Cooling Effect of Thermal Actuator in Hard Disk Drives
This research presents experimental methodology and investigations on the cooling effect on the thermal actuator. Experimental setup was developed and harmonics signal method was introduced to predict the thermal response of the actuator. Two types of disk with different lubricants were used in the experiment.
The thermal effect of a MR head is well known in head-disk interface measurements, notably the in situ flying height (FH), thermal asperity, and pole-tip protrusion measurements. However, with the embedded heater in the slider, the heat generated by the heater has a significant influence on the read back signal of the MR head as the electrical resistance of the MR sensor is highly temperature dependent. Therefore, the dynamic read back signal is a coupling between magnetic signal by head media and the thermal effects due to the proximate heater. Hence, a study of the thermal actuator protrusion using the MR method tends to be inaccurate. Here, a new method is introduced that is able to decouple the effects and detect the actual dynamic changes of the thermal actuator, which is hereby named as the “harmonics method.”
Fig. 1. Experimental setup.
Figure 1 shows the test experimental setup that comprises a spin stand with read/write functionality. A commercial thermal flying-height control (TFC) slider was used to write a 60 MHz track on the disk. The slider FH was varied using a 1 kHz sinusoidal voltage applied to the thermal actuator and the resultant read back envelope was filtered with a 10 kHz low-pass filter. The signal was then translated into frequency domain using fast Fourier transform (FFT) function of the oscilloscope. The magnitudes of the harmonics were recorded as the heater power was varied. Here, the harmonic content is introduced by varying the magnetic spacing via thermal actuation. Any nonlinear contribution of the cooling effect to the temperature variation can similarly be determined by measuring the second harmonic component of the read back signal.
Using the MR to measure the cooling effect, the results indicate that head temperature rise in the flying condition is significantly less than that of the static condition unload. The static (unload) resistance reduction is linearly proportional to the heater power whereas the dynamic (loaded or flying) case is slightly nonlinear — a small divergence from their corresponding linear fitted lines. The observed nonlinearity is attributed to nonlinear pressure rise as the FH is reduced. Given the few nanometers head-media clearance, the gas molecule interaction is in the order of single molecule which indicates possible heat conduction instead of convection between the media and the slider. High pressure corresponds to more molecules per unit volume for heat transport. Simulation results show that pressure change in the protrusion area is nonlinear and, thus, as the protrusion moves closer to the media, the heat generated is dissipated through the air bearing to the media nonlinearly. A greater nonlinearity was observed for the bonded lube disk compared to the bonded + mobile lube disk. Since the former is a poorer thermal insulator, the cooling effect played a more dominant role.
For more information about this research, please refer to the following paper(s):
- K. W. Ng, C. H. Wong, S. H. Leong, Z. M. Yuan, B. Liu, and T. C. Chong, “A method to study the cooling effect of the thermal actuator,” Journal of Applied Physics 103, 07F532, 2008.