Metal Foreign Object Detection and Rejection Principle

Preface

Metal foreign object detection and rejection is a core link in industrial production, especially in quality control and compliance management of food, pharmaceutical, personal care, grain processing and other industries. It is a mandatory critical control point in the HACCP (Hazard Analysis and Critical Control Points) system. Metal foreign objects mixed in products will not only cause personal injury to consumers and damage to brand reputation, but also lead to major operational risks such as compliance penalties and batch recalls.

Based on the core framework of balanced induction coil metal detection technology, this guide systematically sorts out the full-process metal foreign object control system, underlying physical principles of detection, equipment structure and working mechanism, accuracy influencing factors, rejection system selection, industry compliance requirements, operation and maintenance calibration and other comprehensive content, providing complete technical and application guidance for metal foreign object control at the production end.

Full-process Metal Foreign Object Control System

The core logic of metal foreign object control is "source prevention + process control + terminal interception + continuous optimization", rather than relying solely on terminal detection equipment. The complete control process is divided into five core links, and terminal detection and rejection is the ultimate guarantee of the control closed loop.

Sources and Prevention of Typical Metal Foreign Objects

Introduction from Raw Materials: The primary source of metal foreign objects, covering all basic raw materials such as grains, nuts, dried fruits, fruits and vegetables, meat, poultry, seafood, and chemical raw materials. Impurities such as iron wire, iron nails, metal chips, and equipment wear parts mixed in raw materials during planting, harvesting, initial processing, and transportation. Key control points: supplier qualification audit, incoming raw material pre-inspection, raw material screening/filtration pretreatment.
Introduction by Human Factors in Production Process: Falling of prohibited personal items brought into the workshop by production personnel, commonly including pins, paper clips, pen caps, jewelry, coins, keys, watch metal accessories, etc. Key control points: workshop personnel access specifications, centralized management of personal items, metal-free dressing requirements, workshop inspection system.
Generated by Production Equipment Operation Loss: Metal impurities generated by mechanical wear and component fracture of equipment in all production processes, covering all moving equipment links such as stamping, slicing, mixing, stirring, filtration, transportation, filling, etc. Common ones include metal chips, broken blades/screens, screws, springs, bearing wear particles, etc. Key control points: equipment preventive maintenance, regular replacement of wearing parts, equipment integrity inspection before production.
Introduction by Equipment Maintenance Operations: Secondary metal foreign objects generated during equipment inspection and maintenance, as well as tool loss, commonly including metal wires, chips, welding slag, welding rod heads, wrenches, screwdrivers and other small maintenance tools. Key control points: closed-loop management of maintenance work orders, tool inventory before and after maintenance, on-site cleaning after maintenance and pre-inspection before equipment startup.

Full-process Control Closed Loop

Source Prevention: Establish supplier foreign object control standards, incoming raw material inspection specifications, workshop personnel and material access systems to reduce the probability of foreign object mixing from the source;
Process Control: Reduce the risk of foreign object introduction in the production link through equipment preventive maintenance, production process inspection, and workshop environment control;
Terminal Interception: Realize 100% detection and accurate rejection of metal foreign objects in finished products through the closed-loop design of metal detector + rejection system, preventing non-conforming products from flowing into the market;
Traceability Management: Fully archive detection data, rejection records, calibration records, and abnormal events to realize traceability and review of foreign object incidents;
Continuous Improvement: Reverse optimize source and process control measures based on foreign object incident review and detection data statistics to continuously improve the foreign object control system.

Underlying Physical Principles of Metal Detection Technology

For internal use - Confidential

The core technical foundation of the balanced induction coil metal detector is Faraday's Law of Electromagnetic Induction discovered by Michael Faraday in 1831, as well as the derived Eddy Current Effect and Magnetic Effect of Current. These three physical laws together form the theoretical basis for metal foreign object identification.

