Entwicklung und Charakterisierung dehnungsbasierter Kraft- und Momentensensoren für medizinische Anwendungen

• Development and characterization of strain-gauge based force and torque sensors for medical applications

Nolten, Ulrich; Mokwa,Wilfried (Thesis advisor)

Aachen : Publikationsserver der RWTH Aachen University (2013)
Dissertation / PhD Thesis

Aachen, Techn. Hochsch., Diss., 2013

Abstract

Force and torque sensors are widely used in robotics today. But in medicine, many surgeons rather rely on their own senses than on sensors. But also in this field - especially in orthopedics - these sensors can assist the surgeon. Fields of application are the long-term measurement of loads in implants and the intra-operational measurement of forces and torques. Force and torque sensor are based on the following working principle: the elastic deformation of a structure caused by external forces or torques is measured. Therefore, the strain on the surface of the structure is measured with strain gauges. During this work, platinum strain gauges were manufactured and characterized for different applications. Multiple strain gauges were additionally grouped on sensor foils and were combined to form measurement circuits. Moreover, the strain gauges on the sensor foils can be aligned more precisely to each other than single strain gauges. Application-specific sensor foils therefore help to meet the demands of the measurement task in the best possible way. The miniaturized strain gauges for the sensor foil were produced on a polyimide substrate. They have a gauge factor of k = 3 and show a linear temperature coefficient of a = 2.5 • 10-3 K-1. The metal resistor has a length of 1.23 mm and a width of 0.84 mm. Furthermore a special assembly technique for the strain gauges was developed. Additionally, the sensors can be sterilized with hot steam for 30 times. Sensor foils for three different applications were developed and tested. In the first application, a 190 mm long milling tool for the removal of femoral bone cement in revision total hip replacement was realized. A sensor foil on the shaft measured the deformation of the tool in reaction to the milling forces. This information can be used for position correction in navigated surgery. The second application is a sensor clip for uniaxial force measurement on surgical tools. The sensor clip can be attached on various instruments with a quick mount. The sensor clip allows it to measure intra-operational forces acting on the instrument. The sensor clip is capable of measuring forces reproducibly. This was shown on test structures. The third application is a universal force and torque sensor. With this sensor it is possible to measure forces in all spatial directions and also measure the torques around the axes. The working principle of this sensor was shown. Besides the platinum strain gauges, piezoresistive silicon strain gauges were designed and manufactured. The piezoresistors were structured by reactive ion etching of the 2 µm thick device layer of a SOI wafer. By this method, the resistor is fully isolated by the buried oxide and the surrounding air. Ultrasonic flip-chip bonding connects the sensors with gold stud bumps to a flexible circuit board. On these circuit boards, the piezoresistive strain gauges can be connected to sensor foils again. The piezoresistive sensors have a gauge factor of k = 70 and a temperature coefficient of a = 3.6 • 10-3 K-1.

Institutions

• Chair of Materials in Electrical Engineering I and Institute of Materials in Electrical Engineering [611510]