An improved robotic water-jet system for stripping paint from a ship or other large metallic structure is undergoing development. In addition to utilizing a high-pressure water jet to remove paint and a robotic crawler to scan the jet along the painted structure, the system utilizes high-intensity ultrasound to loosen the paint just ahead of the water jet in order to ensure more nearly complete removal. The improved system also includes a quantitative gauging subsystem that measures the thickness of the paint and a qualitative gauging subsystem that generates an approximate map of paint residues; these subsystems provide real-time feedback for control of the crawler, water-jet, and ultrasonic subsystems.

Figure 1. Concentrated Ultrasound blisters and otherwise loosens paint, facilitating the removal of the paint by a high-pressure water jet.
The ultrasonic subsystem exploits a combination of heating and mechanical stresses to loosen paint. In the focal zone, the intense ultrasound can raise the temperature several hundred degrees, causing the paint to blister. In the presence of the mismatch of acoustic impedances between the paint and the metallic substrate, the ultrasound gives rise to tensile and shear stresses that contribute to blistering. The paint is further damaged if ultrasonic cavitation is present.

Figure 2. A Comb of Springy Contact Wires is scanned along the workpiece to test for removal of paint, as indicated by electrical continuity between the wires and the metal substrate.
The ultrasonic paint-loosening subsystem includes a piezoelectric transducer that generates focused ultrasonic waves; the transducer is mounted on the crawler and positioned to concentrate the ultrasound into the surface layer of water on the workpiece near the advancing water jet (see Figure 1). The transducer is excited with a combination of two ultrasonic signals — one at a frequency of several hundred kilohertz (chosen for its shorter wavelength and thus greater amenability to focusing) and one at a frequency of tens of kilohertz (chosen because it is more effective in producing cavitation in water). The more highly focused higher-frequency ultrasound propagates into the lower-frequency ultrasonic field, raising the intensity of the total ultrasonic field in the focal region above the threshold for cavitation (U.S. Pat. No. 5,827,204).

Two candidate transducer concepts for the quantitative thickness-gauging subsystem have been identified. The first concept is that of an eddy-current thickness gauge: one would place a small electromagnet coil in contact with the paint, excite the coil with alternating current at a suitable frequency, measure the impedance of the coil, and deduce the thickness of paint from the known variation of impedance of the coil with distance from the metal substrate.

The second transducer concept is that of an ultrasonic thickness gauge that would give a direct reading of the thickness of the paint: This gauge would include ultrasonic transducers operating in the frequency range of 1 to 10 MHz. The high-pressure water jet would be used as the coupling medium. It would be necessary to compensate the gauge reading for the effects of stripped paint and bubbles. Rapid spectral analysis could be used to reduce the effects of noise and interference.

The qualitative thickness-gauging subsystem would include a comb array of springy wire electrodes that would be scanned along the workpiece behind the water jet. The number of wire electrodes would be chosen to obtain the desired resolution. By simple electrical contact (or lack thereof) with the metal substrate, the electrodes would give indications of the removal or nonremoval of paint from their respective locations. In real time, contact/noncontact signals from the wires could be multiplexed and sent as feedback to a control subsystem. For non-real-time inspection, contact/noncontact signal data acquired by scanning along the workpiece could be used to generate a map of paint residues.

This work was done by Yoseph Bar-Cohen, Xiaoqi Bao, and Neville Marzwell of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Machinery & Automation category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Technology Reporting Office
JPL
Mail Stop 249-103
4800 Oak Grove Drive
Pasadena, CA 91109
(818) 354-2240

Refer to NPO-21063, volume and number of this NASA Tech Briefs issue, and the page number.

Topics:
Acoustics Automation Coatings, colorants and finishes Coatings, colorants, and finishes Marine vehicles and equipment Materials properties Mechanical Components Mechanics Metals Noise Robotics Tensile strength Vehicles Water
This Brief includes a Technical Support Package (TSP).
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Water-Jet/Ultrasonic Removal and Real-Time Gauging of Paint

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NASA Tech Briefs Magazine

This article first appeared in the May, 2001 issue of NASA Tech Briefs Magazine (Vol. 25 No. 5).

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Overview

The document presents a novel technology developed at NASA's Jet Propulsion Laboratory (JPL) aimed at improving the efficiency of robotic paint removal from large metallic structures, such as ships. This innovative system integrates high-pressure water jets with high-intensity ultrasonic waves to enhance the paint stripping process.

The core of the technology lies in its two main components: a paint loosening mechanism and a thickness gauging system. The paint loosener utilizes focused ultrasonic waves to generate cavitation and induce high temperatures at the focal point, which effectively loosens and fractures the paint. This process creates blisters in the paint layer, making it easier for the water jet to remove the loosened material. The ultrasonic waves can be finely tuned to optimize their effects, allowing for targeted energy concentration at the desired location.

The thickness gauging system provides real-time feedback to the robotic crawler, which is equipped with magnetic wheels for adherence to vertical surfaces. This feedback allows for the optimization of the crawler's travel speed and ensures that the paint removal process is efficient and thorough. The integration of these two elements enables the system to adapt to varying conditions and paint types, enhancing its versatility and effectiveness.

The document also describes an experimental demonstration where a focused ultrasonic transducer was used to treat a coated steel plate. The results showed significant damage to the paint layer, indicating the potential of the ultrasonic technology in practical applications. The experiment highlighted the ability of the system to generate sufficient energy to cause visible damage to the paint, showcasing its effectiveness in real-world scenarios.

Overall, this technology represents a significant advancement in robotic maintenance solutions, combining the power of ultrasonic technology with traditional waterjet systems. It addresses the challenges associated with paint removal, particularly in hard-to-reach areas, and offers a more efficient and environmentally friendly alternative to conventional methods. The work is part of a broader initiative supported by the National Robotic Engineering Consortium (NREC) and demonstrates JPL's commitment to innovation in robotic engineering and maintenance technologies.