Tendon-driven Continuum Manipulator Research Articles

Recurrent neural network robust curvature tracking control of tendon-driven continuum manipulators with simultaneous joint stiffness regulation

Recurrent neural network robust curvature tracking control of tendon-driven continuum manipulators with simultaneous joint stiffness regulation

An Operating Stiffness Controller for the Medical Continuum Robot Based on Impedance Control.

Continuum manipulators can conform to curvilinear paths and manipulate objects in complex environments, which makes it emerging to be applied in minimally invasive surgery (MIS). However, different and controllable operating stiffness of the continuum manipulator is required during different stages of surgery to achieve safe access or stable and precise operation. This work proposes an operating stiffness controller (OSC) for the typical tendon-driven continuum manipulator based on the variable impedance control method with Lagrangian dynamic modeling. This controller can adjust the operating stiffness by modifying the driving forces along the driving tendons of the continuum manipulator without changing its material or structure. The proposed OSC converts the damping and stiffness matrices of the impedance control into variable parameters. This merit allows it to dynamically adjust the operating stiffness of the continuum manipulator according to the desired constant or time-varying stiffness. Furthermore, the OSC stability can be proven based on a Lyapunov function, and its stiffness control performances have been analyzed and evaluated in both simulations and experiments. The OSC controller generated average relevant error values of 7.82% and 3.09% for the operating stiffness control experiments with constant and time-varying desired stiffness, respectively. These experimental results indicate that the OSC has high accuracy, stability, and strong robustness in the operating stiffness control tasks.

Design and modeling of a planar-to-spatial tendon-driven continuum manipulator subjected to uncertain forces

Continuum Manipulators (CMs) have gained popularity as a promising solution for exploring unstructured environments due to their dexterity, compliance, and redundancy. Studies on CMs have focused individually on planar and spatial configurations, ranging from single to multi-section constructions. Planar CMs are easier to design, have simpler kinematics equations and possesses less number of tendons with reduced dexterity, redundancy, and reach. Whereas spatial CMs, offer a larger workspace with improved dexterity and redundancy while computationally expensive. This paper introduces a novel two-section tendon-driven continuum manipulator (TDCM) design that uses two planar sections to manipulate the distal end of the manipulator in three-dimensional space to handle external interaction from the environment. The proposed solution offers improved performance in terms of handling payloads and tip deflection under external interactions. This design is intended for agricultural applications such as vegetable harvesting and requires larger workspace and fewer actuators. We demonstrate the effectiveness of our design through kinematic and static analysis. The kineto-static model estimates the profile of the proposed TDCM under external interaction acting at the manipulator's distal end. Finally, the proposed design is experimentally validated by an in-housed prototype of an two-section TDCM, actuated by two tendons/section.

A Two-Segment Continuum Robot With Piecewise Stiffness for Maxillary Sinus Surgery and Its Decoupling Method

With the use of tendon-driven continuum manipulators, it is possible to reach deep-seated small lesions in a constrained anatomical space. This circumvents the difficulties encountered by existing straight endoscopes and surgical instruments, which require direct line of sight access during inspection and operation. However, maintaining adequate stiffness while ensuring smoothness and flexibility during operation for a continuum robot with a small diameter is difficult. In this article, we present the design, modeling, and validation of a new two-segment continuum robot for maxillary sinus surgery. It addresses the coupling effects that can affect the safety and precise operation within the maxillary sinus. To overcome potential distal-to-proximal deviations, a piecewise stiffness structure for the continuum manipulator is proposed. Modified kinematics, based on variable curvature continuum kinematics, is used to resolve motion coupling, and a model of the continuum manipulator with varying stiffness is proposed, with due consideration of tension coupling. Experiments are carried out to validate the proposed design, modeling, and compensation schemes. Results show that the proposed manipulator can flexibly and accurately reach the target positions in the maxillary sinus model. Comparisons of continuum robots with and without the proposed compensation schemes demonstrate the practical value and potential clinical use of the proposed method.

Influence of Antagonistic Tensions on Distributed Friction Forces of Multisegment Tendon-Driven Continuum Manipulators With Irregular Geometry.

