This study proposed a method for simultaneously reducing mass imbalance and vibration in gimbal systems utilizing a tuned mass damper (TMD) as a balancing weight. Finite element analysis (FEA) and experiments were used for testing the method. Mass imbalance in gimbal systems generally causes external disturbance torque. To reduce this, a balancing weight can be used. However, weight increase due to balancing weight causes resonance in the gimbal system, which generates bias error in the gyroscope sensor. This study demonstrated that both mass imbalance reduction and vibration reduction effects could be achieved by utilizing a TMD as a balancing weight. FEA results showed that the mass imbalance reduction effect of the gimbal was not affected by TMD. The magnitude of vibration response at the resonance point was reduced by about 98% with TMD. When a TMD was applied, the magnitude of the vibration response at the resonance point was reduced by 98% to the same level as that of the gimbal. Bias error of the gyroscope sensor was reduced by about 95% or more. These results show that a TMD is useful for effectively reducing mass imbalance and vibration in gimbal systems while improving gyroscope sensor performance.
This paper describes the control method of an electric gripper using a current control system. A current control system was designed and fabricated, and it consisted of a current sensor, an amplifier, and a precision voltage regulator. As a result of calibration by applying the current control system to the electric gripper, the error was less than ±0.34%. The proportional gain for the PI control of the gripper was 0.41, and the integral gain was 0.01. As a result of the control characteristics test, the arrival time was 0.79 s and the steady-state error was ±5 mV (±0.025 N). As a result of the experiment of holding the object, the gripper was able to hold the object safely. Therefore, the electric gripper applied using the current control system performed excellently in current control based on the reference force (grabbing force); thus, it is judged that it can be used to safely grip various objects in various fields.
In this study, the disturbance torque that maintains the gimbal at a specific angle during the centrifugal acceleration test was analyzed. Newton"s Second Law for Rotation was applied, to calculate the disturbance torque. A Theoretic solution for calculating the disturbance torque was derived, by separating the horizontal/vertical components of the moment of inertia. The Theoretic solution was verified, by numerical analysis (RecurDyn) of the simplified Gimbal model. To include the effect of acceleration, the distance between the central axis of the gimbal and the accelerated test equipment was applied as 0 and L (non-Zero). As a result of the analysis, it was found that the main disturbance torque is not related to acceleration, but to self-centrifugal force caused by rotation. A centrifugal acceleration test was conducted, to verify the operational performance of gimbal. The disturbance torque was calculated, by measuring the torque used to operate the gimbal"s motor. The result was compared with the disturbance torque, calculated by the Theoretic solution of the gimbal. The error between the result of test and Theoretic solution of torque was less than 4.5%.
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A study of Tuned Mass Damper (TMD) Application for Mass Imbalance and Vibration Reduction in Gimbal Systems for High-speed Maneuverable Vehicles Jun-Soo Kim, Dong-Kyun Lee, Jong-Kuk Lee, Hyeon-Jun Cho, Ji-in Jung Journal of the Korean Society for Precision Engineering.2024; 41(11): 857. CrossRef
This paper presents an improved formulation, to estimate the sliding friction torque of deep groove ball bearings (DGBBs). Running torque of rolling element bearings, is directly associated with heat generation in rotating machines. Among the components of running torque, sliding friction is a major friction source in ball bearings. For DGBBs, sliding friction is dominated by spinning and differential sliding between balls and races. This paper addresses the sliding friction torque components of DGBBs: Spinning friction, differential sliding friction due to the ball rotation, and differential sliding friction due to the ball orbital motion. An efficient and accurate computational method is proposed for the individual sliding friction sources, based on pure rolling lines in the elliptical contact area between the balls and races. The proposed method applies an updating algorithm, for estimating more accurate information about the pure rolling lines. The proposed method was validated in terms of comparison with other methods, and with the empirical formulae provided by a bearing manufacturer. Simulations were also conducted to investigate the impacts of important parameters on the sliding friction torque in DGBBs.
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Study on Thermo-mechanical Modeling and Analysis of High-speed Angular Contact Ball Bearings Under Oil-jet Lubrication Gilbert Rivera, Shinhyang Park, Chan-sik Kang, Dongjoo Kim, Seong-Wook Hong Journal of the Korean Society for Precision Engineering.2024; 41(7): 569. CrossRef
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One of the problems in inverse dynamics calculation for the inertial measurement unit (IMU)-based joint force and torque estimation is the amplified signal noises of segment kinematic data mainly due to the differentiation procedure and segmental soft tissue artifacts. In order to deal with this problem, appropriate filtering methods are often recommended for signal enhancement. Conventionally, a low-pass filter (LPF) is widely used for the kinematic data. However, the zero-phase LPF requires post-processing, while the real-time LPF causes an unignorable time lag. For this reason, it is inappropriate to use the LPF for real-time joint torque estimation. This paper proposes a Kalman filter (KF) for inverse dynamics of IMUbased joint torque estimation in real time without any time lag, while utilizing the smoothing capability of the KF. Experimental results showed that the proposed KF outperformed a real-time LPF in the estimation accuracy of hip joint force and torque during jogging on the spot by 100 and 29%, respectively. Although the proposed KF requires the process of adjusting covariance according to the dynamic conditions, it can be expected to improve the estimation performance in the field where joint force and torque need to be estimated in real time.
