1. 2008-01-0267
Capacitive sensing in an
automotive environment
Erin Kirby
Nartron Corporation
ABSTRACT second component is a sensor strip positioned onto an
appropriate surface relative to a moving panel. A typical
Capacitance sensing in motor vehicles allows for location would be parallel to the weather seal. A weather
protection in power windows, sunroofs, liftgates and seal could also be manufactured with the sensor
sliding doors. It also provides greater design freedom of embedded within. The system works by measuring the
operator interfaces that include switch entry and change in capacitance of the sensor. When an
fingerprint sensing. Fingerprint sensing adds new levels undesirable change is sensed, the moving panel is
of security for the access and starting of a vehicle thus halted, reversed, or not allowed to move.
allowing for the elimination of keys.
Capacitive sensing can also provide unique
This paper describes the design and operation of various opportunities for control and switch placement both in
systems that could employ capacitive sensing function and layout. This gives greater flexibility to the
technology including keyless entry, operator controls, interior designer regarding surface finishes and general
and safety related sensing. Also discussed are vehicle form.
installation, safety enhancement, and ergonomic
benefits. For example, instead of conventional mechanical
switches including membrane contact switch similar to
DESIGN the ones used on the GM graphic displays of the mid
1980s, touch switches, using capacitive sensing, can be
The design goals of capacitive sensing are: employed. The touch surface can use various textures
and ridges to guide the user to different locations on a
1. Produce automotive sensor systems that will prevent panel. The panel can likewise have different form
trapping, injury, or death due to moving panels such characteristics, such as flat, simple curved, or compound
as windows, sunroofs, and power lift gates curved surfaces. This provides the designer with many
visual and ergonomic possibilities and advantages.
2. Provide unique and ergonomically beneficial
operator interface solid state controls and surfaces Solid state capacitive sensed switching, AC and DC
technology, is described in US Patents 4,731,5482 and
3. Provide for fingerprint recognition of designated 4,758,735.3
individuals for vehicle access and operation
Apple’s iPod and iPhone use capacitive sensing which is
DESCRIPTION described as an “elegant solution” in the article “How
Apple has transformed automotive electronics” from the
With a capacitive sense system in a moving panel August 2007 issue of Automotive Design and
application, a space-charge field is generated in the Production4 magazine.
moving panel opening. This field is used to detect a
person’s presence and provides the information required Biometric sensing is becoming prevalent in many facets
to safely control the panel.1 of society; automotive applications are no exception. It is
now common for computers to have fingerprint
The system is composed of two major components, the scanners, as do some cell phones, PDAs, and flash
first being a module that integrates the motor, gearing, drives. Some banks are using biometrics for account
and electronic control into a single package. This module access control, and portable fingerprint scanners are in
controls and drives the moving panel mechanism. The use by some police departments. While there are
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2. different techniques used for sensing different biometric Occupancy detection could be implemented by
characteristics, this paper will focus on capacitive embedding sensors in a typical automotive seat and seat
sensing. back. The sensors can be used to determine absence or
presence of an occupant as well as determining the
CAPACITIVE SENSING FOR SAFETY approximate location and position of an occupant. The
information provided could enable and control air bag
Capacitive sensing in an automotive environment has deployment based on occupant presence and position
always been a challenge. Wide extremes in temperature detection.
and humidity present formidable obstacles. However, in
recent years techniques have evolved to allow for
reliable detection of small changes in capacitance. This CAPACITIVE SENSING FOR DESIGN FREEDOM
allows capacitive sensing to be used for entrapment
detection in windows and sunroofs as well as for Use of capacitive sensing allows design freedoms that
motorized doors. A capacitive sensor strip embedded in are not possible with conventional through-the-dash
a door window weather seal is shown in Figure 1 and an switches. Designers can create flat or curved surfaces
example of the strip’s construction is shown in Figure 2. with legends imprinted on them defining desired
functions. No thru-hole features are required for switch
and knob shafts. An example of such a design is that of
Figure 3 that shows two touch switches on a control for a
generator that is located on a fire truck. All circuitry
including the switches are located behind a solid plastic
housing. This greatly enhances the reliability of the
switches, and circuitry in general, because multiple leak
paths are eliminated.
Sensor
Module
Figure 1 – Strip sensor embedded in weather seal Figure 3 – Control using touch switches
Touch surfaces can enable the designer to implement
intelligent touch controls that respond to touch, tap, and
slide commands similar to computer touchpads or
touchscreens.
