Research On Digital Twin Self-awaring Technology Of NC Machine Tools

With the breakthrough of cloud computing,big data,artificial intelligence,5G,and other technologies,the fourth industrial revolution has come to the Manufacturing.In this industrial revolution,intelligent manufacturing is both the main innovation direction and the industrial commanding height.The core of intelligent manufacturing is the intelligence of equipment.The key to realizing intelligent equipment is the Cyber-Physical System(CPS).The core of CPS is the Digital Twin which is the support part of CPS system data.This paper comes from the national intelligent manufacturing special project "CNC machine tool interconnection communication protocol standard and test verification",and carries out numerical control machine digital twin,the lightweight of NC machine tools instruction domain big data,NC machine tools interconnection communication specification based on instruction domain big data,and other critical technologies research.The main research contents are as follows.Aiming at the complex structure and timeliness of NC machine tools,a digital twin model of NC machine tools based on the fusion of topology and time series was proposed.A unified model was established in the form of equipment,components and data hierarchies.The model was embedded in the NC system.Digital twin can identify the equipment automatically and store in time series according to their models.And a digital twin system containing the topology and time-series information of the kinematic chain is realized,which makes the digital twin modeling of CNC machine tools standardized and automated.Aiming at the problem of massive data digital twin of NC machine tools and limited network bandwidth resources,a method for synchronizing big data of NC machine tools based on heat preference and equal network flow is proposed.The internal data points of NC machine tools are up to hundreds of thousands,and the data value and data activity are different.By setting sampling channels,the most time-ordered big data of the instruction domain is isolated from other data and collected preferentially.And through online learning methods,the other data is identified as "hot data" with high activity and "cold data" with low activity.The synchronization priority is arranged according to the "data heat".This method can effectively control the occupation of bandwidth resources by data collection and realize the optimizing of data collection.Aiming at a large amount of data of NC machine tools,the complexity and time-consuming of existing compression algorithms,an instruction domain big data compression algorithm based on discrete cosine transform and embedded wavelet zero tree(EZW)was proposed on the basis of the sparse nature of the big data in the instruction domain.As the machine state,program Number,subprogram Number,cutter Number,and G code line Number have good sparsity in the time domain,the stroke compression encoding method can be used to design the expression structure for it.As the load power and load current have characteristic of "frequency domain generation,Time domain acquisition",the digital acquisition signal has good sparsity after discrete cosine transform,and the EZW encoding has good compression effect after discrete cosine transform on those signals.And the command position and actual position need to be lossless during high-precision analysis.It is required to improve the EZW algorithm,and an integer embedded wavelet zero tree lossless compression algorithm(EZW)is proposed,which improves the overall compression ratio and meets the demand for data lightweight.Aiming at the problem that the existing NC machine tool interconnection communication protocol cannot meet the needs of digital twin modeling,a specification of NC machine tool interconnection communication based on big data in the instruction domain is proposed.The three-level data interworking architecture of "Adapter?Agent?Application System" is used to achieve the compatibility of multi-source heterogeneous CNC systems;the equipment model of the level data structure which calls "device ?component?data" is designed to provide unified construction of CNC machine tools model method.This paper provides a digital dictionary that comprehensively describes CNC machine tools in a standardized way,builds a data interaction model in a subscription /publishing manner,supports high-frequency instruction domain big data collection.And as for the device access security and data use security,this paper also designed the internet communication security model.Compared with the existing CNC machine tool communication protocols such as OPC UA,MTConnect,and umati in terms of functions and performance,the protocol specifications designed in this paper have technical advantages such as simple model,large information capacity,and high efficiency.It can meet the needs of high-frequency real-time big data collection in the command domain ofNC machine tools,and has formed the China's NC machine tool interconnection communication protocol specifications,filling the domestic gap.Based on the above research,the theory and method of this paper are experimentally verified by using the big data platform of nc machine tool as the verification environment.The digital twin automatically fuse the kinematic chain topology and data timing to achieve "plug and play".Under the 2M bandwidth limitation,the synchronization of 32,000 data points per second is realized,among which the command domain big data precision reaches the millisecond level.The average compression ratio of big data in the instruction domain reaches 30%,and the algorithm time is less than 30% of the big data collection time in the instruction domain,which meets the requirements of digital twin of NC machine tools.The computer communication specifications for NC machine tools have been experimentally verified in terms of computing resource consumption and data collection performance.Compared with the function and performance of MTConnect protocol,the results show that the communication specifications of the NC machine tool designed in this paper are better than MTConnect comprehensively,which occupy less resources on the CNC system and have excellent performance.At present,the research results of this paper have become the group standard NC-Link of the China Machine Tool Industry Association.In 2018,it was established as a national standard.