Six-dimensional Force Research Articles

Visual inspection and grasping methods based on deep learning

Aiming at the problems of existing robot grasping systems that have high hardware requirements, are difficult to adapt to different objects, and produce large harmful torques during the grasping process, a visual detection and grasping method based on deep learning is proposed. The channel attention mechanism is used to improve YOLO-V3, enhance the network's ability to extract image features, improve the effect of target detection in complex environments, and increase the average recognition rate compared with the original . Aiming at the problem of discreteness of the current pose estimation angle, a minimum area bounding rectangle (MABR) algorithm based on the main network embedded in the Visual Geometry Group 16 (VGG-16) is proposed to perform grasping pose estimation and angle optimization. The average error between the improved grasping angle and the actual angle of the target is less than , which greatly reduces the harmful torque applied by the two-finger manipulator to the object during the grasping process. A visual grasping system was built using UR5 robotic arm, pneumatic two-finger manipulator, Realsense D435 camera and ATI-Mini45 six-dimensional force sensor. Experiments show that the proposed method can effectively grasp and classify different objects, has low hardware requirements, and reduces harmful torque by about , thereby reducing damage to objects. It has good application prospects.

A spatial six-dimensional force measurement method based on acceleration sensors

Accurate measurement of disturbance forces is an essential prerequisite for the simulation and analysis of micro-vibrations of space optical devices. Most of the current force measurement methods are based on force sensors, which have problems such as serious coupling between sensors, and the measurement accuracy is easily affected by the preload force. In order to solve these problems, a six-dimensional disturbance force measurement method based on acceleration sensors is proposed in this paper. First, the basic structure of the force measurement platform is introduced, and then the dynamics model of the platform is established and its force measurement principle is analyzed. Then the simulation analysis of the measurement accuracy of the platform is carried out by the Monte Carlo method. The analysis results show that the measurement accuracy of the disturbance force is proportional to the measurement accuracy of the acceleration sensor and the mass matrix recognition accuracy. When the errors of the acceleration sensor and mass matrix are within the normal range, the measurement error of the force measurement platform is less than 15%. Finally, a single-degree-of-freedom disturbance force measurement verification platform is built, and the test results show that the disturbance force is basically proportional to the acceleration of the platform when it is far from the resonance frequency, which verifies the correctness of the method to a certain extent. The measurement method is simple in structure and easy to implement, and can perform real-time measurement. Moreover, the measurement frequency is wide and the resolution is high, which is suitable for the measurement of disturbance force in the form of sinusoidal vibration or multi-frequency line spectrum.

Six-dimensional constraints and force feedback for robot-assisted teleoperated fracture reduction

Six-dimensional constraints and force feedback for robot-assisted teleoperated fracture reduction

Gravity compensation and output data decoupling of a novel six-dimensional force sensor

Abstract. A shunt three-legged parallel six-dimensional force sensor has been designed for more precise measurement of six-dimensional force/moment information. The theoretical static force model of the sensor was established based on the equivalent of a six-bar closed-loop parallel mechanism. The sensor has been experimentally calibrated under a given external load, and the neural network method has been utilized to nonlinearly fit the experimental data and achieve decoupling. Furthermore, a novel gravity compensation method for the six-dimensional force sensor of the wrist of a robot has been proposed based on the CAD variable geometry method. The positive solution of the position of the parallel robot is simulated through a wire-frame diagram, enabling accurate estimation and correction of the sensor. Experimental validation has confirmed the feasibility of the compensation algorithm.

