Adaptive control for enhancing tracking performances of flexible tendon–sheath mechanism in natural orifice transluminal endoscopic surgery (NOTES)

Tendon–sheath mechanism (TSM) has inherent advantages in the development of flexible robotic systems because of its simplicity, safety, flexibility, and ease of transmission. However, the control of TSM is challenging due to the presence of nonlinearities, namely friction, backlash-like hysteresis and the time-varying configuration of the TSM during its operations. Existing studies of TSM found in the literature only address tendon transmission under the assumption of fixed configuration and a complex inverse model of backlash is required. In order to flexibly use the system in a wider range of applications, the aforementioned nonlinear effects have to be characterized for the purpose of compensation. In this paper, we endeavor to address these issues by presenting a series of controller strategies, namely a feedforward control scheme under the assumption of known backlash-like hysteresis profile, and an adaptive control scheme to characterize the nonlinearities with unknown backlash hysteresis and uncertainties. The proposed control schemes do not require information of the tendon–sheath configurations, which is challenging to obtain in practice, in the compensation structures. In the absence of output position feedback, a simple direct inverse model-based feedforward has been used that efficiently reduce the tracking errors. The feedforward compensation does not require any complex algorithm for the inverse model. In the presence of output position feedback, a nonlinear adaptive controller has been developed to enhance the tracking performances of the TSM regardless of the random change in the tendon–sheath configurations during compensation. In addition, exact values of the model parameters are not required. They are estimated online during the operations. A dedicated experimental setup is introduced to validate the proposed control approaches. The results show that the proposed control schemes significantly improve the tracking performances for the TSM in the presence of uncertainties and time-varying configurations during the operations. There is a significant decrease of 0.0158 rad² (before compensation) to smaller value of 0.0012 rad² (use feedforward control) and 8.2815 × 10⁻⁵ rad² (use nonlinear controller) after compensation.     

Nonlinear identification and optimal feedforward friction compensation for a motion platform

In this study, we present a method of nonlinear identification and optimal feedforward friction compensation for an industrial single degree of freedom motion platform. The platform has precise reference tracking requirements while suffering from nonlinear dynamic effects, such as friction and backlash in the driveline. To eliminate nonlinear dynamic effects and achieve precise reference tracking, we first identified the nonlinear dynamics of the platform using Higher Order Sinusoidal Input Describing Function (HOSIDF) based system identification. Next, we present optimal feedforward compensation design to improve reference tracking performance. We modeled the friction using the Stribeck model and identified its parameters through a procedure including a special reference signal and the Nelder–Mead algorithm. Our results show that the RMS trajectory tracking error decreased from 0.0431 deg/s to 0.0117 deg/s when the proposed nonlinear identification and friction compensation method is utilized.     

钢丝绳传动的三轴转台伺服控制的设计

针对某机载平台中钢丝绳传动带来的非线性误差及外部扰动等因素直接影响平台相机成像质量的问题,提出一种模糊自适应前馈补偿的控制策略.首先对钢丝绳传动机构和高精度直流伺服电机进行了建模,并建立了摩擦模型,为转台速度环控制回路引入模糊自适应PID控制器.设计出前馈补偿和模糊自适应控制器的复合控制策略.Matlab仿真结果以及实...     

Hysteresis modeling and position control of tendon-sheath mechanism in flexible endoscopic systems

Tendon-sheath mechanism has “revolutionized” the use of flexible endoscopic systems, by its many advantages of high maneuverability, lightweight, low cost, and simple design. However, nonlinear phenomena such as friction and backlash hysteresis present major challenges for motion control of the tool tips. This paper introduces a new mathematical model and a control scheme for the tendon-sheath mechanism for use in endoscopic systems. The asymmetric backlash hysteresis model that characterizes the transmission phenomena of the tendon-sheath mechanism in the loading and unloading phases is presented and discussed. An efficient parameter identification method is used to estimate the model parameters. Comparisons between the proposed model and experimental data validate the adoption of this new approach. A feedforward compensation method based on the asymmetric backlash hysteresis model is proposed and explored. The proposed model and control scheme are validated by experimental studies using a suitable experimental setup. The results show that the proposed model and the control scheme can improve the accuracy of tendon-sheath mechanism without using any output feedback and can be easily implemented in surgical robots using tendon-sheath mechanism as the main mode of transmission.     

