Inner Magnet U-Yoke

Background

In the magnetic circuit system of electrodynamic (moving-coil) loudspeakers, besides the commonly seen external magnetic structure, the Internal Magnetic structure features compact layout, low magnetic flux leakage and enclosed high-efficiency magnetic circuit. It is widely adopted in space-limited applications including headphone/earphone drivers, receiver speakers for mobile phones and communication devices, loudspeakers for laptops and tablets, smart wearable equipment, small vehicle-mounted speakers, square and miniature electroacoustic components.

A complete internal magnetic circuit generally takes the U-yoke (U cup / pot-shaped iron housing / Bottom Cup) as the magnetic conductive framework. The U-yoke is cup-shaped with a protruding center pole extending upward from the center of the cup bottom, and the surrounding cup wall forms the outer frame of the magnetic circuit. An annular permanent magnet (ferrite or NdFeB ring magnet) is fitted around the center pole and attached to the bottom and inner wall of the U-yoke. The top plate is pressed on top of the magnet. Together with the outer circle of the U-yoke center pole and the cup wall, it forms an even, symmetrical magnetic gap normally ranging from only 0.3–0.8 mm in width. The voice coil reciprocates within this magnetic gap to convert electric energy into acoustic energy.

Note: To improve magnetic efficiency and positioning accuracy, some internal magnetic structures adopt a three-layer stacked assembly of "bottom plate + ring magnet + top plate" (e.g., 14-core internal magnetic drivers). The three components must be strictly concentric and positioned inside the cavity of the U-yoke simultaneously.

All components of the magnetic circuit are bonded together by adhesive. A typical process involves dispensing AB adhesive onto the bottom/inner wall of the U-yoke and the upper and lower bonding surfaces of the ring magnet. The magnet and top plate are then installed and rotated to achieve uniform adhesive distribution, followed by positioning with magnetic gauges or voice coils and curing (the typical curing time for industrial internal magnetic production lines is approximately 8 minutes). The continuity, uniformity, positional accuracy and adhesive overflow volume of the adhesive layer directly determine the uniformity of the magnetic gap and prevent failures such as adhesive discontinuity, core offset and adhesive overflow into the magnetic gap, which further affect loudspeaker sensitivity, distortion and long-term reliability. For internal magnetic U-yokes, the magnetic circuit is enclosed inside the cup cavity. Any core offset or adhesive overflow into the gap is extremely difficult to detect via visual inspection inside the cup, making it a recognized "hidden critical risk for quality".

Objectives

Centered on Songqi Intelligent’s vision-following dispensing machine, this solution delivers a fixture-free, labor-free, deep-cavity anti-collision, high-precision, flexibly convertible and data-traceable automated dispensing solution for the magnetic circuit section of internal magnetic U-yokes. The specific objectives are listed as follows:

• Replace manual labor for circular adhesive dispensing and positioning between U-yoke and magnet, magnet and top plate (and multi-layer three-component assemblies). A single unit can steadily replace one to several skilled workers (the traditional manual workflow for the main magnetic section of internal magnetic products normally requires 8 workers).
• Leverage visual recognition and dynamic tracking to cut the rework rate caused by adhesive overflow, missing dispensing and core offset from over 5% to below 0.5%, and raise the yield rate of magnetic circuits to above 99%.
• Eliminate reliance on precision fixtures. Product model switching supports minute-level calibration, delivering an 80% increase in production line changeover speed and a 20%+ improvement in overall equipment efficiency compared with conventional solutions.
• Achieve seamless connection with existing production lines and support MES system integration and process data traceability.

Gluing for the magnetic circuit of inner-magnet U-yoke components still relies heavily on manual operations. Due to its characteristics of "deep cavity, multi-layer structure and small dimension", its pain points are more concealed and intractable compared with outer-magnet structures:

Cumbersome Procedures, Labor-intensive Operations and Output Restricted by Manpower

• The traditional manual workflow for the main magnet part of inner-magnet products is generally as follows: manual terminal crimping → manual frame dispensing → manual magnet assembly → manual dispensing → manual magnet assembly → manual magnet dispensing → manual washer assembly → manual magnetic gauge insertion → curing → manual magnetic gauge removal. This process requires 8 operators in total, with an output of only 1,200–1,500 PCS.
• Multiple dispensing processes together with magnetic gauge insertion and removal are highly dependent on skilled workers. The annual salary of proficient dispensers exceeds 100,000 RMB, yet staff turnover remains frequent. Operators leaving posts will cause fluctuations in production line tact time and sharply increase delivery risk.

