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The Skagit River bridge collapse on Interstate 5 in Washington State, USA, got me thinking about using the Arducopter for large, inaccessible infrastructure inspections. I drove my family across this bridge many times so I took its unanticipated collapse harder than most who heard the news in far away places. Granted, metal fatigue or corrosion didn't directly lead to the collapse. Current data (witnesses and surveillance camera footage) point to repeated collisions between a large truck-ferried load and the overhead bridge structure as the truck passed through that section of bridge. But the bridge dates from the 1950s and didn't include redundant load-bearing paths. It's classified as a  "fracture critical" bridge. There are more "fracture critical" bridges in the USA and we don't necessarily know their structural health in detail.

The quadcopter platform appears well suited for bridge inspections from beneath or above the bridge deck. I figure it needs to stay in line of sight of an operator's RC controller, needs to provide video on-screen display so the operator sees what the drone sees, and some level of "standoff protection" to keep the quad from striking the bridge structure. The bridge is more dangerous to the quad than the quad is to the bridge. Maybe ultrasonic ranging for structure following or standoff might do the trick. The quadcopter must keep its distance from the structure while under control of the operator, though. I lean toward a "hard envelop limiter" in the control law or control mode software so the quad won't hit the inspected item no matter what the operator commands ( or a variable limiter that requires a lot of operator override to get the quad closer, that's more in keeping with my Boeing bias.)

The quad blades may require a perimeter shield round the blades to keep them from contacting bridge structure, too. Just in case a wind gust shove the "airbot" inspector too close to the structure.

Protecting the quadcopter from inadvertent structure contact is an interesting problem in itself. It seem tractable on the surface. The greater challenge is remote sensing of the structure. What kind of sensors can a quad carry near to a structure to probe its integrity? Is it limited to passive sensing (optical) or active sensing (induction from a distance, or via a probe, to measure structure response, or sampling (wow, take a flake), or ultrasound? We can learn a lot by dumping some energy into the structure and measuring its response, in addition to watching it in parts of the visual spectrum.

Remote sensing satellites solved a lot of these problems. They may offer direct of indirect advice on the sensor approach.  Satellite sensors can measure  the reflected light spectrum from soils, plants, rocks, for example. Making sense of the spectra requires "ground truth" measurements. The spectra need to be calibrate to soil or plant conditions and mineral types.

But paint can mask rust. And fatigue cracks may not show at the structure surface. If this is the case, then energy needs to be directed into the structure, from the quad, and it's effects need to be measured, by the quad. The wikipedia entry on non-destructive metal testing lists some interesting methods that could (in theory) be adapted to a quad - such as magnetic particle testing, liquid penetrate testing, and methods that require some form of physical contact during the measurement, like eddy current testing, ultrasound testing.

Physical contact testing would be a challenge in that the quad need to station keep, not hit the structure, and simultaneously extend one or more devices to make contact with the structure during the test. Is that a great controls problem or what?  And more - the quad measurement device needs energy (battery) and that's weight. The quad itself needs energy from a battery to fly for a while - 10 minutes or so. This may be the biggest challenge to quad copters that actively probe structures. The energy need to fly and actively probe the structure must be provided by batteries. It may be a difficult optimization to allow a reasonable inspection and reasonable mission time.

Passive structure observation seems like a shoe-in with a quad and one or more cameras, with respect to passively probing the structure and mission time. A quad can take close-up video and pictures of structures illuminated from beyond the quad by the operator. That can address the battery issue - if the operator can use a different device to illuminate the structure with ultraviolet or infrared or laser, and the quad can measure the reflected result, then the "active probe" is not on the quad, doesn't need to be lifted and doesn't need to be powered by the quad. 

But the kinds of light we can shine on the subject may not illuminate all possible defects we want to detect.

Overall an interesting set of engineering problems, when we think about using a quad copter to inspect structures...