A positioning system with long-range high-precision displacement sensor integrated for measuring molecular bonding forces

Abstract

The working area of ultra-precision position-control devices has been expanding to meet the demands of a wide range of scientific and industrial applications. To achieve the requirement for a high-dynamic-range position-control system with reduced size and complexity, a contact-type linear encoder-like capacitive displacement sensor (CLECDiS) was developed to measure displacements over the millimeter range with nanometer resolution. However, small changes in the contact conditions due to the surface profile or friction, which are inherent characteristics of contact sensors, lead to significant distortion of the output signal. Thus, reliable contact conditions during CLECDiS measurement must be achieved to enable practical application of the device. In this thesis, in order to design an instrument with reliable contact conditions, the contact conditions were analyzed by characterizing the signal distortion, observing the pressure distribution between the contacting surfaces, and measuring the motional errors of the sensor using a laser Doppler vibrometer (LDV). A mechanism based on the analyses is proposed and a manufactured prototype instrument enabled the CLECDiS to measure displacement with improved reliability. The prototype was integrated into a dual-actuated stage to develop an ultra-precision long-travel-range position-control system. A null motion control strategy was used for this redundant system, and a fringe subdivision technique using quadrature phase signals from the CLECDiS was applied to convert the capacitance variable signal to a displacement. As a result, the developed positioning system based on the CLECDiS achieved an accuracy and repeatability of hundreds of nanometers over several millimeters of working area

it also showed the ability to provide nanometer-scale position control if the read-out or pattern design of the CLECDiS is enhanced. In addition, a new method that requires an ultra-precision long-range position-control system is proposed to increase the reliability of bio-molecular bonding force measurements using scanning probe microscopy (SPM) and a micro-machined nanodot. When using the probing method to measure the unbinding molecular interaction forces, unlike conventional techniques, the substrate is treated so that a single molecule is likely to attach to the nanodot. Then, the unbinding forces can be measured by drawing force–distance (F–D) curves while probing around the nanodot. The unbinding force between a single protein (vibrio cholera toxin B subunit) and a carbohydrate (GM1 pentasaccharide) was successfully measured around a nanodot, demonstrating the potential of the developed technique and instrument to increase the reliability of molecular bonding force measurements. However, in this application, a dual-stage made of commercially available coarse and fine stages was applied to a laboratory-made SPM because the developed position-control system using CLECDiS has not yet achieved the required nanometer-scale accuracy and repeatability.