DA & Control

This Solid-State Recorder Rides in a Bullet

March 1, 2005 By: Bruce Kaufman Sensors

The development and proof testing of many types of medium-caliber ammunition entails measuring the round's performance while it is moving inside the gun barrel. These in-barrel events—propagation of the propellant pressure wave, acceleration, balloting, spin, and electronic functions—occur at very high speeds. (Balloting refers to off-boresight movement of a round in a gun, typically due to gas leakage around the ammunition and the elasticity of both barrel and round under the high pressures associated with firing.) RF-based telemetry techniques do not work well inside a gun's metal barrel. An onboard recorder (OBR) is the only way to collect the desired performance data.

While large OBRs have been used to obtain data on larger rounds [1], it is not a trivial matter to pack the necessary functions into a volume small enough to fit inside medium-caliber ammunition such as a 25 mm round. Controlex has accomplished this with the CM346 miniature solid-state recorder (see Figure 1).

 Figure 1. From left to right are the bare printed wiring assembly, a 25 mm host round, and the encapsulated recorder ready for placement in the round.
Figure 1. From left to right are the bare printed wiring assembly, a 25 mm host round, and the encapsulated recorder ready for placement in the round.

Other Technical Challenges

The short time that the fired round is in the barrel, accelerating from rest to supersonic speeds in a few milliseconds, creates very short transient events with durations of a few milliseconds and rise times in the microseconds. Capturing sensor waveforms with good fidelity demands a very fast sampling rate.

The rapid acceleration of the fired round imposes very high g-forces on the recorder riding in the round. These forces can reach 100,000 g for a short duration, so the recorder's mechanical design must provide considerable structural integrity.

The small space allotted to the data recorder leaves no room for a battery. The Flash memory devices often incorporated in OBRs typically do not write fast enough to meet the bandwidth necessary for capturing the fast signals generated by medium-caliber ordnance. So a nonvolatile memory device capable of writing at a minimum of 1 mHz is needed to handle the recorded data for subsquent analysis.

Design Considerations

Perhaps the most important design decision is the use of ferroelectric RAM (FRAM) to store the sampled data. FRAMs, now commercially available, use a layer of ferroelectric ceramic film with a square D-E loop as the storage element for each memory cell of the chip. Storage is accomplished by retention of polarization charge in the film, with one unique state for 1's and another for 0's. In contrast to Flash devices, which have endurances of 105 cycles and require careful management of memory use, FRAMs exhibit endurance of more than 1010 cycles, far in excess of the recorder's expected life. FRAM devices can operate at 5 mHz read and write cycles, fast enough to support 1 mHz sampling. Further, compared to Flash, FRAMs do not require erasing to a known state before writing new data. Their interface is quite like a conventional static RAM, greatly simplifying system design.

To provide power normally supplied by a battery, the recorder runs on energy stored in a capacitor to drive DC/DC converters that deliver regulated supply voltages. Charging the capacitor with a power supply external to the recorder right before a test round is fired gives it enough energy to run the recorder for the duration of the bullet's travel through the barrel. An important design requirement for the converters is that they deliver regulated output voltage even as the voltage on the capacitor falls when the charge is removed.

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