Study of vibrations and instability in a robotic grinding process

The vibratory dynamics of the grinding process performed by a robot arm is studied in this thesis. The robotic grinding process under development at Hydro-Québec’s research institute (IREQ) for maintenance operations on hydropower equipment is a high material removal rate task used for profiling large parts and complex geometries. The profiling application is unlike conventional grinding in which, a thin layer of material is removed as a finishing procedure. One major hurdle for this technology is the significant vibrations inherent to the process. However, the maneuverability that the robot brings into the operations makes it, sometimes, the only practical solution for machining in hard-to-reach areas. The main features of the robotic tool holder which affect its vibratory dynamics, i.e. the robot’s high compliance and its configuration-dependent dynamics are studied. The objective is to investigate vibrations and instability based upon appropriate understandings of the instantaneous dynamics of the material removal process performed by the articulated multi-body robot arm. Since vibrational instability in material removal is caused by the interactions between the dynamics of the cutting process and the tool holder’s structural dynamics, two lines of research are conducted accordingly. \n \nAn experimental investigation substantiated by numerical simulations is carried out on the steady vibratory dynamics of the process. Due to the compliance of the robot arm, material removal is found governed by vibro-impacts, occurring mainly at the spindle’s rotational frequency, between the cutter and the workpiece. The “impact-cutting” behavior is characterized through angular analysis of the cyclic impacting oscillations. The measured instantaneous rotational frequency of the spindle during robotic grinding is mapped into a representation suited for monitoring the dynamic evolutions in the impacting regime. The “impact-cutting map” was also used to validate a plausible hypothesis for uniform disk wear when exhibiting an impact-cutting operation. The measured drop in the instantaneous angular speed, as a transient which is excited impulsively by the cutting impacts, was found well correlated to grinding power. The practical significance of this latter result is considered as to integrate the real-time measurement of the speed drop and the number of impacts per spindle revolutions into the robot control strategy in order to improve the metal removal estimation. \n \nIn a following step, an impact-cutting model for metal removal was used to estimate the grinding power required for a grinding task performed by the robot. Constant coefficients of the model were first identified experimentally. Robotic grinding tests were performed while setting the target grinding power in the control strategy based upon the impact-cutting model. It was demonstrated that a uniform cut with a target rate of metal removal and a target cutting depth can be achieved in presence of stabilized impacting oscillations. The waviness amplitude on the finished surface is found to be much smaller than the amplitude of vibro impact oscillations. The knowledge about vibro-impact oscillations present in the process helps improving the strategy of controlled material removal rate employed in the robot control strategy. The iterative procedure of grinding/profile scanning to reach the desired tolerance level on the surface can be improved based upon the estimation of the material removal rate by the impact-cutting model. \n \nThe limit of stable impact cutting due to regenerative chatter was investigated next. The investigation resulted into understanding that the high compliance of the robot arm locates the problem of robotic grinding regenerative chatter on the far upper right of the first lobe on the stability chart. In this region, the limit of stable operation is defined by very large gain values. This is different from traditional machining which is located inside the “lobes zone” on the stability lobes diagram. The cyclic impacting dynamics of material removal is invoked to investigate instability in this region. The limit of stable operation is identified from numerical simulations of impact-cutting. The boundary is found to be very close to the margin predicted using the traditional approach for regenerative chatter analysis. It is concluded that the large gain is typical for robotic grinding. The impacting dynamics of material removal due to robot compliance must be considered to understand such large gain values, never occurring in conventional grinding. Experiments are performed to substantiate the new understanding regarding the problem of regenerative chatter in robotic grinding. \n \nA second line of research was focused on the robotic tool holder’s structural dynamics. The goal was to provide a modeling tool for an investigation of the effect of robot’s configuration-dependent dynamics on vibrations and instability in the process. A 6-DOF multi-body dynamic model was developed for the robot manipulator. Experimental modal analysis on the robot structure was used to validate the mode shapes and natural frequencies predicted by the model. A discussion is provided about how the developed modeling tool can serve an investigation of mode-coupling chatter in robotic machining. \n \nThe study of vibrations and instability in this thesis contribute into understanding the vibratory dynamics that govern the robotic grinding process. Further development of the robotized technology for precision profiling of industrial parts relies on such understanding bases.

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