Robert J. Anderson

Bilateral control of teleoperators with time delay

A control law for teleoperators is presented which overcomes the instability caused by time delay. By using passivity and scattering theory, a criterion is developed which shows why existing bilateral control systems are unstable for certain environments, and why the proposed bilateral control law is stable for any environment and any time delay. The control law has been implemented on a single-axis force-reflecting hand controller, and preliminary results are shown. To keep the presentation clear, a single-degree-of-freedom (DOF) linear time-invariant (LTI) teleoperator system is discussed. Nevertheless, results can be extended, without loss of generality, to an n-DOF nonlinear teleoperation system. >

Hybrid impedance control of robotic manipulators

The inclusion of force information in the control of robots increases their adaptability to uncertain environments, such as are found in deburring, grinding, and assembly tasks. The authors present a foundation for force control strategies, in view of the fact that the type of control strategy that is employed depends fundamentally on the characteristics of the environment. A general control approach is introduced, called hybrid impedance control which in its simplest forms reduces the operational space control of O. Khatib and J. Burdick (1986), or to N. Hogans (1985) impedance control. The control law is formulated in a general enough fashion, however, to allow for higher order controllers. >

Compressive response of NiTi single crystals

The deformation of NiTi shape memory single crystals are reported under compression loading for selected crystal orientations and two diAerent Ti3Ni4 precipitate sizes. For the (148) orientation, selected for highest recoverable strains, the peak aging treatment decreased the transformation stress from austenite to martensite. At the same time, peak aging raised the flow stress of both the austenite and martensite compared to the overaged case by increasing the resistance of the material to dislocation motion. The transformation proceeds beyond the stress plateau region and extends until martensite yielding occurs. This results in recoverable strain levels equivalent to the theoretical estimate of 6.4%. The (112) orientation was chosen to produce two variant formations and in this case, the transformation proceeded over an ascend- ing stress-strain curve compared to the nearly plateau response for the (148) case. Since the austenite and martensite yield levels are reached at a smaller strain level in this case, the maximum recoverable strain was limited to 3.5% even though the theoretical estimates are near 5.1%. The theoretical estimates of transformation strains were established for Type I and Type II twinning cases to cover all possible habit plane and twin systems. TEM investigations support that slip in austenite occurs concomitant with increas- ing transformation strains. In the (001) orientation, the unfavorable slip systems for dislocation motion in the austenite inhibit slip and permit recoverable strains similar to the theoretical estimates (nearly 4.2%). The (001) orientation exhibits a continuous increase of flow stress with temperature beyond 360 K unlike any other orientation. The results point out that in order to optimize the material performance, close atten- tion must be paid to the selection of the crystallographic orientation, and the precipitate size through heat treatment. 7 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.

On the mechanical behavior of single crystal NiTi shape memory alloys and related polycrystalline phenomenon

Room temperature monotonic and cyclic stress–strain curves for single crystal and polycrystalline NiTi shape memory alloys containing Ti3Ni4 precipitates are presented. The tensile and compressive single crystal results illustrate the importance of crystallographic texture, the unidirectional nature of the martensitic transformation, and martensite detwinning on tension-compression stress–strain asymmetry in polycrystalline NiTi. Moreover, results on the fatigue of NiTi single crystals demonstrate the fundamental characteristics of cyclic deformation in NiTi alloys and the importance of texture on the fatigue of polycrystalline NiTi.

Cyclic deformation behavior of single crystal NiTi

Abstract Single crystals of NiTi (with 50.8 at.% Ni) were subjected to cyclic loading conditions at room temperature which is above the M s (martensite start) temperature of −30°C. The single crystals exhibited remarkable cyclic hardening under zero to compression strain control experiments. The stress range under strain control increased by as much as a factor of 3 in compression. The increase in stress range is primarily due to the increasing strain hardening modulus. In the tension case, loop shape changes occurred but the increase in stress range is rather small. The fatigue cycling was undertaken with a strain range of 3% which is far below the theoretical transformation strains levels exceeding 6%. The maximum stress levels reached in the experiments are below those that cause martensite slip. Therefore, the stress–strain response is governed by transformation from the austenite to the martensitic phases and the dislocation structure evolution in the austenite domains. Two single crystal orientations [148] and [112] were examined during the experiments with single and double CVP (correspondent variant pair) formations respectively. The strain hardening in compression cases is rather substantial with the stress range in the double CVP case surpassing the single CVP case. Two heat treatments were selected to produce coherent and incoherent precipitates in the microstructure respectively. The influence of the coherent precipitates on the stress–strain response is significant as they lower the transformation stress from austenite to martensite, and at the same time, they raise the flow stress of the austenite and martensite domains leading to higher saturation stresses in fatigue.

RAILROAD DERAILMENT FACTORS AFFECTING HAZARDOUS MATERIALS TRANSPORTATION RISK

U.S. freight railroad accident and hazardous materials release rates have declined substantially since 1980. Ironically, this trend has made the identification and implementation of further safety improvement options more challenging because less empirical information exists on which accident causes present the greatest risks. Consequently, more sophisticated methods are needed to identify the best options for transportation risk reduction. Of particular interest is identifying the principal causes of accidents that can result in a tank car release of hazardous materials, which can harm people, property, and the environment. Because large hazardous materials release accidents are relatively rare, railroads cannot effectively manage safety improvement efforts solely in response to the causes of specific accidents. Instead, a risk-based approach is needed to better understand predictive factors for conditions that can cause a release. Railroad derailment data were analyzed to identify the conditions most likely to lead to a release accident. The objective was to identify proxy variables that can be used as performance measures. The speed of derailment and number of derailed cars highly correlated with hazardous materials releases. Some accident causes are much more likely to lead to release conditions than others. Accident prevention efforts to reduce these causes are more likely to reduce the risk of major railroad hazardous materials release accidents.

Passive computed torque algorithms for robots

Computed torque algorithms are used to compensate for the changing dynamics of robot manipulators in order to ensure that a constant level of damping is maintained for all configurations. There are three significant problems with existing computed torque algorithms. First, they are nonpassive and, as shown, can lead to unstable behavior; second, they make poor use of actuator capability; and third, they cannot be used to maintain a constant end-effector stiffness for force control tasks. A new class of passive computed torque controller which have guaranteed stability properties, utilize actuators effectively, and maintain constant end-effector stiffness is introduced. Network models are used to motivate and illustrate the approach.<<ETX>>

A note on the asymptotic variance of the gray and williams measure of partial association

The correct asymptotic variancef or the partial association analog of Goodman and Kruskals T measure of associati o n based on proportional prediction is derived for both full multinomial sampling and product multinomial sampling. These results are illustrated within the context of an example data set.