
Do not treat the sensor’s quantization count as controllable levels when the range is large. The useful curve is hit rate without prompts at N = 2, 3, 4, 5, and where it falls toward chance. Holds are harsher than taps because they expose effort fatigue.
Where it stops holding
Keys with a mechanical second stage (two-stage shutter, a distinct second travel) turn “level” into a travel event, so controllable steps can exceed pure pressure by one—and that is no longer a pure force channel. A stylus on paper, with a stable tip and friction, can do continuous force (stroke width) better than a button does discrete bins, because the task is continuous matching, not hitting invisible slots. Children, arthritis, and grip-strength spread cut the already small count further. Three levels measured with the forearm in a lab rest may collapse to two in an unsupported phone grip.
Applying it
- Default to two meaningful force states: below threshold and above. If a third is required, the second must have a distinct mechanical or haptic step; do not ask people to remember “medium.”
- Do not bind five menu commands to five pressure levels.
- Use continuous force only to drive a continuous result (harder means denser). Do not cut a row of invisible command slots into that continuum.
- Verify: with no numerals, only the product’s own feedback, have people hit each claimed level 20 times and watch adjacent confusions. If a level falls toward chance, delete it rather than nudging thresholds. Repeat unsupported, in gloves, and as a three-second hold.