| A single contact stress sensor, sensor package, components of a sensor array and acquisition electronics have been created to improve the study of interface load distributions. The instruments may be placed at an interface to measure the interactions between contacting bodies. Example applications include cartilage, gasket, roller, tire, shoe, tactile sensing and crash-test dummy studies. The single-axis, silicon-based MEMS sensor is designed to respond only to solid contact stress normal to the contact plane. A reproducible process has been established that forms the silicon sensor chip of dimensions 2mm x 2.5mm x 50 micron thickness. Smaller and thinner devices have also been demonstrated. The wafer-scale process allows for cost-effective mass-manufacturing. The silicon die includes a diaphragm structure with embedded piezoresistors arranged in a wheatstone bridge. Deformation of the diaphragm produces a bridge output voltage proportional to applied loads. Device load ranges may be designed from 0-70 kPa to 0-7+ MPa (1000psi) by choice of diaphragm dimensions. The MEMS device is entirely encapsulated in a multi-layer flexible-film package of total thickness of 100 microns and of arbitrary length and form. Typical performance for a packaged chip with a 700 micron diameter and 15 micron thick diaphragm shows +/- 6% accuracy, 2% hysteresis, 0.03 mV/V/psi sensitivity and +/- 0.46%/°C thermal dependence with a load range of 0-3.5MPa. The devices are capable of performing indefinitely under repeated loads and can be used dynamically. All of these parameters can be selectively optimized for specific applications. Additionally, processing necessary to expand the single sensors into very large, two-dimensional arrays capable of conforming to complex curvatures has been demonstrated. Flexible and extensible wafer-scale interconnects combined with polymer packages allow for the fabrication of large sensor arrays. At present, 900-sensor arrays on 860 micron centers capable of simple curvature conformity of less than 4mm radius and complex curvature conformity of less than 12mm radius have been demonstrated. Finally, data acquisition and control circuitry have been modeled, built and tested to dynamically measure the arrays' output with a row-column multiplexing scheme. This scheme minimizes the interconnects necessary to communicate with an array and introduces less than 1% error. |
