Chi haur Wu

Robot accuracy analysis based on kinematics

The positioning accuracy problem of robot manipulators has long been one of the principal concerns of robot design and control. In a previous work, a linear model that described the robot positioning accuracy due to kinematic errors was developed. However, the previous work considered only the small errors by ignoring the higher order terms and did not address the special case of two consecutive parallel joints. In this work a more detailed model is given that applies to consecutive parallel joints and includes the second-order terms. By comparing the results of the linear model and the second-order model, the accuracy of the linear model can be evaluated for a given manipulator and range of input kinematic errors. The error envelopes obtained using the linear model and the developed second order model for the Puma 560 are plotted and compared for various sets of input kinematic errors. A comparison of the computation complexity for the two models is also given.

A Kinematic CAD Tool for the Design and Control of a Robot Manipulator

The correct relationship between two connective joint coordi nates of a robot manipulator is defined by four link parame ters ; one being the joint variable and the other three the geometric values. The basis for all open-loop manipulator control is the relationship between the Cartesian coordinates of the end-effector and the joint coordinates; therefore, the fidelity of the Cartesian position and orientation of the end- effector to the real world depend on the accuracy of the four link parameters of each joint. In this paper, a linear analytic error model describes the six possible Cartesian errors and the four independent kinematic errors from which the Carte sian error envelopes due to any combination of four kinds of kinematic errors can be uniquely determined. From a design standpoint, this error model can be used as a guide to mini mize the open-loop kinematic errors of the robot manipulator. Finally, a new calibration technique based on this model has also been developed that can be used to correct the kinematic errors of the robot manipulator.

Robot calibration and compensation

A method is presented for calibrating and compensating for the kinematic errors in robot manipulators. A method of selecting a set of independent kinematic errors for modeling any geometric errors in a manipulators structure is developed. A calibration algorithm is presented for finding the values of these kinematic errors by measuring the end-effector position. These kinematic errors are experimentally determined for a PUMA 560 six-joint manipulator. Two general-purpose compensation algorithms are developed and the improvement in the Cartesian position of the end-effector is experimentally measured and these results are presented. The results show that the positioning accuracy of a robot manipulator can be substantially improved using a relatively simple technique for measuring the Cartesian position of a tool attached to the end of the manipulator. >

Total knee replacement

Describes a computer-assisted surgical system which uses a calibrated robot. The authors consider: total knee replacement surgery; motivation for robotic systems in total knee replacement; system operation; CT scan and preoperative planning; femoral fiducial location; femoral component placement; tibial fiducial location and component placement; fixturing; immobilization of the knee; immobilization of the pelvis and ankle; fiducial identification and registration; landmark pins; center of the femoral head; robot system and calibration; calibration using a telescopic ball-bar. >

A method for calibrating and compensating robot kinematic errors

This paper presents a method of calibrating and compensating for the kinematic errors in robot manipulators. A method of selecting a set of independent kinematic errors for modeling any geometric errors in a manipulators structure is developed. A calibration algorithm is presented for finding the values of these kinematic errors by measuring the end-effector Cartesian position. These kinematic errors are experimentally determined for a PUMA 560 six joint manipulator. Two general purpose compensation algorithms are developed and the improvement in the Cartesian position of the end-effector is experimentally measured and these results are presented.

Intracranial 4D Flow MRI: Toward Individualized Assessment of Arteriovenous Malformation Hemodynamics and Treatment-Induced Changes

