Servo Control

Program Overview

Due to the presence of various vibrations in the hard disk drive (HDD) such as disk wobbling, air-flow induced suspension/slider vibration, spindle vibration and external vibrations, it is a great challenge for servo engineers to work out feasible servo control mechanisms which can keep track mis-registration (TMR) budget and provide a high positioning accuracy in nanometer level for the Read/Write head to maintain off-track capability. An advanced servo control system is developed to improve servo positioning accuracy and system robustness to achieve ultra-high track density needed in future high density magnetic recording.

Block diagram of a servo control loop.

nominal servo control is designed to guarantee stability and robustness, its design is a compromise between performance and robustness, feedforward control is designed to improve positioning accuracy without degradation of robustness, and intelligent algorithms are used for learning and updating of parameters online.

Research Focus

  • Develop a universal experimental platform to demonstrate servo performance. advantages with dedicated servo technology at drive level.
  • Servo control scheme and algorithms for dual-actuator disk drives.
  • High bandwidth control of dual-stage actuation system.
  • Identification and control of critical mechanical resonances above Nyquist frequency.
  • Active vibration control with self-sensing technology.
  • Control design and implementation with different configurations for millactuator and microactuator-based tri-stage actuation system.

Research capabilities

1) Vibration rejection

  • Narrowband vibrations at certain frequencies: peak filter, H2 optimal control, KYP Lemma, blending control
  • High frequency resonance & vibrations: KYP Lemma, phase-stabilized control, blending control
  • Known-frequency vibrations: adaptive feedforward, adaptive resonance compensation
  • Unknown vibrations: adaptive estimation and compensation

2) Dual-stage control

  • High bandwidth control: resonance compensation
  • Shaping sensitivity function: H∞ loop shaping
  • TMR optimization: LQG/LTR, H2 method
  • Reduce position error: peak filtering
  • Saturation control: anti-windup; optimal reset
  1.  Active vibration control with sensors: active damping, feedforward compensation
  2. Uncertain resonances: robust control, adaptive identification and control
  3. Nonlinearity (friction, bias): modeling, neural network
  4. Impulse disturbance: nonlinear filtering
  5. Limited sampling rate: multi-rate control, identification and control of resonances above Nyquist frequency
  6. Fast and smooth seeking: current profile/reference trajectory, time optimal control, optimal reset control, nonlinear control
  7. Drive level servo control system

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