Difficult Positioning of Multiple Components in Deep Cavities; Concealed Eccentricity Brings Severe Consequences

• For small-size inner-magnet products, eccentricity accounts for a larger proportion of the magnetic gap. Eccentricity directly causes unilateral magnetic gap, non-linear BL (force factor), increased distortion, and even rolling of voice coil tails that collide with the center pole (commonly known as "coil scrubbing against the base").
• Industrial process standards explicitly warn that for inner-magnet magnetic circuits (e.g., three components consisting of two washers and one magnet sheet), all three components must be positioned simultaneously via voice gauge tooling with sufficiently deep inner steps. Otherwise, magnet offset and coil scrubbing against the base are highly likely to occur. Traditional manual operation relies on deep-step voice gauges plus workers’ operating experience, leading to poor consistency.
• The magnetic circuit is enclosed inside the U-yoke cup. Once overflow glue, iron filings or foreign matter enters the magnetic gap, visual inspection inside the cup can hardly detect such defects. Defects such as rattling, squeaking and coil collision noise are only identified during pure acoustic testing, resulting in extremely high rework costs.

High Concentricity Requirements for Multi-layer Bonding and Complicated Tooling

• Inner-magnet products often require stacking three components (bottom washer, ring magnet and top washer) inside the U-yoke cavity with strict concentricity. Deviations in glue volume or position at any bonding interface will accumulate and amplify eccentricity. Conventional solutions require customized deep-step voice gauges and magnetic circuit bonding tooling, featuring high cost per set and slow model changeover.

Challenges in Controlling Tiny Glue Volumes and Strict Requirements on Glue Bead Shape

• Micro-sized products require an extremely small amount of glue per dispensing point, with strict requirements that the glue should be string-free, non-sagging and form full beads. Manual dispensing leads to inconsistent glue volume and unstable mixing ratio of A/B glue, which may result in incomplete curing and insufficient bonding strength. Other risks include glue strings bridging gaps and burning out voice coils, as well as glue sag contaminating the effective area of diaphragms.

Poor Accessibility for Deep Cavity Operations; Dispensing Nozzles Prone to Colliding with Center Poles or Cup Walls

• The U-yoke adopts a cup-shaped structure with a center pole. Dispensing nozzles need to extend deep into the cavity for operation, yet they face obstructions from the center pole and interference from cup walls. Traditional fixed-path teaching easily causes nozzle collision and workpiece scratching. Without dynamic visual avoidance capability, stable automated dispensing inside deep cavities is nearly unachievable.

Untraceable Quality Data

• Manual dispensing lacks records of process parameters including glue volume, dispensing position and dispensing speed. When batch defects such as coil rubbing or abnormal noise emerge, root cause analysis becomes difficult, and quality improvement cannot be supported by effective data.

Gluing for inner magnet U-yoke components has long relied on skilled veteran operators, stemming from the combination of three stringent requirements: deep cavity structure, multi-layer assembly and miniature dimensions.

Deep Cavity Geometry: Path Planning Requires Column Avoidance

• The gluing surfaces are located inside the U-yoke cup cavity (the bottom ring and upper/lower fitting rings of the ring magnet). The dispensing needle descends into the cavity from the cup opening for operation. A central pillar protrudes from the middle of the cup bottom and is surrounded by the cup wall. The movement path must automatically avoid the central pillar and cup wall within three-dimensional space, while preventing workpiece scratches caused by vertical descent of the Z-axis. This is the core challenge distinguishing inner magnet structures from flat-flange outer magnet structures.

Extreme Magnetic Gap Tolerance: Zero Margin for Gluing Errors

• The magnetic gap ranges from 0.3 to 0.8 mm (even narrower for micro products). The adhesive must avoid both interrupted dispensing (leading to poor adhesion) and overflow (causing coil jamming). Extremely high standards are imposed on dispensing needle positioning accuracy, adhesive output control and adhesive bead profile. For miniature parts, tolerance accounts for a larger proportion and the system becomes far more sensitive.

