Thayer Scale manufactures the most accurate weigh belts and conveyor belt scales on the market. With that said we are always looking for ways to improve on the products we provide our customers. Sometimes the influencing factors that cause inaccuracy in a weighing system are not due to the inaccuracy of the scale but due to such things as:
- splice-impact shocks
- gusts of wind
- non-uniformity of conveyor belting weight
- varying belt loading effects that are due to impressed belting curvatures (lack of flatness) that tend to decay or change under operation.
- idler “wobble” effects due to T.I.R (total indicator run out) and numerous other extraneous disturbances that can cause instantaneous measurement errors.
The length of a weigh span to weigh material on a conveyor belt plays a key role in establishing weighing accuracy and control response limits of a belt weighing system. Long weigh spans are more accurate for weighing the material. However, because of their longer transport time, long weigh spans are not as responsive to rapid loading changes as short weigh spans
Because short weigh spans have improved responsiveness due to their short transport time, short weigh spans tend to be more useful for certain control and alarming functions than long weigh spans. However, short weigh spans are more reactive to many extraneous influences (because they are more responsive to rapidly changing conditions.
Virtual Weigh Span (VWS) is a patented algorithm that effectively adds length to the Scale Weigh Span, thereby providing the benefits of both a short and a long weigh span simultaneously:
Effective Weigh Span = Scale Weigh Span + Virtual Weigh Span (EWS = SWS + VWS)
VWS works differently than regular filters used on conveyors in that it is completely independent of the speed of the belt. The most common filter is to simply add a time average to the loading signal, but this does not give consistent results when the belt changes speed. A quickly moving belt will average a very short span of belt, while a slowly moving belt will average a very long span of belt. VWS allows the user to determine the distance of belting being filtered, regardless of the belt speed. The user is always guaranteed the distance desired.
How it works.
The Scale Weigh Span (SWS) is the length of belting over which the loading measurement is made by the physical scale. This length is the sum of the length of belting directly supported by the scale mounted idler(s) plus ½ the lengths of belting extending toward the two fixed idlers to either side. The foregoing can be reduced to a simple mathematical definition:
“The Scale Weigh Span (SWS) is equal to the product of the number of scale mounted idlers (N) and the distance between the idlers (S), or SWS = N*P”
In the example above, there is only one scale mounted idler with all idlers spaced on 3 ft centers. The scale weigh span is therefore 1 X 3 = 3 ft. If the scale had 3 idlers, the scale weigh span would have been 3 x 3 = 9 ft.
We can achieve the benefits of a 9 ft weigh span by simply programming our virtual weigh span for 6 ft.:
EWS = SWS + VWS or 9 ft = 3 ft + 6 ft
The second term is the Virtual Weigh Span (VWS), which is the addition distance the user wants to average the load over. In the example above, the VWS is set to a length equivalent to 2 idler spacings, or 6 feet.
Effective Weigh Span (EWS), which is the Virtual Weigh Span (VWS) added to the Scale Weigh Span (SWS). In this example, the SWS is 3’ and the VWS is 6’. Added together results in an EWS of 9’, which is the same as a 3 idler weigh bridge previously mentioned.
Virtual Weigh Span is available from Thayer scale and is provide as standard feature in all Conveyor Belt Scale and Weigh Belt Integrators.
Contact Thayer for more information