Study on Framework of STEP-NC Controller with On-Machine Inspection

Junzhe Tan, Chengrui Zhang, Riliang Liu, Xihui Liang

School of Mechanical Engineering, Key Laboratory of High Efficiency and Clean Mechanical Manufacture

(SDU) Shandong University, Ji’nan 250061, P.R. China

e-mail: tanjunzhe1972@gmail.com

Junzhe Tan School of Engineering, Ocean University of China Ocean University of China, Qingdao 266100, P.R.

China e-mail: tanjunzhe1972@gmail.com

Abstract—On-Machine Inspection (OMI) is an effective way in NC machining processes to improve the quality of machined parts and reduce assistant machining time. In this paper, STEP-NC （ ISO 14649 ） data model is introduced to the integration of NC machining and On-machine Inspection in order for Closed-loop Machining. First a STEP-NC Controller with OMI to realize integrated machining and inspection of parts is proposed, along with its architecture, functional modules and working procedure. Then the implementation method of such a system on Java platform is presented, and preliminary modules are developed including the interpreter, machining simulation and probing simulation. A case study of an example part is also provided focusing on the integration of the machining and the probing processes, which demonstrates the feasibility of the proposed STEP-NC Controller with OMI.

Keywords- OMI (On-machine Inspection), STEP-NC, CNC, CLM (Closed-loop Machining), touch probing

I. INTRODUCTION With the development of Advanced Manufacturing

Technology (AMT), On-machine Inspection (OMI), or In-process Measurement, etc. has been studied globally in order to obtain low cost, high efficiency and high quality machining. In-process measurement is the measurement of workpiece size, whilst the component is located on the machine tool and preferably whilst the machining process is in progress [1]. As OMI technology is integrated into NC machining, the inspection results can be fed back into CNC controller, the motion parameters can be changed according to the error to ensure the quality of product.

On conventional NC machine tools, however, the data model of OMI is based on G-code (ISO 6983). The inspection operation cannot be fully integrated with the manufacturing processes and the inspection results cannot be fed back to the CNC controllers directly. Post processors are necessary to update NC program according to the inspection results [2].

How to integrate OMI process into the NC machining process to realize intelligent control has become a novel

research topic. This integration was accepted world wide in the middle of the 1980’s when the STandard for the Exchange and sharing of Product (STEP) model data was launched under the approval of ISO 10303[3].

STEP-NC is the extension of STEP to the digital manufacturing fields. It is a new data interface based on STEP between CAD/CAM and CNC. In 2003, the draft International Standard ISO 14649 was developed in Europe [4]. The STEP-NC data model provides a higher level of information in the manufacturing process including the geometric information, enables a bi-directional information flow.

STEP-NC data model provides the possibility for OMI being integrated into NC machining when the draft of ISO 14649 part 16 is developed. The data model of ISO 14649 part 16 is built upon the basic STEP-NC process model ISO 14649 Part 10 and ISO 10303[5]. ISO 14649 part 16 focuses on touch probing operations, which can either be executed with a touch probe on a CNC machine tool or on a CMM (Coordinate Measuring Machine). Currently, ISO14649 part 16 is mainly used on CMM [2].

II. BACKGROUND AND DEVELOPMENT OF STEP-NC COMPLIANT OMI TECHNOLOGY

OMI is the automatic inspecting process on CNC machine tools when the part is still on it. Because the mechanical structure of milling machine and its motion control principle is similar to that of CMM, inspecting process on CNC milling machine tools is similar to the measuring process on CMM. Therefore, research on automatic measurement on CMM has provided plenty of methods for the study on OMI technology.

