HOW DOES A BELT SCALE WORKS?

Not sure if this is what you are referring to but one system I am familiar with is a series of conveyors that will stop and start by sensors, weigh a package or product, scan a bar-code and send it to a particular area for sorting.

The first belt transports the package blocking a sensor and after a set time stops the package on the "weigh" belt or scale belt. The scale section is separated from the feed and take-away belts to have a more accurate reading. The weight is measured by the scale built into the system under the belt. It is zeroed out frequently to have a more accurate reading and calibrated usually on a daily basis. From there a label applicator usually will be used to place the info on the package or may be stored in memory to be added to the sum of items sent to a particular runout or Sortation bin. Once the bin or runout reaches near maximum it will alert the operator that maximum weight is almost reached with a light or other alarm. The operator clears the weight stored by the computer to start a new allotment just through pushing a "clear" button.

If not cleared a second alarm stating maximum reached and that particular runout or bin is taken out of the system. The take-away belt transfers the load to the machine from the scale belt, usually through a timing sequence to correctly load a tray, hopper, or associated conveyor.

Principle of operation is very simple

In a word one of belt conveyor idlers' is resting on load cell (weigh sensor), therefore signal from its sensor Uq does correspond to mass of bulk material/per idler's span q measured in [kg/m],[p/f]. I.e.

Uq=k1*q, k1 - transfer coeff.

There is as well speedometer either if you wish belt's speed sensor which produce signal Uv corresponding to speed of traveled belt V measured in [m/s],[f/s] i.e.

Uv=k2*V.

When we multiply these two signals we shall get

U_Q=k1*k2*q*V=k*q*V, [kg/s] either [p/s].

i.e. U_Q is corresponding to instantaneous flowrate. For getting total result of how much of bulk material had been passed through conveyor we must integrate results by means of integrator[totalizer], i.e.

Q=k∫U_Qdt, [kg] or [ton]

Either for simplicity

Q=q*v*t.

As for conveyors we have flowrate measured in tons per hour. Last expression could be written as

Q=3.6*q*v*t,

There were explained principle of simplest belt scales (belt weigher) which using analogue technique of measurement.

First BW had been appearing at 1908 and its been designed as completely mechanical. There were further developing of BW based on electromechanic, electric, hydraulic, analogue electronic, digital electronic and microcontroller basis.

Main advantages of BW are --- relatively low cost, ease for installing everywhere you need.

Main disadvantages --- its very very unstable in terms of accuracy, here's as well impossibility for most cases to perform properly adjustment tests.

On my opinion to call this device "a scale" is very controversial practic, so I do prefer to call its a weigher.

How one idler BW looks you could see here

Block Diagram of uC implementation

Hope it helps

Here's one more application. When someone is keeping a steady diet having pursued the idea to drop abundant weight, belt might be used as a scale.