3-DOF Translation Research Articles

Type Synthesis of a Novel Class of One-DOF Multi-Mode Parallel Mechanisms

Abstract Multi-mode parallel mechanisms (PMs) are a class of reconfigurable mechanisms that can switch between different operation modes without the need for disconnection and reassembly. Although a number of multi-DOF multi-mode PMs have been presented in the literature, very few 1-DOF multi-mode PMs have been proposed. This paper deals with the type synthesis of a novel class of 1-DOF multi-mode PMs, which have one 1-DOF translation mode and at least one 1-DOF planar motion mode, using a construction method by leveraging symmetry of mechanisms and merging two multi-mode mechanisms. By inserting a revolute (R) joint into Sarrus-6R like six-joint mechanisms, 1-DOF two-mode two-legged PMs, which are composed of a moving platform and a base connected by one three-joint leg and one four-joint leg, are constructed first. From each 1-DOF two-mode two-legged PM, one can construct a 1-DOF two-mode three-legged PM by adding a third four-joint leg which is the mirror image of the four-joint leg about the plane of motion of the three-joint leg. Subsequently, 1-DOF three- and four-mode three-legged PMs are constructed by merging two 1-DOF multi-mode three-legged PMs. The instantaneous DOF of the above multi-mode PMs in a transition configuration is analyzed using screw theory. This work complements the existing approaches to the type synthesis of multi-mode PMs and hybrid reconfigurable mechanisms.

Comparing and Contrasting Near-Field, Object Space, and a Novel Hybrid Interaction Technique for Distant Object Manipulation in VR

In this contribution, we propose a hybrid interaction technique that integrates near-field and object-space interaction techniques for manipulating objects at a distance in virtual reality (VR). The objective of the hybrid interaction technique was to seamlessly leverage the strengths of both the near-field and object-space manipulation techniques. We employed bimanual near-field metaphor with scaled replica (BMSR) as our near-field interaction technique, which enabled us to perform multilevel degrees-of-freedom (DoF) separation transformations, such as 1~3DoF translation, 1~3DoF uniform and anchored scaling, 1DoF and 3DoF rotation, and 6DoF simultaneous translation and rotation, with enhanced depth perception and fine motor control provided by near-field manipulation techniques. The object-space interaction technique we utilized was the classic Scaled HOMER, which is known to be effective and appropriate for coarse transformations in distant object manipulation. In a repeated measures within-subjects evaluation, we empirically evaluated the three interaction techniques for their accuracy, efficiency, and economy of movement in pick-and-place, docking, and tunneling tasks in VR. Our findings revealed that the near-field BMSR technique outperformed the object space Scaled HOMER technique in terms of accuracy and economy of movement, but the participants performed more slowly overall with BMSR. Additionally, our results revealed that the participants preferred to use the hybrid interaction technique, as it allowed them to switch and transition seamlessly between the constituent BMSR and Scaled HOMER interaction techniques, depending on the level of accuracy, precision and efficiency required.

