Robert Brookhuis

Miniature force-torque sensors for biomechanical applications

Promotion date: October 17.

Promotor: Gijs Krijnen

Assistant Promotor: Remco Wiegerink

To quantify the exerted forces on e.g. a hand, miniature force sensors are essential. Such a sensor must be accurate, with sufficient force range and capable of measuring forces in all directions. Moreover, the sensor must be small enough to be able to place it on a fingertip. In this thesis the design, realization and characterization of these force sensors are presented. The purpose is to measure the interaction forces on the hand while handling objects, so the sensors can be applied in sports or in rehabilitation processes.

The presented force sensor is fabricated in silicon consisting of a movable part which is mechanically connected to a fixed part by many thin silicon pillars. Force applied to the movable part results in a displacement, which is measured capacitively by a novel electrode structure. In total 8 electrodes are used to determine the applied load in all degrees of freedom. For capacitive read-out, a reference capacitor is integrated in the force sensor, to compensate for common-mode changes in the sensor capacitance and for drift in the read-out electronics. Besides the main design, research to other force sensor concepts has been performed and proof-of-principles are made. First measurements show that the presented principles are working.

For read-out of the sensor, a compact read-out system is realized which enables accurate measurements of the internal sensor capacities. The complete system of the read-out system and the sensor is suitable for placement on a fingertip such that the forces exerted on a fingertip can be measured.

Was your PhD research application oriented right from the start?

The project was a STW Open Technology Project, which is besides scientific research also focused on the use of the results in a practical application. The main goal of the project was to realize a small and accurate multi-axis force sensor which can be used in biomechanical applications such as the assessment of motor tasks carried out with the hand. In the project there was also a so-called user committee in which experts from companies were seated which are also interested in the application of the results. They were instituted to guide, steer and facilitate the research processes. It motivates that these companies are truly interested in the research you are doing and are eager to hear about the results.

Our work resulted in a completely novel approach, in which capacitive read-out is used with a special electrode pattern to detect the load applied to the sensor in all directions. The final sensor has three key features: it is compact, accurate and has a wide force range. Compared with the smallest commercial available force sensor with similar force range, it is at least a factor 20 smaller in volume. This makes it possible to place the sensor on a fingertip to measure interaction forces. The sensor does not only measure the total force, but it can also measure the direction in which a force is applied, which is important for a wide range of applications such as monitoring motor functions in patients undergoing rehabilitation.

The sensors are fabricated in the Mesa+ cleanroom, were we have all the technology and equipment available to realize the sensor. Fabrication of the sensor was a challenging part in my PhD project, since it takes many steps which all need to be carried out with great care and precision: if one step fails the sensor will not work as designed and the process must start from the beginning. Therefore, each fabrication step has been tested prior to the final fabrication run.

Besides the final sensor, research has been performed to other force sensor concepts which are also realized as a proof of principle. Furthermore, a compact capacitive read-out system has been realized, which can accurately measure the internal sensor capacitances. The principle of the read-out system is based on a known concept but we discovered that we could also measure the difference between two capacitances in a single measurement by a simple modification. With the combination of both - the compact read-out system and the force sensor - ambulant monitoring applications are feasible in the near future which can be used in rehabilitation processes and sports. Besides that, it can be used in robotics, to give robots a sense of touch.

Can you recall some special moments during your PhD period?

It was a great moment to see that, after spending a lot of time in the cleanroom, the sensor was actually working as well as we had hoped for. Although a systematic preliminary period preceded the fabrication – in which analyses, simulations and calculations were carried out – it was a great relief that the sensor actually did show the functionality it was designed for. This moment marked the end of an uncertain period - from the first design on - in which a lot of problems had to be overcome.

What new experiences did you gain while working on the project?

Halfway through the project we felt the need to patent the measurement principle of the sensor. Here, the input of the user committee was helpful in making this decision. Writing a patent application is not a part of our education program. Therefore, it is nice to experience such a process: to understand the important aspects. In contrast to writing a publication, it is important for a patent application to be as generic as possible. Furthermore it is important that anything claimed in the patent is not published before. For this reason we ensured that the patent was applied before publishing the results in a journal.

At the end of the project, we received a STW Demonstrator grant to develop a robust demonstrator of the sensor. With this sensor we are attracting the interest of companies to commercialize the sensor. It is a fact that if a patent on the sensor exists, it helps to convince companies. A patent has a certain communication power and helps to build a status for the technology involved and its potential for future applications and products. The importance of this aspect was something I learned while working on the project.

In what journals did you publish your results?

Articles were published in Sensors and Actuators, and the Journal of Micromechanics and Microengineering.

What are your future plans?

I like to work on novel sensor designs and the challenges involved. I enjoyed the freedom of spending as much time as needed to do the research during my PhD, and to push the limits of what is possible in the given technology. I prefer a job in industry, because it is more focused on the application of the results from research. A major difference from the academic world is the time scale, I suppose. Sometimes research and development periods of several weeks are asked for, which is significantly shorter than academic research in general.

What is important, in your opinion, for Mesa+ to stay successful in future?

It is important that the knowledge and expertise which is gained by the researchers is as much as possible maintained in the institute. Usually when a PhD project is finished, the person who did the research will leave the institute and therefore also a part of the knowledge and expertise leaves the building. To prevent this process, a clear overview of all the research topics within Mesa+ and the hands-on knowledge in the Nanolab is essential. Especially the hands-on knowledge is something which is not described in a publication, but plays a crucial role in applied research. I think the expertise and the facilities are key factors in attracting companies to do their research and development within Mesa+. In turn these companies create job opportunities for those who finished their PhD.