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Robust Sensing of Human Proximity for Safety
Applications
Markus Neumayer, Boby George, Thomas Bretterklieber, Hubert Zangl, and Georg BrasseurInstitute of Electrical Measurement and Measurement Signal Processing
Graz University of Technology, Kopernikusgasse 24/IV, A-8010 Graz, Austria
Email: [email protected]
AbstractThis paper presents a robust capacitive proximitysensor that facilitates the safety features of power tools. Powertools (e.g. pneumatic metal forming machines) enable easierand faster work but they can also cause severe injuries, ifnot operated carefully. However, accidents and related injuriescan be prevented to a large extent by providing an automaticshut-off functionality. In order to realize such a functionality,a sensing mechanism which detects the presence of a humanin the dangerous working areas of the power tool is necessary.A capacitive sensor which is suitable for this application has
been developed. In most of the cases, the tools are made ofmetallic parts and have good electrical conductivity. Typically,the tools have a fixed part and a movable part. In the proposedscheme, the proximity sensor is attached to the movable part ofthe tool. Depending on the position of the movable part, changesof the electric field distribution across the electrodes will occur.Hence, comparatively large changes of the sensor capacitance areintroduced compared to a capacitance change for the presence ofa human hand. The current position of the movable part and theprior knowledge of the capacitance values for a vacant conditionare used to solve this issue. The practicality of the proposedsensor system is demonstrated with detailed simulation studiesand results from the measurements conducted with a prototypesensor installed on a typical metal forming machine.
I. INTRODUCTIONPower tools of various kinds are extensively in use in the
industry and workshops to support the work and to increase
the efficiency of production. Examples of such power tools are
metal and plastic forming machines, portable and fixed power
saws, power nailers, power drills and wood chippers. Even
though the power tools are very useful for easier and faster
work, they can cause severe injuries to the operator, unless
used carefully. Large number of serious power tool injuries
are reported to occur annually in each country. The number of
such accidents can be substantially decreased by employing
suitable techniques to turn-off (or to trigger a power brake)
the device when the presence of a human (e.g. a hand or a
human finger) in the dangerous area of the tool is detected.In state of the art applications, the detection is done e.g.
by light barrier sensors, which are attached at appropriate
positions of the tool. However, such systems may fail in
dusty and polluted environments. Also, they typically have a
limited sensing coverage volume, in which trapping of humans
or objects may occur. Compared to state of the art sensor
principles for such protection application, systems based on
capacitive sensing techniques offer less expensive and reliable
operation [1], [2], [3]. Capacitive sensors that improve the
personal safety features of a chain saw have been reported
[2], [4]. A seat occupancy sensor based on a capacitive
sensing principle has been developed [3]. A proximity sensor
using microelectromechanical systems technology has been
proposed [5]. A capacitive sensor has been constructed for
tamper resistant enclosures to prevent unauthorized intrusion
[6]. A capacitive proximity sensor using two measuring elec-
trodes with a middle grounded electrode has been reported for
safety applications [7]. Reliable measurement techniques forcapacitive sensors, suitable even for harsh environments have
been reported in [8], [9].
In this paper, we propose a capacitive sensor suitable for
human proximity sensing, for power tool applications. We
have chosen a metal forming machine as one important device
that needs attention. We have developed a capacitive proximity
sensor to sense the presence of a human in the vicinity of the
crucial working areas of the tool. The output from the sensor
is given to the control unit of the tool. Once the presence
of a human is detected at a crucial position, the control
unit stops the movable part from moving forward and hence
to prevent an accident. Even though the developed scheme
is specific for metal forming devices, the method can beadapted for similar applications. The feasibility of the method
is demonstrated and discussed in the following sections of the
paper by measurements on a developed prototype system and
detailed simulation studies.
I I . THE CAPACITIVE PROXIMITY SENSOR
A. Working Principle of Capacitive Sensing
In the following, the basic sensing effects, which can be
exploited for capacitive sensing, are discussed.
