Abstract
Abstract. An important property of high-precision mechanical sensors such as force transducers or torque sensors is the so-called creep error. It is defined as the signal deviation over time at a constant load. Since this signal deviation results in a reduced accuracy of the sensor, it is beneficial to minimize the creep error. Many of these sensors consist of a metallic spring element and strain gauges. In order to realize a sensor with a creep error of almost zero, it is necessary to compensate for the creep behavior of the metallic spring element. This can be achieved by creep adjustment of the used strain gauges. Unlike standard metal foil strain gauges with a gauge factor of 2, a type of strain gauges based on sputter-deposited NiCr-carbon thin films on polymer substrates offers the advantage of an improved gauge factor of about 10. However, for this type of strain gauge, creep adjustment by customary methods is not possible. In order to remedy this disadvantage, a thorough creep analysis is carried out. Five major influences on the creep error of force transducers equipped with NiCr-carbon thin-film strain gauges are examined, namely, the material creep of the metallic spring element (1), the creep (relaxation) of the polymer substrate (2), the composition of the thin film (3), the strain transfer to the thin film (4), and the kind of strain field on the surface of the transducer (5). Consequently, we present two applicable methods for creep adjustment of NiCr-carbon thin- film strain gauges. The first method addresses the intrinsic creep behavior of the thin film by a modification of the film composition. With increasing Cr content (at the expense of Ni, the intrinsic negative creep error can be shifted towards zero. The second method is not based on the thin film itself but rather on a modification of the strain transfer from the polyimide carrier to the thin film. This is achieved by controlled cutting of well-defined deep trenches into the polymer substrate via a picosecond laser.
Highlights
Many sensor types to determine mechanical quantities like force, pressure or torque are based on strain gauges (SGs)
Strain gauges based on granular NiCr − C thin films offer the advantage of higher sensor signals, which allow for enhanced freedom of design
In order to compensate the creep behavior of the metallic spring elements of force transducers, it is necessary to adjust the creep behavior of the strain gauges
Summary
Many sensor types to determine mechanical quantities like force, pressure or torque are based on strain gauges (SGs). An applied load (force, pressure, torque) causes elastic deformations in certain areas and surfaces of the spring element These tensile and compressive strain fields lead to electric resistance changes of the attached SGs, and as a result, the sensor signal increases or decreases, respectively. The emerging stress leads to relaxation of the polymer material beneath the metal grid (polymer foil and adhesives); the metal grid draws back (Keil, 2017) This results in a decreasing sensor signal. This paper presents an in-depth study of the major influence factors on the creep behavior of force transducers with granular NiCr − C thin-film strain gauges, such as material creep of the spring element, the relaxation of polymer carrier, the composition of the thin film, the strain transfer to the thin film and the strain field on. Since it is hardly possible to separate the creep contribution of the polymer carrier and the strain gauge adhesive, we kept the influence of the adhesive constant by usage of the same adhesive type and application routine in all our experiments
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