CEEM'201S/HangZhou
Proceedings
Modeling Human Body Walking Voltage By Human Body Capacitance Yu-nan Han, Jun-chao She, Zheng-lin Wen
David Pommerenke
School of information science and technology, Beijing University of Chemical Technology, Beijing China
[email protected]
Electromagnetic Compatibility Laboratory, Electrical and Computer Engineering Department, Missouri University of Science and Technology, Rolla, MO 65409 USA
Lin Dai
Liaohe Petroleum Exploration Bureau Communication Company, Panjin Liaoning, China To our knowledge, there are a few papers [3][4] devoted to the investigation of charging of the human body by walking in different thermal conditions. While an analysis of characterization of human metal ESD is given in paper [5 - 6 . In this paper, on the basis of the previous investigation 7 -[ , according to the standard IEC 6l340-4-5[91 and ANSIIESD STM97.ilO], the experiment on condition that different temperature and humidity and a various types of floor and shoes is performed and model of human walking is deduced. Model of equivalent RC shows that body potential to time is in accordance with an exponential function and is in agreement with the data measured.
Abstract-Human body potential caused by walking on floors has been modeled by a RC equivalent circuit. Based on the measurement of the charging voltage by human body walking,
i f. �
the RC equivalent model has been deduced and the influence of the parameters on the human bod voltage has also been analyzed. The human body equivalent circuit corresponds with the RC network.
As a consequence, an exponential
function
can be
obtained to describe the walking voltage. The model of human body voltage charging by walking combines two main processes: an exponential increase of human body potential due to charging by the soles repeatedly detaching from the floor during walking, and
for
discharging,
the
human
body
potential
has
an
exponential retention. The time constant is determined as a series combination of body resistance and capacitance relative to the ground. During the walking process, the periodic changes of walking paces cause the periodic changes of the body capacitance
DESCRJPTlON OF EXPERIMENT
to ground, then the human walking voltage can be determined by the
capacitance.
The
walking
voltage
modeled
has
reached
Selecting a Template (Heading 2)
agreement with experimental data.
The test setup for human body voltage charging by walking can be shown in fig. 1 according to IEC 6l340-4-5 and ANSI/ESD STM97.2. The experiments were performed using a charge plate monitor (b) and a hand electrode (a). In this experiment, the recording device collects the voltage data through the electrode while the person walks on a floor in a fixed pattern. The pattern of the walking is shown in Fig.2.
Keywords-electro-static discharge (ESD); human body voltage; walking voltage
INTRODUCTION
Charging of the human body by walking is an electrostatic phenomenon often associated with electrostatic discharge (ESD) upsets and damage[l]. Generally, ESD events in both manufacturing and processing facilities are controlled to protect the microelectronic components or explosive and inflammable substance[2]. The ESD control methods, such as humidity controlling, personal grounding, facility grounding, and personal footwear and floor controlling are required to achieve the low human body charging voltage levels needed for safe component and PCB handling etc. As basic ESD control methods must be implemented due to the sensitivity of modem electronic equipment, the question of the remaining risk needs to be answered. What is the likelihood of ESD events occurring that have a severity higher than the level assured by ESD testing in accordance to the IEC 61000-4-2 standard? What is the waveform of human body walking voltage? To answer this question, the modeling and measurement of human body potential charging by walking may be a useful method.
The measurement method is outlined as follows: a person, wearing a specific type of shoes on a specific type of floors repeats the walking pattern. The person waits for two seconds when he finishes each six step. The induced voltage can be recorded by the recording device every 20ms. On each condition, we take 10 times at least. Fig.3. shows the scheme of the measuring circuit. The left part is the equivalent circuit of the measuring system and the right part is the equivalent circuit of the human body. CM: input capacitance of the measuring system. Cs: capacitance of person relative to the ground. Cw: capacitance of person relative to the objects surrounded, including walls and ceiling. RM: input resistance of the charge plate monitor. RB: the person resistance.
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Proceedings
time constant
Rs: resistance of person relative to the ground. Generally, RM is larger than 1014Q and RB is about IMQ. And according to the industry standard S7.1, Rs of person
T
Rs, Cw and Cs:
will be determined as a series combination of (3)
relative to the ground is smaller than 109Q, Therefore, Rf,1 » R8
The walking human body equivalent circuit shown in figure 3 supposes that the feet of a walker and the grounded construction of the floor there are two layers: a sole (e.g. rubber) of the walker's footwear and a surface layer (e.g. PVC) of the floor. The resistance Rs of the human body to the ground, which consists of the shoes resistance and the floor resistance, is a function of time during walking. Lifting up and treading down the feet when walking cause periodic increase and decrease the resistance Rs. The directly standing resistance Rso of the human body to the ground with a largest contact surface So of feet to the ground respected with a lowest resistance Rs. Compared with standing, human body lifting up feet when walking, the resistance can be higher than standing resistance Rso, if the human body standing on one foot, the resistance is nearly 2 times than standing on two feet. The resistance Rs of walking can be negative correlated with the contact surface areaS
(1)
+ Rs
I So Rs=p-=RsoS S Fig. l . Experiment
The capacity Cs of the human body to the ground, which consists of the shoes capacity and the floor capacity, is also a function of time during walking. Compared with walking resistance Rs, lifting up and treading down the feet when walking cause periodic decrease and increase the capacity Cs. The directly standing capacity of the human body to the ground compared with a highest capacity Cso. Compared with standing capacity Cso, human body lifting up feet when walking, the capacity can be lower than standing on the floor with two feet. The walking human body capacity Cs can be modeled as plane parallel capacitors system as shown in figure 6, which consists of a plane-parallel capacitor respected the contact with ground part capacity C[ and a plane-parallel capacitor C2 respected the lifting with distance x to the ground.
