Low-frequency vibration is a harmful factor that affects the accuracy of micro/nano-measuring machines. Low-frequency vibration cannot be completely eliminated by passive control methods, such as the use of air-floating platforms. Therefore, low-frequency vibrations must be measured before being actively suppressed. In this study, the design of a low-cost high-sensitivity optical accelerometer is proposed. This optical accelerometer mainly comprises three components: a seismic mass, a leaf spring, and a sensing component based on a four-quadrant photodetector (QPD). When a vibration is detected, the seismic mass moves up and down due to the effect of inertia, and the leaf spring exhibits a corresponding elastic deformation, which is amplified by using an optical lever and measured by the QPD. Then, the acceleration can be calculated. The resonant frequencies and elastic coefficients of various seismic structures are simulated to attain the optimal detection of low-frequency, low-amplitude vibration. The accelerometer is calibrated using a homemade vibration calibration system, and the calibration experimental results demonstrate that the sensitivity of the optical accelerometer is 1.74 V (m·s-2)-1, the measurement range of the accelerometer is 0.003⁻7.29 m·s-2, and the operating frequencies range of 0.4⁻12 Hz. The standard deviation from ten measurements is under 7.9 × 10-4 m·s-2. The efficacy of the optical accelerometer in measuring low-frequency, low-amplitude dynamic responses is verified.

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Development of a High-Sensitivity Optical Accelerometer for Low-Frequency Vibration Measurement.

Humanoid robots are equipped with humanoid arms to make them more acceptable to the general public. Humanoid robots are a great challenge in robotics. The concept of digital twin technology complies with the guiding ideology of not only Industry 4.0, but also Made in China 2025. This paper proposes a scheme that combines deep reinforcement learning (DRL) with digital twin technology for controlling humanoid robot arms. For rapid and stable motion planning for humanoid robots, multitasking-oriented training using the twin synchro-control (TSC) scheme with DRL is proposed. For switching between tasks, the robot arm training must be quick and diverse. In this work, an approach for obtaining a priori knowledge as input to DRL is developed and verified using simulations. Two simple examples are developed in a simulation environment. We developed a data acquisition system to generate angle data efficiently and automatically. These data are used to improve the reward function of the deep deterministic policy gradient (DDPG) and quickly train the robot for a task. The approach is applied to a model of the humanoid robot BHR-6, a humanoid robot with multiple-motion mode and a sophisticated mechanical structure. Using the policies trained in the simulations, the humanoid robot can perform tasks that are not possible to train with existing methods. The training is fast and allows the robot to perform multiple tasks. Our approach utilizes human joint angle data collected by the data acquisition system to solve the problem of a sparse reward in DRL for two simple tasks. A comparison with simulation results for controllers trained using the vanilla DDPG show that the designed controller developed using the DDPG with the TSC scheme have great advantages in terms of learning stability and convergence speed.

A Multitasking-Oriented Robot Arm Motion Planning Scheme Based on Deep Reinforcement Learning and Twin Synchro-Control

A four-point error separation technique integrating three-point roundness and two-point straightness error separations is proposed to measure and reconstruct a cylindrical profile for the accurate evaluation of cylindricity. Four probes mounted onto two sections in the probe carriage target the surface of the cylinder to detect the differential vector of the least-squares centers in two adjacent cross-sectional profiles of the cylinder. The vector of the least-squares center in each cross-sectional profile is extracted by accumulation. The spatial curved median line of the cylinder is determined by fitting the extracted least-squares centers. Theoretical analysis and numerical validation prove that radial error motions of the spindle and straightness error motions of the slider are eliminated during reconstruction of the cylindrical profile. The influence of tilt error motions of the spindle on the accuracy of error separation can be weakened by reducing the distance between the two sections in which the four probes are located. The spatial median line profile of the cylinder is accurately determined even if the error motions of the spindle are not repeatable in each revolution. An experimental system was constructed to perform comparison experiments. The results validate the fact that the technique has higher precision in reconstructing cylindrical profiles owing to its robust anti-interference ability. The technique is suitable for in situ measurement of cylindricity under unrepeatable radial error motions of the spindle and indeterminate straightness error motions of the slider.

https://iopscience.iop.org/article/10.1088/1361-6501/aac15b/pdf

Four-point error separation technique for cylindricity

Roundness error separation based on singularity detection and exact-stop of spindle in on-machine measurement of spindle rotation error

An efficient method for minimum zone cylindricity error evaluation using kinematic geometry optimization algorithm

