Browse Prior Art Database

Capacitive Servo Detector

IP.com Disclosure Number: IPCOM000047456D
Original Publication Date: 1983-Nov-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 4 page(s) / 77K

Publishing Venue

IBM

Related People

Klaassen, KB: AUTHOR

Abstract

The track density that can be obtained in today's magnetic disc files is mainly determined by mechanical limitations on the positioning accuracy of the recording heads (such as skew, backlash and thermal expansion). To avoid these mechanical problems, the positioning signal for the head should be derived from the same track as the data signal; the quantity to be measured is the relative displacement of the recording head with respect to a preformatted reference track. There are several ways to create such reference tracks: magnetically in the form of dedicated servo tracks buried under the data tracks, optically in the form of reflecting patterns, or capacitively in the form of conductive patterns.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 34% of the total text.

Page 1 of 4

Capacitive Servo Detector

The track density that can be obtained in today's magnetic disc files is mainly determined by mechanical limitations on the positioning accuracy of the recording heads (such as skew, backlash and thermal expansion). To avoid these mechanical problems, the positioning signal for the head should be derived from the same track as the data signal; the quantity to be measured is the relative displacement of the recording head with respect to a preformatted reference track. There are several ways to create such reference tracks: magnetically in the form of dedicated servo tracks buried under the data tracks, optically in the form of reflecting patterns, or capacitively in the form of conductive patterns. The detection method disclosed herein pertains to the detection of the position of a recording head in a disc file with respect to a pattern of conductive material deposited onto, in, or under the magnetic coating of a disc by means of one or more capacitive pick-up electrodes located on the head slider in close proximity to the recording head (or formed by the yoke of the recording head itself). The anticipated capacitance variations associated with the positioning of the head are very small. The parasitic capacitance (to ground) should therefore be kept small. This makes it necessary to locate the detection circuit close to the pick-up electrodes preferably on the slider of the recording head. An integrable detection circuit in the form of an IC is therefore imperative. To that end, the detector should be simple and may not employ components such as transformers, inductors, large capacitors, etc. The detection method presented here satisfies these requirements. The prior art uses impedance (capacitance) measurement techniques that rely on AC excitation of the capacitance which is to be measured. This gives the total value of the capacitance comprised of the desired capacitance changes due to the head positioning as well as the parasitic capacitance to ground and the change in capacitance due to flying height changes with track radius. Since the desired capacitance signals (order of magnitude 10-16 F) are small compared to the undesirable capacitance signals (respectively 10-12 F and 10-15 F), these impedance detectors must exhibit a very large dynamic range (ratio of largest to smallest signal). The three methods most used are: -RF Oscillator: This method employs an RF oscillator whose resonance circuit comprises the capacitance to be measured. For detection of small capacitance changes the oscillator frequency must be high. Current capacitive video recorders use a 1 GHz oscillator for this purpose. This necessitates the use of the stripline type of tank circuits and high frequency transistors. This detector therefore cannot be easily integrated. -RF Bridge: This method employs a balanced bridge circuit at high frequencies. The balancing is delicate and the frequencies involved are high (100 MHz-1 GHz) so tha...