A differentially amplified Hall effect displacement sensor for positioning control of a long-range flexure stage.

This paper presents a novel positioning feedback sensor using a pair of Hall effect elements on a long-range flexure stage. The proposed Hall effect positioning feedback sensor eliminates error and uncertainty by measuring the center of the flexure stage, where a machine tool or measurement probes would take place in the industrial application. A pair of Hall effect elements were amplified in a differential configuration as the cylindrical permanent magnet enclosed in the center of the shuttle in the flexure stage that moves back and forth, generating a uniform gradient magnetic flux intensity. Nonlinear magnetic flux characteristics of a single Hall effect element were eliminated, and high-quality sensor sensitivity was achieved by differential amplification of the two Hall effect elements. The magnetic field analysis to characterize the linearity of the proposed displacement sensor was simulated using the finite element method to prove that the non-linearity of a single hall effect element may be mitigated by employing the differential amplification technique. The flexure stage was additively manufactured into a monolithic structure, and the permanent magnet was fitted into the shuttle of the flexure stage. Each Hall effect element was placed on either side of the magnet at a certain distance on the axis of shuttle movement. The proposed sensor was characterized by performing dynamic system identification of the flexure stage: open-loop response and closed-loop response. The Laser Displacement Sensor (LDS) with the 10 nm resolution was used for baseline comparison and datum line with respect to the proposed sensor. The proposed sensor responses agreed well with LDS in various dynamic inputs. The sensor response was analyzed with two differential amplification signal processing techniques. The maximum sensitivity of the two signal processing techniques was determined to be 16.55 mV/µm, and the resolution was observed as 2.5 µm. In sum, the differentially amplified Hall effect displacement sensor achieved positioning feedback with high sensitivity and linearity and minimized the sensor placement error while maintaining low cost and simple configuration.

Geometric part error and material property profile separation technique of the additively manufactured and post-processed rods.

This paper presents novel surface profilometry for both geometric part error and metallurgical material property distribution measurements of the additively manufactured and post-processed rods. The measurement system, the so-called fiber optic-eddy current sensor, consists of a fiber optic displacement sensor and an eddy current sensor. The electromagnetic coil was wrapped around the probe of the fiber optic displacement sensor. The fiber optic displacement sensor was used to measure the surface profile, and the eddy current sensor was used to measure the change in permeability of the rod under varying electromagnetic excitation conditions. The permeability of the material changes upon exposure to mechanical forces, such as compression or extension and high temperatures. The geometric part error and material property profiles of the rods were successfully extracted by using a reversal method that is conventionally used for spindle error separation. The fiber optic displacement sensor and the eddy current sensor developed in this study have a resolution of 0.286 µm and 0.00359 µr, respectively. The proposed method was applied not only to characterize the rods but also to characterize composite rods.

Robust, Ultrathin, and Highly Sensitive Reduced Graphene Oxide/Silk Fibroin Wearable Sensors Responded to Temperature and Humidity for Physiological Detection.

Skin temperature and skin humidity are used for monitoring physiological processes, such as respiration. Despite advances in wearable temperature and humidity sensors, the fabrication of a durable and sensitive sensor for practical uses continues to pose a challenge. Here, we developed a durable, sensitive, and wearable temperature and humidity sensor. A reduced graphene oxide (rGO)/silk fibroin (SF) sensor was fabricated by employing a layer-by-layer technique and thermal reduction treatment. Compared with rGO, the elastic bending modulus of rGO/SF could be increased by up to 232%. Furthermore, an evaluation of the performance of an rGO/SF sensor showed that it had outstanding robustness: it could withstand repeatedly applied temperature and humidity loads and repeated bending. The developed rGO/SF sensor is promising for practical applications in healthcare and biomedical monitoring.

Smart Flexible 3D Sensor for Monitoring Orthodontics Forces: Prototype Design and Proof of Principle Experiment.

There is a critical need for an accurate device for orthodontists to know the magnitude of forces exerted on the tooth by the orthodontic brackets. Here, we propose a new orthodontic force measurement principle to detect the deformation of the elastic semi-sphere sensor. Specifically, we aimed to detail technical issues and the feasibility of the sensor performance attached to the inner surface of the orthodontic aligner or on the tooth surface. Accurate force tracking is important for the optimal decision of aligner replacement and cost reduction. A finite element (FE) model of the semi-sphere sensor was developed, and the relationship between the force and the contact area change was investigated. The prototype was manufactured, and the force detection performance was experimentally verified. In the experiment, the semi-sphere sensor was manufactured using thermoplastic polymer, and a high-precision mold sized 3 mm in diameter. The change in the contact area in the semi-sphere sensor was captured using a portable microscope. Further development is justified, and future implementation of the proposed sensor would be an array of multiple semi-sphere sensors in different locations for directional orthodontic force detection.

A monolithic linear motion platform driven by a piezoelectric and fluidic pressure-fed dual mechanism.