Figure 1 sketches the principle of operation of a capacitive sen-
sor suitable for proximity sensing applications. In Figure 1(a),
a simplified equivalent circuit is shown. Two electrodes, say
T and R, are mounted in such a way, that the capacitancebetween them is mainly influenced by the objects within the
region of interest. The electrode T is connected to a sinu-
soidal voltage source, the receiver electrode R is connected
to a current to voltage converter. The coupling capacitance
between the electrodes is changed due to the presence of an
object O and is determined by measuring the displacement
current i. According to Figure 1, the coupling capacitance is
mutually influenced by capacitances CTO, CRO and COG,
which depend on several parameters like the position of the
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T Ri
Object O
~
COGCOG
CTO CRO
+-
VT
RF
VR
D
(a) Equivalent Circuit.
D
vR,s
D0
0
vR,0
vR
(b) Typical Measurement Sig-
nal.
Fig. 1. Sketch of the principle of a capacitive sensor used for proximitysensing. (a) Equivalent circuit of the sensor arrangement. Several couplingcapacitances are encountered. Depending on ratios of these capacitances, dif-ferent coupling effects can be observed. (b) Typical trend of the measurementsignal for an approaching object. vR,0 denotes the signal level in the absenceof an object, vR,s denotes a signal level in the presence of an object.
electrodes, the geometry, the material values, etc.
Figure 1(b) depicts a typical capacitance trace for an approach-
ing object. As long as the capacitance COG between the object
to ground remains high with respect to capacitances CTO and
CRO (this is valid for moderate distances between the object
and the sensor plane), the sensor operates in shielding mode.
The effective capacitance decreases with decreasing distance
between sensor and object. When the object is sufficiently
near to the sensing area, the capacitances CTO and CRObecome large compared to the capacitance COG. A coupling
path is formed and a significant part of the dielectric current
enters electrode R, thus increasing the received signal. In this
configuration, the sensor operates in coupling mode.
A general problem of this sensing principle is ambiguity, i.e.
a distant object with a high permittivity may cause the same
measuring signal as a near object with a low permittivity.
As one can see by the trend depicted in Figure 1(b), also
the signal corresponding to the distance D of object O isambiguous. This problem can be shirked by the use of several
receiver electrodes. As CTO, CRO and COG are different for
all receivers, different measurement effects can be observed.
By using this information it is possible to distinguish between
several objets.
B. Capacitive Sensing for Safety Applications and Description
of the Sensor Setup
Figure 2 depicts a general sketch for the application of a
capacitive proximity sensor for safety applications, as it can be
applied to various working machines where people can get in-
jured, accidently. Figure 3 depicts a photography of the work-
ing machine (a press), which was used for the investigations.The electrodes of the proposed capacitive sensor are mounted
near or on the moving part of the arrangement as shown in
Figure 2. The sensor has a common transmitter electrode T
and two receiver electrodes R1 and R2 as illustrated in the
exploded view of the sensor part in Figure 2. The capacitance
between T and R1 is indicated by C1 while that between T
and R2 is named as C2. There is also a capacitance between
each electrode and ground as indicated in the equivalent circuit
in Figure 2. By the use of two receivers it is possible to solve
ambiguities. An object (a human) within the sensing volume
shields part of the electric field lines when it is not in the close
proximity (but in the vicinity) of the sensor [3]. Also, when
the human is very close to the sensor the coupling capacitance
will get altered [3]. The effect of shielding and coupling due
to an object will be different for C1 and C2, as the receiver
electrodes are kept at different distances from the transmitter
electrode. The fact that working machines in general and our
arrangement in particular are made of metallic parts, causes
large parasitic capacitances to ground as these machines have
large surface areas compared to a human body, which needs
to be detected. As the frame is fixed, the change of the
electric field pattern and the resulting capacitance variation
as a function of the position of the movable part can be
measured in a vacant condition and stored. Deviation of the
measured capacitances from the stored value helps to indicate
the presence of an object in the vicinity of the power tool.
The overall system has been simulated using a finite element
solver environment before developing a prototype. The details
are given in the following sections.
R2
T R1
R2
T R1
T R1
T R1 R2
C2
C1
CGT CG1 CG2
T R1 R2
C2
C1
CGT CG1 CG2
Movingpart
Tool (fixed part)
HandObject
Sensor
Fig. 2. Sketch illustrating the application of the capacitive proximity sensorfor a metal forming machine. The electrodes are mounted on the moving partof the machine. The resulting sensor capacitances and an electrical equivalentcircuit are also illustrated.
