1
Chatter reduction through active vibration damping
Abhijit Ganguli
Department of Civil Engineering, IIT Delhi
2
Motivation of the work:
• To demonstrate active damping as a chatter control strategy
Organization of the presentation • Introduction • Regenerative chatter in turning
• Regenerative chatter in milling
• Active control of chatter
• Experimental investigations
Realization of Demonstrator for chatter in turning
Realization of Demonstrator for chatter in milling
Application of active damping
• Conclusion
3
Applications of machining- • Aircraft & Automobile industries • All kinds basic metallic components & parts • High precision tiny equipments
Introduction
Typical machining operations
Workpiece
Tool
Turning
Milling
4
Chatter Marks Problem affecting machining:
Machine tool chatter instability
Broken tools
Effects of chatter
• Low precision and low surface quality • Damage to tool or workpiece system • Low efficiency • Rising costs
Requirement for the machined end product
• High quality of surface finish
5
Organization of the presentation
• Introduction
• Regenerative chatter in turning
• Regenerative chatter in milling
• Active control of chatter
• Experimental investigations
Realization of demonstrator for chatter in turning
Realization of Demonstrator for chatter in milling
Application of active damping
• Conclusion
6
The regeneration process
T = 60/N = Period of one revolution
Dynamic Chip Thickness
Oscillation during previous pass Oscillation during
current pass
Cutting Force Cutting Constant
Axial width of cut
Chip Thickness
Kcut
7
Feedback Loop Model for chatter in turning (Merrit 1965)
Stable
Marginally Stable
Unstable
Machine vibration
Dynamic chip thickness
Cutting stiffness
Cutting force
Machine tool transfer function
Time delay
+
+
+ _ y(s) Kcut G(s)
Feed
Structural Dynamics
Cutting Process
Displacement Forces
8
Solve characteristic equation
1 + (1 - e-sT ) Kcut G(s) = 0
Pade Approximation
Methods of stability analysis • Time domain simulations • Root Locus Method
Outline of Root Locus Method • Choose spindle speed i.e., T • Solve characteristic equation with increasing Kcut • Identify the value of Kcut for which at least a couple of conjugate roots lie on the imaginary axis
9
Spindle speed
Kcu
t / k
Unstable
Stable
The Traditional Stability Lobe Diagram
Parameters governing regenerative instability:
• Spindle Speed • Kcut
Delay
Structural Mode
10
Organization of the presentation
• Introduction
• Regenerative chatter in turning
• Regenerative chatter in milling
• Active control of chatter
• Experimental investigations
Realization of demonstrator for chatter in turning
Realization of Demonstrator for chatter in milling
Application of active damping
• Conclusion
11
Δx = x – x(t – T); Δy = y – y(t – T) T = tooth passing period = (Period
of rotation/number of teeth)
Regenerative chatter in milling
Periodic Regenerative Forces
Regenerative Displacements
Cutting forces
Axial width of cut
Cutting Constant
Periodic feed
forces
Governing equation: Linear DDE with periodic coefficients
12
Complexities of milling chatter • Existence of tooth passing frequency harmonics (Nm/60) • Multiple mechanisms of chatter
• Stability limits dependent on the kind of milling operation, direction of feed etc.
13
Flip Bifurcation
Hopf Bifurcation
Characteristics of chatter frequencies in milling • Harmonics of tooth passing frequency
• Hopf Bifurcation : Basic chatter frequency close to structural modal frequency
• Flip Bifucation : Chatter frequencies located at odd multiples of one half of tooth passing frequency
14
Observation • Hopf Bifurcation in low stability regions • Flip Bifurcation in certain parts of high stability regions
Downmilling
Downmilling
Upmilling
Stability lobes in milling
15
Organization of the presentation
• Introduction
• Regenerative chatter in turning
• Regenerative chatter in milling
• Active control of chatter
• Experimental investigations
Realization of demonstrator for chatter in turning
Realization of Demonstrator for chatter in milling
Application of active damping
• Conclusion
16
Active control of chatter
• Online control of spindle speed • Application of damping
Spindle speed
Kcu
t / k
Unstable
Stable
High stability Region (control spindle speed)
Augment damping
H(s)
17
Active Damping
Colocated actuator/sensor
Alternating poles & zeros
Actuator
sens
or Control laws with
guaranteed stability
Source – Vibration Control of Active Structures- An Introduction André Preumont
19
Demonstration of enhanced damping effect
Stability enhancement more pronounced
Damping Coefficient Feedback
gain
Chatter Frequency
20
Effect of Active Damping of both directions (Numerical Simulation)
• Enhancement of stability more pronounced in low stability regions
21
Organization of the presentation
• Introduction
• Regenerative chatter in turning
• Regenerative chatter in milling
• Active control of chatter
• Experimental investigations
Realization of demonstrator for chatter in turning
Realization of Demonstrator for chatter in milling
Application of active damping
• Conclusion
22
Motivation • To investigate active damping as a chatter control strategy
Difficulties in real machining • Perform numerous cutting tests • Chatter needs to be actually triggered • Life of the machine endangered • Repeatibility of results
Intermediate Platform
A laboratory demonstrator
Technological advances • Chatter model well developed • Advances in signal processing technologies
Potentials • Realistic simulation of regenerative chatter • Repeatability • Platform for active damping
23
Conceptual model of a mechatronic simulator (Hardware in the Loop concept)
Basic parameters governing regenerative instability: Tooth passing period (Spindle speed) • Kcut • Angle of entry & angle of exit (Type of milling)
User defined
Experimental stability Analysis • Choose cutting conditions • Increase Kcut step wise • Monitor displacements • Store Kcut value at instability
Structural Dynamics
Cutting Process
Displacement Forces
24
The Chatter Demonstrator for 1 DOF turning
Structural Dynamics
Software Layer
Hardware Layer
Cutting process
25
Comparison between Experimental & Numerical Simulations
Stability Lobe Diagram
Chatter Frequency Diagram
26
Organization of the presentation
• Introduction
• Regenerative chatter in turning
• Regenerative chatter in milling
• Active control of chatter
• Experimental investigations
Realization of demonstrator for chatter in turning
Realization of Demonstrator for chatter in milling
Application of active damping
• Conclusion
27
Demonstrator for milling
Cutting process simulation
Voice coil Actuators (electro- magnetic)
Displacement signal
DSP
Cutting force signals
Cutting Conditions
Monitor Displacements
x y
z
Manufacturer: Micromega Dynamics
29
Organization of the presentation
• Introduction
• Regenerative chatter in turning
• Regenerative chatter in milling
• Active control of chatter
• Experimental investigations
Realization of demonstrator for chatter in turning
Realization of Demonstrator for chatter in milling
Application of active damping
• Conclusion
31
Effect of active damping (Experimental)
Bending X 2.1 % 6.7 % 8.2 %
Bending Y 2.7 % 4.5 % 5.8 %
Uncontrolled Damped 1 Damped 2
33
Application of active damping on an industrial setup
Source: Huller-Hille & Micromega Dynamics
Active Mass Damper
34
Conclusions
• Phenomenon of regenerative chatter investigated
• Laboratory mechatronic setups realized
• Demonstrators simulate regenerative chatter realistically
• Active damping as chatter reduction strategy demonstrated
35
Acknowledgements
• Prof. André Preumont
• Drs. M . Horodinca, I. Romanescu, A. Deraemaeker
• All members of the ASL team
• The SMARTOOL Consortium
• The European commission
• Micromega Dynamics