Jo W. Spronck

A capacitive tactile sensor for shear and normal force measurements

Abstract This paper describes a new capacitive tactile sensor for shear and normal force measurements. The spatial sampling principle has been applied to measure the capacitance values efficiently and to transfer the spatial capacitance distribution into the time domain. The shear and normal forces are determined by measuring variations in the phase and amplitude of the output signal, respectively. A first experimental model of the sensor shows a resolution of 0.05 N over the range of 10 N for both shear and normal force measurements.

High resolution heterodyne interferometer without detectable periodic nonlinearity

A high resolution heterodyne laser interferometer without periodic nonlinearity for linear displacement measurements is described. It uses two spatially separated beams with an offset frequency and an interferometer configuration which has no mixed states to prevent polarization mixing. In this research, a simple interferometer configuration for both retroreflector and plane mirror targets which are both applicable to industrial applications was developed. Experimental results show there is no detectable periodic nonlinearity for both of the retro-reflector interferometer and plane mirror interferometer to the noise level of 20 pm. Additionally, the optical configuration has the benefit of doubling the measurement resolution when compared to its respective traditional counterparts. Because of non-symmetry in the plane mirror interferometer, a differential plane mirror interferometer to reduce the thermal error is also discussed.

A smart capacitive absolute angular-position sensor

Abstract This paper describes a novel low-cost smart accurate capacitive absolute angular-position sensor with high accuracy and high resolution (18 bits r.m.s.). One of the novelties of this sensor is that, for each measurement, it uses all 24 segment electrodes along the circumference, each of them ranging from 0 to 0.3 pF. The capacitance values between these electrodes have a lot of redundancy with respect to the position to be measured. The electronic processing uses this redundancy to compensate for mechanical errors in the sensor in order to obtain an accurate measurement and to reduce the mechanical demands on the sensor. This novel approach can also be applied directly to capacitive absolute linear-position sensors. A simple and cheap oscillator is used to measure the various sensor capacitances. The oscillator converts the analog capacitance information into the time domain and is directly digitized by a microcontroller, eliminating the need for an expensive AD converter.

Simple heterodyne laser interferometer with subnanometer periodic errors

We describe a simple heterodyne laser interferometer that has subnanometer periodic errors and is applicable to industrial fields. Two spatially separated beams can reduce the periodic errors, and the use of a right-angle prism makes the optical configuration much simpler than previous interferometers. Moreover, the optical resolution can be enhanced by a factor of 2, because the phase change direction is opposite between reference and measurement signals. Experiments have demonstrated the periodic errors are less than 0.15 nm owing to the frequency mixing of the optical source. The improvements for reducing the frequency mixing of the optical system are also discussed.

A simple capacitive displacement sensor

Abstract This paper presents a simple capacitive displacement sensor based on the application of spatial sampling and Doppler measurement techniques. The theoretical performance of the sensor has been predicted and simulated on a PC, and confirmed experimentally. The prototype sensor shows a resolution and repeatability of 1 μm. A linear measurement range up to a few meters can be achieved.

Fiber-coupled displacement interferometry without periodic nonlinearity

Displacement interferometry is widely used for accurately characterizing nanometer and subnanometer displacements in many applications. In many modern systems, fiber delivery is desired to limit optical alignment and remove heat sources from the system, but fiber delivery can exacerbate common interferometric measurement problems, such as periodic nonlinearity, and account for fiber-induced drift. In this Letter, we describe a novel, general Joo-type interferometer that inherently has an optical reference after any fiber delivery that eliminates fiber-induced drift. This interferometer demonstrated no detectable periodic nonlinearity in both free-space and fiber-delivered variants.

A new two-dimensional capacitive position transducer

Abstract A new capacitive position-measurement system with the capability of simultaneously measuring displacements in x- and y-directions is presented. It is based on measuring the position of a movable sensor head relative to a matrix of reference electrodes, which constitutes a form standard. A laboratory model using printed-circuit-board (PCB) technology shows the following features: a range of 85 mm × 60 mm, a resolution better than 100 nm × 100 nm and a repeatability better than 200 nm × 200 nm (peak-to-peak).

Finite-element modelling as a tool for designing capacitive position sensors

Abstract In this paper a short review is given of the methods that are available for modelling the capacitance response of capacitive sensors. This includes the influence of alignment errors and electrode errors (geometrical errors and defects) as well as parasitic capacitances. Three methods are presented: direct calculation of the capacitance by solving the electric-field equations; approximation of the capacitance by simplifying the electric-field distribution; and modelling using the finite-element method. The suitability of each of these methods for different purposes is discussed. Conclusions are that in most cases approximations based on simplification of the electric field can be used to get an impression of the capacitance response, while for more complicated cases and for higher accuracy the more time-consuming finite-element method can or even must be used.

Relative optical wavefront measurement in displacement measuring interferometer systems with sub-nm precision

Many error sources can affect the accuracy of displacement measuring interferometer systems. In heterodyne interferometry two laser source frequencies constitute the finally detected wavefront. When the wavefronts of these source frequencies are non-ideal and one of them walks off the detector, the shape of the detected wavefront will vary. This leads to a change in measured phase at the detector resulting in increased measurement uncertainty. A new wavefront measurement tool described in this publication measures the relative phase difference between the two wavefronts of the two source frequencies of a coaxial heterodyne laser source as used in commercial heterodyne interferometer systems. The proposed measurement method uses standard commercial optics and operates with the same phase measurement equipment that is normally used for heterodyne displacement interferometry. In the presented method a bare tip of a multimode fiber represents the receiving detection aperture and is used for locally sampling the wavefront during a line scan. The difference in phase between the beating frequency of the scanning fiber and a reference beating frequency that results from integration over the entire beam, is used for the reconstruction of the wavefront. The method shows to have a phase resolution in the order of ~25 pm or ~λ/25000 for λ 632.8 nm, and a spatial resolution of ~60 µm at a repeatability better than 1 nm over one week.

Photolithography on bulk micromachined substrates

Photolithography on high topography substrates, such as the sidewalls or the bottom of cavities and trenches created by bulk micromachining, enables the design of complex three-dimensional structures. When a contact lithography system is used to pattern such substrates, local gaps exist between the mask and the substrate. In this paper we investigate the deformation of patterns as a result of these local gaps. We determine the position accuracy and the minimum size of features that can be patterned as a function of the gap distance. Deformations introduced by the optical system are quantified for a common exposure tool, and compared to pattern deformation due to variations in photoresist layer thickness. Finally, methods to improve the quality of patterns transferred through gaps up to 350 ?m are discussed