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Managing your Belt Weigher Fleet
Establishing that your Belt Weighing equipment is fit for purpose.Common Belt Weigher accuracyCommon Belt Weigher accuracy problems. (& The special problem represented by Stockpile figures )represented by Stockpile figures.)
Effective Belt Weigher Maintenance.Testing and Auditing Belt Weighers
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Is the equipment Fit for Purpose ?
Process Control, 1% or 5% might be OK, , g ,but the equipment should be stable.
Unstable Equipment is a maintenance andUnstable Equipment is a maintenance and downtime cost.
Product Reconciliation 0 25% or betterProduct Reconciliation, 0.25% or better, only the best will do.
Stockpile Figures are a constant accuracy issue.
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Is the equipment Fit for Purpose ?Wh f dWhen performance does not meet expectation, the reasons can be;
Weigh Frames that are unstable and affected by belt tension. Poor Choice of Location, High belt tensions, Long Conveyors.Tachometer Systems that are unstable.Zero Error, which can turn a 0.25% belt weigher into a 5% weigher.Shoddy Calibration References and poor
lib i / i icalibration/maintenance practices.
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Weigh Frame Considerationsll d d ll h bFully Suspended type are usually the better
performer. Fully Suspended designs are sensitive only to loads perpendicular to thesensitive only to loads perpendicular to the conveyor belt and are insensitive to roll turning moment. g
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Fully Suspended Weigh Frame• Some Examples of Fully Suspended Designs
CST PFS4-4
Ramsey 10-14
Schenck BMP
CST PCS2-2
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Weigh Frame PerformanceffFactors Affecting Accuracy
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Weigh Frame Error SourcesWeigh Frame Error SourcesBelt Tension, the Higher and More variable, the worse is the performance.Longer Weigh Lengths support more of the material we
t t i h d l b lt t iwant to weigh and see less belt tension.Good idler Alignment is important to stable weigh frame
fperformance. Weigh Frames which are rugged and do not deflect much do not go out of alignment under load Conveyormuch do not go out of alignment under load. Conveyor Stringers should also be robust and well supportedLonger idler spacing works better than narrow idlerLonger idler spacing works better than narrow idler spacing. Artificially reduced spacing should not be used and negates the advantage of multiple idlers.and negates the advantage of multiple idlers.
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Conveyor RequirementsConveyor RequirementsTo perform correctly a belt weigher requires:
A straight length of conveyor typically 10 times longer than the weigh length, and 10 sets from skirts, samplers, magnets and the like.Belt tension control, usually achieved with a gravity take up device.The material should stay fixed on the conveyor. Sliding
lli i l b i h d lor rolling material cannot be weighed accurately.Accurate alignment of weigh idlers and two fixed idler
t h id f th i h f A d lsets each side of the weigh frame. A gradual ramp down from the weigh zone over at least 5 idler sets is desirabledesirable.
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W i h F A E ti tWeigh Frame Accuracy EstimatesSi l IdlT IdlF Idl F llWeigh Frame Accuracy
20 0%10 0%5 0%Extreme Belt tension
Single IdlerTwo IdlerFour Idler Fully Suspended
Weigh Frame Accuracy Estimates.
20.0%10.0%5.0%Extreme Belt tensionLength > 1000m
Location Near HeadMultiple or moving feed
10.0%5.0%2.0%High Belt TensionLength > 300m High Lift,
location mid conveyor
5.0%1.0%0.5%Medium Belt TensionLength>200m
Location mid conveyor
2.0%0.5%0.25%Low Belt TensionLength<100mTail Location
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Weigh Frame CalibrationStatic Masses simulate a conveyor loading, units kg/mg, g/
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Weigh Frame Calibration Systems
SIP Masses
Hand Masses
Calibration Chain
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Static Weight CalibrationBest to apply 100% of full scale mass to Weigh FrameUsing a small percentage of full scale static mass can lead to extrapolation error. Factors which affect the precision of the result are;
Is the System a Single Point or can it be applied in steps to y g pp pcheck linearity?Can the mass be easily applied several times to check repeatability?repeatability?
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C lib ti Ch iCalibration Chain Simulates a fixed kg/m of belt loading, usually S u a es a ed g/ o be oad g, usua yaround 70% of full capacityMust extend beyond the weight sensitive zone
Are used with the belt running
Are used over complete belt revolutions as areAre used over complete belt revolutions as are static masses
Are a single point load, represent maintenanceAre a single point load, represent maintenance and OHS issues.
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Calibration with <100% Mass
Best Accuracy Calibration
Loss of Calibration Calibration Ca b at oPrecision
Low Accuracy Calibration
20% 100%
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The value of a Linearity test
1. Single 100% Calibrtaion pointCalibrtaion point
2 Any Calibration
Response can be Non-Linear due to jambing of weigh f ibl2. Any Calibration
Error is obvious at 50% of full scale
frame or possible electronics range issues.
