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Automatic Notch Frequency Control for Stabilization

IP.com Disclosure Number: IPCOM000087775D
Original Publication Date: 1977-Mar-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 4 page(s) / 65K

Publishing Venue

IBM

Related People

Palmer, RS: AUTHOR

Abstract

Servo positioning systems are often limited in bandwidth by mechanical resonances occurring in the mechanical portions of the system. In the case of a head positioning servo system for a disk file, such resonances are usually several kHz in frequency, requiring closed loop bandwidths to be limited to a few hundred Hz. Fixed filters have not been successful in removing these resonances from the servo systems because the resonance frequency changes with head position. A typical mechanical transfer function for such a system is shown in Fig. 1A.

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Automatic Notch Frequency Control for Stabilization

Servo positioning systems are often limited in bandwidth by mechanical resonances occurring in the mechanical portions of the system. In the case of a head positioning servo system for a disk file, such resonances are usually several kHz in frequency, requiring closed loop bandwidths to be limited to a few hundred Hz. Fixed filters have not been successful in removing these resonances from the servo systems because the resonance frequency changes with head position. A typical mechanical transfer function for such a system is shown in Fig. 1A.

Fig. 2 shows a head positioning system in which a tracking notch filter system automatically tracks shifting resonance frequencies and ensures stability of the servo loop by having an attenuation factor greater than the magnification
(2) factor of the resonance. The notch filter system 1, which comprises a tunable notch filter 19 and a frequency discriminator 12, can advantageously be the oscillating notch filter described on pages 3858-3859 of this issue.

The mechanical moving parts are schematically illustrated as a linear electric motor 2, a head carriage 3, and a head 4. Mechanical resonance is represented schematically by a spring 5. A position error signal representing the position error between head 4 and a position reference, for example a servo pattern on one of the disks, is generated by position detect circuits 6. Assuming that the notch filter 19 is tuned to the resonance frequency, in a manner to be described, the position error signal is filtered to remove the resonances and applied to compensation and power amplification circuitry 7. The output of circuit 7 is fed back by way of summing junction 8 to motor 2 to maintain the heads at a position determined by the position reference.

In order for the notch filter to track the resonance, the resonance frequency F(R) must be continuously monitored by an external accelerometer or tachometer, or by recovering F(R) from the position error signal itself. To this end the head position error signal which will include a resonance frequency component, is applied to bandpass filter 9 having a transfer function as shown in Fig. 1B.

Any mechanical resonance component which is present will appear at the filter 9 output as in Fig. 1C. The resonance frequency F(R) is limited at 10, squared in 11 and applied to frequency discriminating circuitry 12, in particular to a single-shot 18. A...