Production Counting Without Sensors: Using PLC Signals You Already Have

Signals That Indicate a Completed Cycle

Most PLCs produce at least one of these signals at cycle completion: a rising edge on a digital output (actuator returning to home position), a specific M-bit flag set by the PLC programme, a register increment (part counter maintained by the PLC), a conveyor pulse (part passing a sensor). Work with your maintenance team or the machine documentation to identify which signal your PLC generates.

Edge Detection vs Register Reading

If your PLC maintains an internal counter register (D100 = parts produced today), read it directly. If you only have a pulsing digital signal, you need edge detection — count rising edges of the signal at your gateway device. Both approaches give you accurate counts without touching the PLC programme.

Dealing With Faulty Signals

Machine vibration sometimes causes false pulses on digital signals. Filter these by imposing a minimum cycle time — if two pulses arrive within 500ms and your normal cycle is 8 seconds, the second pulse is noise. Ignore it.

Calibration

For the first week, compare your automated count with the operator hand count at end of shift. Discrepancies reveal signal issues or counting logic errors. Do not trust automated counts until they match manual counts for 5 consecutive shifts.