Steel-ball-type six-dimensional dynamic force measurement platform with fault-tolerant performance

To satisfy the requirement of dynamic vibration testing of aerospace equipment on the ground prior to orbiting in space to guarantee the stable functioning of precision components, a steel-ball-type dynamic force measuring platform is suggested. First, based on two traditional platforms, a novel dynamic force measuring platform with fault-tolerant function, higher load capacity, and higher fundamental frequency is proposed, with its fault-tolerant measurement method theoretically deduced. Next, the study focuses on improving the measuring performance of redundant platforms by transforming conventional hard-connected branches into steel-ball branches. The structural parameters of these branches are examined theoretically, and simulations are used to confirm the theory's accuracy. Ultimately, the prototypes of the redundant steel-ball and hard-connected platforms are fabricated. Comparative experiments demonstrate that the steel-ball platform has superior performance, with static and dynamic measuring errors of 1.2% and 2.3%, respectively, and static and dynamic average coupling errors of 0.3% and 1.5%, respectively. The fault-tolerance function's validity is also confirmed.

Design and analysis of parallel six-dimensional force sensor based on near-singular characteristics

The high sensitivity six-dimensional force sensor is the core component of intelligent robot force feedback perception, which is related to the structure of the sensor. This paper proposes a new modeling method to chain the near singular eight-branch Stewart mechanism, which indicates the relationship between structural parameters and sensitivity. The new method gains 16 new configurations by innovating the traditional Stewart institution. A configuration with high sensitivity in Fz, Mx and My directions is selected for further study. The theoretical stiffness model and force mapping model show that the sensitivity of the new structure can be amplified by 4 times in specific degrees of freedom. The results show that the nonlinear error and coupling error are 2.77% and 2.26%, respectively. The proposed method can be widely applied in the field of parallel six-dimensional force sensors.

A novel 6-dimensional force measurement based on the electric 6-DOF loading device

The 6-degrees-of-freedom (6-DOF) platform has gained popularity in industry owing to its advantages, especially in indoor simulation experiment requiring six-dimensional force environments. However, up to date, the measurement of such six-dimensional forces is challenging due to complex structures, difficult decoupling, and high costs. In this study, an electric 6-DOF loading device designed by ourselves is presented, with an ability to load high precision displacement and force. To measure the six-dimensional force of the device’s moving platform, this paper proposes a scheme involving embedding tension-compression sensors in each leg of the device and describes the mechanical model of the device. Then a novel method with a variable transformation matrix is proposed, complemented by a decoupling study that optimizes the variable transformation matrix. Finally, we validate the proposed method through an external force loading experiment. The results indicate that the error in Fy experiment is less than 2.7%, and the error in Fz experiment is less than 1.6%. The novel method exhibits high accuracy, easy installation, fast response, and low cost.

Task Space Compliant Control and Six-Dimensional Force Regulation Toward Automated Robotic Ultrasound Imaging

<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Objective:</i> We propose a general control framework for task space compliant motion and six-dimensional (6-D) force regulation towards automated robotic ultrasound (US) imaging. The framework endows a position-controlled robotic manipulator with the capability of accurate compliant motion in free space and accurate force control in motion-constrained environment. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Methods:</i> An intuitive six degree-of-freedom (6-DoF) admittance control model expressed in an arbitrary Cartesian body frame is mathematically derived with closed-form task space error mapping. Its practical implementation on widely-used collaborative manipulators is proposed to achieve full task space compliant behaviors and accurate 6-D force control. A hybrid control law is presented to achieve good motion accuracy in free space and improved coupled stability in motion-constrained environment. The coupled model of physical human-robot interaction is established and the reason for the improved coupled stability is analyzed through simulation. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Results:</i> Evaluation experiments on the proposed control framework were performed to show the effectiveness. The mean error of compliant trajectory following was less than 0.30 mm in free space. The mean relative force and moment control accuracy in three orthogonal directions was better than 0.5% and 0.8%, respectively. The improved coupled stability under the same model parameters was also confirmed by human-robot interaction experiments. Finally, an automated robotic US imaging experiment on a human volunteer in a real clinical scenario was carried out to show the potential application of our proposed framework. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Conclusion:</i> Experimental results have shown the advantages of the control framework, including satisfied force control accuracy, high accuracy of compliant motion, improved coupled stability, and system effectiveness on a human volunteer. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Note to Practitioners</i> —This paper was motivated by the increasing needs of automated ultrasound (US) scanning for both diagnostic and interventional purpose. Clinical sonographers suffer from repeated workload when performing diagnostic US imaging, which could benefit from automated robotic scanning. Robotic US imaging involves physical interaction between the robot end-effector (i.e., US transducer) and the human body. The dynamics of the interaction is regulated by the control law to guarantee the contact of the US transducer and the safety of the procedure. Most existing works have focused on regulating in-plane contact force in terms of the position without considering the compliance in other dimensions. However, it is not a trivial work to extend the positional compliance to six degree-of-freedom (6-DoF) compliance. As the prevalence of low-cost collaborative robotic arms in medical scenarios, how to perform 6-DoF compliant trajectory following and accurate six-dimensional (6-D) force control on these robotic arms becomes increasingly important. This paper gives a complete general solution to achieve 6-DoF compliant control and 6-D force regulation with accurate kinematics on a position-controlled robotic arm. A hybrid control law is proposed to switch the government of “instantaneous model” and “theoretical model” to achieve compliant motion accuracy in free space and improved coupled stability in motion-constrained environment. No expensive torque sensors and torque control interface are required. And no prior geometric knowledge about the scanning object is needed. We have demonstrated the application for robotic US imaging in a real clinical scenario.

