A Simple Transverse Motion Detector for Railguns

This paper addresses the accurate measurement of the transverse armature-sabot motion during a railgun shot. One way to make such measurements is to use three orthogonally arranged piezorestive accelerometers made by ENDEVCO. Model 7270A-200k is a tiny unit rated for measurements up to 200 kGee. Each unit costs about $ 1,500 and has a specified 4-kGee uncertainty due to "non-linearity and hysteresis" and a 5 percent "transverse sensitivity." There are "Zero shifts" - an output voltage corresponding to 0 acceleration, and the unit can be highly susceptible to electromagnetic interference (EMI). Assuming a railgun-launched armature experiences a 10 percent transverse/longitudinal acceleration ratio, a 100-kGee axial launch will result in a transverse measurement uncertainty on the same order of magnitude as the measurement itself. Expense and measurement uncertainty are the principal liabilities of the piezorestive-accelerometer method for measuring transverse acceleration. The new method, below, addresses these liabilities.