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We report the real time characterization and measurement results of a new tri-axial capacitive sensing array. Each unit cell of the Kapton based sensor has an overlapping electrode geometry that allows for the measurement of normal and shear force magnitudes, and the estimation of the 2D and 3D angle of the shear force. The use of a patterned Ecoflex dielectric layer further improves the shear force response and the tunability of the sensor. A custom readout circuit was developed to selectively read the differential capacitances in the sensor array. The local (unit cell) and global (array) force response of the sensor was characterized using a tilt stage, allowing us to determine both the normal and the shear response at different angles in the range of 0-20N. The results show that the Unit Cell response is uniform in the linear range of 0-10N for normal forces (4.6fF/N for all four differential capacitances) and shows good sensitivity for shear forces (1.4 fF/N difference between the right and left differential capacitances at the minimum) and low cross talk. Similar results were seen when a global force was applied across the sensor array. 2D and 3D models were developed to extrapolate the shear force angle from the capacitance data. We also characterized the flexible patterned dielectric layer to understand the effect of its structural dimensions on the effective relative permittivity and developed an analytical model to easily calculate the effective relative permittivity, which showed excellent correlation with the experimentally obtained results. [2020-0382]

The interfacial properties of n-alkylsilane monolayers on silicon were investigated by normal force spectroscopy, lateral force measurements, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Monolayers of (CH3( CH2)n - 1SiCl3) with chain lengths n = 5, 8, 12, and 18 were prepared and NEXAFS spectra were used to compute the dichroic ratio, RI. As n decreased from 18 to 5, the film structures change from ordered (RI = 0.41) to disordered (RI = 0.12) states. Normal force spectroscopy data were analyzed with a modified elastic adhesive contact model to extract Young's modulus, Efilm, and the work of adhesion, w, of the film; Efilm decreased from 1.2 to 0.67 GPa, and w increased from 48.6 to 60.1 mJ·m-2 as n decreased from 18 to 5. Lateral force measurements quantified the reduction in friction via an interfacial shear strength, τ , and a lateral deformation analog, η. Monolayer adsorption reduced τ from 3500 MPa for SiO2 to less than 50 MPa for n = 12 and 18 alkylsilanes and was dependent on contact pressure. Conversely, η was pressure invariant, with values of ≈3500 MPa for n = 5 and 8 and ≈1000 MPa for n = 12 and 18.

A microfabricated mechanical test platform has been designed, fabricated, and operated. This system consists of a reusable chip capable of large-displacement actuation, which interfaces to a test coupon chip compatible with synthesis conditions for many nanomaterials. Because only normal forces are used for mechanical interfacing, the two chips are not permanently connected, allowing exchange of the test coupon chips. The actuated test platform chip contains a thermal actuator driving a compliant displacement amplification transmission, and a bulk-micromachined well in which the test coupon chips may be placed and removed. The displacement amplification structure provides 40 of output displacement, extending a probe over the well and into contact with the test coupon. The test coupon contains compliant structures that are actuated by the probe from the test platform.

This paper reports the development of a bioinspired 3-D tactile sensor for minimally invasive surgery. Inspired by the principle of hair cells, the sensor is composed of a central silicon post and a piezoresistor-embedded polyimide diaphragm to which the silicon post is attached. The high-aspect-ratio silicon post helps to increase the sensitivity to shear force significantly. Another unique advantage is that the silicon post is surrounded by a cylindrical cavity that provides a safety stop for the excessively large force/displacement. A fabrication process using deep reactive-ion etching, as well as a simple but effective packaging scheme, was demonstrated. The packaged tactile sensor was characterized by monitoring the resistance change of the piezoresistors when the central post was pushed in different directions. The sensor exhibited a shear force sensitivity of 10.8 N-1 and a normal force sensitivity of 3.5 N-1. The measured displacement sensitivities in shear and normal directions are 1.2 × 10-3 and 6.0 × 10-3, respectively. The responses of single and multiple sensors to the rubber scratching test were studied to demonstrate the capability of the sensor to detect the direction and other information of the scratching.

