302 終端状態制御によるハードディスクのショートシーク制御(精密・情報機器の制御I)(OS 精密・情報機器の制御)

This paper discusses an improvement of final-state control (FSC). FSC is a control method for transferring the state of a dynamic system to specified states at a specified terminal time by using an external input. So far, it is mainly applied to motion control problems such as servo control of hard disk drives. However, FSC cannot deal with time-varying specified states. For example, when an operator pushes a cart and attempts to connect with another stopped cart without any stop, the stopped cart should accelerate to the same velocity as the running cart within a specified time. Here, the velocity of the running cart is time-varying, and a control method which can deal with such time-varying specified states is required. In this paper, in order to deal with the time-varying states, an updating final-state control (UFSC) technique is introduced. UFSC updates the initial and ideal final states at every sample. However, the equation of UFSC algorithm includes inverse matrix computations and the computation process is complicated. Moreover, most computed inputs will not be applied because of updating. So, in order to reduce the computational load, this paper also introduces a device to extract the requisite computation for simple controlled models. The effectiveness of the proposed method is verified by simulations in a case of connection control problem of two carts.

For information devices such as hard disk drives, actuators must be controlled as quickly and precisely as possible. The design of the feedforward input is very important in satisfying these requirements. Therefore, we have proposed design methods based on final-state control so as to reduce its spectrum at the desired frequency points. Furthermore, the final-state control method has been extended to generate the feedforward input using a polynomial. By using a polynomial, the required memory size can be drastically reduced because only the coefficients of the polynomial must be stored. However, when the step number of the input increases, the method may not produce a correct solution because of a numerically ill-posed problem. In this paper, we propose a design method for the final-state control based on a time-symmetric polynomial input. The simulation results show that the proposed method contributes to the improvement of not only the numerically ill-posed problem but also the performance.

High-precision stages require high-speed and high-precision control to improve their production throughput and quality. However, it is expected that their motion speed and accuracy will reach a limit in the near future if the structure of the conventional high-precision stage is used. Therefore, the authors designed and fabricated a stage called the catapult stage which has a decouplable structure consisting of a fine stage and a coarse stage. This stage is different from conventional dual stages in which the fine stage would be disturbed by the coarse stage since they contact with each other. This paper proposes a novel control system design for the catapult stage, and a settling time shortening control method using final-state control (FSC). So far, FSC is mainly applied to the applications such as hard disk drives whose initial states are the zero. However, it is important to consider the initial states for the catapult stage since the initial position, velocity and acceleration of the catapult stage are not equal to zero. Simulations and experimental results demonstrate the effectiveness of the proposed methods.

In this paper, the authors develop a new design method of the reference trajectory for hard disk drives. The control input is assumed to be generated through zero-order-hold and the reference trajectory is designed so that the frequency component of the control input is minimized over the desired frequency points. Input constraints can also be incorporated into the design. Simulation results are shown to evaluate the effectiveness of the proposed method.

This paper proposes a new design method of track-seeking trajectory for hard disk drives by using final-state control method. The proposed method can obtain a feedforward control input whose frequency components are minimum at the desired frequency points. In our approach, the control input is evaluated in continuous-time domain, and the frequency components higher than the Nyquist frequency can be also optimized. Simulation and experimental results are shown to evaluate the effectiveness of the proposed method.

In hard disk drive (HDD) servo systems, the short-span track seeking control may be viewed as a reference tracking problem. However, the perfect tracking control (PTC) is difficult to realize, because the voice coil motor (VCM) has a double-integrator behavior with a relative degree of two and it may also have nonminimum-phase (NMP) zero. These characteristics prevent the plant model inversion for the PTC design. A dual-stage HDD servo system uses VCM as the primary actuator and places a secondary micro-actuator piggyback on the VCM. This paper proposes a two-degree-of-freedom (2DOF) control scheme for fast short-span seeking such a system. The control design utilizes the parallel structure of two actuators to relax the restriction of plant order and NMP zero for perfect reference tracking. Experiment results show that the time of short-span seeking can be greatly shortened with the proposed 2DOF control design with dual-stage actuators.

