3D sensor solution 3D Profile Unit
One-sided optical thickness measurement on glass
Sensors that can measure the distance to an object have many applications in the manufacturing industry. The requirements on the measurement system may differ substantially depending on the application. The distance to be measured, the accuracy required and the type and surface of the object are just a few of the numerous factors that determine the choice of sensor. Sensors based on an optical measurement principle have the advantage that they are not subject to the negative effects of electromagnetic interference fields, nor to the effects of wear. In addition, optical measurement principles enable very high resolution, accuracy and repeatability.
The new optical measurement system optoNCDT 2401 (Figure 1) from Micro-Epsilon GmbH & Co. KG, Ortenburg, is based on an optical measuring principle, which specifically exploits the so-called chromatic aberration of light. This refers to the different refraction of light in relation to its wavelength. For example, the effect leads to the creation of a rainbow or the splitting of white light by a prism.
Confocal measurement principle
The innovative optical sensor system consists of a controller and a sensor. It uses an LED, whose light is passed from the controller through an optical fibre to the sensor. This optical fibre can be up to 50 m long, enabling the controller to be installed remote from the sensor. In hazardous or explosive environments, this is an invaluable benefit. In the sensor, the polychromatic (white) light of the LED passes through an optical system of multiple lenses, which focus the light onto the surface of the object to be measured. The lenses are arranged such that a specific chromatic aberration occurs. This means that light with a different wavelength or colour is focused to a different degree. The focus for blue light, for example, is located closer to the sensor than that for red light (Figure 2). The light reflected from the surface returns to the controller where it, in turn, passes through an optical system that images it onto a light-sensitive sensor element. The sensor element detects the spectrum of the reflected light and determines the distance to the measurement object by evaluating the colour. Through a calibration already carried out by the manufacturer, each wavelength of light or colour is related to a certain distance to the measurement object.
Technique offers many advantages
A significant advantage of this technique is that the condition of the surface does not have any influence on the measurement accuracy. The system is even capable of precise measurements of reflecting or transparent surfaces, which can quickly lead to errors with other more conventional measurement techniques. Depending on the sensor type, the measuring spot is only a few µm in size and also remains constant even with varying measurement distances. Consequently, very high resolution can be realised in all directions. In the measurement direction, the minimum measurable change of distance is 3 nm. The sensor system is therefore ideally suited to three-dimensional topographical measurement of surfaces. In contrast to systems that operate with laser light, no precautions are required. Since an LED is used as standard for the light source, the user does not need to consider laser protection class.
Innovative controller for a high performance measurement system
Along with the new sensors, two new controllers are also being introduced. Both of them operate as standard, with an LED as the light source. Used in this way, both controllers achieve a measuring rate of 2 kHz. In addition to the LED, the IFC 2431 Controller also has a plug-in module for an external xenon light source. With the high light intensity of this xenon light and in conjunction with a fast CMOS matrix, the measuring rate increases to 30 kHz. Equipped with two analogue interfaces, USB, RS232 and RS422, integration into existing systems is greatly simplified.
Even difficult measurement tasks can be realised
The beam path of the sensor is compact and concentric. This enables the system, for example, to measure inside drilled holes, which other optical methods, such as the triangulation technique, find difficult or even impossible due to shadow formations. For measurements of this nature, the confocal miniature sensors optoNCDT2402, which have a sensor diameter of only 4 mm (Figure 3), are particularly suitable. A further potential application is in the thickness measurement of transparent film, boards or layers. In contrast to other methods, the system only needs one sensor for a measurement of this type (Figure 4). For this measurement, the reflection of the focal points on the front and rear surfaces are evaluated.
