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This paper proposes a new algorithm for estimating the resonant frequency of adaptive notch filters used in servo systems. Notch filters and adaptive notch filters are widely used in commercial servo systems for suppressing a resonance which is a major obstacle in improving their performance. However, the conventional frequency estimation algorithm gives a dynamic behavior that is proportional to the difference between the square of the estimated frequency and the square of the actual frequency. This can cause the estimation dynamics to be too slow for low-frequency resonances, if both low- and high-frequency resonances are present and if the estimator gain is designed for a high frequency. This paper develops a new algorithm to give a dynamic behavior that is proportional to the difference in the estimated frequency and actual frequency. This allows selecting the estimation parameters independent from the value of resonant frequencies. The developed algorithm is implemented to a production servo controller and applied to a production printed-circuit-board inspection system. The experimental results show that the developed algorithm is much more effective in suppressing the resonances of both low- and high-frequencies, compared to the pervious algorithm. Furthermore, the ability to suppress vibration allows increasing the feedback gain, which in turn allows improving the tack time performance from 190 ms to 100 ms. All experiments reported in this paper were performed in an actual industrial environment using a production system, and the developed algorithms are applied to the production system.

In this paper, we propose a new control scheme that provides position and velocity profile tracking control for next-generation servo track writing (STW). Whereas conventional servo track writers require controllers that perform fast positioning control with fast track seeking and regulation, spiral servo track writers require accurate position and velocity profile tracking control to achieve high quality servo patterns on the media disk. Because STW timing eventually renders geometrically accurate servo patterns, both position and velocity error signals should be regulated within small bounds in a constant velocity region. Regulation via an integral sliding mode controller (SMC) is known to provide good tracking performance; however, use of a high switching gain is inappropriate for an actuator with resonance modes. In this paper, we therefore apply integral sliding mode control with a disturbance observer to STW. The relationship between eigenvalues and control gains is mathematically analyzed to improve dynamic tracking response. To verify the utility of the proposed position and velocity profile tracking control, we perform a comparative study between the proposed and conventional control methods and experimentally validate the performance of the proposed method.

As being independent of the magnitude of reference input signal, the conventional concept of servo bandwidth stemming from electronics fails to reflect the ability of the multiple degree-of-freedom (DOF) mechatronic system to perform vibration control. Considering the magnitude and frequency of reference input signal, a novel definition of mechatronic servo bandwidth of the Stewart platform-based active vibration isolator for a super antenna is proposed firstly. Then its mechatronic servo bandwidth is theoretically evaluated according to electrical and mechanical performances by using an optimization method. Experiments were conducted on the Stewart platform prototype, and the results validate the definition of and solution to the mechatronic servo bandwidth. From the application point of view, the mechatronic servo bandwidth can be employed to measure the manipulating rapidity of a multiple DOF mechatronic system and the range of vibrations that an isolator is able to overcome. HighlightsTo describe the rapidity of multiple-DOF mechatronic system, we presented a novel definition on mechatronic servo bandwidth of Stewart platform.We developed a general method of analytically determining the mechatronic servo bandwidth according to the dynamical formulations.We determined the mechatronic servo bandwidth analytically and experimentally.The results mentioned above agree with each other.

The bit-error-rate performance of free space optical communication systems with binary phase shift keying modulation and coherent homodyne detection is performed. Besides the turbulence-induced wave-front phase error and the amplitude fluctuation, the servo bandwidth of adaptive optics system is investigated. It is shown that Greenwood frequency is large enough for the servo bandwidth of adaptive optics system when the detected photons per bit are more than 100. However, if the photons per bit are less than 70, the Greenwood frequency is only sufficient for weak scintillation. We should increase the servo bandwidth to almost twice the value of Greenwood frequency at least in order to obtain an acceptable BER performance when the scintillation index is larger than 0.7. In addition, we also investigate the aperture averaging effects when the receiving aperture is larger than the coherent length.