Simultaneous Concentric Positioning of Multi-Layer Components (Washer + Magnet Sheet + Washer, Three Components)

• The three stacked components must maintain precise concentricity, as eccentricity errors accumulate and amplify. Vision systems are required to simultaneously detect the posture of the U-yoke cup opening, central pillar and all stacked parts for dynamic compensation, instead of rigid positioning relying solely on dedicated fixtures.

Metering, Mixing and Micro-Dispensing of Two-Part AB Adhesive

• The two-part AB adhesive demands precise proportional metering, homogeneous mixing, on-demand micro-volume dispensing without stringing or dripping. Instant suck-back is needed to cut off the adhesive flow upon completing dispensing; otherwise, residual adhesive cures and clogs precision valve bodies.

Double-Sided Gluing on Cup Bottom and Inner Wall

• Adhesive paths may form annular patterns on the inner bottom plane of the U-yoke cup for bonding the magnet to the cup bottom and inner wall. Side dispensing, tilted dispensing valves or spiral descending paths need to be programmed to ensure the adhesive layer continuously covers mating surfaces without accumulation or sagging.

Vision Positioning for Miniature Workpieces

• Workpieces can be as small as φ9. Features occupy a low proportion of the field of view under standard camera settings. High-resolution ROI at the camera center must be activated, or macro lenses/narrow field-of-view modules selected to guarantee positioning accuracy for tiny features and fully eliminate physical fixtures.

Strict Standards for Adhesive Bead Profile

• The adhesive must be string-free, non-sagging and fully filled: adhesive strings bridging gaps can cause coil burnout, sagging adhesive contaminates diaphragms, and insufficient adhesive results in insecure fixation and component detachment.

• Capabilities: Automatically identify the precise position, angle and posture of U-iron cup openings, center posts, ring magnets and washers on the production line with a positioning accuracy of ±0.15 mm; automatically mark the center of the center post for motion avoidance. Products can be placed randomly, and the vision system completes positioning and deviation correction automatically, eliminating the need for physical fixtures entirely.

Motion Control ("Hand")

• Controller: Self-developed YMC-08-01 motion control card. Vision coordinates and motion trajectories are directly linked at the bottom layer to avoid latency and precision loss caused by protocol conversion in intermediate layers.
• Platform: All-steel KK modules (X-axis KK8620 / Y-axis KK6020 / Z-axis KK6010) equipped with AC servo motors (400 W for XY axes, 100 W with brake for Z axis), featuring high rigidity and fast response. The brake-equipped Z axis prevents workpiece scratching caused by shaft dropping upon unexpected power failure during deep cavity downward probing operations.
• Rotary Axis: Songqi patented 360° rotating R axis. Combined with self-developed algorithms, it enables circular glue path tracking and posture adjustment at any angle, perfectly suited for circular gluing on cup bottom rings and ring magnet rings.
• Deep Cavity Anti-collision Function: Based on the contours of center posts and cup walls identified by vision, the software automatically plans Z-axis avoidance height and spiral trajectories around posts to prevent needle collision and surface scratching.
• Precision: Motion accuracy of XYZ axes: ±0.02 mm; rotation accuracy of R axis: ±0.15° (full 360° rotation).

Glue Supply System

• Dual-barrel Glue Supply + Low Material Alarm: Enables non-stop glue supply during production. Optional screw pump glue supply improves efficiency by 20%–30% compared with conventional glue feeding methods, while reducing glue waste as well as dispenser valve wear and maintenance costs.
• Precise Metering & Anti-drip Design: Equipped with an instant automatic suck-back system that cuts off the glue path immediately after dispensing, preventing residual glue dripping and valve blockage. Compatible with various two-component AB glues including epoxy resin and modified acrylate (1:1 mixing ratio and adjustable ratios supported).
• Precision Valve for Micro Glue Volume: Fine-tip precision dispensing valve is available for miniature inner magnet products to accurately control ultra-small glue output, ensuring no stringing, no sagging and full glue beads.
• Glue Compatibility: Compatible with two-component epoxy/acrylate structural adhesives commonly used in magnetic circuits, meeting the requirements for multi-layer bonding of inner magnets.

