Search

We present the first demonstration of a battery-free untethered wirelessly powered sub-gram jumping robot on an insect-scale. In order to operate the insect-sized robot autonomously, the limitation in battery use emphasizes the need for a wireless power transmission system as an onboard power solution. We designed a wireless power transmission system based on inductive coupling to power the Shape Memory Alloy (SMA), which serves as an elastic energy storage element and actuator for the jumping robot. The assembled mechanical structures, onboard power and electronics yield a 2 mm (high) × 24 mm (long) × 12 mm (wide) robot with ∼a weight of 216 mg. The experiments show that our jumping robot wirelessly lift-off up to 5.75 times its body length and repeats the jump around 7 times per minute. To date, out of the several untethered sub-gram insect-scale jumping robots with onboard power, this is the first wirelessly powered robot with the highest jumping performance. The novelty in this work, which addresses the engineering challenges in insect-scale jumping robots, is an untethered wirelessly powered design that achieves dynamic jumping maneuvers, and has self-righting ability.

We study the interaction of one-dimensional acoustic solutions with an inhomogeneous elastic medium containing a spherical void. We investigate the influence of the in homogeneity on the reflection of incident acoustic solutions using a hybrid model consisting of finite-element and discrete element methods. The reflection of incident acoustic solutions and the time of flights of reflections are found to be dependent upon the changes in the contact interaction between the last particle and the inhomogeneous medium. Furthermore, the time of flights of the reflected acoustic solutions are sensitive to the size and the location of a void.

Cellular structures like hexagonal honeycombs have been of particular interest because of their unique dynamic behavior. These periodic structures can control wave propagation through exhibiting pass and stop bands over certain frequency ranges. Geometrical modifications of unit cells in these hexagonal honeycombs have been explored to achieve different properties such as a negative Poisson's ratio, which is also known as auxetic materials. It is found that a characteristic angle of the unit cell determines the value of the Poisson's ratio. In this study, we investigate different topologies of unit cells through the Timoshenko beam-based finite element method and the principles of Floquet-Bloch theorem. We find that the characteristic angle of the unit cell has a significant effect on the in-plane wave propagation behavior and its directionality.

Periodic woodpile structures composed of slender cylindrical rods, defined as woodpile mechanical metamaterials herein, show versatile wave dynamics due to local resonances of slender rods. We numerically and experimentally investigate such resonance effects on nonlinear wave propagation in the woodpile mechanical metamaterials under impact excitations. We report these woodpile architectures exhibit variable speed and amplitude of transmitted nonlinear waves depending on the geometrical configurations of the assembled woodpile architectures.

We numerically and experimentally investigate the nonlinear wave dispersion in 3D woodpile periodic structures consisting of slender cylindrical rods. The slender rods have low frequency resonance modes that are coupled with the propagating wave. Depending on the coupling of local modes, the propagating waveform in woodpile can change significantly. Also, the wave dispersion in temporal and spatial domain depends significantly on the mode coupling. The coupling of the local modes can be tuned by adjusting the architecture of the woodpile.

Double squirrel cage rotor design is employed in induction motor applications that require high starting torque and high efficiency operation. The outer cage of double cage rotors is vulnerable to fatigue failure since it must withstand the large thermal/mechanical stresses experienced during a loaded startup due to the high starting current and long acceleration time. However, there are only a few publications that investigate broken bar detection for double cage induction motors. In this paper, the detectability of broken outer cage bars in double cage motors for the most commonly used rotor bar test methods is evaluated. A finite element and experimental study show that the sensitivity of on-line MCSA is significantly decreased, whereas that of off-line standstill tests is not influenced for broken outer cage bars. This suggests that one should be aware of the insensitivity of MCSA for double cage rotors, and there is a need for development of new on-line monitoring methods.

It is difficult to apply conventional online motor current signature analysis techniques for diagnosis of rotor faults for closed-loop induction motor drives for many applications due to the masking effect of the feedback current controller and/or variable frequency or load operation. Relying solely on traditional offline inspection techniques during regular maintenance does not allow frequent monitoring of rotor problems and is inconvenient since it requires rotor disassembly and/or manual rotor rotation. In this paper, a new automated technique for testing voltage source inverter-fed squirrel-cage induction machines at a standstill for rotor faults is proposed. The main concept is to use the inverter to excite the machine with a pulsating field at a number of angular positions to observe the variation in the impedance pattern due to broken rotor bars, whenever the motor is stopped. An experimental study on a 7.5-hp induction motor verifies that broken bars can be detected with high sensitivity and reliability. It will be shown that the proposed method can provide automated and reliable assessment of rotor condition frequently without motor disassembly, manual rotation, or additional instrumentation. The proposed test can also provide rotor quality assessment independent of variations in motor or load operating conditions since it is a standstill test.

A new strategy for monitoring the condition of adjustable speed induction machine drive systems is presented in this paper. The main concept of the new method is to use the inverter to perform off-line tests for quality assessment of the vulnerable components in the inverter, cable, and induction motor automatically, whenever the motor is stopped. This new approach is suitable for monitoring system components that deteriorate gradually to failure, if the degradation is observable. Off-line test methods for monitoring the quality of the 1) dc link aluminum electrolytic capacitor, 2) electrical connections, 3) cable and stator winding insulation, 4) stator core, and 5) rotor bar are presented along with a summary of the failure mechanism and existing test methods. It will be shown that the new strategy can be implemented with minimal hardware requirements, and that it has many advantages over existing off-line and on-line tests. An experimental study performed under simulated fault conditions for each component shows that the method provides a reliable and sensitive indication of drive system problems for reliable, efficient, and safe operation.

The lifetime of the stator winding insulation is reduced when operated with PWM inverters, since insulation degradation is accelerated due to increased thermal and electrical stresses on the motor. This paper focuses on a new approach for monitoring the condition of the stator insulation for failure-prone inverter-fed machines. The main concept of the proposed technique is to apply a variable frequency AC test voltage to the stator insulation using the inverter, to perform standard off-line insulation tests whenever the motor is not operating. This allows the stator insulation to be tested more frequently compared to off-line tests performed once every 3-6 yrs. during periodic maintenance. A method for testing the insulation with minimal modifications to the inverter circuit is proposed and verified experimentally. The results show that the proposed technique provides a simple low cost solution for detecting stator insulation degradation at an early stage for reliable operation of inverterfed machine systems, since the insulation condition can be monitored frequently. This not only helps prevent forced outages and safety risks due to insulation breakdown, but also helps perform maintenance in an efficient manner since individual motor maintenance can be prioritized and scheduled based on its present condition.