Core Physical Laws

Faraday's Law of Electromagnetic Induction: A conductor in a changing magnetic field generates an induced electromotive force, and an induced current is formed in a closed conductor. The transmitting coil of the metal detector is fed with high-frequency alternating current, which generates a high-frequency alternating primary magnetic field; when a metal foreign object enters the magnetic field, the magnetic field has two effects on the metal foreign object: direct magnetization effect on ferromagnetic metals, or induced electromotive force on non-ferromagnetic good conductors, thereby forming closed induced currents (i.e., eddy currents) inside the metal.
Magnetic Effect of Current: Any conductor carrying current generates a secondary magnetic field around it; magnetized ferromagnetic metals themselves form an additional magnetic field, and eddy currents inside non-ferromagnetic metals also generate a reverse secondary magnetic field. Both types of secondary magnetic fields interfere with the original alternating magnetic field of the metal detector, changing the distribution and intensity of the original magnetic field.
Eddy Current Effect: When a high-frequency alternating magnetic field acts on a metal conductor, the eddy current generated inside the conductor forms a reverse magnetic field that offsets part of the primary magnetic field. At the same time, the size of the eddy current is directly related to the electrical conductivity, magnetic permeability, size and shape of the metal, as well as the frequency of the magnetic field, which is also the core reason for the difference in detection sensitivity of different metals.

Magnetic Field Response Mechanism of Different Metals

Different metals have essential differences in magnetic permeability and electrical conductivity, so their response mechanisms in the detection magnetic field are completely different, which is also the core root of the difference in detection difficulty:

Ferromagnetic Metals (Iron, Carbon Steel, Low Carbon Steel, etc.): Magnetic permeability is much higher than air (relative magnetic permeability can reach hundreds to thousands), dominated by magnetization effect, with high secondary magnetic field intensity and strong interference to the primary magnetic field, making them the easiest metal type to detect;
Non-ferromagnetic Nonferrous Metals (Copper, Aluminum, Brass, Gold, Silver, etc.): No ferromagnetism, relative magnetic permeability close to 1, but extremely high electrical conductivity, dominated by eddy current effect, secondary magnetic field interference intensity is second only to ferromagnetic metals, with low detection difficulty;
Austenitic Stainless Steel (304, 316 and other food-grade stainless steels): Austenitic structure at room temperature, no ferromagnetism, and electrical conductivity much lower than nonferrous metals such as copper and aluminum. Both magnetization effect and eddy current effect are weak, with small interference to the primary magnetic field, making it the most common and difficult metal type to detect in industrial production; martensitic transformation occurs only after cold working, producing weak magnetism, and detection sensitivity will be slightly improved.

Structure and Working Mechanism of Balanced Induction Coil Metal Detector

Core Structure of the Whole Machine

The balanced induction coil metal detector (commercially referred to as "metal detector") is composed of five core modules, which work together to complete the whole process of detection, alarm and rejection of metal foreign objects:

Detection Head Core Module: The "brain" and "sensory organ" of the metal detector, consisting of three groups of coaxially wound induction coils, as well as supporting signal generation, signal processing and analog-to-digital conversion units. The three groups of coils are divided into 1 group of transmitting coil in the middle and 2 groups of receiving coils symmetrically distributed on both sides. The two groups of receiving coils have exactly the same parameters and are arranged in mirror symmetry relative to the transmitting coil, which is the core carrier for metal identification.
Material Conveying System: Responsible for stably and uniformly conveying the products to be detected through the detection channel, divided into three categories according to product form: belt conveying system (suitable for packaged products and independent parts), pipeline conveying system (suitable for liquids, pastes, sauces), and gravity falling conveying system (suitable for powders, granules, capsules).
Rejection Execution System: The "execution terminal" of the metal detector. After receiving the rejection signal from the control system, it separates non-conforming products containing metal foreign objects from the normal production line within an accurate time window to avoid flowing into the next link. Different product forms are adapted to different types of rejection mechanisms.
Human-Machine Interaction and Control System: Responsible for equipment parameter setting, operation status monitoring, detection data display, alarm information output, and timing control of the whole process of detection, conveying and rejection, which is the operation center of the equipment.
Data Storage and Traceability Module: Responsible for storing detection data, rejection records, calibration records and abnormal event logs to meet industry compliance and traceability requirements. High-end models can connect to the factory MES/ERP system to realize digital management.