Self-developed Software & Data Closed-loop System

• Fully Self-developed Vision Software: Custom-built for equipment configuration with deep coupling between the "hand (dispensing valve)" and "eye (camera)". No licensing fees for third-party commercial software such as Halcon, as well as no version upgrade fees or additional secondary development charges.
• Template-based Programming: Product changeover only requires calling or creating new vision templates, with calibration completed within minutes. Offline programming and storage of multiple sets of process parameters (including deep cavity avoidance trajectory templates) are supported.
• MES Connection & Traceability: Records real-time parameters including glue volume, speed, position and temperature. Supports in-depth connection with MES/ERP systems to provide complete data support for quality tracing and process optimization.

Integration with Existing Production Lines

• Embedded Model: Mounts directly above the customer’s existing conveyor belt and utilizes the on-site belt conveying system for material circulation without conveyor replacement or modification. The deployment workflow is simplified to "workstation confirmation → equipment placement → signal connection → parameter calibration", and commissioning from unpacking to mass production normally takes only several hours.
• Cabinet Model: Comes with an integrated conveyor belt and can be flexibly deployed at any section of the production line as an independent workstation. It can operate as an independent closed-loop unit or coordinate with upstream and downstream automatic loading/unloading stations (e.g. automatic washer feeding → automatic washer gluing → automatic magnet assembly → automatic magnet gluing → automatic gasket mounting → drying line).

Comprehensive Economic Benefits: Equipment efficiency exceeds market competitors by over 20%, customer repurchase rate reaches 83.2%+. The payback period in the Pearl River Delta region is generally 6–12 months. Fluctuation of production cycle is narrowed from ±20% to within ±3%, supporting stable 24-hour non-stop operation. The workforce for the inner magnet magnetic section can be cut from the traditional 8 workers down to approximately 1 worker, while the output can remain steadily at 1200–1500 PCS.

Main Technical Specifications of Equipment (Taking GP-26-740 Embedded Model as an Example)

RX Series (Cabinet Type with Fixed-Point Function) delivers longer travel while maintaining equivalent accuracy (e.g., RX-26-940 with travel of 760×660 mm and bandwidth of 500 mm), together with integrated conveyor options. The effective dispensing travel and overall dimensions vary by model configuration.

Recommended Dispensing Process Parameters (Inner Magnet U-Yoke)

Q1: What is the fundamental difference between the inner magnet U-yoke solution and the outer magnet T-yoke solution?

A: The outer magnet T-yoke features a flat flange plus center pole with good openness. The inner magnet U-yoke adopts a deep cup structure with a center pole. The dispensing head needs to go deep into the cup cavity for operation, bringing challenges in deep cavity accessibility and collision prevention caused by center pole obstruction and cup wall interference. Besides, it mostly adopts multi-layer stacking (washer + magnetic sheet + washer). Eccentricity exerts a more severe impact on small-sized products, and adhesive overflow or foreign matter inside the cup is harder to detect. The Songqi solution specially addresses the pain points of multi-layer assembly in deep cavities of inner magnet products via visual identification of the center pole, automatic pole-circumventing motion trajectories, braked Z-axis, micro-volume precision valves and dynamic concentric compensation. The two solutions share identical capabilities including fixture-free operation, reduced manual work, non-stop line production and traceability.

Q2: Which specifications of inner magnet U-yokes / speakers are applicable to this solution?

A: It applies to inner magnet round speakers (≤ φ52 mm), inner magnet square speakers (length ≤ 200 mm × width ≤ 52 mm × inner height ≤ 30 mm) and square speakers (38×38 mm with inner height ≤ 30 mm). The applicable magnet size ranges from φ9 mm to φ24.5 mm, center pole from φ9 mm to φ24.5 mm, and washer from φ11 mm to φ25 mm. Larger sizes can be supported based on machine model expansion.

Q3: How to prevent the dispensing head from colliding with the center pole / cup wall during deep cavity dispensing?

A: The vision system automatically identifies the contours of the U-yoke cup opening, center pole and cup wall. Self-developed software generates Z-axis avoidance height and spiral descending trajectories around the pole accordingly. Cooperated with all-steel KK modules and braked Z-axis servo system, it avoids needle collision or accidental dropping that scratches workpieces during deep cavity operation.

Q4: How to guarantee concentricity and prevent voice coil rubbing against the base during three-component stacking (washer + magnetic sheet + washer) for inner magnet assemblies?

