A variable rate feeder is a piece of industrial control equipment used to deliver solid material at a known rate into some process.

A number of different techniques and transport mechanisms are used, but these broadly split into these categories, in descending order of accuracy:

• closed-loop feedback systems, where the output from some variable feeder is measured and the feed rate varied by a local control system ensuring the desired rate is maintained
• volumetric feeders, such as screw conveyor
  Screw conveyor
  A screw conveyor or auger conveyor is a mechanism that uses a rotating helical screw blade, called a "flighting", usually within a tube, to move liquid or granular materials. They are used in many bulk handling industries...
  
  s, delivering a predictable volumetric feed, driven at a variable rate
• open loop systems where a feeder (e.g. a belt feeder) is driven at a given rate which has been established in advance
• open loop systems where a door or valve on a feed hopper is opened by a graduated amount, delivering an approximate rate of feed

Any of these may be combined with subsequent checks from beltweigher

Beltweigher

A beltweigher or belt weigher is a piece of industrial control equipment used to gauge the mass or flow rate of material travelling over a troughed conveyor belt of any length which is able to adequately contain the material being weighed. These are also known as belt scales, dynamic scales,...

s or (in the case of grains, powders and liquids) flowmeters to provide a more accurate flow of material.