Core Working Principle

The core working logic of the metal detector is to identify the existence of metal foreign objects through changes in the balance state of the induction coil. The complete working process is divided into four core stages:

Establishment of Initial Balance State: After the equipment is powered on, the transmitting coil is fed with high-frequency sinusoidal alternating current to generate a high-frequency alternating primary magnetic field; the two groups of mirror-symmetric receiving coils induce induced electromotive forces of exactly the same magnitude and opposite directions in the primary magnetic field. After the two groups of coils are connected, the induced currents cancel each other out, the output signal is zero, and the equipment is in a electrical balance state with no alarm signal output.
Balance State is Broken: When the product to be detected passes through the detection channel normally without metal foreign objects, it will not interfere with the primary magnetic field, and the equipment remains in a balanced state; when the product containing metal foreign objects passes through the detection channel, the secondary magnetic field generated by the metal foreign objects will break the symmetrical distribution of the magnetic field in the detection channel, causing changes in the induced electromotive force of the receiving coil close to the foreign object side. The induced currents of the two groups of receiving coils cannot cancel each other out, the equipment balance state is broken, and an alternating voltage signal is output.
Signal Processing and Threshold Judgment: The weak voltage signal output by the coil is sent to the processing unit of the equipment for analysis after amplification, filtering and analog-to-digital conversion; the processing unit compares the processed signal amplitude with the user-set detection threshold (Vth): if the signal amplitude is lower than the threshold, it is judged as a normal product and the equipment takes no action; if the signal amplitude is higher than the threshold, it is judged that there is a metal foreign object, and an alarm is triggered immediately.
Rejection Action Execution: At the same time as the alarm is triggered, the control system calculates the accurate delay time according to the production line speed, product position and installation distance of the rejection mechanism. Within the accurate time window when the non-conforming product reaches the rejection mechanism, it sends a trigger signal to drive the rejection mechanism to act, separating the product containing metal foreign objects from the production line and completing the foreign object rejection closed loop.

Detection Characteristics and Sensitivity Rules of Different Metals

The detection sensitivity of metal foreign objects is corely determined by the inherent properties of the metal itself, and is also affected by the detection environment and equipment parameters. There are significant differences in detection difficulty and sensitivity of different metals.

Core Influencing Properties

Magnetic Permeability: The higher the magnetic permeability, the stronger the magnetization effect, the greater the interference to the magnetic field, and the higher the detection sensitivity;
Electrical Conductivity: The higher the electrical conductivity (the lower the resistivity), the stronger the eddy current effect, the greater the secondary magnetic field interference, and the higher the detection sensitivity;
Geometric Size and Shape: The larger the volume and effective cross-sectional area of the metal foreign object, the stronger the detection signal and the easier it is to be detected; the detection sensitivity of spherical foreign objects is higher than that of flake and filamentous foreign objects of the same volume.

Detection Characteristics Table of Common Metals

Metal Type	Magnetization Performance	Electrical Conductivity	Resistivity at 20℃ (μΩ·cm)	Detection Difficulty	Reference for Minimum Detectable Size Under Normal Conditions
Iron/Carbon Steel	Strong Ferromagnetic	Good	9.78	Very Easy to Detect	0.3mm
Copper	Non-magnetic	Excellent	1.72	Easy to Detect	0.5mm
Aluminum	Non-magnetic	Good	2.82	Easy to Detect	0.6mm
Brass	Non-magnetic	Good	5~7	Relatively Easy to Detect	0.7mm
Zinc	Non-magnetic	Moderate	6.1	Relatively Easy to Detect	0.8mm
Tin	Non-magnetic	Moderate	11.5	Moderate Difficulty	1.0mm
Lead	Non-magnetic	Poor	21.4	Relatively Difficult to Detect	1.2mm
304 Stainless Steel	Non-magnetic (Room Temperature)	Poor	73	Difficult to Detect	1.5mm
316 Stainless Steel	Non-magnetic (Room Temperature)	Poor	74	Most Difficult to Detect	1.8mm

Under normal operating conditions and the same detection conditions, ferromagnetic metals have the highest detection accuracy, followed by non-ferromagnetic nonferrous metals, and austenitic stainless steel has the worst accuracy; however, when the detection channel opening size is too large, a sensitivity reversal phenomenon occurs: the detection accuracy of non-ferromagnetic metals will deteriorate significantly, while the detection accuracy of austenitic stainless steel will relatively improve, even exceeding that of some non-ferromagnetic nonferrous metals. Mainstream brand metal detectors are integrated with stainless steel accuracy optimization technology, which can greatly improve the detection sensitivity of austenitic stainless steel and weaken the sensitivity reversal effect.

Core Influencing Factors and Optimization Schemes of Metal Detector Detection Accuracy

Detection accuracy is the core performance indicator of the metal detector. In addition to the inherent properties of the metal itself, it is also affected by equipment design, installation environment, product characteristics, operating parameters and other multi-dimensional factors. Clarifying the influence mechanism of each factor is the core premise to achieve optimal detection accuracy.