In this paper, we thoroughly analyze the effect of single-tendon and antagonistic tendons actuation on tension loss of multi-segment tendon-driven continuum manipulators (TD-CMs) with irregular geometry. To this end, we propose a generic analytical modeling approach and iterative algorithm that can solve the unknown correlation between distributed friction force, tendons' tension transmission loss, and planar deformation behavior of TD-CMs during tendons' pulling and releasing phases. The proposed generic model solely relies on known input tendons' tensions and does not require a priori knowledge of the manipulator's shape and/or other experimental conditions. To investigate the influence of actuation type on tension loss and deformation behavior of TD-CMs, we fabricated two different TD-CMs and performed various simulation and experimental studies with single-tendon and antagonistic tensions actuations. The obtained results indicate the importance of considering the effect of distributed friction force and actuation type on tension(s) loss of multi-segment TD-CMs. Moreover, it clearly demonstrates the efficacy and accuracy of the proposed model in providing insights and understanding of tension transmission process in various types of actuations in multi-segment TD-CMs with irregular geometry.

Impact of Generic Tendon Routing on Tension Loss of Tendon-Driven Continuum Manipulators with Planar Deformation.

We present a novel physically-intuitive mathematical formulation to investigate the effects of a fully-constrained generic tendon routing (GTR) on the correlation between tension loss and deformation behavior of a variable-curvature tendon-driven continuum manipulator (TD-CM). The proposed model can account for distributed friction forces/moments along a GTR path that have been typically ignored in the previous approaches (e.g., the well-known frictionless Cosserat rod model). For the first time, the internal distributed forces on a GTR are expressed using three physically-intuitive generic functions. Solely relying on the known actuation input(s), the proposed mathematical formulation can also solve the entangled and unknown correlation between GTR, internal distributed forces, tension loss and deformation behavior of TD-CMs. To evaluate the performance of the proposed approach, we performed various simulation studies using eight different GTR paths. Additionally, we fabricated two different types of TD-CMs with different GTRs to experimentally evaluate the efficacy and performance of the proposed mathematical framework. The results demonstrate the proposed model can successfully and accurately (i.e., about <10% error) capture the trends of substantial tension loss (e.g., about <50%) on fully constrained GTRs, which reveals the importance of considering tension loss in modeling these TD-CMs.

A learning-based tip contact force estimation method for tendon-driven continuum manipulator

Although tendon-driven continuum manipulators have been extensively researched, how to realize tip contact force sensing in a more general and efficient way without increasing the diameter is still a challenge. Rather than use a complex modeling approach, this paper proposes a general tip contact force-sensing method based on a recurrent neural network that takes the tendons’ position and tension as the input of a recurrent neural network and the tip contact force of the continuum manipulator as the output and fits this static model by means of machine learning so that it may be used as a real-time contact force estimator. We also designed and built a corresponding three-degree-of-freedom contact force data acquisition platform based on the structure of a continuum manipulator designed in our previous studies. After obtaining training data, we built and compared the performances of a multi-layer perceptron-based contact force estimator as a baseline and three typical recurrent neural network-based contact force estimators through TensorFlow framework to verify the feasibility of this method. We also proposed a manually decoupled sub-estimators algorithm and evaluated the advantages and disadvantages of those two methods.

Effect of External and Internal Loads on Tension Loss of Tendon-Driven Continuum Manipulators

We present a novel analytical modeling approach to investigate the combined effects of external loads (i.e., contact and gravitational loads) and internal distributed friction forces on tension loss of a generic variable-curvature tendon-driven continuum manipulator (TD-CM). Unlike previous studies, in our proposed approach, we do not assume the deformation behavior (e.g., bending shape/curvature) and friction distribution profile are known a priori and/or can be experimentally obtained/tuned before or during the experiments. Instead, we only assume the tendons' tensions at the base of manipulator and magnitudes and locations of external forces are available while both deformation behavior and internal distributed forces are unknown and correlated. To evaluate the efficacy of the proposed approach, we performed different sets of simulation and experimental studies in the presence of both concentrated point and distributed surface contact loads. We compared the results of the performed studies with the results obtained by a friction-less modeling technique. This comparison clearly demonstrates the phenomenal performance of the proposed modeling approach in modeling tendon tension loss of a TD-CM experiencing both internal and external loads.

A Model-Based Method for Predicting the Shapes of Planar Single-Segment Continuum Manipulators With Consideration of Friction and External Force

Abstract The shape prediction of tendon-driven continuum manipulators is a challenging problem due to the effect of inner friction and external force. Many researchers use actuation displacement or actuation force as model input to predict the shapes of manipulators, but very few consider their relations and models able to predict the status of friction. This paper proposes a model-based method that combines the mechanics model with the kinematic model to predict the shapes of planar single-segment manipulators with consideration of external force and friction. Finally, the shape prediction of manipulators is converted to an optimization problem with actuation displacement and actuation force as the inputs of our algorithm. The distribution of tendon force and the situation of friction can be calculated by using the feedback data of the actuation unit even when actuation direction changes and hysteresis occurs. Experimental results indicate that the method has good performance in predicting the manipulator’s shapes.

