The performance of a virtual reality (VR) system can be assessed from two aspects in the human-VR interaction loop. One aspect is the degree of immersion, which objectively quantifies the performance of the VR system using metrics such as the display field of view or the refresh rate. The other aspect is presence, which measures the user response to the VR system. This article presents a study that compares the impact on presence by changing immersion through enabling and disabling of the head tracking ability on a VR headset. The study quantitatively assesses this change by taking objective measurements of posture and subjective ratings of the VR experience, in terms of presence and motion sickness, after participants have gone through two versions of a roller coaster simulation; one with head tracking on and the other with head tracking off. The results indicate that a loss of immersion, caused by turning the head tracking feature off, results in a significant reduction in postural sway. This loss of immersion also affected presence, as shown through the user surveys. The survey responses indicate that the simulation with head tracking off was less enjoyable and caused more motion sickness compared to when head tracking is kept on.

Virtual holonomic output constraints define a set in the output space of a robot to which the end-effector should be confined. We propose the use of set stabilization for implementing virtual constraints, conducting analyses and experimental comparisons between existing control schemes and a set stabilization controller to justify our choice. The existing methods combine geometric projections and PD control, which ignores the higher-order properties of the constraint like curvature. The set stabilization method inherently incorporates constraint curvature information, allowing it to decouple the dynamics towards and along the set. It is shown that the two methods are equivalent on a line constraint, but that only the set stabilization method simultaneously guarantees asymptotic stability on a circle.

This paper presents a novel bilateral teleoperation algorithm for n degree of freedom nonlinear manipulators connected through communication networks with time delays. Central to this approach is the use of second order sliding mode unknown input observers for estimating the external forces acting on the manipulators. The use of these observers removes the need for both velocity and force sensors, leading to a lower cost hardware setup that provides all of the advantages of a position-force teleoperation algorithm. A proof of stability for each of the master and slave manipulators and their associated observers and controllers is given, as well as stability results for the entire closed loop in the presence of time delays. Experimental results are presented, confirming the validity of this approach in practice.

This paper presents the design of a tactile actuator that creates rough textures directly at the fingertip of the user. The purpose of the tactile actuator is to convey varying magnitudes of surface roughness to the users. The stimulus is created by a rotating spiral brush. The results of a psychophysical experiment show that the speed of the brush is directly related to roughness perception. However, there are two groups of users that perceive roughness in an opposite manner from each other.

This paper studies the problem of improving the fidelity of bilateral teleoperators under the stability constraint. First, a new fidelity measure for teleoperators is proposed. Then the teleoperator design problem is formulated as the optimization problem of maximizing the proposed fidelity measure under the stability constraint. The robust stability theory for unstructured uncertainty is applied to analyze the stability of bilateral teleoperators. Third, a new scheme to estimate adaptively the environmental impedance is developed to improve the fidelity of the systems. Finally, a case study is presented to validate the introduced approach.

This paper develops a systems- and control-oriented intelligent agent framework called the hybrid intelligent control agent (HICA), as well as its composition into specific kinds of multiagent systems. HICA is essentially developed around a hybrid control system core so that knowledge-based planning and coordination can be integrated with verified hybrid control primitives to achieve the coordinated control of multiple multimode dynamical systems. The scheme is applied to the control of teams of unmanned air and ground vehicles engaged in a pursuit-evasion war game. Results are demonstrated in simulation.

This paper presents a systematic procedure for constructing a multi-input multi-output fuzzy controller that guarantees identical performance to an existing stabilizing linear controller. An algorithm is devised that generates a fuzzy controller which is functionally identical to a given time-invariant or time-varying finite-dimensional linear controller. The benefit of this transformation is that it provides an automated technique for the initial fuzzy controller setup while vital knowledge-based attributes are integrated afterwards. An important result of this work is that once a linguistic mapping into the fuzzy domain has been performed, one can see in linguistic terms how the linear controller operates. The effectiveness of this approach is demonstrated with a model for a flexible robot that exhibits nonminimum phase characteristics. An extension is outlined to use this deterministic approach for the case of a general dynamic control-law, and several applications to nonlinear control problems are discussed.

We describe a pursuit-evasion scheme involving multiple coordinated vehicle teams in which each vehicle is modeled as a class of hybrid intelligent agents. This is the hybrid intelligent control agent (HICA), a control-oriented enhancement to the intelligent agent paradigm which conceptually wraps an agent around a hybrid control system core. The problem is formulated as two supervisory teams of HICA to synthesize relevant control/coordination logic. Simulations demonstrate the results of such an approach.