Stiffness analysis and parameter optimization of six-dimensional force sensor with the novel circular flexible spherical joint

As robot haptic provider, six-dimensional force sensors are an indispensable component of industrial intelligence. To address the disadvantages of the existing six-dimensional force sensor, a novel circular flexible spherical joint is proposed, its stiffness model is derived by numerical analysis method and microelement, and the stiffness model of the circular flexible spherical joint is verified by finite element simulation. A parallel six-dimensional force sensor is designed based on a circular flexible spherical joint, and a mathematical model of the sensor was established. Based on the genetic algorithm, the structural optimization design of the circular flexible spherical joint was performed, and the structural parameters of the circular flexible spherical joints are obtained by analyzing isotropy. Finally, according to the optimization results, a sensor was developed and calibration experiments were carried out. The experimental results show that the maximum error of the sensor is 1.85%, which verifies the effectiveness of the mathematical model of the sensor, which has high measurement accuracy and can accurately measure the six-dimensional external force and moment applied to the sensor.

Study on Agricultural Machinery-Load-Testing Technology and Equipment Based on Six-Dimensional Force Sensor

Tractor traction power consumption is one of the main causes of energy consumption in agricultural production. Scientific and accurate control of tractor traction power consumption can obviously save energy and reduce consumption. In view of the backward load-testing technology and low measurement accuracy in field work, this study designed an array test equipment, which formed a measurement matrix based on a six-dimensional force sensor to accurately measure tractor hauled load, which could provide a reference signal for intelligent operation. In this paper, the static calibration test was carried out on the six-dimension force sensor, and the linearity, sensitivity, and zero drift were analyzed. The static characteristics of the test unit meet the measurement requirements. A static decoupling model was established. The decoupling errors of each channel were stable at 0.02%FS, 0.02%FS, 0.8%FS, 0.36%FS, 0.018%FS, and 0.06%FS, respectively. Finally, the whole hanging test of the measuring equipment was carried out—the error was 1.24%, −1.2% respectively—to verify the accuracy of the measurement of the sensor device under different working conditions.

Accuracy Analysis and Experimental Study of a Six-Dimensional Force Sensor with Circular Flexible Spherical Joints.

This paper proposed a circular flexible spherical joint to reduce the processing difficulty and manufacturing cost, and design a six-dimensional force sensor based on it. This paper analyzes the influence of the structural parameters of the flexible spherical joints on the accuracy of the six-dimensional force sensor by comparing the force mapping matrix of flexible spherical and ideal spherical joints. As the force mapping matrix is related to the stiffness matrix, the stiffness matrix of a six-dimensional force sensor based on a circular flexible spherical joint and the force mapping matrix of the generalized external force acting on the sensor was derived. Finally, a set of optimal parameters was selected to create a prototype and was verified through finite elements and experiments. The results show that the six-dimensional force sensor with a circular flexible spherical joint has good accuracy and provides some guidance for the design and analysis of the sensor.