Input-dependent stability of joint torque control of tendon-drivenrobot hands

The input-dependent stability observed during torque control experiments using the first joint of the Darmstadt-HAND is discussed. Friction and compliance existing in tendon-sheath drive systems introduce a hysteresis nonlinearity between the joint torque output and the actuator displacement. Although this transmission characteristic is close to the well-known backlash behavior of the gears situated between a motor and a load shift, this hysteresis loop exhibits input-dependent characteristics in the backlash region of the transmission system, with springlike behavior within a portion of the backlash region. Experiments confirmed that there is a close relationship between the input-dependent backlash characteristics and the input-dependent stability. Based on these experiments, the authors describe the transmission characteristic using a simple model and explore the system stability using sinusoidal-input-describing-functions (SIDF). A nondimensional stability-criterion-map that successfully predicts the experimental results is presented     

Friction modeling and adaptive compensation using a relay feedbackapproach

In this paper, the application of a dual-relay feedback approach toward modeling of frictional effects in servomechanisms is addressed. The friction model consists of Coulomb and viscous friction components, both of which can be automatically extracted from suitably designed relay experiments. At the same time, the dynamical model of the servomechanical system can be obtained from the experiments. Thus, a proportional-integral-derivative feedback motion controller and a feedforward friction compensator can be automatically tuned in this manner. The friction model obtained is also directly applicable to initialization of an adaptive control scheme proposed. Results from simulation and experiments are presented to illustrate the practical appeal of the proposed method     

Robust motion controller design for high-accuracy positioningsystems

This paper presents a controller structure for robust high speed and accuracy motion control systems. The overall control system consists of four elements: a friction compensator; a disturbance observer for the velocity loop; a position loop feedback controller; and a feedforward controller acting on the desired output. A parameter estimation technique coupled with friction compensation is used as the first step in the design process. The friction compensator is based on the experimental friction model and it compensates for unmodeled nonlinear friction. Stability of the closed-loop is provided by the feedback controller. The robust feedback controller based on the disturbance observer compensates for external disturbances and plant uncertainties. Precise tracking is achieved by the zero phase error tracking controller. Experimental results are presented to demonstrate performance improvement obtained by each element in the proposed robust control structure     

A DSP-based adaptive controller for a smooth positioning system

The implementation of a self-tuning regulator for the positioning of a direct-drive servomotor is described. The servo motor is a permanent magnet DC motor in which no speed reducer is used. The auto-tuning regulator consists of two major loops. The inner loop contains a feedback (PD or PID) regulator with additional feedforward terms. The parameters of the feedforward compensation are adjusted by the outer loop, which contains an online parameter estimator. The estimator is based on a recursive least-squares equation, and the estimated parameters are the load inertia and viscous friction. This self-tuning regulator has been simulated with PC.MATLAB, and the results demonstrate the high performance of the scheme. Experimental results obtained with a small DC motor (Electrocraft E-576) are presented, and these results show good agreement with the digital simulation results. There are two innovative aspects to this work. First, parameter estimation is used to adapt the feedforward compensation terms instead of the gains of the feedback controller, as usually is the case in conventional indirect self-tuning regulators. Secondly, the complete adaptive controller has been implemented using a single-chip digital signal processor (DSP), which results in the reduction of system hardware and cost     

An adaptive controller for a direct-drive SCARA robot

A direct adaptive controller for trajectory tracking of high-speed robots such as a direct-drive SCARA robot is presented. In this robot, nonlinear effects due to centrifugal, Coriolis, and inertial forces are more important than friction and gravity forces, unlike most industrial robots. The control law of the adaptive scheme consists of a PD regulator plus feedforward compensation of full dynamics. The feedforward terms are adjusted by an adaptation law so that the steady-state position errors are zero. With this adaptive controller, the joint acceleration measurement is not required and no inversion of the estimated mass matrix is involved. The tracking performances of the controller applied to a two-degree-of-freedom SCARA is illustrated by a real-time implementation based on a single-chip digital signal processor (DSP)     

Piecewise affine modeling and compensation in motion of linear ultrasonic actuators

Ultrasonic actuators used in high-precision mechatronics possess strong frictional effects, which are among the main problems in precision motion control. Traditional methods apply model-based nonlinear feedforward to compensate the friction, thus requiring closed loop stability and safety constraint considerations. In this article, model-based parametric controllers are developed to obtain an optimal positioning control for these motors. A systematic approach which uses piecewise affine models greatly simplifies the friction model compared to the traditional methods. Issues about the nonlinear effects of the friction are addressed by designing a robust control law near zero speed. These developments result in a gain-scheduling optimal input, which is simple to carry out in real-time applications. The controller is expected to improve the safety constraints and the tracking performance for actuator operation.     

