New CETR Acoustic Emission Amplifier with RMS,  Peak Detection and Raw outputs

In order to stay ahead of the ever changing testing requirements of the disk drive industry CETR has developed a new Acoustic Emissions Amplifier module which features several enhancements over the previous model.


Improved performance for measuring pico sliders

The maximum amplifier gain has been increased by a factor of 10. The filter design has been changed to a 10 pole Butterworth band-pass filter with plug-in modules for different frequencies. The improvements in gain and filter design are particularly important when testing head sliders that have a natural resonance greater than 1 MHz, where signal to noise ratio becomes more critical. The smaller sliders have higher natural frequencies, and produce lower acoustic energy. 


Peak detection of momentary head-to-disk contacts

The RMS and Peak Detection outputs of the amplifier module are switch selectable. The Raw amplifier output is also provided. The RMS output is useful for assessing the stability of the air bearing between the head slider and the disk during spindle spin up and during near contact flight. In these cases the acoustic energy is sustained long enough to make RMS measurements.
The new Peak Detection mode is useful for detecting momentary head-to-disk contacts such as those caused by disk surface asperities or dynamic head loading. The acoustic energy generated by these kinds of contact are too short lived to produce a measurable RMS output. The peak detector captures the peak value of an acoustic pulse and holds it long enough to be sampled by the data acquisition system. 
Specifications:

Amplifier:

AC Gain
Switch selectable to 80 dB, 86 dB, 94 dB, or 100 dB
Frequency Response
Without band-pass filter
±1dB from 100 kHz to 5 MHz
Band-pass filters
Type 10 pole Butterworth plug in PCBs
600-1900 kHz
600-1000 kHz
900-1300 kHz
1200-1600 kHz
1500-1900 kHz

Outputs:

Raw amplified and filtered signal
RMS or Peak Detection switch selectable
RMS and Peak Detector Gain
Potentiometer adjustable from 2 to 100
Full scale 10 volts 
RMS Converter Time Constant 0.5 msec.
Peak Detector hold time 2 msec.

All switches and potentiometer adjustments are easily accessible by the user through the mounting panel.

Peak Detection for Ramp load testing

A series of tests were performed to demonstrate the effect of the seek velocity and the spindle RPM when loading a head onto the disk using a CETR Olympus System tester equipped with an Iomega style ramp.
In the first test an HGA with a 50 % slider and a negative pressure air bearing was used. The AE amplifier was set to the peak detect mode. The first sequence set the seek velocity to 0.5 IPS and did 5 load/unload cycles at each RPM setting starting at 500 RPM, then 1000 RPM and advancing in 1000 RPM steps to 8000 RPM. 

Ramp Head Loading Peak AE vs Seek and Spin Velocities with Negative Pressure 50% Slider

Ramp Head Loading Peak AE vs Seek and Spin Velocities with Tri-pad 50% Slider

This sequence was repeated five more times increasing the seek velocity by 0.5 IPS each time until 3.0 IPS was reached. The peak value of the AE signal was captured during every head-loading seek. The chart above shows the results. At low seek velocities and low spindle speeds there is little or no AE output. Increasing seek velocity or spindle speed produced increases in peak AE output indicating greater head to disk contact.
The same test was repeated using an HGA with a 50 % slider and a tri-pad air bearing. The chart below shows the results. At low seek velocities speeds there is little or no AE output. Increasing seek velocity produced increases in peak AE output indicating greater head to disk contact. However, increasing spindle speed had less of an effect. 

Contact Start Stop testing

The AE profile below of a 50 % tri-pad slider during a CSS cycle when using the RMS mode shows the customary rise and fall of acoustic energy as the head transitions from sliding to flying and vice versa.

AE RMS output during CSS cycle 50% - Tri-pad slider

The spectral energy during these two RMS peaks is shown below.

The natural frequency of the slider can be seen as a peak in response at 884 kHz. The band-pass filter separates this small band of frequencies from other noise and resonance frequencies in the system. The signal is then sent to the RMS detector and from there to the data acquisition system which displays and stores the data.

CSS testing of a 30% slider with a negative pressure air bearing

In the figure below the bottom trace (red) shows the AE RMS profile of a pico negative pressure slider during a CSS cycle. 

AE output during CSS cycle 30% - negative pressure slider

Oscilloscope image of the amplifier output during the takeoff transition

Little change can be seen in the RMS mode when the head begins to fly or land. However, the Peak detection mode trace (blue) shows sharp peaks at those same points indicating a sudden transition from sliding to flying. The spindle velocity profile (green) is shown for a three second ramp to 5400 RPM and a three second ramp down to a stop.
The frequency of this brief “ringing” is slightly above 1 MHz. The duration is too short to register any RMS energy, but the peak detector captures the peak value.

Conclusion

The new AE option on CETR HDI testers with Peak Detection represents the excellent means to evaluate low-flying and load/unload characteristics of the head-disk interface.

High Frequency Acoustic Emission Sensor

The original AE sensor has frequency response from 0.05 to 5.0 Mhz. 
Certificate of frequency response available upon request. Sensor weight is 2 grams.  Each proprietary high performance, light weight sensor is assembled by hand and is made of the new quality materials with the use of the latest technology. Their dimensions are diameter 6.8 mm the very tip 1 mm long, and diameter 6.5 mm the rest of the sensor, which is 5.5 mm long.

CETR offers the AE sensors and amplifiers either as a part of our tribology testers or as standalone. In the testers, the RMS / Peak Detect output is connected to the data-acquisition system.
Both the in-tester and standalone designs allow for easy connection, via a coaxial cable, of both the raw and RMS / Peak Detect outputs to oscilloscope or a spectrum analyzer. 

The standalone AE systems can be ordered in the following configurations:
1. Sensor with cable,
2. Small electronic box (112mm x 63mm x 30mm) which includes an amplifier with a customer-specified hardware filter,
3. Power supply (optional).