Günter Niemeyer

CHARACTERIZING THE HUMAN WRIST FOR IMPROVED HAPTIC INTERACTION

Haptic displays provide the user with a sense of touch in both simulation of virtual environments and teleoperation of remote robots. The instantaneous impedance of the user’s hand affects this force interaction, changing the transients experienced during activities such as exploratory tapping. This research characterizes the behavior of the human wrist joint while holding a stylus in a three-fingered grasp. Nonparametric identification methods, evaluating frequency-and time-responses, support a second-order system model. Further analysis shows a positive linear correlation between grip force and wrist impedance for all subjects, though each individual’s trend is unique. These findings suggest that a quick calibration procedure and a realtime grip force measurement could enable a haptic display to predict user response characteristics throughout an interaction. Such knowledge would enable haptic control algorithms to adapt continuously to the user’s instantaneous state for improved performance.Copyright

Induced master motion in force-reflecting teleoperation

Telerobotic systems have persistently struggled to provide users with realistic force feedback; high-frequency contact transients convey important information about the remote environment but are typically attenuated to avoid the contact instability they incite. This undesirable behavior can be traced to high-frequency induced master motion, movement of the master device that is caused by force feedback rather than user intention. Such motion is interpreted as a position command to the slave, closing an internal control loop that is unstable under high gain. This paper examines the phenomenon of induced master motion in position-force teleoperation, presenting a new approach for achieving stable, high-gain force reflection using model-based cancellation. Requirements for the model of the induced motion dynamics and methods for its characterization are described, focusing on successive isolation of inertial and connecting elements. The sixth-order nonlinear model obtained for a one-degree-of-freedom user-master system is validated and then tested in a cancellation controller. Canceling high-frequency induced master motion during teleoperation is shown to improve the stability of impacts, allowing significantly higher force reflection levels and a more authentic user experience.

Force and Position Scaling Limits for Stability in Force Reflecting Teleoperation

Many telerobotic systems require the use of a slave robot with large inertial and frictional properties. Using a force sensor on the end effector can hide the slave’s inertia and friction from the user providing a more accurate sense of the environment, but introduces dangers of system instability. Both the position and force scale directly affect the system loop gain and hence stability. This opens up the possibility of trading off between them based on the environment and task. In this paper we derive explicit limits for their product. In particular we consider varying environment stiffnesses, as well as distinguishing impact and contact phases. The theoretical limits closely align with experimental results using a large slave telerobotic system interacting with very soft to nearly rigid environments.© 2008 ASME

Canceling Induced Master Motion in Force-Reflecting Teleoperation

Providing the user with high-fidelity force feedback has persistently challenged the field of telerobotics. This paper presents a new approach for achieving stable, high-gain force reflection via cancellation of the master mechanism’s induced motion. In a classic position-force controller, high force-feedback levels drive the system’s internal master-slave loop unstable during contact with the remote environment. Lowering the force-feedback gain ensures stability but diminishes the haptic cues available to the user, masking contacts and preventing hard objects from feeling appropriately stiff. The proposed cancellation approach permits high levels of force feedback by attenuating only the controller’s internal loop. Using a model of the master mechanism’s response to applied force feedback, an estimate of induced high-frequency movement is substracted from the master’s measured position to approximate the user’s intended path for the slave. The cancellation technique is described, modeled, and validated herein, including testing on a one-degree-of-freedom telerobotic system. It is shown to improve the feel of the system, tripling the testbed’s achievable force-feedback gain without compromising stability.Copyright

Predicting Blood Glucose Levels Around Meals for Patients With Type I Diabetes

Insulin pumps and continuous glucose monitors enable automatic control of blood glucose (BG) levels for patients with type 1 diabetes. Such controllers should carefully assess the likely future BG levels before injecting insulin, since the effects of insulin are prolonged, potentially deadly, and irreversible. Meals pose a strong challenge to this assessment as they create large, fast disturbances. Fortunately, meals have consistent and predictable effects, if their size and start time are known. We present a predictive algorithm that embeds meal detection and estimation into BG prediction. It uses a multiple hypothesis fault detector to identify meal occurrences, and linear Kalman filters to estimate meal sizes. It extrapolates and combines the state and state covariance estimates to form a prediction of BG values and uncertainties. These inputs enable controllers to assess and trade off the acute risks of low and chronic risks of high BG levels. We evaluate the predictor on simulated and clinical data.Copyright