Using a high-performance polycrystalline-silicon micromachined actuator that also functions as a friction tester, we have found frictional forces that cannot be explained by Amontons' law with a constant coefficient of friction. In our friction test, a constant tangential force is applied while normal load is ramped down at a rate \mathdotF n after a hold time t h at a hold load F h. When coated by a monolayer of FOTAS (CF3 C5F10C2H4Si(N(CH3)2)3), we find that there is no unique coefficient of static friction mus , but instead that mus depends on all three of these parameters. The dependence on t h implies static friction aging, but the rate of static friction aging can be suppressed by greater hold force. When sliding motion begins, we have identified a critical normal force to shear force ratio such that any motion initiating above the critical ratio proceeds with time-dependent frictional creep over several hundred nanometers, whereas any motion initiating below the critical ratio proceeds with a large inertial jump. These effects demonstrate that contact aging effects extend from the micrometer to the nanometer scale and are relevant to micromachined interfaces.

Insects run with far greater speed and agility for their size than even the most advanced legged robots produced to date. The single-leg ground reaction forces of running insects such as the cockroach B. discoidalis provide valuable insight into the biomechanical basis for this rapid robust locomotion. To better study the running kinematics and biomechanics of these insects, a multiaxis silicon micromachined force sensor has been fabricated. The sensor consists of a 5.3-mm square plate that is supported at its corners by thin springlike beam elements. Each flexure beam is instrumented with two piezoresistive strain gauges, allowing the determination of both normal and in- plane bending force components. Typical unamplifled normal and in-plane flexure force sensitivities of 55 and 12 V/N, respectively, have been demonstrated for a sensor with 18-mum-thick flexures and a mechanical bandwidth of 1.3 kHz. Nominal normal force resolution is 2.2 nN/Hz1/2 at 1 kHz. This paper details the design, fabrication, calibration, performance, and analytical modeling of the first-generation micromachined ground reaction force sensor. Preliminary data obtained from running cockroaches show that this sensor represents a marked improvement in performance over the techniques previously available for studying small-animal biomechanics.

Static friction between sidewall contact surfaces of polycrystalline silicon micromachines was investigated under different contact pressures, vacuum conditions, relative humidity levels, and temperatures. The static coefficient of friction exhibited a nonlinear dependence on the external contact pressure. A difference between in-contact and pull-out adhesion forces was observed due to the elastic recovery of the deformed asperities at the contact interface. The true static coefficient of friction was determined by considering the effects of the dominant adhesion forces (i.e., van der Waals and capillary forces) on the normal force applied at the sidewall contact interface. The roles of van der Waals and capillary forces in the sidewall friction behavior were analyzed in light of results for the interfacial shear strength and the adhesion force. The major benefits of the present friction micromachine and the developed experimental scheme are discussed in the context of static coefficient of friction and adhesion force results obtained under different environmental and loading conditions

This work demonstrates a polycrystalline silicon surface-micromachined inchworm actuator that exhibits high-performance characteristics such as large force (/spl plusmn/0.5 millinewtons), large velocity range (0 to /spl plusmn/4.4 mm/sec), large displacement range (/spl plusmn/100 microns), small step size (/spl plusmn/10, /spl plusmn/40 or /spl plusmn/100 nanometers), low power consumption (nanojoules per cycle), continuous bidirectional operation and relatively small area (600 /spl times/ 200/spl mu/m/sup 2/). An in situ load spring calibrated on a logarithmic scale from micronewtons to millinewtons, optical microscopy and Michelson interferometry are used to characterize its performance. The actuator consists of a force-amplifying plate that spans two voltage-controlled clamps, and walking is achieved by appropriately sequencing signals to these three components. In the clamps, normal force is borne by equipotential rubbing counterfaces, enabling friction to be measured against load. Using different monolayer coatings, we show that the static coefficient of friction can be changed from 0.14 to 1.04, and that it is load-independent over a broad range. We further find that the static coefficient of friction does not accurately predict the force generated by the actuator and attribute this to nanometer-scale presliding tangential deflections.