Torsional vibrations are inherently present in every rotating powertrain. In resonant conditions, torsional vibrations can be significantly amplified. A typical method to reduce the torsional vibration particularly at resonance is to modify the torsional natural frequencies with the component design. Commonly, a straightforward method for the modification is the adjustment of the torsional stiffness of a coupling. This study presents a method to modify the torsional natural frequencies using a coupling design with continuously adjustable torsional stiffness. The presented coupling design is investigated with torsional analysis and experimental measurements. Torsional analysis was utilized to predict the effects of varying the coupling stiffness to the torsional natural frequencies of a powertrain. The experimental measurements were conducted by attaching the adjustable stiffness coupling to the powertrain and measuring the change in the torsional natural frequencies while the torsional stiffness of the coupling was adjusted. The torsional natural frequencies were determined from the measurements by identifying the resonance induced torsional vibrations from the vibration response of the powertrain. The torsional vibrations were excited to the system by a load motor. The measurements showed that the first torsional natural frequency of the powertrain changed from 15.6 Hz to 43.0 Hz as the torsional stiffness of the coupling was adjusted. The results of the torsional analysis and the experimental measurements were compared to determine the performance of the realized coupling. The results indicated that the torsional natural frequencies determined by torsional analysis agree well with the experimentally measured results. The prediction errors were generally below $\pm 5\%$, which is typically considered as a margin for accurate torsional analysis.

Eddy current probes are widely used for proximity and displacement measurement in mechatronics and for defect detection in nondestructive testing. These probes induce eddy currents to an electrically conductive surface. Intensity of these currents varies relative to the changes in material properties, displacement, and surface irregularities. Therefore, changes in these properties can be detected. Surface roughness is a common surface irregularity that can cause noise and measurement errors. This article investigates the correlations between eddy current probe impedance responses and areal surface roughness parameters. Effect of increasing and decreasing roughness was investigated by creating roughness variations using brushing and polishing treatments. Parameters Sdr and Sdq were found to be the most robust predictors of the impedance response. Average roughness height-based parameters (Sa and Sq) were significantly weaker predictors for the impedance response. Many standards related to eddy current measurements use these average height-based parameters for roughness control. Hence, surface-related measurement errors and noise may be experienced even when following the standards. The results of this study imply that roughness control in relevant standards could be improved by additional criterion using Sdr or Sdq. These findings could be utilized in high-accuracy applications, such as displacement measurement, condition monitoring, and nondestructive testing.

Eddy current probes are common in various applications, such as nondestructive testing, displacement measurement, and condition monitoring. However, they have not been extensively applied to machining tool condition monitoring. The present study addresses this gap. This study investigates a tool condition monitoring method using an eddy current probe. An eddy current probe is used to scan face-milled surfaces, and the tool condition is inferred from the probe response. Two probes with different shapes were used: a round-shaped coil and a racetrack-shaped coil. The probes are used to scan surfaces milled using face milling inserts with different wear states. The insert wear is determined from the eddy current probe responses that were processed by filtering and discrete Fourier transform (DFT). The peak height of the resulting frequency spectra was used to determine the tool condition. The results indicate that the method is suitable for tool condition monitoring. In general, the response peak height increased as the inserts were more worn. The results also show that the eddy current probe shape has a significant influence on the wear measurement. The racetrack coil was significantly more sensitive for wear measurement compared to the round coil. Future studies could investigate the use of different coil configurations, such as reflection probes or probe arrays.

Surface roughness can create noise and errors in eddy current measurements. This study investigates the effect of small-scale roughness variations on the eddy current probe response, which are known to influence the current flow when the skin depth is close to the roughness height. Small roughness variations were made using silica carbide paper on two test pieces made from steel. Treated areas were measured using eddy current probe with frequencies ranging from 0.125 to 8 MHz. High frequencies and ferromagnetic test pieces lowered the skin depth close to the roughness height. Significant eddy current probe responses were observed in the treated areas. Changes in surface roughness were measured using the white light interferometer. The treatments also induce residual stresses and work hardening, which were estimated using X-ray diffraction measurements. Test pieces were annealed after the measurements to remove the residual stress and work hardening. Consequently, the response magnitudes were significantly reduced. The results implied that the residual stresses had the most significant influence, and the roughness changes themselves had a weak frequency-dependent effect on the probe response, which was observed after annealing. It was concluded that the influence of small-scale roughness variations itself is weak enough to be relevant only in specific high-accuracy applications.