press_mechanical — count_cycles (inductive-proximity)

Physics

An inductive proximity sensor drives an oscillating coil that generates a high-frequency electromagnetic field at its sensing face. A ferrous target (a bolt-on tab or the existing crank/flywheel keyway) passing through that field induces eddy currents in the target, damping the oscillator’s amplitude; the sensor’s internal circuit detects that damping and switches its digital output once per pass, with no contact and no wear. Mounted so the target crosses the face once per press cycle (once per revolution of the crankshaft, or once per ram stroke on the flywheel/crank), this gives an exact digital pulse-per-stroke count, independent of load, unlike a current-based method whose amplitude varies with die tonnage. The sample rate only needs to resolve one pulse per stroke plus the pulse’s rise/fall time, not the stroke’s internal dynamics: for a press running up to 150 strokes/min (2.5 Hz), sampling the sensor’s discrete output at 500 Hz gives ~200x oversampling of the pulse rate, comfortably above Nyquist for the pulse frequency and fast enough to resolve a pulse width of a few milliseconds without aliasing two adjacent strokes into one.

Sensor

• Spec: inductive proximity switch, M18 barrel, 8 mm sensing range, NPN/PNP, NO
• Signal: counter ({count}), range n/a

Example parts

• ifm electronic IGS204 M18 inductive sensor (8 mm range, PNP NO, IP67)
• SICK IME18-08BPSZW2S M18 inductive sensor (8 mm range, PNP NO)
• Pepperl+Fuchs NBB8-18GM50-E2 M18 inductive sensor (8 mm range, PNP NO)

Mounting

Mount the sensor on a rigid bracket off the press frame, facing a ferrous target welded or bolted to the crankshaft, flywheel, or ram so the target passes directly in front of the sensing face once per cycle, within the sensor’s rated sensing range (typically 2-8 mm). Set the air gap at the low end of the rated range to maximize signal margin, and verify the target does not contact the sensor face at any point in its travel.

Wiring

3-wire DC (24 V, 0 V, signal) to a digital input / counter channel on the edge box’s I/O module; use a shielded cable on runs over a few meters to avoid noise pickup from the press’s motor starter and solenoid wiring.

Validation

run the press for 50 manually-counted strokes at its normal production speed and compare the detector’s counter delta over the same window

Expected: counted strokes exactly equal to the manual count (a correctly mounted, debounced proximity sensor gives an exact 1:1 pulse count, unlike load-dependent methods)

Safety notes

• de-energize and lock out the press per LOTO before working near the crankshaft, flywheel, or ram to fabricate and install the target and mounting bracket
• mount the bracket so it cannot be struck by the moving crank, flywheel, or ram at any point in the stroke, including at the sensor’s minimum air gap
• verify the sensor’s voltage rating matches the 24 V DC supply before wiring; do not wire a DC-rated sensor to an AC supply

Cost

35–120 USD, difficulty 3/5, ~45 min install downtime.