Fast and deterministic (3+1)DOF point set registration with gravity prior

Point set registration is the technology used to estimate the spatial transformation between two LiDAR scans, which is challenging in the presence of outlier correspondences and noise. Our focus is on 4 degrees of freedom (DOF) point set registration, in which 1DOF rotation and 3DOF translation need to be estimated. It is commonly found in practical scenarios, such as arbitrarily mobile robots equipped with an inertial measurement unit (IMU), terrestrial LiDAR scanners, or planar moving vehicles in urban environments. Recently, many solutions have leveraged branch and bound (BnB) in global and deterministic approaches to solve the registration problem with performance guarantees. However, BnB-based methods are usually time-consuming since their convergence speed is exponential to the dimensionality of the solution domain, and existing methods estimate these 4DOF simultaneously. Our key idea is to speed up BnB-based methods by decoupling the joint pose into separate translation and rotation with the aid of known gravity directions. This effectively reduces the search domain to 3DOF+1DOF, thereby enhancing algorithm efficiency. Specifically, we propose a novel BnB-based consensus maximization method for a fast 3DOF translation search and derive the specific lower and upper bound functions. We then propose an efficient global voting method for estimating the rotation with 1DOF. To demonstrate the superiority of our proposed method, we conduct extensive experiments on both synthetic and real-world datasets. The experimental results show that (1) our proposed method is more robust against outliers and noise than several existing methods and far faster than the existing BnB-based 4DOF method by almost an order of magnitude, (2) our proposed method is robust against the biases in gravity directions, such that the general error of the IMU is acceptable, and (3) thanks to its significant robustness, our proposed method can solve the challenging problem of simultaneous pose and correspondence registration (SPCR). Moreover, the proposed approach is also more robust and accurate than several SPCR benchmark methods. Code is available at https://github.com/Xinyi-tum/Fast-and-Deterministic-Registration.

Evaluating the feeling of control in virtual object translation on 2D interfaces

Abstract Computer-aided design (CAD) plays an essential role in creative idea generation on 2D screens during the design process. In most CAD scenarios, virtual object translation is an essential operation, and it is commonly used when designers simulate their innovative solutions. The degrees of freedom (DoF) of virtual object translation modes have been found to directly impact users’ task performance and psychological aspects in simulated environments. Little is known in the existing literature about the sense of agency (SoA), which is a critical psychological aspect emphasizing the feeling of control, in translation modes on 2D screens during the design process. Hence, this study aims to assess users’ SoA in virtual object translation modes on mouse-based, touch-based, and handheld augmented reality (AR) interfaces through subjective and objective measures, such as self-report, task performance, and electroencephalogram (EEG) data. Based on our findings in this study, users perceived a greater feeling of control in 1DoF translation mode, which may help them come up with more creative ideas, than in 3DoF translation mode in the design process; additionally, the handheld AR interface offers less control feel, which may have a negative impact on design quality and creativity, as compared with mouse- and touch-based interfaces. This research contributes to the current literature by analyzing the association between virtual object translation modes and SoA, as well as the relationship between different 2D interfaces and SoA in CAD. As a result of these findings, we propose several design considerations for virtual object translation on 2D screens, which may enable designers to perceive a desirable feeling of control during the design process.

Classification of 3-Degree-of-Freedom 3-UPU Translational Parallel Mechanisms Based on Constraint Singularity Loci Using Gröbner Cover

Abstract A 3-degree-of-freedom (DOF) 3-UPU translational parallel mechanism (TPM) is one of the typical TPMs. Despite comprehensive studies on 3-UPU TPMs in which the joint axes on the base and the moving platform are coplanar, only a few 3-UPU TPMs with skewed base and moving platform have been proposed, and the impact of link parameters on constraint singularity loci of such TPMs has not been systematically investigated. The advances in computing comprehensive Gröbner system (CGS) or Gröbner cover of parametric polynomial systems provide an efficient tool for solving this problem. This paper presents a systematic classification of 3-UPU TPMs with skewed base and moving platform based on constraint singularity loci. First, the constraint singularity equation of a 3-UPU TPM is derived. Using Gröbner cover, the 3-UPU TPMs are classified into 12 types. Finally, a novel 3-UPU TPM is proposed. Reconfiguration analysis shows that unlike most existing 3-UPU TPMs which can transit from a 3-DOF translational mode to two or more 3-DOF operation modes, the proposed 3-UPU TPM can only transit from a 3-DOF translational mode to one general 3-DOF operation mode. The singularity locus divides the workspace of this 3-UPU TPM into two constraint singularity-free regions. As a by-product, a 3-UPU parallel mechanism that the moving platform can undergo 3-DOF translation and 1-DOF infinitesimal rotation is revealed. This work provides a solid foundation for the design of 3-UPU TPMs and a starting point for the classification of 3-UPU parallel mechanisms.