To increase a servo bandwidth of a head-positioning control system in hard disk drives, a triple-stage actuator system was developed with thermal actuators. This positioning control system consists of a voice coil motor (VCM), piezoelectric (PZT) actuators, and a thermal-positioning actuator. The thermal-positioning actuator is good for control in high frequency range because it has little negative impact caused by mechanical resonances. However, the thermal-positioning actuator causes flying-height fluctuations that may lead to worsening of the magnetic recording performances. To overcome this issue, this paper presents a control system that employs the feedforward control scheme on a thermal flying-height control system. Simulation results showed that the proposed method was able to compensate for the flying-height fluctuations during the track-following control by about 87 %.

The accuracy of the servo system is dependent on the speed or band width of the closed loop control system. To achieve the high speed closed loop operation, hardware oriented control is preferred and it is achieved by means of an ASIC (Application Specific Integrated Chip). The servo system is used to drive a two axis robotic arm moving in a plane where two PMSM motor with servo control provides the required motion. Robotic applications require quick and accurate changes in driving torque so that the required motion behaviour is attained. Quick change in operating flux is required for fast and accurate servo control. Conventional scalar control drives cannot achieve quick response time and hence vector control is employed. The IRMCK201 ASIC used in the application has the Vector Control algorithm hard wired into it for complete servo control of a single PMSM. It takes the position and current feed back by using shaft encoder, hall sensor and IR2175 Current sensing IC. It can set up communication with a host controller through either high speed serial, SPI or Parallel protocols. The inverter unit is implemented using an Intelligent Power Module (IPM) and it is driven by the Space Vector Modulation signals from IRMCK201. The work shows that highly efficient servo drive can be implemented using IRMCK201 and Host controller with PMSM.

In this paper, we propose a new control scheme, position and velocity profile tracking control for new generation servo track writing (STW). While conventional servo track writers need controllers that perform fast positioning control with fast track seek and regulation, spiral servo track writers require accurate position and velocity profile tracking control for high quality servo patterns on the media disk. Since STW timing eventually renders geometrically accurate servo patterns, in constant velocity region both position and and velocity error signals should be regulated within small bounds. It is known that regulation via integral sliding mode controller (SMC) provides a good tracking performance, but using a high switching gain is not appropriate for a system having resonance modes. In this paper, thus we apply sliding mode control with disturbance observer to STW. To verify utility of the proposed position/velocity profile track control, we conducted some experiments using a state-of-the-art STW system. Comparative study with the conventional control method was made. The performance of proposed method was experimentally validated.

This paper presents a mode switching control (MSC) scheme in discrete-time domain for fast and precise set-point tracking in servo systems subject to control saturation and unknown disturbance. The basic idea is to combine the proximate time-optimal servomechanism (PTOS) and the composite nonlinear feedback (CNF) control, using the output position as the only measurable information for feedback. The PTOS is responsible for fast targeting in servo systems when the tracking error is large, and once the system trajectory enters into some specified region, the CNF will take over the control to ensure a smooth settling without compromising the fast transient performance. A reduced-order extended state observer is adopted to estimate the speed signal for feedback and the disturbance for compensation. The asymptotical stability of the proposed MSC scheme is analyzed and the switching conditions are provided. Simulation and experimental results on a permanent magnet synchronous motor (PMSM) servo system verify that the proposed control scheme is effective in improving the tracking performance for a wide range of target set-points.

This work proposes a novel strategy for middle frequency resonance detection and reduction for the speed control of industrial servo systems. The report includes an analysis of the drawbacks of the traditional resonance reduction method based on an adaptive notch filter in the middle frequency range, and the main drawback is summarized as the difference between the resonance frequency and the oscillation frequency. In the proposed method, a self-tuning low-pass filter with a corner frequency determined using FFT results and several self-tuning rules is introduced in the speed feedback path. Consequently, the effective range of the adaptive filter is extended across the middle frequency range. The simulation and experimental results show that the frequency detection is accurate, and the resonances are successfully reduced during steady-state and dynamic speeds.

