Diagnostics for machine tool monitoring
The control law in the synthesis of machine tool adaptive control systems (adaptive CNC systems) is determined by observing changes in the properties of the controlled process (milling or turning). These changes, if appropriately taken into account with a designed safety margin, enable a robust control algorithm to be created. The control system used for machine tool feed axes drives (as well as other subsystems) needs to be robust for machining quality and for safety.
To achieve a constant high machining quality, the control systems should exhibit insensitivity to:
- Variations in environmental conditions (vibrations, changes of temperature)
- Variations in machining parameters (rate of travel composed of rates for individual axes, accelerations, travel profiles, etc.)
- Nonlinearities of selected machine modules (friction, backlash)
- Geometrical errors of the machine.
When designing a machine tool control system specifications include: machine kinematics (horizontal or vertical axes, dimensions of the machine), types of motors applied to feed drives (rotary or linear) and machining parameters, hence, motion parameters for individual axes axes (typical machining, high-speed machining, high-precision machining).
Specifications for control platform
The above-mentioned specifications determine requirements for hardware and software components of the control platform. These are:
- The way the position/velocity is to be measured, as well as the measurement resolution and the desired accuracy
- Communication protocols between the CNC processing unit and drive modules (for ‘typical’ machines it can be the CAN network, the MODBUS network, the nondeterministic Ethernet TCP/IP protocol to exchange data with the company office layer, or in the case of high-precision multi-axial applications it can be only the real-time industrial Ethernet: EtherCAT, Ethernet Powerlink, SERCOS, EtherNet/IP, Profinet or MODBUS-IDA),
- Software architecture for sampling times, real-time operation including the way computations are to be allocated between many processor cores or processors themselves operating sometimes in the network, degree of advancement of control algorithms to be used in view of the designed precision level, the degree the machine tool operation monitoring and active supporting are to be integrated, hardware/software fusion concerning information from additional sensors.
The new approach within the open architecture CNC systems is the integration of the condition monitoring sub functionalities within the system. The main idea is to shorten the information roads (as well as to introduce the sensor fusion) between the CNC components to enable new possibilities for constant high quality assurance during machining.
Figure above shows future open architecture CNC system modules. Typically motion control subsystem communicates with graphical user interface as well as with additional diagnostic modules through the tasks within the CNC RTOS (Real-Time Operating System) kernel (1). With the presented approach integration between the condition monitoring system is made on the basis of direct throughput of diagnostic modules computations and the motion control subsystem (2). This approach is the main idea of the Polish Ministry of Science and Higher Education grant no. N N502 336936 (acronym M.A.R.I.N.E. – multivariable hybrid ModulAR motIon coNtrollEr), conducted in close cooperation with one of the Polish machine tool builders.
Prototyping and testing of new hardware-software condition monitoring modules and possibilities of its integration within the open architecture CNC system is one of the goals of the research team of Centre of Mechatronics WPU of Technology, Szczecin. At now it is possible while the equipment of team has extended with new high-end system, called SAGITTARIUS – ICM. Its software-hardware architecture is shown in figure below.
The system presented here is financed by the Polish Ministry of Science and Higher Education grant. The resulting system is an ancronym: SAGITTARIUS – ICM (univerSAl inteGrated open architecture diagnostIc sysTem for research on machine Tool dynAmics and micRo-electromechanIcal systems in USage for development of Integrated Condition Monitoring modules for CNCs). The effort enables the research group from Centre of Mechatronics at West Pomeranian University of Technology to tighten already close cooperation with machine tool builders in Poland and other countries. It will formulate the fundaments for future development of integrated condition monitoring modules for CNCs. The team will propose a family of simple condition monitoring modules and set of rules for its integration for the interested companies within next five years.
Krzysztof Pietrusewicz, PhD is with the West Pomeranian University of Technology, Szczecin, Faculty of Electrical Engineering. He is also editor for Control Engineering Poland. His research work is connected with robust open architecture control and integrated condition monitoring approach for machine tool. This article appeared in the October Control Engineering North American edition’s Control Engineering International page.
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