Jacqueline Rausch

Printed resistive strain sensors for monitoring of light-weight structures

In this paper we present the design and test of printed strain sensors, which can be integrated in light-weight structures for monitoring purposes. We focus on composite structures consisting of metal substrate as well as insulating and conductive ink layers for sensing normal strain at the surface. Both, inkjet and screen printing technology are used to realize resistive topologies that can be evaluated using a Wheatstone bridge configuration. In a first step, we analyze electrical properties of functional inks: electrical impedance and breakdown electrical field strength in case of insulation inks, resistance in case of conducting inks. Silver and PEDOT:PSS based suspensions are printed as sensing layer. To determine the resistance change due to plastic deformation of the metal substrate, tensile tests are performed up to 30% strain and subsequent resistance change is measured. In a second step, the sensing effect of printed conductive structures is investigated. Resistive sensing topologies are designed for detecting longitudinal and transversal normal strain. Meander structures, which form single resistors as well as bridge configurations, are printed on test specimens and analyzed in a four-point bending set up. Performing loading and unloading cycles, gauge factor, cross sensitivity, nonlinearity and hysteresis error of the sensors are measured.

Experimental comparison of piezoresistive MEMS and fiber bragg grating strain sensors

We report on the experimental comparison of piezoresistive MEMS sensors and optical fiber BRAGG grating sensors (FBGS) for strain measurements in force sensors. To our knowledge, this is the first direct comparison of piezoresistive and FBG transducers as force sensors. Cantilevers are used as deformation elements. The sensors are bonded on top and bottom side of the cantilever using a heat-curing epoxy adhesive. The piezoresistive ones are micro machined silicon chips with decoupled adhesive areas, and four ion implanted piezoresistive areas (R<inf>sq</inf> = 125 Ωcm, (1 × 2)mm²×350 µm). The FBGS are draw-tower-gratings (0.78mm² × 9 mm). The measured values are compared by analyzing nominal strain, sensitivity, resolution, measurement uncertainty and thermal behavior. The MEMS sensor is more sensitive than the FBGS (0.28%, /N ≫ 0.004%/ N), its measurement uncertainty is lower (2% ≪ 5%) and the resolution Δ∈ = 10⁻³ µm /m is 100 times higher than in case of FBGS Δ∈ = 1.38 µm / m.

Analysis of mechanical properties of liver tissue as a design criterion for the development of a haptic laparoscopic tool

In this paper we present a mechanical soft tissue model serving as a design criterion for a novel haptic telemanipulation system for laparoscopic liver surgery. Haptic properties of this tool can be characterized by transparency T. T is defined as the ration of the mechanical impedance of the manipulated tissue Z Tissue detected by an intracorporal force sensor to the mechanical impedance generated by the haptic human-machine interface Z feedback. In order to use T as a design criterion a mechanically interpretable closed form description of the mechanical impedance is desired. Assuming visco-elastic behaviour we adopt a mechanical network with lumped elements, based on an extended Kelvin model. Since human fingers need to sense vibrations during skilful manipulative tasks up to several kilohertz we present a new experimental setup which enables us to determine lumped network parameters in a broad bandwidth: A sinusoidal force sweep from 10 Hz to 10 kHz is generated by an electrodynamic shaker. The speed and force response of the liver tissue is detected by an impedance head and parameters of the lumped network are determined by a fit, using a trust-region algorithm (RSME ≦ 0.7). Comparison measurements were taken on ex vivo porcine liver and a haptic rubber phantom.

Sheet metal hydroforming of functional composite structures

This paper studies the formability of functional composite structures, consisting of a metal substrate, insulating plastic foils, flat copper conductors and printable conductive polymers. The aim is the production of smart components in a sheet metal hydroforming process. In addition to their mechanical properties, these components can also transfer energy and data. Conventional boundaries between mechanics and electronics will be relaxed expediently. The challenge of this study is the design of the forming process, so that all elements of the multi-layer composites will withstand the process conditions. In this context, an analytical method for estimating the formability of these smart components is presented. The main objectives are the definition of basic failure modes and the depiction of the process limits.

Strain measurement on stiff structures: experimental evaluation of three integrated measurement principles

This paper presents an experimental evaluation of three different strain measuring principles. Mounted on a steel beam resembling a car engine mount, metal foil strain gauges, piezoresistive silicon strain gauges and piezoelectric patches are investigated to measure structure-borne forces to control an active mounting structure. FEA?simulation determines strains to be measured in the range of 10?8 up to 10?5?m???m?1. These low strains cannot be measured with conventional metal foil strain gauges, as shown in the experiment conducted. Both piezoresistive and piezoelectric gauges show good results compared to a conventional piezoelectric force sensor. Depending on bandwidth, overload capacity and primary electronic costs, these principles seem to be worth considering in an adaptronic system design. These parameters are described in detail for the principles investigated.

Piezoresistive Dehnungsmesselemente für adaptronische Systeme

Zusammenfassung Es werden zwei Sensortechnologien für den Einsatz zur Dehnungsmessung in adaptronischen Systemen vorgestellt. Inhomogen dotierte, piezoresistive Siliziumelemente und gedruckte Sensorschichten werden hinsichtlich ihrer Sensoreigenschaften verglichen. Die zum Verständnis nötigen Grundlagen beider Sensortechnologien werden erläutert. Neben dem statischen Übertragungsverhalten wird auch das Rauschen der Sensoren untersucht. Abstract Two sensing technologies for realizing strain sensors, which can be integrated into adaptronic systems, are presented. Inhomogeniously doped silicon chips based on the piezoresistive principle and printed strain sensors are compared. The operating mode of each sensor type is explained. The static characteristics and the noise as one of the main internal error sources are discussed.