Proximate Time-optimal Servomechanism Research Articles

Proximate Time-Optimal Servomechanism Based on Transition Process for Electro-Optical Set-Point Tracking Servo System

Set-point tracking servo systems encounter the problem of the trade-off between the swiftness and smoothness in tracking task. To deal with this problem, the proximate time-optimal servomechanism based on transition process (PTSTP) is proposed in this paper. The PTSTP control scheme incorporates a transition process (TP) into the framework of proximate time-optimal servomechanism (PTOS) to eliminate the conservatism of the original PTOS without the controller changing. The target position signal amplitude and the ultimate ability of actuator are utilized to design the time-optimal TP to make the jumping target position signal turns to a smooth signal, which can significantly reduce system overshoot. Therefore, the system swiftness and smoothness performance are guaranteed by PTSTP. Then, the stability of the proposed method is analyzed theoretically. Finally, the experimental results show that the controlled system is able to track the target position signal with different amplitude fast and smoothly in an electro-optical set-point tracking servo system.

A discrete-time framework for proximate time-optimal performance of damped servomechanisms

This paper proposes nonlinear control strategies for the discrete-time near time-optimal control of damped servomechanisms. Both the well established Proximate Time Optimal Servomechanism (PTOS) and its dynamically damped version (ddPTOS) are extended for damped systems. Stability results are presented in a general discrete-time framework suitable for both damped and undamped systems. These results allow the reduction of constraints in the traditional PTOS – particularly regarding the acceleration discount factor – while encompassing the nonlinear damping of the ddPTOS. Experimental results demonstrate the efficiency of the proposed controllers.

An Observer-Based Mode Switching Control Scheme for Improved Position Regulation in Servomotors

A mode switching control scheme is proposed to achieve fast and precise set-point tracking in motor servo systems. The control scheme incorporates a composite nonlinear feedback (CNF) control law into the framework of proximate time-optimal servomechanism (PTOS). The CNF control law consists of a linear feedback part for achieving fast response and a nonlinear feedback part for suppressing the overshoot, so as to improve the transient performance in short span positioning, while the PTOS control law is responsible for fast acceleration and deceleration when the tracking error is large. A reduced-order extended state observer is designed to estimate the unmeasured velocity and unknown disturbance for feedback and compensation. The closed-loop stability is analyzed theoretically. The control scheme is then applied to the position-velocity control loop in a permanent magnet synchronous motor servo system. Experimental studies have been carried out using the TMS320F2812 digital signal controller board. The results verify that the controlled system is capable of tracking a wide range of target positions fast, smoothly, and accurately, with some degrees of performance robustness against disturbance and uncertainty.

Robust proximate time-optimal servomechanism with speed constraint for rapid motion control

A control scheme is proposed to achieve fast and accurate set-point servo motion in typical double integrator systems subject to disturbances and speed constraint. The controller consists of a Proximate Time-optimal Servomechanism (PTOS) control law and a compensation term for the unknown disturbance. An extended state observer is adopted to estimate the un-measured velocity signal and the unknown disturbance. In the presence of speed constraint, a speed regulation stage is incorporated between the PTOS acceleration and deceleration stages. The closed-loop stability is analyzed theoretically. The control scheme is then applied to the position-velocity control loop in a permanent magnet synchronous motor servo system for set-point tracking. MATLAB simulation has been conducted, followed by experimental verification based on the TMS320F2812 DSP controller board. The results confirm that the proposed control scheme can track a wide range of target references fast and accurately, and has good performance robustness with respect to the disturbance and parameter variations.

Two-dimensional proximate time-optimal servomechanism

This paper focuses on the development of a proximate time optimal control method for two-dimensional rigid body systems, eg. XY positioning tables. Our approach is based on the traditional proximate time-optimal servomechanism (PTOS), which starts with a near-time-optimal non-linear controller and then switches to a linear controller as the system output approaches the desired target. This work proposes to expand the PTOS, a unidirectional control law to twodimensional systems, in order to perform linear point to point trajectories in minimum time.

Two-Dimensional Proximate Time-Optimal Servomechanism

This paper focuses on the development of a proximate time optimal control method for two-dimensional rigid body systems, eg. XY positioning tables. Our approach is based on the traditional proximate time-optimal servomechanism (PTOS), which starts with a near-time-optimal non-linear controller and then switches to a linear controller as the system output approaches the desired target. This work proposes to expand the PTOS, a unidirectional control law to two-dimensional systems, in order to perform linear point to point trajectories in minimum time.

Discrete-time mode switching control with application to a PMSM position servo system

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.