Position of Metal Foreign Object in Detection Channel

Influence Mechanism: The magnetic field intensity distribution in the detection channel of the metal detector is uneven. The magnetic field intensity at the edge of the channel around the coil is the highest, and the magnetic field intensity at the center of the channel is the weakest; therefore, the detection accuracy is the best when the metal foreign object is at the edge of the channel, the side accuracy is better than the upper and lower bottom, and the center of the channel is the blind area with the worst detection accuracy.

Optimization Scheme: Products should pass as close to the side wall of the channel as possible during transportation to avoid concentrating in the center of the channel; when multiple rows of products are transported in parallel, a lane-dividing design should be adopted to avoid product stacking; a matching detection channel should be customized according to the product size to avoid the channel size being much larger than the product size.

Detection Frequency and Electromagnetic Environment

Influence Mechanism: Under normal operating conditions, the higher the detection frequency of the metal detector, the higher the induction sensitivity to metal foreign objects and the better the detection accuracy; however, the higher the frequency, the more sensitive the equipment is to external electromagnetic interference, and it is very easy to be interfered by surrounding frequency converters, high-power motors, metal structure vibration, other high-frequency equipment, resulting in frequent false alarms and false rejections.

Optimization Scheme: Match the optimal detection frequency according to the product type (low frequency for wet products, high frequency for dry products); select equipment with real-time self-balancing technology to offset signal drift caused by external interference in real time; install the equipment away from high-power electrical equipment, ensure independent grounding of the equipment, and avoid sharing the grounding loop with other equipment; fix the surrounding metal structures to avoid magnetic field interference caused by vibration.

Product Effect

Influence Mechanism: Product effect is the core factor affecting detection accuracy and causing false alarms in industrial sites. It refers to the signal similar to metal foreign objects generated by the product to be detected due to its conductivity and dielectric properties when passing through the detection channel, which masks the signal of tiny metal foreign objects or even directly triggers false alarms.

Optimization Scheme: Adopt multi-frequency synchronous detection technology to separate product effect and metal foreign object signals through signal comparison of multiple groups of frequencies; adopt product effect suppression algorithms (such as 3S algorithm) to offset the signal interference of the product itself through product feature self-learning; distinguish the signal phases of product effect and metal foreign objects through phase adjustment technology to filter product effect signals; for metalized packaged products, select special free-fall and pipeline metal detectors to complete product detection before packaging.

Other Key Influencing Factors

Detection Channel Size: The larger the channel size, the more dispersed the magnetic field distribution, the lower the overall detection sensitivity, and the smallest matching channel should be selected according to the maximum product size;

Product Conveying Speed: The faster the conveying speed, the shorter the time for metal foreign objects to pass through the detection channel, the fewer effective signals that can be collected by the equipment, and the detection accuracy will decrease slightly;

Product Stacking and Arrangement: When products are stacked or passed in parallel without intervals, the product effect will be superimposed, and metal foreign objects may be blocked, leading to missed detection;

Ambient Temperature and Humidity: Extreme temperature and humidity will affect coil parameters and electronic component performance, causing signal drift.

Types, Selection and Application Specifications of Metal Foreign Object Rejection Systems

The rejection system is the core link of the metal detector closed-loop control, and its core requirements are accuracy, reliability and no omission. It is necessary to match the appropriate rejection mechanism according to product form, packaging form, production line speed and capacity requirements, while meeting the hygiene and compliance requirements of the corresponding industry.

Rejection systems are divided into two categories: conveyor-type rejection systems and non-conveyor-type rejection systems, which are respectively adapted to different conveying scenarios and product forms.

Conveyor-type Rejection System (Adapted to Belt Conveyor Lines)

Air Blast

Working Principle: Control high-pressure air flow through a high-speed solenoid valve, and spray it within an accurate time window to blow lightweight products containing metal foreign objects into the rejection bin;

Core Advantages: Fast response speed, non-contact, little damage to products, simple structure and easy maintenance;

Applicable Scenarios: Narrow belt, lightweight, single-row, rigid independent products, such as biscuits, candies, chocolates, small packaged snacks, capsules, tablets, etc.;

Selection Specifications: Working air source pressure should be stable at 0.4~0.6MPa, with supporting air source triple unit and air storage tank; the nozzle should be aligned with the center of gravity of the product, with supporting photoelectric gate timer.

