Nondestructive testing and online inspection are both essential inspection methodologies for modern, automated industrial production. By customizing precision instruments, we provide more convenient inspection solutions that not only deliver high-efficiency measurements but also generate greater economic benefits.

Nondestructive testing refers to a method that, without damaging or impairing the service performance of the test object and without harming its internal structure, utilizes changes in thermal, acoustic, optical, electrical, magnetic, and other responses caused by internal structural anomalies or defects. Employing physical or chemical techniques and leveraging modern technologies and equipment, this method enables the inspection and evaluation of the internal and surface structure, condition, and characteristics of test specimens—specifically the type, quantity, shape, nature, location, size, distribution, and evolution of any defects. Nondestructive testing is an indispensable and effective tool for industrial development; to a certain extent, it reflects a nation’s level of industrial advancement, and its importance is now universally recognized.

Online monitoring involves deploying equipment directly on the production line and leveraging soft-sensing technology to perform real-time measurement and feedback, thereby optimizing production control and minimizing unnecessary waste.

Online nondestructive testing has become a highly favored inspection method in modern industrial manufacturing. The following provides a brief overview of several online NDT solutions:

01 Optoelectronic Nondestructive Testing Solution

The fundamental principle of photoelectric detection is to convert changes in light intensity or the direction of light propagation into corresponding changes in electrical signals, thereby achieving the objectives of detection (display) or control. It is widely used in numerous fields, including communications, security inspection, anti-theft systems, and healthcare.
Optoelectronic nondestructive testing is used to inspect external diameter, width, thickness, straightness, springback, inner-diameter rib height of threaded steel bars, and the length, height, and position of small rolled products. The testing method is non-contact, nondestructive, and performed online, and can be categorized into linear-type and reflective-type optoelectronic inspection. The commonly employed high-precision online inspection approach is the collimated-beam method; alternatively, the two methods can be combined for high-precision inspection of wide-width plates.

1. Linear Optoelectronic Detection
The characteristic of linear photoelectric detection is that the light emitter and the light receiver are not on the same side. Its basic principle can be illustrated by the slot-type photoelectric detection shown in Figure 1a: when the light emitted by the light emitter D is not blocked by an object, the light receiver U exhibits low impedance because it receives the emitted light, and its collector output is a low logic level. Conversely, when the light from the emitter is blocked, the light receiver U becomes high impedance due to the absence of received light (the application circuit is shown in Figure 1b).
Sometimes, for detection purposes, the distance between the light emitter and the light receiver may be considerable—on the order of several meters or even tens of meters (as shown in Figure 1c). Compared with the slot-type photoelectric detection described above, this configuration has the advantage of being able to detect relatively large objects.

2. Reflective Photoelectric Detection
Reflective photoelectric detection is characterized by integrating the light emitter and receiver into a single unit, and can be classified into three types: mirror-reflection photoelectric detection, diffuse-reflection photoelectric detection, and total-internal-reflection photoelectric detection.
(1) Specular Reflection Type Photoelectric Detection
The basic operating principle can be illustrated by the reflective photoelectric detection shown in Figure 2a: when the light emitter projects a beam toward the reflector, if the beam is not obstructed by the object being measured, it is specularly reflected back by the reflector and subsequently detected by the receiver. However, if an object enters the light path, the receiver will fail to detect the beam and will output a control signal (as depicted in Figure 2b; note that the control signal may be either a high level or a low level), thereby triggering the operation of the associated equipment.
(2) Diffuse Reflection Photoelectric Detection
The principle of diffuse reflection photoelectric detection is essentially the same as that of specular reflection photoelectric detection described above. In both cases, the light emitter and receiver are housed in the same unit; however, no reflector is mounted in front of the unit. Under normal conditions, the light emitted by the source does not reach the receiver. When a target object enters the light path, some of the light is reflected back and detected by the receiver, as illustrated in Figure 2c. This triggers the receiver to output a control signal, thereby completing the detection process. A drawback of this method is that, because only a portion of the reflected light is received, the detection accuracy is relatively low.
(3) Total Reflection Optoelectronic Detection
Total-internal-reflection optoelectronic detection involves guiding the light emitted by an optical transmitter to the detection point via the total-internal-reflection property of optical fibers, and then routing the reflected light signal from the detection point back to the photodetector through the same fiber. The photodetector subsequently converts the returned optical signal into an electrical output, as illustrated in Figure 2d. A significant advantage of this approach is the extremely low optical loss during propagation, enabling the detection of very small objects and operation under harsh conditions such as high temperatures—glass optical fibers can function reliably at up to 200°C—and highly corrosive environments.

