Delays in the transmission of sensory and motor information prevent errors from being instantaneously available to the central nervous system (CNS) and can reduce the stability of a closed-loop control strategy. On the other hand, the use of a pure feedforward control (inverse dynamics) requires a perfect knowledge of the dynamic behavior of the body and of manipulated objects. Sensory feedback is essential both to accommodate unexpected errors and events and to compensate for uncertainties about the dynamics of the body. Experimental observations concerning the control of posture, gaze and limbs have shown that the CNS certainly uses a combination of closed-loop and open-loop control. Feedforward components of movement, such as eye saccades, occur intermittently and present a stereotyped kinematic profile. In visuo-manual tracking tasks, hand movements exhibit velocity peaks that occur intermittently. When a delay or a slow dynamics are inserted in the visuo-manual control loop, intermittent step-and-hold movements appear clearly in the hand trajectory. In this study, we investigated strategies used by human subjects involved in the control of a particular dynamic system. We found strong evidence for substantial nonlinearities in the commands produced. The presence of step-and-hold movements seemed to be the major source of nonlinearities in the control loop. Furthermore, the stereotyped ballistic-like kinematics of these rapid and corrective movements suggests that they were produced in an open-loop way by the CNS. We analyzed the generation of ballistic movements in the light of sliding control theory assuming that they occurred when a sliding variable exceeded a constant threshold. In this framework, a sliding variable is defined as a composite variable (a combination of the instantaneous tracking error and its temporal derivatives) that fulfills a specific stability criterion. Based on this hypothesis and on the assumption of a constant reaction time, the tracking error and its derivatives should be correlated at a particular time lag before movement onset. A peak of correlation was found for a physiologically plausible reaction time, corresponding to a stable composite variable. The direction and amplitude of the ongoing stereotyped movements seemed also be adjusted in order to minimize this variable. These findings suggest that, during visually guided movements, human subjects attempt to minimize such a composite variable and not the instantaneous error. This minimization seems to be obtained by the execution of stereotyped corrective movements.

Does the brain use sliding variables for the control of movements

Present virtual reality (VR) systems present powerful graphical information with human interaction capability but may be missing corresponding motion or force sensing inputs to the human operator. The concept of "virtual force" is introduced herein as a way to provide some of this missing sensing information. The "virtual force" environment is created in this paper using a displacement stick controller (joystick) with the reflective force on the controller being coordinated, in a spatial sense, with a visual scene displayed on a monitor. Force type fields are built about visual objects to provide proprioceptive feedback to the operator to improve his telepresence about the virtual scene. Since this study involves both information and control theory, certain equivalences are derived in the appendixes which extend to different plant dynamics not excluding small time delays which may appear anywhere in system. Data from a study of disabled patients and normals are presented to illustrate the efficacy of "virtual force" in terms of a performance measure related to transinformation (capacity or bits/second). >

A study on spatially induced "virtual force" with an information theoretic investigation of human performance

A methodology for designing force-reflecting active controllers in cases involving teleoperation is presented. Certain types of force loop configurations nested within a position loop are shown to provide more information about the environment to the human, and that improves interaction. The procedure of modulating the controller to match the integral of its inverse mechanical impedance to the plant is given. Experimental data involving subjects are used to validate the approach. >

http://ieeexplore.ieee.org/iel1/5488303/3194/00103357.pdf

Active force reflection devices in teleoperation

A driver's body functions as a pathway that transmits vehicle motion and inertia forces to the vehicle's manual control interface, producing unintended commands and degrading driver/vehicle performance In this paper, we apply a model-based cancellation controller to mitigate this effect, called biodynamic feedthrough Using measurements of vehicle acceleration and hand/interface interaction force, we first construct a model of the body transmittance In operation, the controller processes vehicle acceleration measurements through a regression model of the biodynamic transmittance and a motor imposes the result as a torque on the manual interface We tested individually fit cancellation controllers by quantifying the tracking performance of 12 human subjects using a joystick to control displacements of a single-axis motion platform The addition of the cancellation controller reduced the root mean square tracking error by 45%, significantly reduced spectral energy in the 1-7 Hz band, and had a marked effect on the ability of the driver to reject exogenous disturbances

/pdf/cancellation-of-biodynamic-feedthrough-in-vehicle-control-2jh37qljgk.pdf

Cancellation of Biodynamic Feedthrough in Vehicle Control Tasks

A statistical algorithm is developed to study human tracking behavior in a precognitive tracking task. The algorithm presented here determines the point in time when a tracking task becomes too difficult for the human to follow. Consequently, different behavior responses are observed to occur. A decision rule based on a statistical test of normality is used to delineate the two regions of tracking behavior. The proof of convergence of this algorithm to a unique solution is given. Data from a good and poor tracker are analyzed using this algorithm to illustrate how to utilize the approach presented here.

An Algorithm to Ascertain Critical Regions of Human Tracking Ability

: Data from an antiaircraft artillery simulator is used to validate the dependence of tracking performance measured by the closed loop error and parameters obtained from a Proportional Integral Derivative (PID) model of the man A comparison of three tracking teams is used as the data base with an azimuth and elevation operator composing each team Although this problem is nonstationary, precognitive (a learned tracking task), two axis, and very difficult to model by conventional means, the PID model provides a simple approach to relate the error in the closed loop to the model parameters from each team In order to provide adequate statistical estimates of these model parameters, identification techniques are applied to the same team (replications) tracking a similar arc tangent flyby trajectory Statistical analysis was applied to the identified parameters between trackers to evaluate the credibility of this approach

Earl J. Hartzell

Papers

An Algorithm to Ascertain Critical Regions of Human Tracking Ability

Performance Measures of Human Tracking Utilizing PID Modeling and a Closed Loop Error Metric