Pneumatic Punch/Pusher

Working Principle: Drive the push rod through a high-speed cylinder to push non-conforming products out of the conveyor line horizontally into the rejection bin within an accurate time;

Core Advantages: Large thrust, accurate action, strong stability, wide application range;

Applicable Scenarios: Narrow belt, light to medium weight, independent packaged products with fixed shape, such as boxed food, bagged snacks, hard-packaged personal care products, etc.;

Selection Specifications: Must be equipped with photoelectric gate synchronization system to ensure the push rod always hits the center of the product; the push rod stroke matches the belt width.

Diverter Arm

Working Principle: Drive the arc-shaped diverter arm through a cylinder to slide obliquely across the belt surface, guiding non-conforming products to the rejection bin;

Core Advantages: Gentle action, little impact on products, adaptable to products without fixed orientation;

Applicable Scenarios: Narrow belts with width ≤350mm, light to medium weight, independent products without fixed orientation, such as bagged sauces, irregular packaged snacks, etc.;

Selection Specifications: The diverter arm material should be wear-resistant and food-grade; the angle and stroke of the diverter arm should match the product size.

Overhead Sweep

Working Principle: Drive the horizontal paddle through a rodless cylinder to slide across the belt surface at a right angle, pushing non-conforming products into the rejection bin;

Core Advantages: Long stroke, uniform thrust, adaptable to wide belts and heavy products;

Applicable Scenarios: Wide belts with width ≤650mm, medium to heavy weight, products with fixed orientation, such as large packaged grain and oil, barreled products, etc.;

Selection Specifications: The height of the paddle should match the maximum height of the product; the rodless cylinder should be equipped with a speed control valve to ensure smooth action.

End Flap/Dump

Working Principle: A reversible flap is set at the end of the conveyor line. When a non-conforming product is detected, the flap quickly flips downward, and the non-conforming product directly falls into the rejection bin;

Core Advantages: No jamming risk, adaptable to bulk materials, simple structure;

Applicable Scenarios: Small independent parts, loose bulk materials (dry/sticky), granules, nuts, etc.;

Selection Specifications: Reserved production line height drop; the flap should be sealed to avoid powder leakage.

Retracting Belt

Working Principle: The roller at the end of the conveyor line can quickly retract backward to form a falling gap, and non-conforming products fall into the rejection bin from the gap;

Core Advantages: Accurate rejection, high reliability, adaptable to multi-row products, little damage to products;

Applicable Scenarios: Multi-row parallel conveyed products, fragile products, bulk materials, small-sized products;

Selection Specifications: The reset speed of the roller must be faster than the belt running speed; knife-edge end rollers should be selected for small products.

Non-conveyor-type Rejection Systems

Pipeline-type Liquid/Paste Rejection System: Adapted to the pipeline conveying scenario of liquid, paste and sauce products, the detection head is a ring-shaped pipeline structure, and the rejection mechanism is a sanitary three-way reversing valve; when a metal foreign object is detected, the reversing valve quickly switches the flow channel to guide non-conforming products into the waste barrel, which must meet CIP/SIP cleaning requirements.
Gravity Falling Rejection System: Adapted to freely falling products such as powders, granules and capsules, the detection head is a vertical channel structure, and the rejection mechanism is a high-speed flap or spray valve; products fall freely through the detection channel by gravity, and when a foreign object is detected, the rejection mechanism acts within milliseconds to divert non-conforming products to the waste port.
Vertical Packaging Machine Integrated Rejection System: Adapted to on-line detection of vertical bag-making packaging machines, the detection head is installed at the feeding port of the bag-making machine, and the rejection mechanism is synchronized with the control system of the packaging machine. When a metal foreign object is detected, it triggers the packaging machine to not seal and directly reject.

Core Supporting Technologies of Rejection Systems

Photoelectric Gate Timing Technology: Accurately identify the real-time position of the product through photoelectric sensors, and the control system accurately controls the trigger time of the rejection mechanism according to the product position to ensure accurate rejection;
Variable Speed/Start-stop Production Line Adaptation Technology: Real-time collection of production line running speed through a pulse generator, detection signals of multiple consecutive foreign objects can be stored in a shift register, and rejection is triggered sequentially according to the production line speed;
Rejection Confirmation and Anti-tampering System: Confirm whether non-conforming products are successfully rejected through sensors at the rejection port, and immediately trigger a shutdown alarm if rejection fails; at the same time, encrypt and store equipment parameters and detection records.