BACKGROUND AND PURPOSE: Arteriovenous malformations are an important etiology of hemorrhagic stroke. However, current imaging modalities and risk do not provide insights into individual AVM hemodynamics and its role in pathophysiology. The aims of this study are to determine whether intracranial 4D flow MR imaging can provide insights into arteriovenous malformation hemodynamics independent of the Spetzler-Martin grade and to report the changes in flow observed during staged embolization. MATERIALS AND METHODS: Intracranial 3D blood flow was assessed in 20 patients with AVM (age = 39 ± 15 years, Spetzler-Martin grade ranging from 1–4) with the use of 4D flow MR imaging (temporal resolution = 45 ms, spatial resolution = [1.2–1.6mm]3). AVM hemodynamics were visualized by means of time-integrated 3D pathlines depicting the AVM arterial feeding and venous draining patterns over the cardiac cycle. Analysis included the grading of feeding and draining velocities on a 3-point scale (0 = low <25 cm/s, 1 = medium <50 cm/s, 2 = high >50 cm/s). For 4 of 20 patients undergoing 4D flow MR imaging follow-up after staged embolization, peak velocities were quantified in arterial feeders, draining veins, the sagittal sinus, and contralateral arteries. RESULTS: In 50% of the cases with Spetzler-Martin grade >2, heterogeneous flow (velocity grade differences >1) was found across arteries and veins. Velocities in draining veins increased from Spetzler-Martin grade = 1 (grading = 0.5 ± 0.6) to Spetzler-Martin grade ≥3 (1.1 ± 0.6), whereas arterial velocities were similar (1.7 ± 0.6 versus 1.5 ± 0.6). In the postembolization subgroup of 4 patients, 4D flow MR imaging demonstrated successively more compact AVM and redistribution of velocities. Changes in arterial and venous velocities during treatment were highly different among individuals. CONCLUSIONS: Spetzler-Martin grade does not reflect differences in 3D AVM arterial and venous hemodynamics, and an individual assessment of AVM hemodynamics may be needed for improved lesion characterization. Four-dimensional flow MR imaging may have the potential to monitor and guide embolization treatment planning.

Nonlinear Damping of Limb Motion

The muscle-reflex mechanisms form a feedback system called the motor servo (Houk and Rymer, 1981; Gielen and Houk, 1987), which consists of a muscle, its spindle receptors, and the corresponding reflex pathways back to the muscle. This neuromuscular system mediates the stretch and unloading reflex of the muscle by the feedback. Motivated by a desire to understand the capabilities of biological arms, many researchers have studied limb movements (Asatryan and Feldman, 1965; Feldman, 1966; Freund and Budingen, 1978; Gottlieb et al., 1989a,b; Hasan, 1983,, 1985,, 1986; Hogan, 1984a,b; Polit and Bizzi, 1979; Stein, 1982) and findings have been applied to arm control and task planning (Atkeson and Hollerbach, 1985; Bizzi et al., 1978,, 1982,, 1984; Flash, 1987; Flash and Hogan, 1985; Hogan, 1985; Hollerbach, 1982; Hollerbach and Flash, 1982; Mussa-Ivaldi et al., 1985,, 1988; Uno et al., 1989).

Voluntary movements for robotic control

A neuromuscular-like nonlinear mathematical model for controlling robotic limbs is described. The model consists of two muscle-reflex models representing a pair of muscles acting as an agonist and an antagonist in moving a load. The responses of the model have been compared with experimental data from human wrist movements in order to identify command signals in the form of a series of rectangular pulses for effecting the measured movements with two different loads. These results can be used to obtain several empirical rules for muscle modulation control. The proposed control strategy can also compensate for unexpected disturbances, which is an essential capability for compliance control.<<ETX>>

Automatic generation of assembly instructions using STEP

An automatic method for generating assembly instructions using CAD files is presented. Algorithms for extracting geometrical information of objects stored in a non-proprietary format, ISO-10303, STEP-CAD data file are explained. The developed algorithms form an important link between design and manufacturing. In our previous work, (Mok et al., 2000, 1999, Wu and Kim, 1994, and Kim and Wu, 1990) we designed a hierarchical assembly model that allows designers to evaluate products for manufacturing cost using a structured assembly coding system (SACS). Each SACS code represents a sequence of assembling/disassembling operations for mating two parts. The developed algorithms in this paper generate the required SACS codes based on the geometrical and topological information extracted from the STEP files of a product in a CAD environment. Based on the generated SACS codes, the proper assembly operations can then be derived for assembling the designed product. An example of assembly with simple parts is presented to verify the method.

Compliance Control of a Robot Manipulator Based on Joint Torque Servo

The application of a robot manipulator to the task of assem bling manufacturing products requires compliance control, which means the control and monitoring of both forces and positions of the robot. To increase the operation speed of compliance control, a fast, reliable force servo system based on a proper sensing technique is imperatively needed. In this paper, current existing force-sensing techniques are analyzed, and the results suggest that a joint torque-sensing technique will give the best performance in a force servo system. A prototype joint torque sensor system is designed for our ex periment. As a result, a new, simple, high-gain, wide-band width torque servo system based on a joint torque sensor system is developed and the systems performance is verified by a single joint manipulator. To provide compliance, a method of selecting the force servo joints for a robot is also provided.