The basic loss-in-weight operating principle requires that special enhancements to its control scheme be made if it is to be successfully applied in practice. These enhancements are introduced below.
In loss-in-weight feeding, measured feeder-system weight is continually compared against the setpoint’s consistently declining target weight value. Any difference between the two values triggers an appropriate adjustment in metering speed. Thus, any external disturbance experienced by the weigh system (such as a coffee cup placed on the feeder or a broom bumping against it) will temporarily perturb the system, likely producing a control action in response.
While some disturbances may be readily suppressed through mounting isolation and appropriate filtering of the weight signal, any transient disturbances that do result in spurious control actions require immediate compensation to remedy the induced error.
Special perturbation control programming provides this capability. By continually detecting and accumulating any and all differences between measured and desired system weight, small metering speed adjustments can be applied to quickly and accurately compensate for any transient under- or over-feed condition resulting from the disturbance.
When the feeder hopper's charge of material nears depletion, it must be resupplied. However, since feeding accuracy relies on the ability to continuously weigh the entire feeding system, direct gravimetric control during this period is not possible. Even though refill should occur quickly, it is important that feeder performance be maintained during hopper replenishment.
As illustrated on the left in the accompanying diagram, lacking any basis for gravimetric control during the brief refill period, metering speed has historically been held constant throughout refill (A), reverting, in effect, to volumetric control. However, two problems arise from this approach. First, the density of material discharged during refill can rise due to increasing headload (B), up to 10-15% for some compressible powders. This compaction briefly but potentially significantly compromises feeder performance (C). Second, after refill is complete and the weigh system again senses a consistently declining weight, re-entry to legitimate gravimetric control will result in an abrupt slowing of metering speed (D) as the feeder senses and adjusts to the increased density of the headload-compressed material.
At right, to minimize feed rate errors during refill, K-Tron’s Smart Refill Control technology enables metering speed to be gradually reduced during the refill process (E) to precisely compensate for increasing material density (F). Using material density data extracted from the previous gravimetric feeding phase, metering speed is gradually slowed as the hopper is refilled, resulting in near-gravimetric performance throughout the refill operation (G) while avoiding the abrupt, steplike change in metering speed experienced in the traditional approach’s re-entry to true gravimetric operation. (Download Smart Refill Technology white paper)
Single screw feeders, particularly at low rates, can produce non-uniform discharge within one revolution of the screw, resulting in mass flow pulsation. In critical applications where optimal momentary feeder performance is required, such pulsation can significantly increase second-to-second discharge variability. Possible mechanical remedies include conical screws, rotating tubes, extended tubes, threaded spiral ends, paddles, etc. Depending on the application some are feasible, others not. And despite additional costs for these solutions, there is always a residual pulsation.
To address this pulsation effect without mechanical means, however, a new screw speed modulation algorithm produces increased accuracy over short sample durations as well as at low rates (below 60rpm). By recording the periodic pulsation and appropriately adjusting screw speed to compensate for the effect, mass flow variability associated with screw position can be substantially reduced, even during the feeder’s volumetric refill phase or during weight disturbances. (Download Screw Speed Modulation info)