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Dynamic control of spindle motors for milling applications

Dynamic control of spindle motors for milling applications
Qinwei Pan

2017

Department of Electronic and Electrical Engineering, The University of Sheffield, Sheffield, South Yorkshire S10 2TN, UNITED KINGDOM.

ABSTRACT

This programme of research is concerned with investigating novel methods for enhancing the machining of metals in terms of increasing material removal rates while maintaining surface finish and dimensional tolerance. This thesis provides a detailed investigation of the interaction between the drive control system, the power converter and the cutting process. Especially the coupling between the controller and power converter behaviour and the dynamic cutting conditions is the key novelty in this research.

Specifically, the research explores the merits of high speed and precise torque modulation of the drive and its influence on the cutting tool to suppress chatter in milling operations. The study focuses on the following areas:
  1. The open-loop torque matching to achieve precise speed control. A high frequency component of the machine load is generated due to the milling process, resulting in speed fluctuation. This research demonstrates a method for precise speed control using a low bandwidth PI controller by applying a torque correction signal to the control loop.
  2. Rapid torque modulation for chatter suppression in milling operations. The past research of variable speed milling on chatter suppression is limited by industrial milling machines. The industrial speed controller limits the frequency of speed variation that can be applied around the operating condition. This thesis increases the speed variation frequency capability for high speed milling process.
  3. Investigating the complex interactions between speed control settings and cutting performance. By applying torque match and rapid torque modulation the speed variation frequency capability is significantly increased. A detailed investigation of the dynamic behaviour with a high frequency speed variation and the effect of chatter suppression on high frequency variable speed milling (HFVSM) is presented.
  4. Detecting the onset of chatter using speed measurements. The cutting torque fluctuation is reflected as a disturbance of the motor speed. Chatter can then be identified by monitoring the motor speed. The application of this technique benefits the milling industry by providing a more robust chatter detection compared with conventional techniques.
  5. Combined dynamic operation. The combined operation of torque match, rapid torque modulation and chatter detection is demonstrated and has potential application to a milling machine capable of detecting and auto suppressing chatter during milling.
By combining auto chatter detection identification technique and high frequency variable speed milling, this research provides a detailed study with significant potential for industrial application. The key enabling methods demonstrated increase the operating speed capability for milling activities and hence reduce the time taken to manufacture a part and consequently increase the productivity of a milling machine.