Development and testing of an application specific force sensor for snowpack assessment

Abstract

Snow penetrometers create a specific application for a force sensor with specifications that are over achieved at great cost when using traditional load cells. A moderately priced, compact force sensor suitable for this application does not exist, and numerous force-sensing technologies were investigated, from spring and displacement transducer combinations, to strain gauges mounted on plastics, to pressure transducers. The high dynamic range, linearity, and electrical simplicity of low-cost piezo-resistive pressure sensors lent well to rapidly developing a working design on a small budget. Preliminary prototypes show promising results where the natural frequency requirement is 60% above the target specification at wn = 10497 rad/s +/- 92 rad/s. The sensor is nearly ideally damped with damping ratio, [zeta] = .43 +/- .04. A linear model results in acceptable hysteretic error for mid and upper range force values, but the sensor performs inaccurately below 2.8 N (greater than 20% error). While the sensor's performance isn't perfect for all criteria, snowpack simulation tests suggest that the sensor's shortcomings are acceptable for the targeted application of measuring relative snow hardness.