Peter D. Lawrence

A transformation method for the reconstruction of functions from nonuniformly spaced samples

The reconstruction of functions from their samples at nonuniformly distributed locations is an important task for many applications. This paper presents a sampling theory which extends the uniform sampling theory of Whittaker et al. [11] to include nonuniform sample distributions. This extension is similar to the analysis of Papoulis [15], who considered reconstructions of functions that had been sampled at positions deviating slightly from a uniform sequence. Instead of treating the sample sequence as deviating from a uniform sequence, we show that a more general result can be obtained by treating the sample sequence as the result of applying a coordinate transformation to the uniform sequence. It is shown that the class of functions reconstructible in this manner generally include nonband-limited functions. The two-dimensional uniform sampling theory of Petersen and Middle ton [16] can be similarly extended as is shown in this paper. A practical algorithm for performing reconstructions of two-dimensional functions from nonuniformly spaced samples is described, as well as examples illustrating the performance of the algorithm.

Robotically assisted medical ultrasound

A system for medical ultrasound in which the ultrasound probe is positioned by a robot arm under the shared control of the ultrasound operator and the computer is proposed. The system comprises a robot arm design suitable for diagnostic ultrasound, a passive or active hand-controller, and a computer system to co-ordinate the motion and forces of the robot and hand-controller as a function of operator input, sensed parameters and ultrasound images.

A single camera eye-gaze tracking system with free head motion

Eye-gaze as a form of human machine interface holds great promise for improving the way we interact with machines. Eye-gaze tracking devices that are non-contact, non-restrictive, accurate and easy to use will increase the appeal for including eye-gaze information in future applications. The system we have developed and which we describe in this paper achieves these goals using a single high resolution camera with a fixed field of view. The single camera system has no moving parts which results in rapid reacquisition of the eye after loss of tracking. Free head motion is achieved using multiple glints and 3D modeling techniques. Accuracies of under 1° of visual angle are achieved over a field of view of 14x12x20 cm and over various hardware configurations, camera resolutions and frame rates.

Tracking control of an electrohydraulic manipulator in the presence of friction

Analysis and estimation of friction and compensation for its effects in the control of an electrohydraulic manipulator is addressed. The specific hydraulic manipulator is an integral part of an automated fish processing machine which has been developed in our laboratory. The analysis reveals that considerable static and dynamic friction exists in the system. An available nonlinear observer for Coulomb friction, is modified to simultaneously estimate friction, velocity, and acceleration. A novel observer-based friction compensating control strategy is developed for improved tracking performance of the manipulator. The approach is based on acceleration feedback control. Experimental investigations show that this controller significantly outperforms the conventional PD controller. The general approach presented in this paper, may be applied to compensate for friction in any servomechanism, particularly when the actuator dynamics is not negligible.

Questions regarding the sequential neural generator theory of the somatosensory evoked potential raised by digital filtering

Digital bandpass filtering (300-2500 Hz) designed for zero phase shift was applied to somato-sensory evoked potentials recorded with cephalic bipolar montages. Four consistent negative and corresponding positive peaks with latencies of about 16, 18, 19, and 20 msec were elicited with median nerve stimulation. Peroneal nerve stimulation also elicited 4 reproducible negative-positive peaks having latencies of about 24, 26, 28, and 30 msec. Interpeak latencies measured 1.3 +/- 0.2 msec and 1.8 +/- 0.25 msec for median and peroneal elicited SEPs respectively. Because cephalic bipolar recordings cancel most far-field potentials, multiple generators cannot account for all the additional components seen. It is hypothesized that some of the high frequency components recorded are due to activity in recurrent intrathalamic neuronal networks.

Impedance control of a teleoperated excavator

Earth-moving machines such as hydraulic excavators are usually used for carrying out contact tasks. Impedance control can be employed as an approach for achieving compliant motion in such tasks. This paper describes a position-based impedance controller that has been developed in our laboratory for excavator-type manipulators, and presents the supporting experimental results. First, the problem of impedance control for a single hydraulic cylinder is addressed and a method is presented to analyze the system stability. The steady-state position and force tracking accuracy of the closed-loop system is also studied. Next, the problem of impedance control for a multi-link hydraulic excavator is addressed and the arm Jacobian and accurate estimates of the arm inertial terms are employed to map the desired impedance of the end-effector (bucket of the excavator) onto the hydraulic cylinders. Various contact experiments carried out using an instrumented mini-excavator demonstrate that the proposed impedance controller has very good performance for both single-link and multilink cases.

Application of force feedback to heavy duty hydraulic machines

The design and implementation of a force-reflecting controller for conventional heavy duty hydraulic machines are described. The unsuitability of direct force feedback with rate control has been shown analytically and confirmed on a simulator of a typical hydraulic machine. A stiffness control scheme to circumvent this problem was developed and used successfully in controlling the endpoint force on a CAT 215 log loader to better than 10% of a typical working load. A magnetically levitated wrist was used as a force-reflecting master, and the endpoint forces were obtained from hydraulic cylinder pressures.<<ETX>>

A non-contact device for tracking gaze in a human computer interface

This paper presents a novel design for a non-contact eye detection and gaze tracking device. It uses two cameras to maintain real-time tracking of a persons eye in the presence of head motion. Image analysis techniques are used to obtain accurate locations of the pupil and corneal reflections. All the computations are performed in software and the device only requires simple, compact optics and electronics attached to the users computer. Three methods of estimating the users point of gaze on a computer monitor are evaluated. The camera motion system is capable of tracking the users eye in real-time (9fps) in the presence of natural head movements as fast as 100^o/s horizontally and 77^o/s vertically. Experiments using synthetic images have shown its ability to track the location of the eye in an image to within 0.758 pixels horizontally and 0.492 pixels vertically. The system has also been tested with users with different eye colors and shapes, different ambient lighting conditions and the use of eyeglasses. A gaze accuracy of 2.9^o was observed.

Identification of inertial and friction parameters for excavator arms

A novel yet simple approach for experimental determination of the link (mass and inertia-related) parameters and friction coefficients is developed for a typical excavator arm. The parameters are needed for indirect measurement of the external forces, compensation for the link weights in the operators hand in a force-feedback teleoperation setup, impedance control of the arm, simulation of the manipulator dynamics, and model-based fault detection. Treating the machine arm as an open kinematic chain, its dynamic equations are presented symbolically. The static torque equations are derived from these equations and the gravitational parameters are defined accordingly. A new method for decoupled estimation of the gravitational parameters from static experiments is presented. Furthermore, the arm dynamics are expressed in a form which is linear in the inertia and friction-related parameters. The results obtained show that the identified model predicts the joint torques, in both static and dynamic conditions, with a very good accuracy.

Coordinated and Force-Feedback Control of Hydraulic Excavators

The human interface of a Caterpillar 325FB feller-buncher was modified to allow the operator to use (i) a 5-DOF joystick, and (ii) a 6-DOF magnetically-levitated joystick with stiffness feedback. While the operator commanded the velocity of the endpoint, an onboard computer system managed total system power, solved the inverse kinematics, servoed the joint actuators, and controlled the magnetically-levitated joystick.