The next issue in producing a tap test image is position encoding. To generate an image, one needs to acquire contact time data as a function of position over the area of inspection. Although there are numerous motorized, computer-controlled scanning systems that are capable of acquiring such data, the experience of the CNDE composite group has been that the airline inspection community tends to shy away from complex automated systems due to cost, speed and operator training considerations. The composite group has therefore opted for a simple method of position encoding:  a thin plastic sheet with a printed square grid taped over the area of inspection.  To take data, each grid box was tapped once by hand. The tapping mass was an accelerometer fitted with a hemispherical tup. The output voltage signal of the accelerometer was processed in an accelerometer-computer interface circuit and the contact time data were then fed into a laptop PC for generating the tap test images. A considerable and sustained effort was made to develop versatile software for data acquisition and processing. To initiate a scan, the operator would specify the width and length of the scan area and the row and column convention on the PC and then proceed with the tapping. Figure 2 shows a manual tap test conducted on the wing of an MD-80 over an aluminum honeycomb structure known as a heater blanket. The scan image shows small disbonds in a 2-foot by 3-foot inspection area. Although the manual mode of tap test is intrinsically tedious, it still remains a valuable technique for inspecting curved, irregular parts and in hard to reach places.

Figure 2  Grid overlay used as a position encoder in manual tap test on an MD80 wing.  The resulting image shows a 2-foot by 3-foot area.

Although the manual tap test system using the grid overlay was successfully applied to on-aircraft inspections and produced useful images, the slow speed and tedious data taking clearly needed improvements. To make the tapping faster and more uniform, Dan Barnard and John Peters, two engineers in the FAA-funded project, invented an elegant “magnetic cam” that used the repulsive force between small but strong permanent magnets to throw the accelerometer toward the part surface. As shown in figure 3 and in the linked movie, (requires plug-in support) magnets embedded in the circumference of a wheel pushed a magnet in a rocker arm, and hence the accelerometer, up and down as the wheel rolls over the surface. Because the magnetic cam is based on action-at-a-distance, it is largely free from wear and alignment problems and hence superior to a mechanical tapper. The magnetic cam makes uniform, equally spaced taps along the rolling direction and only requires an overlay with a set of parallel lines to guide its indexing motion as it scans an area. The spacing between taps can be changed by having different number of magnets on the wheel. With a pitch of ¾-inch, a 6-foot² area can be mapped out in less than 10 minutes with the magnetic wheel.