Tech Briefs

Technological improvements make pulsed-power experiments with gunpowder- or air-driven guns safer.

After extensive review from the US Army Research Laboratory (ARL) Electrical Safety Office, it was determined that the existing firing system in Experimental Facility 167 (EF 167) was not adequate to safely perform pulsed-power experiments with gunpowder- or air-driven guns. This firing line used solid copper wire, which provided a continuous electrical conduction path between the high-voltage capacitor in the test chamber and the firing/control room, where personnel are stationed when experiments are performed (Figure 1). This poses a safety risk since high voltage can travel from the test chamber and potentially result in personnel injuries and damaged equipment.

To mitigate this electrical risk hazard, the firing system was refurbished to operate using a fiber-optic line, which would electrically isolate the firing room from the test chamber. Doing so required a cost-efficient solution to update the hardware involved in the firing process.

Figure 1. Schematic of the EF 167 layout. The copper firing line - delineated in red - provided the offending current path in the old setup.

As a means of mitigating cost, much of the old equipment was repurposed for the safer firing module. Changes made to the housing and wiring to simplify the interface of the system are shown in Figure 2. Since the walls between the control, assembly, and experimental chamber rooms are made of steel, electrically insulating polyvinyl chloride (PVC) through pipes as well as nonconducting cable trays to the firing solenoid were installed and used to thread the fiber-optic lines between the rooms. New equipment was also added to the system including the fiber-optic line with associated converters and an arbitrary waveform generator.

Figure 2. Rear-face view, old (left) and new (right) fire switch and relay box. Note the reduced wiring on the new fire switch and relay box, elimination of finger protection, and elimination of the electric bus bar.

The new fiber-optic firing system has a similar configuration to the old copper wire system. A single switch initiates the process, which fires the gun. The firing box contains 2 relays with various inputs and outputs. An interlock system ties a switch from the door between the assembly room and the experimental chamber to the first relay in the firing box. Unless the door to the experimental chamber is closed, the relay in the firing box will remain open to prevent accidental firing. If the interlock switch on the door is engaged, triggering the fire switch closes the second relay in the firing box. When this relay closes, it completes the circuit running from the firing box to the make-screen box. The make-screen box recognizes the completed circuit as a “made” connection and outputs a 5V transistor-transistor logic (TTL) pulse to the external trigger on the arbitrary waveform generator. Receiving this pulse triggers the waveform generator to output a 5V pulse for a duration of 1.5 s.

The inclusion of the waveform generator is necessary because the output from the make-screen box is of insufficient duration to properly fire the solenoid. The pulse from the waveform generator is fed into a BNC-to-fiber converter and is transmitted via fiber to the experimental chamber room. In the experimental chamber, the fiber is connected to another converter located in a lockout box. During preparation and experiment, the single key to open the otherwise closed lockout box is only retained by the approved gunner.

This work was done by Robert Borys Jr. and Colby Adams for the Army Research Laboratory. ARL-0212