Flexure-based multi-degrees-of-freedom in-vivo force sensors for medical instruments

This paper presents novel multi-degrees-of-freedom force sensors based on flexures used as mecano-optical transducers (named flexure body) and white light interferometers used as opto-electrical transducers. Together, these transducers make up a load cell exploiting the nanometric accuracy of Fabry-Pérot interferometric measurement to reach milli-Newton force accuracy. The design focuses on the flexure body composed of three sections: a base (attached to the measuring device), a compliant section which deforms under applied forces and a pointed rigid section whose tip touches tissues during surgery. The fiber interferometer measures the distal displacement with respect to the base using one 125 μm diameter optical fiber for each load cell DOF. The key advantages of this design are: compact design (1 to 4 mm diameter shaft), simple optical alignment during assembly, scalability from Newton down to milli-Newton force levels, insensitivity to electrical charge and compatibility with sterilization procedure. These properties satisfy the requirements of in-vivo force measurements during surgery. The paper presents analytical stiffness estimation of 1 DOF flexure bodies and finite element stiffness analysis of multiple-DOF structures followed by the design, manufacturing and assembly process. The realized sensors are then characterized experimentally on a specifically designed motorized test-bench, which allows application of calibrated forces from various directions onto the senor tip. A specific calibration strategy was developed improving measurement accuracy of the sensor.

Development of a New Embedded Dynamometer for the Measurement of Forces and Torques at the Ski-Binding Interface

Frédéric Meyer, Alain Michel Prenleloup, Alain Schorderet

In alpine skiing, understanding the interaction between skiers and snow is of primary importance for both injury prevention as well as performance analysis. Risk of injuries is directly linked to constraints undergone by the skier. A force platform placed as an interface between the ski and the skier should allow a better understanding of these constraints to be obtained to thereby develop a more reliable release system of binding. It should also provide useful information to allow for better physical condition training of athletes and non-professional skiers to reduce the risk of injury. Force and torque measurements also allow for a better understanding of the skiers' technique (i.e., load evolution during turns, force distribution between left and right leg ...). Therefore, the aim of this project was to develop a new embedded force platform that could be placed between the ski boot and the binding. First, the physical specifications of the dynamometer are listed as well as the measurement scope. Then, several iterations were performed on parametric 3D modeling and finite element analysis to obtain an optimal design. Two platforms were then machined and equipped with strain gauges. Finally, the calibration was performed on a dedicated test bench. The accuracy of the system was between 1.3 and 12.8% of the applied load. These results show a very good linearity of the system, which indicate a great outcome of the design. Field tests also highlighted the ease of use and reliability. This new dynamometer will allow skiers to wear their own equipment while measuring force and torque in real skiing conditions.

2019

Load cell with positive, negative and zero stiffness operating modes for milli- to nanonewton contact force monitoring in electrical probing

Michal Stanislaw Smreczak

Monolithic integrated circuits (ICs) have been miniaturized over the past five decades, and today their components range in size from hundreds of microns to several nanometers. Making point contact with electrical samples under a microscope is referred to as microprobing or nanoprobing and is carried out using micromanipulators equipped with sharply-ended probe needles. Probe landing on the sample is a critical step in this process since physical contact with the sample can damage the probe tip or the sample itself. Hence, this research was motivated primarily by the desire to find a novel method of monitoring and controlling physical contact between probe tip and sample, increasing its reliability in micro- and nanoprobing.Micro- and nanoprobing often involve qualified operators using microscope visual feedback to land the probe. This method, however, has several limitations, including a constrained field of view and no qualitative contact assessment. A more versatile solution is to measure the contact force between the probe tip and the sample since it does not rely on microscope images and provides an additional feedback channel that complements visual-servoing and facilitates micromanipulation. This work reviews a number of known methods of measuring force on a small scale, but none fully meets the needs of micro- and nanoprobing. The main challenges of these applications include, among others, measuring forces in the nanonewton to millinewton range, using different probe tips based on the type of sample, controlling contact force to obtain stable reliable conditions for electrical measurement, and protecting the probing system against excessive pressure exerted by the probe tip.The core objective of the thesis is to find a method for measuring and controlling the contact force between a probe tip and a sample that addresses the micro- and nanoprobing requirements. The proposed concept is based on a compliant mechanism that transduces the contact force applied to the probe tip into an output displacement. Through stiffness adjustment, the mechanism is capable of handling a wide range of forces that occur during micro- and nanoprobing. As a result of stiffness adjustment, the load cell can be designed at mescoscale, which provides high stiffness in transverse directions, and facilitates the interchangeability of probe tips. Using different probe tips necessitates analyzing gravity effects on the operation of the load cell and compensating for zero offset. Additionally, the study examines active force control implemented through stiffness and offset force adjustment, which provides a constant-force characteristic, bi-stable mode with overload protection, and allows fine probe tip positioning.Several polymer demonstrators and metal prototypes were fabricated as part of the work, allowing the experimental validation of subsequent stages. The performance of these mechanisms was compared with that of the analytical models and the finite element analysis developed within the research. The obtained results demonstrate the successful fulfillment of the requirements, including the ability to measure static contact forces over 60 mN with a resolution of 10nN (dynamic range of 6x10^6). A wide range of stiffness and offset force adjustments enable probe tip positioning with a resolution of 12 nm over a range of 5.64 mm. Finally, the new load cell was used for microprobing on a sample of micropillars to validate its different operating modes.

EPFL2022

Construction material monitoring with "optical hair" hygrometers

Pascal Kronenberg

Moisture is an essential parameter in the behaviour of capillary-porous construction materials such as timber and concrete. It affects liquid and gas transport phenomena, chemical and biological degradation processes, mechanical properties and, in the case of concrete, hydration. From a scientific point of view, moisture monitoring is essential in order to improve the understanding of material behaviour. Moisture measurements may increase the predictive accuracy of established material behaviour models. In practice, the increased knowledge of material behaviour improves structural maintenance planning. The main objective of this work is to propose a measurement method for non-destructive, in-depth moisture monitoring of construction materials. In this context, an intrinsic point and an averaging fibre optic relative humidity sensor have been developed and tested. The sensors are based on optical fibres that are coated with a hygroscopically swelling transducer polymer. When wet, the swelling of the coating strains the fibre (analogy with the hair hygrometer). The induced strain is assessed with conventional fibre optic strain sensing techniques such as fibre Bragg gratings and Michelson interferometry. Theoretical and experimental studies lead to a detailed understanding of the influence of humidity and temperature on the steady and transient state sensor behaviour. The sensors have an accurate, linear, reversible and reproducible response to relative humidity between 5 and 95 %RH and between 13 and 60 °C, at least. The sensor response time is in the order of 20 minutes. However, when packaged, the sensor responds slower. The temperature cross-sensitivity of the fibre Bragg grating sensor may be compensated with an additional non-hygroscopic grating, while the Michelson interferometric sensor provides an auto-compensation of temperature effects. Tests in mortar and timber samples demonstrate that the sensors preserve their sensing ability when embedded. These tests have also clarified the multiplexing potential of fibre Bragg gratings for forming a multi-point RH sensor. Multi-point sensors are particularly useful for profile measurements.