Optimized friction drive controller for a multi-DOF ultrasonic nanopositioner

This paper presents a new generation of compliant multi-degrees-of-freedom piezoelectric nanopositioner for positioning, transport, alignment of micro-objects under the field of view of a microscope. It is based on the cooperation of arrayed direct-drive standing-wave ultrasonic actuators (microSWUMs). A number of nonlinearities exist in the actuator due to its macro- and microdynamics. An optimized friction drive multidimensional controller is proposed based on a closed-loop electromagnetic field-based preload controller ensuring optimal preload, and a feedforward pulsewidth modulation (PWM) controller with input shaping for driving force control. These techniques are applied to reduce the effects of low-speed-low-force instabilities due to stick-slip and friction pairs which lead to output oscillations during nanometric stepping motion. The closed-loop positioning system designed with microSWUMs produced 10-nm resolution and 5% displacement repeatability in a low-speed-low-force region; unlimited travel with velocities of 0.3 m.s/sup -1/ and driving forces around 2 mN in a high-speed-high-force region.     

Interaction control of an industrial manipulator using LPV techniques

This paper presents the design and implementation of a hybrid force/motion control scheme on a six-degrees-of-freedom robotic manipulator employing a gain-scheduled linear parameter-varying (LPV) controller. A nonlinear dynamic model of the manipulator is obtained and the unknown parameters are estimated. The manipulator is decomposed into an inner and a wrist submodel, and a practical way is proposed to investigate the coupling between them. The motion control part of the hybrid controller which is the main focus of this paper is formed by a combination of an LPV controller and a model-based inverse dynamics controller for the inner submodel and the wrist joints, respectively. A quasi-LPV model with a reduced number of scheduling parameters is derived for the inner submodel, and a polytopic LPV gain-scheduled controller is synthesized in a two-degrees-of-freedom structure including feedback and feedforward parts, which is augmented by a friction compensation term. A PD controller with a feedforward path is designed to control the interaction force. The proposed hybrid force/motion scheme is implemented on the 6-DOF CRS A465 robotic manipulator to perform a writing task. Comparison of the results with those of a hybrid force/motion controller with a plain model-based inverse dynamics motion control and the same force control shows that the proposed controller improves the position tracking performance significantly.     

Backlash characterization and position control of a robotic catheter manipulator using experimentally-based kinematic model

This paper deals with the problem of position control of a robot-assisted catheter manipulator for intracardiac interventions. Kinematic controllers based on the Jacobian inverse are not suitable for the catheter because of multiple singularities in the workspace and significant backlash of the system. To tackle these issues, first, the backlash of the system in axial translation and twist motion due to the interaction of the catheter with the veins through which it passes, are characterized. The effects of variations in clinical settings on the backlash parameters are identified through extensive experiments simulating the clinical procedure. Next, to properly compensate for the backlash behavior of the catheter distal shaft, an inverse kinematic model is proposed based on experimental data. A position controller is developed and implemented using the experimentally-based inverse kinematics and the identified inverse backlash model for the twist and axial motion. The results of experiments performed using a robotic catheter manipulation system show that with a proper choice of controller gains, the proposed scheme is able to guide the catheter tip to the goal with the desired behavior. The tracking performance of the controller has also been evaluated under the dynamic external force simulating the blood drag force. The empirical results demonstrate the improved performance of the proposed approach against the existing kinematic-based and uncompensated control schemes.     

Position control of shape memory alloy actuator based on the generalized Prandtl–Ishlinskii inverse model

Hysteresis and significant nonlinearities in the behavior of Shape Memory Alloy (SMA) actuators encumber effective utilization of these actuator. Due to these effects, the position control of SMA actuators has been a great challenge in recent years. Literature review of the research conducted in this area shows that using the inverse of the phenomenological hysteresis models can compensate the hysteresis of these actuators effectively. But, inverting some of these models, such as Preisach model, is numerically a complex task. However, the generalized Prandtl–Ishlinskii model is analytically invertible, and therefore can be implemented conveniently as a feedforward controller for compensating hysteresis nonlinearities effects in SMA actuators. In this paper a feedforward–feedback controller is used to control the tip deflection of a large deflected flexible beam actuated by an SMA actuator wire. The feedforward part of the control system is based on the generalized Prandtl–Ishlinskii inverse model while a conventional proportional–integral feedback controller is added to the feedforward controller to increase the accuracy together with eliminating the steady state error in position control process. Experimental results show that the proposed controller performs well in terms of achieving small overshoot and undershoot for square wave tracking as well as small tracking errors for sinusoidal trajectory. It has also great capability for tracking hysteresis minor loops.     

