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Click Here to Learn More!We have a system using 3 load cells (total capacity 675 lbs) and a Hardy Process Solutions weight controller to measure weight and rate on our feed conveyor. The material feed rate continuously varies from 1000 pounds per hour to 10,000 pounds per hour. The speed of the conveyor is fixed at 80 feet per minute +/- 3%. At the end of the day they totalize, via the PLC, the amount fed. It seems accuracy is around 10%.
What accuracy can we expect, assuming our programming is correct? Can you offer suggestions to optimize that?
The following answer applies to the HI 1746WS, HI 1756WS, HI 1756-2WS, HI 1769WS, HI 1769-2WS, HI 1771WS, HI 200DNWM, HI 6300 series, HI 2080, HI 2110 WI, 2151/30WC, HI 3010, HI 3030, and the HI 4050.
(NOTE: The HI 2160RCplus, HI 1756FC, HI 1769FC, and the HI 4060 controller do all of these calculations internally, but in a different format and application type. These units cannot be used in a continuous feed belt weighing type application. These controllers use loss in weight and can control the speed of the belt and determine/control rate and from that to determine a total weight lost. The belt and fill hopper must be included on a scale and isolated so to provide a LIW.)
We will use the HI 3030 for the example in our explanation.
Your Hardy weight controller calculates weight readings periodically and is called the update rate. For our example, the HI 3030 update rate is 55 updates per second. The update rate for other Hardy controllers can be found in other WebTech answers. If the application requires faster readings (finer definition), that will be explored later as an accuracy and resolution calculation.
Some ways to improve the output:
Perform calculations to determine the amount of weight passing over a scale for a time period and add that to a Totalizer. You should always verify the timing and speed calculations are accurate using a manual catch method and verify the weight on a certified scale. Start with WAVERSAVER set at 3.5 Hz and Averages of 10 - 20. Observe the application performance and make small changes to these parameters to improve the totaled weight.
Verify the decimal point is correct. To determine the decimal point, divide the total load cell capacity to fit into the one part in 10,000 guidelines. Where 0.01 is one part in 67,500, this would be too many decimal points. 0.1 is one part in 6,750 which is within the optimum range.
Changing back to two decimal points and the grad size to read in 0.05 increments will give you a resolution of one part in 33,750. A little over the top, but gives you more decimal points to work with. Depending on the application and scale installation, this may or may not work. The one part in 6,750 setting may be your equipment limit and produce sufficient resolution for your application. Performing tests to determine your best setting is recommended.
The weighed conveyor section should be separated from the non-weighed conveyor. You can install load cells in a section of conveyor, but the weight will be influenced by the up stream and down stream effects of the belt and material. Material tends to be lumpy and have different density from time to time and is generally difficult to use in a totalized weight applications.
First, collect the weight data at a starting point. When the belt has moved one full length, read the weight again and add that weight to a totalizer. Wait for one more full length of belt movement and read that weight again and add that weight to the totalizer. This would be the same concept as weighing and totalizing one bucket or belt section of material at a time.
Belt scale weighing methods rely on knowing the precise belt speed or position. For example, if we use an 8 foot belt scale at 80 foot per minute, the belt makes one complete material change every 1/10 of a minute. That means that every 6 seconds, a fresh pile of material will be on the belt. Every 6 seconds you can collect the weight reading and multiply by 10 to convert to Minutes. This is a flow rate and can be used to control the speed of material transfer. Also this same 1/10th of a minute reading can be added to a totalizer and used as an accumulation for batching or product tracking.
Overall accuracy of a scale system is normally 1 part in 5000 (0.02%) but is dependent on many variables. If the application is a fill and discharge, it would be using
