Journal of the Korean Society for Precision Engineering

Position Control of a Linear Motor Motion Stage Using Augmented Kalman Filter: Keun-Ho Kim, Hyeong-Joon Ahn; J. Korean Soc. Precis. Eng. 2025;42(11):887-892.
Published online November 1, 2025; DOI: https://doi.org/10.7736/JKSPE.025.011

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The rapid growth of semiconductor and display manufacturing highlights the demand for fast, precise motion stages. Advanced systems such as lithography and bio-stages require accuracy at the μm and nm levels, but linear motor stages face challenges from disturbances, model uncertainties, and measurement noise. Disturbances and uncertainties cause deviations from models, while noise limits control gains and performance. Disturbance Observers (DOBs) enhance performance by compensating for these effects using input–output data and a nominal inverse model. However, widening the disturbance estimation bandwidth increases noise sensitivity. Conversely, the Kalman Filter (KF) estimates system states from noisy measurements, reducing noise in position feedback, but it does not treat disturbances as states, limiting compensation. To address this, we propose an Augmented Kalman Filter (AKF)–based position control for linear motor stages. The system was modeled and identified through frequency response analysis, and DOB and AKF were implemented with a PIV servo filter. Experimental validation showed reduced following error, jitter, and control effort, demonstrating the improved control performance of the AKF approach over conventional methods.

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Customized Current Control of a Linear Motor Motion Stage: Kyung Ho Yang, Hyeong-Joon Ahn; J. Korean Soc. Precis. Eng. 2024;41(11):875-880.
Published online November 1, 2024; DOI: https://doi.org/10.7736/JKSPE.024.085

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In the 4^th Industrial Revolution, advancements in semiconductor technology demand high performance, efficiency, and precision, highlighting the importance of high-speed and ultra-precise motion stages. To improve positioning performance of a motion stage, robust torque generation by current controllers alongside position control is crucial. This paper explored a custom current control for linear motor motion stages. We built a linear motor motion stage with a 560 mm stroke, 5 m/s speed, and 280 N continuous thrust supported by air bearings and equipped with a passive reaction force compensation. Custom user code for position and current controls of PowerPMAC motion controller was developed for the motion stage. The position control code included frequency domain system identification, disturbance observer, and repetitive learning control while the current control code featured vector or d/q-axis current controllers and disturbance observer. We developed a current control tuning GUI to adjust the current control gain by injecting an excitation signal into the motion controller and measuring the frequency response of the open-loop transfer function. Experimental results confirmed the effectiveness of the custom current controller for evaluating static and dynamic performance.

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Control Boost of a Magnetic Levitation System with Disturbance Observers: Yupeng Zheng, Hyeong-Joon Ahn; J. Korean Soc. Precis. Eng. 2024;41(4):273-278.
Published online April 1, 2024; DOI: https://doi.org/10.7736/JKSPE.023.142

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A magnetic levitation system (MLS) controls the position of a steel ball with the magnetic force of the electromagnetic actuator. A disturbance observer (DOB) could improve the disturbance rejection and command tracking performance of the voltage-controlled MLS. This paper studied control boost of MLS using current and position DOB. The current-controlled MLS had a higher control performance than the voltage-controlled MLS. The combination of PID position and PI current controls provided stable levitation and a wide operation range of MLS. When DOB was applied to PI current control, it could compensate for inductance change according to the position of the steel ball. In addition, when another DOB was introduced to the PID position control, it improved the disturbance removal performance. Finally, we discussed the effectiveness and limitations of the DOB-based current and position control by measuring closed-loop frequency responses.

