Andy Pye discusses with Micro-Epsilon how confocal sensors are being used in order to measure texture and surface finish on laser-machined components.
According to Chris Jones, Managing Director at Micro-Epsilon UK, non-contact confocal displacement sensors are helping to inspect the shape, size and surface topography of MEMS (Micro Electro Mechanical or Micro Electronic Systems) structures to nanometre resolution.
The latest confocal chromatic sensors offer extremely high sensitivity and sub-micrometre resolution, providing significant advantages when it comes to inspecting the shape, size and surface topography of MEMS structures during or post-production. These sensors can be integrated to linear X-Y stages, machine tools or special purpose inspection systems with closed loop feedback control.
How do confocal sensors work?
The confocal chromatic measuring principle works by focusing polychromatic white light onto the target surface using a multi-lens optical system. The lenses are arranged in such a way that the white light is dispersed into a monochromatic light by controlled chromatic deviation (aberration). A certain deviation (specific distance) is assigned to each wavelength by a factory calibration. Only the wavelength that is exactly focussed on the target surface or material is used for the measurement. This light reflected from the target surface is passed through a confocal aperture onto a spectrometer, which detects and processes the spectral changes.
Confocal measurement offers nanometre resolutions and operates almost independently of the target material. A very small, constant spot size, typically <10 µm, through the measurement range of the sensor is achieved. Miniature radial and axial confocal versions are available for measuring the internal surfaces of drilled or bored holes, as well as the measurement of narrow apertures, small gaps and cavities.
Inspecting highly reflective and dark diffuse surfaces
Precision laser micromachining companies are benefiting from the use of non-contact confocal chromatic sensors. UK-based firm OpTek Systems, for example, has installed non-contact confocal chromatic sensors from Micro-Epsilon on its laser processing machines. These are used to measure the surface finish and groove depth of difficult materials, ranging from highly reflective, mirrored surfaces to dark, diffuse surfaces.
The machines that OpTek builds need to work to extremely high tolerances, typically to sub-micron accuracy. This means that any measurement and inspection systems installed on these machines also need to operate at these levels of accuracy. On a recent project, OpTek had some challenges to overcome in terms of the in-situ measurement capability of its machines, which were required to measure the depth of features laser-etched in the surface of disc-shaped components to accuracies of 0.25 microns, as well as measuring the surface finish [Ra] of these components to less than 0.1 micron accuracy.
These same measurement systems also needed to cope with changes in the texture and surface finish of the components, which could be high precision air bearings or seals. Typically, these components are machined from metals or hard ceramics, such as silicon carbide or tungsten carbide, and so the sensors needed to measure on both dark, diffuse surfaces, as well as shiny reflective, mirrored surfaces.
OpTek did consider employing some of the contact-based, stylus measurement systems typically used for off-line measurements, but the practicalities of integrating these rather delicate touch probe sensors within the automated environment of the machine were problematic. Moreover, the ability of the non-contact probe to be able to measure a range of different sizes and shapes of parts and features with small tolerances, both in depth and laterally without concern for mechanical crashes, made the non-contact approach very attractive. Also, the ability to avoid probe-tip cleaning and replacement provided more reliability and long-term stability.
OpTek considered various non-contact displacement measuring principles, including laser triangulation sensors. However, it was concluded that laser sensors would not provide the measurement accuracy needed in this application, particularly as the surfaces were changing from dark, diffuse to highly reflective.
Micro-Epsilon’s confocal DT IFS 2405 non-contact confocal displacement sensor benefit from large standoff distances (up to 100mm), providing users with greater flexibility in terms of the variety of applications in which they can be used. In addition, the tilt angle of the sensor has been increased significantly (up to 34 degrees), which provides better performance when measuring across changing surface features.
OpTek machines – and indeed laser-machines in general – already have line-of-sight of the component that needs to be measured and so installing a confocal sensor close to this target was relatively straightforward. The confocal sensor can be positioned well away from the debris field produced by the laser machining process, and then brought in to measure the result almost in real time.
In addition, being able to supply laser-processing machines with in-situ measurement capabilities for difficult, changing surface textures, added new quality assurance features on Optek’s machines.
Compact laser triangulation with auto-target compensation
Meanwhile, Micro-Epsilon UK has launched a range of laser triangulation sensors with integrated controller. The sensors offer machine builders, systems integrators and OEMs a combination compact size, performance (up to 4kHz measuring rate) and ease of installation.
Two new sensor ranges are available: the OptoNCDT 1320 and the OptoNCDT 1420 laser displacement sensors. The OptoNCDT 1320 is the entry-level version for high precision measurement of displacement, distance and position. The controller is integrated in the sensor housing, which simplifies installation and set up. The sensor is extremely compact (46 x 30 x 20mm), lightweight (30g) and so can be mounted in tight spaces on machines or other difficult-to-access locations. The sensor is ideal for highly automated environments and for applications in which high accelerations occur, including machine tools, robot arms and automated pick-and-place systems.
The OptoNCDT 1320 offers high accuracy and adjustable measuring rates up to 2kHz. The unique Auto Target Compensation (ATC) feature ensures stable distance signal control, regardless of target colour or brightness. A small, sharply projected laser spot size ensures that even extremely small objects can be measured reliably.
The OptoNCDT 1320 is designed for rapid set up and installation, which is achieved with just a few clicks of the multi-function sensor button. The user can select the zero setting/mastering function or the trigger set up. An intuitive web interface enables the user to carry out extended sensor set up and configuration. For the most common surface types, the user can select from a menu of predefined settings. A ‘quality slider’ enables the sensor to be adapted to static and dynamic processes.
With identical dimensions, the OptoNCDT 1420 is designed for high precision, high speed, dynamic displacement, distance and position measurement applications. The measuring rate is adjustable up to 4kHz and the sensors offer an excellent price-performance ratio, particularly for high volume OEM applications.
A range of different output signals enable easy integration of the sensor into plant or machine control systems. As well as analogue voltage and current outputs, a digital RS422 interface provides distance information from the sensor. For ease of installation, users can select from either a 3m length integrated cable with open ends, or a 0.3m long pigtail with M12 connector.
All OptoNCDT 1420 sensors operate using a web interface for fast sensor set up and configuration. Up to eight user-specific sensor settings can be stored and easily exported to other sensors. Additional features include video signal display, signal peak selection and freely adjustable signal averaging, which enables optimisation of the measurement task. A region of interest (ROI) function allows background signal noise to be filtered out.