For information devices such as hard disk drives, actuators must be controlled quickly and precisely as much as possible. To satisfy these requirements, feedforward input design is very important. For this problem, we have proposed a frequency-shaped final-state control method. This method can obtain a feedforward input that has lower spectrum at desired frequency points so as to reduce residual vibration after positioning. Furthermore, the method has been extended to generate the feedforward input by a polynomial. By using the polynomial, the required memory size can be drastically reduced because it only requires to store the coefficients of polynomial. However, in the frequency-shaped final-state control method by polynomial input, it is not guaranteed that the solution always exist theoretically. In order to solve this problem, final-state control methods in which the feedforward input is generated by both of a polynomial and time-series data are proposed. The effectiveness of the proposed methods are verified by simulation using the plant model of the HDD benchmark problem. The simulation results show that the proposed method outperform the conventional methods.

A time optimal based discrete-rime sliding mode control (DSMC) approach is developed in this paper. In this approach, a proximate time optimal switching curve is adopted for the time varying sliding mode design and a controller, consisting of an equivalent part and a discontinuous one, is designed such that the trajectory from any initial point is driven into a sliding region in the vicinity of the switching surface without chartering and thereafter remains inside it. By using the time-varying switching surface, one unified framework for both track seeking and track following control in hard disk drives (HDDs) is provided and the smooth transition from seeking control to track following control is achieved. In addition, a time-varying sliding region is also used instead of rime-invariant one to eliminate the chattering and overshoot and improve the tracking performance. Simulation studies in HDDs demonstrate that the proposed control scheme provides better performance during track seeking than the commonly used discrete-time proximate time-optimal servomechanism (PTOS) and sliding mode proximate time optimal servomechanism (SMPTOS). Moreover, the unified controller achieves better disturbance rejection in track-following than the SMPTOS control.

In this paper, novel multirate two-degree-of-freedom controllers are proposed for digital control systems, where it is restricted that the speed of the A/D converters are slower than that of the D/A converters. The proposed feedforward controller assures the perfect tracking at M inter-sampling points. Next, the proposed method is extended to systems with time delay. The proposed scheme is applied to the seeking control for hard disk drive, and advantages of this approach are demonstrated.

In this paper, we propose a new multi-rate state estimator for seeking control of hard disk drives, which has a proportional, integral, and discontinuous error feedback structure to improve robustness. Simulations and experiments have been carried out to validate our proposed multi-rate state estimator. Finally, an experimental example of seeking control for a single stage actuated hard disk drive is demonstrated.

Seek control of hard disk drive head via reference goveror

To meet an increasing demand for high performance and large-capacity in magnetic hard disk drives, both fast response and precise positioning are strongly required for the head positioning control. A mode switching control (MSC) system, which includes a track seeking controller, a track following controller, and a switching function, is widely used to meet this requirement. The issue for MSC is to confirm a design method of servo mode switching. This paper proposes the initial value compensation method (IVC). The IVC inputs certain initial values in the controller state variables at mode switching in order to improve the transient response after switching. There are two approaches to determine the values: (1) minimizing a linear-quadratic function of the plant state variables; and (2) cancelling the unfavorable poles of the transfer function between initial values and head position by relocating zeros. Some experimental results are shown to be effective.

In Hard Disk Drives, there are two seek modes. One is a short distance seek mode, the other is a long distance seek mode. There is not saturaliton in the short distance seek mode, so many methods were proposed to realize a low acoustic noise and low vibration seek. However, in order to realize a low acoustic noise seek in Hard Disk Drives, the long distance seek mode is very important. So, a new method for the long distance seek is proposed in this paper. In conventional methods, a reference velocity curve is used. This reference velocity curve can be considered a switching line in the phase plane. In the proposed method for a long distance seek, the sliding mode controller with a frequency-shaped switching line is designed for a feedforward controller. The advantage of this method is a hign frequency ingredient in the feedforward input can be eliminated by setting the frequency-shaped switching line. The validity of the proposed method was confirmed by numerical and experimental results using a miniature 2.5-inch Hard Disk Drive.