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16 March 2017 | PBEI Weight balancing your feeder and filtering the load cell weight readings can help to separate out plant vibration noise from true material weight loss. Modern load cells and control algorithms are able to discriminate between the load to be measured and transient forces imposed by vibration. Sophis- ticated digital filtering algorithms can be applied to identify and extract frequency components characteristic of in-plant vibration. Such technology, along with proper feeder isola- tion during installation, can greatly improve feeder accuracy. 2. Control motor speed during feeder refill. The traditional method of maintaining the material feed while refilling the feeder in a continuous process is to use a constant metering speed throughout the hopper refill phase. The feeder essentially functions volumetrically during refill with the feeder operating speed the same as it was just prior to the refill phase. When the refill phase is complete and the material has settled, the feeder senses the declining system weight and returns to gravimetric operation with the meter- ing speed once again being determined by the LIW. This method has two problems. First, during refill the feeder can't compensate for fluctuations in material bulk den- sity because the material is being fed volumetrically. Second, when the feeder returns to true LIW control at the end of the refill phase, the feeder speed can change abruptly, sometimes resulting in an extended period of off-spec mass flow until the feeder settles into the new proper speed. Using a constant feeder speed that ignores bulk density variations during refill can significantly reduce feeder accu- racy. An alternative method avoids this problem by storing the feeder's weight-to-speed ratio during gravimetric feed- ing as the hopper empties and using that data to control the feeder speed during volumetric feeding in the refill phase. This method is illustrated by the graphs in Figure 2. The top graph plots the net hopper weight during operation ver- sus time. Beginning with a full hopper (where the net hopper weight equals the refill phase stop weight) the feeder operates gravimetrically as previously explained. As the net hopper weight declines, however, the controller also determines and stores an array of up to 100 feed factors. A feed factor is a mea- sure of the average bulk density of the material discharged at a given hopper weight. A low feed factor indicates that a higher number of screw revolutions were required to dis- charge a given weight, implying a reduced material bulk density. A higher feed factor indicates that fewer screw revo- lutions were required to deliver that same weight, implying a higher material bulk density. The middle graph in Figure 2 plots the motor speed during operation versus time. As shown in the graph, during the early portion of the gravimetric feeding phase, the motor Figure 3 LIW feeder pressure curve Force (grams) Time (seconds) 1,000 800 600 400 200 0 -200 0 50 100 150 200 250 300 350 Figure 2 Refill speed correction Refill complete weight Mass flow Motor speed Net hopper weight Refill start Feed factor array Refill request weight Refill time With refill speed correction Mass flow remains constant throughout refill with refill speed correction Mass flow error without refill speed correction Time Without refill speed correction Time Time Refill stop Figure 4 LIW feeder pressure curve Close-up on refill cycle Force (grams) Time (seconds) 10 20 30 40 50 60 70 210 190 170 150 130 110 90 70 50 30 10 -10 -30 -50 Gradual pressure drop