Industry Applications and Compliance Requirements of Metal Detectors

Metal foreign object detection is a mandatory compliance requirement in multiple industries, and there are significant differences in detection standards, accuracy requirements and equipment adaptability in different industries.

Food and Beverage Industry

Core Compliance Standards: China's "Food Safety Law", "General Hygienic Specifications for Food Production" (GB 14881), international standards BRC, IFS, HACCP, FDA 21 CFR Part 11.

Core Requirements: All prepackaged foods and bulk foods must set critical control points for metal foreign objects to achieve 100% on-line detection; detection accuracy must meet industry general standards: iron ≥0.8mm, non-iron ≥1.0mm, stainless steel ≥1.5mm;

Equipment Adaptation: Leisure food is adapted to belt type + air blast/pusher rejection, sauces and beverages are adapted to pipeline type, milk powder and flour are adapted to gravity falling type, fresh meat products are adapted to multi-frequency anti-product effect models.

Pharmaceutical Industry

Core Compliance Standards: China's "Good Manufacturing Practice for Pharmaceutical Products" (GMP), "Pharmacopoeia of the People's Republic of China", US FDA cGMP, EU GMP.

Core Requirements: Zero tolerance for metal foreign objects, detection accuracy requirements are much higher than the food industry, conventional requirements: iron ≥0.3mm, non-iron ≥0.5mm, stainless steel ≥0.8mm; equipment must meet sterile production requirements, can be cleaned/sterilized in place, and data records must meet traceability, anti-tampering and audit trail requirements;

Equipment Adaptation: Tablets and capsules are adapted to gravity falling type, oral liquid and injection are adapted to pipeline type, ointments and patches are adapted to special belt type models.

Other Core Application Industries

Grain Processing Industry: Adapted to large-capacity, large-channel belt type and gravity falling type models, detecting metal impurities in raw grains, flour and feed to protect subsequent grinding and tableting equipment from damage;
Rubber and Plastic Industry: Adapted to pipeline-type detection machines for raw material conveying, detecting metal impurities in plastic particles and rubber raw materials to avoid damage to injection molding machine and extruder molds;
Personal Care Industry: Adapted to belt type and pipeline type models, detecting metal foreign objects in toothpaste, cosmetics and personal care products to meet product quality control requirements.

Installation, Operation & Maintenance and Calibration Specifications of Metal Detectors

Installation Specifications

Site Requirements: The equipment installation location should be away from strong electromagnetic interference sources such as high-power frequency converters, motors and welding machines, and away from large moving metal structures; the ground should be flat and solid to avoid vibration during equipment operation;
Grounding Requirements: The equipment must be independently and reliably grounded for protection, with a grounding resistance ≤4Ω; it is strictly prohibited to share the grounding loop with other equipment to avoid grounding interference;
Conveying Connection: The conveyor lines before and after the equipment should be horizontally aligned with the detection channel without height difference to avoid vibration and jumping of products when passing through; the racks of the front and rear conveyor lines should be insulated from the equipment rack.

Commissioning and Calibration Specifications

Initial Commissioning: After the equipment is powered on, complete no-load balance calibration first, then conduct product feature learning to optimize product effect, set detection threshold according to compliance requirements, and complete sensitivity test and rejection synchronization calibration;
Calibration Standards: Use industry-standard test pieces (iron, copper, stainless steel) to test at four positions (center, up, down, left, right) of the detection channel, test 10 times continuously at each position, and achieve 100% detection and 100% accurate rejection;
Calibration Frequency: Sensitivity inspection must be conducted before and after daily production; calibration must be conducted after product changeover, equipment shutdown and restart, and parameter modification; third-party metrological calibration must be conducted once a year;
Record Requirements: All calibration, inspection and test data must be completely recorded and archived, and the archiving period must meet industry compliance requirements (not less than 2 years for the food industry and not less than 5 years for the pharmaceutical industry).