This paper presents a design methodology for designing position/force controllers for a single flexible link (SFL) manipulator. The objective is to design a single controller that stabilizes the system during both constrained and unconstrained motion, avoiding problems that arise when using switching controllers. A coherent system identification based robust control design methodology is presented. As a robust controller, we consider the quantitative feedback theory (QFT) based design. The proposed method is very suitable since linear model of constrained SFL manipulator is a gross simplification and includes significant uncertainties that can be handled well in the QFT frame work.

There is an increasing interest in examining the use of flexible link manipulators in tasks where there is contact with the environment. Presently, there has been limited work examining the stability of force control strategies for such manipulators, especially in the case where there is a switching transition between the unconstrained and constrained environments. In this paper, the modeling and stability of a single flexible link under proportional derivative control contacting an environment is studied. Intuitively, since the system only has passive elements, one would expect the system to be stable. With a few very reasonable assumptions, the problems associated with finite-dimensional approximations are solved by using a novel infinite-dimensional approach. The resultant infinite-dimensional switching system is shown to be asymptotically stable using an energy-based method.

A bilaterally controlled teleoperation system usually involves contact tasks at a remote location. Dynamic interaction between the slave robot and the environment may cause severe chattering instability. This paper presents a contact transition stability analysis for a position-error-based bilateral teleoperation system. The scenario studied is when the slave robot collides with a stationary stiff environment. Under a few assumptions and conditions, the teleoperation system is shown to be asymptotically stable. The analysis includes the dynamics of the human arm and of the environment.

The primary objective of this research is to model the biomechanical control system employed by the central nervous system (CNS) to maintain posture and balance of the head-arms-torso (HAT) during gait. More specifically, the intent is to stabilize a model of the upper body so that the HAT response is similar to that found experimentally in human subjects during gait. This is accomplished by using appropriate physiological parameters as feedback to generate realistic control signals at the hip musculature. The modelling includes the HAT musculoskeletal characteristics as well as pure neural time delays. The CNS control system is modelled by a linear state feedback controller as well as a hybrid fuzzy controller that has been adapted from a linear quadratic regulator. In addition, a newly derived feedforward component is demonstrated. It is expected that a better understanding of the human fait process will prove valuable in designing assistive devices and bipedal robots.

There is increasing interest in examining the use of flexible link manipulators in tasks where there is contact with the environment. Presently, there has been limited work examining the stability of force control strategies for such manipulators, especially in the case where there is a switching transition between the unconstrained and constrained environments. In the paper, the stability of a single flexible link under PD control contacting an environment is studied. The problems associated with finite-dimensional approximations are outlined and an infinite-dimensional proof of asymptotic stability is described.

To design and test large flexible manipulators, dynamically equivalent scale model manipulators can be built which will fit inside a conventional research lab. By constructing dynamically equivalent prototypes, all the dynamic properties of the manipulator are scaled, including gravity and flexibility effects. Dimensional analysis is used to determine the nondimensional groups which define the dynamic equivalence conditions for general flexible-link manipulators. The scaling laws for natural frequencies, mode shapes, and friction effects are also presented. An example illustrates how two physically different flexible-link manipulators with two degrees of vibration can be constructed to be dynamically equivalent. The example also illustrates how, with the application of scaled PD controllers, the responses of the two systems can be scaled versions of one another.

Dimensional analysis is used to determine the nondimensional groups which define the dynamic equivalence conditions for rigid-link manipulators. Scaling laws for general actuators and nonlinear controllers are also presented. As an example, simulation results are presented which illustrate how a large two-link elbow manipulator operating on the Moon can be designed by constructing a dynamically equivalent prototype in Earth gravity, and also, how a sliding mode controller designed for the prototype Earth robot can be scaled for implementation on the Moon-based manipulator.

In recent years, the Preisach hysteresis model (1935) has emerged as the model of choice for the behaviour of many smart materials, such as shape memory alloys (SMA). This research treats the identification of Preisach models for a differential SMA actuator. The traditional identification technique is applied, and several models are derived for the actuator. It is seen that the classical Preisach model is able to model its behaviour. However, the results raise questions concerning the robustness of the traditional identification technique, as well as the conservative nature of current passivity-based control results for the Preisach model.

In this paper, we present an inexpensive force feedback virtual reality system using a 3-DOF five-bar-linkage manipulator. The direct drive manipulator has a gravity balanced structure. An implicit force control strategy is used to determine the virtual force feedback. The effects of virtual wall and virtual free space controllers are studied. A graphics system is developed to provide the visual feedback to the user. Experimental results are presented to show the user interaction with a virtual wall and a virtual object.

The motivation for this research arises from interest in actuators made from "smart materials" such as shape memory alloys and piezoceramics. One common characteristic among these new materials is that they exhibit complex hysteresis nonlinearities. The input-output nature of the Preisach model of hysteresis results in a simple model for which identification strategies are known, and implementation is straightforward both in simulation and real-time control. This paper examines the passivity of the Preisach model, and the stability of velocity feedback control for hysteretic actuators.

