Tegoeh Tjahjowidodo

A New Approach to Modeling Hysteresis in a Pneumatic Artificial Muscle Using The Maxwell-Slip Model

Two main challenges in using a pneumatic artificial muscle (PAM) actuator are the nonlinearity of pneumatic system and the nonlinearity of the PAM dynamics. The latter is complicated to characterize. In this paper, a Maxwell-slip model used as a lumped-parametric quasi-static model is proposed to capture the force/length hysteresis of a PAM. The intuitive selection of elements in this model interprets the unclear, but blended contributing causes of the hysteresis very well, which are assumed to originate from the dry friction of the double helix weaving of the PAM braided shell, the friction of the weaving and the bladder, the elasticity of the bladder and/or the deformation of the conical parts of a PAM close to the end caps. The obtained model is simple, but physically meaningful and easy to handle in terms of control.

Identification of pre-sliding friction dynamics

The hysteretic nonlinear dependence of pre-sliding friction force on displacement is modeled using different physics-based and black-box approaches including various Maxwell-slip models, NARX models, neural networks, nonparametric (local) models and dynamical networks. The efficiency and accuracy of these identification methods is compared for an experimental time series where the observed friction force is predicted from the measured displacement. All models, although varying in their degree of accuracy, show good prediction capability of pre-sliding friction. Finally, we show that even better results can be achieved by using an ensemble of the best models for prediction.

FRICTION IDENTIFICATION AND COMPENSATION IN A DC MOTOR

Abstract Friction modeling and identification is a prerequisite for the accurate control of electromechanical systems. This paper considers the identification and control of friction in a high load torque DC motor to the end of achieving accurate tracking. Model-based friction compensation in the feedforward part of the controller is considered. For this purpose, friction model structures ranging from the simple Coulomb model through the recently developed Generalized Maxwell Slip (CMS) model are employed. The performance of those models is compared and contrasted in regard both to identification and to compensation. It turns out that the performance depends on the prevailing range of speeds and displacements, but that in all cases, the CMS model scores the best.

Adaptive control of position compensation for Cable-Conduit Mechanisms used in flexible surgical robots

Natural Orifice Transluminal Endoscopic Surgery (NOTES) is a method that allows for performing complex operations via natural orifices without skin incisions. Its main tool is a flexible endoscope. Cable-Conduit Mechanisms (CCMs) are often used in NOTES because of its simplicity, safety in design, and easy transmission. Backlash hysteresis nonlinearities between the cable and the conduit pose difficulties in the motion control of the NOTES system. It is challenging to achieve the precise position of robotic arms when the slave manipulator inside the humans body. This paper presents new approaches to model and control for pairs of CCMs. It is known that the change of cable-conduit configuration will affect the backlash hysteresis non-linearities. To deal with such change, a new nonlinear and adaptive control scheme will be introduced. The backlash hysteresis parameters are online estimated under the assumption of availability of output feedback and unknown bound of nonlinear parameters. To validate the proposed approach, a prototype of single-DOF-Master-Slave system, which consists of a master console, a telesurgical workstation, and a slave manipulator, is also presented. The proposed compensation scheme is experimentally validated using the designed system. The results show that the proposed control scheme efficiently improves the tracking performances of the system regardless of the change of endoscope configuration.

Dynamic Friction-Based Force Feedback for Tendon-Sheath Mechanism in NOTES System

Tendon-sheath mechanism (TSM) is commonly used in flexible endoscopic systems because of its high flexibility, light weight, and easy transmission. Due to the size constraints and sterilization problems, traditional sensors cannot be placed at the tool tips of robotic arms. In addition, nonlinear friction and backlash hysteresis cause many challenges to predict the desired force when the system inside the humans body. It is extremely difficult to provide the force information to haptic devices and subsequently to the users. In this paper, a new scheme of dynamic friction model for a pair of TSMs is proposed to estimate the force at distal end of endoscopic system. In comparison with current approaches in the literature, our model is able to provide continuous force information. The model is able to predict distal force with any sheath shapes. An experimental setup is designed to measure the friction force in the TSM. Finally, the validity of the proposed model approach is confirmed with a good agreement between the estimated values and real experimental data.