Dosimetry Between CBCT-Guided Translational Correction vs MR-Guided Online Adaptation for Prostate Ultra-Hypofractionated Radiotherapy

Purpose: Ultra-hypofractionated (³6Gy/fraction) radiotherapy for localized prostate cancer is delivered using daily image-guidance, with either Conebeam-CT (CBCT) or Magnetic Resonance Imaging (MRI). CBCT-Linac allows pre-delivery 3 degrees of freedom (3DOF) translation correction whereas MR-Linac enables treatment adaptation based on daily anatomy. Herein, we compared the dose to prostate and organs at risk (OARs) by these two systems.

Instant Visual Odometry Initialization for Mobile AR.

Mobile AR applications benefit from fast initialization to display world-locked effects instantly. However, standard visual odometry or SLAM algorithms require motion parallax to initialize (see Figure 1) and, therefore, suffer from delayed initialization. In this paper, we present a 6-DoF monocular visual odometry that initializes instantly and without motion parallax. Our main contribution is a pose estimator that decouples estimating the 5-DoF relative rotation and translation direction from the 1-DoF translation magnitude. While scale is not observable in a monocular vision-only setting, it is still paramount to estimate a consistent scale over the whole trajectory (even if not physically accurate) to avoid AR effects moving erroneously along depth. In our approach, we leverage the fact that depth errors are not perceivable to the user during rotation-only motion. However, as the user starts translating the device, depth becomes perceivable and so does the capability to estimate consistent scale. Our proposed algorithm naturally transitions between these two modes. Our second contribution is a novel residual in the relative pose problem to further improve the results. The residual combines the Jacobians of the functional and the functional itself and is minimized using a Levenberg-Marquardt optimizer on the 5-DoF manifold. We perform extensive validations of our contributions with both a publicly available dataset and synthetic data. We show that the proposed pose estimator outperforms the classical approaches for 6-DoF pose estimation used in the literature in low-parallax configurations. Likewise, we show our relative pose estimator outperforms state-of-the-art approaches in an odometry pipeline configuration where we can leverage initial guesses. We release a dataset for the relative pose problem using real data to facilitate the comparison with future solutions for the relative pose problem. Our solution is either used as a full odometry or as a pre-SLAM component of any supported SLAM system (ARKit, ARCore) in world-locked AR effects on platforms such as Instagram and Facebook.

Independent Electromagnetic Field Control for Practical Approach to Actively Locomotive Wireless Capsule Endoscope

Toward wireless medical microrobot applications driven by an electromagnetic actuation (EMA) system, challenges associated with movability, the electromagnetic force, and the coil system size must be addressed. This paper presents an enhanced EMA system with a higher magnetic field via new coil configurations, an independent magnetic field control method, and application to the multi-degree-of-freedom (DOF) motion of an untethered capsule endoscope. The magnetically actuated capsule endoscope (MACE) system proposed herein consists of an endoscopic capsule with a permanent magnet in the body, eight air-cored stationary electromagnetic coils, and a control system. The coil system is designed to maximize the working space available within a limited equipment space. The MACE is designed to perform full 5-DOF motion, including 3-DOF translation and 2-DOF rotation. The independent magnetic field control method with the new coil configuration enables orientation-independent-driving (OID) control of the capsule endoscope that could not be accomplished by previous EMA systems. The developed system performance was verified by simulations and experiments. The MACE motion in the spatial domain was evaluated with a robotic endoscopic procedure and diagnostic performance by in-vitro and ex-vivo experiments.

Registration of Magnetic Resonance Tomography (MRT) Data with a Low Frequency Adaption of Fourier-Mellin-SOFT (LF-FMS).