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Improvement of the Transient Levitation Response of a Magnetic Levitation System Using Hybrid Fuzzy and Artificial Neural Network Control
Yupeng Zheng, Hyeong-Joon Ahn
International Journal of Precision Engineering and Manufacturing.2025; 26(5): 1159. CrossRef

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Active Control of Pneumatic Vibration Isolator with Pressure Observer: Jae-Min Shin, Hyeong-Joon Ahn; J. Korean Soc. Precis. Eng. 2024;41(3):169-174.
Published online March 1, 2024; DOI: https://doi.org/10.7736/JKSPE.023.108

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The pneumatic vibration isolator is economical, has no risk of contamination, and attains high vibration isolation performance by lowering the natural frequency. Pressure feedback control is used to improve the response speed of the pneumatic vibration isolator and keep the internal pressure of the pneumatic actuator constant. In this paper, the vibration isolator was actively controlled by estimating the internal pressure of the pneumatic actuator with the displacement signal. A pneumatic actuator was modeled and its dynamic characteristics were identified through frequency response measurements. A pressure observer based on relative displacement was designed, and the observer control gain was adjusted with nominal model and experiments. Pressure estimation performance and active vibration suppression performance using a pressure observer were verified through experiments. The pressure of the pneumatic actuator was estimated by the observer, and measurement noise was eliminated effectively. In addition, vibration isolation performances of direct and estimated pressure feedback showed no difference, verifying the effectiveness of the pressure observer.

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Model-based Motion Control Design of a Linear Motor Stage in Frequency Domain: Hee Won Jeon, Hyeong-Joon Ahn; J. Korean Soc. Precis. Eng. 2024;41(1):55-60.
Published online January 1, 2024; DOI: https://doi.org/10.7736/JKSPE.023.107

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The fourth industrial revolution led to advanced servo systems, enhancing productivity across industries. However, designing these systems remains challenging due to the performance-stability trade-off. This paper presents a model-based motion control of a linear motor motion stage in frequency domain. A user-code for the PowerPMAC commercial controller was developed to excite motion control system so that we could get a frequency response. The theoretical frequency response of the servo algorithm was compared with the experimental frequency response. Based on this, a tuning graphical user interface (GUI) was developed to predict performance when the servo loop gain is changed. Especially, to compensate for residual vibrations caused by high acceleration and deceleration and to improve tracking error, DOB (Disturbance Observer) and ILC (Iterative Learning Control) control techniques were applied in the frequency domain. Through the design of the frequency domain motion controller, the control performance of the linear motor motion stage could be predicted with over 96% accuracy, resulting in a 54.32% improvement in tracking error and a 93.56% improvement in settling time, 85.29% in RMS error.

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Fuzzy Neural Network Control for a Reaction Force Compensation Linear Motor Motion Stage
Kyung Ho Yang, Hyeong-Joon Ahn
International Journal of Precision Engineering and Manufacturing-Smart Technology.2024; 2(2): 109. CrossRef
Customized Current Control of a Linear Motor Motion Stage
Kyung Ho Yang, Hyeong-Joon Ahn
Journal of the Korean Society for Precision Engineering.2024; 41(11): 875. CrossRef

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Adhesion Force of the Modular Permanent Magnet Wheel-leg according to the Posture of a Wall Climbing Drone: Dong Hyo Lee, Hyeong-Joon Ahn; J. Korean Soc. Precis. Eng. 2023;40(6):493-498.
Published online June 1, 2023; DOI: https://doi.org/10.7736/JKSPE.022.132

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Improving battery performance is crucial for increasing drone flight time. However, developing individual parts can also enhance mission performance and extend operating time. By attaching a drone to a wall instead of hovering in the air, the operating time and range of task performance can be extended. This study focuses on the adhesion force of a modular permanent magnet wheel leg for wall climbing drones. The wheel leg comprised several spokes without a rim. It could climb obstacles higher than wheel radius and provide a large adhesion area. An equation for the adhesion force of the wheel leg was derived, considering mechanical factors such as drone size, inclination of the ferromagnetic wall, and drone posture. A simple experimental model was created to verify the validity of the adhesive force equation. The effectiveness of the derived equation was confirmed by experimentally measuring the angle of the ferromagnetic wall that losT adhesion according to mechanical factors and comparing it with the derived adhesion force.

