Le Song

Highly sensitive, precise, and traceable measurement of force

ABSTRACT Force in the micronewton range may be traced to the International System of Units by an electrostatic force balance weight system. However, there is a conflict between range and sensitivity. To solve this problem, a lever-type force metrology system based on the null-balance method is reported. The force is loaded on one end of the lever, causing elastic torsion of the central rod, and an electrostatic force is applied to the other end. With a variety of mass loading positions and standard weights, a wide range of moments may be produced to calibrate the system. The electrostatic force was generated by a pair of coaxial cylindrical capacitors where the applied voltage was converted into electrode displacement. The design of the inner and outer cylindrical capacitors was deduced considering the material and miniaturization requirements, and concentric alignment was ensured by a vision system. The null balance was achieved by a proportion-integration-differentiation control system, so that the external force was compensated by the electrostatic force. A method to increase the center of gravity was used to improve the resolution. The mechanism was analyzed based on stiffness, strength, stability, and frequency. Furthermore, the impact on the capacitance gradient due to the capacitor tilt caused by the applied force was estimated. Standard weights were utilized to evaluate the system performance. The results showed that a stiffness of 0.8 N/m and a force resolution of 10−8 N were achieved.

Spiral tool path generation for diamond turning optical freeform surfaces of quasi-revolution

Space Archimedean spiral is defined firstly in this paper. Thereafter, a new spiral tool path generation based on space Archimedean spiral is proposed for diamond turning optical freeform surfaces of quasi-revolution, which is defined as a surface close to some surface of revolution. By projecting the space Archimedean spiral onto the freeform surface along the normal direction of the base surface instead of a fixed direction like traditional method, a quasi-uniform spiral tool path on the freeform surface can be obtained. This method can be used on diamond turning optical freeform surfaces. Finally, two examples are presented to prove its effectiveness and adaptability. Space Archimedean spiral is defined.A new spiral tool path generation based on space Archimedean spiral is proposed to machining freeform surfaces with big slope.The proposed method can be used for 3-axis and 4-axis ultraprecision diamond turning optical freeform surfaces of quasi-revolution.

Improving environmental noise suppression for micronewton force sensing based on electrostatic by injecting air damping.

A micro/nano force can be traced to the International System of Units by means of an electrostatic force balance weight system. However, the micro/nano force measurement system is susceptible to environmental disturbances. Various methods have been proposed to reduce the effect of environmental disturbances and obtain high resolution and fast response. In this paper, we introduce a combination of air damping and inherent damping from the internal molecular friction of spring suspension. This will optimize system stability and improve environmental noise suppression. Results from the air damping model show that the damping ratio increases from 0.0005 to 0.1, which improves the vibration resistance. We found that the system with air damping has the advantages of fast response and low scatter.

The multi-position calibration of the stiffness for atomic-force microscope cantilevers based on vibration

Calibration of the stiffness of atomic force microscope (AFM) cantilevers is critical for industry and academic research. The multi-position calibration method for AFM cantilevers based on vibration is investigated. The position providing minimum uncertainty is deduced. The validity of the multi-position approach is shown via theoretical and experimental means. We applied it to the recently developed vibration method using an AFM cantilever with a normal stiffness of 0.1 N m−1. The standard deviation of the measured stiffness is 0.002 N m−1 with a mean value of 0.189 N m−1 and the relative combined uncertainty is approximately 7%, which is better than the approach using the single position at the tip of the cantilever.

Bionic compound eye for 3D motion detection using an optical freeform surface

To achieve a system combining large field of view (FOV), high image quality, high 3D detection accuracy, and high-speed tracking in a close shot, a single-layered compound eye with seven ommatidia is developed. The functional relationship among the FOV, optical axis angle, and overlapping area is established mathematically. Based on the relationship, one center ommatidium and six hexagonally arranged marginal ommatidia with large apertures are set on a spherical substrate to guarantee a large FOV and sufficient overlapping information for 3D imaging. To control the off-axis aberration of the marginal ommatidia, an optical freeform surface is applied. The algorithms of regions of interest and Kalman filter are applied to achieve high-speed capturing. Experiments show that the system can obtain a high-quality image with sufficient overlapping information for 3D tracking in shifting distance.