Fourier-Mellin-SOFT (FMS) is a rigid 3D registration method, which allows the robust registration of 3 degrees-of-freedom (dof) rotation, 1-dof scale, and 3-dof translation between scans on discrete grids. FMS is based on a spectral decomposition of these 7-dof. This complete spectral representation of the input data enables an adaption to certain frequency ranges. This special property is used here to focus on relevant mutual 3D information between bone structures with a Low Frequency adaptation of FMS (LF-FMS), that is, it is utilized for matching and concurrently determining corresponding transformation parameters. This process is applied on a set of Magnetic Resonance Tomography (MRT) data representing the hand region, in particular the carpal bone area, in a sequence of different hand positions. This data set is available for different probands, which allows a comparison of resulting parameter plots and furthermore matching in between bone structures.

Development of Vision Stabilizing System for a Large-Scale Flapping-Wing Robotic Bird

Bio-inspired flapping-wing flying robots have huge application potential in military reconnaissance, environment exploring, disaster rescue and so on. Vision sensors are usually mounted to provide image information of targets. However, the position and attitude of this kind of robot changes drastically during flapping flight, resulting in more challenges for stable imaging. In this paper, a vision stabilizing system (also called stabilizer or camera gimble) is developed to supply stable imaging condition for a large-scale flapping wing robotic bird with 2.3m wingspan and 650g weight. Based on theoretical analyses and experiments, flight characteristics of the robot are first obtained. Then a 3-DOF stabilizer is designed to supply 2-DOF rotation and 1-DOF translation movement. The kinematics and dynamics equations are then derived to analyze the disturbance on the fuselage caused by the stabilizer. Furthermore, the key parameters of the stabilizer are determined according to the constraints on the flight capability. To deal with the interaction between the stabilizer and the fuselage, a coordinated control strategy is proposed and the onboard controller is developed to control the compounded system. Finally, the prototype of the stabilizer is assembled and integrated to a real robotic bird. Practical flight experiments are carried out and the results show that the developed stabilizer has good working performance.

PinNPivot: Object Manipulation Using Pins in Immersive Virtual Environments.

Object manipulation techniques in immersive virtual environments are either inaccurate or slow. We present a novel technique, PinNPivot, where pins are used to constrain 1DOF/2DOF/3DOF rotations. It also supports 6DOF manipulation and 3DOF translation. A comparison with three existing techniques shows that PinNPivot is significantly more accurate and faster.

Analysis and Experiment of 5-DOF Decoupled Spherical Vernier-Gimballing Magnetically Suspended Flywheel (VGMSFW)

Due to the capacity of outputting both the high precision torque and the instantaneous large torque, the vernier-gimballing magnetically suspended flywheel (VGMSFW) is regarded as the key actuator for spacecraft. In this paper, a 5-DOF active VGMSFW is presented. The 3-DOF translation and 2-DOF deflection motions of the rotor are respectively realized by the spherical magnetic resistance magnetic bearings and the Lorentz magnetic bearing. The mathematical model of the deflection torque is established, and the decoupling between the 2-DOF deflections is demonstrated by the numerical analysis method. Compared with the conventional cylindrical magnetic bearings-rotor system, the spherical system is proven to eliminate the coupling between the rotor translation and deflection. In addition, a set of spherical magnetic resistance magnetic bearings with six-channel decoupling magnetic circuit are adopted to achieve the 3-DOF translation decoupling. The rotor dynamic model is derived, and the control system is established. The decoupling experiments and the torque experiments of the prototype are carried out. The results show that the decoupling among 5-DOF motions is realized and the instantaneous large torque can be obtained, which indicates that the requirements of the spacecraft can be highly satisfied by the spherical VGMSFW.

