Measuring Small
Gary Collins, P.E., Collins Consulting
Published in Computer Technology Review, Vol. XXV Number 3, 2005, pages 36-38
How do you measure movement as small as 1/100th of a human hair? Yet track widths and track movement of that size will need to be measured if tape cartridge capacities are to increase into the 1 to 10 TB areas. Track widths will have to be less than 1 micron. But before tape drives can achieve this, one must be able to accurately measure movements in the order of 0.1 micron in the track position, Figure 1. If you can’t measure it, you don’t know if you’ve achieved it.
Up to this time, such precision was not needed. Track pitches were wide, as much as 500 microns in early 3490s. The tape could wander laterally half the track width and the signal could be read without servos because the track was so wide compared to the reader head. Guiding was simple.
However, today tracks are typically 20 microns wide in LTO drives. Researchers are presently working on drives with track widths in the 5 to 1 micron region. For these small widths, tape wander, properly called Lateral Tape Movement (LTM), must be less than 1 micron. This is because the track following head always lags the moving target and produces a position error signal. If the position error signal gets to be larger than about 1/10 the track width, the track cannot be read. However, it follows that the smaller the LTM, the smaller the error is. This allows more tracks can be placed on the tape.
With the advent of precision tape drive paths such as the porous air bearing deck, which yields about .7 microns of non-repeatable LTM , the need for precision measurement of tape position and wander has arrived. The question is, what methods are suitable for such measurement and how practical are they?
Measurement Methods
A rule of thumb is that LTM should be about the same as the width of a track. If then, tracks of 1 micron are desired, LTM should be 1 micron or less. To measure this, the measurement system should have an accuracy of 1/10th the measured object, or in this case .1 micron.
Measuring small in the computer hardware field is just part of challenge. The advent of nanotechnology and microscopic machines in other fields makes the capability to do this even more necessary. Measurement of this sort may be possible now on a limited basis. The challenge is to do it cheaply, conveniently, routinely, and accurately. Therein lays the problem.
Four methods of measuring will be discussed. First is the Fotonic probe, a method using fiber optics. Second is the position decoding of the pre-written servo track. Third is writing a high frequency track next to a low frequency track and measuring the Hf/Lf ratio. The fourth is to measure the movement of the tape surface with a laser vibrometer.
Method 1: A Fotonic Edge Probe
This instrument by MTI of Albany NY gives tape edge displacement by measuring the amount of light blocked by the tape. The fiber optic probe directs a curtain of light past the measurement target edge to a receiving bundle, Figure 2. The intensity of light received changes with the edge position. This translated directly into microns of tape edge motion via the appropriate calibration setup.
The following calibration curve, Figure 3, is typical of the sensitivity one gets from the Fotonic probe.
From this curve one can see that the linear portion spans about 10 volts across 100 microns, or 10 microns/volt or 10 nm/mV. (MTI’s own literature claims 2.5 nm/mV across 100 microns. ) This is what is needed for submicron track location.
This instrument provides the sensitivity needed, but what about the resolution and bandwidth? After all, tape movement can be rapid enough that the track following servo cannot keep up. These are typically frequencies above 800 Hz. What, then, is the accuracy of the Fotonic probe for high frequency movement?
Tests of accuracy were conducted using a notched rotating disk that gave mechanically stepped square wave input, Figure 4. The tests showed that, with the 10K low pass filter, the Fotonic probe faithfully reproduced the input pattern up to 1500 Hz. Beyond that distortion of the step corners was evident.
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In practical terms of measuring tape edge quality or lateral tape motion in the 800 Hz to 1500 Hz region, a region of concern to tape drive developers, the MTI Fotonic probe would give the resolution and bandwidth needed to record the motion and waveforms. It is a very good choice. However, the MTI probe is expensive, and most labs can afford only one. If multiple measurements are simultaneously needed, an alternative is needed.
A low cost alternative is to use a common photo switch with appropriate noise reducing circuitry. This has been successfully used in various tape drive development labs including Carnegie Mellon University. Mountain Engineering II has developed a similar device to the place where it matches the resolution and low noise of the more expensive Fotonic probe and is cheap enough to locate multiple devices along the tape path, Figure 5.
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The disadvantage of this method is that it measures the edge of the tape, not the surface of the tape which carries the track. It follows then that if the edge of the tape were rough but the surface of the tape moved along smoothly due to good guiding, edge sensors would measure roughness that one might misinterpret as lateral tape motion.
Note: the following is from another report that shows how I generated the square wave that was “read” by the probes.
Test method
Using the notched wheel shown, each sensor was given an input profile of defined steps. The frequency was varied by increasing the rpm. The output was compared to the known input.
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Increasing the disk rpm mechanically increased the alternating pattern frequency until pattern detail became unclear. This happened beyond 1500 Hz.
Contact Segway Systems, Littleton CO, for test results. www.segwaysystems.net
www.mtiinstruments.com
Done at Mountain Engineering II, Inc. in Longmont CO , www.MountainEngineering.com
Contact Dr. Jonathan Wickert of CMU at wickert@cmu.edu
Contact Mountain Engineering II for specifications and availability. sensor@MountainEngineering.com
