The first generation of three-axis silicon micromachined ground reaction force sensors sized for measurements of the cockroach Blaberus discoidalis have been successfully fabricated. Each sensor consists of a rigid central plate supported at each of its four corners by a thin flexure. Each flexure is instrumented with a pair of boron-doped piezoresistors, one located along the centerline of the flexure, and the other along its edge. Flexures Comparison of the signals in these eight piezoresistors allows for the determination of normal and in-plane forces exerted by the insect when it contacts the sensor plate.

In previous measurements taken in the Full lab at Berkeley, insects would run down the length of a track into which openings were cut to expose the force sensing elements. In the interest of improving measurement efficiency (reducing the number of runs per usable dataset), microfabricated sensors are configured in linear arrays which may be positioned across the width of the running track, thereby increasing the likelyhood that the insect will contact a sensor. As the device wafer photo shows, interconnect wires for power and signal (white regions) are routed to the ends of the arrays where they may be wire-bonded to off-chip signal conditioning and data acquisition. This configuration keeps wire-bonds out of the running path of the insects. Single test sensors used for characterization and calibration are located in the periphery of the device wafer.

The micrograph of the single square sensor element shows the relative sizes of the sensor plate and the flexures which support it. Each plate is 5mm on a side, while each of the four flexures measure 450um long, 70um wide, and between 50um and 100um thick. Both plate and flexures are defined by DRIE plasma etching from both the front and back of the wafer. Additional areas of silicon are etched away on the backside of the sensor plate to reduce its mass and thereby increase the mechanical bandwidth of the sensor. The top surface of the plate is covered with an array of small posts roughly 15um square and 1-3um tall. These traction structures provide surface roughness to help prevent slipping when an insect steps onto the sensor.

Sensor characterization and testing with the Blaberus cockroach is ongoing, and future work will explore the viability of such sensors for measuring forces produced by even smaller insects.