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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