MEMS and Sensors

Confocal chromatic sensors enable new measurement perspectives

07 May 2021

In this era of Industry 4.0, sensors deployed in the manufacturing environment must perform high speed measurements and deliver high precision results so as to ensure reliable quality assurance. Controlling processes on modern production lines requires constant feedback of the machines and robots fabricating parts, as well as the status of the parts themselves. Distance sensors play a critical role in these applications, detecting positions and displacements to ensure everything remains on track.

A relatively new technique to optically gauge distances is confocal chromatic measurement. This non-contact, wear-free method provides a high measurement rate and high accuracies demanded in today’s industrial plants. Confocal chromatic systems are ideal feedback devices for both automated processes and quality control inspection systems in a wide range of industries. In particular, they are used in cases where laser triangulation sensors provide poor results, such as in confined spaces, or where high reflective surfaces or transparent objects need to be measured.

Confocal chromatic measuring principle

Figure 1: Confocal chromatic distance measurement is made possible by the controlled chromatic aberration of a series of lenses that split white light into a spectrum of monochromatic wavelengths. Source: Micro-EpsilonFigure 1: Confocal chromatic distance measurement is made possible by the controlled chromatic aberration of a series of lenses that split white light into a spectrum of monochromatic wavelengths. Source: Micro-EpsilonConfocal chromatic measurement systems consist of a sensor and a controller. The controller feeds white light from an internal source to the sensor over an optical fiber. Inside the sensor, a set of lenses splits the white light into different colors (wavelengths) as a result of the lenses’ chromatic aberration — an optical distortion that occurs because the lenses have different refractive indexes for different wavelengths. Each wavelength has a characteristic focal length, each of which is focused at a different distance ahead of the sensor.

The sensor projects the various colors onto the target and receives each of the wavelengths as backscattered light reflected from the surface. Only a focused beam of backscattered light, consisting of the specific wavelength that was focused on the measurement target, reaches the optical fiber in the sensor head. Each of the other wavelengths is scattered over an area, entering the optical fiber with less intensity. As a result, the light that is returned to the controller is essentially monochromatic. Inside the controller, a spectrometer detects the wavelength of the light signal and a signal processor calculates the distance corresponding to the wavelength.

Measurement capabilities

In addition to measuring the distance between a sensor and its target to determine object positions and displacements, confocal chromatic sensors are capable of measuring the thickness of transparent materials. Indeed, they can evaluate thickness from one side of a sample with a single sensor, detecting the dimension in both single- and multi-layer glass applications, like solar cells and smartphone screens.

Typically, the distance to the target in these cases must be carefully controlled due to errors introduced by varying refractive indices of the target material, depending on the distance between the sensor and the target. Confocal chromatic sensors from Micro-Epsilon solve this problem by automatically compensating for these errors. A thickness calibration calculation driven by a database of refractive indices for a range of materials stored in the controller enables accurate thickness measurements regardless of the distance to the target within the measuring range.

Figure 2: Micro-Epsilon’s confocal chromatic sensors gauge distances with high accuracy but can also reveal fine surface details and material differences by measuring signal intensity. Source: Micro-EpsilonFigure 2: Micro-Epsilon’s confocal chromatic sensors gauge distances with high accuracy but can also reveal fine surface details and material differences by measuring signal intensity. Source: Micro-EpsilonBeyond distance and thickness measurements, confocal chromatic sensors offer a unique capability to generate detailed surface topographies by tracking the intensity of the signal as the sensor scans across an object. This process shows even the finest surface structures, revealing flaws such as scratches or a damaged finish. Material differences are also highlighted due to changes in reflectivity as the sensor passes over various substances in the target.

Micro-Epsilon’s confocal chromatic sensors feature ultra-compact light spots, enabling high lateral resolutions. Resolution along the Z-axis is also excellent because the width of the measuring spot remains nearly unchanged from the top of the measuring range to the bottom. These sensors are ideal for inspecting the smallest elements in semiconductor fabrication, such as bonding wires or IC pins.

Versatile, high-speed reliability

An engineer might expect that a sensing technology that can detect even the finest details falls short when it comes to measuring rate. On the contrary, Micro-Epsilon’s confocal chromatic controllers are among the fastest in the world, operating at up to 70 kHz to keep up with high-throughput industrial production lines.

Figure 3: On this glass production line, simultaneous thickness gauging at two positions is enabled by a confocal chromatic measurement controller with dual-channel capability. Source: Micro-EpsilonFigure 3: On this glass production line, simultaneous thickness gauging at two positions is enabled by a confocal chromatic measurement controller with dual-channel capability. Source: Micro-Epsilon

With automatic exposure control, Micro-Epsilon’s controllers rapidly adapt to a variety of surfaces from matte to shiny. For each measurement cycle, the system automatically adjusts the exposure time on the couple-charged device by evaluating the previous cycle. As a result, measurement accuracy remains consistently high even if surface reflectivity changes. Whether rough or mirror-like, the system can handle a wide range of materials, including glass, stainless steel and ceramic.

Confocal chromatic sensors benefit from a beam path aligned with the axis of the sensor, eliminating the shadowing effect seen in other optical distance measurement methods with offset light transmission and reception locations such as laser triangulation. This allows for confocal chromatic sensors to measure inside recesses with no loss of accuracy. For example, they can measure the fill levels inside a tray of narrow test tubes, a situation that laser triangulation could not handle because the reflected light would be obstructed by the test tube’s walls. Micro-Epsilon also offers 90° models for precisely gauging dimensional accuracy inside boreholes or cylinders.

Figure 4: The compact sensor design with 90° beam enables the integration in restricted spaces. Figure 4: The compact sensor design with 90° beam enables the integration in restricted spaces.

Summary

Micro-Epsilon’s confocal chromatic sensors are passive devices with no moving parts and no elements that generate heat or electromagnetic noise. All electronic components are contained in the controller unit, separated from the sensor by a fiber optic cable. Consequently, confocal chromatic sensors have an inherent advantage over other measurement techniques that are vulnerable to measurement inaccuracies from these sources.

Offering a wide range of sensors and controllers that span a variety of measuring ranges and rates, Micro-Epsilon’s confocal chromatic measuring systems provide highly accurate distance, position, displacement and thickness measurements. In diverse applications, from semiconductor fabrication to medical engineering, they deliver reliable, non-contact optical gauging with submicrometer resolution.

Discover more about Micro-Epsilon’s confocal sensors for measurement tasks in numerous industries.



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