Technology Zone

Instrumentation and Control

Sensors, analyzers and software turn a recycling line into a measured process. This zone covers material identification, mass-flow and moisture measurement, radiation detection and the control software that ties them together.

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Instrumentation and Control: how it works, key numbers and troubleshooting

Fundamentals

Turning a sorting plant into a measured process

Most recycling plants run on operator experience: the loader driver sees the pile, the line manager hears the shredder. Instrumentation makes that knowledge measurable — mass flow per stream, composition, moisture, machine load, purity — and control uses it to keep the line at its design point. The gains are in throughput stability, purity, energy and the ability to prove quality to a buyer.

Measuring flow and level

Belt scales on every major conveyor give mass balance across the plant; radar level sensors in bunkers and hoppers keep feeders supplied; speed monitors, motor current and vibration sensors watch every drive. A mass balance that closes to within a few per cent is the foundation for every other measurement; without it, purity and recovery numbers are guesses.

Measuring composition and quality

Sensor-based sorters already carry NIR, VIS, X-ray and AI cameras; the same sensors mounted over a belt as monitors report the composition of the input and the product continuously. Moisture sensors on organics and RDF, metal detectors on product streams, and camera-based counting of contaminants (batteries, PVC, glass) turn manual sampling into a live signal. Laboratory sampling stays as the reference.

Control and automation

PLC and SCADA systems sequence the plant: interlocks (a conveyor runs only if the next one runs), metering control on the feed bunker, load-dependent speed on shredders, automatic reversing on jams, and recipe control on sorters. Data historians record every signal; dashboards show purity, throughput and downtime per shift; alarm management stops the operator drowning in nuisance alarms.

Radiation, batteries and safety instrumentation

Scrap yards and incinerators screen incoming loads for radioactive sources with portal monitors; X-ray and AI cameras find lithium batteries at reception; gas detectors watch organic storage and landfill gas; thermal cameras and spark detectors feed the fire system. Safety functions run on dedicated safety controllers, separate from the process PLC.

Key parameters

ParameterTypical rangeRule of thumb
Belt scale accuracy±0.5–1 % with weekly zero checkIdler alignment and belt tension decide more than the electronics
Plant mass balance closureWithin 2–5 % over a shiftIf it does not close, find the unmeasured stream
Radar level in bunkers80 GHz; ±5–10 mm; dust-tolerantAim away from the fill stream and the loader
Online moisture (organics, RDF)NIR or microwave; ±1–2 % after calibrationCalibrate per material; heterogeneous waste scatters
Composition monitorNIR/VIS camera over belt; results per minuteTrend, not absolute; verify with manual samples
Radiation portalAlarm threshold a few % above background; vehicle speed ≤ 8 km/hEvery load, both directions of travel
Speed and rotation monitoringAlarm below 80 % of nominal within secondsStandard on every conveyor and elevator
Data logging1 s resolution for drives; 1 min for flows and levelsKeep at least a year for trend analysis

Troubleshooting

SymptomLikely causesWhat to do
Mass balance does not closeUnmeasured streams (fines, reject, rework); belt scale drift; double countingMap every stream, calibrate scales, define measurement points
Level sensor in bunker gives wrong levelSensor sees the loader or the fill stream; build-up; heap shapeReposition, use two sensors, camera as backup
Nuisance alarmsThresholds set at commissioning; sensors on unstable signals; no alarm managementAlarm review, deadbands and delays, prioritisation
Composition monitor disagrees with labMonitor sees surface only; burden depth; calibration setUse as trend; calibrate against lab samples from the same time window
Shredder trips on overloadNo load-dependent feed control; feed surgesFeed metering on motor current, automatic reversing, buffer bunker
Radiation alarms with no source foundNaturally occurring radioactive material (fertiliser, ceramics, some scrap); medical isotopesSpectroscopic identification, procedure for isolation and re-check
Data collected but not usedNo dashboards; no KPIs; nobody owns the dataShift dashboards with purity, throughput, downtime, energy; weekly review

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What this zone covers

Frequently asked about instrumentation and control

Where do I start with instrumentation in a recycling plant?

With a mass balance: belt scales on the input, the main products and the residue, plus level sensors on the feed bunkers. Once the tonnes are known per stream and per shift, purity, recovery, energy per tonne and downtime become meaningful numbers, and every later sensor has something to be compared with.

Can I measure the composition of my input continuously?

Approximately: NIR and camera monitors above the input belt report the share of the main material classes in the surface layer every minute. Use them as trends and for detecting a change in the input, and keep manual sampling as the reference — the monitor sees the top of the burden, not the whole stream.

How accurate are belt scales in recycling?

±0.5–1 % on a well-installed scale with aligned idlers, constant belt tension, a weekly zero check and a periodic span check with test weights or a material test. Sticky, wet material on the belt and misaligned idlers are the usual causes of drift; the electronics rarely are.

Should the safety systems run on the process PLC?

No. Emergency stops, guard interlocks, spark extinguishing, fire suppression and explosion protection belong on a certified safety controller with its own wiring and diagnostics; the process PLC receives their status. That separation is required by the machinery and ATEX rules and expected by insurers.

What should a shift dashboard show?

Throughput per stream, purity and recovery of the main products, downtime by cause, energy per tonne, filter and shredder condition indicators, and open alarms. Fewer numbers looked at every shift beat a hundred numbers nobody reads. Owning the data — one person responsible for each KPI — matters more than the software.

How do I detect batteries and radioactive sources at reception?

Radiation: portal monitors at the weighbridge that every load passes at low speed, with a procedure for isolating and identifying alarms. Batteries: visual inspection on the tipping floor supported by X-ray or AI camera systems on the reception conveyor, with a KPI on batteries found. Both are cheaper than the fire or the contaminated load they prevent.

Is a level sensor enough to control feeding?

A level sensor tells the operator when to load and lets the PLC hold the bunker in a band; the actual feed rate to the line comes from the metered discharge (floor or drum speed) and, ideally, a belt scale directly after the bunker that closes the loop. Level alone does not keep the burden depth constant.

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