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2/2021 - Present


This project develops a design approach for gravity compensators using planar four-bar mechanisms and a linear spring. This work enables the development of a class of gravity compensators encompassing 42 different types, which are characterized by high performance and kinematic simplicity. The gravity compensators are constructed by combining planar four-bar mechanisms with a rotating mass and attaching a linear spring to each mechanism, then permuting the springs. The parameters of the gravity compensators are derived from an optimization procedure that minimizes the actuator torque within a specified balancing zone. The performances of the gravity compensators were demonstrated via both numerical examples and experiments. The results showed their design feasibilities and high performances in which a torque reduction rate of 87.8% was practically achieved. Lastly, an application of the proposed gravity compensators to serial robots is described. It was found that the actuator torque of a serial robot over a prescribed workspace could theoretically be reduced by 98.2%. A prototype of a serial robot was also built to validate the applicability of the gravity compensators.


  1. V.L. Nguyen*. A design approach for gravity compensators using planar four-bar mechanisms and a linear spring. Mechanism and Machine Theory, 172, p. 104770, 2022. (Link)

  2. V.L. Nguyen. Gravity compensation of articulated robots using spring four-bar mechanisms. The 7th IFToMM International Symposium on Multibody Systems and Mechatronics (MuSMe), Córdoba, Argentina, Oct. 12–15, 2021, pp 201–209. (Link)


2/2021 - Present


This research aims to present an analytical model to compensate for the compliance errors of a Delta parallel robot as the mini robot, which is mounted at the end effector of an articulated robot to form a macro-mini manipulator for milling operations. This model is derived from a passive compliance design via mechanical springs for the robot considering uncertainties in the joint stiffness. The significance of this design is that it allows determining the compliance parameters of the model by analytical formulas. Quantitative criteria, probabilistic error models, and numerical examples with milling-like trajectories are given to evaluate the effectiveness of the proposed model. Simulation analysis was performed for the Delta robot that identified the sensitivity of its compliance errors over the workspace. The positioning accuracy reliability of the robot was improved with the model, particularly its deflection accuracy along a prescribed trajectory was theoretically increased by 82.6 percent under an estimated process force. The amplification of the compliance errors was diminished when the standard deviation of the joint stiffness was varied.


  1. V.L. Nguyen*, C.-H. Kuo, and P. T. Lin. Compliance error compensation of a robot end-effector with joint stiffness uncertainties for milling: An analytical model. Mechanism and Machine Theory, 170, p. 104717, 2022. (Link)

  2. V.L. Nguyen*, C.-H. Kuo, and P. T. Lin. Reliability-based analysis and optimization of the gravity balancing performance of spring-articulated serial robots with uncertainties. ASME Journal of Mechanisms and Robotics, 14(3), p. 031016, 2022. (Link)

  3. V.L. Nguyen, C.-H. Kuo, and P.T. Lin. Reliability and sensibility analysis of gravity-balanced robotic manipulators with uncertain parameters. ASME International Design Engineering Technical Conferences & Computers and Information in Engineering Conference (IDETC/CIE), Virtual, Online, Aug. 17–19, 2021, p. DETC-66762. (Link)

Research: Grants


8/2020 - 8/2021


This research aims to develop a robotic manipulator with performance augmentation for an intelligent automatic guided vehicle (AGV). The robot augmentation is realized through the principle of gravity compensation for reducing the actuation torques and energy consumption. The robot’s dexterity and flexibility are improved by adding a parallel platform under its base, thereby creating a hybrid serial-parallel robotic manipulator. The proposed hybrid robotic manipulator is mounted on the AGV, which is used for coordination and navigation in industrial environments. The AGV could suggest several services, such as multi-sensor data fusion, mission assignment, time synchronization, visual data representation, global navigation, and local path planning. Besides, an intelligent gripper equipped with sensors and instruments is also implemented to the proposed hybrid manipulator to provide high-level system decision-making in the grip and release of objects. This gripper could offer gripping-force accuracy and sampling rate at a minimal cost through data acquisition and processing, layout design, and embedded electronic circuit design.