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Development of Drone-attached Spraying Device for Active Maintenance of Structures
Seung-Han Yang, Kwang-Il Lee
Journal of the Korean Society for Precision Engineering.2023; 40(12): 975. CrossRef

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Frequency Domain Identification and Model-based Disturbance Observer for a Mini Drone: Kyu-Hwan Chung, Hyeong-Joon Ahn; J. Korean Soc. Precis. Eng. 2023;40(5):383-388.
Published online May 1, 2023; DOI: https://doi.org/10.7736/JKSPE.022.134

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Drone is an innovative industry that can combine the application of various technologies in the fourth industrial era, such as big data, artificial intelligence, and ICT. Although the synergy effects of these technologies will be great in various industrial ecosystems, drones are vulnerable to gusts such as "building wind" or "valley wind". Herein, the frequency domain of a mini drone was identified and a model-based disturbance observer (DOBs) was applied to implement the drone robust resistance against gusts. The frequency response of the Parrot Mambo or mini drone was measured with multi-sine excitation and the system dynamic parameters were identified. Based on the identified model, DOBs were designed and applied to the drone’s altitude, position, and yaw control. The effectiveness of the DOBs was verified with a sinusoidal disturbance. With the model-based DOB, 84.5% of the drone altitude responses, 50.7% of x responses, 52.1% of y responses, and 79.7% of yaw responses against sinusoidal disturbances were reduced. Flight responses were measured against wind disturbances with changing speed and direction. With the model-based DOBs, the drone"s altitude decreased by 87.7%, the x position by 53.0%, the y position by 60.6%, and the yaw angle by 56.2%.

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Control Performance Improvement of a Nonlinear Magnetic Levitation System with a Disturbance Observer: Yupeng Zheng, Hyeong-Joon Ahn; J. Korean Soc. Precis. Eng. 2023;40(4):329-334.
Published online April 1, 2023; DOI: https://doi.org/10.7736/JKSPE.022.133

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Magnetic levitation system (MLS) is a typical nonlinear system that controls the position of a steel ball with the magnetic force of the electromagnetic actuator. Since disturbances, due to various external forces and modeling errors, may cause excessive vibration or poor command following, disturbance suppression is necessary to improve the control performance of the MLS. This paper presents a control performance improvement approach of an MLS with a disturbance observer (DOB). First, a mathematical model of the MLS was introduced and validated with the measured frequency response. The MLS steel ball was levitated with a proportional–integral–derivative (PID) controller and a DOB was designed based on the physical model of the MLS. Both disturbance rejection and command tracking performances of the MLS with the DOB were investigated with several design parameters such as PID gains and Q filter. The disturbance rejection and command tracking performances were improved by 76.1% and 64.7%, respectively by using DOB. Finally, the disturbance rejection and command-following performances of the MLS with the DOB were verified experimentally. The effectiveness and limitations of DOB were explained with measured closed-loop frequency responses.

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Control Boost of a Magnetic Levitation System with Disturbance Observers
Yupeng Zheng, Hyeong-Joon Ahn
Journal of the Korean Society for Precision Engineering.2024; 41(4): 273. CrossRef

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A Collocated Eddy-Current Displacement Sensor for Magnetic Bearings: Sheng He Jin, Vo Ngoc Vu, Hyeong-Joon Ahn; J. Korean Soc. Precis. Eng. 2022;39(1):29-33.
Published online January 1, 2022; DOI: https://doi.org/10.7736/JKSPE.021.091

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Magnetic bearings use electromagnetic force to support the rotating shaft without any mechanical contact and actively control shaft vibration; hence, there is no mechanical friction and wear due to contact during the operation, and it has a semi-permanent lifespan. Because magnetic bearings are unstable by themselves, a gap sensor is necessary to stably control the position of objects. However, there is a limit to the improvement in control performance because the sensor is installed on one side of the bearing and is not aligned with the electromagnet. This paper presents a newly developed collocated eddy-current PCB displacement sensor for magnetic bearings. The PCB sensor is designed and built to install between the poles of a magnetic bearing and to minimize the electromagnetic interference. A sensor calibration test is performed to evaluate the sensitivity and noise of the collocated PCB sensor. In addition, the control performance of the collocated PCB sensor is evaluated by measuring the closed-loop sensitivity function of a 1 DOF magnetic suspension test rig. The collocated PCB sensor has noise within ±1 μm and excellent vibration suppression performance.