An image appearance based optimization scheme for monocular 6D pose estimation of SOR cabins

Monocular pose estimation for the surface of revolution (SOR) structure is a classical computer vision issue in aerospace applications. Existing pose estimation methods solve the 3-DoF translation and 2-DoF attitude reliably. However, they still require external constraints to solve the remained 1-DoF rotation and optimize the full 6D pose. This work proposes a general full 6D pose estimation method based on the image appearance feature and a non-linear optimization scheme. First, the position and attitude are solved from the imaged elliptic cross-sections. Then, an error objective function is designed to describe the appearance difference of the cross section caused by the pose error. Finally, the attitude is determined, and the full pose is solved by sorting and iteratively minimizing the objective function. Experiment results show that the proposed method is accurate, and is adaptive to view changes, dark illumination, and partial occlusion conditions. The average absolute translation and rotation error is 0.434 mm and 0.661° respectively, outperforming the keypoint-based baseline. The mean image difference on real images is 0.306, yielding a 17% error reduction from that of the fiducial marker benchmark.

6-DOF motion blur synthesis and performance evaluation of light field deblurring

Motion deblurring is essential for reconstructing sharp images from given a blurry input caused by the camera motion. The complexity of this problem increases in a light field due to its depth-dependent blur constraint. A method of generating synthetic 3 degree-of-freedom (3-DOF) translation blur on a light field image without camera rotation has been introduced. In this study, we generate a camera translation and rotation (6-DOF) motion blur model that preserves the consistency of the light field image. Our experiment results show that the proposed blur model can maintain the parallax information (depth-dependent blur) in a light field image. Furthermore, we produce a synthetic blurry light field dataset based on the 6-DOF model. Finally, to validate the usability of the synthetic dataset, we conduct extensive benchmarking using state-of-the-art motion deblurring algorithms.

Preliminary design and development of an active suspension gravity compensation system for ground verification

A new active suspension gravity compensation system (ASGCS) is developed to offload the gravity of spacecraft or satellites for ground verification. The ASGCS is of six degrees of freedom (6-DOF), including a compensation stage of 3-DOF translation and a gimbal of 3-DOF rotation. A buffer-assisted pinion-rack mechanism is developed to substitute the traditional cable suspension unit, which enables the proportion of gravity compensation is tunable. A new 3-DOF gimbal that is suitable for objects of various sizes and arbitrary shape is developed. Hence, the test object attached to the ASGCS can be free-floating as it is in the outer space. Furthermore, a prototype of the ASGCS is developed, and the dynamic model and kinematic model are derived. Experiments are conducted with the aid of a 6-DOF hybrid coordinate manipulator. The results demonstrate that the ASGCS successfully tracks 6-DOF trajectories of a typical docking task and compensates for 95% of the gravity.

Monocular Vision- and IMU-Based System for Prosthesis Pose Estimation During Total Hip Replacement Surgery.

The average age of population increases worldwide, so does the number of total hip replacement surgeries. Total hip replacement, however, often involves a risk of dislocation and prosthetic impingement. To minimize the risk after surgery, we propose an instrumented hip prosthesis that estimates the relative pose between prostheses intraoperatively and ensures the placement of prostheses within a safe zone. We create a model of the hip prosthesis as a ball and socket joint, which has four degrees of freedom (DOFs), including 3-DOF rotation and 1-DOF translation. We mount a camera and an inertial measurement unit (IMU) inside the hollow ball, or "femoral head prosthesis," while printing customized patterns on the internal surface of the socket, or "acetabular cup." Since the sensors were rigidly fixed to the femoral head prosthesis, measuring its motions poses a sensor ego-motion estimation problem. By matching feature points in images of the reference patterns, we propose a monocular vision based method with a relative error of less than 7% in the 3-DOF rotation and 8% in the 1-DOF translation. Further, to reduce system power consumption, we apply the IMU with its data fused by an extended Kalman filter to replace the camera in the 3-DOF rotation estimation, which yields a less than 4.8% relative error and a 21.6% decrease in power consumption. Experimental results show that the best approach to prosthesis pose estimation is a combination of monocular vision-based translation estimation and IMU-based rotation estimation, and we have verified the feasibility and validity of this system in prosthesis pose estimation.