Fig.2. Fixed walking pattern
(a)
...
R·IC. rw �
(4)
...
c,
...
Fig.3. Scheme of human body model for and measuring circuit. I.
MODELLING OF HUMAN BODY WALKING VOTAGE
(a)
As shown in Fig.3, the human body equivalent circuit corresponds with the RC network. The charging function UB(t) can be expressed as follows:
(b)
Fig. 4. The walking human body modeled by plane-parallel capacitor system with step capacitor. (a) plane-parallel capacitor model of human body directly standing on the floor with two feet; (b) plane-parallel capacitors model of human body walking on the floor.
(2)
The capacitance C[ of the soles contacted with the floor can be calculated by the basic model of a plane-parallel capacitor
The body potential UB increases due to charging when human starts walking. After time t over tp, the body potential UB decreases to the zero gradually because of discharge. The
S C1 =CsoSo
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(5)
CEEM'201S/HangZhou
Proceedings
The capacitance C2 of the soles lifting with a certain height above the floor can be calculated as the capacitance of two capacitors connected in series, the first of which corresponds to the soles surface area (S-So) contact with the floor, and the second of which corresponds to the capacity of air gap with the permittivity Ga between the sole and the floor:
conclude that the changes of the capacitance are not under consideration in the RC fitting curve. Thus, the shape of the curve is also the charge Q of the tester.
x
C2
=
B.
( So - S ) Csoca
-"--"------.£.----''-' ''-'----
SOca +xCSO
(6)
The capacitance Cscan be calculated as the capacitance of two capacitors C[ and C2 connected in parallel:
(7) And the time constant rw can be deduced from equation (3), (4) and (6), and it will be given as follows:
'IV
where
rw
_
S20
_
Period of the walking voltage andfitting
Fig.6 shows the variations of walking voltage in charging process at each period and the average curve respectively. The waveform at each period has a good similarity and the variations of fluctuation are in a reasonable range. It is nearly around the average curve. Each period curve shows a satisfactory reproducibility. Suppose the normalized average curve is the function of u(t), then u(t) = u(t + nT) (9) where T is the period of person walking step. Fig.7 shows the comparison of the fitted curve and measured data with the normalized average curve u(t}. Visual inspection informs us that it reaches an agreement of the fitting data with the measured data. �O
ca +xCso
S Soca +xCso
'0
--�--�--�--��r=�==�
� '-
----e---- average sixstep
(8)
can be over 10 seconds on the condition that the
resistance Rs person relative to the ground is large. The equation of represents the time constant
rw
is negative
correlated with the contact surface area of the sole and the floor. That means the human body can be charged much slower when walking than standing. RESULTS OF
A.
-1�O -1
THE EXPERIMENT
---'-.' 5� --�2---'2�.5 --�--,-L3 5. '----'
400 L-�O.5'---� 0
Time(s)
Fig.6. Periodogram of walking voltage. ffiO
Walking voltage
--�--�--�--��== �==�
1-
� ,-
measured data -·_·-fittingdata
Fig. 5 shows the walking voltage and RC fitting curve under the environmental condition (27°C, 25%RH). The floor person stands on is high pressure laminate and shoes person wears is Sperry.
1
���--�--�--��r=�==�� walking voltage -.-.oRe fitting
-�OO��'0��2�O--�30��40--�5�O--�60'----=70--�8'O Time(s)
Fig. 7. Average curve of walking voltage fitted to measured data.
C. Capacitance relative to the ground
As shown in Fig.2, during the walking process, the period of the capacitance relative to the ground is in accordance with the period of walking voltage. As it has been mentioned in the previous section, time constant Tw can be over ten seconds, thus in a short period of time, charge Qs of walker's body is approximately as a constant. The body potential Us(t) is negative correlated with the capacitance Cs(t):
-�O��'0��2�O--�3�O--�40��5�O--�60��70��OO Time(s)
Fig.5. Comparison of walking voltage and RC fitting curve.