An innovative parallel error separation technique for on-line measurement and reconstruction of cylindrical profile is presented. A five-probe system configured to target three adjacent cross-sections of the cylinder is proposed to simultaneously execute the three-probe roundness error separation and three-probe straightness error separation. The curved median line is determined by fitting through the least squares centers of circular cross-sectional profiles. The cylindrical profile conforming to the mathematical definition can then be reconstructed for the cylindricity error evaluation. Theoretical analysis and numerical validation have been performed. The results verify that the spindle’s radial and tilt error motions as well as the probe carriage’s straightness deviations are removed, and the least squares center of each cross-sectional profile of the cylinder is accurately extracted even if the spindle’s error motions are not repeatable in each rotation. This method has thoroughly solved the inherent problem in conventional multi-probe error separation techniques: the suppression of the first-order harmonic in separating the cross-sectional profile of the cylinder and the error motions of the spindle.

A Parallel Error Separation Method for the On-line Measurement and Reconstruction of Cylindrical Profiles

Low-frequency vibration is one of the harmful factors that affect the accuracy of micro-/nano-measuring machines because its amplitude is significantly small and it is very difficult to avoid. In this paper, a low-cost and high-precision vibration generator was developed to calibrate an optical accelerometer, which is self-designed to detect low-frequency vibration. A piezoelectric actuator is used as vibration exciter, a leaf spring made of beryllium copper is used as an elastic component, and a high-resolution, low-thermal-drift eddy current sensor is applied to investigate the vibrator's performance. Experimental results demonstrate that the vibration generator can achieve steady output displacement with frequency range from 0.6 Hz to 50 Hz, an analytical displacement resolution of 3.1 nm and an acceleration range from 3.72 mm s−2 to 1935.41 mm s−2 with a relative standard deviation less than 1.79%. The effectiveness of the high-precision and low-cost vibration generator was verified by calibrating our optical accelerometer.

https://iopscience.iop.org/article/10.1088/1361-6501/aa9f6d/pdf

High-precision and low-cost vibration generator for low-frequency calibration system

https://iopscience.iop.org/article/10.1088/1361-6501/aab705/pdf

Measurement method of rotation angle and clearance in intelligent spherical hinge

A new high-precision two-dimensional (2D) micro-accelerometer is proposed in this study to measure low-frequency vibration. A flexure hinge with low crosstalk and high axial stiffness is used as a spring, and a 2D optical angular displacement sensor is used to detect the motion of the inertial mass. The accelerometer is calibrated using a homemade one-dimensional horizontal micro-vibration calibration system. Furthermore, the accelerometer is dynamically identified through the adaptive least-square method. The resonance frequencies of the accelerometer in X- and Y-axes are identified as 43.16 and 40.31 Hz, respectively. The working frequency ranges of the accelerometer, which are approximately 20% of their resonant frequency, are obtained according to the calibration experimental results. The sensitivities of the accelerometer in the X- and Y-axes are 25.17 and 25.55 V/g, respectively. The resolutions of the accelerometer in the X- and Y-axes are 0.07 and 0.06 mg, respectively. These findings are consistent with the theoretical analysis results. The effectiveness of the accelerometer is verified by measuring low-frequency and low-amplitude dynamic responses.

A high-precision two-dimensional micro-accelerometer for low-frequency and micro-vibrations

A three-probe error separation technique with a novel V-block for measuring and reconstructing a cylindrical profile is presented. The V-block has two tandem V-shaped planes and three displacement probes mounted in one section. During the measurement, the V-block is put onto the measured cylinder, and the two V-shaped planes contact with the cylinder when the cylinder rotates, which make the measurement of cylindrical profile immune to the radial error motions of the spindle. Three probes target each cross-section of the cylinder to collect the data. From the outputs of the probes, the least squares circle center vector of each cross-section is determined via an error separation model built in accordance with the respective relationships of the cross-sectional profiles of the cylinder and error motions of the V-block. Theoretical analysis and numerical validations proved that this technique can accurately detect the median line profile of the cylinder, and the anti-interference capability can be moderately improved by the modified error separation models based on two measuring strategies. This technique realized theoretically full harmonic error separation with the minimum number of probes, accurately detected the spatial median line of the cylinder. Further, high-precision reconstruction of the cylindrical profile was achieved. Integrating the V-block with a laser measurement system can yield a portable system for in-situ measurement cylindricity. It is foreseeable that this technique will garner more attention for future applications.

Peng-Hao Hu

Papers

A Parallel Error Separation Method for the On-line Measurement and Reconstruction of Cylindrical Profiles

High-precision and low-cost vibration generator for low-frequency calibration system

Measurement method of rotation angle and clearance in intelligent spherical hinge

A high-precision two-dimensional micro-accelerometer for low-frequency and micro-vibrations

A V-block three-probe error separation technique for portable measurement of cylindricity