Research On Functional Safety And Related Key Techniques For Computer Numerical Control Systems

The high-end Computer Numerical Control (CNC) System is the enabling technology and basic equipment of developing new and hi-tech industry and advanced industrial science and technology. To meet the high-end CNC demands of safety, which is proposed by the performance of high velocity, high accuracy, composition, networking development trends and multiple channels, this dissertation focuses on the research of the functional safety of CNC system and related key technologies.Functional safety is part of the safety of the machine and the machine control system which depends on the correct functioning of the safety-related electrical control system, other technology safety-related systems and external risk reduction facilities. In accordance with related international standards, the implementation of functional safety includes two procedures, the first one is the risk assessment procedure, and the other one is the risk reduction procedure. Therefore, this dissertation focuses on solving related risk assessment method, safety communication method and safe supervisory control method.The main contributions of the dissertation are described as following:1. We present a hierarchical risk assessment method for CNC system.To identify risk factors and estimate the safety level of CNC system effectively, a hierarchical risk assessment method was presented. First of all, a hierarchical model of CNC system was proposed to disintegrate and simplify the complex CNC system, and an assessment sequences set matrix was deeply researched with the corresponding fuzzy mapping algorithm to mix the expert experience together and to quantize the expert experience. Secondly, a layered diffusion method of factor weights was proposed to locate key risk factors, and a horizontal fragmentation method was adopted to calculate the risk degree of CNC system. Finally, case studies were adopted to compare the performance between the digraph and matrix method and the new mechanism. The results proved that the new mechanism could decrease the similarity performance, reduce the execution time, and be more suitable for the application of risk assessment process in CNC systems. 2. We present a safety communication method based on CNC Dual Ring Fieldbus.To meet the requirements of safety signals in transmission security and real-time performance of CNC system, a message safety communication method which adopting ring coding based on CNC Dual Ring Fieldbus was presented. Corresponding to the proposed method for safety communication, one ring field bus is used to transmit original messages, and the other one is used to transmit selfreparing messages which can be caculated to achieve original messages. The structure of CNC Dual Ring Fieldbus safety message was proposed, and safety communication mechanism based on Dual Ring and related algorithms were deeply researched. And then mathematic method and experiment tests were adopted to compare the performance between the new and traditional mechanisms. The results proved that the new mechanism could enhance the redundancy performance, reduce the retransmission probability, meet the real-time performance, and be more suitable for the application of message safety-critical communication in CNC Fieldbus.3. We present a safe supervisory control method under observability failure for Discrete Event System (DES).CNC system is a typical DES, so the robustness of safe control function of CNC system can be enhanced by the research of safe supervisory control methods based on the DES. Firstly, we summarized and described traditional modeling methods in detail for DES. Moreover, to solve the problem of safe supervisory control under possible loss of observability and repair of observability failure for DES, a step-fusion algorithm was proposed. Based on the step-fusion algorithm, a new safe supervisory control model was presented. And then mathematic method and case study were adopted to compare the performance between the new and traditional mechanisms. The results proved that the new mechanism could obtain a safe controller that avoids disaster states, and be more permissive than the traditional supervisor.