The RC fitting curve is in accordance with the upper outline of the band, which means person stands on the floor without lifting feet up. Since the period of walking steps cause the period of capacitance relative to the ground, it is possible to
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Proceedings
� (t) C,. (t) In a certain cycle, expression of equation (10) applicable as follows: q(t) U 8 (t)
u(t)
=
c(t )
thermal conditions, floor types and shoes types according to standard of IEC 61340-4-5. Secondly, the human body walking voltage wavefonn has been analyzed, then the human body walking voltage equivalent RC circuit model was deduced, and exponential function can be obtained to describe the human body walking voltage, the time constant 1" is determined by the human body resistance R and capacity C to the ground. Thirdly, the human body capacity is periodic changing with human body steps, so the human body voltage is also negatively changing with human body capacity. Influence of different environmental conditions on the capacitance is concerned and analyzed in this paper.
(10)
=
IS
also (11)
Fig. 8 shows the shape of capacitance c(t). .2 6
0 1 .10 r--:-=-�-�-�-r--;:========il 1 --changes ofcapacitance I X
REFERENCES
� 1.6
§
z
1.4 12
1 � O 5 -�--1�5-�--,-2�5 -�--,-3�5----' 0L-----,Time(s)
[I]
LJU Shanghe, SONG Xuejun. Progress in electrostatic and related research [1]. Chinese Joumal of Nature,2007(2): 63-68+60.(in Chinese)
[2]
Mori, I.; Fujiwara, 0., "Severity evaluation of immunity test for air and contact discharges of ESD generators," Environmental Electromagnetics, 2009. CEEM 2009. 5th Asia-Pacific Conference on , voL, no. , pp.110,113, 16-20 Sept. 2009.
[3]
Fangming Ruan; Dan Shi; Yougang Gao; Zeyun Song, "Influence on discharge properties of esd subprocess from large speed of moving electrode," Environmental Electromagnetics, 2009. CEEM 2009. 5th Asia-Pacific Conference on , voL, no. , pp. l 18,121, 16-20 Sept. 2009.
[4]
Huang Jiu-Sheng; Deng Qi-Bin; Liu Fang; Fu Peng-Cheng; Liu Pei-Zhu, "The test of electromagnetic field radiated by electrostatic discharge (ESD) from the real charged human body in the office," Environmental Electromagnetics, 2000. CEEM 2000. Proceedings. Asia-Pacific Conference on , voL, no., pp.412,416, 2000.
[5]
Chundru, R.; POImnerenke, D. ; Kai Wang; Van Doren, T. ; Centola, FP. ; Jiu Sheng Huang, "Characterization of human Metal ESD reference discharge event and con'e1ation of generator parameters to failure levels part I: reference event," Electromagnetic Compatibility, IEEE Transactions on , voL46, no.4, pp.498,504, Nov. 2004.
[6]
Kai Wang; POImnerenke, D.; Chundru, R. ; Van Doren, T. ; Centola, F.P.; Jiu Sheng Huang, "Characterization of human metal ESD reference discharge event and correlation of generator parameters to failure levels part II: correlation of generator parameters to failure levels," Electromagnetic Compatibility, IEEE Transactions on , voL46, no.4, pp.505,511, Nov. 2004.
[7]
MORADIAN M, PATNAIK A, HAN Yunan, et aLDetennination of the Effect of humidity on the probability of ESD failure or upset in data centers [J]. ASHRAE Transactions, 2014, 119(2): 1-17.
[8]
TALEBZADEH A, MORADIAN M, HAN Yunan, A.Patnaik, D. Swenson, and D. Pommerenke. Dependence of ESD charge voltage on humidity in data centers (partI - test methods) [J]. Accepted for ASHRAE Transactions, 2014.
[9]
intemational Electro technical COImnission. IEC 61340-4-5 Electrostatics -Part 4-5: Standard test methods for specific applications - Methods for characterizing the electrostatic protection of footwear and flooring in combination with a person [S]. Geneva, Switzerland: Typeset and printed by the IEC Central Office, 2007.
Fig.8 Peliodogram of capacitance.
It is similar to the function of u(t). As shown before, the local capacitance is maximum when person stands on the floor. The walking voltage increases and the capacitance decreases simultaneous as tester starts to walk. Therefore, the maximum of the peak value and the minimum of the trough value of u(t) correspond to the minimum of the trough value and the maximum of the peak value of c(t) respectively. From the above, the capacitance is related to the walking voltage and the period of capacitance is the same as the walking step. Fig.9 shows the capacitance under different temperature and humidity. It verifies that environmental condition has less influence on the capacitance related to the ground. ---18t45%RH . 27'\: 25%RH -- 27t45%RH
08 O .60�----;O.';:-5 ----:---:-.1 ';:-5 ----:---::.2 ';:-5----:---::3.';:-5----' O Time(s)
Fig.9. Capacitance under different environmental conditions.
[10] American National Standards Institute (ANSI). ANSJlESD STM97.2-. Floor Materials and Footwear-Voltage Measurement in Combination with a Person [S]. Arlington, VA: Telecommunication Industry Association, 2006.
CONCLUSION
Based on the measurement of human body walking voltage, the RC equivalent circuit model for human body voltage charging by walking has been deduced and the model parameters affection on the time domain voltage waveform has also been analyzed. Firstly, human body voltage charging by walking has been measured in different condition of
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