Zi Chen

Prediction of Cutting Chatter Based on Hidden Markov Model

Self-chatter is a serious problem in cutting process. This paper aims to solve the problem by establishing time series model of vibration acceleration signal in cutting process based on Hidden Markov Model (HMM) technology and achieve the purpose of chatter recognition and prediction. Features which can indicate cutting state are extracted from the acceleration signal. HMM parameters are obtained by model training, and the reference models database is built. Then cutting state recognition is performed according to the feature matching level. Simulations and experiments are conducted, and the results show that the proposed method is feasible and it could get high recognition

Automatic Visual Inspection for Leather Manufacture

The visual inspection system was developed for defects detection on leather surfaces, which is an important component of automatic CAD/CAM cutting systems. The main functions of the system are quality control and raw material cutting. An efficient algorithm, which combines multiresolution approach, energy and entropy matrices, is presented for detection of defects embedded in leather surface images. A wavelet band selection procedure was developed to automatically determine the number of resolution levels and decompose subimages for the best discrimination of defects and removals of repetitive texture patterns in the image. An adaptive binary thresholding is then used to separate the defective regions from the uniform gray-level background in the restored image. The proposed methodology is able to efficiently detect several types of defects that current approaches cannot detect, and is fast enough to be used for real-time leather inspection.

A Planar Capacitive Sensor for Large Scale Measurement

A planar capacitive sensor (PCS) is proposed for X and Y directions measurement. One axial displacement can be measured without the coupling of another axial movement. Two groups electrodes with 90 degree out-of-phase, forming triangle sine and triangle cosine wave, can be applied for direction detection and interpolation, aiming at high resolution. Simulation results show that proposed PCS is available for planar measurement.

A Horizontal Vibration Wave Model for High Speed Elevators

Horizontal vibration has critical influence upon elevators’ ride quality. Based on the wave theory of one-dimensional string vibration, a horizontal vibration wave model is built to simulate the dynamic performance of a high speed elevator system. Through coordinate transformation, Galerkin’s method is used to discretize the governing partial differential equations into a dumped-mass system, which can be solved effectively with classic methods. Numerical results show that the horizontal vibration behaviors are closely interconnected with the location, velocity and acceleration of elevator cab.

Research on the Thermal Property of Machine Tool Slides Based on Cellular Structures

To decrease the thermal error in the machining of high speed machine tools, a new design idea based on the cellular metal structures was proposed. With forced convection in cellular metals, the heat generated in machining will be dissipated and the machining precision can be guaranteed. The thermal property of machine slides using 2D rectangle cellular structures was studied with theoretical analyses and numerical simulation. The theoretical predictions are in good agreement with the finite element simulation results. The overall heat transfer coefficient and the thermal performance were acquired with the volume-averaging method and the relationships between the cellular porosity, cellular sizes and the thermal performance were discussed. The analyzed results show that the maximum thermal performance can be got when the cellular porosity equals about 0.85 and with the increase of cellular sizes, the thermal performance decreases.

An Effective EMD-Based Feature Extraction Method for Boring Chatter Recognition

Chatter often occurs during precision hole boring, it results in low quality of finished surface and even damages the cutting tool. In order to identify chatter rapidly and gain the precious time for chatter suppression, a chatter monitoring system was established and an effective feature extraction method for boring chatter recognition was presented. According to the characteristic of chatter signal, empirical mode decomposition (EMD) was introduced into chatter feature extraction, and its basic theories were investigated. The vibration signal was decomposed by EMD, then the intrinsic mode functions (IMF) was got. Finally, the feature of chatter symptom was extracted by analyzing the energy spectrum of each IMF. The results show that feature extracted from vibration of boring bar by EMD can indicate chatter outbreak symptom, and it can be used as feature vectors for rapidly recognizing chatter.

Wavelet Denoising of the Horizontal Vibration Signal for Identification of the Guide Rail Irregularity in Elevator

Wavelet denoising method was introduced to improve the validity of identifying the guide rail irregularity in elevator. The horizontal acceleration signal measured from a test elevator was processed by wavelet denoising. Then the denoised signal was decomposed up to four levels using Daubechies 4 mother wavelet to identify the guide rail irregularity. The results indicate that the impulses due to the guide rail irregularity are prominent in wavelet decompositions. And the causes resulting in those characteristics of the vibration signal can be well revealed. So discrete wavelet transform (DWT) can be used as an effective tool for denoising the horizontal vibration signal of elevator and diagnosing faults in the guide rail.

Wireless Sensor Failure Identification Technology for Thermal Monitoring System of NC Machine Tools Based on Bayesian Networks

Wireless sensors are increasingly adopted in mechanical systems to acquire and wirelessly transmit sensed information for machine condition monitoring. For wireless sensors on spindle, the reliability of the sensors system at high rotary speed is the key factor to guarantee the validity of monitoring. How to identify sensor failure accurately and timely is essential to enhance reliability of monitoring system. To address this issue, a novel method based on the BNs was presented to distinguish sensor failure from other transmission errors. The method described causal relationships of factors inducing sensor failure by graph theory and deduced sensor failure by Bayesian statistical techniques. Experiments carried on NC machining center prove the validity of this approach.

A Reliability Analysis Method Based on Execution Scenario for Modular NC Software

To describe the modular structure of numerical control (NC) system and deal with the problem of interference between modules in software reliability analysis, a module dependency graph (MDG) was presented and a scenario-based scheme was proposed. MDG describes reliability of module in different application environment, module dependency and structure of software with establishing maps among modules, execution scenarios and reliability. According to the information shown in MDG, an algorithm based on the idea of traversing directed graph in graph theory was proposed. The algorithm calculates the reliability of scenario with breadth-first principle. The reliability of system could be obtained from reliability and running frequency of scenario. The scheme eliminates the defects of the conventional methods which ignore module correlation. Case study proves the scheme could evaluate the reliability of NC software effectively.

NURBS Interpolator Based on Dynamic Property of the Machine Tool

The proposed Axis-based Look-ahead NURBS Interpolator (ALANI) strictly confines the component acceleration and jerk at each axis during every interpolating period based on the mechanics of the machine tools, so that the acceleration and jerk at axes can not exceed the limit in order to avoid immoderate vibrations and shocks. Contour precision is also strictly guaranteed while interpolating. The interpolator can trace back and recalculate previous data if necessary after forwards calculation and verification. And the recalculation algorithm is able to choose optimized machining parameters to attain high efficiency. The introduction of ALANI and simulation experiment is present in this paper.