Common Food Metal Detector Faults and Troubleshooting Tips

Common Food Metal Detector Faults and Troubleshooting Tips

Introduction: Why metal detector uptime matters for food safety

A reliable food metal detector is essential to protect consumers and brands, reduce recalls, and meet retailer and regulatory standards. Downtime or poor performance can cause costly production halts or, worse, product contamination. This article explains the most common faults, how to troubleshoot them, and practical maintenance and validation steps. Throughout we reference real-world operating recommendations and industry best practices so your detector performs consistently.

1. Symptom: Loss of Sensitivity or Sensitivity Drift

What it looks like

Sensitivity drift presents as a gradual inability to detect small metal particles that were previously caught, or inconsistent detection thresholds across shifts. You might notice occasional missed rejects on product lines or failing routine test-piece checks.

Common causes of sensitivity loss

Typical causes include product effect (product conductivity/encapsulation camouflages metal), coil degradation, electronic component aging, environmental changes (temperature/humidity), conveyor instability, and electromagnetic interference (EMI) from nearby equipment.

Troubleshooting steps

1) Run a standardized test-piece check using the manufacturer’s approved test pieces at the operating line speed and product presentation. 2) Isolate product effect: test with and without product on the belt. If detection improves without product, adjust product compensation or increase sensitivity within acceptable limits. 3) Inspect coils and connectors for damage or contamination; clean coils per the manual. 4) Review environmental factors — allow the detector to stabilise after temperature shifts and control humidity. 5) Check for new EMI sources (VFDs, welders, large motors) and relocate or shield them. 6) If drift persists, schedule calibration/service; many operations require full calibration every 6–12 months or after major mechanical changes.

2. Symptom: Frequent False Rejects

What it looks like

False rejects occur when good product is ejected as contaminated. This reduces yield, increases waste, and can mask real faults if operators begin to ignore rejects.

Common causes of false rejects

Causes include excessive sensitivity, unstable product presentation (overlapping or inconsistent spacing), conveyor vibrations, incorrect conveyor speed settings, transient EMI pulses, and dirty or misaligned reject mechanisms that trigger incorrectly.

Troubleshooting steps

1) Verify reject timing and mechanical operation — ensure the reject actuator moves smoothly and timing matches product transit. 2) Reduce sensitivity slightly and re-test with the test piece; use the lowest sensitivity that still reliably detects your minimum contaminant. 3) Improve product presentation: add infeed guides, increase spacing, or use product flattening to reduce variable product effect. 4) Check conveyor motor mounts and belt tension to eliminate vibration. 5) Log false reject events and correlate with other equipment cycles to identify intermittent EMI. 6) If the reject system is pneumatic, check air pressure stability and valve response.

3. Symptom: Missed Contaminants (False Negatives)

What it looks like

Missed contaminants are when known metal pieces pass through the detector without triggering a reject. This is the most critical fault for food safety.

Common causes of missed contaminants

Causes include detector set too low (sensitivity reduced), improper calibration for product effect, aperture size too large for the contaminant size, orientation of the metal piece (long axis parallel to coil), and actual hardware faults such as a damaged coil or failing electronics.

Troubleshooting steps

1) Immediately run test-piece validation with traceable test pieces representing the smallest contaminant you must detect. 2) Check settings: verify aperture size, product compensation, and conveyor speed are set as specified for the product SKU. 3) Inspect coils, connectors, and grounding. 4) Evaluate aperture choice — if you routinely miss small particles, consider switching to a smaller aperture model or upgrading detection electronics. 5) If problems persist, stop the line and request a qualified service technician for in-depth electrical and coil testing.

4. Symptom: Conveyor or Mechanical Failures Affecting Detection

What it looks like

Mechanical issues include belt mistracking, torn belts, uneven speed, or jammed reject arms — all of which impair metal detector performance or lead to inconsistent detection.

Common causes of mechanical problems

Belt wear, incorrect tension, misaligned rollers, foreign material buildup, or weakened bearings on rollers and reject mechanisms are typical causes. Poorly maintained conveyors transmit mechanical noise and vibration into the detector electronics.

Troubleshooting steps

1) Perform a mechanical inspection: check belt condition and tension, roller alignment, and cleanliness. 2) Replace worn belts and bearings promptly. 3) Verify conveyor speed with a tachometer and compare to detector configuration. 4) Ensure reject mechanisms are lubricated and actuators respond within specified times. 5) Maintain a preventive maintenance schedule: daily visual checks and monthly mechanical inspections reduce unplanned downtime.

5. Symptom: Power, Communication, or Software Errors

What it looks like

Common symptoms are error codes on the HMI, unresponsive controls, intermittent resets, or missing data to the factory network/PLC.

Common causes of electronic errors

Power surges, unstable mains, poor grounding, damaged power supplies, outdated firmware, or faulty communication cables and connectors can all disrupt normal metal detector function.

Troubleshooting steps

1) Check mains voltage stability and use surge protectors and UPS where appropriate. 2) Inspect and clean connectors; replace any corroded cables. 3) Confirm proper grounding and equipotential bonding between the detector and conveyor frames. 4) Review error codes in the manual and apply manufacturer-recommended fixes. 5) Keep firmware updated but follow manufacturer instructions — test updates on a non-production unit if possible. 6) For PLC/SCADA communication faults, verify protocol settings, baud rates and network health.

6. Symptom: Electromagnetic Interference (EMI)

What it looks like

EMI typically causes sporadic false alarms, sensitivity drift, or complete inability to stabilise. It often correlates with nearby equipment cycles (e.g., forklift chargers, welders, motors starting).