New hybrid controller for systems with deterministic uncertainties

In this paper, a new hybrid controller for systems with deterministic uncertainties is developed. The proposed controller identifies and compensates deterministic uncertainties simultaneously. It is the combination of a time-domain feedback controller and a frequency-domain iterative learning controller. The feedback controller decreases system variability and reduces the effect of random disturbances. The iterative learning controller shapes the system input to suppress the error caused by deterministic uncertainties such as friction and backlash. The control scheme use only local input and output information, no system model is required. The uncertainties can be structured or unstructured. The effectiveness of the proposed controller is experimentally verified on a servo system with gearbox     

Adaptive dynamic surface control with sliding mode control and RWNN for robust positioning of a linear motion stage

Ball-screw-driven system provides high precision and long stroke range for positioning and tracking control of a linear stage. Friction and backlash nonlinearities in this system act often the main obstacles for high precision control. It is difficult to achieve effective compensation of these types of nonlinearities by traditional linear control methodology without the aid of a proper compensation schemes. Here, we present an adaptive dynamic surface control scheme combined with sliding mode control to compensate for friction and backlash nonlinearities in a linear stage motion system. The adaptive laws of the recurrent wavelet neural networks and friction estimation are derived to approximate and compensate for the backlash and friction nonlinearities. The boundedness and convergence of the closed-loop system are guaranteed from a Lyapunov stability analysis. The performance of the proposed control scheme was verified through simulations and experiments on the ball-screw-driven linear stage.     

Geared PM coreless motor modelling for driver’s force feedback in steer-by-wire systems

Gearboxes allow for changes in the motor’s torque and speed ranges, thus allowing the motor to function at its best operation ranges. As a consequence, smaller and low cost motors can be used in comparison with direct drive motors. Nevertheless, gearboxes introduce nonlinearities to the system such as friction, backlash and flexibility. When geared motors are employed, a controller using a torque sensor or a torque observer is normally required to compensate for the drawbacks associated with the gearbox and the servo-amplifier. The design of such a controller requires the comprehensive knowledge of the system’s dynamics. In this paper, a general approach to model accurately amplifier–motor–gearbox assemblies has been developed. This approach that takes into account backlash, flexibility, friction for stiction and sliding, identification procedures, is applicable to a wide range of amplifier–motor–gearbox assemblies. It is explained by applying it to a particular case: an amplifier–motor–gearbox assembly for a driver’s force feedback system. In the design of driver’s force feedback systems for steer-by-wire systems or for high fidelity Human in the Loop (HiL) driving simulators, either direct drive motors or geared motors are used, independently of the motor type. The assembly considered here is composed of a two stage planetary gearbox, a coreless PMDC motor and a linear four quadrant servo-amplifier. It is installed in an X-by-wire (XBW) vehicle prototype, Both the amplifier and the mechanical components were built in the model. The four quadrant operating modes of the amplifier were taken into account. Friction within both the motor and the gearbox are modelled using a modification of the LuGre friction model that allows friction to be considered as load-dependent. Backlash and flexibility in the gearbox are considered together using a fifth order polynomial for each rotational direction. The identification procedures necessary to calculate the parameters of the model are presented. Because all the parameters of the model have a direct physical significance, these identification procedures are easy to realize. Comparisons between simulations realized with Simulink and the experimental data for three typical driving situations show that the model is highly accurate at representing real system dynamics.     

A relay-based method for servo performance improvement

This paper proposes a relay-based performance-improving method for servo mechanism systems. The method first utilizes a relay-based feedback technique to identify the model parameters and the Coulomb friction value. Then, based on the identified results, a control algorithm, which consists of a feedforward controller, a time-delay compensator and a sliding mode controller, is designed. The feedforward controller and the time-delay compensator are used to compensate the system dynamics and the external disturbances respectively. Their parameters are decided directly according to the identified values. The sliding mode controller is to stabilize the system, two of whose parameters are one-to-one mapping to the closed-loop characteristic roots. Thus, this method avoids the complicated parameters tuning process, which is attractive in practice to the control engineers. Experimental studies on a linear-motor-driven table illustrate that the proposed method is capable of improving the servo performance greatly and canceling the external disturbances effectively.     

Design and modeling of a flexible test bed for use in control system analysis and verification

This paper describes a test bed specifically designed for studying the effects of nonlinear friction, backlash, and drive train flexibility on structurally flexible electro-mechanical systems, and for developing control algorithms that will compensate for nonlinear friction, backlash, and drive train flexibility in structurally flexible electro-mechanical systems. The test bed has a modular and adjustable design. Friction, backlash, and flexibility can all be varied or disabled. A mathematical model of the dynamics of the test bed is also described. This model includes stiction, Coulomb friction, backlash and drive train, and link flexibility. The model is verified via comparisons of model-predicted and experimental responses.     

Use of synchronised dither in pseudorandom-sequence testing

A method of applying synchronising dither signals to pseudo-random-binary-sequence measurements is described. This enables the linear response of certain systems to be obtained when small movements of their output are affected by stiction, Coulomb friction and/or backlash.