injection_moulding — guard_state (gate-switch-tap)
Physics
Section titled “Physics”An injection-moulding machine’s safety gate (covering the clamp unit and mould area) carries a certified safety gate switch whose primary contacts feed the machine’s safety relay; many such switches also expose a separate auxiliary/monitoring contact for exactly this kind of passive status tap. This recipe wires ONLY that auxiliary contact to a digital input, read-only and electrically isolated from the safety-rated contact pair, so it never touches the certified safety circuit. The digital 0/1 signal feeds the generic run/idle/off/fault threshold-state detector, with “off” mapped conceptually to gate-open (alarm side) and “run” to gate-closed; the detector’s off/run field naming does not literally mean gate-open/closed, it is reused here for binary classification. Gate actuation is a human-timescale mechanical event (an operator opening the gate takes >=0.5 s), so sampling the debounced digital level at 10 Hz gives >>2x Nyquist margin over any realistic gate-swing transient while resolving a sub-second debounce window.
Sensor
Section titled “Sensor”- Spec: isolated digital input tap on an existing safety gate 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 contact tapped) — tap the auxiliary/monitoring contact only; never splice into the safety-rated contact pair feeding the machine’s safety relay
- 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 gate: wire into the EXISTING certified safety gate switch’s auxiliary/monitoring contact terminals only, clearly marked separate from the safety contact terminals on the switch’s terminal block per its datasheet. Route the 2-conductor tap lead through a cable gland into the machine’s electrical cabinet or a nearby terminal strip, strain-relieved and kept physically separate from the safety-circuit wiring.
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 machine’s safety relay inputs or outputs at any point, and use a dedicated conductor pair rather than sharing a return with the safety circuit.
Validation
Section titled “Validation”with the machine powered but not cycling, open and close the safety gate 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 moulding machine’s certified safety gate interlock or safety relay in any way, and must never be wired into the safety circuit
- de-energize and lock out (LOTO) the machine’s main disconnect before opening the gate switch’s terminal enclosure to identify and wire the auxiliary contact
- keep hands and tools clear of the clamp unit, mould, and gate-locking mechanism travel path while testing the gate open/close cycle with the machine 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.