Too Fast or Too Slow: Matching Product Transit Time to Metal Detection Timing

Metal detection relies on the signal waveform formed as a product passes through the detection field, and the instrument needs a certain time to build up and evaluate that signal. If a product passes too quickly, the signal lasts too briefly for the discriminator to capture it fully and sensitivity suffers; if it passes too slowly or spacing is insufficient, signals from adjacent products may overlap. Before purchase, provide the line rate, product length and pitch to the supplier for calculation.

The key figure is dwell time in the detection field, usually estimated as product length divided by line speed, then compared with the sensitivity available at that setting in the manual. Where line speed is adjustable, check both normal and peak speeds and confirm at the less favourable condition. For mixed production, use the shortest product with the smallest gap as the basis rather than the main specification alone.

Mounting position also affects timing. Sufficient conveyor distance must be left between the detector and the reject device so that, once the signal is confirmed, the rejection action aligns with the target product; too short a distance risks hitting neighbouring items, too long a distance risks misalignment at speed. During commissioning, run marked test pieces to measure transit time and reject position, record the results in the work instruction, and re-verify after changeover or speed increases.