A JAG in La La Land

As the development of highways, it is quite normal for buses running in a speed around 100km/h. When buses are running in a high speed, they may suffer from the influence of side wind disturbances at anytime. Sometimes, it may result in traffic accidents. Therefore, the study of bus stability under side wind disturbances becomes more and more important. Due to restrictions of real tests, computer simulation can be used to study this subject. The bus side wind response character is reflected through the driver’s manoeuvre , so open-loop analysis is hard to give a comprehensive evaluation of the side wind stability of the bus. Therefore, closed-loop analysis is studied in this thesis. An ADAMS bus model and a side wind force model are developed in this thesis, along with two driver models, the PID control model and the preview curvature model. The driver models are built in Simulink and co-simulation between ADAMS/View and Simulink is conducted. The results of co-simulation show that the two driver models can both control the bus from deviating from the desired course under side wind disturbances. The PID control model is simple and shows a very good control effect. The maximum lateral displacement of the bus by PID control model is just 0.0205m under maximum side wind load 1000N and 2500Nm when preview time is 1.2s, while it is 0.0702m by preview curvature model, however, it is difficult to determine the coefficients Kd, Kp, and Ki in the PID controller. The preview curvature model also shows a good control effect in terms of the maximum lateral displacement and yaw angle of the bus. Comparing these two models, the PID control model is more sensitive to deviations, with quicker response and larger steering input. The bus model system is stable under side wind disturbances. Through driver ’s proper steering manoeuvre, the bus is well controlled. The closed-loop analysis is a good method to study the bus stability under side wind disturbances

Exploiting Passive Stability for Hierarchical Control

The dynamics of a Spring Loaded Inverted Pendulum (SLIP) \template” [1] approximate well the center of mass (COM) of running animals, humans, and of the robot RHex [2]. Running control can therefore be ierarchically structured as a high level SLIP control and the anchoring of SLIP in the complex morphology of the physical system. Analysis of the sagittal plane lossless SLIP model has shown that it includes parameter regions where its gait is passively stabilized, i.e. with the discrete control input | the leg touchdown angle | held constant. We present numerical evidence to suggest that an open loop \clock” excitation of a high degree of freedom hexapedal robot model can lead to asymptotically stable limit cycles that \anchor” [1] the SLIP model in its self stabilizing regime. This motivates the search for completely feedforward SLIP locomotion control strategies, which we now speculate may be successfully used to elicit a self-stabilizing running robot such as RHex. For more information: Kod*La

Feedback Control of a Bipedal Walker and Runner with Compliance.

This dissertation contributes to the theoretical foundations of robotic bipedal locomotion and advances the experimental state of the art as well. On the theoretical side, a mathematical formalism for designing provably stable, walking and running gaits in bipedal robots with compliance is presented. A key contribution is a novel method of force control in robots with compliance. The theoretical work is validated experimentally on MABEL, a planar bipedal testbed that contains springs in its drivetrain for the purpose of enhancing both energy efficiency and agility of dynamic locomotion. While the potential energetic benefits of springs are well documented in the literature, feedback control designs that effectively realize this potential are lacking. The methods of virtual constraints and hybrid zero dynamics, originally developed for rigid robots with a single degree of underactuation, are extended and applied to MABEL, which has a novel compliant transmission and multiple degrees of underactuation. A time-invariant feedback controller is designed such that the closed-loop system respects the natural compliance of the open-loop system and realizes exponentially stable walking gaits. A second time-invariant feedback controller is designed such that the closed-loop system not only respects the natural compliance of the open-loop system, but also enables active force control within the compliant hybrid zero dynamics and results in exponentially stable running gaits. Several experiments are presented that highlight different aspects of MABEL and the feedback design method, ranging from basic elements such as stable walking, robustness under perturbations, energy efficient walking to a bipedal robot walking speed record of 1.5 m/s (3.4 mph), stable running with passive feet and with point feet. On MABEL, the full hybrid zero dynamics controller is implemented and was instrumental in achieving rapid walking and running, leading upto a kneed bipedal running speed record of 3.06 m/s (6.8 mph).Ph.D.Electrical Engineering: SystemsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/89801/1/koushils_1.pd

