Measuring Displacement with Confocal Sensors
Published on : Friday 08-12-2023
Displacement measurement using the colour of the light has numerous benefits.
Measuring displacement by using confocal chromatic sensors is used in a wide range of applications. The first products that made use of this technology utilised sensors that were only available in a size of approximately 50mm diameter. Since then, new light sources have become established and sensors with diameters from 4mm are now available. Displacement measurement using the colour of the light has numerous benefits and makes it possible to measure displacement in the most unlikely of applications.
Every form of light is composed of many different wavelengths (a colour spectrum). White light overlaps all visible wavelengths. The visible range for humans starts at 400nm (blue) and ends at 700nm (red). Not all the various wavelengths can be bundled into exactly one point by lenses. This is called the chromatic lens error or the chromatic lens aberration. This can be compared with the depth of field when using microscopes or cameras. It is precisely this effect that is used in confocal measurement technology. The blurring of the focal point of the different colours is extended using special lenses. This means that depending on the distance to the lens, there is precisely one wavelength in the focal point. Only this information is included for measurements.
New technology, new possibilities
Extremely high resolutions are possible when using confocal chromatic measurement technology. Resolution in the nanometre range is possible by expanding the colour spectrum. As colour in the focal point is used for the distance information, confocal sensors have an extremely small measuring spot, which also makes measurements on very small objects possible. Therefore, even the finest surface scratches can be detected reliably.
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 to achieve due to the formation of shadows. For measurements of this nature, Micro-Epsilon’s optoNCDT2402 confocal miniature sensors, which have a sensor diameter of just 4mm, are ideally suited. Another interesting potential application is measuring the thickness of transparent films, boards or layers. In contrast to other methods, the system only requires one sensor for a measurement of this type.
Gap between two layers
For many years, safety glass has been used in the production of window glass for cars. Only laminated safety glass can be used for car windscreens. This type of glass consists of two or more layers with an adhesive plastic film between these, which prevents glass splinters from flying around when an accident occurs. If this windscreen was to fracture and cause an accident, the manufacturer would not have fulfilled its legal obligations, as splinters could fly around and injure passengers inside the vehicle. Therefore, the thickness of the plastic layer must be subjected to random sample inspection.
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 to customers.
Micro-Epsilon is among the leading sensor manufacturers worldwide. For more than 50 years the company has continuously offered reliable, high performance, unique solutions particularly when high precision measurement or inspection is required. The product range covers sensors for the measurement of distance and displacement, sensors for IR temperature measurement and color detection, as well as systems for dimensional measurement and defect detection.
Micro-Epsilon India Private Limited, Pune. Tel.: +91 20 4913 1030. Fax: +91 20 4913 1001. Email: [email protected]
