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.
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.
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.