Hybrid Data-Driven Optimization Design of a Layered Six-Dimensional FBG Force/Moment Sensor With Gravity Self-Compensation for Orthopedic Surgery Robot

This paper developed a layered six-dimensional FBG force sensor to detect the interaction force between drill and tissue in robot-assisted bone drilling. Eight unique C-shaped beams are arranged in layers within the structure to constitute a force-sensitive 3D printed flexure-body and eight FBGs have been tensely suspended on the concave side of beams, leading to low chirping risk and low cost of the designed sensor. A mathematical sensing model of the designed sensor has been derived by response surface methodology (RSM) with the hybrid data obtained from experiments and the FEM. Multi-objective optimization model of the sensor with consideration of machining deviation has been built, and its measurement isotropy for force and moment has increased by 31.4% and 30.7%, respectively, as well as the enhancement of the capabilities of the inter-dimensional decoupling and toleration for machining deviation. Experimental results demonstrate a low coupling error and a high resonant frequency of more than 232 Hz. The relative error of the sensor is less than 8.27%. A robot-assisted bone drilling experiment has been implemented to further confirm the feasibility and reliability of the sensor. By innovatively integrating a gravity self-compensation method, the sensor's detected force agrees with the reference result with a less error of 7.4%.

Surface electromyogram, kinematic, and kinetic dataset of lower limb walking for movement intent recognition

Surface electromyogram (sEMG) offers a rich set of motor information for decoding limb motion intention that serves as a control input to Intelligent human-machine synergy systems (IHMSS). Despite growing interest in IHMSS, the current publicly available datasets are limited and can hardly meet the growing demands of researchers. This study presents a novel lower limb motion dataset (designated as SIAT-LLMD), comprising sEMG, kinematic, and kinetic data with corresponding labels acquired from 40 healthy humans during 16 movements. The kinematic and kinetic data were collected using a motion capture system and six-dimensional force platforms and processed using OpenSim software. The sEMG data were recorded using nine wireless sensors placed on the subjects’ thigh and calf muscles on the left limb. Besides, SIAT-LLMD provides labels to classify the different movements and different gait phases. Analysis of the dataset verified the synchronization and reproducibility, and codes for effective data processing are provided. The proposed dataset can serve as a new resource for exploring novel algorithms and models for characterizing lower limb movements.

Design and calibration of spoke piezoelectric six-dimensional force/torque sensor for space manipulator

The large manipulator outside the space cabin is a multi-degree of freedom actuator for space operations. In order to realize the automatic control and flexible operation of the space manipulator, a novel spoke structure piezoelectric six-dimensional force/torque sensor with redundancy ability, high stiffness and good decoupling performance is innovatively proposed. Based on the deformation coordination relationship, the redundancy measurement mechanism is revealed. The mathematical models of the sensor with and without branch fault are established respectively. The finite element model is established to verify the feasibility of structure and redundancy measuring principle of the sensor. Depending on the theoretical analysis and simulation analysis, the prototype of the sensor is developed. Static and dynamic calibration experiments are carried out. The actual output voltage signal of the six-dimensional force/torque sensor is collected to establish the equation between the standard input applied load and the actual output voltage signal. Based on ant colony optimized BP algorithm, performance indexes of the sensor with and without branch fault are analyzed respectively. The experimental results show that the spoke piezoelectric six-dimensional force/torque sensor with the eight-point support structure has good accuracy and reliability. Meanwhile, it has strong decoupling characteristic that can effectively shield the coupling between dimensions. The nonlinear errors and maximum interference errors of decoupled data with and without branch faults are less than 1% and 2%, respectively. The natural frequency of the six-dimensional force sensor can reach 2856.45 Hz and has good dynamic characteristics. The research content lays a theoretical and experimental foundation for the design, development and application of the new six-dimensional force/torque sensors with redundancy. Meanwhile, it will significantly improve the research level in this field, and provide a strong guarantee for the smooth implementation of force feedback control of the space station manipulator project.