Fig. 3. Photography of the machine under investigation.
III. DETAILS OF THE SIMULATION MODEL
Due to the large dimensions of the arrangement and the
mounting of the electrodes, which are fully exposed to the
environment, electromagnetic radiation effects are of concern
if the measurement frequency is high. To provide an estimate
of the occurrence of wave propagation effects, the wave length
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, velocity of light c and measurement excitation frequency f
can be related as given below.
=c
f(1)
For a given measurement frequency of f = 250kHz, thisresults in a wave length of about 1200 m. Thus, Faradays lawof induction
E= B
t (2)
can be set to zero and a static formulation of the Maxwells
equations given by
() = 0 (3)
can be used, where denotes the permittivity and is theelectric scalar potential. To complete this Laplace type par-
tial differential equation, we applied Dirichlet type boundary
conditions given by
R = 0 (4)
T = VTR (5)
FB = 0 (6)where R and T denote the surface of the receiver and thetransmitter electrodes and FB denotes a far boundary whichterminates the problem domain. To compute the capacitance
between two electrodes, Gauss law is used, i.e.
C= 1
VTR
R
nd, (7)
where R denotes the surface of the receiver electrode. VTRis the voltage which is applied between the electrodes.
Figure 4 depicts the model used for the simulation studies.
It consists of the moving part carrying the electrodes and
the fixed part. The far boundary, which is used to terminate
the problem domain is not plotted for clarity. The modelprovides a plane of symmetry. In general, symmetry should
be incorporated into the model during the preprocessing phase
as its consideration decreases the number of finite elements.
However, for simulations where also objects are included into
the model, the symmetry of the sensor geometry can not be
exploited to reduce the model size. Consequently, we applied
the full 3D model depicted in Figure 4 for the simulation based
investigations.
A. Prototype Sensor and Measurement Setup
A prototype sensor has been developed and mounted on
the moving part of our arrangement. The electrodes are made
out of thin copper plates. They are firmly fixed to a dielectricmaterial with a relative permittivity of about 2. The dielectric
material has a thickness of 2 mm. A thin copper sheet is placed
below the dielectric material and is connected to measurement
circuit ground. Accurate measurements of the capacitances
C1 and C2 with high resolution are crucial for the sensor
system. For the experiments, we used a measurement system
developed using a sigma-delta capacitance-to-digital converter
IC AD7143 [9]. The measurement and simulation results
obtained are given below.
Fig. 4. Overall view of the 3D FEM model for simulation studies.
IV. RESULTS
This section provides the results of the simulation based and
experimental investigations. We also demonstrate the usability
of the simulation model and evaluate the sensing region of the
sensor by providing a sensitivity map.
A. Capacitance as a Function of Position
In our first investigations we evaluated and recorded the
value of the capacitances when the moving part moves from
its open position to the closed position. Figure 5 depicts the
results. In Figure 5, the trends of the two capacitances obtained
by the simulation model are compared with the measured
trends of the capacitances.
Comparing the trends of the simulated data (dashed line) and
the measured capacitances (marked by circles), it can be seen
that the deviation between them increases for an increasing
distance between the moving part and the fixed part. The
accurate computation of capacitance values for the given typeof model is a general problem. Especially the fact, that the
ratio of the extension of the problem domain compared to the
size of the smallest model structures (i.e. the thickness of the
electrodes) is large causes problems for accurate computations.
Thus, in general only changes of the capacitance can be
computed with high accuracy. Therefore, numerical methods
are more appropriate to compute general trends but not to
compute absolute values. For the given problem we found that
the difference between simulation and measurement is given
by a single gain error. Multiplying the simulated trend with
this gain leads to the bold drawn trends in Figure 5, which
provide good accordance with the measured values. Thus the
model can be used for simulation based investigations of thesensor.
