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Specifying Accuracy in an In-Motion Scale

In-Motion Scale Accuracy

You want to implement an in-motion weighing system to help get control of your packaging processes. Data on throughput, package sizes is in-hand. What accuracy should you specify?  That’s a good question, but simply adding more zeroes behind the decimal point may not be the best way to do it.

Standard_deviation_diagram_svg

Bell Curve Distribution

How Accuracy is Measured. The most common metric is to use “2 sigma” metric, based on a weight and a standard bell curve.  In other words, if a scale manufacturer says their unit will weigh to ±0.1 lb accuracy at a specified weight, then you can expect the scale will report the actual weight within 0.1 lb 95% of the time, and 5% of the time it may be reported very slightly higher or lower.

How Checkweighers Work.  Most checkweighers utilize conventional, “strain gauge” load-cell type technology.  Several load cells can be used to support the conveyor bed.  The signals are digitized and combined to get the total weight.  The checkweigher’s software records a series of weighments from the time a product enters the scale either for a fixed time or when its departure is sensed, and provides averaging as well as “trimming” functions to help discard “noisy” data.

Using this type of technology, an accuracy of .1 lb or as good as  .02 lb for a 40 lb product is easily achievable.  Accuracy is subject to a number of operational considerations, such as air currents (e.g. from HVAC systems or open dock doors), regular calibration, floor vibration, conveyor speed, freeboard of moving liquid in the product, etc.  Suppliers have many options and techniques to help provide the best accuracy possible, ranging from minimal to moderate cost for standard features and/or enhancements:

  • protective air current tunnels
  • extending the conveyor length and therefore “settling” time
  • manufacturing for minimum internal vibration (e.g. balanced pulleys)
  • additional software calibration

An “enhanced” conventional checkweigher can typically be capable of accuracies in the range of 0.5 – 1.0 grams (.001-.002 lb) for a 25 lb. product.

EMFR Checkweighers. If your accuracy needs are much greater, then an EMFR (Electromagnetic Force Restoration) – based scale may be appropriate. EMFR charges an inductive coil, floating the scale bed in an electromagnetic field. Weight variance causes the movement of a ferrous material through that coil creates a fluctuation in the coil current proportional to the weight of the object.  This technology is much more expensive than either standard or enhanced load-cell scales.

Be Realistic.  In most logistics application, the weight variability of the box or packaging will be significant.  Moisture content due to humidity changes, for example, may change the weight of the package.  So, as long as the weight reported conforms to the carrier’s rate table, there is no need to try to obtain accuracy down to the last tenth of a gram.

Operational Benefits of Checkweighers

Packaging Operations can easily run scenarios on the cost savings around “give back” based on filler or other packaging accuracy. It is straightforward to calculate the savings and therefore, benefit of this application. A checkweigher in this environment can provide a couple of other benefits.  First, many checkweighers can provide closed-loop feedback to a filler for automatic adjustment as its performance varies.

Logistics Operations may be concerned about parcel rating; look at the rate weight tiers but consider the variability in the product and packaging.  For example, the weight of cardboard boxes can vary a surprising amount due to irregular cuts and moisture due to humidity.  If you are packaging, consider your customer and filling accuracy, as well as material cost and cost of errors. A one-ounce (parcel) or one-pound (freight) tier is pretty large.  Combining a checkweigher with an automated shipping system generates return through parcel shipping savings.

Post-Implementation.  In order to validate your installation, run a statistically significant test (30-40 samples).  What is the performance in your application?  If it is not up to par, look for the standard problems:  Air Currents, Vibrations, and a smooth transfer of product on and off the scale.

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