Template Based Control of Hexapedal Running

In this paper, we introduce a hexapedal locomotion controller that simulation evidence suggests will be capable of driving our RHex robot at speeds exceeding five body lengths per second with reliable stability and rapid maneuverability. We use a low dimensional passively compliant biped as a template -- a control target for the alternating tripod gait of the physical machine. We impose upon the physical machine an approrimate inverse dynamics within-stride controller designed to force the true high dimensional system dynamics down onto the lower dimensional subspace corresponding to the template. Numerical simulations suggest the presence of asymptotically stable mnning gaits with large basins of attraction. Moreover, this controller improves substantially the maneuverability and dynamic range of RHex\u27s running behaviors relative to the initial prototype open-loop algorithms

Small Mercury Ion Clock for On-board Spacecraft Navigation

I.Small Ion Clock Approach and Heritage: a) No lasers, uwave cavities, cryogenics, atomic beams, etc. b) Ions are electrically shuttled between separate optical and microwave traps. II. Each trap is optimized for its task: quadrupole for optical state selection; multi-pole for microwave clock. a) Very good stability shown in USNO. Timescale running "open loop." III. "Open loop" operation means no self-measurements of frequency offsets: (Zeeman, ion temperature,... etc.) a) Fewer parts and procedures, produces stable output continuously. IV. Ion clock is not so sensitive to temperature fluctuations a) Measured u:nshielded temperature coefficient of few 10(exp -15) per C. b) No bulky temperature isolation needed

Remote bistatic receiver synchronization using DLL techniques

Pulse to pulse staggering in the pulse reference interval (PRI), usually used to increase the radar unambiguous range and MTI blind speeds, complicates notably the synchronization of the pulsed waveform. A free running open loop technique may be used for this purpose, adjusting a stable clock at the same frequency as the transmitted pulses, which is synchronized by the received sequence during direct illumination. When the received signal can be contaminated by interferences, noise, or multipath effects, a more robust closed loop approach is desirable. This paper presents a closed loop solution to the synchronism process, based on a delay-lock loop configuration, using the special properties of the pulse to pulse staggered signal, that can be considered as a digital pseudorandom code modulation with a period equal to the PRIPeer ReviewedPostprint (published version

Cascaded multiplexed optical link on a telecommunication network for frequency dissemination

We demonstrate a cascaded optical link for ultrastable frequency dissemination comprised of two compensated links of 150 km and a repeater station. Each link includes 114 km of Internet fiber simultaneously carrying data traffic through a dense wavelength division multiplexing technology, and passes through two routing centers of the telecommunication network. The optical reference signal is inserted in and extracted from the communication network using bidirectional optical add-drop multiplexers. The repeater station operates autonomously ensuring noise compensation on the two links and the ultra-stable signal optical regeneration. The compensated link shows a fractional frequency instability of 3 \times 10-15 at one second measurement time and 5 \times 10-20 at 20 hours. This work paves the way to a wide dissemination of ultra-stable optical clock signals between distant laboratories via the Internet network

Large N Field Theory and AdS Tachyons

In non-supersymmetric orbifolds of N =4 super Yang-Mills, conformal invariance is broken by the logarithmic running of double-trace operators -- a leading effect at large N. A tachyonic instability in AdS_5 has been proposed as the bulk dual of double-trace running. In this paper we make this correspondence more precise. By standard field theory methods, we show that the double-trace beta function is quadratic in the coupling, to all orders in planar perturbation theory. Tuning the double-trace coupling to its (complex) fixed point, we find conformal dimensions of the form 2 + i b, as formally expected for operators dual to bulk scalars that violate the stability bound. We also show that conformal invariance is broken in perturbation theory if and only if dynamical symmetry breaking occurs. Our analysis is applicable to a general large N field theory with vanishing single-trace beta functions.Comment: 26 pages, 6 figures. v3: small changes, version published on JHEP

The Coleman-Weinberg Mechanism and First Order Phase Transitions

Revised and reduced the size of the text. Removed figure 2 in previous text. So, old figure 3 and 4 become new figure 2 and 3, respectively. Added figure 4 for first order phase transition observation in the strong coupling region. Postscript files for the figures are available upon request. To be published in Phys. Lett. B.Comment: 8 pages, 4 figures. BUHEP-93-5, hep-lat/930301