Based on the above described single-spring model, several
commercial instrument systems have appeared. The
most popular, at least in the aircraft inspection application
that we know best, are the Mitsui “Woodpecker” [1]
(advocated by Airbus for nondestructive testing of composite
laminated aircraft skin components) and the
WichiTech “RD3” instrumented hammer [2] (a commercial
version of apparatus developed by Georgeson et al at
Boeing [3]). The Mitsui product uses a solenoid-driven
hammer and the WichiTech product uses a hand-wielded
hammer; however both instruments measure essentially
the output of an accelerometer embedded in the hammer
head.