Project Title: Dynamic Performance Characterization and Enhancement of High-Speed Machine Tool Drives - Rapid Tracking for High Speed Machining

Principal Investigator: Tsu-Chin Tsao

Background/Description

The advances of tooling materials, such as carbide, ceramic, polycrystalline diamond and cubic boron nitride, has dramatically increased metal removal rate potential. This has made the machine tool performance, in particular dynamic rigidity and motion control accuracy, the limiting factor in the exploitation of higher cutting speeds. To increase the speed and response, there is a trend in using direct-drive ball screw slides and direct-drive linear motors. Recent industrial development of direct-driven machine tools using current CNC and servo control technologies has experienced problems such as inaccurate high-speed continuous path motion, residual vibration motion at high acceleration/deceleration point-to-point motion, vibration due to feed motion and dynamic cutting forces and chatter instability at certain combinations of machine/workpiece configurations, cutting conditions and servo gains. These problems are mainly resulted from the direct dynamic interactions among the machining process, the direct-drive servo control loop dynamics and the machine/tool/workpiece structural dynamics and from the inability of the servo control to handle high bandwidth motion and to account for those dynamic interactions. Current CNC servo control technologies are based upon low to medium bandwidth motion of the decoupled prime drives and hence problems arise when they are used to control the high bandwidth motion of the process coupled direct-drives. Servo control is thus a critical factor that has limited current direct-drive machine tool performance. The purpose of this project is to address these problems so that the accuracy and speed potential of direct-drive machine tools can be further extended for high speed machining applications. The objective of this work is to apply motion control to high speed multi-axis machining operations with both direct and indirect feed drives in order to achieve reduced machining times with increased accuracy. Direct drive actuators can provide the high bandwidth response that is necessary for accurate high speed machining. Three aspects of motion control that can benefit from improvements in high speed servo control are point to point control, axis tracking, and multi-axis coordinated motion. Implementation of control strategies for these problems will take place on a Bridgeport vertical CNC machining center. A system will be developed to allow open-system architecture at the servo control level. This will incorporate the vertical CNC machining center, a high performance PC, an industrial motion control card, and the hardware and software interfacing. Ultimately, we like to demonstrate that advanced motion control can be implemented on an industrial machine with substantial performance improvement over the existing system.

Research Objectives

This project's research objectives are: 1.to continue the modeling and identification effort to develop a procedure to characterize machine tool drive dynamic performance,

2.to develop hardware and software interfacing to allow implementation of open-system architecture control at both the servo control level and the process control level on a vertical CNC machining center, and

3.to develop and implement effective control strategies for point to point motion control, single axis tracking, and multi-axis coordinated motion control for high speed machining applications.

Expected Results/Deliverables

This project will result in the development of software and hardware interfacing to allow open-system architecture at the servo control level. Effective strategies to counteract nonlinear dynamics and disturbances will be developed. Advanced servo control methods for high speed motion control in machining applications will be developed on a industrial platform and in a format that can be combined with existing servo control systems.

Progress to Date

The following items have been accomplished:

Dynamic performance characterization: We developed and compared several system identification procedure for machine tool drives. Both ball screw drives and direct linear motor drives have been tested with dynamic model prediction error less than 10 um. We are conducting more comprehensive tests to validate our approaches. The dynamic characterization of machine tool servo drives was not comprehensively covered by the ASME B5.54-1992 standard. Our development here could potentially adds to this standard.

Rapid tracking motion control algorithms: We have been developing several high-speed motion control algorithms for rapid tracking. A contour tracking algorithm using receding horizon optimal control approach has been developed. Optimal feedforward tracking controller design which minimize the maximum time domain error with constraint of maximum trajectory speed or maximum acceleration has been developed. Currently, we are working on incorporating system modeling errors in the feedforward controller design.

PC based machine tool controller: We have been using a TMS320C30 digital signal processor to test our motion control and system identification algorithms. We have recently acquired from Delta Tau, through a substantial donation by the company, a complete PC based CNC system running the PMAC motion control board. Using this new system, our immediate goals are first to develop a universal cable interface so that a variety of our laboratory hardware, including Bridgeport machining center, linear motors, and electrohydraulic servo actuator, can be interfaced to this system simply with plug-and-play, and second to implement our motion control and system identification algorithms in the PMAC motion control board. We believe that the accomplishment of this two tasks will demonstrate the important concept and fact that open architecture machine control system can be readily developed down to the servo level using a commercial software/hardware platform and hence higher performance advanced control algorithms can be implemented on the factory floor with reliable commercial hardware platforms.

Machine tool drives: We have developed an interface to our Bridgeport vertical machining center in the velocity loop and current loop level. This allows external control of the machine tool drives at those levels. We have integrated a brushless linear motor slide for high-speed motion control and high bandwidth machining process control such as drilling force regulation. We will use these two drives to conduct experiments on rapid tracking for high-speed machining.

Member Company Benefits

Servo control in industry relies primarily on simple PID control. This means slow feed-rates are necessary to machine parts with high accuracy. A control strategy incorporating advanced techniques designed for high feed-rates can reduce machining times. The use of direct drive linear motors allows the high bandwidth response that is required for high accuracy at high speed. Efficient industry use of the linear motors for feed drives has been limited by the use of controllers designed for ball and screw drives. The identification of control strategies that are particularly effective for linear motor systems will allow industrial users to take full advantage of the advances in actuator technology. Some of the strategies that will be examined can be applied in an add-on format that can be combined with existing controllers to improve overall performance. By having control schemes that have been designed in this modular format, a manufacturer can combine the control aspects that are deemed important for a particular process to create a optimal overall servo control strategy. Furthermore the implementation of these strategies on the open-system architecture environment will show that they are appropriate for direct application for systems in industry.