The need to accurately and continuously weigh in a process environment presents specific challenges to a weigh belt feeder. The particular types of demands result from the feeder’s operating principle. These weighing challenges are introduced below for weigh belt feeders.
Unlike the loss-in-weight feeder, the required measurement range of a weigh belt feeder is limited. This is due to the fact that a roughly consistent bed of material is formed upon entry to the feeder, and feed rate is varied through belt speed adjustment. Thus, belt loading (weight per unit belt length) is reasonably constant for a given setup.
In weigh belt feeding small corrective adjustments to belt speed can occur many times per second. To determine the basis for these adjustments the weigh system must accurately gauge the weight of material passing above it at speeds anywhere from a slow crawl to quite a quick clip. For each measurement this requires the accurate weighing of a cross sectional slice of material whose length depends on belt speed. This, in turn, requires high resolution weighing for optimal feed rate control.
Measurement repeatability in weigh belt feeding is just as important as it is in loss-in-weight feeding or any other form of gravimetric feeding for that matter. However, the issue becomes complicated by the fact that, unlike loss-in-weight feeding, weigh belt feeding involves sensing the weight of material through the moving conveyor belt as it crosses a supporting (stationary) weigh deck in its transport to discharge. Measurement repeatability demands a deflection-free weigh system as well as an effective taring and belt management system as addressed below.
Because of the comparatively limited weighing range required in weigh belt feeding, the linearity performance of weighing systems suited to this principle is not of as much concern as in other feeding principles.
Because weigh belt feeding relies on accurate weighments through the belt, any change in belt weight due to wear, material impregnation or adherence will directly produce a proportionate feed rate error. Historically this necessitated frequent and time-consuming re-taring involving emptying the feeder and running it through several belt rotations so the feeder could ‘learn’ its new tare value. More recently, however, the taring process has been fully automated and can be performed continuously without process interruption, eliminating variations in belt weight as a source of feed rate error.
Notwithstanding the automation of the taring process, the belt itself remains a possible source of weighing error. To weigh accurately through the belt, a constant tension must be maintained at all speeds, and the belt must not be allowed to come into contact with the flanking side skirts whose job it is to prevent material from falling off the side of the conveyor. Appropriate mechanical systems accomplish these tasks, augmented by belt scraping and cleanout systems, but checks of their continuing effectiveness should be part of any weigh belt feeder maintenance regimen, especially when handling dusty materials.
With today’s digital belt feeders corrective adjustments to belt speed are applied several times per second. This requires the weigh system to perform an accurate, high-resolution weighment with the same frequency. In turn, this demands a highly responsive weigh system that can precisely gauge the weight of each cross sectional slice of material as it passes over the weigh deck. Note that all corrective adjustments based on these weighments are automatically delayed precisely long enough for the material slice that warranted the adjustment to reach the point of discharge. This is known as transport lag adjustment.
Shock and Vibration
Again due to its operating principle, the weigh belt does not require the same high level of isolation as is required in loss-in-weight feeding. However, shock mounts and flexible inlet/outlet connections are often employed as a conservative measure.
All gravimetric feeders must contend with the issue of temperature (its value and its variability) on two fronts: the material itself and the ambient environment. In weigh belt feeding, unlike loss-in-weight, the weigh system is close to the material, and is thus influenced more by its temperature and variability than it is by the external environment. Handling materials whose temperature is high and/or highly variable, may necessitate the option of insulating and controlling the temperature of the weight sensor at an elevated but stable value.