A. Feature-based automatic measurement based on CMM CMM has been developed quickly due to the feature-

based automatic inspection issues were studied on. These issues include automatic inspection process planning, CAD-directed optimization of inspection planning, intelligent inspection system, etc. Related research on feature-based measurement began from 1990s. Gyula Hennann et al. described an off-line programming system for CMM according to the geometric model produced by a CAD system [6]. S.G. Zhang et al. proposed a feature-based

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DOI 10.1109/AICI.2009.396

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inspection process planning system according to CAD model for CMM [7]. F.S.Y. Wong et al. proposed algorithmic methods for characterizing and extracting inspection features [8].

With the development of the standard ISO 10303, STEP has been used in various industry domains, STEP compliant inspection, which is feature-oriented, has become one of the utilization of STEP standard.

B. Closed-loop Machining integrated measurement process and OMI Closed-loop Machining (CLM), which is the integration

of CMM and NC machining process, is the basic technology of OMI. However, most of the closed-loop machining integrated measurement process is based on CMM. The inspection process was carried out on CMM instead of on NC machine tools. Therefore, the workpiece has to be moved from the machine tools to CMM when measurement was necessary and the reposition of workpiece was needed after the inspection process.

Currently, lots of the manufacturers carried out measurement of parts by using tactile or optical probes on NC machine tools. And many research groups in advanced countries are studying on OMI on CNC machine tools. Tyler A et al. proposed a Direct Machining and Control (DMAC) system which can conduct in-process dimensional inspection of parts [9]. Yongjin Kwon et al. investigated the closed-loop measurement error in CNC milling by using spindle probe measurement [10]. OMI on CNC machine tools has become a key method to improve the quality of products and save lead-time.

However, the data model of OMI mentioned above is still based on G-code (ISO 6983), and the inspecting process was controlled by G-code. Because NC machining and inspecting process based on G-code do not support bi-directional data exchange in CNC system, the inspection results cannot be fed back into CNC controller directly, and seamless connection between NC machining and inspecting process cannot be achieved. New data model standard that supports bi-directional data flow is needed to realize the integration of NC machining and inspecting process.

C. STEP-NC compliant On-Machine Inspection Currently, much research work has been done on Closed-

Loop Machining system based on STEP-NC in developed countries all over the world. In 2004, Brecher, et al. in the Laboratory for Machine Tools and Production Engineering (WZL) at Aachen University, Germany proposed the first prototype of a closed process chain, which integrated inspection into the STEP-NC information flow. They proposed a feature-oriented STEP-NC compliant program interface which supported bi-directional data exchange between CAM and CNC [11]. Figure 1 shows a prototype implementation for on-line inspection according to ISO 14649 part 16.

Figure 1. Prototype implementation for inspection at Aachen

University, Germany [11] In 2005, Newman et al. integrated STEP-NC (ISO14649)

and AP219 and developed an inspection framework for closing the inspection loop through integration of information across the CAX process chain [12]. In 2006, Xun W Xu, et al. proposed a STEP-NC data model for on-line inspections by using ISO 14649 part 16 [2]. In 2007, High-level Measurement Process Planning activity model was presented in the conference of STEP-NC Machining and Measurement in Ibusuki, Japan. High-level Measurement Process Planning (HIPP) was proposed to integrate into AP238 and the demonstration of STEP-NC machining and measurement was also held during the conference [13].

Applying STEP-NC data model to NC machining assures bi-directional machining process information flow and forms a closed-loop machining chain so that automatic, seamless data feedback of the measurement results into the product model can be achieved. The results of inspection can be used for adjustment of NC machining for quality assurance.

III. FRAMEWORK DESIGN OF THE STEP-NC CONTROLLER WITH OMI FUNCTION

In this section, the framework of a STEP-NC Controller with OMI according to the ISO 14649 is presented. This STEP-NC controller, which is designed based on Ethernet Field Bus, can execute contact dimensional measurement for the machined parts by using touch trigger stylus on CNC milling machine tools. Figure 2 and figure 3 shows the architecture of STEP-NC controller with OMI by using Ethernet Field Bus and the information flow among STEP-NC controller and its peripheral device.

