Development of a milling testbed platform with open architecture controller.

Staroveski, Tomislav ; Brezak, Danko ; Udiljak, Toma 等

1. INTRODUCTION

In the last two decades, a lot of efforts have been made in the development of open control systems for machine tools. According to the IEEE, "an open system provides capabilities that enabled properly implemented applications to run on a variety of platforms from multiple vendors, interoperate with other system applications and present a consistent style of interaction with the user" (Pritschow et al., 2001).

The first Open Architecture Controller (OAC) solution was proposed by National Institute of Standards and Technology (NIST). This project has evolved over time, and is currently focused on development of OAC system named as Enhanced Motion Controller, EMC (Proctor & Michaloski, 1993). After this first initiative, several other projects have started in the Europe, USA and Japan, among which the most important are: OSACA (Open System Architecture for Controls within Automation System), OMAC (Open Modular Architecture Controllers), OSEC (Open System Environment for Controller) and JOP (Japanese Open Promotion Group). Beside aforementioned, other university research activities in hardware and software area of open architecture CNC systems were also conducted and resulted in systems such as Open Real-Time Operating System (Erol et al., 2000) or Soft-CNC system based on OSACA principle. However, despite all of these efforts, a universal open CNC architecture still remains undefined.

In this paper, a concise description of activities regarding the development of 3-axis bench-top mini milling machine controlled by OAC is presented. This project was motivated by the necessity for open controlled machine tool testbed platform, intended to be used in the analysis of different control algorithms and process monitoring techniques, as well as educational purposes. Among several proposed OAC solutions, EMC was implemented. It runs on Linux based operating systems with real-time extensions. Characteristics of such software platforms, which primarily include stability and performance, were main reasons for choosing this type of OAC. Up to this time, EMC has been successfully used in a several CNC retrofitting projects, including applications with complex kinematic chains (wiki.linuxcnc.org).

2. ENHANCED MACHINE CONTROLLER

First version of EMC was originally developed by the Intelligent Systems Division at the NIST (Proctor & Shackleford, 2001). Current EMC version, EMC2, (The EMC Team, 2009) is actively developed and community maintained software package, presenting an effort to simplify, organize and continuously extent the original work.

EMC is composed from four components: Motion Controller (EMCMOT), Discrete I/O Controller (EMCIO), Task coorinating module (EMCTASK), and Text-based and graphical user interfaces (GUI).

In the four modules of EMC, only EMCMOT is a realtime module. The communications between non-realtime modules are implemented by Neutral Messaging Language (NML) channels (Proctor et al., 2000), and the communications between the realtime module (EMCMOT) and the non-realtime module (EMCTASK) is implemented etither by shared memory or RT-Linux FIFO mechanisms.

EMCMOT executes cyclically in real-time and performs trajectory planning, direct and inverse kinematic calculations and computation of desired output to motor control subsystems. This process includes sampling of controlled axis positions, computation of next trajectory point and interpolation between these trajectory points. Interpolation is done by means of cubic interpolation routines, and a trapezoidal velocity profile generator is used during computation of desired position references. Programmable software limits are also supported, as well as interfaces to hardware limit and home switches.

All I/O functions, which are not directly related to the actual motion of machine axis, are handled within EMCIO module. EMCIO is implemented as a single I/O controller, consisting of hierarchy of subordinate controllers for main spindle, automatic tool change (ATC), coolant, auxiliary functions (e.g., E-STOP chain, lubrication, etc.) and other user-defined subsystems. It is based on a hierarchy controller classes written in C++ using NML Library.

EMCTASK is task level command handler and program interpreter for the RS-274 NGC machine tool programming language (Marietta, 1994), commonly referred as G code. As coordinating module in the architecture of EMC, EMCTASK is hierarchically placed above EMCMOT and EMCIO, and under GUI. EMCTASK monitors the status of subordinate modules (EMCMOT and EMCIO) and coordinates them. It also receives and analyzes the commands, either from the operator through GUI or from another process (locally or remotely in both cases), interprets them into NML messages and dispatches them to EMCMOT, EMCIO or EMCTASK itself at appropriate times. EMCTASK is coded similarly to the EMCIO using the NML Library.

Several user interfaces have been developed for EMC: keystick, xemc, tkemc, mini and AXIS. All of these programs natively run under Linux based operating systems and all run in X11 environment (X Window System), with exception of keystick, which is character-based. AXIS is the most advanced GUI, featuring interactive G-code previewer. GUI-based programs can be expanded and adopted to match specific application needs by means of virtual control panels (VCP), which is supported with pyVCP package. Besides above mentioned user interfaces, telnet based program emcrsh is also provided for running remote sessions.