50% 100%
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Errors from Weigh Frame CalibrationMultiple Stored In
Place, Conveniently Applied Calibration
M
Multiple hand weightsSingle Application of Single Point Load or
Calibration Chain
Weigh Frame Calibration Methods
0.3%0.4%0.5%30% of Full Scale
Masses
0.15%0.2%0.3%70% of Full Scale
0 1%0 1%0 2%100% of Full Scale 0.1%0.1%0.2%
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TachometerTachometer – A Length measuring device
The length measurement device, commonly called g , ythe tachometer, is attached to a relatively large diameter pulley in contact with the clean side of the conveyor beltthe conveyor belt. The tachometer produces a digital electronic pulse, each pulse represents a calibrated length ofpulse, each pulse represents a calibrated length of conveyor belt which has passed over the weigh frame. The tachometer is usually calibrated in meters per pulse (m/pulse).
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Tachometer Calibration
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S E l f T h tSome Examples of Tachometers
Ramsey, Prox Type Schenck Standard
Shaft Driven on CSTCST Spiral Pulley
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Tachometer Accuracy EstimatesPoor tachometerMediumGood TachometerTachometer Poor tachometer,
Diameter <=100mmTangential contact
Medium tachometerDia 200-300mmContact 5Deg to
i l
Good TachometerDiameter >= 600mm180 Degree Angle of
Wrap
Tachometer Accuracy Estimates
2.0%1.0%0.5%Calibrate by
tangential
Pulley Measurement
alone
0.5%0.2%0.1%Calibrate by Measuring the belt
l h dlength and capturing the pulses
in one belt revolutionrevolution
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Belt Weigher System Schematic
LCD and
ElectronicsWIM (Weigher Interface Module)
Electrical
LCD and Keypad
CPUElectrical
Junction Box
AI1 Tachometer
(Central Processor Unit)
AIO2
AIO3
AO4
DIO1WEIGH
Load
Load
Analogue and Digital In/Outputs
DIO2
DIO3
DIO4
DIO5
Proximity Sensor
FRAME
dcells
dcells
In/Outputs
DIO5
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Zero ConceptsManual / Auto Zero processManual / Auto Zero process
Old zero set point (too high)
+ + + +Exactly one belt revolution
+ + + +
- - -- -
+
New zero set pointError
Auto Zero Tracking
-New zero set point(avg. of + and – weights)
Error(Sum of + and – weights)
If outside of limit alarm is raised andg
MATERIALZero Tracking timer One Revolution
Zero tracking window+
alarm is raised and new zero is not set.
Zero set point of last zero process
+%
-
ZT limit
last zero process-∞ New Zero constant
if inside limits
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The Accuracy Sum, ‘0.5%’ System
3.0%-0.1%Zero Error
Worst Case?
Best CaseError Source
0.5%0.1%Dead Weight Repeatability, manifesting as systematic error
3.0%0.1%Zero Error
4.0%0.5%Live load to Dead Load Error, systematic error
0.3%0.1%Tachometry Error, systematic error
1.0%-0.1%Live Load repeatability, random error
9.8%0.5%Total Error
Idler Roll Buildup and alignment Error 0.0% 1.0%
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D D A IDay to Day Accuracy IssuesLarge errors of consistent size 5%Large errors of consistent size, 5% to 10% (Alignment) Smaller errors, spoiling 0.25% performance (Calibration system)performance (Calibration system)Zero Stability ErrorIdler roll problemsPLC ConnectionPLC Connection
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Large Errors, Alignment Problem
When everything else hasWhen everything else has been checked and verified, its alwaysverified, its always alignment that causes the large errors.large errors.A 3mm alignment error can be 6% errorcan be 6% error.
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S ll E Ch k ClSmall Errors, Check Clearances
SIP Calibration Masses, check the chain supports, they need to be fully off when off and fully on when on.
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Zero Stability and buildupZero Stability and buildup'Normal' buildup can beNormal buildup can be taken care of by Zero Tracking, but there mustTracking, but there must be an opportunity.Water sitting over theWater sitting over the weigh frame will give a false zero adjustmentfalse zero adjustment.Buildup on rolls affects
d hzero and geometry with large errors as a result.
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PLC Connection ErrorsPLC Connection Errors
Mismatch, belt weigher display to Citect (DCS) screen can rob performance.Use built in test routines Stretch pulses soUse built in test routines. Stretch pulses so PLC scan matches, test PLC ability to count
lpulses.PLC Scan 70ms
Pulse would need to be 100mS
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Stockpile Accuracy
Errors in and out of stockpiles add and accumulate, Error will include belt weigher , gerror and moisture content changes.All the error on all the incoming materialAll the error on all the incoming material adds to stockpile errorAll the error on all the outgoing materialAll the error on all the outgoing material adds to stockpile errorStockpiles contain the sum of the errors
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Stockpile contains Sum of Errors
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Effective Belt Weigher MaintenanceWeigh Frame
Maintenance of Idler Alignment (annual) Idler Roll condition and significance (bi-annual changeout) Calibration checks frequency and needCalibration checks, frequency and needMaintenance of a good working zero
TachometerTachometerReasonable Maintenance, good and bad practice
Electronics SystemyChecking frequency and reliability of Load Cells and Electronic weighing systems.
