Gestures

Every gesture is the same two-machine dance: the X/Y arm drives the stylus to the bbox center, then a solenoid drops the tip onto the glass and lifts it. The Z axis is not a servo and there’s no depth to calibrate — the tip sits at a fixed height, and the only thing that varies is timing. What makes a tap a tap and a long-press a hold is how long the coil stays energized.

The solenoid, in one paragraph

The stylus tip is pushed down by a solenoid wired to the GRBL spindle PWM pin, so it’s driven with spindle G-code: M3 S<duty> energizes the coil at a PWM duty on a 0–1000 scale, and M5 releases it. A return spring lifts the tip when the coil cuts. A solenoid burns its coil out if held at full current, so PhysiClaw uses a hit-and-keep profile: strike hard to pull the iron core in, then drop to a lower hold current to keep it seated.

Constant	Value	What it is
HIT_S	1000	strike duty — pulls the core in (brief only)
HOLD_S	750	hold duty — safe to sustain (long-press, swipe slide)
SETTLE_MS	80ms	dwell after the strike before dropping to the hold level
RELEASE_MS	200ms	spring-rebound dwell after M5, before any XY move
DOUBLE_TAP_GAP_MS	100ms	brief lift between the two strikes of a double-tap

Why HOLD_S is below HIT_S

The strike duty (S1000) is for the ~80ms pull-in only; sustaining it would cook the coil. HOLD_S (S750) is the only level safe to hold continuously — and it’s deliberately high: at a cooler S500 the tip eased off at the strike→hold transition and the panel read a long-press as a single tap. The driver refuses to start if HOLD_S isn’t strictly below HIT_S.

After every M5, PhysiClaw dwells RELEASE_MS so the spring can lift the tip clear before the next XY move starts — otherwise the tip would drag a line across the screen.

Tap

A momentary press: strike at HIT_S, hold for the tap duration (80ms), release. The press is short enough that the reduced hold current isn’t needed — S1000 for 80ms gives a crisp contact without cooking the coil.

G-code

G0 X.. Y..        ; arm moves to the bbox center
M3 S1000          ; strike (stylus down)
G4 P0.08          ; hold 80 ms
M5                ; release
G4 P0.2           ; spring-rebound dwell (tip clears the glass)

Double-tap

Two strikes separated only by a brief contact-breaking lift — not the full spring-clear dwell. The whole pair runs as one gesture so down-to-down stays well under the iOS ~300ms double-tap window; chaining two independent taps (each with its 200ms release) pushed it to ~280ms, and timing jitter then split the gesture into two single taps.

G-code

M3 S1000          ; strike 1
G4 P0.08          ; hold 80 ms
M5                ; lift briefly…
G4 P0.1           ; …100 ms, just enough to break contact
M3 S1000          ; strike 2
G4 P0.08          ; hold 80 ms
M5                ; release
G4 P0.2           ; spring-rebound dwell

Long-press

Here the hit-and-keep profile earns its name: strike, settle, drop to the hold current, and keep the tip seated for the full long-press duration (1.2s) before lifting. iOS/Android register a long-press at ~500ms; 1.2s leaves comfortable margin.

G-code

G0 X.. Y..        ; arm moves to the bbox center
M3 S1000          ; strike
G4 P0.08          ; settle 80 ms
M3 S750           ; drop to hold current
G4 P1.2           ; hold 1.2 s
M5                ; release
G4 P0.2           ; spring-rebound dwell

Swipe

A swipe presses, then moves under load. The tip is held down at HOLD_S for the whole slide while the arm runs a controlled G1 move (not a rapid G0) from the origin to the endpoint, then lifts. Stroke length comes from the size argument; the speed argument sets the G1 feed rate:

speed	Feed	Feel
slow	F3000	careful scroll / drag
medium	F6000	normal swipe (~100 mm/s)
fast	F10000	fling, page switch

G-code

G0 X.. Y..        ; arm moves to the start
M3 S1000          ; strike
G4 P0.08          ; settle 80 ms
M3 S750           ; drop to hold current (tip stays down for the slide)
G1 X.. Y.. F6000  ; drag to the endpoint at the chosen feed
M5                ; release
G4 P0.2           ; spring-rebound dwell

Why the hold current, not the strike, drives the slide

A stationary hold needs S750 to stay registered, but a moving tip re-establishes contact as it slides — S500 is fine for a swipe yet not for a still hold. PhysiClaw uses the one safe-to-sustain level (S750) for both so there’s a single hold path to reason about.

Summary

Gesture	Down	Hold	Move	Up
Tap	S1000	80ms	—	M5 + 200ms dwell
Double-tap	S1000 ×2	80ms each, 100ms gap	—	M5 + 200ms dwell
Long-press	S1000 → S750	1.2s	—	M5 + 200ms dwell
Swipe	S1000 → S750	for the slide	G1 at F3000/6000/10000	M5 + 200ms dwell

Safety nets

Every gesture de-energizes the coil before returning — even on an exception or Ctrl-C — so a fault mid-tap can never leave the coil hot. As a hardware backstop, the FluidNC firmware (off_on_alarm: true) drops the spindle PWM whenever the board enters an alarm state.