Archival demands for digital tape storage systems require smaller spacing between data tracks on the magnetic tape to allow for more data storage on a single cartridge. Increasing track density while maintaining track following provides a challenge for the servo control of the tape drive. In particular, interchangeable tape cartridges and time-varying tape reel diameters during operation cause variable disturbances during servo control. In this paper we show how a tape servo control algorithm can be regulated to handle changes in disturbance dynamics during servo operation of the tape drive. The approach is based on adaptive regulation by exploiting a Youla-Kucera parameterization of the servo controller and an explicit minimization of the Position Error Signal (PES) during servo operation in real-time. Theoretical results on the stability of the feedback system and real-time application results on an Linear Tape-Open (LTO) drive are included in the paper. The result show a significant reduction of the variance of the PES over different tape cartridges and a constant PES variance during a complete reel-in operation of the tape drive.

Design and implementation of servo controller for a hydraulic servo system with a load of large inertia, low stiffness and multi-stage resonance is considered. To achieve a set of desired requirements, this paper is divided into three parts of work. First, notch filters and peak filters extracted from the general phase stabilized controller are applied in order to restrain the resonance peak of the load. Second, a disturbance observer in the feedback path is used for rejecting low-frequency disturbance and high-frequency noises. Third, as the complete inversion of the closed-loop dynamics at the whole frequency band does not necessarily give the best performance, low-pass filter, proportional derivative (PD) compensation, and lead correction are adopted in the feedforward path. The work proposed here overcomes the challenge of improving amplitude margin and phase margin while eliminating resonance simultaneously. The effectiveness of the combined control is investigated through simulation and experiment results, which show that the control strategies raised here successfully complete resonance rejection and also give ideal performances that meet the indices.

In a tape drive, the head assembly hosting servo readers and data writers/readers must initially be moved to a target position, so that information can be written to or read from the desired tracks. In tape systems with flangeless rollers, as recently introduced in IBM’s tape drives for improved performance and extended drive and tape lifetime, the initial positioning of the servo readers over the servo bands, from which the position-error signal for steady-state track-following operation is obtained, has to be fast. This requirement is dictated by the nonnegligible lateral tape motion experienced in tape paths with flangeless rollers. In this paper, a method is proposed to utilize both the servo readers and the data readers in a drive to detect the presence of a valid servo signal. If the servo readers are not positioned over a servo band, the distance between servo readers and servo band can thereby be continuously estimated by identifying the data reader detecting the presence of a servo signal. This information is then provided to a control element that controls the joint operation of a coarse stepper motor and a voice-coil motor based fine actuator with large stroke in a feedback or feed-forward configuration to achieve fast servo reader positioning. In the feedback-based approach, all data channels are continuously monitored to detect the presence of a valid servo signal and a pseudo position-error signal is generated and fed back to the control unit to drive the actuator towards the target position. In the feed-forward approach, a trajectory for the actuator is determined as soon as the servo band is in the capture range of the fine actuator and a control signal is generated so that the servo reader rapidly lands over the servo band with optimum control effort.

Motion synchronization of servo systems carries great importance in various precision manufacturing operations. For double-sided (duplex) milling of thin plates, in particular, the cutting tools must engage with both sides of workpiece in a precisely synchronized manner to avoid forced vibrations. A continuous sliding mode controller (SMC) is developed to deal with the problem of position synchronization for dual spindle servo systems. Synchronization error is defined as the differential position error between the two servo systems that follow same reference motion command. Hence, the developed continuous SMC penalizes 3 error states; namely the individual servo positioning errors, and the synchronization error between them. As a result, individual trajectory tracking as well as motion coupling objective is introduced. The final control law is implemented in the form of a PID controller in real time, and double-sided milling experiments are performed to verify its effectiveness.

