Fringe Field Sensors
Figure 1: Fabric inspection system with flexible sensors and digital telemetry.
For conductive objects, capacitance sensors are used to measure both separation and surface profile. Applications of separation measurement technology include seam tracking, instrument positioning, collision avoidance, and vehicle occupant detection. As surface profilers capacitance sensors are used for such tasks as surface imaging, burr detection, precision machining, crack detection, and automated part inspection.
Non-conductive components are investigated by impedance sensors. The penetrating electric fields can be used to measure both localized and bulk dielectric properties. Applications of this technology include textile inspection, delamination/void detection, and cure-state determination.
Capacitance and impedance sensors can benefit a wide range of industrial manufacturing processes. These Sandia developed fringe-field sensors are already being used in industrial applications:
Figure 2: Sensor data showing full-width, on-loom, real-time detection of missing pick flaw.
Fringe-field sensors can be both cost-effective and rugged. Designed directly onto both standard and flexible printed wiring boards, fringe-field sensors are quickly integrated with drive and detection circuitry to provide a robust sensor system. Standard circuit board manufacturing allows sensor electrode patterning small enough to accurately inspect individual cloth fibers less than 10 mils diameter. Sensor arrays are also easily patterned onto a single board.
Both the capacitance and impedance sensors measure perturbations in spatially resolved, fringing electric fields. These fields are generated by applying a voltage across precisely patterned metal electrodes. As the sensor is brought near or into contact with the workpiece, the electric field is altered. This small perturbation is measured as a change in capacitance or impedance by the sensor. In the case of the capacitance sensor, the sensor capacitance is lowered as it approaches a grounded workpiece due to the effective shorting of some of its electric field lines. In the case of the impedance sensor, penetration of the workpiece by the sensor's electric fields allows precise determination of the object's dielectric properties. Novel measurement software can be applied to the sensor output which allows real-time determination of manufacturing parameters such as flaw location in textile production.
Last modified: August 23, 1999
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