packing — scrap_weight (checkweigher)
Physics
Section titled “Physics”An in-line dynamic checkweigher carries each packed unit across a short conveyor section mounted on a load cell; as the unit transits, the load cell’s strain gauge bridge outputs a voltage proportional to the instantaneous weight on the platform. Filtering that signal over the unit’s known dwell time (set by belt speed and platform length) yields a stable net-weight reading per unit, which is compared against a target weight band to classify the unit as good or reject (underweight/overweight). Each classification increments a good or reject counter, and the resulting good/reject counts feed an SPC p-chart detector that tracks the reject fraction against a statistically-derived control limit, flagging a shift before it drifts into a sustained high-scrap run. The line’s throughput sets the required sample rate: the load cell signal must be sampled fast enough that the digital filter can settle to a stable reading within each unit’s dwell window. For a line running up to 120 units/min (2 Hz unit rate) with a typical 150-200 ms per-unit dwell on the weigh platform, sampling the load cell amplifier’s analog output at 500 Hz gives >200x oversampling of the unit rate, comfortably satisfying Nyquist and leaving ample margin for the settling-time digital filter to produce one clean weight reading per part without aliasing adjacent units.
Sensor
Section titled “Sensor”- Spec: dynamic checkweigher load cell platform, strain-gauge bridge, mV/V output
- Signal: scalar (g), range n/a
Example parts
Section titled “Example parts”- Mettler-Toledo XS3 dynamic checkweigher load-cell module
- Hardy Process Solutions HI 6500 checkweigher weight processor with load-cell platform
- Thermo Fisher Scientific (Ramsey) InCheck 4.2 in-motion checkweigher
Mounting
Section titled “Mounting”Install the checkweigher platform in-line on the packing conveyor immediately after the fill/seal station, leveled and isolated from adjacent conveyor sections with flexible couplings so vibration from upstream/downstream drive motors does not couple into the load cell. Set the platform length and guide-rail spacing to the packed unit’s footprint so each unit is fully supported and centered while crossing the weigh zone.
Wiring
Section titled “Wiring”Load-cell bridge output (mV/V) to the checkweigher’s own signal-conditioning amplifier, then the amplifier’s calibrated analog or digital output to the edge box’s DAQ module; use a shielded cable between the load cell and amplifier and ground the shield at the amplifier end only to avoid a ground loop with the conveyor drive.
Validation
Section titled “Validation”run 50 units of known good weight and 10 units intentionally under/overweight through the checkweigher at normal line speed, and compare the edge system’s good/reject classification against the known weights
Expected: classification matches the known good/reject status for all 60 units, and the SPC detector’s reject-count signal increments only for the 10 known-bad units
Safety notes
Section titled “Safety notes”- de-energize and lock out the conveyor drive (LOTO) before working near the belt, drive rollers, or guide rails to install the checkweigher platform and load cell
- keep hands and loose clothing clear of the conveyor’s pinch points (belt-to-roller nip, guide-rail adjustment points) whenever the line is powered, even during calibration
- verify the load-cell amplifier’s supply voltage matches its rating before wiring; do not wire a DC-rated amplifier to an AC supply
3500–18000 USD, difficulty 3/5, ~90 min install downtime.