A waste recycling plant is using a conveyor belt scale to measure the total weight of the product coming into the plant. They report that the scale totalizer is counting product running across the scale even when the belt is running empty. The issue causes accounting errors affecting their suppliers and worker pay.
Can the accuracy be improved?
The typical recycling waste is a blend of paper, cardboard, plastics, aluminum cans and some ferrous metal. The large size and light weight products require a suitable belt scale with a very long weigh span distance on a wide conveyor belt. This application uses a belt scale with unusually low capacity load cells with respect to the belt width. The dead load component is a high percentage of the weighbridge load cell capacity compared to the live load component. The belt loading is both variable and intermittent.
The belt scale system includes the Siemens MSI weighbridge, WS300 speed sensor, and the BW500L integrator.
Here’s a summary from the start-up commissioning report:
This is clearly a tough application for any conveyor belt scale. Some performance compromises must be expected. The compromise set-up for this customer requires turning on the “negative totalization” feature.
This is done in the BW500L by setting parameter P619=0. On typical belt scale systems, the totalizer will report zero change continuously for the period of time that the belt runs empty. When the “negative totalization” feature is turned on, the totalizer value can increase and decrease continuously when the belt is running empty. The net change to the totalizer value will be zero over a time interval of one or more belt circuits. If there is a change to the totalizer value when the belt is running empty then perhaps recalibration is required or there is another issue affecting the scale performance. When running product, the totalizer accuracy errors are greater for very short time intervals. When the product run time is long enough, say an 8-hour shift, the totalizer accuracy can be acceptable. All of the factors that contribute to the accuracy error must be considered and addressed for the best conveyor belt scale performance.
What sort of belt scale applications have you been challenged with?
What sort of belt scale applications have you been challenged with?
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