Common sources and detection

Sources include variable frequency drives (VFDs), high-current welding, radio equipment, and fluorescent light ballasts. EMI is often intermittent and correlates with other machinery operation.

Troubleshooting and mitigation

1) Temporarily stop suspected equipment and observe detector behaviour. 2) Increase physical distance or re-route cable runs away from the detector. 3) Install ferrite cores, cable shielding, or grounded metal barriers. 4) Ensure the detector installation follows the manufacturer’s separation guidelines from noisy equipment. 5) Where EMI cannot be eliminated, consider models with better EMI immunity or additional filtering modules.

7. Calibration and Validation Best Practices

Routine check frequency

Industry best practice is to perform daily in-line test-piece checks at start-up and after product changeovers. Document results for HACCP, BRC, or customer audits. Perform a complete calibration and service at least every 6–12 months, or sooner if sensitivity drift is observed.

Using correct test pieces and records

Use manufacturer-specified test pieces that mimic the smallest contaminant you aim to detect. Keep a log of test-piece passes/fails, date, operator, and corrective actions. This audit trail demonstrates compliance and helps identify trends before failures.

8. Choosing the Right Detector and Aperture for Your Application

Why aperture and detector class matter

Aperture size and detector electronics determine the best detectable contaminant size. Smaller apertures provide better sensitivity for small metal pieces but may limit throughput or product size.

Typical detection capabilities by aperture (industry guidance)

Below are typical, conservative detection ranges you can expect under normal conditions. Actual performance depends on product effect, orientation of contaminant, and machine model.

Interpreting the table

These are typical ranges; high-moisture, conductive products and product mixing can reduce detectability. If your application requires detection near the lower end of these ranges, select a detector designed for high sensitivity and consult your supplier for validation on your product.

9. Preventive Maintenance Checklist

Daily tasks

Perform start-up self-test and test-piece validation; visually inspect belts and reject devices; record results in the logbook.

Weekly tasks

Clean coils and conveyor surfaces; inspect electrical connectors and cable routing; check belt tracking and tension.

Monthly tasks

Run full functional tests, verify firmware versions, inspect rollers and bearings, and review logs for emerging trends.

Annual tasks

Schedule a certified service visit for calibration, coil testing, and firmware review. Replace worn mechanical parts and review the detector’s installation relative to new equipment added on the factory floor.

10. When to Contact Manufacturer or Certified Service

Indicators you need professional support

If sensitivity cannot be restored through calibration, if coils or electronics show visible damage, persistent EMI cannot be traced, or error codes indicate hardware faults, contact your detector manufacturer or an authorized service partner. For Kenwei multihead and detector customers, use the support channels listed at https://www.kenweigroup.com/ for prompt service and spare parts.

What to provide to service engineers

When contacting support, supply the HMI error codes, last successful test-piece log, environmental changes, recent maintenance actions, and a description of process changes or nearby new equipment. This information speeds diagnosis.

Company Note: Kenwei’s experience in integrated weighing and detection

Why Kenwei’s approach helps improve metal detection outcomes

Kenwei is a leading manufacturer of multi-head weighers and automated packaging systems. With expertise in metal detectors, check weighers, and linear weighers, Kenwei supplies integrated lines where detector placement, conveyor design, and product presentation are engineered together. This systems approach reduces product effect issues and simplifies troubleshooting. For support, customization, and after-sales service, Kenwei offers full installation, training, and maintenance capabilities from its Guangdong facility. Visit https://www.kenweigroup.com/ for contact and service details.

FAQ: Questions users commonly ask about food metal detector faults

How often should I run test-piece checks on a food metal detector?

Run test-piece checks at every shift start and after product changeovers; log results. Full calibrations should be carried out at least every 6–12 months or as recommended by the manufacturer and your food safety management system (HACCP/BRC/IFS).

Why does my detector detect metal in some product batches but not others?

Product effect is the most common cause—differences in moisture, particle size, salt content, or packaging can change the detector’s baseline. Recalibrate for each SKU and use product compensation features.

Can environmental temperature really affect detection?

Yes. Rapid temperature changes can shift coil characteristics and detector electronics, causing drift. Allow the detector to stabilise after temperature shifts and use environmental control where possible.

What test pieces should I use for validation?

Use test pieces specified by the detector manufacturer that represent the smallest contaminant you need to detect. Keep traceable records and follow retailer or regulatory requirements for frequency and documentation.

When is it better to replace a detector rather than repair it?

Replace if coils are irreparably damaged, electronics are obsolete, sensitivity cannot meet product needs despite adjustments, or if downtime and repair costs exceed the investment in a modern unit with better EMI immunity and diagnostics.

Who should I contact for support on Kenwei systems?

Contact Kenwei through https://www.kenweigroup.com/ for spare parts, calibration, and service. Provide equipment model, serial number, error codes, and recent test logs to speed the response.

Closing: Practical next steps to improve detector reliability

Immediate actions to take today

1) Implement daily test-piece checks and logging. 2) Inspect mechanical and electrical connections. 3) Review detector settings for each SKU. 4) Identify and minimise nearby EMI sources. 5) Schedule a preventive maintenance and calibration if one has not been done in the last 6–12 months.

Long-term improvements

Adopt a systems approach — align conveyor design, product presentation, and detector selection. Consider upgrading to detectors with enhanced sensitivity and advanced product compensation if you manage high-moisture or mixed-product lines. Regular training for operators on validation and basic troubleshooting reduces false rejects and missed contaminants.

Keeping your food metal detector performing reliably protects consumers and Kenwei. If you need technical support, calibration, or a tailored solution integrating metal detectors with multi-head weighers, Kenwei can provide experienced design and after-sales service to meet your requirements.