Belt Weigher Technical

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Conveyor

Belt Scales

Dealer BuildsTrust In

Marketplace

Conveyor

Belt Scales

Dealer BuildsTrust In

Marketplace

A&D Weighing18

Allied Equipment14

Cardinal ScaleB.C.

CAS27

CCi Scale6, 7

Excell Precision35

HBM 17

INCELL35

INSCALE39

Intercomp23

JadeverWeightech

24

Microframe16

National Scale14

Ohause15

Rice LakeWeighingSystems

29

ShimadzuScientific

31

Shinko Denshi33

SouthwesternScale

27

Totalcomp Scales& Components

5, 19, 26, 28

Troemner13

Universal ScaleService

16

VISHAY20-21

Yamato25

MAY/JUNE

Serving The Industry Since 1914

2003

wammag.com


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Scales are gravimetric devices. In other words, they aredevices that measure the force that gravity exerts onmass. Scales can be classified as two types: differential and

integral weighing devices.

Differential Weighing DevicesDifferential weighing devices are the most common.

You begin at one weight on a scale and then add or sub-tract weight to give you the difference between the begin-

ning and ending reading. For example, a truck scale startsat one reading, generally zero, and then goes to a differentweight reading when you drive a truck onto the scale plat-form. The difference between the two readings is theweight of the truck. Truck scales, rail scales, platformscales, package scales, postal scales, counting scales, etc.all operate on this principle.

Batching scales weigh ingredients into a weigh hopper

on a differentialbasis, one ingre-

dient at a time. Bulk weighers weigh and dump into andout of a bulk material weigh hopper to record the high

weight, the low weight, and the difference which is thetotal weight transferred of bulk materials. Loss-in-weightfeeders use a feeding device to empty a weigh hopper ata controlled rate using the rate of change of weight (thedifferential weight change) as the measuring element.

Integrating Weighing DevicesIntegrating weighing devices are the second category

of scales. Conveyor belt scales are integrating weighingdevices that use a simple integral calculus summationprocess to measure a conveyed quantity of material. Twovariables are involved: weight and speed. A weight func-tion measures the weight of a small section of a convey-

or. The gross weight on the scale is the weight of the belt,the belt conveyor idler and the material on the belt. Thenet weight of material only is the gross weight less theweight of the supported section of the belt and the scaleidler. The speed function is the second variable to be mea-sured. Most modern speed sensors are rotary digital pulsegenerators. These can be optical, magnetic or other on/offsensing units. They are mounted on a pulley or wheel thatrotates as the belt moves. They generate an on/off signalas they move which is directly proportional to the dis-tance the belt moves and the speed of the belt.

In order to better understand this belt movement com-bined with the weighing function, let us look at a singleweight traveling along a conveyor as it crosses over aweighing idler section (see example). A triangular shapedweight function is generated as it goes from zero to the fullweight and back to zero. Every particle of material thatgoes over the scale generates its own triangular function.The total weight sensed at any particular position of theconveyor is the sum of all particles in the weighing areawith respect to their individual triangular waveforms andtheir position in the scale weighing area. This weight func-tion is a representation of the weight per unit distance atany one point on the conveyor belt. This is usually repre-sented in lbs./ft. or kg/m.

W & M/June 036

Application and

Operating Principles

Of Conveyor

Belt Scalesby Richard D. Linville, Jr.


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Formulas used for commonly displayed Data:

Total weight = Weight/unit distance x distance

Rate = Change of total weight

Time

Belt Speed = Distance

Time

As the conveyor moves a small discrete distance asmeasured by the speed sensor a portion of the weight istotaled. If the belt loading is 50 lb./ft. and the belt moves1/100 of a foot then the totalizer will add 0.5 pound tothe total. This happens at a relatively high speed. A con-veyor travelling at 300 ft./min. will generate 30,000 addi-tions per minute or 500 readings per second. We use

multiple readings per pulse to gain higher resolution.This equates to 4000 analog weight readings per secondin the above example.

Using integral calculus, there are several ways to addup the weight between pulses. These are left-handapproximation, right-hand approximation, trapezoidalapproximation and Simpsons rule approximation. Theleft- and right-hand approximations sum rectangularareas that intersect the curve at leading or trailing sidesof the curves intersection between two pulses. Thetrapezoidal approximation give the area of a trapezoidthat goes between the leading and trailing curve inter-

section points toform a trape-zoidal shape.Simpsons Ruletakes two-pulsewidth spaces anduses the begin-ning, middle andend points to fit aparabolic curve

to approximate the area under the two-pulse width area.This is the most accurate mathematical model, but itinvolves more extensive computational algorithms.