Classical and modern methods for time-constrained energy optimal motion – Application to a badminton robot

Two control approaches are presented to improve the energy efficiency of a robot which has to perform point-to-point motions during a fixed time interval. One is called Proximate Energy Optimal Servo (PEOS) and the other is called Energy-Optimal Model Predictive Control (EOMPC). The PEOS approach is based on the Proximate Time Optimal Servo (PTOS) control algorithm whose parameters are optimized in order to achieve energy efficient behavior. The EOMPC approach developed based on the Time-Optimal Model Predictive Control (TOMPC) approach achieves energy optimal performance by minimizing the energy losses in the object function. The two developed approaches are applied to a robot playing badminton and compared to two existing control approaches: the Proximate Time Optimal Servo (PTOS) and the Time-Optimal Model Predictive Control (TOMPC). The robot is still able to intercept most of the opponent shuttles on time, while a significant reduction of the energy consumption is demonstrated in both cases.

Improved Control Design Methods for Proximate Time-Optimal Servomechanisms

It is well known that minimum time-optimal control for servomechanisms can generate chattering in the presence of measurement noises, feedback delays, or model uncertainties; thus, it is not practical in applications. Maybe, the most popular alternative approach is the so-called proximate time-optimal servomechanism (PTOS). This approach starts with a near-time-optimal controller and, then, switches to a linear controller when the system output is close to a given target. However, the chattering problem is avoided at the expense of a slower time response. In this paper, two methods for eliminating the conservatism present in the PTOS are proposed. The first method applies a dynamically damped controller that allows the so-called acceleration discount factor to be pushed arbitrarily close to 1. The second method applies a continuous nonlinear control law that makes use of no switching. Experimental results show that the proposed designs practically eliminate the conservatism in the traditional PTOS.

Analysis of sliding mode proximate time-optimal disk drive servo control

An analysis of a sliding mode proximate time-optimal servomechanism (SPTOS) conducted through simulations and experiments is presented. The sliding mode is utilized to deliver proximate time-optimal performance naturally. We also examine why the SPTOS scheme is superior to the proximate time-optimal servomechanism that is widely used in the disk drive industry. The SPTOS is shown to produce bounded motion along the reference profile, which contributes to accurate reference profile tracking even in the presence of plant uncertainties. The application of the SPTOS yielded accurate reference velocity profile tracking as well as uniformly excellent seek performance regardless of the seek length.

Improved Servomechanism Control Design – Nonswitching Case

AbstractThis paper attempts to achieve time-optimal performance for servomechanisms via a practical feedback control law. An improvement over the traditional Proximate Time Optimal Servomechanism (PTOS) is proposed in order to eliminate the conservatism present in the controller. The original PTOS switches from the nonlinear approximation of Time Optimal Control (TOC) to a linear controller as the system approaches the reference point. This switching results in a compromise in performance inasmuch as the linear controller is not sufficiently aggressive to settle the system output with acceptable levels of overshoot. The proposed controller eliminates the necessity of the switching function by making use of an elaborate nonlinear control law. Simulations and experimental results show that the proposed design achieves levels of performance comparable to that of the theoretical TOC.

An Improved 2-DOF Proximate Time Optimal Servomechanism

The proximate time optimal servomechanism (PTOS) is a well established algorithm for the control of linear systems where control bounds and the speed of response are important. The PTOS approach modifies the time-optimal switching curve by including an unsaturated ldquoslabrdquo region that leads to a linear control regime for small errors. The control parameters are required to satisfy the continuity and the stability constraints resulting in only a single independent design parameter. In this note, the PTOS theory is reviewed and the stability conditions are derived. An improved version of the PTOS algorithm, called the MPTOS, is presented that results in a performance improvement over the original PTOS by providing two independent control parameters in the design. As a result, the system response can be shaped in a more efficient way in both continuous and discrete time domains. The stability conditions are derived for the proposed MPTOS scheme for continuous and discrete time implementations. Further extensions are discussed including a new discrete algorithm suitable for systems with slow sampling and a model reference tracking method also known as a feedforward/feedback structure for reducing the response of flexible modes. The latter method can be extended to dual actuator systems as are being introduced to the disk drive read/write head assembly fabrication and control. An approach is also presented for combining the vibration reduction filters with the model reference structure to further enhance the performance.

Discrete-Time Sliding Mode Control With Time-Varying Surface for Hard Disk Drives

A time optimal-based discrete-time sliding mode control (DSMC) scheme with application to hard disk drives (HDDs) is developed in this study. In this approach, an approximate time optimal switching curve is adopted for the time-varying sliding mode design and the controller, which consists 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 chattering and thereafter remains inside it. By using the time-varying switching surface, one unified framework for both track seeking and following control is provided and smooth transition from seeking control to track following control is achieved as well. Simulation and experimental studies on the design of controller in HDDs were conducted to illustrate its feasibility and effectiveness. The simulation/experimental results demonstrate that the proposed 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 suppression in track-following than the proportional-integral-differential (PID) control.

A Discrete-Time Modified Sliding Mode Proximate Time-Optimal Servomechanism for Scanning-Probe-Microscope-Based Data Storage

In this paper, we present a discrete-time modified sliding mode proximate time-optimal servomechanism (MSMPTOS) applied to scanning probe microscope (SPM)-based data storage (SDS) systems. Three conditions on the global asymptotic stability are introduced. Time-varying sliding mode with variable convergence rate factor is used to eliminate the overshoot problem and reduce the settling time compared to the conventional sliding mode proximate time-optimal servomechanism (SMPTOS). For long track seek, its trajectory is bounded by a velocity profile that is close to time-optimal velocity profile. Its settling performance is hardly affected by seek distance whereas reducing seek time. We have obtained improved seek performance over SMPTOS for short and long seeks. Its effectiveness is verified with simulation results.

