Gregg D. Larson

State-Switched Absorber for Semi-Active Structural Control:

A system that has the capability to make instantaneous changes in its mass, stiffness, or damping may be termed a state-switchable dynamical system. Such a system will display different dynamical responses dependent upon its current state. For example, state-switchable stiffness may be practically obtained through the control of the termination impedance of piezoelectric stiffness elements. If such a switchable stiffness element is incorporated as part of the spring element of a vibration absorber, the change in stiffness causes a change in the resonance frequencies of the system, thereby instantaneously “retuning” the state-switched absorber to a new frequency. This paper briefly develops the fundamental analysis tools for a Single-Degree-of-Freedom state-switchable device, and then considers the application of such a device for the purpose of vibration control in a 2-DOF system. Simulation results indicate that state-switched vibration absorbers may be advantageous over classical passive tuned vibration absorbers under certain conditions.

State switched transducers: A new approach to high-power, low-frequency, underwater projectors

In order to produce high-amplitude, low-frequency signals, an underwater transducer must generate a relatively large volume displacement. Since water exerts a large reaction force back on the transducer, “conventional wisdom” dictates that such a transducer would have to be a high Q resonant device and thus not be broadband. However, a transducer does not have to be broadband in the conventional sense to meet the requirements of communication and sonar systems. A transducer that is capable of instantaneously switching between two discrete frequencies is adequate for communication and transmission of coded signals; one that is capable of switching among several frequencies could produce the chirp signals commonly used in active sonars. Ordinarily, a broadband transducer is needed to accomplish the frequency switching rapidly. A way around this difficulty is the “state-switched” source concept originally proposed by Walter Munk in 1980 which permits instantaneous frequency switching of a high Q resonant tra...

Field testing and development of a seismic landmine detection system

A technique for the detection of buried landmines, which uses a seismic probing signal in conjunction with a non-contact radar-based surface displacement sensor, has been studied for several years at Georgia Tech. Laboratory experiments and numerical models have indicated that this technique shows great promise for imaging a large variety of mine types and burial scenarios. In order to develop a detection system based on this technique, recent studies have focused on transitioning the experimental work from laboratory models to realistic field environments, which poses several challenges for system development. Unknown soil properties at field sites as well as the presence of local inhomogeneities, vertical stratification, and surface variations make the propagation and the modal content of the seismic probing signal more difficult to predict. This complicates the processing required to image buried mines. The small-scale surface topography and naturally-occurring ground cover impede the function of the systems non-contact sensor, which must be capable of looking through the ground cover and spatially averaging its measurement over the irregular surface. A prototype detection system has been tested at several field sites with widely disparate soil properties. Problems were encountered that required modifications to the system sensor, scanning technique, and signal processing algorithms. Following these changes, system performance comparable to that observed in laboratory models was demonstrated during field testing.

Elastic waves interacting with buried land mines: a study using the FDTD method

A three-dimensional (3-D) finite-difference model for elastic waves in the ground has been developed and implemented. The model has been created to supplement the development of a sensor that uses elastic waves to detect buried land mines. The model is used to investigate the propagation characteristics of elastic waves in the ground and to explore the interaction of elastic waves with buried land mines. When elastic waves interact with a buried mine, a strong resonance occurs at the mine location. The resonance can be used to enhance the mines signature and to distinguish the mine from clutter. Results presented in this paper explain the features of elastic wave propagation in the ground and show the interaction of elastic waves with both an anti-personnel mine and an anti-tank mine.

State switched acoustic source

To produce high amplitude low‐frequency signals, an underwater transducer must generate a relatively large volume displacement. Since water exerts a large reaction force back on the transducer, ‘‘conventional wisdom’’ dictates that such a transducer would have to be a high Q resonant device and thus not be broadband as seemingly required for many applications. However, a transducer does not have to be broadband in the conventional sense to work in communication and SONAR systems. A transducer capable of switching between two discrete frequencies is adequate for communication and one capable of switching among several frequencies could produce chirp signals for active sonars. Ordinarily, a broadband transducer is needed to switch frequencies rapidly. It is theoretically possible, however, to instantaneously switch frequencies with a high Q resonant system provided that the system’s resonant and drive frequencies are altered simultaneously. Such a ‘‘state‐switched’’ transducer [Munk, Webb, Birdsall, unpubli...

Modeling the measured em induction response of targets as a sum of dipole terms each with a discrete relaxation frequency

Broadband electromagnetic induction (EMI) sensors have been shown to be able to reduce false alarm rates and increase the probability of detecting landmines. To aid in the development of these sensors and associated detection algorithms, a testing facility and inversion technique have been developed to characterize the response of typical targets and clutter objects as a function of orientation and frequency.

