The need to accurately and continuously weigh in a process environment presents specific challenges to a loss-in-weight feeder. The particular types of demands result from the feeder’s operating principle.
These weighing challenges are introduced below for loss-in-weight feeders.
Since, in loss-in-weight feeding, the entire feeder and hopper is weighed, a significant portion of the weigh system’s range may be consumed by the feed system itself. Thus, the measurement range of the weigh system must be sufficient to accommodate this larger weight while at the same time offer enough unused range to weigh the material charge as well.
Key to weighing precision in any feeding application, measurement resolution must be especially high in order to reliably detect the exceedingly small, moment-to-moment changes in total system weight characteristic of loss-in-weight feeding. Underscoring the need for the highest possible ‘at-the-sensor’ resolution is the frequently required use of a force-reducing weighbridge that proportionally reduces weighing resolution. To achieve high gravimetric feeding performance, weight measurement (not display) resolutions comfortably in excess of 1:4,000,000 over a time base of 80ms are usually required.
The repeatability of the feeder’s weigh system strongly contributes to overall feeding performance. Comprised not just of the weight sensor or load cell itself, but also of the structures, systems, or weighbridges that carry, transmit and apply the load to the point of measurement, the weigh system as a whole must consistently report the same result whenever an identical load is applied. For this reason, any movement, deflection, distortion or displacement experienced within the weigh system is to be avoided or at least minimized. In loss-in-weight feeding where accurate weighments must be made over a significant range of loads, this concern becomes especially important.
Whenever applied loads vary over a range, as is the case in loss-in-weight feeding, the weight sensor’s linearity performance commands attention. Historically, the linearity of a weight sensor largely reflected the linearity of its underlying operating principle. (For example, a simple spring scale is linear within its elastic range, but not if its operating principle is literally stretched too far!) Fortunately however, the advent of the microprocessor made it possible, convenient and economical to electronically linearize weight measurement. This, along with other advancements made possible by the microprocessor, facilitated the development of new alternatives to weight sensor designs such as the vibrating wire whose ‘resonant-frequency-versus-tension’ measurement principle, although non-linear, can be accurately linearized through post-measurement processing.
Taring or ‘zeroing out’ the weight sensor is not a crucial issue in loss-in-weight feeding performance because measurement and subsequent control actions are based on moment-to-moment differences in weight, not absolute weight values.
In loss-in-weight feeding, when the feed hopper approaches depletion, measured weight drops to a preset minimum level, triggering a refill request. During the refill cycle the feeder continues to discharge material as the hopper is filled with material, either automatically or manually. Lacking weight measurement during this intentionally brief period, other techniques (such as utilzing previous gravimetric mode trended and stored data) must be employed to maintain accurate feed rate control during the refill operation.
Unlike other gravimetric principles such as weigh belts or weigh meters, loss-in-weight feeding involves weighing the complete feeder/hopper/material system. As a result, the physical size of a loss-in-weight feeding system will roughly correspond to its maximum rate capacity. Low rates permit small feeder/hopper systems while high-rate units become correspondingly large. This attribute presents a weighing challenge for physically large systems. While small, low-rate systems can be readily weighed using a conventional platform type weighbridge, such an approach becomes progressively less feasible as system size increases due to concerns of load stability related to the system’s higher center of gravity. As shown in the accompanying diagram the solution is simply to suspend larger systems from multiple (summed) weight sensors where measurement stability is restored.
In order to consistently maintain a high level of feeding precision over short timescales, such as is required in many extrusion and compounding operations, the feeder’s weighing system must be able to capture its measurements very quickly and very accurately. This demands an unusually ‘stiff’ and responsive weigh system with a very low stabilization time, especially in loss-in-weight feeding where the applied weight is constantly declining. As a result, any weigh system that relies on physical deflection to produce its measurement impairs performance potential in this crucial aspect.
Shock and Vibration
A process line is not an ideal weighing environment. In both principle and practice, loss-in-weight feeders require effective isolation from their surroundings via proper mounting and the use of flexible inlet and outlet connections. Necessarily sensing and residual induced inertial forces as well as legitimate weight data, some of today’s advanced systems apply sophisticated filtering and signal processing techniques to isolate and extract meaningful weight signals. Such measures are especially important in loss-in-weight applications where the feeding system is located near heavy operating machinery such as an extruder.
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 loss-in-weight feeding, material temperature/variability is not usually an obstacle to accurate weighing simply because, as typically configured, the weighing system is sufficiently separated from the material so as not to be affected by its temperature. However, ambient temperature always requires consideration in extreme or uncontrolled environments. Viable solutions are readily available, but the concern must be identified before action can be taken.