Different types of sensor available
A large number of sensor types are available for the system. The possible measurement range varies from 120 µm to 25 mm. The miniature sensors, which only have a diameter of 4 mm, are available in both axial and radial versions. The resolution of the sensors is 0.004% of the measurement range. Mounting the sensors is comparatively straightforward, because, particularly with diffusely reflecting surfaces, a large tilt angle of 80° or 50° for the miniature version is possible. With highly reflective surfaces, the tilt angle can be between 1.5° and 43° depending on the sensor used. The measurement system can be operated at temperatures from +10°C up to +50°C. Here, the temperature stability is 0.01% of the measurement range per 1°C. With an external xenon lamp, the sensor system achieves a measuring rate of up to 30 kHz. It is therefore possible to use the system for dynamic measurement tasks.
Safety test on laminated safety glass
Safety glass has been used for many years in the production of window glass for cars. Only laminated safety glass may be used for the windscreen. This glass consists of two or more layers with an adhesive plastic film between, which prevents glass splinters flying around when an accident occurs. This plastic consists of either polyvinyl butyral or casting resin, both of which are strongly adhesive materials that firmly join together the layers. In production, the plastic is stretched as a film between the layers. In an autoclave the plastic film fuses under high temperature and pressure with the two glass layers. During this process, the thickness of the plastic layer may not be constant over the complete windscreen. When the windscreen fractures, this situation would not fulfil the legal requirements. Splinters could fly around and injure passengers in the vehicle. Consequently, the thickness of the plastic layer must be subjected to random sample inspection.
With glass panes, laser triangulation sensors must only be used with reservation. The laser beam would penetrate the windscreen and cannot then determine the layer thickness inside transparent materials.
Therefore, confocal sensors with a measurement range of 10 mm were used for this special application. Here, the sensor achieves a resolution of 0.4 µm. To keep the offset distance of the sensor constant, a small construction around the sensor was developed so that it only needs to be placed on the windscreen before starting the measurement. The measurement results are transferred via an optical fibre to the controller located 10 m away, which represents a safe working distance.
The sensor acquires the distance of the individual layer transitions in the windscreen, such as glass-to-plastic. Four points are calculated with each measurement. The difference between the inner points represents the thickness of the plastic film. It is only when the film is of constant thickness, that the production is deemed to be running correctly and the windscreens can be shipped.
Conclusion: A flexible, versatile system
The measurement system is suitable for a wide variety of applications. With its very high resolution, it is suitable for the measurement of thickness, filling levels and surface topology. Aside from the examples already mentioned, the system is being used for applications in electronics production, dimensional inspection, in the process manufacturing industries and in laboratories.
The company sees many future applications for this product in the pharmaceuticals sector.
Confocal measurement principle
The innovative optical sensor system consists of a controller and a sensor. It uses an LED, whose light is passed from the controller through an optical fibre to the sensor. This optical fibre can be up to 50 m long, enabling the controller to be installed remote from the sensor. In hazardous or explosive environments, this is an invaluable benefit. In the sensor, the polychromatic (white) light of the LED passes through an optical system of multiple lenses, which focus the light onto the surface of the object to be measured. The lenses are arranged such that a specific chromatic aberration occurs. This means that light with a different wavelength or colour is focused to a different degree. The focus for blue light, for example, is located closer to the sensor than that for red light (Figure 2). The light reflected from the surface returns to the controller where it, in turn, passes through an optical system that images it onto a light-sensitive sensor element. The sensor element detects the spectrum of the reflected light and determines the distance to the measurement object by evaluating the colour. Through a calibration already carried out by the manufacturer, each wavelength of light or colour is related to a certain distance to the measurement object.
Technique offers many advantages
A significant advantage of this technique is that the condition of the surface does not have any influence on the measurement accuracy. The system is even capable of precise measurements of reflecting or transparent surfaces, which can quickly lead to errors with other more conventional measurement techniques. Depending on the sensor type, the measuring spot is only a few µm in size and also remains constant even with varying measurement distances. Consequently, very high resolution can be realised in all directions. In the measurement direction, the minimum measurable change of distance is 3 nm. The sensor system is therefore ideally suited to three-dimensional topographical measurement of surfaces. In contrast to systems that operate with laser light, no precautions are required. Since an LED is used as standard for the light source, the user does not need to consider laser protection class.
