The purpose of band extractor control is to provide as stable and accurate material flow rate as possible, leading to finished product quality. The product in the mixer should be constant and meet the proportions of each component materials.

To do so, 2 types of controls are used :
       Volumetric control
       Weight control

Volumetric control : extractor speeds up and detects the material flow (seam sensor).
The weight is assumed to be fixed and correct, and only the speed is controlled and kept as constant as possible.

Weight control : Extractor speeds up and controls the weight.

The weight is measured n kilograms/metre.
The speed is expressed in metres/minute.
The ton flow rate per hour is expressed as follows :

Flow rate (t/h) = weight* speed (60 / 1000)

Control principle: 
    If the weight drops, the speed should go up to provide a constant flow rate.
    If the weight goes up, the speed goes down to provide a constant flow rate.
      (Within high and low limits thresholds set depending on the materials).

Weight control helps take into account :
    Material density.
    Material humidity.
    Material flow problems (allows to manage a vibration generator on automatic).

As a result, weight control provides better control that volumetric control.

Scope :
 With  level 2 and 3 central units, French standards provide for the use of volumetric hoppers to dose granulates with finecontents equal to or less than 2%. To dose granulatres with fine contents higher than 2%, humid binders and dry bulk, the dosing should be provided by gravity hoppers.
(Cf NF P 98-115)