Model Mediated Telemanipulation

A telemanipulation system allows a human user to manipulate a remote environment using a local interface (master robot) to control a remote (slave) robot. In doing so, it is desirable to provide users with appropriate sensory feedback, most often taking the form of visual and force information. In the presence of communication delays, however, a force feedback telemanipulation system must overcome detrimental effects caused by the delay, both on the quality of feedback to the user and the stability of the control system. For large delays, like those experienced in space telerobotics, the users perceptive abilities are distorted and challenged by the lag between action and response. With this paper, a user-centered approach is proposed which seeks to simultaneously provide stable master-slave interaction as well as a natural user experience, tolerant of large delays. Rather than directly sending sensory information from the slave robot to the user, the goal is to use this information to create a real-time virtual model of the remote environment, which then serves as the users interface. Maintaining a dynamic, virtual model locally at the master-side, the user is provided with immediate visual and haptic responses to his/her actions through the master device. At the remote site, the slave robot tracks the users continuous and natural motion commands, while providing new information needed to update the virtual model. This method abstracts the data transmitted between the sites and creates greater delay tolerance. The basic principles of the approach are demonstrated on a simple one-degree of freedom telerobotic system, with a rigid, stationary slave environment.Copyright

Exploiting Inate Motor Dynamics for High Stiffness Haptic Display on Brushless DC Motors

The maximum stiffness that can be rendered by impedance-controlled haptic devices has traditionally been limited by quantization, discretization, and delay in the digital loop. Recent research has shown that performance can be improved by utilizing the natural inductive stiffness inherent in brushed DC motors. This study extends the concept of exploiting a motor’s dynamics to the three phase brushless DC (BLDC) motor. It is analytically shown that the inductances of a BLDC motor’s windings map to a high physical stiffness. This stiffness is made available at frequencies important to haptic interaction by cancelling the resistance in each winding with analog feedback, effectively slowing the motor’s electrical dynamics. Experimental verification is obtained by implementing the proposed spring drive in analog circuitry in combination with a digital position feedback loop. The final results support the analytic solution and compare the spring drive favorably with traditional current and voltage drives for haptic applications.Copyright

Experiments in Local Force Feedback for High-Inertia, High-Friction Telerobotic Systems

Many telerobotic systems include a slave robot with much larger inertial and frictional properties than the master robot and/or a non-backdrivable slave acting as an admittance device. Passive controllers, which are known for their stability and robustness, display the large dynamic forces to the user and/or become insensitive to contact forces. In effect, the user feels the large inertia and friction of the slave robot but does not feel the force of the environment. Force sensors can isolate the environment forces. In this paper, we experiment with local force feedback for an admittance type slave robot. We use the local controller to convert the slave to an apparent impedance device, restoring its sensitivity to environment forces. This will allow the application of stable passive teleoperation controllers. The control structure is validated on a single axis of a large, non-backdrivable, industrial Adept robot operating as a slave in contact and in free space.Copyright

High Frequency Acceleration Feedback in Wave Variable Telerobotics

The human hand is very sensitive to the high frequency accelerations produced by tool contact with a hard object, yet most time delayed telerobots neglect this feedback band entirely in order to achieve stability. We present a control architecture that both incorporates this important information and provides the ability to scale and shape it independently of the low frequency force feedback. Leveraging the clean power flows afforded by wave variables, this augmented controller preserves the passivity of any environment that it renders to the user but is not subject to the limitations of being passive itself. This architecture guarantees stability in the presence of communication delay while achieving a level of feedback not possible with a passive controller. We show experimentally that this feedback augmentation and shaping can present a high frequency acceleration profile to the user’s hand that is similar to that experienced by the slave end effector. We anticipate that these natural haptic cues will make teleoperative systems easier to use and thus more widely applicable.© 2005 ASME

Perception of Prototypical Environments in Model-Mediated Teleoperation

In teleoperation, master and slave robots directly exchange position and force data to establish a connection between local and remote sites. Model-mediation interjects an environment model rendered to the user to create a sense of remote presence. The slave continually updates this model from actual interactions. Here we evaluate a dual-mode implementation, which classifies the environment into one of two models describing rigid contact or free motion. Performance is measured against various prototypical environments, observing master force outputs to motion inputs to evaluate user’s perception of the environment. We find performance to depend on system lag as well as model error, such that multiple-model systems fundamentally outperform traditional architectures.© 2008 ASME