Sensitivity force sensors

Conceptually, predecessors of the scanning force microscope are the surface force apparatus (SFA) [73,74] and the stylus profilometer [75,76]. The SFA enables measurement of normal and friction forces between molecularly smooth surfaces as small as 1 nN as a function of distance with a resolution of 0.1 nm. In addition to the local force measurement, the profilometer provides a topographic map of the surface by scanning the surface with a sharp probe. However, the profilometer is not suitable for structure characterisation because of the large radius of the tip (about 1 pm) and the low sensitivity of the force sensor (in the range of 1(T2 to 1(T5 N). 

Semiconductor fabrication processes permit construction of small, sensitive, stress sensors. In fact the levers used in atomic force microscopes are almost ideal for this purpose. The combination of the mechanical properties of silicon nitride and the geometry of the cantilever mean that the lever has a high resonant firequency and a low spring constant [32]. The low spring constant is beneficial for sensor applications because it means that a small applied force can be transduced to a measurable deflection, which lies at the heart of any sensor [33]. When combined with the highly sensitive optical lever AFM detection system, both of these factors mean that this arrangement is a fast and highly sensitive stress sensor. 

This technique uses an extremely sensitive AFM force sensor that is able to detect forces in the range of pico to nano Newtons. The relationship between... 

When die surfaces arc in contact die motion of the piezoelectric tube is transmitted directly to die force sensor. This results in a linear increase of die force sensor signal with the expansion of die piezoelectric tube. The sensitivity of the force sensor can be calibrated from this straight line, and this measuring procedure allows determination of forces as a function of separation from a hard wall contact with a high precision (within 1-2 A in distance resolution). Note, however, that the assumption of a hard wad contact is not always correct (29). 

In contrast to strain gauge force sensors, piezoelectric sensors are the other type used to measure forces in the die. Basically, a piezoelectric force sensor incorporates a highly sensitive element typically made of a crystalline or ceramic... 

The micro-force sensor, attached at the end of the IPMC-PVDF beam, has a similar structure as the IPMC-PVDF composite beam shown in Fig. 8.16, except that the IPMC layer is replaced by a (relatively) rigid passive film. In the prototype, we used 200 pm thick polyester film as the middle layer. The same charge amplifier circuit as in Fig. 8.17, with possibly different gains, is used for the force sensor. Analogous to the case of measuring the bending displacement, one can derive the sensitivity of the force sensor in terms of the electromechanical properties and dimensions of the layers [Chen et al. (2008)]. 

Shen, Y., Xi, N., Wejinya, U. and Li, W. (2004). High sensitivity 2-D force sensor for assembly of surface MEMS devices, in Proceedings of lEEE/RSJ International Conference on Intelligent Robots and Systems (Sendai, Japan), pp. 3363-3368. 

Viry, L., et al., 2014. Flexible three-axial force sensor for soft and highly sensitive artificial touch. Adv. Mater. 26 (17), 2659-2664. 

Figure 5.12 describes the advantages of this principle. Standard cavity pressure sensors are shown in the top row where the first one is installed correctly (left) and the second one touches the sensor bore due to an inclined installation (right). As a consequence, the second sensor loses sensitivity (force shunt). This is reflected in the measured values, which are too low (shown schematically by the two internal pressure curves). The so-called PRIASAFE sensors, which are protected by a sleeve, are shown in the lower row. Both the correctly installed sensor (left), as well as the sensor with the sleeve touching the sensor bore (right), provide identical, problem-free measurement signals. This principle considerably facilitates the handling of cavity pressure sensors. 

The technological importance of thin films in snch areas as semicondnctor devices and sensors has led to a demand for mechanical property infonnation for these systems. Measuring the elastic modnlns for thin films is mnch harder than the corresponding measurement for bnlk samples, since the results obtained by traditional indentation methods are strongly perturbed by the properties of the substrate material. Additionally, the behaviour of the film under conditions of low load, which is necessary for the measnrement of thin-film properties, is strongly inflnenced by surface forces [75]. Since the force microscope is both sensitive to surface forces and has extremely high depth resolntion, it shows considerable promise as a teclnhqne for the mechanical characterization of thin films. 

Inertial sensors are useful devices in both science and industry. Higher precision sensors could find practical scientific applications in the areas of general relativity (Chow et ah, 1985), geodesy and geology. Important applications of such devices occur also in the field of navigation, surveying and analysis of earth structures. Matter-wave interferometry has recently shown its potential to be an extremely sensitive probe for inertial forces (Clauser, 1988). First, neutron interferometers have been used to measure the Earth rotation (Colella et ah, 1975) and the acceleration due to gravity (Werner et ah, 1979) in the end of the seventies. In 1991, atom interference techniques have been used in... 

The simplest IR sensor would consist of a source, a sample interface and a detector. Although quite sensitive, such an arrangement would have no selectivity as any IR absorbing substance would cause an attenuation of the detected radiation. To get the selectivity that is a main driving force behind the application of IR systems, the radiation has to be spectrally analysed. This can be accomplished either by measurement at discrete wavelengths or, for multi-component sensors or samples containing (potentially) interfering substances, by full spectral analysis of the collected radiation. 

AFM has been used to study surface molecules under different conditions. Colloidal system studies by AFM AFM has allowed scientists to be able to study molecular forces between molecules at very small (almost molecular size) distances. Further, it is a very attractive and sensitive tool for such measurements. In a recent study, the colloidal force as a function of pH of Si02 immersed in the aqueous phase was reported using AFM. The force between an Si02 sphere (ca. 5 mm diameter) and a chromium oxide surface in the aqueous phase of sodium phosphate were measured (pH from 3 to 11). The Si02 sphere was attached to the AFM sensor as shown in Figure 10.3. 

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