# Application of Tooth Gear Impact-Echo System for Repeated and Rapid Data Acquisition
## 3 Tooth Gear Testing System  
### Impact Source Excitation by Tooth Gear  
In this study, tooth gears were used as impactors for the air-coupled IE test. While the gear rolls and moves, continuous contact on the surface of concrete generates an impact source. Therefore, the gear has the advantage of a moving platform where the impactor itself moves and generates sources simultaneously. Moreover, the impact position is regular, based on the pitch of the gear.  
Two different tooth gears were used to evaluate the excitability as impactors, including 11 and 30 teeth gears, respectively, as shown in Figure 2. The tooth width and thickness were 5.8 and 2.5 mm for the 11-tooth gear and 4.2 and 1.1 mm for the 30-tooth gear. Therefore, the potential contact area of the gears was 14.5 and 4.6 mm\({}^{2}\), respectively. The contact duration of the tooth gears was compared with that of a conventional steel ball impactor. The diameter of the steel ball was 12 mm. The motion of impact onto the surface of a concrete element was recorded using a high-speed camera (MEMRECAM GX-1). The videos were captured at 10,000 fps (320 \(\times\) 240 resolution). Each frame was analyzed manually, and the contact duration and velocity of each impactor was estimated. The contact durations obtained were 0.1, 32, and 16 ms for the steel ball, the 11-tooth gear, and the 30-tooth gear, respectively. It is noted that the sampling rate was limited to measure the contact duration accurately; however, the different mechanism from the impactors were clearly shown. Using Equation (3), the contact duration of the steel ball was calculated as 0.08 ms (assuming the density of the steel ball to be 7850 kg/m\({}^{3}\), and the elastic modulus to be 200 at 30 GPa for steel ball and concrete, respectively).  
As shown in Figure 3, the contact mechanism of the tooth gears is somewhat different from a hammer-type impactor. While the tooth gear continuously moves, the contact is sequentially made from one tooth to another. It is noted that the second contact was executed after waiting for the  
Figure 1: Illustration of air-coupled impact-echo (IE) test; (a) Field image of simulation, (b) synthetic signal data of simulation.  
finish of the first contact. Therefore, the contact duration was longer than the contact duration from a conventional hammer-type impactor, implying low frequency excitation. The contact duration of a tooth gear depends on the pitch of the tooth (or the number of teeth) and the moving speed of the gear. The pitch of the gear (\(p\)) is defined by the radius of the gear (\(r\)) and the numbers of teeth (\(N\)), as follows:  
\[p=\frac{2\pi r}{N} \tag{4}\]  
Figure 3: Photo of the contact mechanism: (**a**) 12 mm ball hammer, (**b**) 11-tooth gear.  
Figure 2: Tooth gears used as impactors including the 11-tooth (silver) and 30-tooth (black) gears.  
Therefore, increasing the number of teeth reduces the pitch and the contact duration. From Equation (4), the pitches of the 11- and 30-tooth gears are 22.9 and 10.1 mm. With the same moving speed, the contact duration of the 30-tooth gear is 2.3 time shorter than that of the 11-tooth gear, implying that much a higher frequency range is excitable using the 30-tooth gear. With practical assumption of rolling speed as 250 m/s, the contact duration of 11- and 30-tooth gears are 0.09 and 0.04 ms, where corresponding excitation frequency ranges are 16.9 and 36.4 kHz.