This paper presents a novel dual-mode motion mechanism capable of achieving nanopositioning on a monolithic linear motion platform. Unlike conventional dual-mode stages that use piezoelectric (PZT)- and electromagnetic-combined or similar actuation mechanisms comprising two separate motion axes, the dual-mode actuation was developed by combining a PZT for a coarse motion and a fluidic pressure-fed mechanism (FPFM) for a fine motion and was implemented in a monolithic flexure stage fabricated by metal additive manufacturing. The FPFM actuates the flexure stage by pressuring the media in the fluidic channels created inside the flexure spring structures. Experimental tests were performed to investigate the performance of the dual-mode linear motion platform. The stiffness, damping, and frequency response functions of the dual-mode stage were experimentally characterized. The proportional-integral-differential control combined with dual-mode control was employed to control the position of the flexure stage while bidirectionally controlling the flow of compressed air for a fine motion. The FPFM motion showed a good response to 1 nm stepwise input (every 10 psi), and it was implemented to provide up to ∼10 nm fine motion along with the PZT coarse motion (1 µm). The hysteresis characteristics of the FPFM were also characterized and compensated to track the positioning error.

Knife-edge interferogram analysis for corrosive wear propagation at sharp edges.

This paper presents a novel noncontact measurement and inspection method based on knife-edge diffraction theory for corrosive wear propagation monitoring at a sharp edge. The degree of corrosion on the sharp edge was quantitatively traced in process by knife-edge interferometry (KEI). The measurement system consists of a laser diode, an avalanche photodiode, and a linear stage for scanning. KEI utilizes the interferometric fringes projected on the measurement plane when the light is incident on a sharp edge. The corrosion propagation on sharp edges was characterized by analyzing the difference in the two interferometric fringes obtained from the control and measurement groups. By using the cross-correlation algorithm, the corrosion conditions on sharp edges were quantitatively quantified into two factors: lag and similarity for edge loss and edge roughness, respectively. The KEI sensor noise level was estimated at 0.03% in full scale. The computational approach to knife-edge diffraction was validated by experimental validation, and the computational error was evaluated at less than 1%. Two sets of razor blades for measurement and control groups were used. As a result, the lag will be increased at an edge loss ratio of 1.007/µm due to the corrosive wear, while the similarity will be decreased at a ratio of 5.4×10-4/µm with respect to edge roughness change. Experimental results showed a good agreement with computational results.

Enhancement of knife-edge interferometry for edge topography characterization.

This paper introduces an optical measurement technique to enhance knife-edge interferometry (KEI) for edge topography characterization with a high resolution by shaping a beam of light incident on the sharp edge. The enhanced KEI forms spherical wavelets as a new light source by focusing a beam before the sharp edge by using an objective lens, and those wavelets interfere with the secondary wavelets diffracted at the sharp edge along the propagation direction. Unlike a conventional KEI that is limited to low spatial resolution due to a relatively large beam diameter, the enhanced KEI can increase the fringe spatial frequency and produce more data necessary for fringe analysis toward edge topography characterization. Edge samples with various edge conditions were used for validation. As a result, the enhanced KEI improved the resolution of edge topography characterization compared to the conventional KEI. This study has the potential to be utilized in high-resolution optical microscopy for edge topography characterization.

A novel sensor for two-degree-of-freedom motion measurement of linear nanopositioning stage using knife edge displacement sensing technique.

This paper presents a novel method for measuring two-degree-of-freedom (DOF) motion of flexure-based nanopositioning systems based on optical knife-edge sensing (OKES) technology, which utilizes the interference of two superimposed waves: a geometrical wave from the primary source of light and a boundary diffraction wave from the secondary source. This technique allows for two-DOF motion measurement of the linear and pitch motions of nanopositioning systems. Two capacitive sensors (CSs) are used for a baseline comparison with the proposed sensor by simultaneously measuring the motions of the nanopositioning system. The experimental results show that the proposed sensor closely agrees with the fundamental linear motion of the CS. However, the two-DOF OKES technology was shown to be approximately three times more sensitive to the pitch motion than the CS. The discrepancy in the two sensor outputs is discussed in terms of measuring principle, linearity, bandwidth, control effectiveness, and resolution.

A curved edge diffraction-utilized displacement sensor for spindle metrology.

This paper presents a new dimensional metrological sensing principle for a curved surface based on curved edge diffraction. Spindle error measurement technology utilizes a cylindrical or spherical target artifact attached to the spindle with non-contact sensors, typically a capacitive sensor (CS) or an eddy current sensor, pointed at the artifact. However, these sensors are designed for flat surface measurement. Therefore, measuring a target with a curved surface causes error. This is due to electric fields behaving differently between a flat and curved surface than between two flat surfaces. In this study, a laser is positioned incident to the cylindrical surface of the spindle, and a photodetector collects the total field produced by the diffraction around the target surface. The proposed sensor was compared with a CS within a range of 500 µm. The discrepancy between the proposed sensor and CS was 0.017% of the full range. Its sensing performance showed a resolution of 14 nm and a drift of less than 10 nm for 7 min of operation. This sensor was also used to measure dynamic characteristics of the spindle system (natural frequency 181.8 Hz, damping ratio 0.042) and spindle runout (22.0 µm at 2000 rpm). The combined standard uncertainty was estimated as 85.9 nm under current experiment conditions. It is anticipated that this measurement technique allows for in situ health monitoring of a precision spindle system in an accurate, convenient, and low cost manner.