A. Framework of STEP-NC Controller with OMI Main framework of this STEP-NC controller with OMI

consists of interpreter, database, machining and inspecting controller, etc. In this STEP-NC Controller with OMI, both the machining and inspecting process are implemented through the same servo control system. The software structure of STEP-NC Controller with OMI based on Ethernet Field Bus is also shown in Figure 2.

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As Ethernet Field Bus was applied in this STEP-NC controller, servo control driver, inspecting I/O card can be connected through Ethernet Field Bus and other devices can be added easily if necessary. All the servo drive controlling information and analog or digital data are transferred between STEP-NC Controller and milling machine tools through Ethernet Field Bus.

Figure 2. Architecture of STEP-NC Controller with OMI by using

Ethernet Field Bus

B. Software design of each function module The software of the STEP-NC Controller with OMI is

modularized designed, that is, the STEP-NC Controller with OMI includes different modules to realize different functions. Function modules include interpreter module, machining process plan module, probing process plan module, inspecting results analysis module, motion control module and communication module, etc.

(1) The main function of interpreter module can extract project information from the STEP-NC files. STEP-NC data model provides more information than G and M codes, linked with geometric parameters and technological information. Extracting information from STEP-NC (include the machining and inspecting process, geometric parameters of machining features, workingsteps, workpiece, operation. etc.) is the first step for STEP-NC Controller.

(2) Machining process plan module and probing process plan module have the similar function, which is to generate machining process plan and probing process according to the machining and inspecting workingstep of each feature respectively.

Figure 3. STEP-NC Controller with OMI and milling machine tools

In ISO 14649, the specification of tool path and probing

path is “optional”. The optimized tool path or probing path can be generated in this module, according to the geometric information of machining features and tools, operations, workingstep, machining strategies, etc.

(3) The main function of Motion Control Module is to execute interpolation process, which is to calculate the increment feed value of each axis of machine tool in each interpolation period. Other control parameters such as spindle speed, digital value for I/O, etc. are also generated in this module.

(4) Communication module can transfer motion control data from Motion Control Module and Inspecting Module to Ethernet Card, which is the bridge of STEP-NC Controller and its peripheral device (include servo driver, I/O Card and other devices.

(5) Inspecting Module can collect the probing results and transfer them to the Inspecting Result Analysis Module, which analysis the machining error based on the measurement data.

C. Working procedure of STEP-NC controller with OMI When the STEP-NC files of parts are input into the

STEP-NC Controller, interpreter module then extract the project information (include geometric size of features, machining operation, etc.). Machining process plan module plans the machining working step and generates tool paths according to ISO 14649 part 10, 11and 12. Then motion control module transfer the motion control data through Ethernet Field Bus to drive the servo motors to carry out the machining process.

Servo Motor X

CNC Milling Machine ToolTouch

Trigger Stylus

Servo Driver X

I/O Card

Servo Motor Y

Servo Driver Y

Servo Driver Z

Servo Motor Z

Servo Driver

Spindel Motor

Other Control Cards

Other Devices… …

STEP-NC Controller with OMI

Ethernet Card

Interpreter Module

Inspecting Module

Motion Control Module

Probing Process

Plan Module

Inspecting Result

Analysis Module

Machining Process

Plan Module

STEP-NC File

STEP-NC Controller with OMI

Ethernet Card

Communication Module

ISO 14649 part 16

ISO 14649 part 10, 11,12

Knowledge Libary

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During the automatic probing process, the touch trigger stylus moves like a milling cutter on spindle along the probing paths generated according to ISO 14649 part 16. Once the touch trigger stylus approach a feature of the part, it is triggered, STEP-NC controller then collect current position of the stylus. After all the required data of a feature are collected, the inspecting results analysis module calculates the measurement result automatically. Then the measurement results can be fed back into the STEP-NC controller, and the machining process is regenerated according to the deviation that the measurement results compared with the original design dimension of the parts.