Kinematics Modelling of Tendon-Driven Continuum Manipulator with Crossed Notches

Single port surgical robot (SPSR) is a giant leap in the development of minimally invasive surgical robot. An innovative manipulator with high control accuracy and good kinematic dexterity can reduce wound, expedite recovery, and improve the success rate. This paper presents a tendon-driven continuum manipulator with crossed notches. This manipulator has two degrees of freedom (DOF), which possesses good flexibility and high capacity. Then based on cantilever beam theory, a mechanics model is proposed, which connects external force and deformation of a single flexible ring (SFR). By calculating the deformation of each SFR, the manipulator is considered as a series robot whose joint numbers is equal to SFR numbers, and the kinematics model is established through Denavit-Hartenberg (D-H) procedure. In this paper, the total manipulator is described as a curve tube whose curvature is increased from tip to base. Experiments were conducted and the comparison between theoretical and actual results proved the rationality of the models.

Kinematic comparison of surgical tendon-driven manipulators and concentric tube manipulators

Robot manipulators are increasingly used in minimally invasive surgery (MIS). They are required to have small size, wide workspace, adequate dexterity and payload ability when operating in confined surgical cavity. Snake-like flexible manipulators are well suited to these applications. However, conventional fully actuated snake-like flexible manipulators are difficult to miniaturize and even after miniaturization the payload is very limited. The alternative is to use underactuated snake-like flexible manipulators. Three prevailing designs are tendon-driven continuum manipulators (TCM), tendon-driven serpentine manipulators (TSM) and concentric tube manipulators (CTM). In this paper, the three designs are compared at the mechanism level from the kinematics point of view. The workspace and distal end dexterity are compared for TCM, TSM and CTM with one, two and three sections, respectively. Other aspects of these designs are also discussed, including sweeping motion, scaling, force sensing, stiffness control, etc. From the results, the tendon-driven designs and concentric tube design complement each other in terms of their workspace, which is influenced by the number of sections as well as the length distribution among sections. The tendon-driven designs entail better distal end dexterity while generate larger sweeping motion in positions close to the shaft.

Steering of Multisegment Continuum Manipulators Using Rigid-Link Modeling and FBG-Based Shape Sensing

Accurate closed-loop control of continuum manipulators requires integration of both models that describe their motion and methods to evaluate manipulator shape. This work presents a model that approximates the continuous shape of a continuum manipulator by a serial chain of rigid links, connected by flexible rotational joints. This rigid-link model permits a description of manipulator shape under different loading conditions. A kinematic controller, based on the manipulator Jacobian of the proposed rigid-link model, is implemented and realizes trajectory tracking, while using the kinematic redundancy of the manipulator to perform a secondary task of avoiding obstacles. The controller is evaluated on an experimental testbed, consisting of a planar tendon-driven continuum manipulator with two bending segments. Fiber Bragg grating (FBG) sensors are used to reconstruct 3-D manipulator shape, and is used as feedback for closed-loop control of the manipulator. Manipulator steering is evaluated for two cases: the first case involving steering around a static obstacle and the second case involving steering along a straight path while avoiding a moving obstacle. Mean trajectory tracking errors are 0.24 and 0.09 mm with maximum errors of 1.37 and 0.52 mm for the first and second cases, respectively. Finally, we demonstrate the possibility of FBG sensors to measure interaction forces, while simultaneously using them for shape sensing.

Configuration Tracking for Continuum Manipulators With Coupled Tendon Drive

Robotic control of flexible devices can enhance and simplify many medical procedures. We present a method for controlling a tendon-driven continuum manipulator by means of specifying the shape configuration. The basis for control is a linear beam configuration model that transforms beam configuration to tendon displacement by modeling internal loads of the compliant system. An essential aspect of this model is the inclusion of both the mechanical and geometrical coupling among serial articulating sections. Important capabilities of this model are the general forward kinematics and the decoupled inverse kinematics that allow for independent control of multiple sections. Tracking results are presented for a cardiac catheter with two articulating sections.