# Prediction Methods and Experimental Techniques for Chatter Avoidance in Turning Systems: A Review
## 4 Passive and Active Suppression Techniques
### Active Chatter Suppression Techniques  
Active chatter suppression schemes, with suitable sensors/actuators installed on the spindle or tool holder, have demonstrated a potential performance improvement in the machine tool. This dynamic solution can adjust the parameters of the process depending on the feedback from the sensors. However,  
Figure 6: Cantilever beam with attached spring-mass-damper system and distributed load [104].  
it usually requires high computing power and the effectiveness of the vibration suppression system depends on the strategies installed in the device controller.  
An active dynamic absorber reduces vibration by using, for example, a piezoelectric actuator generating inertia forces that counteract the disturbance produced in the turning process [106]. Active damping dissipates energy once the vibration is measured and an actuator introduces a controlled force as a response. In a typical application a piezo actuator is bonded on the cutting tool. [PERSON] et al. [107] optimized a passive shunt electrical circuit, tuned according to the natural frequency of the cutting tool for damping optimization. In accordance with experimental modal testing, the stability limit is doubled when piezoelectric is used. Similarly, [PERSON] et al. [108] demonstrated, in boring experiments with shunt circuits in both directions, the surface finish was improved with a roughness reduction of 30% on a peak-to-peak value.  
Various active systems used to mitigate chatter are based on controllers with several inputs. [PERSON] et al. [109] developed an active chatter suppression method that expands significantly the stability lobe diagram based on an active sliding mode controller that requires only a displacement measurement. This robust controller is suitable for non-linear uncertainties and disturbances such as permutations found in chatter vibrations. [PERSON] et al. [110] analyzed the dynamic behavior of a constrained layer damping (CLD) tool holder that has frequency-dependent dynamics. These authors proved that the developed tool holder improves stable margins when the chatter frequency is far from the natural frequency.  
Another type of device could be considered as a semi-active technique, it is based on a magnetorheological (MR) fluid-controlled boring bar (Figure 7) requiring lower energy. The MR fluid-controlled boring bar changes the stiffness of the tool by varying the strength of the applied magnetic field. [PERSON] et al. [111] used a square wave current as a control signal showing that chatter could be significantly suppressed. For a high aspect ratio boring process, [PERSON] et al. [112] used an electromagnet to adjust the damping force of an MR damper showing an average attenuation of 30% in high-frequency ranges. When the rotational speed is low it is easier to suppress chatter by adjusting the natural frequency. For high rotational speeds an increased damping effect is advantageous [113].  
Recently a novel non-contact magnetic actuator [114] was developed to compensate the static deflection measured of a long boring bar. The bending mode was actively dampened, and the stiffness was increased for stable cutting at high depth cuts.