B. Sensitivity Map
Sensitivity maps are useful illustrations, which depict the
spatial dependent behavior of a system. This map shows the
change or the sensitivity of a measured quantity for a certain
inclusion, which is placed in the sensing region. As depicted
in Figure 6, in the simulation, we used a rod as inclusion in the
3D model. In the quasi-static case, the complex conductivity is
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0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.164
3
2
1
0x 10
13 Receiver 1 (Near electrode)
Distance (m)
C1
(C)
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.1615
10
5
0
5x 10
14 Receiver 2 (Far electrode)
Distance (m)
C2
(C)
Measured data
Simulated trendSimulated trend with gain
Measured dataSimulated trend
Simulated trend with gain
Fig. 5. Simulated and measured values of the capacitances for variouspositions of the movable part with respect to the fixed part.
given by (+j). Due to the low frequency, the conductivity() is dominant for the observed electric effects. As weused static simulations, the permittivity of the rod was set
to r = 1000 to model the (partly conductive) behavior ofhuman tissue. Furthermore the distant end of the rod acts
as electrode, which was set to ground potential. Thus, the
same behavior as caused by the capacitance COG (compare
Figure 1) is modeled. To obtain the sensitivity map, the rod
was moved such that the tip of the rod reaches most points of
the sensing volume.
Fig. 6. Overall view of the 3D FEM model for the determination of thesensitivity maps. To obtain the sensitivity map, the rod was moved in forwarddirection and in up/down direction.
Similar to the simulations, measurements have been per-
formed to obtain the sensitivity map. Thus, a test object has
been moved within the sensing volume in the same manner,
as explained for the simulations and the capacitance readouts
were recorded. The test object was a copper pipe of 2 cm in
diameter and 8 cm in length. This copper pipe was connected
to the circuit ground of the measurement system. We marked
the test space for each 1 cm. The test space has 10 cm length
in front direction and a width of 11 cm to each side. Thus, a
10 by 10 matrix map is obtained leading to 100 measurement
positions. At each position of the test object, 50 independent
readings were recorded and the average of these 50 values
was used at the corresponding position to obtain the sensitivity
graph. Figure 7 depicts the computed and measured sensitivity
maps for the near and the far electrode of the sensor. For
the illustration, the investigated changes of the capacitances
were plotted in a 3D plot at the position of the tip of the test
object (the near end of the pipe/rod). For the experiment and
the simulation the moving part of the system was moved to
the fully opened position. The contour of the moving part
is plotted in the sensitivity maps for illustration. For the
measured data the readings are normalized with respect to
the maximum capacitance change. As shown in Figure 7,the
sensing volume of the far electrode reaches up to 8 cm inmoving direction of the mobile part of the arrangement and
up to 6 cm in the orthogonal direction.One can see good accordance between the simulated and themeasured sensitivity maps. In the simulated sensitivity map for
the far electrode (Figure 7(c)), an increased coupling effect for
a certain close distance between the inclusion and the moving
part is observed. Thus, a system with only one electrode would
be ambiguous. By the use of the information from two receiver
electrodes, this ambiguity is avoided.
C. Functionality Test of the Sensor Arrangement
The capacitance values of C1 and C2 were measured for
various positions of the moving part. Initially, the movable
part was kept 30 cm away from the fixed part. The capacitance
values corresponding to this position are taken as zero. Then itwas gradually moved towards the fixed part. The capacitance
readings were taken for each 1 cm of displacement of the
movable part of the power tool. The change in reading was
also recorded for the presence of a human finger for each 1
cm of displacement of the moving part. This is plotted along
with the readings for the vacant condition for each 1 cm of
movement and given in Figure 8. As can be seen in Figure 8,
the presence of a human finger can be detected reliably even
when the movable part is very close to the fixed part.
Tests were also carried-out to evaluate the performance
of the sensor under high humidity conditions. We applied a
water spray to the sensor and recorded the drift in the output
readings. The capacitance readings for this condition with andwithout the presence of a hand and a finger were also recorded
and analyzed. It is found that the sensor has low sensitivity
towards variation in humidity or moisture layer on the surface.
