The advantages and disadvantages of the glass disc screening machine, how to choose - Xiang Hongyu

Core workflow

1. Product placement: The product to be inspected is placed on a transparent glass disc rotating at a uniform speed.

2. Multi-angle imaging: Multiple industrial cameras distributed around the glass disk are triggered at the moment to capture images of different sides of the product.

3. Image processing: The image is transmitted to the processing system for defect identification, dimensional measurement, etc.

4. Sorting execution: According to the results, the system controls the downstream mechanism (such as air nozzle and robotic arm) to remove defective products.

Notable advantages

1. 360° Blind Spot Detection (Core Advantage):

• Multi-camera coverage: A cluster of cameras can simultaneously or sequentially capture the top, side, and even bottom surface of the product (if the glass is transparent and there is a camera/light source at the bottom), enabling near-all-round inspection, especially for complex products that need to be inspected on multiple sides (e.g., electronic components, precision parts, bottle caps, small containers).

• Single-pass finish: The product can be rotated for one week to complete imaging from all angles with high efficiency.

2. The lighting design is relatively convenient (especially for light-transmitting products):

• The advantages of bottom transmitted light are significant: the transparency of the glass disc allows for easy transmitted illumination from the bottom light source. This is very effective for detecting internal defects, fill levels, impurities, etc. of transparent and translucent objects (glass bottles, plastic parts, chemical impurities, label bubbles) and has a simple optical path.

• Flexible arrangement of side light sources: The camera is set up around to provide convenient space for arranging ring light, bar light, coaxial light, etc., which is used to illuminate the side contours, characters, and specific features of the product. The light source can be mounted near the camera lens or on a stand-alone stand.

• Top light source optional: An additional light source can be set up above the glass disc to supplement the top surface lighting.

3. Relatively stable structure and easy maintenance:

• Fixed camera position: Compared with robot grasping detection or accompanying fixture detection, the fixedly installed camera is more stable, reduces the impact of vibration, and is not easy to deviate after calibration.

• Clear maintenance channels: Cameras and light sources are distributed around the periphery of the equipment, usually with good accessibility, making it easy to clean lenses, change light sources, and adjust angles.

4. High Throughput Potential: The rotary design allows for continuous feeding, and with high-speed cameras and image processing systems, high inspection speeds can be achieved.

Main disadvantages and challenges

1. Glass reflection interference (greater pain point):

• Ambient light and stray reflections: Smooth glass surfaces are highly resistant to reflecting the surrounding environment (equipment frame, light source housing, and even the operator) and the camera itself, forming spots and ghostings that interfere with image quality.

• Affect detection accuracy: Reflection may mask the true characteristics of the product or form false defect signals (such as mistakenly treating reflective points as scratches), which greatly increases the difficulty of algorithm processing and reduces the reliability of detection.

2. Dynamic imaging blurring risk:

• Motion blur: The glass disc is constantly rotating and the product is in motion. If the camera exposure time is too long or the trigger timing is not accurate, the image will be blurred, affecting the accuracy of detail recognition.

• High synchronization requirements: high-precision encoders are required to provide real-time feedback on the turntable position, and the camera must be triggered when the product moves to a preset angle. Synchronization is also required between multiple cameras to avoid positional deviations.

3. Defects in the glass disc itself introduce interference:

• Scratches, stains, bubbles: Glass plates will inevitably produce fine scratches, stains or bubbles during use. These imperfections periodically appear in the background of the image when rotated and can be mistaken for product defects.

• High cleaning and maintenance requirements: Frequent and careful cleaning of glass plates is required to maintain their light transmission and surface cleanliness, increasing maintenance costs.

4. Product Positioning and Fixing Challenges:

• Position offset: When the product rotates on the smooth glass disk, it may slide, shift, or even tip due to inertia, vibration, or irregular shape (especially for small and light parts), resulting in inconsistent imaging positions and increasing the complexity of the detection algorithm.

• Fixed Requirements: For products that are easy to move or require a specific posture, additional fixtures or vacuum adsorption devices may need to be designed, adding to the complexity of the system.

5. Space Occupancy and Cost:

• Large radial space requirements: The cameras are distributed around the turntable, and the overall diameter of the equipment is large, which has certain requirements for site space.

• Large number of cameras: Multiple cameras may be required to achieve full coverage, especially if there are many inspection surfaces or large products, increasing hardware costs.

Key selection considerations

1. Product features are fundamental:

• Size vs. Weight: Determine the diameter of the glass pan, weight capacity, and the number of cameras/field of view required.

• Material and transparency: Transparent/translucent products can best play the advantage of transmitted light at the bottom and are the scene. Highly reflective metal parts require extreme care to handle reflections.

• Testing requirements: Clarify which aspects need to be tested? What are the flaws? Precision requirements? This directly determines the number of cameras, type (resolution, frame rate), light source type, and layout.