The Differential Method for Force Measurement Based on Electrostatic Force

The small force measurement is very important with development of the technology. The electrostatic force is adopted, in which a pair of coaxial cylindrical capacitors generate the electrostatic force when a voltage is applied across the inner and outer electrodes. However, the measured force will be covered by noise (creep, ground vibration, and air flow) and could not be measured accurately. In this paper, we introduce the differential method to reduce the effect of noise. Two identical parallelogram mechanisms (PM) serve as the mechanical spring. One of the PM serves as the reference and another serves as the force sensor. The common signal will be offset, and the difference signal will serve as output. In this way, the effect of the creep will be reduced. The measurement system of the electrostatic force was characterized by applying mechanical forces of known magnitude via loading weights of calibrated masses. The uncertainty from voltage, laser interferometer, and capacitance gradient was estimated. For the measured force, the relative uncertainty is less than 4% .

Curved compound eye imaging system with a large field of view based on a plano-concave substrate

A novel curved compound eye imaging system is put forward in this paper. Non-uniform hexagonal lens array is arranged on the inner surface of a plano-concave substrate. Based on the geometrical optics, the parameters of each microlens are set according to the position of the lens, and even orders of aspheric lens are used to correct some primary aberrations. Optical parameters of this configuration are entered into numerical ray-tracing simulations (ZEMAX). The result shows that the new curved compound eye can enlarge the field of view (FOV) approximately 50% compared to the lateral compound eye, and the FOV can be up to 150°. The principles and functions of all parts of system are described in detail. At last, the feasibility of ultra-precision machining is studied in this paper.

Optimization of Electrostatic Force System Based on Newton Interpolation Method

The measurements of micro/nanoforces are of great importance in both science and engineering. We developed a traceable system for micro/nanoforces based on electrostatic force using two electrodes. Noises (creep, ground vibration, and airflow) are one of the limitations for force resolution. The forces are distorted by noise and cannot be measured accurately. Although ABA method can be used to eliminate linear creep, it is invalid for nonlinear noise. In this paper, a new method known as the Newton interpolation method (NIM) has been adopted in capacitance gradient and the calibration of cantilever stiffness to reduce the effect of nonlinear noise. The results show that the capacitance gradient, with a relative standard deviation of 0.004%, is stable and has good repeatability. The stiffness of cantilever was measured using electrostatic force. The typical value of stiffness ranged from 5.1 to 48 N/m. The relative standard deviation was small, i.e., less than 0.6% owing to Newton interpolation method. These results show that our system is very stable and repeatable. This research may assist in the designing of force measurement systems based on electrostatic force.

Vision measurement method to promote calibration precision of capacitance gradient in micro-force measurement system

The micro-force measurement system based on electrostatic force using a pair of precision coaxial cylindrical electrodes as a force generator is proposed. However, there is axes deviation between the inner and outer electrodes inevitably, which can cause a change of the capacitance value. In this paper, a system is designed to measure and adjust the coaxiality of the coaxial cylindrical electrodes. After that the multidimensional adjustment stage is used to adjust the inner and outer electrodes to make them coaxial, it has a precision of 5 µm to adjust the eccentric error, and 0.030° to adjust the tilt error. A reasonable scheme is designed to calibrate the capacitance gradient, the experiment results show that after adjusting the inner and outer electrodes coaxial, the relative standard deviation in multiple measurements can be less than one over a thousand. With this measurement and adjustment scheme, the capacitance gradient value can get a good repeatability.

Study on viscosity measurement using fiber Bragg grating micro-vibration

It is now ascertained that traditional electric sensors are vulnerable to electromagnetic interference when measuring viscosity. Here, we propose a new viscosity-sensitive structure based on the fiber Bragg grating (FBG) sensing principle and a micro-vibration measurement method. The symmetric micro-vibration motivation method is also described, and a mathematical model for compensational voltage and fluid viscosity is established. The probe amplitude, which is produced by reciprocating stimulation, is accessible by means of an FBG sensor mounted on an equal-strength beam. Viscosity can be therefore calculated using a demodulation technique based on linear edge filtering with long period grating. After performing a group of verifying tests, the sensor has been subsequently calibrated with a series of standard fluids to determine uncertain parameters in the mathematical model. The results of the experiment show that the relative measurement error was less than 2% when the viscosity ranged from 200 to 500 mPa s. The proposed architecture utilizes the characteristics of anti-interference, fast response speed, high resolution and compact structure of FBG, thereby offering a novel modality to achieve an online viscosity measurement.