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Maintenance of Weigh Frame CalibrationWeigh Frame CalibrationMaintenance of a good working zero
Significant zero errors of almost any size can easily occur and can make a 0.25% b l i h i %belt weigher into a 5% system.Most belt weighers use Automatic Zero
k d h f d h hTracking to adjust the zero if and when the belt runs empty, however, these systems need to be carefully set up to avoid beingneed to be carefully set up to avoid being tricked by trickle flow of material or water.
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Tachometer MaintenanceIn most cases, checks at three month or six month intervals will adequately account formonth intervals will adequately account for pulley wear.If a conveyor belt is changed tachometer re-If a conveyor belt is changed, tachometer re-calibration is advisable, but may not be as significant as first thought because the pulleysignificant as first thought because the pulley is the ‘calibrated’ part.If ll d t h t i h dIf a pulley used as a tachometer is changed, the tachometer must be re-calibrated.
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Electronics System Maintenance
Checking frequency and reliability of Load C ll d El t i i hi tCells and Electronic weighing systems.
Modern electronic weighing systems do not show i bl d if d d d lappreciable drift and do not need regular re-
calibration. (We have seen 0.1% repeatability over 5 years – but weigh frame must be good)5 years – but weigh frame must be good)Mounting electronics sets inside MCCs rather than in harsh outdoor conditions can significantlyin harsh outdoor conditions can significantly extend their life.
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Maintenance Regimes
Day to Day MaintenanceThere is a need for a specialist on most large sites to keep a regular check on the b l i hbelt weighers.The Zero of the system should be checked d l h l d ldaily at the electronics display.Any alarms generated on the electronics screen must be acknowledged and acted upon.
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Maintenance Regimes
Three Monthly Calibration CheckAll setup data as found should be recorded.The tachometer sub-system should be checked at least by observing that the correct belt speed is displayed.Aft h ki th f th t h ldAfter zero checking, the span of the system should be checked by applying at least 50% and 100% calibration mass Re-span only if there is acalibration mass. Re span only if there is a credible reason, otherwise declare faulty.Record all setup data as left.Record all setup data as left.
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Maintenance Regimes
Annual Belt Weigher Maintenance CheckLift belt, check and remedy idler alignment if required. Replace Rolls as appropriate.
d li b iRecord alignment observations.Measure the conveyor belt length and the
h l h f ll hweigh length. Carry out full tachometer calibration procedure.Carry out three monthly calibration procedure.
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M i t i i K l d RMaintaining Knowledge ResoucesInstallation and ReplacementInstallation and Replacement
At engineering level, the manufacturer can provide expert assistance.
Repair and Regular MaintenanceRepair and Regular MaintenanceAvoid 'Knowledge Dropout', Use factory trained specialists for maintenancetrained specialists for maintenance.
Day to Day maintenanceTraining of site electricians or instrument specialists by factory specialists will savespecialists by factory specialists will save un-necessary down time.
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Testing and AuditingA t id ti i t idAn outside perspective is necessary to avoid operational and maintenance blind spots or in-
i t i t ltappropriate maintenance culture.Live Load testing is sometimes the only way to establish confidence. (CST has a portable truck weigh bridge for this purpose.) It is necessary to bring a belt weigher up to its full potential before addressing deeper problemsfull potential before addressing deeper problems such as live load/dead load discrepancy. This is part of a belt weigher auditpart of a belt weigher audit.
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Ian BurrellManaging DirectorControl Systems Technology Pty Ltd37 Stanley St37 Stanley StPeakhurst, NSW, 2210
Mobile: 0419 292 604Work Direct: 02 9584 4507Work Direct: 02 9584 4507
www controlsystems com auwww.controlsystems.com.au
Ian Burrell has worked in the design, installation, maintenance and troubleshooting of belt scales for more than 30 years Ian started Controltroubleshooting of belt scales for more than 30 years. Ian started Control Systems Technology Pty Ltd in 1984, the companies products are now popular in Australia and are exported to many overseas economies. CST equipment is trade certified to 0.25% in Australia, to 0.1% in Canada, to 0.25% in the USA and has an OIML Class 0.5 Certificate for Europe –0.25% in the USA and has an OIML Class 0.5 Certificate for Europe 0.25%. Control Systems Technology Pty Ltd supplies and maintains belt weighers throughout Australia. They have offices in Sydney, Mackay and Perth.