Automatic force-controlled 3D photoacoustic system for human peripheral vascular imaging.

Photoacoustic (PA) imaging provides unique advantages in peripheral vascular imaging due to its high sensitivity to hemoglobin. Nevertheless, limitations associated with handheld or mechanical scanning by stepping motor techniques have precluded photoacoustic vascular imaging from advancing to clinical applications. As clinical applications require flexibility, affordability, and portability of imaging equipment, current photoacoustic imaging systems developed for clinical applications usually use dry coupling. However, it inevitably induces uncontrolled contact force between the probe and the skin. Through 2D and 3D experiments, this study proved that contact forces during the scanning could significantly affect the vascular shape, size, and contrast in PA images, due to the morphology and perfusion alterations of the peripheral blood vessels. However, there is no available PA system that can control forces accurately. This study presented an automatic force-controlled 3D PA imaging system based on a six-degree-of-freedom collaborative robot and a six-dimensional force sensor. It is the first PA system that achieves real-time automatic force monitoring and control. This paper's results, for the first time, demonstrated the ability of an automatic force-controlled system to acquire reliable 3D PA images of peripheral blood vessels. This study provides a powerful tool that will advance PA peripheral vascular imaging to clinical applications in the future.

Experimental study on influence of sharkskin denticles structure on the hydrodynamic performance of airfoil

The hydrodynamics properties of sharkskin have been studied for the past few decades, demonstrating the drag-reducing effect of the sharkskin surface denticle structure. This study explores the influence of the sharkskin denticles structure on the hydrodynamic performance of airfoil and the drag reduction mechanism using time-resolved particle image velocimetry (TR-PIV) by arranging the sharkskin denticles on the National Advisory Committee for Aeronautics (NACA) 0012 airfoil. The change in lift and drag of the airfoil was measured by a six-dimensional force sensor. The effects of angle of attack, sharkskin denticles location, Reynolds number, and arrangement on the hydrodynamic performance of the airfoil were mainly analyzed. The results show that for the single-row sharkskin denticles structure, the closer the bionic structure is arranged to the trailing edge of the airfoil, the better the drag reduction effect is. The maximum drag reduction is up to 25.4% by arranging the bionic denticles at 0.38L of the airfoil, when Re = 1.62 × 105 (U = 0.8 m/s) and α = 0°. For the double-row sharkskin denticles structure, the staggered arrangement is less effective in reducing drag than the aligned arrangement due to the influence of form resistance. The velocity field of the turbulent boundary layer on the airfoil surface was obtained by TR-PIV. The experiment results show that the sharkskin denticles structure can change the boundary layer of the airfoil, and the static flow area formed at the tail can reduce the disturbance of the airfoil by the turbulent flow.

Fast peg-in-hole assembly policy for robots based on experience fusion proximal optimization

Background: As an important part of robot operation, peg-in-hole assembly has problems such as a low degree of automation, a large amount of tasks and low efficiency. It is still a huge challenge for robots to automatically complete assembly tasks because the traditional assembly control policy requires complex analysis of the contact model and it is difficult to build the contact model. The deep reinforcement learning method does not require the establishment of complex contact models, but the long training time and low data utilization efficiency make the training costs very high. Methods: With the aim of addressing the problem of how to accurately obtain the assembly policy and improve the data utilization rate of the robot in the peg-in-hole assembly, we propose the Experience Fusion Proximal Policy Optimization algorithm (EFPPO) based on the Proximal Policy Optimization algorithm (PPO). The algorithm improves the assembly speed and the utilization efficiency of training data by combining force control policy and adding a memory buffer, respectively. Results: We build a single-axis hole assembly system based on the UR5e robotic arm and six-dimensional force sensor in the CoppeliaSim simulation environment to effectively realize the prediction of the assembly environment. Compared with the traditional Deep Deterministic Policy Gradient algorithm (DDPG) and PPO algorithm, the peg-in-hole assembly success rate reaches 100% and the data utilization rate is 125% higher than that of the PPO algorithm. Conclusions: The EFPPO algorithm has a high exploration efficiency. While improving the assembly speed and training speed, the EFPPO algorithm achieves smooth assembly and accurate prediction of the assembly environment.