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Measurement and Evaluation of Tooth Profiles of a Harmonic Drive: Min Choi, Hyeong-Joon Ahn; J. Korean Soc. Precis. Eng. 2021;38(8):589-594.
Published online August 1, 2021; DOI: https://doi.org/10.7736/JKSPE.021.028

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Due to the recent 4^th industrial revolution and explosive interest in smart factories, the demand for industrial robots and related technologies is rapidly increasing. In order to develop a precision reducer for robots, a measurement technology of the small teeth of a gear is very important. This paper developed a method to measure the tooth shape of a precision harmonic reducer for robots and to evaluate the tooth shape measurement error quantitatively. The tooth shape of the flexspline of a harmonic drive was measured using three instrumental devices: a stylus, a projector, and a laser line scanner. A mathematical tool was also developed to evaluate the error by comparing the designed and measured teeth. The measured tooth profile was quantitatively assessed with maximum, average, and root mean square error. The measurement method using a stylus has disadvantages of careful measurement effort and a small measuring range. However, it has better precision. On the other hand, the projection method shows a moderate performance without much effort during measurement.

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The Prediction of the Angular Transmission Error of a Harmonic Drive by Measuring Noncontact Tooth Profile and Considering Three-dimensional Tooth Engagement
Beom-Seok Kim, Seung-Tae Jeong, Hyeong-Joon Ahn
International Journal of Precision Engineering and Manufacturing.2023; 24(3): 371. CrossRef

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Evaluation of Input Shaping Methods for the Nonlinear Vibration System Using a Furuta Pendulum: Anh-Duc Pham, Hyeong-Joon Ahn; J. Korean Soc. Precis. Eng. 2020;37(11):827-833.
Published online November 1, 2020; DOI: https://doi.org/10.7736/JKSPE.020.056

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There are many nonlinear vibration problems of mechanical structures because of various reasons such as geometric parameters, impact loads, or property of materials. One simple solution for the suppression of nonlinear structural vibration is input shaping that generates a command signal to cancel its vibration. However, a motion platform to evaluate the performance of input shapers for nonlinear vibration is rare. This paper presents the evaluation of input shaping methods for the nonlinear vibration system using a Furuta pendulum. First, the mathematical model of the Furuta pendulum is introduced and its nonlinear vibration characteristic is analyzed. Then, commands for canceling the nonlinear vibration of the Furuta pendulum are generated with various input shapers such as ZV, ZVD, and ZVDD. Finally, we illustrate the effects of input shapers on the nonlinear Furuta pendulum by comparing the pendulum overshoot, settling time, and vibration-reduction ratio. The Furuta pendulum is shown to be a good motion platform to evaluate input shaping methods for nonlinear vibration systems.

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Analysis of the parametric configuration impact on BallBot control performance
Anh‐Duc Pham, Ba Hoa Thai, Phuoc Vinh Dang, Nhu Thanh Vo
International Journal of Mechanical System Dynamics.2024; 4(4): 446. CrossRef

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Controlling the Fluid Induced Instability of a Supercritical CO₂ Compressor Supported by Magnetic Bearing: Sheng-He Jin, Jae-Eun Cha, Jee-Uk Chang, Sang-Hyun Choi, Hyeong-Joon Ahn; J. Korean Soc. Precis. Eng. 2020;37(10):737-742.
Published online October 1, 2020; DOI: https://doi.org/10.7736/JKSPE.020.049

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Since sCO₂ (Supercritical Carbon Dioxide) turbomachinery are generally small and operate at high rotational speed, the bearings remain a significant challenge to the design of the turbomachinery for the sCO₂ power cycles. However, a fluid induced instability similar to the oil whirl may occur even with the magnetic bearing under high pressure and high speed conditions of the sCO₂ turbomachinery. This paper presents experimental investigation on the instability of a sCO₂ compressor supported by the magnetic bearing. First, we introduce the sCO₂ compressor supported by the magnetic bearing. The procedure to guarantee the rotordynamic performance of the sCO₂ compressor supported by the magnetic bearing is provided. Then, the effects of the working condition such as the pressure and rotating speed on the fluid induced instability are investigated experimentally. Finally, a strategy to resolve the fluid-induced instability with conventional PID control is proposed and experimentally verified.