Full-DOF Calibration of a Rotating 2-D LIDAR With a Simple Plane Measurement

This paper proposes a calibration method that accurately estimates six parameters between the two centers of 2-D light detection and ranging (LIDAR) and a rotating platform. This method uses a simple plane, and to the best of our knowledge, it is the first to enable full-degree-of-freedom (DOF) estimation without additional hardware. The key concept behind this method is a decoupling property, in which the direction of a line on a plane does not contain 3-DOF translation terms. Based on this, a cost function for rotation is constructed, and 3-DOF rotation parameters are estimated. With this rotation, the remaining 3-DOF translation parameters are calculated in a manner that minimizes the cost function for translation only. In other words, an original 6-DOF problem is decoupled into two 3-DOF estimation problems. Given these cost functions, degenerate cases are mathematically analyzed for known cases (incomplete), and the robustness is numerically tested for all possible cases (complete). The performance of the method is validated by extensive simulations and experimentations, and the estimated parameters from the proposed method demonstrate better accuracy than previous methods.

Optical Detector Array Design for Navigational Feedback Between Unmanned Underwater Vehicles (UUVs)

Designs for an optical sensor detector array for use in autonomous control of unmanned underwater vehicles (UUVs), or between UUVs and docking station, are studied in this paper. Here, various optical detector arrays are designed for the purpose of determining and distinguishing relative 5 degrees-of-freedom (DOF) motion between UUVs: 3-DOF translation and 2-DOF rotation (pitch and yaw). In this paper, a numerically based simulator is developed to evaluate varying detector array designs. The simulator includes a single light source as a guiding beacon for a variety of UUV motion types. The output images of the light field intersecting the detector array are calculated based on detector hardware characteristics, the optical properties of water, and expected noise sources. Using the simulator, the performance of planar and curved detector array designs (of varying size arrays) are analytically compared and evaluated. Output images are validated using empirical in situ measurements conducted in underwater facilities at the University of New Hampshire, Durham, NH, USA. Results of this study show that the optical detector array is able to distinguish relative 5-DOF motion with respect to the simulator light source. Furthermore, tests confirm that the proposed detector array design is able to distinguish positional changes of 0.2 m and rotational changes of 10 $^{\circ}$ within 4–8 m range in x-axis based on given output images.

Robust design for a flexible bearing with 1-DOF translation using the Taguchi method and the utility concept

A flexible bearing with compliant mechanisms for high-precision mechanism applications has been established as an efficient structure. We have attempted to realize a novel robust design of a flexible bearing with one degree of freedom (1-DOF) translation for highprecision mechanisms. First, the relationship between five design variables and the stress of the displacement responses is illustrated using the response surface method. Next, the Taguchi method, combined with a utility concept, is adopted to determine the optimal parameter combination to minimize stress while simultaneously maximizing displacement. An orthogonal array L27 is used in the experimental work. The experimental results show that displacement and stress values measured 0.11332 mm and 37.625 MPa, respectively. The confirmation results fall within 95% of the CICE. The proposed methodology is useful for the robust design of flexible bearings and related mechanisms.

A Flexible Bearing with 1-DOF Translation for High-Precision Mechanism

Traditional bearings with one degree of freedom (1-DOF) translation are due to sliding between rigid bodies; however, the wear, backlash, and low precision are existing defects. For high-precision mechanism to overcome these limitations, a flexible bearing with 1-DOF translation in this paper is designed alternatively by the use of the concept of compliant mechanism because its motion replies on elastic elements. Besides, the fatigue strength, fracture, and crack are frequently appeared as mechanical failures due to high stress at the fixed end of flexible hinges. To reduce mechanical failures, experiments are conducted by an L27 orthogonal array of the Taguchi multiple quality method to optimize design parameters, including an applied force and the length, width, thickness, and filleted radius of flexible hinges considering the stress concentration. The results demonstrate that the resulting stress of the new design flexible bearing is almost 99.7% smaller than that of the original design.