Mitigation of Chatter Instabilities in Milling by Active Structural Control

Abstract

This report documents how active structural control was used to significantly enhance the metal removal rate of a milling machine. An active structural control system integrates actuators, sensors, a control law and a processor into a structure for the purpose of improving the dynamic characteristics of the structure. Sensors measure motion, and the control law, implemented in the processor, relates this motion to actuator forces. Closed-loop dynamics can be enhanced by proper control law design. Actuators and sensors were imbedded within a milling machine for the purpose of modifying dynamics in such a way that mechanical energy, produced during cutting, was absorbed. This limited the on-set of instabilities and allowed for greater depths of cut. Up to an order of magnitude improvement in metal removal rate was achieved using this system. Although demonstrations were very successful, the development of an industrial prototype awaits improvements in the technology. In particular, simpler system designs that assure controllability and observability and control algorithms that allow for adaptability need to be developed.

Adaptive chatter mitigation control for machining processes with input saturations

“…Hence, a higher MMRR can be expected by applying the proposed control method. The present control approach is developed for the regenerative chatter dynamics derived from real cutting processes [14,15,19]. Our future work will focus on applying the method to experimental and actual machining systems.…”

“…When the lower actuator saturation is activated, that is, u u max 1 2 , we can obtain that OE 3 ; 4 T 0 (otherwise ı 3 0 and u u max 1 2 C "1 2 ı 3 u max 1 2 , which is a contradiction) and e u D u C u max 1 2 "1 2 ı 3 based on left inequality (19) in the positive invariant set O 1 and O 2 , which is guaranteed by Lemma 3.1. Therefore, one has…”

“…Consequently, the productivity of passive control-based milling systems is still limited.By comparison, the active control is a more promising one, which installs suitable sensors and actuators (like electrostrictive actuators [16], piezoelectric motors [17], or active magnetic bearing [18]) onto spindles or tool holders to generate desired chatter suppression forces. As representative works, Dohner et al [19] applied active damping on a milling spindle equipped with two orthogonal pairs of electrostrictive stack. By this means, he developed a linear-quadratic-Gaussian control law to achieve a good balance between performance and robustness.…”

“…The performance of the proposed adaptive control method is given in Figure 4 with control parameters given in Table II. The spindle speeds and depths of cut used in this case study are comparable with those of [19]. The stability boundary is calculated under Assumption 3.1 and the residual tracking error satisfies j 1;2 j 6 50 m [22].…”

Fixed‐structure robust controller design for chatter mitigation in high‐speed milling

“…Moreover, vibration absorbers require accurate tuning of their natural frequencies and, consequently, lack robustness to changing machining conditions. Active chatter control in milling has mainly been focused on active damping of the machine dynamics [3,4] or workpiece [5]. Damping the machine or workpiece dynamics, either passively or actively, results in a uniform increase of the stability boundary for all spindle speeds.To enable more dedicated shaping of the stability boundary (e.g., lifting the SLD locally around a specific spindle speed at which one desires to operate for the manufacturing of a certain product), the regenerative effect, which is inherent to the metal cutting process and which is the root cause for chatter, should be taken into account during chatter controller design.…”