The positioning accuracy achievable by track-follow servo for flexible tape media is investigated using advanced concepts for several elements of the system. First, the servo pattern geometry in the timing-based servo system is designed in conjunction with a digital synchronous servo channel to minimize the measurement error in the position-error signal. Second, a flangeless tape path is introduced to mitigate high-frequency components of the lateral tape motion. Finally, a track-follow controller is designed based on the H ∞ approach, taking into account the disturbance and noise characteristics of tape path and servo-channel and the measured plant transfer function. Combining these elements with a new high SNR magnetic tape based on perpendicularly-oriented BaFe particles, we demonstrate a closed-loop track-follow performance with a standard deviation of the position-error signal of less than 15 nm.

In a tape drive, the head assembly, which hosts servo readers and data writers/readers, must initially be moved to a target position, so that information can be written to or read from the desired tracks. The positioning of a servo reader over a servo band, from which the position-error signal for steady-state track-following operation is obtained, has to be fast, because of the moving-target nature of the problem. This is especially true in the presence of non-negligible lateral tape motion, as found in tape paths with flangeless rollers. In this paper, a method is proposed whereby not only the servo readers but also the data readers in a drive are used to detect the presence of a valid servo signal. If a servo reader is not positioned over a servo band, the distance between servo reader and servo band is continuously estimated by identifying the data reader detecting the presence of a servo signal. This information is then provided to a control element, which determines a trajectory for the actuator and outputs a control signal so that the servo reader rapidly lands over the servo band with optimum control effort. A realization of the proposed method is illustrated by simulation as well as experimental results.

This paper takes into consideration the rejection of narrow-band disturbances at two frequencies higher than servo bandwidth in order to obtain good positioning accuracy for a micro-actuator. A phase stabilized feedback controller combined with a disturbance observer structure is applied to achieve the rejection of the two disturbances. A high frequency bandpass filter is adopted in the disturbance observer in order to reject the high frequency disturbance, and simultaneously the stability of the feedback control loop is remained when the disturbance observer is added on to the external stabilized controller. The effectiveness of the combined control is investigated through simulation and experiment results, which show that this control scheme can reduce position error significantly and is better than using phase stabilized feedback control only or proportional-integral-derivative (PID) controller with notch filters.

Generally, tracking control for optical disk system is accomplished by the feedback control designed by using PID control, repetitive control and so on. However, it is difficult to reduce the tracking error on condition of high disk rotation speed. In the near future, the disk rotation speed of the optical disk system becomes higher than 10000[rpm]. Therefore, the tracking servo system for optical disk system must have the new structure and highly performance. This paper proposes a new free parameter and desgin method of feedback servo system, which suppresses the high speed periodic disturbance. It has the widebandwidth and high gaim margin. Moreover, in order to suppress this periodic disturbance, this paper proposes a new control structure of robust feedforward servo system, which includes the prediction of tracking error, a new design method of disturbance observer and the zero error phase tracking control. The experimental results point out that the proposed system well regulates the high precision tracking servo control on condition of the disk rotation speed 8000[rpm], which is equal to CD system of 40 X speed.

This paper presents a digital servo driver that realizes a novel multirate feedback controller based on position, velocity and acceleration feedback. The velocity and acceleration signals are firstly estimated by state observer using optical encoder information, and the estimated velocity is fed into discrete disturbance observer (DDOB) to estimate and compensate the external disturbance. In this scheme, controller with acceleration feedback can be realized by replacing the current loop with acceleration loop. When the DC servo motor is controlled by the proposed acceleration feedback control with disturbance compensation, the total servo system from acceleration to position becomes the acceleration controlled system which is fixed to a nominal double integral dynamics in the presence of parameter variation and torque disturbance. Hence, the fast and precise position control can be carried out easily. The proposed acceleration feedback controller and PD position feedback controller are evaluated experimentally on a DSP, which is implemented multirate control scheme, controlled DC servo motor positioning system. The experimental results show that this digital servo system is robust and remarkably sustains the same performance compared to the single fast rate control scheme.

