press_mechanical — guard_state (door-switch-tap)
Physics
Section titled “Physics”A guard-locking switch on the press’s point-of-operation door already produces a digital open/closed signal and, on most modern safety switches, exposes a separate auxiliary or monitoring contact alongside the safety-rated contact pair that feeds the safety relay. This recipe taps ONLY that auxiliary/monitoring contact with a passive digital input, read-only, electrically isolated from the certified safety circuit: it observes the same mechanical state the safety switch already senses without adding any load, splice, or jumper to the safety wiring. The resulting 0/1 signal is fed to the generic run/idle/ off/fault threshold-state detector, mapped conceptually so the “off” state represents door-open (the alarm-worthy side) and “run” represents door-closed; the detector’s off/run naming does not literally mean guard-open/closed, it is reused for this binary classification. A door guard changes state on human timescales (an operator opening a door takes >=0.5 s), so no high-frequency sampling is needed: a 10 Hz sample rate gives >>2x Nyquist margin over any realistic door-actuation transient and comfortably resolves a few-hundred-ms debounce window on the switch’s mechanical contact bounce.
Sensor
Section titled “Sensor”- Spec: isolated digital input tap on an existing guard-switch auxiliary/monitoring contact, 24 V DC, dry contact or PNP/NPN output
- Signal: state (1), range n/a
Example parts
Section titled “Example parts”- Schmersal AZM201-I1-24VAC/DC guard-locking switch (auxiliary monitoring contact tapped, existing switch on the guard door) — tap the switch’s non-safety auxiliary contact only; never splice into the safety-rated contact pair
- Pepperl+Fuchs INS2 safety hinge switch (auxiliary monitoring output tapped)
- Allen-Bradley 440G-MT guard-locking switch, 440G series (auxiliary contact tapped)
Mounting
Section titled “Mounting”Install no new sensor on the guard itself: wire into the EXISTING certified guard-door interlock switch’s auxiliary/monitoring contact terminals only (clearly marked separate from the safety contact terminals on the switch’s terminal block or datasheet). Route the new 2-conductor tap lead through a cable gland into the press’s junction box or a nearby terminal strip, strain-relieved and separate from the safety-circuit wiring run.
Wiring
Section titled “Wiring”Auxiliary contact (dry contact or 24 V DC digital output, per the switch model) goes to a single isolated digital input channel on the edge box’s digital-io module; do not connect this tap to the safety relay’s inputs or outputs at any point. Use a dedicated conductor pair, not a shared return with the safety circuit, to keep the monitoring path fully independent of the safety path.
Validation
Section titled “Validation”with the press powered but not cycling, open and close the guard door 10 times at a normal operator pace and compare the detector’s logged open/closed transitions and timestamps against a stopwatch-timed manual log of each open/close event
Expected: all 10 open and 10 close transitions logged within +/-0.5 s of the manual timestamps, with no missed or duplicate transitions
Safety notes
Section titled “Safety notes”- this tap is a PASSIVE MONITORING-ONLY reading of an existing auxiliary contact; it is NOT a safety-rated device, it does not replace, duplicate the function of, or substitute for the press’s certified safety interlock or guard-locking relay in any way, and must never be wired into the safety circuit
- de-energize and lock out (LOTO) the press’s main disconnect before opening the guard switch’s terminal enclosure to identify and wire the auxiliary contact
- keep hands and tools clear of the ram, die, and guard-locking mechanism travel path while testing the door open/close cycle with the press powered
- verify with a multimeter, before connecting, that the auxiliary contact terminals are electrically isolated from the safety contact pair per the switch’s datasheet
15–60 USD, difficulty 2/5, ~30 min install downtime.