The concept and progress of intelligent spindles: A review

Intelligent spindles can be described as spindles with the capabilities of sensing, decision making and control, which guarantee the optimum machining process and reliable operations. Drs. Hongrui Cao, Xingwu Zhang and Xuefeng Chen Xi’an Jiaotong University discussed in great detail recently the concept state-of-the-art and progress of intelligent spindles technologies. The work was published in the journal, International Journal of Machine Tools & Manufacture.

According to the researchers focus has been shifted to smart production, where intelligent machines, systems and networks are capable of independently exchanging and responding to information, to manage industrial production processes. A key to building smart factories is to turn traditional machines into more intelligent machines. The basis of this next generation machine tool is the ability to monitor and control multiple process modules. They pointed out that core components of machine tools, spindles have direct effects on the material removal rates and machined quality of workpieces.

Spindle technologies have been developed extensively to improve accuracy and abilities, such as higher speeds, precision and reliability. The article also reviewed the historical development, recent challenges and future trends of machine tool spindles in detail, noting that further development would be required to allow sensor integration to make the spindle unit an inherent quality insuring system.

Most studies have focused on several intelligent functions of machine tool spindles, and a consensus on the concepts and characteristics of intelligent spindles has not been reached. The lead author Hongrui Cao said no intelligent spindle has been practically use in the industry. Intelligent spindles should be able to actively suggest task arrangements and modify operational parameters to optimize machining processes and spindle performance. When comparing traditional spindles, the authors highlighted that intelligent spindles need to possess new characteristics, including autonomy, self-learning, compatibility and openness. In order to design and manufacture intelligent spindles, there should be a focus on developing the key enabling technologies, which include sensing, decision making and control.

 They consider six expected functions of intelligent spindles, which was later categorized into two groups. The first group is related to the spindle-process interaction, which treats machining processes and spindle structures in an integrated way. In this group, expected intelligent functions include the monitoring and control of tool condition, chatter and spindle collision. The second group is related to the spindle itself, including functions such as monitoring and controlling thermal errors, spindle balance and spindle health. In the sensing subsystem, various types of signals are monitored with integrated sensors, forming a sensor network to supervise the spindle performance. If an abnormality is identified, then the control subsystem is activated, and the embedded actuators, CNC system and operator are the executors in achieving the control purpose.

The key enabling technologies of intelligent spindles have improved significantly, and spindles with certain intelligent features have been developed, such as active balancing, chatter control, and fault diagnosis. The characteristics, such as autonomy, self-learning, compatibility and openness, have not been fully addressed. The key issue facing the development of intelligent spindle include, lack of a top-down design for intelligent spindles, lack of integration, and databases and knowledge bases are far from sufficient. According to the current limitations, the future trends of intelligent spindles are likely to move toward the following direction: The top down design of intelligent spindles; integrated smart sensors and actuators; advanced real-time data processing and decision making; prognostics-centered control and maintenance; integration into the industrial big data environment.

The researchers explained that advancement in sensors, actuators, data processing algorithms, artificial intelligence technologies, control technologies and maintenance strategies have significant contribution to the development of intelligent spindles.

There are still gaps in directly applying work of machining process monitoring and control to intelligent spindles, explained Dr. Hongrui Cao. The development of intelligent spindles is expected in the future, which could be promoted by several potential technologies, including top-down design, smart sensors and actuators, real-time data processing and decision making, prognostics-centered control and maintenance.