This paper is concerned with disturbance rejection for data storage systems which applies microactuator to expand its servo bandwidth. An approach based on the generalized Kalman-Yakubovic-Popov (KYP) lemma together with the Youla parameterization is proposed which allows us to suppress disturbances of particular frequencies within or beyond the servo bandwidth via convex optimization. The rejection of narrowband high-frequency disturbances is achieved by using this approach. Simulation together with implementation results demonstrate the simplicity and effectiveness of the design method as compared with the traditional frequency weighting method.

We present a new MEMS-based milli-mirror for precise tracking in high-density optical disk drives (ODDs). The device consists of a torsionally suspended mirror plate, one pair of torsion springs, which support the mirror plate and offer a restoring torque, and two pairs of electrodes attached to the mirror plate and glass substrate. The dimensions of mirror plate and torsion springs were determined so that a 5 V dc bias ±4.5 V ac drive voltage would provide the mirror with ±0.02° rotation to transmit laser beam spot on spinning disk. The MEMS-based milli-mirror was successfully fabricated using MEMS technology. Displacement–voltage linearization scheme was implemented by differential voltage driving. The static and dynamic performances of mirror prototype, such as capacitance versus driving voltage, rotation angle versus driving voltage, and resonant frequency were characterized and compared well with the simulation solutions. The mechanical resonant frequency of the mirror is expected to be high enough to satisfy the requirement of the servo bandwidth of precise tracking-control in high-density blue-laser optical disk drive.

To achieve frequency separation between actuators of a dual-stage servo actuator system in a hard disk drive, the PQ control design method can be used. In the PQ method, the servo controller is designed by focusing on structural interference of the actuators. This paper provides a specific parametrization for fixed order feedback controllers used in the PQ-method for which the structural interference between actuators can be minimized. The proposed control design method is applied to a dual-stage instrumented suspension, where the additional sensor of the instrumented suspension is used to attenuate high frequency disturbance caused by windage turbulence in a hard disk drive.

We consider in this paper the design and implementation of a dual-stage actuated hard disk drive (HDD) servo system, in which an additional piezoelectric actuator is mounted on top of the conventional voice-coil-motor (VCM) actuator to provide a faster and finer response. More specifically, the dual-stage actuated HDD servo system consists of a primary VCM actuator and a secondary suspension piezoelectric microactuator. The VCM actuator is used to quickly move the read/write (R/W) head of the HDD servo system to a target track, whereas the microactuator is used to fine-tune the R/W head position when it is getting closer to the target. In our design, the primary actuator is controlled by a newly developed composite nonlinear feedback (CNF) control law, which has fast rise time and small overshoot, and the microactuator is controlled through a simple static gain together with a low-pass filter and a notch filter. Simulation and implementation results show that in track following, the dual-stage actuated HDD servo system has outperformed the single-stage actuated counterpart by more than 47% in settling time and by 39% in position error signal (PES) tests.

A new thin-film PZT micro-actuator bonded to a stainless steel (SUS304) suspension for positioning a magnetic head for high-density hard disk devices has been designed, fabricated and investigated. This PZT micro-actuator in dual-stage serve system was made using a sol–gel technique deposited thin-film PZT and applying reactive ion etching processes to fabricate the micro-actuator. The SUS304 cantilever beam integrated with the PZT micro-actuator was also simulated in order to test the driving mechanics. The novel piezoelectric multi-layer micro-actuators possess a useful compromise performance in displacement, resonance frequency and generative force. The results reveal that the new design concept provides a valuable alternative for multi-layer piezoelectric actuators. When a double-layer thin-film PZT micro-actuator was used, an applied voltage of ±15 V was sufficient to obtain 0.9866 μm tip displacement of the cantilever beam and high-resonance frequency over 16.67 kHz, yielding the required servo bandwidth (13 kHz) of several kHz.

