Human Proximity

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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.

REFERENCES

[1] L.K. Baxter. Capacitive Sensors, Design and Applications. IEEE Press,1997.

[2] B. George, H. Zangl, and Th. Bretterklieber. A warning system for chain-saw personal safety based on capacitive sensing. In IEEE InternationalConference on Sensors, Leece, Italy, October, 26-30 2008. in print.

[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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