A: Traditional solutions rely on deep-step gauges to rigidly position the three components simultaneously. The Songqi solution uses vision to detect the poses of the U-yoke and all stacked components at the same time, implements dynamic concentric compensation, and achieves uniform lamination via 360° rotation of the R-axis. It fundamentally eliminates accumulated eccentricity and avoids voice coil rubbing caused by offset magnetic sheets.

Q5: Micro products require extremely small adhesive volume. How to control adhesive to avoid stringing and dripping?

A: Equip with precision micro-volume dispensing valve with fine needle and instant automatic suck-back system to cut off the adhesive flow immediately after dispensing. Dual-barrel precision metering (screw pump optional for 20%~30% efficiency improvement) ensures uniform mixing and consistent volume of two-part AB adhesive, delivering non-stringing, non-sagging and full adhesive beads.

Q6: What types of adhesives are supported? How is the mixing ratio of AB adhesive controlled?

A: It is compatible with two-part magnetic circuit AB adhesives such as epoxy resin and modified acrylate, with a standard mixing ratio of 1:1. The adhesive feeding system adopts dual barrels plus precision metering; a screw pump is optional, and suck-back function prevents adhesive dripping.

Q7: How to prevent adhesive / foreign matter from overflowing into the magnetic gap and resulting in voice coil rubbing?

A: ① The vision system accurately locates the inner diameters of the U-yoke center pole, ring magnet and washer to confine the dispensing trajectory outside the center pole. ② Closed-loop control of dispensing volume and bead height based on process parameters; the annular overflow adhesive is strictly controlled within ≤1.5×1.5 mm without offset. ③ Optimized adhesive morphology (no stringing or sagging). ④ Full-process data recording to facilitate abnormality tracing and process fine-tuning. ⑤ For deep cavity structures, stricter control over incoming material cleanliness and dust prevention is required to stop iron filings and foreign particles from entering gaps.

Q8: How long does product / line changeover take?

A: Repeated mechanical teaching and custom fixtures are unnecessary. Operators can call up corresponding visual templates or create new calibrations (including deep cavity avoidance trajectory templates) in the software. The equipment completes line change preparation within minutes, lifting changeover efficiency by approximately 80% compared with traditional methods.

Q9: What is the single-machine output and cycle time?

A: Visual recognition only takes 0.1–0.3 seconds. The dispensing head runs synchronously with the assembly line belt without line stoppage. The equipment outperforms competing products on the market by more than 20% in efficiency. The workforce for the main magnetic assembly section of inner magnet products can be reduced from the traditional 8 operators to roughly 1 operator, while stably maintaining an output of 1200–1500 PCS under 24-hour continuous operation.

Q10: Does it support MES / data traceability? What after-sales support is available? Can sample trials be conducted first?

A: Deep integration with MES/ERP is supported, with real-time recording of parameters such as adhesive volume, speed, position and temperature. Songqi adopts fully self-developed vision software and self-developed YMC-08-01 control card with no annual licensing fees, no version upgrade fees and no additional secondary development charges. After-sales services include dedicated on-site support (1-to-1 / 1-to-2), 7×24 multi-channel technical support (response within 15 minutes), lifetime maintenance and regular follow-up visits. We also provide free process solution design and free sample testing. After customers send samples, trials will be carried out on actual equipment with a complete test report issued, allowing customers to evaluate performance before making decisions.

• Company: Dongguan Sukiauto Intelligent Technology Co., Ltd. (SUKIAUTO)
• Address: Unit 4, 9th Floor, Building 2, Dongguan Daotong Intelligent Valley, approximately 160 meters northeast of the square of Dongcheng Foreign Economic Industrial Park, Huijing Road, Dongcheng Subdistrict, Dongguan City, Guangdong Province
• Phone: 13761860627
• Email: service@sukiauto.cn
• Services: Free sample testing · Dedicated one-on-one after-sales service · 7×24-hour response · Lifelong maintenance

The equipment parameters, efficiency and yield data listed in this solution are based on internal tests and sales statistics of Sukiauto Intelligent. The industry process standards and inner magnet assembly line data are sourced from public production specifications of the electroacoustic industry, patents and literatures, as well as actual measurements of automated inner magnet production lines. The specific model selection and process parameters shall be subject to actual sample verification and the process plan confirmed by both parties.