Quarter-Cycle Switching Control for Switch-Shunted Dampers

Significant interest has been generated by the possibilities of active vibration control through the implementation of state switching, with a specific implementation embodied through piezoceramic shunting. A state-switched absorber (SSA) is a vibration absorber that has the unique ability to change its resonant state amongst multiple distinct resonant states while in motion, thereby increasing the effective bandwidth over that of a single frequency device and thereby allowing control of multi-frequency, transient, and time-varying disturbances. In contrast, a switch-shunted damper (SSD) is a variant of an SSA that is used to increase the damping of the structure to which the damper is applied. Active vibration control applications discussed in the literature indicate the potential advantages of SSDs which employ piezoelectric ceramics as switchable springs with control algorithms that require switching states at points of non-zero strain. However, consideration of the constitutive equations for piezoelectric materials indicates a discontinuity in the electrical and mechanical conditions imposed by switching the stiffness at non-zero strains. A prototype SSD has been built and tested to experimentally investigate switching control logic and electrical and mechanical discontinuities at switching points; experimental measurements with this prototype SSD indicate that quarter-cycle switching algorithms which include switching states at a condition of maximum strain yield enhanced damping effectiveness but also leads to the generation of potentially undesirable mechanical transients.

Ultrasonic displacement sensor for the seismic detection of buried land mines

A system is under development that uses seismic surface waves to detect and image buried landmines. The system, which has been previously reported in the literature, requires a sensor that does not contact the soil surface. Thus, the seismic signal can be evaluated directly above a candidate mine location. The system can then utilize small amplitude and non-propagating components of the seismic wave field to form an image. Currently, a radar-based sensor is being used in this system. A less expensive alternative to this is an ultrasonic sensor that works on similar principles to the radar but exploits a much slower acoustic wave speed to achieve comparable performance at an operating frequency 5 to 6 decades below the radar frequency. The prototype ultrasonic sensor interrogates the soil with a 50 kHz acoustic signal. This signal is reflected from the soil surface and phase modulated by the surface motion. The displacement can be extracted from this modulation using either analog or digital electronics. The analog scheme appears to offer both the lowest cost and the best performance in initial testing. The sensor has been tested using damp compacted sand as a soil surrogate and has demonstrated a spatial resolution and signal-to-noise ratio comparable to those that have been achieved with the radar sensor. In addition to being low-cost, the ultrasonic sensor also offers the potential advantage of penetrating different forms of ground cover than those that are permeable to the radar signal. This is because density and stiffness contrasts mediate ultrasonic reflections whereas electromagnetic reflection is governed by dielectric contrast.

An investigation of surface-contacting sensors for the seismic detection of buried landmines

Techniques have been studied for the detection of buried landmines with acoustic/seismic interrogation signals. Much of this work has involved full wave-field imaging from local measurements of ground motion using noncontact sensors. These offer inherent safety for the system operator and accommodate the need to make measurements over rough ground surfaces. The system requirement is, however, only that a sensor does not intrude on the measurement rather than that it not contact the ground. An experimental investigation was conducted into the feasibility of an array of ground-contacting sensors for use in a seismic landmine-detection system that exploits full wave-field imaging. The main considerations in the design of the array sensor were safety, sensitivity, fidelity, reproducibility, and sensor-to-sensor interaction. A relatively simple and inexpensive sensor was demonstrated in an experimental simulation of a landmine-detection system. The sensor, which is suitable for inclusion in a large planar array that could be used for detection confirmation, exerts a safe normal force at the point of contact and enables detection performance comparable to that which could be achieved using noncontact techniques.

Technical issues associated with the detection of buried land mines with high-frequency seismic waves

An array of radars is developed as a stand off sensor for use in elastic/seismic mine detection systems. The array consists of N radar sensors which operate independently to sense the displacement of the surface of the earth due to elastic waves propagating in the earth. Each of the sensors consists of a lens-focused, conical, corrugated, horn antenna and a homodyne radar. The focused antenna allows the sensor to have greater standoff than with the previous unfocused antenna while maintaining the spatial resolution required for a mine detection system. By using an array of N sensors instead of a single sensor, the scan rate of the array is improved by a factor of N. A theoretical model for the focused antenna is developed and an array of two radars is developed and used to validate the theoretical model. This array is tested in both the experimental and the field models for the elastic mine detection system. Results from both systems are presented.