• Stability: Can the product remain stable during rotation? Is assisted immobilization required?

2. The lighting scheme is the core difficulty:

• Light source selection: Give priority to light sources with good controllability and strong directionality (such as high-angle ring light, coaxial light, polarized bar light).

• Polarized light applications: Using a polarizer (in front of the camera lens) and a polarized light source is one of the most effective means of suppressing glass reflections. The polarization angle needs to be carefully adjusted.

• Light shading and environmental control: Add a light shield inside the equipment to isolate external ambient light and reduce stray reflection sources.

• Fine angle of the light source: Repeatedly test the angle of incidence of the light source to avoid the strong reflection angle of the camera's field of view on the glass.

• Prioritize the use of transmitted light: If the product has good light transmission, bottom transmitted light is the most effective and priority scheme.

• Overcoming Reflection Problems:

• Multi-light source fusion: It is often necessary to combine the transmitted light at the bottom, multiple light sources on the side, and even the top light source to work together.

3. Camera Setup and Imaging Strategy:

• Resolution: Calculated based on the inspection accuracy requirements and the field of view covered by a single camera.

• Frame rate: It must be able to capture the moving product clearly at higher rotation speeds (considering exposure time and motion blur). There needs to be a margin.帧率需求 ≈ (转盘转速 RPM * 相机数量) / 60

• Global shutter: Essential to avoid the distortion caused by the rolling shutter on moving objects.

• Interfaces: GigE Vision, USB3 Vision, etc., to meet the requirements of speed and stability.

• Quantity and location: determined based on product size and inspection surface requirements. Common layouts include: evenly distributed (4 units at 90° intervals, 6 units at 60° intervals) or focused on specific surfaces.

• Height and Angle: The height of the camera determines the size and resolution of the field of view. The installation angle needs to be carefully designed, avoiding the normal direction of the glass (which is prone to strong reflection), and often using a certain inclination angle (such as 30°-60°).

• Camera selection:

• Lens selection: Match the sensor size and desired field of view/working distance, considering whether the depth of field meets the product height variation.

4. Dynamic processing and synchronization are guaranteed:

• High-precision encoder: directly connected to the turntable spindle to provide real-time angular position feedback, the resolution must be high enough.

• Precise triggering: The control system calculates and triggers each camera to take pictures at a preset angle position based on the encoder signal.

• Motion blur control: Use a short enough exposure time (with a strong enough light source) or a strobe light source (highlight at the moment of exposure).

• Processing speed: The processing power of the image processing system (usually industrial computer + GPU) must keep up with the acquisition speed of the camera to avoid becoming a bottleneck.

5. Selection and maintenance of glass plates:

• Material: Optical glass with high light transmittance, low self-illumination (less impurities/bubbles), and high hardness (scratch resistance).

• Surface Treatment: Consider anti-glare (AG) and anti-reflective (AR) coatings, which can effectively reduce reflective interference (but increase costs).

• Cleaning plan: Plan convenient cleaning methods (such as automatic air knives, cleaning brushes, shutdown to wipe windows).

6. Supplier Experience and Validation:

• Case matching: Priority is given to suppliers with rich successful experience in similar products (materials, sizes, testing items).

• Reflective processing capabilities: focus on the supplier's technical solutions (polarization application, light source design, shading) and practical effects in suppressing glass reflection.

• On-site testing (highly recommended): Provide a sufficient number of representative samples (including various defective products) and conduct actual lighting, imaging, and algorithm testing at the supplier to verify the effect. This is the surest way to reduce risk.

Summary and selection suggestions

• Suitable for scenarios: Glass disc screening machines are particularly suitable for applications that require multi-angle, high-precision inspection, and the product size is moderate, especially with light transmission (such as precision parts, electronic components, small packaging containers, pharmaceutical packaging, light-transmitting material products). Its bottom transmitted light advantage is irreplaceable when detecting internal defects in transparent objects.

• Not suitable for scenarios: oversized, overweight, products that are highly prone to tipping or sliding in rotation; Products with extremely highly reflective surfaces that cannot be effectively suppressed by polarization or other means (e.g., high-brightness metal spheres).

• Select the core:

1. Thoroughly analyze product characteristics and testing needs.

2. Overcoming glass reflection is the primary technical challenge, focusing on evaluating the supplier's lighting solutions (especially polarization applications) and actual suppression effects.

3. Pay attention to dynamic synchronization accuracy and motion blur control.

4. Adhere to on-site proofing and testing, and use actual samples to verify the imaging effect and algorithm feasibility.

5. Choose suppliers with successful experience with similar products.

The glass disc screening machine is a powerful multi-angle inspection platform, but its successful application is highly dependent on the effective solution of core challenges such as glass reflection and dynamic imaging. A deep understanding of their principles, pros and cons, combined with product characteristics and rigorous validation processes for selection, is key to ensuring the effectiveness of equipment investments and the reliability of testing.