  1. Z.-Y Chen, P.-R. Liaw, V.L. Nguyen, P.T. Lin. Design of a high-payload Mecanum-wheel ground vehicle (MWGV). Robotic Systems and Applications, 1(1), p. 24-34, 2021.

  2. V.L. Nguyen, C.-H. Kuo, C.-Y Lin, and P.-T. Lin. Measurement and Uncertainty Assessment of Torque Reduction of A Five-Bar Gear-Spring Mechanism (GSM). National Conference on Theoretical and Applied Mechanics (CTAM), November 26 – 27, 2020, Yilan, Taiwan.

  3. V.L. Nguyen, C.-H. Kuo, C.-Y. Lin, and P.T. Lin. Improved Accuracy of a Delta-Based Macro-Mini Manipulator with Passive Compliance. The 23rd National Conference on Mechanism and Machine Design (CSMMT), November 13, 2020, Tainan, Taiwan. 

  4. Z.-Y. Chen, P.-R. Liaw, V.L. Nguyen, and P.T. Lin. Design and Manufacturing of a High-Payload Mecanum-Wheel Ground Vehicle (MWGV). The 23rd National Conference on Mechanism and Machine Design (CSMMT), November 13, 2020, Tainan, Taiwan.



2019 - 2020

This research applied the gear-spring module (GSM) for gravity compensation of the renowned Delta parallel robots. Design examples are implemented through a theoretical model and a real industrial Delta robot, the FANUC M-3iA/12H. A comparative study between the uses of the gear-spring modules, torsion springs, and tension springs for gravity compensation is provided. As a result, the proposed GSM design suggests a compact mechanical solution for gravity compensation of the Delta robots without a compromise of the compensation performance and robot workspace.


  1. V.L. Nguyen, C.-Y Lin, and C.-H. Kuo. Gravity Compensation Design of Delta Parallel Robots Using Gear-Spring Modules. Mechanism and Machine Theory, 154, p. 104046, 2020. (SCI, IF = 3.312) (Link)

  2. .L. Nguyen, C.-H. Kuo, and C.-Y Lin. Gravity Compensation of Delta Parallel Robot Using a Gear-Spring Mechanism. The 23rd CISM IFToMM Symposium on Robot Design, Dynamics and Control (ROMANSY), September 20 – 24, 2020, Sapporo, Japan. (Finalist: Best Student Paper Award) (Link)

Research: Grants


2017 - 2019

Gravity compensation using mechanical springs allows eliminating the gravitational effect on robotic manipulators, thereby reducing the actuation energy and improving accuracy, safety, and robot performance. In this research, a novel gravity compensation design, called the “gear-spring module (GSM)”, is proposed and installed on serial robotic manipulators for gravity compensation. The proposed GSM-based design is characterized by structure compactness, less assemblage effort, ease of modularization, and high performance of gravity compensation.


  1. V.L. Nguyen, C.-Y Lin, and C.-H. Kuo. Gravity Compensation Design of Planar Articulated Robotic Arms Using the Gear-Spring Modules. ASME Journal of Mechanisms and Robotics, 12(3), p. 031014, 2020. (Link)

  2. V.L. Nguyen and C.-H. Kuo. An Analytical Stiffness Method for Spring-Articulated Planar Serial or Quasi-serial Manipulators Under Gravity and Arbitrary Load. Mechanism and Machine Theory, 137, pp. 108–126, 2019. (Link)

  3. V.L. Nguyen and C.-H. Kuo. A modularization approach for gravity compensation of planar articulated robotic manipulators. Gravity Compensation in Robotics, edited by Vigen Arakelian, Springer Nature Switzerland AG, Cham, Switzerland. (Link)

  4. V.L. Nguyen and C.-H. Kuo. Gravity compensation of a palletizing robot using a gear-spring mechanism. Australasian Conference on Robotics and Automation (ACRA), Virtual, Online, Dec. 6–8, 2021, p. 105. (Link)