Compliance and control characteristics of an additive manufactured-flexure stage.

This paper presents a compliance and positioning control characteristics of additive manufactured-nanopositioning system consisted of the flexure mechanism and voice coil motor (VCM). The double compound notch type flexure stage was designed to utilize the elastic deformation of two symmetrical four-bar mechanisms to provide a millimeter-level working range. Additive manufacturing (AM) process, stereolithography, was used to fabricate the flexure stage. The AM stage was inspected by using 3D X-ray computerized tomography scanner: air-voids and shape irregularity. The compliance, open-loop resonance peak, and damping ratio of the AM stage were measured 0.317 mm/N, 80 Hz, and 0.19, respectively. The AM stage was proportional-integral-derivative positioning feedback-controlled and the capacitive type sensor was used to measure the displacement. As a result, the AM flexure mechanism was successfully 25 nm positioning controlled within 500 µm range. The resonance peak was found approximately at 280 Hz in closed-loop. This research showed that the AM flexure mechanism and the VCM can provide millimeter range with high precision and can be a good alternative to an expensive metal-based flexure mechanism and piezoelectric transducer.

Novel design and sensitivity analysis of displacement measurement system utilizing knife edge diffraction for nanopositioning stages.

This paper presents a novel design and sensitivity analysis of a knife edge-based optical displacement sensor that can be embedded with nanopositioning stages. The measurement system consists of a laser, two knife edge locations, two photodetectors, and axillary optics components in a simple configuration. The knife edge is installed on the stage parallel to its moving direction and two separated laser beams are incident on knife edges. While the stage is in motion, the direct transverse and diffracted light at each knife edge is superposed producing interference at the detector. The interference is measured with two photodetectors in a differential amplification configuration. The performance of the proposed sensor was mathematically modeled, and the effect of the optical and mechanical parameters, wavelength, beam diameter, distances from laser to knife edge to photodetector, and knife edge topography, on sensor outputs was investigated to obtain a novel analytical method to predict linearity and sensitivity. From the model, all parameters except for the beam diameter have a significant influence on measurement range and sensitivity of the proposed sensing system. To validate the model, two types of knife edges with different edge topography were used for the experiment. By utilizing a shorter wavelength, smaller sensor distance and higher edge quality increased measurement sensitivity can be obtained. The model was experimentally validated and the results showed a good agreement with the theoretically estimated results. This sensor is expected to be easily implemented into nanopositioning stage applications at a low cost and mathematical model introduced here can be used for design and performance estimation of the knife edge-based sensor as a tool.

Phase-locked loop based on machine surface topography measurement using lensed fibers.

We present the phase-locked loop (PLL)-based metrology concept using lensed fibers for on-machine surface topography measurement. The shape of a single-mode fiber at the endface was designed using an ABCD matrix method, and two designed lensed fibers-the ball type and the tapered type-were fabricated, and the performance was evaluated, respectively. As a result, the interferometric fringe was not found in the case of the ball lensed fiber, but the machined surface could be measured by utilization of autofocusing and intensity methods. On the other hand, a very clear Fizeau interferometric fringe was observed in the case of the tapered lensed fiber. Its performance was compared with the results of the capacitance sensor and a commercially available white-light interferometer. We confirmed that PLL-based surface profile measurement using the tapered and ball lensed fibers can be applied for on-machine surface topography measurement applications.

Design of retrodiffraction gratings for polarization-insensitive and polarization-sensitive characteristics by using the Taguchi method.

We present the design of retrodiffraction gratings that utilize total internal reflection (TIR) in a lamellar configuration to achieve high performance for both TE and TM polarized light and polarization-sensitive performance for gratings behaving as polarizer filters; the design was based on rigorous coupled wave analysis (RCWA) and the Taguchi method. The components can thus be fabricated from a single dielectric material and do not have to be coated with a metallic or dielectric film layer to enhance the reflectance. The effects of the structural and optical parameters of lamellar gratings were investigated, and the TIR gratings in a lamellar configuration were structurally and optically optimized in terms of the signal-to-noise ratio (S/N) and a statistical analysis of variance (ANOVA) of the refractive index, grating period, filling factor, and grating depth as control factors and the estimated efficiency by RCWA as a noise factor. For more accurate robustness, a two-step optimization process was used for each purpose. For TIR gratings designed to perform similarly for TE and TM incident polarization, the -1st-order efficiencies were estimated to be up to 92.0% and 88.5% for TE and TM polarization, respectively. Also, for the TIR gratings designed to achieve polarization-sensitive performance when behaving as a polarizer filters, the -1st-order diffraction efficiencies for TE and TM polarization were estimated to be up to 95.5% and 2.7%, respectively. From these analysis results, it was confirmed that the Taguchi method shows feasibility for an optimization approach to a technique for designing optical devices.