Miniature force-torque sensors for biomechanical applications

Abstract

The forces involved in interactions between the human body and the environment are important for many tasks in daily life. For example in sports, the magnitude, application and effectiveness of forces involved in a given task are a key factor for success. Likewise, in many situations, such as physical labour, loadings need to stay within safe limits to prevent injuries. 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 force sensors which satisfy the aforementioned requirements is presented. The purpose of these sensors is to measure the interaction forces on the hand while handling objects. If this is combined with motion sensors which are also placed on the hand, an estimation of the mechanical power involved in a given task can be made. With such a system a given task can be optimized for optimal mechanical power transfer, which can for example be applied in sports or in rehabilitation in case a functional motor task involving physical interaction with the environment needs to be learned. The research described in this thesis can be divided in two parts: the realization of an accurate miniature force sensor and the realization of a capacitive read-out system for this sensor. The realized force sensor is fabricated in silicon and consists of two parts, a movable part which is mechanically connected to a fixed part by many thin silicon pillars. A force applied to the movable part results in a displacement which is measured capacitively using electrode structures. From the measured capacitances, the load applied to the sensor can be determined. By changing the number of pillars, their diameter and length, the sensor can be optimized for a given force range. This initial force sensor showed limitations in the design, and an improved force sensor design was made. In this new design a corrugated ring is placed completely around the movable part such that it seals the interior of the sensor ensuring that the electrode area cannot be contaminated. Furthermore, a part of the shear force exerted to the sensor will be carried by the ring, enabling a higher shear force range. 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. The aforementioned sensors show good performance but their fabrication processes are relatively complex, therefore another force sensor is investigated which is easier to fabricate. The force range of this sensor is significantly smaller, but first measurements show that the principle is working. Further research is required to increase the force range of this sensor and to determine the ultimate accuracy that can be obtained. For read-out of the force sensor without the use of lab equipment, a small read-out system is realized which enables accurate measurement of the internal sensor capacitances. For this system a relaxation oscillator is used which is adapted such that it can measure differential capacitance. The noise performance of the oscillator in the system is analysed and measured, revealing the influence of individual component values on the noise performance of the system and providing a design guide for obtaining good noise performance. As a proof-of-principle the read-out system is interfaced with a force sensor and the measurement results are compared with measurements obtained from dedicated lab equipment. The complete system of the realized force sensor in combination with the capacitive read-out system is suitable for placement on a fingertip such that the forces exerted on a fingertip can be measured.