The basic weigh belt 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.
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 average tare value.
More recently, however, the taring process has been fully automated and can be performed continuously, without process interruption, and with much greater precision than traditional manual taring. Although available only as an optional enhancement, the improvements in operational efficiency offered by this convenience warrant its inclusion here.
Transport Lag Delay
In weigh belt feeding, material is weighed through the belt as it traverses a short weighdeck positioned midway between the feeder’s inlet and discharge points. As the loaded belt crosses the weighdeck, the weigh system (with belt weight tared out) registers a moving average of material weight, providing the basis for any required adjustment to belt speed to maintain the desired discharge rate.
For precise control especially at lower belt speeds or when the feeder is proportioning to variable or wild (uncontrolled) material streams, corrective adjustments to belt speed must be time-delayed to permit the material that caused the need for the adjustment to reach the point of discharge.
Belt Slip Detection & Compensation
In weigh belt feeding, any belt slippage will understandably result in a direct feed rate error and will also compromise the transport lag delay adjustment discussed above. To avoid this possibility, many weigh belt feeders employ a high traction drive roller along with some form of mechanical belt tensioning. Also, scraper and diversion systems help minimize the accumulation of dust and residual material to the drive region which could cause or aggravate belt slippage.
These design features, combined with recommended inspection and cleaning, guard against belt slip. However, additional protection is provided when the belt speed sensor is located on the idler roller. In this way, if belt slip does occur, the condition can be immediately sensed, indicated and automatically compensated for by appropriate control actions until the underlying condition can be corrected.