Surface inspection

In a surface inspection, the visible shell of an object is inspected. Here, irregularities on the surface are to be found, such as dents, cracks, scratches, dents. Also color changes and flat changes in the surface structure due to dirt or corrosion should be found.

Surface inspection is used in industries that manufacture products where aesthetics are important, such as stainless steel kitchen lines, or car bodies. Surface inspection is also used in sheet metal and steel fabrication where surface defects can lead to strength reduction. In metal welding, surface inspection is used to assess the quality of the weld and the strength of the joint.

Surface inspection can ensure important quality characteristics of the starting material in the run-up to product manufacture and thus avoid expensive rejects of end products later on.

Surface inspection: Surface defects of a metal roll as a 3D point cloud

Basics & Methods of Surface Inspection

There are various methods of surface inspection that are still used today, but are gradually being replaced by automated methods such as inline surface inspection directly in the production process as part of quality assurance in industrial companies.

Methods of surface inspection:

The classic method is the visual inspection of a workpiece by humans. This often involves the use of specific lighting at different angles, which allows to increase the contrast of the surface.
These procedures are subject to fluctuations due to lack of concentration, personal condition, breaks, etc. In particular, this test method only allows a purely qualitative test, i.e. not quantitative. In other words, humans can distinguish between good/bad, but they cannot, for example, detect the depth of a dent in µm.

Automated optical surface inspection (inline inspection), on the other hand, is increasingly being used in industrial production processes. The inspection is performed with the help of line scan or area scan cameras. With the help of image processing, the captured lines or 2D images are processed. There are three approaches to defect detection:

1. First, the image of a defect-free workpiece is taught in (golden sample). Then all images are compared with this golden sample and the differences are displayed.
2. In the analytical method, the defects are measured in length and width and compared with a tolerance field. If the defects are larger than the allowed tolerance, the part is rejected. This method requires complex and robust algorithms and is a quantitative method without measuring the defects in all 3 dimensions. Since these are 2 D cameras, the defects can only be measured in width and length, but not in depth.
3. The surface is recorded, for example, with the aid of a 3D laser line scanner in the form of a three-dimensional point cloud. Defects can be measured quantitatively on this point cloud, i.e. each defect has a length, width and depth, i.e. a defect volume. The 3D measurement of surface defects therefore allows quantitative statements to be made about the surface defects in all 3 dimensions.

3D surface inspection with QuellTech

The surface inspection of QuellTech is carried out with the help of 3D laser line scanners. The laser scanner is usually moved by means of a linear axis. The laser scanner projects a laser line of up to 1,000 mm in length vertically onto the surface to be inspected. The receiver unit of the laser scanner receives the diffuse reflection of the laser lines at an angle of approx. 40°. Since the distance and angle between the laser line and the receiving unit are known, a 3D profile of the measured surface can be calculated with high accuracy. If many such profiles are recorded sequentially in one movement, a 3 D point cloud is obtained.

The 3 D point cloud is then converted to mm values by a calibration algorithm. The calibrated point cloud is then read in by the QuellTech image processing software. The image processing software then performs various calculations on the number, size, length, depth and volume of surface defects. These results are compared to a tolerance field and sorted into good/bad.

Due to the high scan rates of the Quelltech laser scanners of up to 200,000 profiles/s, many laser profiles can be acquired 100% in one pass without any loss of time. Resolutions in the range of a few µm can be achieved.

The result of a QuellTech 3D surface inspection is then displayed on the screen as a colored surface, with a good/bad traffic light, number of defects, and saved as a report with the raw data. Furthermore, a process capability analysis can be output. The result output can be flexibly adapted to the customer’s needs and displayed in many languages.

3D and 2D imaging of surface detection on a metal surface

3D and 2D imaging of surface defects on a metal surface

Advantages of QuellTech’s surface inspection

Compared to camera-based 2D surface inspection, QuellTech’s 3D surface inspection can achieve a true quantitative measurement of surface defects in 3 dimensions. This provides a measured result in numbers for each of the three dimensions: Width, depth and length of each defect.