Innovative controller for a high performance measurement system
Along with the new sensors, two new controllers are also being introduced. Both of them operate as standard, with an LED as the light source. Used in this way, both controllers achieve a measuring rate of 2 kHz. In addition to the LED, the IFC 2431 Controller also has a plug-in module for an external xenon light source. With the high light intensity of this xenon light and in conjunction with a fast CMOS matrix, the measuring rate increases to 30 kHz. Equipped with two analogue interfaces, USB, RS232 and RS422, integration into existing systems is greatly simplified.
Even difficult measurement tasks can be realised
The beam path of the sensor is compact and concentric. This enables the system, for example, to measure inside drilled holes, which other optical methods, such as the triangulation technique, find difficult or even impossible due to shadow formations. For measurements of this nature, the confocal miniature sensors optoNCDT2402, which have a sensor diameter of only 4 mm (Figure 3), are particularly suitable. A further potential application is in the thickness measurement of transparent film, boards or layers. In contrast to other methods, the system only needs one sensor for a measurement of this type (Figure 4). For this measurement, the reflection of the focal points on the front and rear surfaces are evaluated.
Different types of sensor available
A large number of sensor types are available for the system. The possible measurement range varies from 120 µm to 25 mm. The miniature sensors, which only have a diameter of 4 mm, are available in both axial and radial versions. The resolution of the sensors is 0.004% of the measurement range. Mounting the sensors is comparatively straightforward, because, particularly with diffusely reflecting surfaces, a large tilt angle of 80° or 50° for the miniature version is possible. With highly reflective surfaces, the tilt angle can be between 1.5° and 43° depending on the sensor used. The measurement system can be operated at temperatures from +10°C up to +50°C. Here, the temperature stability is 0.01% of the measurement range per 1°C. With an external xenon lamp, the sensor system achieves a measuring rate of up to 30 kHz. It is therefore possible to use the system for dynamic measurement tasks.
Safety test on laminated safety glass
Safety glass has been used for many years in the production of window glass for cars. Only laminated safety glass may be used for the windscreen. This glass consists of two or more layers with an adhesive plastic film between, which prevents glass splinters flying around when an accident occurs. This plastic consists of either polyvinyl butyral or casting resin, both of which are strongly adhesive materials that firmly join together the layers. In production, the plastic is stretched as a film between the layers. In an autoclave the plastic film fuses under high temperature and pressure with the two glass layers. During this process, the thickness of the plastic layer may not be constant over the complete windscreen. When the windscreen fractures, this situation would not fulfil the legal requirements. Splinters could fly around and injure passengers in the vehicle. Consequently, the thickness of the plastic layer must be subjected to random sample inspection.
With glass panes, laser triangulation sensors must only be used with reservation. The laser beam would penetrate the windscreen and cannot then determine the layer thickness inside transparent materials.
Therefore, confocal sensors with a measurement range of 10 mm were used for this special application. Here, the sensor achieves a resolution of 0.4 µm. To keep the offset distance of the sensor constant, a small construction around the sensor was developed so that it only needs to be placed on the windscreen before starting the measurement. The measurement results are transferred via an optical fibre to the controller located 10 m away, which represents a safe working distance.
The sensor acquires the distance of the individual layer transitions in the windscreen, such as glass-to-plastic. Four points are calculated with each measurement. The difference between the inner points represents the thickness of the plastic film. It is only when the film is of constant thickness, that the production is deemed to be running correctly and the windscreens can be shipped.
Conclusion: A flexible, versatile system
The measurement system is suitable for a wide variety of applications. With its very high resolution, it is suitable for the measurement of thickness, filling levels and surface topology. Aside from the examples already mentioned, the system is being used for applications in electronics production, dimensional inspection, in the process manufacturing industries and in laboratories.
The company sees many future applications for this product in the pharmaceuticals sector.
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