An interesting alternative to mechanical switches is the
proximity sensor. The term "proximity" refers to the fact that
there is no contact between the medium you're trying to
detect (i.e., finger, liquid, metal, etc.) and the actual sensing
element. Most likely a plate of glass or plastic separates the
two. Although you are likely to touch the separating element,
there is no physical contact with the sensor.
Proximity sensing technology enables adaptive controls,
alleviates isolation issues, improves overall application
robustness, generates almost unlimited design flexibility and
fosters new functionalities.
Proximity Sensing Technologies
Table 1 describes some of the common sensing technologies.
Additional options include Hall effect, magnetoresistive, radar,
sonar and others.
The two technologies most used today for mechanical switch
replacement are optical and capacitive sensing, however,
as market interest indicates, capacitive is the most versatile
and flexible. This article describes some of the theory behind
capacitive detection and shows how this theory can be applied
to the human-machine environment.
||Induced electromagnetic currents
||Operates in harsh conditions
Rapid response time
Detects only movement
Difficult array setups
||Virtually all objects
||Sound wave echo
No idea of size/shape
||Reflection or absorption of light different to background
||Possibility of interference
Pb in fog/smoke/nontransparent materials
||Objects capable of absorbing or creating electric charge
||Permitivity variation to background
||Simple array construction
Detect metal and nonmetal
A capacitor is a device made up of two electrically conducting
materials (called electrodes), each at a different potential,
separated by a non-conductive material (insulator). The
physical value of a capacitor depends on the dielectric constant
of the insulator, the relative permittivity of free air, the area of
each electrode and the distance separating the electrodes.
This value corresponds to the amount of energy the capacitor
is able to hold.
Applying a voltage to one electrode that is different to that
present on the other induces an electric current through
the capacitor, which decreases as the charge builds on the
electrode. This potential difference creates an electric field
between the electrodes.
Capacitive measurement techniques
Time constants: Input a step function to an RC network where R is fixed, measure the time
the output takes to achieve a given voltage.
Phase shifts: Input a periodic signal, measure the delay, due to
the capacitance, on the output signal.
Frequency modulation: Design a circuit whose frequency
depends on the charge and discharge of a capacitor.
Amplitude modulation: The amplitude of an ac waveform
changes due to an RC network, where R is fixed.
Below are simplified schematics of how to perform these
measurements (Table 2).
In the real world, the challenge is finding the trade-offs between
sensitivity, robustness, noise immunity and cost.
Measuring RC time constants off a square wave function
is without doubt the simplest and least expensive solution.
However, the drawbacks are sensitivity, detection frequency/
speed and electromagnetic noise, since you're typically injecting
a mono-pulse step function with a given repetition rate for delay
Freescale has chosen this technique for a new family of
MPR08x proximity sensors based on our S08 microcontroller.
It provides the optimal compromise between performance
and cost-ideal for keypad, tactile screens and simple
Phase shifts have similar sensitivity issues, but tend to have
faster response times. Again, noise may be an issue. This
measurement technique can easily be integrated into an MCU
but does need some external components.
Frequency modulation is a good solution for discrete designs,
especially when using square/triangular waves. An F-to-V
converter then gives information that is easily interpreted by an
MCU. The drawback is noise.
Amplitude modulation is quite design intensive, however it gives
the best performance in terms of electromagnetic robustness,
since you can easily adapt this technique to sine-waves. The
sensitivity is similar to that of frequency modulation.
Freescale has built a portfolio of products based on a
small signal sinusoidal excitation. Due to the virtually perfect
sine-wave, the resultant electromagnetic interference spectrum
is best in class. This portfolio has been in full production for
quite a number of years, demonstrating excellent robustness
So if all we need to do is measure capacitance, where's the
problem? Since the capacitance changes with the environment,
just about anything will influence the measurement-insects or
mud, tropical climates or desert dryness, children's toys or even
a sack of potatoes. The key to resolving these issues is how
you calibrate your sensing system.
Not only can the external environment impact the measurement,
but also the design of the measuring system can play an
important part in the sensitivity and dynamic range. Unwanted
capacitance (or parasitic capacitance) can be created by the
chassis (fixings, metal housings, etc.) or by routing the electrode
path close to other signals (ribbon cables, PCB routing, etc.).
Although there may be certain applications where you want to
detect this, such as tamper proofing or security detection, this
is more of an inconvenience than a benefit for the vast majority
Two options exist to overcome disturbance issues: either
you ensure that the A/D part of the capacitor equation is
so small that the result has little or no impact or you shield
the measurement channel. We have seen previously that an
electric field is created between two points having a different
potential, therefore by creating a shield circuit with nearly the
same amplitude and phase as the electrode signal ensures that
there is little or no potential difference between the two signals,
thereby canceling out any electric field. By ensuring sufficiently
low shield impedance, the parasitic capacitors that now exist
between the shield and the chassis, GND signals, etc., can be
charged and discharged without affecting the signal amplitude.
Applying the Theory: Making Life Easier and Less Power Hungry
Optimizing the man-machine environment
When man and machines work together, there is often a
physical limit or exclusion zone that constrains the machine.
