Smart Operation-Capacitive sensors

Capacitive Sensors work on the principle of electrical capacitance. This describes the interaction between two points in space, such as the electric force field between two electrodes.

The electric flux lines within this force field can be changed by the approach of a conductive object, e.g. a finger tip. Capacitive sensors detect the change in the force field and respond with a voltage change that can serve as a trigger function. The effect of the change in the force field is that sensors do not have to be directly touched, but will also work from close range.

For best performance, all capacitive sensors must be sealed with an outer shell (with no air gap). The best technique for accomplishing this is film insert molding, which has long been used for surface finishing plastic parts. In addition, the composite bond formed with the housing protects the sensitive sensor structures. As the field lines can readily penetrate non-conducting solids, the sensor effect also works in a composite bond with a 5-mm thick cover layer. Rotary and linear sliders can be created by arranging a sequence of capacitive sensors in a straight line or in curved configurations.

plastic electronic GmbH, Linz, Austria, integrates control functions in the surfaces of all kinds of device housings under the brand name Touchskin. Electrically conducting structures are printed in the form of capacitive sensors onto electrically functional plastic film. Depending on the versatility of the printing line, both the design and the position of the sensors on the carrier film can be chosen freely within wide limits. This, and the fact that the printed film can be shaped and cut in any 3-D surface, improves the design freedom for device designers and developers. Currently, thermoforming ratios of up to 200 % can be achieved without detachment of, or interruption to, the conductive layer. The film inserts are then placed into the mold and overmolded or back-molded.

Additional design potential results from the combination of decorative and functional film. For instance, the sensor film on the inside of the housing can be of a universal type, whereas the device variant can be customized on the outside via the decorative sheet. As various decorative films can be injection molded in rapid succession, customization is possible from batch one. The respective functions are programmed via controller software. This need not be mounted directly at each sensor, but rather can be positioned relatively freely on the composite part. Besides the benefit of an ergonomic and logical device operation, the technology provides cost-savings potential in that there is no need for mechanical parts such as rotary switches or pushbuttons, nor for associated assembly operations.