Daily Operation & Maintenance Requirements

Daily Inspection

Complete sensitivity test and rejection function test
Equipment appearance cleaning and photoelectric sensor cleaning
Air source pressure check

Weekly Maintenance

Coil detection channel cleaning and conveyor belt cleaning
Electrical terminal tightening
Rejection mechanism action lubrication and sensor function verification

Monthly Maintenance

Equipment grounding resistance test and electromagnetic interference troubleshooting
Conveyor system roller and bearing inspection
Rejection mechanism cylinder and solenoid valve tightness test

Annual Maintenance

Equipment complete machine disassembly and cleaning, coil performance test
Circuit board function detection, conveyor system wearing parts replacement
Complete machine accuracy calibration and issue maintenance report

Common Fault Troubleshooting and Solutions

Fault Phenomenon	Core Causes	Solutions
Frequent False Alarms and False Rejections	1. Unoptimized product effect; 2. Strong surrounding electromagnetic interference; 3. Poor equipment grounding; 4. Too low detection threshold setting; 5. Contaminated conveyor belt	1. Re-optimize product effect; 2. Troubleshoot interference sources and ensure good grounding; 3. Adjust detection threshold reasonably; 4. Clean/replace conveyor belt
Missed Detection of Metal Foreign Objects, Unable to Detect	1. Too high detection threshold setting; 2. Metal foreign object located in the central blind area of the channel; 3. Excessively strong product effect; 4. Mismatched detection frequency; 5. Coil balance failure	1. Lower detection threshold; 2. Optimize product conveying path; 3. Adopt multi-frequency technology to optimize product effect; 4. Re-calibrate coil balance
Inaccurate Rejection Mechanism Action, Missed Rejection/False Rejection	1. Photoelectric sensor installation/contamination; 2. Incorrect rejection delay parameters; 3. Production line speed fluctuation; 4. Insufficient air source pressure; 5. Too small product spacing	1. Clean/adjust photoelectric sensor; 2. Re-calibrate delay parameters; 3. Enable speed synchronization function; 4. Adjust air source pressure/ensure product spacing
Equipment Unable to Complete Balance Calibration, Startup Alarm	1. Metal foreign objects in the detection channel; 2. Changes in surrounding metal structures; 3. Coil damage; 4. Strong electromagnetic interference; 5. Circuit board failure	1. Clean detection channel; 2. Remove surrounding metal structures; 3. Detect/replace coil; 4. Isolate interference source/repair circuit board

Development Trends of Metal Foreign Object Detection Technology

Multi-frequency and Ultra-wideband Detection Technology: From traditional single-frequency and dual-frequency technologies to ultra-wideband and multi-frequency synchronous technologies, realizing full-scenario adaptation to different metal types and different product effects, greatly improving detection sensitivity and reducing false alarm rate;
AI Artificial Intelligence Signal Recognition: Through AI deep learning algorithms, learn massive product signals, metal foreign object signals and interference signals, accurately distinguish product effect, environmental interference and metal foreign object signals, achieving "zero false alarms and zero missed detections";
Digitalization and Industry 4.0 Integration: Equipment fully supports industrial bus protocols such as Ethernet and Profinet, which can seamlessly connect to factory MES/ERP systems to realize real-time upload of detection data, remote monitoring, remote operation and maintenance, and data analysis;
Breakthrough in Metallized Packaging Detection Technology: Aiming at the detection problems of aluminum foil packaging and metallized film packaging, develop special magnetic field shielding technology and signal separation technology to realize on-line detection of finished products with metallized packaging;
Multi-technology Fusion Detection Scheme: Integrate metal detection technology with X-ray foreign object detection technology to realize synchronous detection of metal foreign objects and non-metallic foreign objects such as glass, stone and plastic.

Appendix: Core Terminology Explanation

Detection Threshold (Vth): The minimum signal amplitude for the equipment to determine the existence of metal foreign objects; the lower the threshold, the higher the detection sensitivity and the higher the false alarm risk;
Product Effect: The interference signal generated by the product to be detected to the detection magnetic field due to its conductivity and dielectric properties, which is the core factor affecting detection accuracy;
Eddy Current Effect: The closed induced current generated inside the metal conductor when a high-frequency alternating magnetic field acts on it, which is the core principle for identifying non-ferromagnetic metals;
Real-time Self-balancing: The equipment real-time monitors the balance state of the coil, automatically offsets the signal drift caused by environmental interference, and is the core technology to ensure the long-term stable operation of the equipment;
Photoelectric Gate Control: A synchronous technology that accurately identifies product position through photoelectric sensors and controls the accurate triggering of the rejection mechanism, which is the core supporting technology to ensure rejection accuracy.