Variable rate feeder

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