This limit is often defined as the limit of "inconvenience" for
the operator, that is to say a position that the operator would
normally have to stretch to reach. The underlying objective of
this is to ensure that under no circumstances can the machine
get too close to someone without that person making a
However, what can be considered "distance of security" in one
case, such as a robotic tool, can be interpreted by the operator
as being just a little too far to be comfortable.
Imagine the improved convenience and machine performance
if the robot was able to adapt to the operator's position. By
detecting the operator's presence at a given distance, the robot
could safely adjust its position with respect to the operator.
Safety measures can also be enhanced in applications where
user presence must be validated before operation, such as a
lawnmower. If the user slips or loses control of the lawnmower
in any way, the mower would stop operating as quickly as
possible. Another example is an industrial stamping machine
where the user must be detected at a safe distance from the
equipment prior to its activation.
The concept of protecting people can equally be applied
to protecting sensitive equipment, such as a camera. If it's
dropped, using proximity detection would enable the equipment
to detect the absence of a human presence and place itself into
a more secure state, such as retracting the lens.
Automatic door openers
One of the most common applications for presence detection is
the automatic door. Typically, as you approach a door you are
detected by an optical sensor, or your weight closes a contact
in the floor.
The electric field sensor can be integrated into the floor and
can detect the presence of a person through different
substances (wood, tile, carpet, etc.). There are no moving
parts and the sensor is impervious to rust and virtually
indestructible, making it a suitable replacement technology
for the mechanical pressure sensor. The physical nature of the
electrode ensures a well defined and limited sensing area,
unlike that of an optical solution where you need to define a
volume and sensitivity threshold.
Alternatively, proximity sensors can be embedded in the wall or
other object to be activated only by voluntary movement. This
also allows the door to be opened without any physical contact.
Optimizing access control can also lead to benefits in energy
consumption. Minimizing the time a doorway remains open
ensures the shortest possible exchange between hot or cold
outside air with the conditioned air in the building.
Occupant and presence detection
If you want to check how many people are on an aircraft, how
many seats are left in a cinema or how many beds are occupied
in a hospital ward you can either count the number of tickets
sold or the number of people present, or you can let the seat
or bed, each with proximity sensing technology, detect by itself
whether it is occupied or not.
By using multiple electrodes per seat, not only will a person be
detected, but also his/her size and position will be measured.
This is particularly useful when employed in conjunction with
automotive airbag safety systems.
Energy consumption in battery powered equipment
There is general concern about the amount of energy wasted by
electronic equipment when not in use. Displays and lights that
remain lit and equipment that continues to draw power, even
when turned off, are just a couple of examples.
Rather than setting a certain time limit before extinguishing
backlights or putting equipment in standby, why not detect
the presence of the user and adapt the energy consumption
Battery powered applications can remain in stand-by mode until
a proximity sensor detects the approach of a user's hand. The
device then automatically powers up. Then, as the hand moves
away, the interface can return to a stand-by low-current mode.
The dielectric properties of water are altered as it changes
state from gaseous to liquid to frozen. Therefore, for instance,
as water vapor between two electrodes changes to ice, the
capacitance value across those two electrodes will vary.
This phenomenon can be used to detect any ice build up in
a freezer, helping prevent the igloo effect, where ice actually
acts as an insulator. Under extreme conditions ice build up will
prevent the compressor from cooling the freezer sufficiently,
resulting in wasted energy and spoiled food.
Choosing the Right Technology
When considering which technology to use for which
application, here's a very rough guide: the RC technique is best
suited to applications expecting a "1" or "0" response. The
amplitude modulation allows the user to identify and monitor
the "fuzzy" bit between the "1" and the "0," or more accurately,
the change in state. Here is a simple table that outlines which
technology is best applied to which applications:
|Liquid level (continuous)
||Discrete presence detection
|Excellent EMI performance
|Presence up to 15cm
|Touch panels in harsh environments
Freescale has been working with electric field measurement in
harsh, security conscious environments for over 10 years, with
particular attention to occupant detection in an automotive
environment. In addition to the product portfolio, we provide
evaluation kits that allow fast and simple experimentation and
The portfolio comprises three product ranges:
- An analog ASSP providing the highest sensitivity
- An MCU-based solution with IP developed to perform
calibration, filtering and other debounce algorithms targeting
touch panel solutions
- A software package for S08 and ColdFire V1 products that
customers can integrate with their own application software
to enable simple button replacement.
The technology described above can be used to enhance
security and automated equipment awareness in the following
- Replacing the traditional mechanical dead man's switch with
proximity sensor technology
- Allowing a robotic system to detect the presence of a
human or animal to modify machine speed and movement
- Integrating access control sensing into flooring or walls
There are many other opportunities to apply electric field
proximity sensing technology, including:
- Hiding light-switches behind the plaster board
- Placing electrodes behind glass to develop interactive touchscreen
- Liquid volume and level detection
- Access control and anti-pinch functions
Datasheet : MC33941.pdf, MC34940.pdf, MPR084.pdf
Application notes : AN1985.pdf, AN3456.pdf
About the Author
Oliver Jones is a Product Marketing Specialist in the Consumer & Industrial Go-To-Market team in EMEA. Based in Toulouse, France, he's been with Motorola/Freescale for 11 years, occupying Product Engineering, Program Management and Marketing positions for Analog and Sensor Products.