The ability to control a robot in both constrained and unconstrained situations is important. One way to do this is using a switching controller which uses a force controller for constrained space and a position controller for unconstrained space. This paper develops a switching controller for a single flexible link manipulator and provides experimental results for a motion from unconstrained to constrained space. Tarn et al. (1996) proposed that positive acceleration feedback be used to reduce the impulse force of contact. The effect of different levels of positive acceleration feedback on the force response, after a collision with an unexpected object occurs, is examined. It is concluded that small amounts of positive acceleration feedback reduces the force overshoot caused by the collision but that too much can cause instability.

When constructing large robotic manipulators or space structures, it is advisable to begin with a small-scale prototype on which to perform the design, analysis, and debugging. To ensure that the results obtained on the scale-model apply directly to the actual manipulator, it is necessary that the prototype and the original robot are dynamically equivalent. This paper examines the single flexible link (SFL) manipulator. Dimensional analysis is used to identify the nondimensional groups for the SFL. These groups are present in the corresponding nondimensional equations of motion, which are also derived. To account for inherent manufacturing imprecision, tolerances are developed for the nondimensional groups. Scaling laws for continuous-time and discrete-time controllers are developed for dynamically equivalent SFL systems. These theoretical scaling laws are verified experimentally for an H/sub /spl infin// and a PD control strategy.

A simple collision detection method for a flexible link manipulator is proposed. The method only requires two strain measurements to determine the position of a collision with the flexible link. Equations which relate the force and position of contact to the strain at two points in the flexible link are derived assuming the link is at static equilibrium. Since the flexible robot is actually in motion, one needs to examine whether these relationships are accurate enough in a dynamic scenario to be useful. Thus the expressions are verified on a robot through collision tests, and used to successfully locate an encountered object so that a path can be planned around it.

In this paper, issues in the design of a hybrid passive/adaptive controller for a single flexible link robot with a payload mass are discussed. It is well known that collocated control of a single flexible link can be accomplished with PD control. It has already been shown that, with an appropriate choice of output variable (for example, the reflected tip position), a simple noncollocated "PD-like" controller, henceforth referred to as a passive controller, can provide good performance when the nominal physical parameters of the manipulator are known and do not vary significantly with time. In practical applications, a flexible link robot must be able to pick up and move unknown payloads. A fixed passive controller may not provide adequate tracking performance. In this paper, an adaptive control strategy is augmented to the passive controller introduced in to improve the tracking performance.

This paper examines the use of noncollocated passivity-based control on a 5-bar-linkage manipulator with the last link flexible. By defining appropriate output variables such that the resulting transfer functions are passive, very simple low-order controllers can be designed. Previous work on this robot allowed only horizontal vibrations in the last link to take place, thus simplifying the problem. In this paper, the last link can have transverse vibrations in two dimensions. Gravitational effects must also be considered. Experimental and simulation results are shown which demonstrate the feasibility of this approach.

The nondimensional groups which characterize a group of dynamically equivalent single flexible link (SFL) manipulators are determined. A scaling law for linear rational controllers is developed for dynamically equivalent SFL systems. As an example of the applicability of this theoretical scaling law to even sophisticated control techniques, it is verified experimentally for an H/sub /spl infin// control strategy.

This paper deals with the L/sub 2/ stability of a shape memory alloy actuated position control system, based on a single wire under constant axial load. Presuming a controller has certain specific dissipativity characteristics, it is shown that system stability can be guaranteed for a large class of controllers. In particular, the results can be used to show stability under approximated PID, PI and PD control. The result is used to find a stabilizing PI controller, and the corresponding experimental data is shown.

In this paper, H/sub /spl infin// control of a single flexible link with a shoulder joint is discussed. The cross-section of the flexible link is such that only horizontal vibrations are possible. Simulations were performed using a simplified model. An H/sub /spl infin// controller was designed that provides a fast step response with low overshoot, but inadequate disturbance rejection. Experiments were performed using a five-bar robot with one side joint locked. Using linear transfer functions obtained via system identification for several beam inclinations, an H/sub /spl infin// controller was found that has a good step response with the exception of large overshoot. Trajectory tracking is also shown. The performance is compared to that of a conventional PID controller.

