VDD Digital Vibrometers - Applications

Fig. 1: Spindle run-out measurement. VibSoft-VDD provides an FFT function for switching from time to frequency domain. The repetitive spindle runout (RRO) is obvious in the frequency domain.
Fig. 2/3: Run-out measurement on the spinning hard disc media. Comparison of the noise floor for an analog LDV system (Fig. 2, upper image) and a digital VDD LDV system (Fig. 3, lower image).

Frequency Response Measurement

Frequency response testing on actuators and head gimbal assemblies (HGA’s) has traditionally relied on a LDV plus a 2-channel FFT analyzer. With the VDD, Polytec provides a complete solution that eliminates the cost of an external spectrum analyzer and improves the resolution of the measurement. Measuring the resonance response of a part has never been easier.

Measurement of Slider Displacement while Track-following

HDD researchers are interested in the precise, time-domain displacement history of a slider. The off-track motion of the slider is very small. For real time measurements, a high resolution analog decoder is often the best choice, but when even higher resolution is required, the post processed digital output is the only alternative. A digital vibrometer reduces signal-to-noise by one or two orders of magnitude compared to analog vibrometers and can easily detect the motion. Using the VDD system, it is possible to make a direct  comparison between the time dependent slider position and the position error signal (PES). Using this comparison, contributions to the PES can be identified and minimized, for example: torsional HGA resonances, air bearing resonances E-block symmetric and asymmetric resonances, airflow induced excitation, different rigid rocking modes from the slider, etc.

Head/Media Interface Studies

The high resolution of the digital LDV permits accurate measurements of head/media interface dynamics. Engineers can characterize a given HGA and slider configuration with a particular media drive component. If not satisfactory, other product combinations can be tried until an appropriate solution is found that offers stable flight and minimal registration errors.

Stable flying heights for a magnetic head are determined with a digital LDV.

For example it is of great importance to know what happens to the slider at different flying heights when using media with certain specifications. With current media products, the average surface roughness and micro waviness of the media shipped today are around 5 nm to 10 nm in height and l  in the order of 100 microns (roughness average). If we fly the head at flying heights of 4 nm and 7 nm; the dynamics of the slider in track-follow state will be quite different.

Even though we would like to fly the slider around 4 nm for better magnetic pick-up, the resonances at such flying height are so dominant in comparison with the ones with the 7 nm flying height that this is best from the dynamics point of view for that particular product. Engineers thus can characterize a given HGA and slider configuration with a particular media product. If this is not satisfactory, engineers could measure other product combinations and evaluate them with the digital LDV. This documentation will support choosing a given HGA, slider and media combination for a given file product which in the end would minimize misregistration errors.

Similar results can be obtained for vertical and tangential slider motion, which will be related to phase data transfer errors. Digital LDV resolution is necessary to detect sway and torsion slider modes with sufficient resolution that arise during head-media contact events, furthermore it also has been shown that the amplitudes of the sway and torsion slider modes are a function of the slider skew angle. Those results hold for measurements at the center, leading and trailing edge of the slider.

	Slider resonances at 7 nm (upper image) and 4 nm (lower image), out-of-plane flying height measured using digital LDV technique.
	Source:

Calibration of Transducers and Sensors

The digital LDV does a direct and highly precise mathematical phase demodulation comparable to the international standard ISO 16063-11 (Method 3). The measurement results are referenced to the wavelength of the HeNe laser, thus being very accurate and long-term stable. Its performance is so outstanding that it can be used as a reference measurement system in primary calibration systems for shock and vibration transducers.

The VDD-650 (predecessor of the VDD-E-600) is the first commercially available LDV system receiving PTB (German (National Bureau of Calibration) certification for use as a primary calibration according to an ISO standard. The certification from the PTB is for the standard head optics only. The VDD-650 Digital LDV has been certified by the PTB to have £ 0.1% error and linearity up to 20 KHz. 

Relative deviation of one VDD system from the nominal vibration values as measured by Germany’s PTB (National Bureau of Calibration)