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Turbomachine Operation with Magnetic Bearings in Supercritical Carbon Dioxide Environment
Alexander Johannes Hacks, Dieter Brillert
International Journal of Turbomachinery, Propulsion and Power.2022; 7(2): 18. CrossRef
A Study on the Efficient Optimization of Controller for Magnetic Bearings Supporting Oil-Free Turbo-Chiller Compressor
Eunsang Kwon, Myounggyu Noh, Namsoo Lee, Seongki Baek, Young-Woo Park
Journal of the Korean Society for Precision Engineering.2022; 39(2): 123. CrossRef

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Input-Shaping Methods for a Linear Motor Motion Stage with a Passive RFC(Reaction Force Compensation) Mechanism: Kang Jo Hwang, Jae Seong Jeong, Hyeong-Joon Ahn; J. Korean Soc. Precis. Eng. 2017;34(12):897-902.
Published online December 1, 2017; DOI: https://doi.org/10.7736/KSPE.2017.34.12.897

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The residual vibration during the high acceleration and deceleration of a motion stage degrades the manufacturingsystem productivity and lifespan. Although a passive RFC mechanism with a movable magnet track reduces the residual vibration of the system base, a magnet track resonance may occur according to the motion profile, and the mover inposition error increases due to the residual vibration of the magnet track. We investigated input-shaping methods for a linear motor motion stage with a passive RFC mechanism. An air-bearing linear motor motion stage with the passive RFC mechanism is built, and the dynamic characteristic of the passive RFC mechanism is identified using a freevibration test. Then, mover velocity profiles are generated using various input-shaping methods. Further, the effects of the input-shaping methods on the air-bearing linear motor motion stage are investigated by comparing the magnet track oscillation, settling time, and mover in-position error. Finally, several input-shaping methods are applied to reduce the mover rise-time delay for the proposed linear motor motion stage. A properly shaped input motion profile removes the residual vibration of the passive RFC mechanism without any additional devices, as well as reducing the transmitted reaction force and the in-position error.

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Fuzzy Neural Network Control for a Reaction Force Compensation Linear Motor Motion Stage
Kyung Ho Yang, Hyeong-Joon Ahn
International Journal of Precision Engineering and Manufacturing-Smart Technology.2024; 2(2): 109. CrossRef
Software-Based Integral Product Architecture for Modular Motion Control System of a RFC Linear Motor Motion Stage: Model-Based DOB for Residual Vibration Suppression
Seong Jong Yoo, Hyeong-Joon Ahn
International Journal of Precision Engineering and Manufacturing.2020; 21(2): 203. CrossRef
Evaluation of Input Shaping Methods for the Nonlinear Vibration System Using a Furuta Pendulum
Anh-Duc Pham, Hyeong-Joon Ahn
Journal of the Korean Society for Precision Engineering.2020; 37(11): 827. CrossRef

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Control Performance Test of an Electrically-Controlled Variable Valve Timing System Using Servo Motors: Chung-Il Cho, Hyeong-Joon Ahn; J. Korean Soc. Precis. Eng. 2017;34(6):397-403.
Published online June 1, 2017; DOI: https://doi.org/10.7736/KSPE.2017.34.6.397

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Recently, technologies related to green cars are gaining attention. A variable valve-timing system (VVT) is widely used in internal-combustion engines to improve fuel efficiency and engine performance by controlling the valve open-close timing. Since conventional hydraulically controlled VVT has problems, such as slow response and low efficiency, an electrically controlled variable valve timing (ECVVT) system was developed as an alternative the conventional VVT. This paper presents a performance test rig for an ECVVT system using servo motors. The performance test rig consists of an ECVVT module with a cycloid reducer, an engine cylinder block, a driving part, and a motion controller. A small servo motor drives the ECVVT module through the cycloid gear, while a large servo motor drives the camshafts by means of a timing belt. We carried out simulations using a mathematical model of the ECVVT module, cam shaft, valve, and motion control. We then built a performance test rig for the ECVVT system, and did experiment of cam phase variations of the ECVVT system to confirm its performance.

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A Review of Recent Advances in Design Optimization of Gearbox
Zhen Qin, Yu-Ting Wu, Sung-Ki Lyu
International Journal of Precision Engineering and Manufacturing.2018; 19(11): 1753. CrossRef