Shape memory alloys (SMAs) are inherently non-linear devices exhibiting significant hysteresis in their stress-strain-temperature characteristic. As a consequence most SMA actuator systems presented in the literature employ some form of PID or PWM controller. Unfortunately, due to the hysteresis of the SMA, the theoretical stability of these systems has been largely ignored. This paper extends the work of Ikuta et al. (1991) to arrive at a new model of an SMA actuator. The model is then used to prove the L/sub 2/-stability of a position control system. The key feature of the model is its ability to model minor hysteresis loops. The model is suitable for computer simulation and control system analysis. The stability result accounts for the complex non-linear behaviour of the SMA actuator, along with non-linear effects such as saturation.>

In this paper, issues in the design of passive controllers for flexible link robots are discussed. It is well known that, for the single flexible link manipulator, the mapping from the input torques to the joint velocities is passive whereas the mapping from the input torques to the derivative of net tip movements (the usual output chosen) is not. However, if an appropriate output variable is selected, the transfer function, can be made passive provided that the hub inertia seen by the flexible link is sufficiently small. Then, by the passivity theorem, using any strictly passive controller with finite gain will result in an L/sub 2/-stable system. However, for an an industrial robot, the effective hub inertia seen by the flexible link is usually large. Alternate approaches are examined for extending these passivity results to large hub inertias. Experimental results are presented for a 5-bar-linkage manipulator with the last link being flexible.>

Unique phenomena that can arise in linear and nonlinear systems that are dynamically and/or statically coupled have been utilised for vibration suppression. In the linear case, static coupling is established between the oscillatory modes of a plant with a single input and a series of second order controllers. The controlled system is characterised by a plant response that approaches the desired state at the rate of a cosine function. In this investigation the single-degree-of freedom coupling control theory is extended to suppress disturbance induced oscillations of a single flexible link. The suppression action is achieved using the hybrid control method which was developed to supply the suppression action, especially for the multidegree-of freedom case. The resulting control algorithm is implemented experimentally.>

A composite control strategy is outlined for a large class of multilink manipulators with one flexible link. The controller design is based on the inverse dynamics of the rigid manipulator and the use of a closed-loop shaped-input filter to reduce or eliminate the vibrations of the flexible link and to reject external disturbances. Both simulation and experimental results confirm the effectiveness of this design strategy. The design algorithm is a linear one, but the simulation results show that it is robust enough to compensate for vibrations in the full nonlinear system.>

Dynamic equations are derived for a class of multilink manipulators with the last link flexible. The manipulators have three degrees of freedom and are moved by three actuators. This class includes the elbow manipulator and the five-bar-linkage manipulator. Some reasonable assumptions that simplify the dynamic equations for control purposes are described, and the simplified dynamic equations are then presented. It is shown that, for pick-and-place operations, a rigid-body robot controller and a single flexible link controller may be combined to control the manipulator in a straightforward manner.>

A nonlinear control strategy for a large class of multilink manipulators with one flexible link is proposed. The design is based on a transformation which almost linearizes the system. A nonlinear observer is proposed, and it is shown that combining this observer with the proposed nonlinear controller results in a system which is input/output stable in a local sense. >

It is demonstrated that, for an appropriately chosen output variable, the transfer function for some single flexible links can be considered to be passive. Therefore, by the passivity theorem, using any strictly passive controller with finite gain will result in an L/sub 2/-stable system. One approach to designing such a controller is outlined, and experimental results are presented. The implications of this work are examined and future research issues are discussed.>

The authors examine some issues in the transfer function modeling of a single flexible link. Using the assumed-modes approach, it is possible to find the transfer function between the torque input and the net tip deflection. It is shown that when the number of modes is increased for more accurate modeling, the relative degree of the transfer function becomes ill-defined. This can greatly affect the performance of a controller designed using the model. In addition, any attempt to identify the transfer function is also affected. An alternative approach that uses the rigid body deformations minus the elastic deformations as the output is proposed. This solves the above problems and results in a transfer function with a well-defined relative degree of two. Even if three or four vibration modes are used, the leading coefficient in the numerator is much larger with the proposed new output.>

A model of a five-bar-linkage root is examined where the top link is flexible. The modeling process is described and applied to the simpler problem of a single link beam. The model of the five-bar-linkage robot with the top link flexible is derived and then simplified using various assumptions which are discussed. It is shown that under these assumptions, it may be possible to control two joints using a typical rigid-body controller while using the third joint to control the vibrations. This greatly simplifies the control problem.>

The objective of this paper is to examine the end-point control of a flexible beam using the stable factorization approach. We begin with a description of the experimental setup at the University of Waterloo and discuss the derivation of the state space model of the beam using the method of modal expansions. The model is an eighth order nonminimum phase system with poles on the imaginary axis. The stable factorization approach is used to obtain the optimal step response of the system in the sense that the mean square tracking error is minimized over all the stabilizing controllers. Since the plant is strictly proper, the optimal step response is not realizable and only suboptimal controllers, whose responses approach the optimal step response, can be constructed. First, the one parameter compensator is studied. The results are discussed and the effect of weighting the error is analyzed. A two parameter compensator is also simulated and the result is compared to the one parameter compensator. The paper concludes with suggestions for future research.