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The aim of this research was to investigate the torque performance of the motor in an electric vehicle depending on the rotor shape and air gap. The research focused on numerical comparison of torque performance of new rotors based on the average torque and torque ripple rate, which appeared according to the number and placement of permanent magnets. This research was numerically analyzed by MAXWELL V21.1. Average torque values in cases 1, 2, and 3 were increased, but vibration and noise in cases 1 and 3 were increased as the torque ripple rate increased. Considering the average torque and torque ripple rate, the torque performance of case 2 was the most optimal. Compared with Model N, the average torque of case 2 was increased by 9.1% and the torque ripple rate was reduced by 1.5%. The torque performance according to the size of air gap was compared with the basic model of case 2, which showed the best performance. An air gap of 0.7 mm applied to Model N showed the best torque performance. An additional magnet on case 2 and air gap of 0.7 mm provided the best torque performance and improved the driving motor performance for motor durability.
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Recently, carbon fiber-reinforced plastic (CFRP) has been attracting much attention in various industries because of its beneficial properties such as excellent strength, modulus per unit density, and anti-corrosion properties. However, there are several issues in its application to various fields. Severe tool wear issues in its machining have been noted as one of the most serious problems because it induces various serious machining failures such as delamination and splintering. In this regard, timely tool replacement is essential for reducing the influence of tool wear. In this study, tool wear, especially flank wear, in the CFRP drilling was investigated and monitored. First, the reproducibility of tool wear under the same machining condition was experimentally evaluated. And it is demonstrated that tool wear may remarkably differ even though the same machining condition is applied to the tools. Then, tool wear monitoring based on the feed motor torque was applied to the detection of tool life ending in the CFRP drilling process. Consequently, it was demonstrated that the average and maximum detection error of the tool life end were less than 7 and 14%, respectively.
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In this paper, an integrated ankle torque sensor and mechanism (Foot Link) of a Tendon driven-type wearing walking aid robot were designed. The foot link consists of an ankle torque sensor and a mechanism connected to the footrest. The size of the sensing part of the ankle torque sensor was designed through structural analysis and assembled by attaching a strain gauge. As a result, the reproducibility error and the nonlinearity error were within 0.04%, respectively. And the calibration result of the ankle torque sensor, reproducibility error, and non-linearity error were identified to be within 1%, respectively. Therefore, it is proposed that the ankle torque sensor presented in this paper can be used to measure the torque acting on the tendon-driven walking aid robot.
To monitor the stirring state by on-line estimation of liquid viscosity instead of stopping the stirring process and measuring the viscosity using viscometer, a basic study clarifying the effect of stirring conditions was carried out. For this purpose, the relationship between liquid viscosity and the stirring conditions, such as stirring torque, stirring speed, the duty ratio of PWM, impeller and beaker type, and the blade position in the beaker was analyzed. The second-order relationship between the stirring speed and torque, the linear relationship between viscosity and torque, and the linear relationship between duty ratio of PWM and stirring speed were observed, thereby indicating that the liquid viscosity could be estimated by an experimental formula based on the duty ratio and the stirring speed. In addition, the type of impeller and beaker was identified using the magnetic field inside impeller and RFID technology. For reducing the error in calculating the liquid viscosity, the employment of an impeller blade with a large rotational resistance and stirring with the blade at the center zone of the beaker were found to be necessary.
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A Study on Contactless Identification of Impellers Using a Digital Hall Sensor Ho-Cheol Lee Journal of the Korean Society of Manufacturing Process Engineers.2021; 20(12): 71. CrossRef
In this paper, the design and fabrication of the calf-link with knee joint torque sensor of a tandem-driven walking-assist robot is described. Tendon-driven walking-assist robots should be designed and constructed with a wire wheel and a torque sensor, as one body to reduce the weight of the calf link. The torque sensor consists of four plate sensing parts crossed 90° around the wire wheel. Structural analysis was performed to determine the size of the torque sensor sensing part, and a torque sensor was built by attaching a strain gauge to the sensing part. As a result of the characteristics test, the reproducibility error and the nonlinearity error of the manufactured torque sensor were less than 0.03% and 0.04%, respectively. As a result of the calibration, the reproducibility error and the nonlinearity error were less than 0.08%, respectively. Thus, it is considered that the knee joint torque sensor of the calf link can be attached to the tandem-driven walking-assist robot.
