April 18
30
S
ensors
Consumers have developed greater expec-
tations with regard to the performance and
intuitiveness of touch sensor interfaces. This
is quite clearly a consequence of the experi-
ences they have had with items of portable
electronics such as smartphones and tablets.
The vast majority of people will therefore not
accept anything less effective when it comes
to other application areas, as touch control
starts to see more widespread proliferation
in a wide and varied selection of end markets.
They want to derive the same sort of seam-
less smooth, glitch-free operation that they
are used to elsewhere. Here we will look at the
implications this has now that touch technol-
ogy is starting to become popular in automo-
tive designs.
The opportunities for touch-based control
within the automotive sector are just starting
to be appreciated by manufacturers and their
tier one technology partners, providing them
with a way to offer product differentiation in
what has become an extremely competitive
market. There are many places within modern
vehicles that have been identified where this
can be applied. Among the most prominent of
these are HVAC controls, smart key entry sys-
tems (based on proximity sensing) and body
electronics functions such as power window
lifters. Of course, the decision-making process
that relates to specifying touch sensing solu-
tions for such functions is very different from
what would be applicable for portable elec-
tronics, home entertainment systems, white
goods and suchlike. There are, as we will see,
a number of important and highly distinctive
aspects that define automotive deployment,
which simply do not appear in other markets.
These need to be given adequate consider-
ation, or the touch system will fail to meet the
application performance, reliability and lon-
gevity requirements.
The automotive environment is typically
harsh and uncompromising. This is why elec-
tronic components that are used on vehicle
systems need to have attributes that address
this. When looking to utilize a touch sensor
within such a setting, there are key criteria
that need to be considered. Firstly, the touch
sensor is going to be exposed to elevated lev-
els of electromagnetic noise - with the various
electric motors present in the vehicle, as well
as the cable harnessing, the alternator coil and
a multitude of other sources all contributing.
If not properly addressed, this noise could
impair the reliable performance of the touch
system. Secondly, the nature of where these
touch sensors are going to be situated means
that they may have to contend with various
physical stresses, such as mechanical shock,
vibration, and elevated temperatures. Rug-
ged construction thus becomes mandatory.
Capacitance touch sensors are, for these rea-
sons, deemed to be the most suitable. Next, as
there can be significant variations in electrical
system level parameters from one vehicle to
another, there needs to be scope for fine tun-
ing before the car comes off the production
line. Finally, given that there is such a plethora
of different applications within the cabin and
on the car exterior which could benefit from
touch functionality, the sensor should include
various features to match specific application
needs; the ability to support different design
arrangements, while keeping the number of
components involved as low as possible, is
important. As an example, some designs may
result in the presence of an air gap between
the sensor/PCB and the protective cover. This
will normally mean that a light guide has to
be incorporated into the set up (with reper-
cussions in terms of both the bill of materi-
als cost and the associated engineering effort).
Employing technology that can alleviate
problems of this kind will prove beneficial.
Capacitance touch sensors can be based on
either of two different sensing technologies -
these are self-capacitance and mutual capac-
itance. With self-capacitance, an increase in
capacitance level is detected when the finger
of the user approaches the sensor electrode.
Though widely used, self-capacitance touch
sensors can be susceptible to parasitic capac-
Mutual capacitance touch sensors
improve vehicle user interfaces
By Tetsuya Tokunaga,
ON Semiconductor
There is huge potential for touch
control technology in the automotive
sector but also major challenges to
overcome. Before embarking on new
designs, engineers need to take these
into account. They need to specify
semiconductor technology with the
signal integrity to boost touch sensor
performance and the configurability
to optimize the system accordingly.
Figure 1. Functional block
diagram of the LC717A30UJ