In some conditions, it is possible that the electrical potential
of the metallic parts of the arrangement are floating. In such
a condition, the sensitivity of the sensor may vary. However,
as the fixed part of the arrangement has in general a quite
large surface area, it exhibits good capacitive coupling to the
ground and therefore helps the system to work as in a grounded
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0.1
0.05
0
0.05
0.1
0.1
0.05
0
0.05
0.17
6
5
4
3
2
1
0
x 1014
X (m)
Sensitivity map for the near electrode
Y (m)
C(
F)
(a) Computed sensitivity map for the near electrode.
0.1
0.05
0
0.05
0.1
0.1
0.05
0
0.05
0.11
0.8
0.6
0.4
0.2
0
0.2
0.4
X (m)
Sensitivity map for the near electrode
Y (m)
(b) Measured sensitivity map (normalized) for the near electrode.
0.10.05
0
0.05
0.1
0.1
0.05
0
0.05
0.1
1.5
1
0.5
0
x 1014
X (m)
Sensitivity map for the far electrode
Y (m)
C(
F)
(c) Computed sensitivity map for the far electrode.
0.1 0.05
0
0.05
0.1
0.1
0.05
0
0.05
0.11
0.8
0.6
0.4
0.2
0
0.2
0.4
X (m)
Sensitivity map for the far electrode
Y (m)
(d) Measured sensitivity map (normalized) for the far electrode.
Fig. 7. Simulated and measured (normalized) sensitivity maps.
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18250
200
150
100
50
0
Changeincapacitance[fF]
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 180
20
40
60
80
Distance between the movable part and the fixed partChangeincapacitance[fF]
Change in output for the presence of the finger (CV
CF)
vacant condition (CV
)
with finger (CF)
Fig. 8. Readings obtained with and without the presence of a human finger near to the fixed part of the power tool (Figure 3) for various positions of themovable part.
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condition. We have conducted a test series to verify the
sensitivity of the developed system towards ground variations
and found that the system has low sensitivity towards ground
variations.
V. CONCLUSION
A capacitive proximity sensor suitable for detecting the
presence of a human in the vicinity of dangerous working
ares of a power tool has been developed and presented. The
sensor uses multiple electrodes and a sigma-delta capacitance
to digital converter to measure the capacitance changes. The
developed system senses the presence of a human and enables
the control unit of the power tool to move backward or stop to
proceed in foreword direction in order to avoid injuries to the
operator. The proposed scheme has been verified with the help
of finite element analysis and measurements on a developed
prototype system. This scheme can be easily adapted for
similar applications to detect the presence of a human in
dangerous regions even in the surrounding of metallic objects.
V I . ACKNOWLEDGEMENT
This work was partially funded by the Austrian Science
Fund (FWF) through the stand-alone project P21855, Safe-
Tom: Safety by Electrical Capacitance Tomography.
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[3] B. George, H. Zangl, Th. Bretterklieber, and G. Brasseur. A novelseat occupancy detection system based on capacitive sensing. In IEEEConference on Instrumentation and Measurement, pages 15151520,Vancouver Island, Canada, May 15-20 2008.
[4] M. Norgia and C. Sevelto. Rf-capacitive proximity sensor for safety
applications. In IEEE Conference on Instrumentation and Measurement,Warsaw, Poland, May 1-3 2007.
[5] Zhenhai Chen and Ren C. Luo. Design and implementation of capacitiveproximity sensor using microelectromechanical systems technology.
[6] Halit Eren and Lucas D Sandor. Fringe-Effect Capacitive ProximitySensors for Tamper Proof Enclosures. In Proceedings of conference onSensors for Industry (SIcon/05), volume 1, pages 22 26, Feb. 2005.
[7] David Kay Lambert. Capacitive proximity sensor. US Patent No.: US6724324 B1, April 2004.
[8] Th. Bretterklieber and H. Zangl. Versatile sensor front end for low-depthmodulation capacitive sensors. In Proceedings of the IEEE Conferenceon Instrumentation and Measurement, pages 830836, Vancouver Island,Canada, May 15-20 2008.
[9] Analog Devices. Ad7143. Technical report, Analog Devices,http://www.analog.com/en/analog-to-digital-converters/capacitance-to-digital-converters/ad7143/products/product.html, September 2009.
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