A fast tool servo is designed and tested to obtain sub-micrometre form accuracy in diamond turning of flat surfaces. The thermal growth spindle error is compensated for real time using a fast tool servo driven by a piezoelectric actuator along with a capacitive displacement sensor. To overcome the inherent non-linearity of the piezoelectric actuator, Proportional Integral (PI) feedback control with a notch filter is implemented. Besides, feed-forward control based on a simple feed-forward predictor is added to achieve better tracking performance. Actual machining data are discussed in detail to prove that the proposed fast tool servo is capable of fabricating flat aluminum specimens of 100 mm in diameter to a form accuracy of 0.10 μm in peak-to-valley error value.

As the recording track density increases in hard-disk drives, the off-track error-rejection capability of the servo controller must also be increased. The high rejection capability is achieved by increasing the servo bandwidth; however, the ability to extend the bandwidth is limited by the actuator's structural resonance. One particular resonance mode of interest is the so-called ‘butterfly mode’ where it exhibits as the primary resonance with a 180° phase in the structural (open-loop) transfer function plot. The factors influencing the butterfly mode were investigated using finite element analysis and confirmed by prototype testing. Although the mass-to-stiffness ratio in bearing cartridge pivot and the sway mode of the actuator structure by themselves were at a high enough frequency range, it was observed that the coupling effect in the pivot and actuator structure limited the frequency that it can be achieved. With the knowledge gained from the analysis, it is now possible to design an actuator to enable the high servo bandwidth required to follow track densities of more than 50 kilotracks per inch (kTPI) without the need for a dual-stage actuator.

This paper proposes a modification in the decoupled master-slave dual-stage hard disk drive (HDD) servo control configuration to enhance its stability when the secondary actuator is saturated. This method utilizes the special structure of the decoupled master-slave dual-stage control design and uses a nonlinear secondary actuator model instead of the usual linear one to predict its output and decouple the dual-actuator loop. This technique relaxes the displacement range requirements on the secondary actuator, and also greatly improves the stability of the servo system during the actuator saturation.

The critical factor for the hard disk manufacturers when designing the new generation of high capacity hard disks is `areal density'. Although linear density has increased dramatically increases in track density are limited by servo bandwidths, which are controlled by the actuator resonance. An approach, which is currently being investigated by the researchers, is the use of a second high bandwidth actuator at the end of the conventional voice-coil actuator. After theoretical analysis and discussions with industry electrostatic design in which the microactuator will sit on the slider between the suspension and the read/write head is selected. The design factors are: higher servo bandwidth, higher resonant frequency and low voltage supply. To microfabricate such microactuator we propose to use the latest technology called microelectromechanical systems technology. After schematic drawing layout design is made, it has been tested for electrical connections in the electrical layout model and the optimized design parameters are obtained. Finally it is shown that the design will satisfy the required specifications.

Perhaps the most often utilized means of closed-loop control of a servo system is proportional-derivative (PD) control. Linear analysis methods suggest the best tracking performance is achieved at maximum possible proportional and derivative gains. Maximum gains, however, drive the actuators into saturation, which renders the system nonlinear and the linear analysis invalid. This paper investigates the effect of actuator saturation on servo system tracking performance by formulating a frequency-based tracking performance measure roughly equivalent to the linear system −3 dB bandwidth. The proposed measure utilizes a series of band-limited pseudo-random tracking inputs to characterize the ‘bandwidth’ of the (nonlinear) saturating system. Numerical simulations based on this measure show that, for a servo system that exhibits actuator saturation, the best tracking performance is not achieved at maximum gain. Instead, performance improves up to a given gain, then begins to recede as the gain is increased further. The simulations also show that avoiding actuator saturation to ensure linear behavior significantly sacrifices tracking performance. The measure of tracking performance is compared with the −3 dB bandwidth utilized in linear analysis techniques, and the two are shown to be well correlated.

