Capacitive touch sensors have made homes i n scores of MP3 players and mobile phones. Of course, the mobile arena is no longer the only bastion for these devices. Today, sensor technology has literally exploded, expanding into many other product categories. However, this expansion produces a new set of design challenges.
White goods, such as electric ranges and dishwashers, is a product area that has spawned one of these new challenges: operation in a wet environment. To combat the problem, this article shows how to design water-tolerant capacitive touch sensors.
A waterproof design implies system performance that’s totally immune to the effects of water. For a water-tolerant design, water levels encountered in normal operation don’t interfere with sensor operation. Splatters and spills on the touch surface are tolerated, but total immersion is not. Water tolerance is a cost-effective solution for operation in a wet environment.
In a water-tolerant design, only the touch of a finger produces a signal large enough to register as a “touch.” However, if a boiling pot overflows on an electric stove, and the touch surface is submerged in hot liquid, the water-tolerant sensor will be challenged to operate normally. By properly configuring the sensor array, the submersion can be detected, and the system would be subsequently alerted that an abnormal event has occurred.
The safest response to such an event is to turn off the burner until the spill can be cleaned up. In contrast, a waterproof design will continue normal operation after the spill. To turn off the burner, the user of a waterproof system needs to touch the sensor through a coating of hot liquid. If the liquid is too hot to touch, the burner stays on, and the pot keeps boiling, only making the situation worse.
The water-tolerant design, on the other hand, creates a system that turns itself off with a major spill. So, when comparing the two approaches for reacting to a spill of hot liquid, the water-tolerant design is obviously the safer and smarter choice.
SURFACE WETNESS INTERACTION
Surface wetness is classified into three categories: dry, droplet, and stream (Fig. 1). When liquid is sprayed or splashed onto a dry surface, surface tension causes the liquid to bead up, forming droplets. A water-tolerant design needs to operate normally when the surface is covered with droplets. For larger amounts of liquid, the droplets merge together and form a stream if set in motion or a puddle if the surface is at a low point.
Fingers are conductive, so they interact with the electric field that’s set up around the touch sensors. Water is conductive, so it interacts with the same electric field when it lands in the active sensing area. This can lead to a report of a finger touch when water splashes onto the sensing surface, even when no finger is present.
Drops of water can produce the same signal level as a finger for a touch sensor that lacks any features for water tolerance (Fig. 2). The “raw count” shown in the figure is the unfiltered output from the sensor. The “baseline” is a continuously updated estimate of the average raw count level when a finger isn’t present. The baseline provides a reference point for determining when a finger is present on the sensing surface.
Fingers and water interact in a similar, but not identical, way with electric fields. However, enough difference exists between the two to develop techniques for discriminating between a touch and a spill.
On printed-circuit boards (PCBs) and flex circuits, a practical level of water tolerance is achievable by using a shield electrode and guard sensor. These electrodes add no material cost to the system, because they’re incorporated into the same circuit-board layout as the touch sensors (Fig. 3).
The purpose of the shield electrode is to set up an electric field pattern around the touch sensors that helps attenuate the effects of water.1 The guard sensor detects abnormally high liquid levels so the system can react appropriately.
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