The ability to recognize touch presence, location, and
motion enable designer’s to implement more
sophisticated functionality in automotive controls. The
added flexibility of the touch switches allows for a few
switches to have multiple functions thus reducing the
total switch count. A block diagram of a typical method
for determining capacitance on a sensor is shown in
figure 4.
Figure 2 – Strip construction
Further uses are now being contemplated for in-vehicle
sensing. These include seat occupancy detection, child
safety seat detection and identification, and occupant
location detection.
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3. many unique biometrics that could be used, a simple
capacitive touch sensor provides the security needed
with less complexity and cost than other types of
systems.
A fingerprint sensor can enable keyless entry designs.
This would make it such that only authorized persons
could unlock and obtain access to a vehicle or perform
various functions. The system could unlatch a door or
even open it automatically. Driver and passenger
environment preferences, such as seat position,
temperature settings, radio stations, etc., could be
Figure 4 – Block diagram of capacitive sense circuitry changed based on recognition of a particular person.
A detailed description of capacitive sensing design Based on the fingerprint authorization, the vehicle could
aspects can be found in US Patent 7,162,9285. also be started by a simple touch. Likewise, lesser
permissions could be given that provide particular
Touch surfaces can also recognize multi-finger touch capabilities such as for entertainment and window
entry similar to the iPhone and character recognition like control.
some PDA (Personal Digital Assistant) devices. For
example, a touch screen could recognize a finger Fingerprint sensing offers new levels of vehicle access
drawing the letter ‘M’ on it and present a map for control but also opens the possibility of harm to the
navigation. The map could then be touched to indicate occupant to gain vehicle access and control.6
“from” and “to” locations so that detailed directions could
be shown. The touchscreen could recognize the To help ensure that the fingerprint is from an attached
pinching or opening of two fingers to zoom out or in on member on a living person, live detection could be used.
the map. The ability to touch provides intuitive navigation The sensor would not only detect fingerprint patterns but
control. would detect that the subject is alive possibly through
hemoglobin/oxygen sensing or other sensing means.
The ability to have slide control opens up many This will reduce the possibility of harm coming to an
possibilities of variable control covering environment, occupant to gain access to the vehicle.
entertainment, and electromechanical items.
Electromechanical functions include window and sunroof RECOMMENDATIONS FOR FURTHER WORK
control as well as mirror, pedal, and seat position.
Environmental functions include cabin / trim lighting Capacitive sensing technology is well established in the
brightness and color, temperature for the cabin and field and can be implemented into the automotive
heated seats, as well as air direction (i.e. floor, defrost, environment now. It is simply up to the various styling or
vent). And entertainment functions include volume, technical teams in the automotive sector to implement its
location / position in a video or audio presentation, use in the myriad applications available including those
brightness / contrast of displays. outlined in this paper.
A touch surface not only can provide touch / no-touch
CONCLUSIONS
information as described above but can also provide
biometric information that can be used to identify an
1. Capacitive sensing provides means to protect
individual.
occupants from danger associated with power
window and door closures.
CAPACITIVE SENSING FOR SECURITY
2. Design freedoms are enhanced by the use of
Capacitive sensing provides enhanced security by capacitive sensing.
linking authorization to a unique biometric. A system that
senses a particular biometric, such as an iris or retina 3. Security and control is improved by the
pattern, or a fingerprint, can be used as authorization to implementation of biometric sensing.
access and use a vehicle. The biometric used should, of
course, be unique so that an extreme level of confidence
can be established that the person requesting an action
is indeed authorized to perform the task. While there are
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4. REFERENCES
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5. 1
Kirby, Erin, Newman, Todd. “Child Safety via anti-trap proximity technology,” 2006-01-1484, SAE
International, Warrendale, Pennsylvania, 2006.
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Ingraham, Ronald D., inventor; Nartron Corporation, assignee. Touch Control Switch Circuit. United
States patent US 4731548. 1988 Mar 15.
3
Ingraham, Ronald D., inventor; Nartron Corporation, assignee. DC Touch Control Switch Circuit.
United States patent US 4758735. 1988 Jul 1.
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Kelly, Kevin. “How Apple Has Transformed Automotive Electronics.” Automotive Design and
Production Aug 2007. Gardner Publications, Inc. 22 May 2007.
<http://www.autofieldguide.com/articles/080701.html >.
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Shank, David W., inventor; Nartron Corporation, assignee. Anti-entrapment System. United States
patent US 7162928. 2007 Jan 16.
6
Kent, Jonathan. "Malaysia car thieves steal finger." BBC News 31 Mar. 2005. 22 May 2007
<http://news.bbc.co.uk/2/hi/asia-pacific/4396831.stm>