02 Laser Detection Solution

Laser-based inspection solutions encompass both contact and non-contact methods, with the specific approach selected based on the requirements of the rolled material. Commonly used sensors include laser displacement sensors, laser distance sensors, two-dimensional laser measurement sensors, and laser photosensitive sensors.
Laser non-contact inspection is commonly used for measuring thickness, width, 3D surface scanning, and detecting surface defects. It can also be applied to inspecting the inner diameter and end-wall thickness of large-diameter pipes, as well as the straightness of the internal bore of long pipes.

Laser holographic nondestructive testing utilizes laser holographic interferometry to detect and quantify surface and internal defects in objects. The principle of this technique is to apply a controlled load to the object without causing any damage; under external loading, the object undergoes deformation that is directly related to the presence or absence of defects. Defects within the object give rise to minute differential displacements on the surface that differ from those of the surrounding material, and the extent of surface deformation varies with the magnitude of the applied load. By employing laser holography to visualize and compare these deformations, and by recording the surface deformation patterns under different loading conditions, one can perform comparative analysis to determine whether internal defects are present, thereby assessing the quality of the inspected object.

Applying laser holographic nondestructive testing technology to aviation maintenance enables the detection of defects and damage on both large surface areas and within the internal structures of aircraft components, while significantly enhancing the accuracy and reliability of inspections.

03 Radiographic Testing Plan
X-ray technology is used to measure the thickness of waterproof boards, plastic sheets, and other sheet materials, and an APC control system adjusts the heating temperature of each die on the production line based on the measured thickness, thereby enabling automated quality control.
X-ray scanning digital online inspection technology enables remote, full-speed online inspection without disrupting normal production. It can generate intuitive radiographic images of the inspected workpiece, which are then subjected to computer-based analysis to precisely locate defects and classify, pinpoint, and assess the severity of damage, with automatic alarm notification. This approach allows for scheduled or on-demand inspections of workpieces at any time, significantly reducing the workload of operators while maximizing the safe and reliable operation of the production process.
Industrial CT differs from conventional X-ray inspection. It performs 3D image reconstruction by acquiring a series of consecutive 2D tomographic slices, thereby capturing more information and delivering superior inspection capabilities. Not only can it detect the shape, location, and size of defects, but it can also employ density-analysis techniques to determine defect characteristics, thus resolving longstanding challenges in comprehensive qualitative assessments such as spatial localization and depth quantification of defects. Industrial CT systems place particular emphasis on detail, with exceptionally high image-processing accuracy—both in terms of motion and rotational precision. For these reasons, the international nondestructive-testing community regards industrial CT as the premier NDT method.

04 Machine Vision Inspection Solution
A vision inspection system consists of hardware and software components. The hardware comprises lenses, lighting sources, and industrial cameras (the image acquisition system), while the software includes image recognition systems and core computational algorithms. Vision systems typically use low-divergence, high-precision lasers as the light source and employ industrial cameras as the measurement sensors. Through a sequence of steps—image acquisition, image processing, data generation, and data computation—the system measures the geometric dimensions of objects.
Machine vision inspection solutions are commonly used for the inspection of diameter, width, length, threaded steel bars, internal bores, end faces, and other features. In automated automotive production lines, vision systems must, when necessary, be integrated with robotic systems and connected to the line’s PLC control system to enable measurement, inspection, positioning, and identification functions.
Machine vision inspection systems exhibit the following characteristics: they employ non-contact measurement, which enhances response speed and minimizes disruption to production lines; they deliver long-term, stable, and reliable repeatable performance, making them well-suited for continuous assembly-line operations in industries such as automotive manufacturing; and they are ideally positioned for use in environments characterized by high safety risks, poor ergonomics, and harsh operating conditions. These numerous advantages have led to the widespread adoption of vision-based inspection systems in the automotive industry.

Sources: Lanpeng Measurement and Control, Instrumentation Network, Electronic News, AI Automotive Manufacturing, Guangxing Tianxia, and the Jinan Institute of Nuclear Technology.