No discussion between suppliers and customers about good/bad. For example, in the case of stainless-steel panels, it is possible to measure precisely and display on the results screen how many defects of which size are located on a surface. Thus, the surface can be automatically divided into defined quality classes.
No time interruption of the continuous product flow, as the inline measurement does not extend the cycle time in production. Continuous recording of the result data additionally enables complete traceability.
Statistical functions enable planning of predictive maintenance of the production machine.

Application areas of surface inspection

With QuellTech laser scanner-based surface inspection, there are a large number of application examples, including these:

Quality classification in sheet metal production

In stainless steel and aluminium sheet production, quality grading of each sheet is achieved by 3 D laser scanner surface inspection. Thus, 100% monitoring is achieved in the sheet metal production.

Surface inspection of metal rolls in paper production

Metal rolls for paper production with a diameter of approx. 3 m and a length of 6 m are inspected for surface defects using QuellTech laser scanners: This enables the customer to verify that the surface is free of defects during final inspection. If defects are nevertheless detected, these defects can be reworked and eliminated before the roll is delivered. Before the use of a QuellTech surface inspection system, it happened that surface defects were only discovered at the customer’s premises and a time-consuming and expensive return shipment had to be made so that the defects could be repaired.

Checking sealing surfaces of flange connections

Sealing surfaces of flange connections must be free of flaws and surface defects in order to achieve optimum sealing. Thus, the QuellTech laser scanners can check not only the surface quality but also the flatness.

Surface inspection with multiple laser scanners for complex geometries

In the production of rubber seal profiles, which often have complex geometries, several laser scanners are used at different angles to subject the complete envelope of the rubber seal to a surface inspection. This procedure can be used for continuous profiles as well as for very special profile segments.

Testing of plastic parts in the automotive industry

When inspecting plastic parts, laser scanners are used to inspect the surface of both continuous material and complex bodies. The main incentive for surface inspection of plastics is aesthetic reasons.

Weld seam inspection

An increasingly important area of application is weld inspection, which is basically a surface inspection. In this process, a whole range of welding defects become visible, such as pores, penetration notches, asymmetries, weld protrusions, cracks, component misalignment, end craters and weld spatter. Automated weld inspection with laser scanners is increasingly replacing manual visual inspection, achieving greater efficiency with a high and consistent defect detection rate.

Surface inspection in battery production

During battery production, very thin copper carrier foils are coated with various materials. It is important not only to measure the coating thicknesses but also to identify surface defects at an early stage, since these surface defects can strongly influence the subsequent electrical behavior of the battery.

Continuous product testing in throughput in Cable production

In cable production, typically 4 laser scanners are used to scan the entire surface on the 360° of the circumferences. These are mostly continuous products. If a surface defect is measured, the length coordinate of the defect location is recorded so that the defective segment can be cut out later. In the production of continuous products, very high throughput speeds are sometimes used, which means that QuellTech laser scanners with a very high scan rate (max. 200 kHz) are also used here.

Examples for Surface Inspection with QuellTech

Result screen surface detection

QuellTech products for surface inspection

Q6-D laser scanner series with a high resolution, large measuring range , robust against extraneous light suitable for high gloss surfaces.

Q6 Laser Scanner

Q5-D laser scanner series with a medium resolution, large measuring range, very high scan rate, robust against extraneous light suitable for high gloss surfaces.

Q5 Laser Scanner

Q6 Laser Scanner

Q5 Laser Scanner

Do you have any questions about surface inspection or would you like Quelltech to advise you on this topic?

If you have any questions about surface inspection or would like advice from QuellTech on this topic, we would be happy to help.

We would like to help you evaluate your specific measurement task in surface inspection. By performing an initial free test measurement of your application, we can we can give you an early estimate of feasibility.

There is always potential for improvement, we will help you with that. Contact us for more information or to make an appointment for a consultation.

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Stefan Ringwald
Tel: +49 89 124 723 75

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