Brian C. Fehrman

Depth mapping using a low-cost camera array

Computer vision has the potential to discern a large amount of information about the environment. This intelligence can be used to make decisions on navigation and obstacle avoidance. One of the core problems in machine vision is determining the distance from the camera to different objects for a given scene. Stereo-vision is one technique for solving this problem. Typically, two cameras are used for this algorithm. Using more than two cameras, however, has the ability to provide even better results. Here, a low-cost array of cameras was used which was built from commonly available, inexpensive hardware. The information from the multiple cameras was combined to provide a dense real-time depth map of the environment. The results of single stereo camera pairs versus multiple stereo camera pairs were compared and it was found that using multiple pairs does provide a denser depth map over that of a single pair.

Targeted delivery of acoustic energy for self-healing

Self-healing materials capable of autonomic crack healing are potentially important in terrestrial and space applications. Where damage-causing forces and healing forces compete, it is of interest to consider ways to increase the healing rate. This article investigates the use of acoustic energy for this purpose. As a means for targeted acoustic energy delivery to the crack site, we study time-reversal mirrors (iterative time reversal and playback) for stress wave focusing at a discontinuity. The article begins with a discussion of the key analytical results. Both pulse propagation and eigenfrequency vibrations are analyzed, and it is argued that if sufficient time is allowed between successive time-reversed playbacks, the focused pulse amplitude grows faster than the eigenfrequency vibrations. Experimental implementations of iterative time-reversed playback on solid circular steel and nylon rods with artificially introduced discontinuities show that even in the one-dimensional propagation studies here, time-reversal mirrors produce satisfactory focusing for a multitone pulse. Based on the significant amplification seen in the steel rods and the more dispersive and dissipative nylon rods, it appears that acoustic energy could be delivered in a targeted manner to a crack using iterative time-reversal mirrors.

Experiments on the focusing and use of acoustic energy to accelerate polymer healing

In this paper, we cover our studies on accelerating the molding process of a polymer by applying acoustic stress-wave time reversal. Tests carried out on an epoxy polymer mixed with a curing agent have shown evidence that the introduction of unfocused acoustic energy during the molding process will accelerate that process. The effects of focusing acoustic energy at a mold discontinuity while curing are explored. We also detail our investigations on focusing acoustic energy at a crack location by iteratively applying time reversal. Multiple types of media were tested.

Experiments on focusing and use of acoustic energy to enhance the rate of polymer healing

We consider the effects of acoustic pressure on the curing of a two-part epoxy, which can be considered analogous to the polymer healing process. An epoxy sample is loaded into a tube and monitored throughout the early stages of curing by measuring its vibrational response upon periodic impulses. By tracing the natural frequencies of the epoxy-tube system and cross-checking the temperature of the epoxy, the progress of the curing can be quantified. Acoustic stimulation at three different frequencies is investigated and compared to the unstimulated case. We find that external acoustic pressure does seem to affect the curing, though much work remains to be completed.

Using Focused Acoustic Excitation to Accelerate Crack Healing

This paper presents recent investigations on an approach to accelerate the natural healing processes occurring in a polymer aggregate. In a space structure where the processes leading to healing must continuously compete with the mechanisms producing structural damage (such as micrometeor strikes), methods aimed at accelerating healing could prove valuable. The technique examined here is based on the use of focused acoustic energy to inuence the crack healing dynamics via increased pressure and temperature at the crack site. No knowledge or interference of crack location are required with this approach. Here we present our recent results on a study of epoxy curing as an analog to the healing process, and in particular, present in detail our experimental application of time reversed acoustics to focus longitudinally propagating pulses and chirp signals in metal rods. Extensions to other media are in progress.

Handling occlusion with an inexpensive array of cameras

The topic of computer vision is of interest due to its ability to provide a great deal of information about the physical world. One problem that plagues many vision applications is that of occlusion. Occlusion is the situation where the view of an object is being blocked by one or more other objects. In this work, a multi-view approach was taken to solve the problem. The tool used was a low-cost array of cameras that was designed and built in house. Results of the project are reviewed and it is shown that the algorithm used has the potential to recover many details that would otherwise be lost due to occlusion.

Low-cost camera array for mitigating lighting range effects

Lighting is a constant issue for computer vision applications. The range of light in a scene typically exceeds what an image can accurately represent. Typically, multiple images are taken with each image at a different exposure level. These images are then combined together in order to display more detail than a single image at a single exposure. For computer vision applications using a mutli-view approach it would require a substantial amount of bandwidth to perform these routines on each camera individually. This work looked at using multiple views in a camera array to each capture a different exposure level and then used that information to mitigate the lighting issues in a central view. A weighted averaging approach and a radiometric calibration based approach were both implemented and compared. In contrast to existing methods, this work looked at handling depth planes that were not considered to be on or near the plane at infinity. It was found that much of the detail that was lost due to lighting issues could be recovered.

Iterative Time Reversal in Dispersive and Non-Dispersive Media

In this paper we further investigate acoustic energy as a tool to enhance the recovery rate of a self healing material. Time reversal is the method used for the focusing of acoustic energy at a recovering location. Our recent tests, which have produced promising results, included applying acoustic time reversal in an iterative fashion in order to focus and amplify a stress-wave at a defect within a solid rod. Two types of rods were used for testing; (i) a solid steel rod (non-dispersive) and (ii) a brass tube lled with a fully cured two-part epoxy (dispersive). The curing of a two-part epoxy is treated as being analogous to the curing of a self healing material. We have continued to look at the eects of acoustic energy on the curing of the epoxy. It was found that the curing rate of the epoxy was accelerated with the introduction of acoustic energy.

Accelerating the self-repair rate of a polymer via acoustic energy

We consider the effects of acoustic pressure on the curing of a two-part epoxy, which can be considered analogous to the polymer healing process. An epoxy sample is loaded into a tube and monitored throughout the curing process by measuring the amplitudes of its natural frequencies in response to periodic mechanical impulses. The progress of the curing process can be quantified by tracing the natural frequencies and temperature of the epoxy-tube system. Studies described in our last report continue and work completed in this reporting period has sought and achieved repeatable test results by making slight modifications to existing procedures and protocol.

Experiments on a Twelve Mode Membrane Mirror with Boundary Located Electrostatic Actuators

This paper continues the investigation of the multiple-mode electrostatic boundary actuation technique and reports on the ongoing mathematical model development, surface metrology testing as well as the continued updating of the system materials and construction techniques. Static testing has shown that the mirror deforms in the desired convex and concave mirror pattern with the actuation of inside and outside ring of electrostatic actuation pads. Design upgrades of the mirror stage as well as the deformable mirror itself are underway and will supply the system with more stability and a higher reflective quality. An extension of our current closed loop control method and its use on our new mirror design will enable the aberration correction of more modes than our existing mirror design.