Slow–fast control of eye movements: an instance of Zeeman’s model for an action

The rapid eye movements (saccades) used to transfer gaze between targets are examples of an action. The behaviour of saccades matches that of the slow–fast model of actions originally proposed by Zeeman. Here, we extend Zeeman’s model by incorporating an accumulator that represents the increase in certainty of the presence of a target, together with an integrator that converts a velocity command to a position command. The saccadic behaviour of several foveate species, including human, rhesus monkey and mouse, is replicated by the augmented model. Predictions of the linear stability of the saccadic system close to equilibrium are made, and it is shown that these could be tested by applying state-space reconstruction techniques to neurophysiological recordings. Moreover, each model equation describes behaviour that can be matched to specific classes of neurons found throughout the oculomotor system, and the implication of the model is that build-up, burst and omnipause neurons are found throughout the oculomotor pathway because they constitute the simplest circuit that can produce the motor commands required to specify the trajectories of motor actions.

Haptics control of an arm exoskeleton for virtual reality using PSO-based fixed structure H∞ control

This article proposes a robust haptics control algorithm for an arm exoskeleton in virtual reality application that guarantees the robustness of the force displaying performance. The developed arm exoskeleton allows the user to sense the profile of virtual objects through joints at wrist, elbow, and shoulder. Forces exerted by the user are directly measured by load cells attached to the links, which are used to determine the corresponding velocity command based on virtual impedance. Due to the imperfection of modeling and the measurement noises from load cells and encoder quantization, the force displaying performance of the device tends to degrade with increasing oscillation, causing permanent damages to the device and user. To solve this problem, a Particle swarm optimization (PSO)-based fixed structure H∞ controller is proposed to control the developed arm exoskeleton. The control performance of the proposed controller is evaluated by both simulation and experiment. The results show that the proposed controller can achieve better tracking performance in comparison with proportional–integral–derivative controller in terms of less oscillation and fast response.

Handheld Robot for Bone Drilling Assistance

Computer navigation systems has provided useful visual guidance for the surgeon to deliberately locate the tools to the anatomy. However, the tool positioning process is still manually performed. Sometimes the tool positioning may cause fatigue, stress and might be of risk to patient too. In this paper we designed a special purpose handheld robot for bone drilling. Meanwhile the coordinated controller assists the surgeon to precisely and safely drill the bone safely. Two force sensors are embedded at the handle and the cutter to measure the human exerted force and bone drilling force, respectively. The velocity command was then computed by the admittance controller for the robot controller. The motion of the control handle is positioned by the surgeon, while the surgical tool driven by the robot end-effector. The coordination between the human operator and the robot was designed so that the bone drilling can be performed more effectively than only imagenavigation scenario. The drill was able to be maintained on the target trajectory with reasonable accuracy within 2 mm although the human operator has deviated the surgical tool up to 5 cm. The compensation function to guide the drill back to the planned path was very useful to prevent the drill’s breakage when penetrating through the holes on the bone plate in bone drilling procedure.

Dynamic performance improvement of direct image-based visual servoing in contour following

Image-based visual servoing (IBVS) has increasingly gained popularity and has been adopted in applications such as industrial robots, quadrotors, and unmanned aerial vehicles. When exploiting IBVS, the image feature velocity command obtained from the visual loop controller is converted to the velocity command of the workspace through the interaction matrix so as to converge image feature error. However, issues such as the noise/disturbance arising from image processing and the smoothness of image feature command are often overlooked in the design of the visual loop controller, especially in a contour following task. In particular, noise in the image feature will contaminate the image feedback signal so that the visual loop performance can be substantially affected. To cope with the aforementioned problem, this article employs the sliding mode controller to suppress the adverse effects caused by image feature noise. Moreover, by exploiting the idea of motion planning, a parametric curve interpolator is developed to generate smooth image feature commands. In addition, a depth observer is also designed to provide the depth information essential in the implementation of the interaction matrix. In order to assess the feasibility of the proposed approach, a two-degrees-of-freedom planar robot that employs an IBVS structure and an eye-to-hand camera configuration is used to conduct a contour following task. Contour following results verify the effectiveness of the proposed approach.

Modelling and Control of Harmonic Drive Applied in Control Moment Gyros

Limited by supply voltage on board, control accuracy of low velocity and available torque of Control Moment Gyros (CMGs) gimbal driving motor are limited. Adding Harmonic Drive (HD) between driving motor and CMGs gimbal can improve CMGs control system performance. The paper establishes a practical HD model considering transmission, bearing friction, tooth meshing friction and compliance of flexspline. The model is properly simplified for analysing the control performance of whole closed loop system. Transfer functions are analysed and the flexspline output velocity is chosen to be the velocity feedback for controller according to the characteristics of CMGs gimbal velocity command. All of the numerical simulation results validate the theory analysis.

Stable contour-following control of wheeled mobile robots

SUMMARYThis paper presents a continuous wall-following controller for wheeled mobile robots based on odometry and distance information. The reference for this controller is the desired distance from the robot to the wall and allows the robot to follow straight wall contour as well as smoothly varying wall contours by including the curvature of the wall into the controller. The asymptotic stability of the control system is proved using a Lyapunov analysis. The controller is designed so as to avoid saturation of the angular velocity command to the robot. A novel switching scheme is also proposed that allows the robot to follow discontinuous contours allowing the robotic system to deal with typical problems of continuous wall-following controllers such as open corners and possible collisions. This strategy overcomes these instances by switching between dedicated behavior-based controllers. The stability of the switching control system is discussed by considering Lyapunov concepts. The proposed control systems are verified experimentally in laboratory and office environments to show the feasibility and good performance of the control algorithms.