Modified Damping Scheduling Proximate Time Optimal Servomechanism for Improvements in Short Strokes in Hard Disk Drives

Proximate time-optimal servomechanism (PTOS) has been widely used as a point-to-point motion control algorithm in many control systems, especially in hard disk drives, because of its simplicity and good performance. A great deal of research for improving PTOS has been reported; however, most of it is complex or needs a time-consuming tuning process. Damping scheduling PTOS (DSPTOS), which maintained the simplicity of PTOS and showed faster settling in long strokes by scheduling a closed-loop damping coefficient, was proposed previously. In this paper, we propose a modified DSPTOS which can schedule a closed-loop damping coefficient not only in long strokes, but also in short strokes. The proposed algorithm improves seek and settling performances significantly in short strokes. The effectiveness of the proposed algorithm is verified through simulations and real-time experiments. Consequently, we achieved over a 25% settling time improvement for a short stroke (10 tracks).

포 구동시스템에 대한 모드 스위칭 제어기 설계

To meet an increasing demand for high performance in gun dynamic plant, both a precise and a fast response positioning are strongly required for the gun servo system. A mode switching control(MSC) scheme, which includes a fine stabilizing controller, fast positioning one and a switching function, is widely used to meet this requirement. Stabilization is performed through PID controller, while a time optimal control method is used for target designation. In this paper, a modified PTOS(Proximate Time Optimal Servomechanism) algorithm is derived so as to accommodate the damping term in the gun plant model. Also, applying a mode switching strategy, the bumpless transfer is made possible when the controller switches from PTOS to PID. To show the effectiveness of the overall control system, simulation results are given including the gun dynamics.

A novel damping scheduling scheme for proximate time optimal servomechanisms in hard disk drives

The proximate time-optimal servomechanism (PTOS) is widely used in hard disk drives because of its simplicity and good performance. There have been many studies to improve PTOSs, though they are complex or need a time-consuming tuning process. Here, we propose a novel damping scheduling PTOS (DSPTOS) for hard disk drives. The proposed algorithm keeps the closed-loop damping coefficient at a predetermined value. Compared to the original PTOS, smaller damping in deceleration will boost the rising speed, and larger damping in the settling phase reduces the overshoot and residual vibrations. Moreover, the DSPTOS maintains the simplicity of the PTOS by introducing just a predetermined damping coefficient. We applied the proposed DSPTOS algorithm to a voice coil motor in a hard disk drive. Through simulations and real-time experiments, we verified the effectiveness of the DSPTOS. In particular, with respect to long stroke seeks, the DSPTOS performs better than the PTOS.

SLIDING MODE PROXIMATE TIME-OPTIMAL SERVOMECHANISM

A discrete-time sliding mode proximate time-optimal servomechanism is developed using nonlinear sliding mode tangent to a reference velocity profile. Instead of a strong nonlinear function causing control chattering in discrete-time, a linear control is designed around the sliding surface. The resulting servomechanism delivers a bounded motion about the sliding surface even in the presence of plant uncertainties. Application results demonstrate accurate reference velocity tracking and uniformly excellent seek performance regardless of seek length via enforced sliding mode.

Shaped Time-Optimal Feedback Control for Disk-Drive Systems With Back-Electromotive Force

In this paper, we describe the design of closed-loop control laws for disk-drive systems that account for back-electromotive force (back-EMF) by using a phase-plane approach. The time-optimal control law for a rigid body is shaped by using input shaping ideas to address the flexible mode, and then the altered rigid body phase-plane trajectories resulting from this shaping are calculated and used to determine a closed-loop controller. Extensions are made to account for damping and slew rate limits. Simulation results show that this control approach leads to near-time-optimal performance without unwanted residual vibrations, good robustness to modeling errors, and faster time response than the previously developed extended proximate time-optimal servomechanism (XPTOS) control, especially when the flexible dynamics are dominant.

Discrete-time composite nonlinear feedback control with an application in design of a hard disk drive servo system

In a typical disk drive servo system, two or more types of controllers are used for track seeking, track following, and track settling modes. This leads to the problem of mode switching among these controllers. We present in this paper a unified control scheme, the discrete-time composite nonlinear feedback control, which can perform all the above functions in hard disk drive (HDD) servo systems with actuator saturation. The proposed scheme is composed by combining a linear feedback law and a nonlinear feedback law. The linear feedback law is designed to yield a fast response, while the nonlinear feedback law is used to increase the damping ratio of the closed-loop system as the system output approaches the command input. In the face of actuator saturation, this control law not only increases the speed of closed-loop response, but also improves the settling performance. Implementation results show that the proposed method outperforms the conventional proximate time-optimal servomechanism by about 30% in settling time.