Non-local memory hysteresis in a pneumatic artificial muscle (PAM)

In a system using PAMs, a big research effort has been carried out to solve the control problem, in which the nonlinear dynamics of a PAM were left behind as a disturbance to that system. The inherent dynamics in a PAM is due to its constitutional materials which cause hysteresis during cyclic contraction/extension. Prior knowledge of the hysteresis behavior in a PAM may simplify the associated control system. In this paper, the hysteretic behavior of a PAM is investigated and the results show much similarity to the presliding regime in the friction of mechanical contacting elements. The PAM hysteresis is thus generalized and represented by a lumped-parameter model, which is useful for control design.

Nonlinear Modeling and Parameter Identification of Dynamic Friction Model in Tendon Sheath for Flexible Endoscopic Systems

Minimally Invasive Surgery (MIS) has established a revolution in surgical communities, with its many advantages over open surgery. The need of more simplicity and high maneuverability makes the tendon sheath a very suitable mechanism in flexible endoscopic systems. Due to the restriction on size constraints and sterilization problems, traditional sensors cannot be mounted on the tool tips of a slave manipulator. Moreover, in the presence of nonlinear friction and hysteresis between the tendon and the sheath, it is extremely difficult to control the precise motion and sense the force during the operation. This paper proposes a new dynamic friction model to estimate the force at the end effector for the tendon sheath mechanism. The proposed friction model can adapt with any initial pretension of the tendon and any configuration of the sheath. The nonlinearities in both sliding and presliding regimes can be captured by using an internal state variable and functions dependent velocity and acceleration. A specific setup has been designed in order to measure the friction force between the tendon and the sheath. Finally, the validity of the identified model is confirmed by a good agreement of its prediction and experimental data.

Control of a pneumatic artificial muscle (PAM) with model-based hysteresis compensation

Due to the inherent hysteresis in a PAM, the accompanying control of such actuator becomes complicated. In this paper, the pressure/length hysteresis is mathematically described by applying a Maxwell-slip model. The model of this hysteresis is then fed forward to compensate for the actuator nonlinearities. The designed controller is therefore just a simple PI-controller. It shows robust performance in PAM positioning control regarding not only at different equilibrium positions but also with different loads.

Position Control of Asymmetric Nonlinearities for a Cable-Conduit Mechanism

Cable-conduit mechanism (CCM) is widely used in robotic hands, rescue robots, rehabilitation robots, and surgical robots because it offers efficient transmission of forces/torques from the external actuator to the end effector with lightweight and high flexibility. However, the accurate position control is challenging in such mechanism due to friction and backlash-like hysteresis between the cable and the conduit. In this paper, a new control approach is proposed to enhance the trajectory tracking performances of the CCM. Unlike current approaches for the CCM in the literature, the proposed scheme considers the position transmission of the CCM as an approximation of backlash-like hysteresis nonlinearities without requiring the exact values of model parameters and their bounds. Online approximation-based robust control laws, which have the capabilities of estimating unknown system parameters, are also established. In addition, the deigned controller can adapt to any changes of the cable-conduit configuration and it is stable. The results of the proposed control techniques have been experimentally validated on a flexible robotic system using a flexible endoscope. Experimental validations show substantial improvements on the performances of position tracking for the use of CCM regardless of the arbitrary changes of the cable-conduit configurations.

Modeling torque-angle hysteresis in a pneumatic muscle manipulator

Hysteresis is inherently present in Pneumatic Artificial Muscle (PAM) Actuators. Our new observation shows that the hysteresis in a PAM is characterized by quasirate independency and history dependency, and is completely described by the Maxwell-slip (MS) model. In this paper, we explain how to model the existing hysteresis in an antagonistic PAM configuration, using the models derived for the individual PAMs. Experimental results show that the pneumatic manipulator hysteresis has the same behaviors as those found in a single PAM. The proposed model is used to predict the output torque of the manipulator joint not only for any arbitrary angular displacements but also for any arbitrary pressure difference.