IV. IMPLEMENTATION OF STEP-NC CONTROLLER WITH OMI

The software of STEP-NC controller with OMI is developed on Java platform according to ISO 14649. The Java class library is designed based on the STEP-NC data model which is defined through EXPRESS language.

In this section, development method of the STEP-NC Controller with OMI is introduced according to the part as shown in figure 4. The machining features of this part are consists of a planar, a hole and a pocket. The following set of codes show part of the program about rough machining Workingstep of the pocket [4]:

#1= PROJECT('EXECUTE EXAMPLE1', #2, (#4), $, $, $); #2= WORKPLAN('MAIN WORKPLAN', (#10, #11, #12, #13,

#14 ), $, #8, $); ... … … … #13= MACHINING_WORKINGSTEP ('WS ROUGH

POCKET1', #62, #18, #22, $); ... … … … #18= CLOSED_POCKET ('POCKET1', #4, (#22, #23), #84,

#65, (), $, #27, #35, #37, #28); ... … … …

#22= BOTTOM_AND_SIDE_ROUGH_MILLING ($, $, 'ROUGH POCKET1', 15.000, $, #39, #50, #41, $, $, $, #51, 2.500, 5.000, 1.000, 0.500);

... … … …

A. Interpreter module design The function of interpreter module is to extract project

information from the STEP-NC program files. STEP-NC program presents a detailed description of machining project, which consists of amounts of product information and manufacturing information needed to be transferred from CAM system to CNC system. Special method or algorithm is needed to extract such information from STEP-NC program. By using recursive algorithm, Interpreter Module is developed to extract STEP-NC project information from STEP-NC files.

Figure 4. Dimension of the machined part [4]

B. STEP-NC project and features show STEP-NC project information can be shown as an

information tree, where “project” is the root of the tree, all STEP-NC information can be shown on the branch, sub-branch or leaves. And the geometric shapes which present the machining features are also shown in 3D format. Figure 5 shows the information tree of the STEP-NC file.

Figure 5. Project tree view of STEP-NC program

C. Machining and probing simulation Machining simulation and probing simulation interface is

designed to demonstrate and verify the tool path and the probing path which is automatic generated by machining and probing process plan module. By using tool path and probing path generating algorithm, machining process and probing process plan module were designed to produce optimized tool path or probing path according to the project information in STEP-NC files. Figure 6 and figure 7 show the rough machining and probing simulation procedure of the pocket feature respectively.

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Figure 6. Rough machining simulation of the pocket

Figure 7. Probing simulation of the pocket

V. CONCLUSION AND FUTURE RESEARCH WORK On-machine Inspection is an important method to find

the machining problem and correct them as early as possible. OMI has become one of the key technologies for quality assurance in manufacturing. By virtue of OMI technology, high product quality and minimum scrap production can be obtained.

ISO14649 Part 16 provides OMI data model, which can be integrated with the NC machining process, to form closed-loop machining process. By using the ISO14649 part 16 data model, STEP-NC compliant OMI technology is studied to realize close-loop NC machining.

The STEP-NC Controller with OMI mentioned in this paper has successfully extracted the machining and probing information from STEP-NC program file, tool path and probing path are also produced automatically according to ISO 14649 part 11 and 16, and the machining and probing simulation interface are also designed to verify the feasibility of the tool path and the probing path generated. However, great effort is still needed to develop machining module and probing module to realize real time machining and probing on milling machine tool in the following research work.

ACKNOWLEDGMENT The study was financial supported by the National

Natural Science Foundation of China (No. 50675123) and Natural Science Foundation of Shandong Province (No. Y2007F47), the R&D Special Fund for Public Welfare Industry (Quality Inspection, No. 2007GYB148-1). Dr. Xu of the Department of mechanical engineering in Aucland University of N.Z. and Professor S.T. Newman of University of Bath, UK gave me many proposals. Thanks to all the authors I’ve referenced, too.

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