As environmental and energy issues increase, energy efficiency is of great significance in the automobile industries. Drag torque of a wet clutch in an automatic transmission system is one of the causes of energy loss. In this work, the drag torque characteristics of a wet clutch as a function of rotational speed was experimentally investigated with respect to the test parameters such as automatic transmission fluid (ATF) temperature and flow rate, clearance between friction disk and separator, and the number of disks in the test system, using two different friction materials. Drag torque was found to decrease with increasing ATF temperature as a result of the decrease in viscosity. Also, drag torque decreased as the clearance between friction disk and separator increased. In addition, the drag torque increased along with the delay in the generation of maximum drag torque as the ATF flow rate increased. Furthermore, it was observed that drag torque increased with the increase in the number of disks in the test system. The drag torque characteristics as a function of rotational speed may be dependent on the friction materials. The results obtained from this work may aid in the design of wet clutch system to enhance performance.
The manufacture of BLAC motor requires the motor stator (electric steel plate) and the motor case (aluminum die casting material) maintain strong contact to support the resistance of rotating moments against the rotational moments of the rotor. If there is small reverse rotational moment, the motor stator rotates easily, and damages the motor. To prevent a strong reverse rotational moment, interference fit between the motor stator and the motor case manufactured through heat shrinkage is required. This study, considers the tolerances that may occur in the manufacturing process of assembling stator and case, the contact pressure range that can occur in the heat shrinkage and the moments that can be supported by the motor stator. These are verified through the finite element analysis. The result validates finite element analysis as compared with theoretical values.
In this paper, we designed and fabricated a hip joint torque sensor that can measure the torque applied to the hip joint of a walking assistant robot that can be used by a leg patient. To do this, we modeled the structure of the hip joint torque sensor so that it can be connected to the thigh link and the body of the walking assist robot. We calculated the rated torque of the hip joint torque sensor using computer simulation and determined the size of the torque sensor using a finite element program. The hip joint torque sensor was made by constructing a Wheatstone bridge and attaching a strain gauge. The characteristic test of the fabricated torque sensor was performed using a calibration device, and the reproducibility error and the nonlinearity error of the torque sensor were both less than 0.04%. Therefore, it is proposed that the developed hip joint torque sensor can be attached to the thigh link of the wearable walking assist robot, and the torque sensor can accurately measure the torque applied to the hip joint.
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Development of Lower Limb Rehabilitation Robot Capable of Adjusting the Size of Leg and Waist, and Analysis of Gait Trajectory Deviation Young-Ho Jeon, Gab-Soon Kim Journal of the Korean Society for Precision Engineering.2021; 38(11): 817. CrossRef
In soccer, sports science aims to prevent injuries and improve performance by biomechanically analyzing a series of the kick processes. In order to understand the kick processes biomechanically, studies on kinematic, kinetic, and EMG have been conducted. However, these studies have limitations due to absence of integrated theory defining interactions between the segments. In the present work, we propose a model to understand dynamic characteristics of the kicking leg based on the biomechanical features of the instep kick. Five healthy men participated in an experiment to perform four-level instep kick. We collected kinematic and kinetic information of the hip and knee joints. Based on the passive dominance of the knee joint, we devised the knee joint torque model proportional to angle and angular velocity. RMSE between simulated and real torque was 4.17%, and exhibited a tendency to decrease linearly with the kick speed. Henceforth, it is apparent that the faster the kick, the greater the load on the hip; and the slower the kick, the greater the load on the knee joint. We anticipate that this model will be applied to the kick monitoring equipment and for the prevention of injuries by measuring the load.
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Ablation Behavior of ZrB2-SiC UHTC Composite under Various Flame Angle Using Oxy-Acetylene Torch Seung Yong Lee, Jung Hoon Kong, Jung Hwan Song, Young Il Son, Do Kyung Kim Korean Journal of Materials Research.2022; 32(12): 553. CrossRef
The self-locking nuts that are used in high-speed railway-vehicle bogies and car-body connections are key components of the fastening system. These bogies and connection systems should withstand the high vibrations and shocks that are generated by high-speed operations. Since the first high-speed railway was developed, the antiloose nuts that are globally used in all of the high-speed rail-vehicle bogies and car-body connection systems are single-use limited to prevent nutloosening accidents during train operations. In this study, we developed a double reusable nut for the self-locking nuts of high-speed rail vehicles with a 100-% lifetime improvement. The proposed nut design was subjected to the KS R 9144 and NAS 3350 vibration-performance evaluation tests, and following the DIN 65151 method, a Junker test was performed for an impact-performance test. As the final step, a practical-application test was performed to assess the reusability of the proposed nut for which the self-locking nut of the HEMU-430X high-speed rail vehicle was utilized, and two reusability tests were subsequently carried out to evaluate the safety
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