3. TESTBED SETUP

At present, 3-axis bench-top mini milling machine is used as testbed. Although this machine is based on very simple Cartesian kinematics structure, it is still nevertheless sufficient for testing most of EMC concepts and overall system stability.

Configuration of feed drives, which applies to all axes, is based on closed-loop servo system, by means of Permanent Magnet Synchronous Motors (PMSM) with integrated incremental encoders (type SB04A, produced by Mecapion Co.), corresponding motor controllers (type DPCANIE 030A400 for X and Y axis, and DPCANIE-060A400 for Z axis, produced by Advanced Motion Controls Co.) and ball screw assemblies.

Selected drives provide multiple modes of operation, variety of common industrial interfaces for acquisition of reference signals, CAN-bus interface with CANOpen protocol for fieldbus connections, as well as multiple general purpose analog and digital I/O ports. Selection of particular types (DPCANIE-030A400, DPCANIE-060A400) is a temporary solution and only serves for initial testing, as power ratings greatly exceed application needs.

In the current setup, feed drives are configured as closed loop velocity controllers, since position loop for each axis is implemented within EMCMOT module. PID gain factors for both current and velocity loops are obtained manually, using royalty-free DriveWare configuration software package.

Interface to drives from PC side is done via Motenc-Lite PCI card (Vital Systems Inc.). With all breakout boards installed, single Motenc-Lite card features 8 Analog Outputs, 8 Analog Inputs, 4 Differential Quadrature Encoder Inputs (DQEI), as well as 16 digital outputs (24VDC) and 32 digital inputs (24VDC). DQEI interface is mapped to internal 32-bit up/down counter with maximum update frequency of 2MHz. Multiple installations (up to four) of this card is possible in order to further expand configuration.

Incremental encoders, which are powered from motor drives and utilize differential quadrature TTL output signals, are used as feedback devices for both drives and EMC. Signals from encoder are first fed to the motor drive as velocity loop feedback, buffered, and then sent to EMC trough DQEI channels of Motenc-Lite interface card. Emulated Hall Effect sensor signals are also provided from encoders, which are used by the drives for commutation.

Testbed in its current configuration is shown on Fig 1. and finished parts of some test runs on Fig. 2.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

4. CONCLUSION

Development of a 3-axis mini milling machine testbed with a Linux based OAC system named as Enhanced Machine Controller has been presented in this paper. Due to stability and robustness in general, Linux based operating systems have already proved their potential in many fields, including industrial automation. Combined with real time extensions, they also provide possibilities for mission critical implementations. EMC is of great academic and educational importance, as it fulfills essential research demands for this field with benefits of robust open source OS, including expansion possibilities with other open source programs.

Testbed configuration presented in this paper is the base on which several ongoing projects will continue. Upgrade of the current testbed to 4-axis milling machine, and retrofitting of large planning machine into the 3-axis CNC machining center in cooperation with HSTec Co.

Future research will be focused on using EMC as a platform for integration and analyzes of various process monitoring and control algorithms on both developing testbeds.

5. REFERENCES

Erol, N.A.; Altintas, Y. & Ito, M.R. (2000). Open system architecture modular tool kit for motion and machining process control, IEEE/ASME Transactions on Mechatronics, Vol. 5, pp. 281-291, ISSN 1083-4435

Marietta, M. (1994). Next generation controller (NGC) specifications for an open system architecture (SOSAS) revision 2.0.--Technical report, National Center for Manufacturing Sciences, Available from: ftp://ftp.isd.mel.nist.gov/pub/NGC_document.pdf, Accesed on: 2009-06-06

Pritschow, G. et al. (2001). Open Controller Architecture--Past, Present and Future, CIRP Annals--Manufacturing Technology, Vol. 50(2), pp. 463-470, ISSN 0007-8506

Proctor, F.M. & Michaloski, J. (1993). Enhanced Machine Controller Architecture Overview, Available from: ftp://ftp.isd.mel.nist.gov/pub/NISTIR_5331.pdf, Accesed on: 2009-06-06

Proctor, F.M. & Shackleford, W.P. (2001). Use of open source distribution for a machine tool controller, Proceedings of the Conference on Sensors and Controls for Intelligent Manufacturing, Peter E. O. & George K. K. (Ed.), pp. 19-30, ISBN 0-8194-3856-1, Boston, MA, Society of SPIE, Bellingham, WA

Proctor, F.M.; Shackleford, W.P. & Michaloski J.L. (2000). The Neutral Message Language: A Model and Method for Message Passing in Heterogeneous Environments, Available from: www.isd.mel.nist.gov/documents/ shackleford/Neutral_Message_Language.pdf, Accesed on: 2009-06-06

The EMC Team (2009). EMC Documentation, Available from: http://www.linuxcnc.org/docs/EMC2_User_Manual.pdf, Accessed on: 2009-06-06