This paper proposes a new design method of the type-1 deadbeat servo system, in which the output settles to a command signal in a finite number of control steps, for linear digital systems. By using a virtual command signal in the period of the transient response, new degree of freedom is induced. Even when the integrator is added to the controlled system, the minimum settling time does not increase. Furthermore, two theorems are induced to obtain the general form of the type-1 deadbeat servo system.

An important aspect of linear controller design is to clarify whether the design reaches the performance limit. Based on an industrial project-a high precision three-axis gyro test table (servo system), a paradigm is presented to demonstrate that the performance limit of the disturbance attenuation design of the servo system can be reached in a systematic way by using the μ -synthesis. Moreover, this paper reveals that the performance limit is determined by the bandwidth. This finding will help the designer to judge whether any given performance specification is achievable.

In various kinds of servo positioning systems, both fast movement and precise positioning are required. To meet these demands, a mode-switching control (MSC) method, which has several controllers for each requirement and switches from one to the other, has been proposed. The issue raised by the use of this technique is the transient response after switching. This paper proposes the use of the initial value compensation (IVC) method to handle this problem. Under the IVC, initial values are placed in the controller state variables, or an additional input is added at switching. From experiments using hard disk drives, it is proved that the IVC improves transient responses after switching.

An algorithm to design servo systems for future magnetic disk drives has been developed. An optimal relationship between the trackwidth and the servo bandwidth of a magnetic disk drive which minimizes the limit signal S/N ratio is proposed which takes into account position signal noise and mechanical vibrations. The idea of required accuracy distribution is introduced. A dual-stage actuator with a single-input single-output (SISO) system is used to confirm the calculated results. This paper describes a method for obtaining the desired servo signal quality and introduces a relationship between trackwidth and servo performance that can make it possible to design head positioning systems for trackwidths under one micron.

When driving a lightly damped system with a servo motor, the design of a controller that reduces disturbance-excited vibration can be undermined by the need to maintain the system bandwidth to achieve good tracking performance. The use of classical lag/lead compensation and conventional filtering techniques to dampen the oscillatory modes often has adverse effects on the system bandwidth. This is the case with precision hydrostatic fluid-bearing slide systems. A control scheme designated as “directional damping control” is developed using classical root locus and frequency response techniques for systems with a lightly damped mode at a frequency considerably higher than the servo bandwidth. The scheme separates the design of the system bandwidth from that of the damping of the highly underdamped mode. The design guidelines are first presented for the general case, followed by a design example of a slide motion controller for precision machining. The scheme is compared with conventional PID control and the H ∞ optimal disturbance rejection controller. The procedure produces a vibration reduction controller similar in nature to the H ∞ optimal solution, but with some practical advantages. Using the scheme, the slide vibration is significantly less than when using conventional PID control, and a corresponding improvement is seen in the surface roughness of machined parts.

In this paper, the position control of an electrohydraulic servo system under both large inertia variations and substantial external loads has been studied. For this purpose, the Time Delay Control (TDC) was applied to the servo system and a stability analysis was made. The results of experiments show that TDC controller achieves a fast and accurate control under inertia variations over wide range of up to about 20 times the design value, and large external loads, thereby proving its robustness. The comparative study with a well-tuned PID control shows that the system with TDC is the faster and the more accurate.

As optical disks are removable and have some distortions, servo systems of optical disk drives must be robust, especially at low frequency. We proposed the high-gain servo controller with complex zeros, which is the same 2nd order controller as conventional controllers, and realizes a much higher gain at low frequency. Decrease of servo error was confirmed, but the controller increased overshoot of initial value responses. We also proposed applying the initial value compensation to suppress the overshoot.

We have stabilized the frequency of a commercial ring dye laser operating in the blue near 486 nm with an internal electro-optic modulator, using optical heterodyne detection of phase-modulated light reflected from a reference cavity. With a servo bandwidth of 10 MHz, the short-term stability has been reduced below 1 kHz. Frequency doubling of this laser will provide highly monochromatic 243 nm radiation for high resolution spectroscopy of the two-photon hydrogen 1S–2S transition.