  5. V.L. Nguyen and C.-H. Kuo. Performance Evaluation of a Class of Gravity-Compensated Gear-Spring Planar Articulated Manipulators. The 6th IFToMM International Symposium on Robotics and Mechatronics (ISRM), Oct. 28 – 30, 2019, Taiwan Tech, Taipei, Taiwan. (Link)

  6. V.L. Nguyen and C.-H. Kuo. A Gear-Slider Gravity Compensation Mechanism: Design and Experimental Study. The International Design Engineering Technical Conferences & Computers and Information in Engineering Conference (IDETC-CIE), August 18 – 21, 2019, Anaheim, CA, USA. (Link)

Research: Grants


2018 - 2019

This research introduced a gravity-balanced reconfigurable mechanism for lower-limb rehabilitation. The unique feature of this design is that it uses a single counterweight to realize a perfect gravity balancing in two working configurations that are relevant to the hip and knee motions during rehabilitation training. The use of counterweight could eliminate the gravitational effect on the patient's legs, thereby reducing their efforts for movements, and improving the training efficiency.


  1. C.-H. Kuo, V.L. Nguyen, D. Robertson, L.-T. Chou, and J.L. Herder. Statically balancing a reconfigurable mechanism by using one passive energy element only: a case study. ASME Journal of Mechanisms and Robotics,13(4), p. 040904, 2021. (Link)

  2. C.-H. Kuo, V.L. Nguyen, and L.T. Chou. Static Balancing of a Reconfigurable Linkage with Switchable Mobility by Using A Single Counterweight. The 4th IEEE/IFToMM International Conference on Reconfigurable Mechanisms and Robots (ReMAR),  June 20 – 22, 2018, Delft, The Netherlands. (Link)

  3. C.-H. Kuo, V.L. Nguyen, and L.-T. Chou, Static Balancing of a Reconfigurable Linkage with Switchable Mobility by Using A Single Counterweight. The Joint Research Workshop: TAIWAN TECH-KYUTECH-MMH-NYMU-TOKYO TECH-TSGH, August 22, 2018, Taipei, Taiwan.

Research: Grants


2015 - 2017

This project aims to develop a robotic manipulator for neuroendoscopy. The manipulator is constructed based on a novel five-bar spherical decoupled mechanism that can provide a remote center of motion of two decoupled degrees of freedom. In this work, the design concept, kinematics, and statics of the proposed mechanism are first studied and then used for demonstrating its superior performance compared with conventional five-bar spherical mechanisms. A multi-objective optimization was taken to find an optimal dimension for the spherical decoupled robotic manipulator with high performance. The optimization is modeled from a Genetic algorithm and Pareto Frontier considering the real operational workspace in neuroendoscopy. Finally, a prototype of the manipulator was manufactured and tested by a surgeon during neuroendoscopy.


  1. T. Essomba, V.L. Nguyen, and C.-T Wu. Optimization of a Spherical Decoupled Mechanism for Neuro-Endoscopy Based on Experimental Kinematic Data. Journal of Mechanics, pp. 1–15, 2019. (SCI, IF = 1.293) (Link)

  2. T. Essomba and V.L. Nguyen. Kinematic Analysis of a New Five-Bar Spherical Decoupled Mechanism with Two-Degrees of Freedom Remote Center of Motion. Mechanism and Machine Theory, 119, pp. 184–197, 2018. (SCI, IF = 3.312) (Link)

  3. T. Essomba and V.L. Nguyen. The Kinematic Analysis of a Novel Spherical Decoupled Mechanism - Toward A Robotic Manipulator for Neuroendoscopy. The International Conference on Advanced Technology Innovation (ICATI), June 30 – July 3, 2016, Bali, Indonesia.

  4. V.L. Nguyen and T. Essomba. Kinematic Analysis of a Spherical Decoupled Mechanism Concept. The 19th CSMMT Congress, October 2016, Pingtung, Taiwan. (Presenter)

Research: Grants
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