Pre-test successful: 3D printed sensors

Additive manufacturing of optical sensors with great potential

For more than ten years, Fraunhofer IOSB-INA has been researching and developing on intelligent Sensorik and solutions for data processing in intelligent technical systems in the production environment. In the area of production data acquisition in particular, increasingly adaptable and specialized technologies and processes are being tested, as well as in the areas of Information transmission and Potonik. A preliminary study has now laid the foundation for a new generation of highly customizable, additively manufactured sensor components. 

Photonics is one of the most important growth and future-oriented sectors of the German economy. This is due on the one hand to the wide range of application fields, and on the other hand to the great technological potential. The optical recording of information can, for example, create an important data basis for automation. In medical technology and production engineering, for example, various optical methods are used to capture data from objects. The challenge is to be able to measure internal properties that are difficult to access, such as the force curve in the component or the internal temperature distribution. 

Production use case:

In industrial production, control electronics are sometimes manufactured. The production requires many process steps (e.g. placement, soldering, testing), where the individual production steps can only be controlled at the end and in total. At the associated partner SRG Elektronik GmbH For example, previously unclassifiable damage occurs on coilformers. It is difficult to localize the source of the defect because the manufacturing processes take place at high speed, in limited installation space and in opaque fluids (wave soldering). Analogously, this problem is also evident in connection technology, represented by the associated user (Wilhelm Böllhoff GmbH & Co. KG). Here, with constant parameters of production machines in the plastics sector, the product quality fluctuates inexplicably until now.

Medical technology use case:

Medical products, such as prostheses and orthoses, are increasingly being manufactured with the help of 3D printers. Here, the prosthesis/orthosis must be adjusted again and again, both during the fitting and in the later course, since movement patterns, body weight and the body circumference are constantly changing. Various adjustments have to be made again and again by specialist personnel in order to ensure the necessary mobility, correct loading and wearing comfort for a short period of time. In particular, musculoskeletal therapies often require gradual loading of the prosthesis/orthosis, which patients today can only estimate, and thus are treated suboptimally. Typically, this is implemented today by medical supply stores that are associated as users (Sanitätshaus ConradySanitätshaus Koenen).

Preliminary study at Fraunhofer IOSB-INA successful

The additive manufacturing process for optical sensors researched at Fraunhofer IOSB-INA could be used in these applications in the future. The basis for this was created in the successful preliminary study: If the geometry of a component is to become a sensor itself, it is necessary that the physical active principles - in the example for force sensors - as well as the electronics can be additively finished, or at least integrated.

 

The fact that the approach of realizing a force sensor is feasible in principle is demonstrated by means of optical active principles. Three different experimental setups were realized, the principles of which are outlined below. In "Setup 1", the light of an LED is guided through an additively manufactured, straight transparent rod and detected by a photodiode. An application of force causes the rod to bend and a smaller amount of light to appear on the detector surface. "Setup 2" uses total internal reflection. When light hits a transition from an optically dense medium to an optically thin medium at a certain angle, no refraction occurs, only reflection. A photodiode detects the amount of refracted light at the medium transition when the light guide is bent so that the entrance angle changes. In Setup 3, the reflected light component is measured in the same setup. The material of the light guides was varied during the experiments. 

Schematische Messaufbauten

In Figure 2, the measured voltage amplitudes of the photodetector (acting force: 0-1N) are shown for overview. Figure 3 shows in parallel the different light conducting structures:

Kraftabhängigkeit des detektierten Lichtes pro Versuchsaufbau

The measurement results make it clear that there is a proportional relationship between force and measured signal. In principle, therefore, the approach pursued in the project can be implemented.

Lichtleitstrukturen A) PMMA basiert mit einseitiger LED-Aufnahme, B) Epoxid basiert mit Fehlern der Druckform (grün), C) PMMA basiert monolithisch mit Luftblaseneinschluss

The results of the measurement variants show the need for further research:

  • Material-dependent suitable measurement design:
    Setup 2, 3 could not be carried out in the variant with epoxy because the material reacts too brittly and breaks under load. 
  • Active principle with minimal geometry changes:
    The effect of the force resulted in a bending of the light guides by a maximum of 24mm. Such a strong compression of the geometry is not permissible in any application.. 
  • 3D printed light guiding structure: 
    Force sensitivity and transmission with a monolithic structure are significantly higher than with the layered structure typical for 3D printing. If the channel was fabricated for epoxy, air entrapment has occurred. Further manufacturing defects due to detached, entrapped filament cause such strong obstructions that the epoxy-based light guides show no force sensitivity at the measurement points.
  • 3D printed coupling and decoupling:
    The lowest sensitivity was measured in setup 3, although the reflectivity is very angle-dependent. However, since the longest optical path is present here, the material attenuation seems too high. In addition, the coupling and decoupling of the light into the light guide is problematic because of the strong reflections. To realize area-covering sensors, a much longer range is necessary. To implement the active principle, a principle or material with high light transmission must be used.