A computation effective singularity avoidance method for the underwater manipulator with a non-spherical wrist

In order to perform increasingly complex underwater tasks, we have designed a hydraulic underwater non-spherical wrist manipulator with a binocular vision system and a six-dimensional force sensor mounted behind the end-effector, which can improve the path accuracy and automation capabilities of the system. The system lays a foundation for the high precision and autonomous deep-sea operation and can improve its intelligence level. In this paper, we focus on solving the singularity avoidance problem of the hydraulic underwater manipulator for accurate control, and propose a computationally efficient singularity avoidance method to improve the path accuracy of the manipulator with the non-spherical wrist. Firstly, the singular configurations of the manipulator are analyzed. Secondly, the Jacobian is decomposed into sub-block matrices to obtain the singular factors according to the singular configurations. Thirdly, the singularities are avoided by the approximate damped reciprocal method. Theoretical analysis shows that the computation costs of the proposed method are only one-third to one-half of that of the traditional methods. The simulation results prove that the proposed method can largely improve the path accuracy of the manipulator with less computation costs, which shows that our method would have a certain significance in improving the precision and real-time response of deep-sea operations.

Multifunctional In-Situ Testing Device for Investigating the Service Performance of Materials

To obtain the change rule of material/structure service performance under complex working conditions, this study developed a material/structure mechanical property testing device with multi-load and multi-physics loading functions. The features of this device include its ability to realize the composite mechanical loading of three loads (tension/compression–bending–torsion), as well as physical field loading, such as current field, magnetic field, and thermal field, to realistically simulate the actual service conditions of materials/structures. To monitor the surface topography change, temperature, and strain distribution of the specimen in real-time, various in-situ monitoring methods, including microscopic imaging, infrared thermal imaging, and digital image correlation method, were integrated into the device. To realize multi-load composite loading, a six-degree-of-freedom parallel drive platform was adopted in the device, and its control system was developed with a distributed double-layer structure to realize the closed-loop control of any combination of load forms. All functional modules of the device were calibrated, and a coaxial calibration method based on a six-dimensional force sensor was proposed. Lastly, several tests were performed to evaluate the effectiveness of the proposed device functions, and the test results revealed the broad application prospects of the device.

Dynamics analysis of knee joint during sit-stand movement

Sit-stand movement is one of the most common movement behaviors of the human body. The knee joint is the main bearing joint of this movement. Thus, the dynamic analysis of knee joint during this movement has deeply positive influences. According to the principle of moment balance, the dynamics of the knee joint during the movement were analyzed. Furthermore, combined with the data obtained from optical motion capture and six-dimensional ground reaction force test, the curve of knee joint torque was calculated. To verify the accuracy of the analysis of dynamic, the human body model was established, the polynomial equations of angle and angular velocity were fitted according to the experimental data, and the knee joint simulation of the movement was carried out. The result revealed that in terms of range and trend, the theoretical data and simulation data were consistent. The relationship between knee joint torque and ground reaction force was revealed based on the variation law of knee joint torque. During the sit-stand movement, the knee joint torque and the ground reaction force were directly proportional to each other, and the ratio was 5 to 6. In the standing process, the acceleration first increased and then decreased and finally increased in reverse, and the maximum knee torque occurred at an angle of about 140°. In the sitting process, the torque was maximized in the initial stage. The results of the dynamics analysis of knee joint during sit-stand movement are beneficial to the optimal design and force feedback control of seated rehabilitation aids, and can provide theoretical guidance for knee rehabilitation training.