# Design and Experiment of Discharge Control Methods for Three-Stage Coil Gun Experiments
## 5 Fabrication and Experiment  
Coil gun circuits were fabricated, and the coil gun's launcher velocity was experimentally measured. Figure 13 shows the experimental environment, and Figure 14 shows the discharging device of the coil gun system.  
The maximum velocity was not obtained from the two coil gun systems at the discharge position of the projectile derived from simulations, so adjustment of the discharge position was required in both systems. For the delay-type discharge control circuit, this adjustment was made using the delay time. For the photointerrupter circuit, the positions of the infrared LEDs and phototransistor sensors were adjusted to measure the launcher velocity. While adjusting the positions, we recorded the delay values and sensor positions at which the maximum velocities were obtained. For the two adjusted coil gun systems, the launcher velocities were the same within the measurement range. Table 2 shows the maximum muzzle velocity when using the two-stage coil only and when using the three-stage coil gun.  
Figure 14: Discharging circuit of the coil gun system.  
Figure 13: Experimental environment for the velocity measuring equipment.  
The differences between the simulation and experimental results of the two models were identical. The differences were attributable to the experimental models having projectiles off the center of the launch tube and the friction between the projectiles and the launch tube. Moreover, the inductance of the solenoid coil calculated in the simulations was different from that of the solenoid coil used in the experiment. Table 3 lists the velocities of the launch experiment with the two coil gun systems. Following the final adjustment, 10 launch experiments were conducted, and the errors in the operations were compared. Regarding velocity measurements, the experiment was performed with the setting in which the maximum velocity was obtained with no changes in the delay time or the position of the optical sensors.  
During measurement, more errors were observed in the delay-based discharge control circuit than in the photointerrupter-based one. Moreover, the differences in the launcher's muzzle velocity were greater when using the delay-based method than when using the sensor-based method. Errors in operability occur for reasons other than those described above, such as recoil during launch. In the delay method, when an error occurred in the second stage, it affected the acceleration in the third stage, and the overall speed of the projectile decreased. If a coil gun has more than three stages, and error due to recoil occurs because the muzzle velocity is high, the acceleration in the next stage is affected. With stacking errors in each stage, the total error in launch velocity will be significant.  
In contrast, the photointerrupter-based circuit showed little error in operability. Even in the event of vibration due to recoil when the coil gun is fired, the error has little effect on the launcher velocity because the launch tube and the fixed sensor move together. However, space is required for the sensors, and higher launcher velocity requires more space for the sensors.