The Journey from NANOMEFOS to NMF

(Original text available in Mechatronica & Machinebouw)
 

In 2002 TNO asked a master student from the Eindhoven University of Technology to develop a concept for measuring aspherical and freeform optics. A prototype was developed during a PhD project, which was successfully used by TNO for years.

Credit: TNO

Aspherical and freeform optics becomes commonly used nowadays. That was very different 17 years ago. At the time, TNO optics manufacturing department saw the potential of aspherical optics and decided to set up a program for it. Designing and producing it was already quite possible, but metrology proved problematic due to the complex shapes and required nanometer accuracy. Since you can only make what you can measure, a solution had to be found. They contacted Nick Rosielle’s Structures & Mechanisms group at Eindhoven University of Technology, and a graduation assignment was created. They asked master student Rens Henselmans (currently CTO of DUI) to come up with a suitable concept.

Henselmans developed a concept called NANOMEFOS (NAnometer Accuracy NO-contact MEasurement or Freeform Optical Surfaces). It is a contactless coordinate measuring machine. Rens compared it with a gigantic CD player that can be fitted with optics of half a meter. The product lies on a spindle that rotates at a constant speed, while an optical distance sensor is moved over the surface.

However, just measuring the distance between sensor and surface is not enough. For a reliable measurement result, one always has to know the precise location of the sensor and the product.  The individual movements of these parts can be measured independently thanks to a separate metrology system. This can be used to correct for the movement errors that the table and the probe make. Laser interferometers determine the position of the sensor and capacitive sensors measure the vibration of the table, both with reference to a Silicon Carbide reference frame.

The focus sensor has only a limited focus depth of a few micrometers, while freeform optics can have a non-rotational symmetry of a few millimeters. In addition, local angles in the surface lead to measurement errors. That is why a dual-stage probe was developed by a fellow PhD student Lennino Cacace, which makes the probe head constantly follow the surface. This makes it possible to also measure freeform shapes at a very high speed, because the fast-moving mass is low.

In addition to 2 PhDs (Henselmans and Cacace), about 20 people worked on the project to develop NANOMEFOS, including students, optical and mechanical designers and instrument manufacturers. The result was a working machine that was able to measure a freeform surface twice, with the same result down to the nanometer. However, there was no time and budget left for developing a user interface and calibration. Hence Henselmans joined TNO, and the machine was moved to the optics manufacturing department there. He worked as a system architect for astronomy and space, and further developed the user interface for NANOMEFOS. Three years later, the device was calibrated and had working software for aspherical forms and freeforms, with which the people of optics manufacturing could independently take measurements.

Between 2009 and 2018 the prototype was used in all kinds of projects at TNO. The lenses of the ESO VLT OTA Ø380 mm beam expander and the ESA TROPOMI freeform telescope on Sentinel 5P were measured with this machine

 

A new beginning

Gerard van den Eijkel (currently CEO of DUI) brought new life into the development. In 2016 he was the Managing Director of Demcon Focal. Demcon was asked by TNO for a possible copy of NANOMEFOS. However, to make a single machine was too expensive. A number of Asian parties showed interest in NANOMEFOS in the same year. With the orders from a Chinese distributor of optical equipment, Demcon created a business together with TNO. 

Demcon and TNO decided to set up a new company, called Dutch United Instruments, in order to emphasize its Dutch origin. In addition, ‘DUI’ means ‘correct’ in Chinese. Van den Eijkel decided that it would be wise to contact Henselmans and to have an advisory role. Henselmans, who then worked at NTS, was immediately enthusiastic, but an advisory role would not be sufficient so he decided to work full-time for DUI.

In retrospect, Henselman’s expertise was indispensable. He devoted himself to redesigning the machine. Because of his extensive experience, he knew exactly what was good and what needed to be better. The new machine branded as NMF is much more user friendly, the cost price is reduced, the robustness is increased and the design is more suitable for series production, while the accuracy of the prototype from 2009 is maintained. Also the design is scalable for larger and smaller variants and there is room to add options in the future.

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