Another factor that effects the weight-sensing ele-ment on a conveyor belt scale is the angle of the con-veyor from a level position. The force seen by the scaleis proportional to the actual weight times the cosine ofthe conveyor angle. A level conveyor has an angle of zerodegrees. (Cos. (0) = 1) Therefore the force of gravity andthe resultant force on the scale would be the same. A 40-pound weight would have a measured force equal to 40pounds. A conveyor at 30 degrees has a cosine = 0.866.Therefore a weight of 40 pounds on the conveyor would

result in a measured force of 34.64 lbs. on the scale.A fixed angle conveyor scale can be compensated by

a change of gain in the scale. In the above example,34.64 lbs./cos.(30) = 40 pounds. This can be a fixednumber if the conveyor angle is fixed. However, somestacker conveyors change angle of elevation as theyoperate. An angle compensator can measure the angle

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of the conveyor and compensate thescale reading to correct for chang-ing angle.

Types of Belt Scale WeighingTechnologies

Many different types of weighing

technologies have been used over theyears for conveyor belt scales.

The first belt scales used mechan-ical integrators. The speed of the beltdrives a disk. A ball or another diskrolls on this disk at right angles to thespeed disk. The position of the ball orsecond disk is controlled by theweight on the scale. If the ball is in thecenter of the speed disk, it does notrotate at all. As the ball is moved tothe outside, it rotates faster and faster.The speed of the ball is a mechanical

multiple of the speed of disk and theposition of the ball on the disk. Theball is connected to a shaft that dri-ves a totalizer. The speed of this shaftis the rate.

Another method of determiningthe load on a conveyor belt was to usea nuclear source on one side of thebelt and a detector on the other side.The more material that is on the belt,the less radiation is detected. You getan inversely proportional signal ofabsorbing mass on the belt. Usingthis along with belt speed you couldmake a belt scale. These were neververy wide spread in actual use.

LVDTs (linear variable differen-tial transformers) are position sens-ing devices that have been used inconjunction with spring mecha-nisms to measure belt weight. Theyare non-contacting sensors andgenerally have spring mechanismsthat are rugged in nature which

gives them very goodoverload characteristics.On the down side, theycan exhibit problems withthermal effects on themechanical springs.These effects can be bothon zero because of ther-mal expansion and on thespan due to modulus of

elasticity changes.Strain gage load cells

are commonly used inscales today. Many havebeen adapted to mechani-cal lever scales using a

lever system to sum forces at a singlepoint where a load cell is used to readthe resultant force electronically atthis point. These scales were com-

mon for retrofit installations on truckscales for example. Also, strain gageload cells were initially very expen-sive and limited to in-line tensionand compression forces. This made

mechanical hybrid load cell mecha-nisms popular. Some belt scales aremade with this technology today, buttheir use is declining.

Today, most scales are full straingage suspension types. They use lit-tle or no mechanical mechanismsand the load platform is directly sup-ported by the load cell system. Mostnew conveyor belt scales use thistechnology. Overload stops are builtinto the load cell mounting systemsto make them rugged and reliable.Also, they are simple in their overalldesign. Single point suspension loadcell designs allow for off-center load-ing. This also compensates for anyextraneous side loading that couldeffect the true weight component ofthe belt load forces that we are mea-suring.

There are advantages and disad-vantages when considering convey-or belt scales. As an integrating

device, a conveyor belt scale is a verycompact device that fits into a con-veying system. The scale handleslarge quantities of raw materials on acontinuous basis. It does not requirelarge batch hoppers, truck scales orrail scales. It can handle rates thatmake the application of differentialscales almost impractical. Loadingand unloading ships is one example.

Belt scales have their weak points.First, belt scales can run for extend-ed periods of time without returningto zero. This means that any buildupon the scale that affects the zero willnot be detected until the scale is runempty. A zero error is not a one-timeerror. This error is continuouslyadded along with the weight of actu-al material. Also, extraneous forcessuch as wind loading, changing belttension, excessive vibration, andmechanical interference with the

scale can cause errors in the weigh-ing system. The scale does not knowthe difference between the forcesfrom actual material we are trying toweigh and the forces from externalnoise. Although this is true of allscales, the ratio of signal to noise ismuch better on differential weighingdevices than on integrating devices.Integrating belt scales are weighing asmall portion of weight on a continu-ous basis and external noise can besignificant. Care must be taken to

correctly install a belt scale and toprotect it from external influences.

About the AuthorRichard D. Linville, Jr. is president

of Sauk Valley Systems, Inc. locatedat One Belt Way, Rock Falls, IL 61071;phone 815/625/5573; web site:www.saukvalleysystems.com.

Editors Note: This article is a pre-sentation taken, with permission,from the National Industrial ScaleAs so ci at ion s 20 00 Te chn ic alPublication. The publication com-prises the presentation from NISAsSpring and Fall Conferences of 2000.The 52-page publication is availablefrom NISA, 1932 Industrial Drive,Libertyville, IL 60048. The cost is $25.

W & M/June 038

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