- Email: [email protected]
Ion imbalances on an ionizer-controlled work surface
ARTICLE IN PRESS
Journal of Electrostatics 64 (2006) 310–315 www.elsevier.com/locate/elstat
Ion imbalances on an ionizer-controlled work surface M.A. Noras, D. Pritchard Trek, Inc., 11601 Maple Ridge Rd., Medina, NY 14103, United States Received 18 December 2003; received in revised form 5 July 2005; accepted 7 July 2005 Available online 6 September 2005
Abstract This paper presents results of investigations carried out to test the influence of charged objects introduced into the vicinity of an ionizer-controlled work surface. The qualitative and quantitative distribution of ions on the ionizer-controlled surface was the main focus of the experiment. Tests were conducted using an ionizer equipped with a feedback control signal loop from a sensor at the output of the ionizer (factory installed internal feedback). r 2005 Elsevier B.V. All rights reserved. Keywords: Ionizer balance; Ion distribution; ESD; Air flow; Charge decay
1. Introduction In many electronic production and assembly environments, there exists a problem of static electricity. Static charges present within the work area, on tools, parts, etc., produce a risk of damage to sensitive materials, components and equipment, therefore every effort is being made to avoid this type of hazard [1–8]. Among several methods for static charge elimination, neutralization of static charges with ionizers has become one of the most popular. There are many kinds of ionizers: radioisotope, X-ray, ultraviolet, corona discharge—all those devices have been shown to be effective as static electricity eliminators. The corona discharge ionizer is frequently preferred over other ionizers, because it is relatively easy to control the balance between amounts of positive and negative ions being produced. This is usually done by adjusting the voltage applied to electrodes that produce the corona discharge. Ion balance is very important, and if not established properly and maintained, excess of ions of either polarity may lead to charge accumulation at various Corresponding author. Tel.: +1 585 798 3140; fax: +1 585 798 5033. E-mail address: [email protected] (M.A. Noras).
0304-3886/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2005.07.004
places within the electrostatic discharge (ESD) protected area, defeating the purpose of the ionizer. In order to maintain control over the ion balance, corona discharge ionizers are frequently equipped with an internal feedback system. Sensors mounted at the ionizer output are supposed to detect all imbalances and enforce appropriate corrections. Unfortunately the balance is controlled not at the ESD protected work surface, but at the source (output of the ionizer). For example, an overhead bench ionizer is usually placed 0.5–1 m above the work surface. As a result, by the time when ions reach the protected surface, the ion balance is often distorted. Factors such as air flow patterns, air flow obstructions, ambient humidity and temperature may heavily influence ion balance at the ESD protected area [9]. Fig. 1, for example, shows the influence of temperature on the ion balance recorded by an ion balance monitor. This measurement was performed with a commercial threefan ionizer placed 60 cm above the test surface (Fig. 2). The ionizer was factory-equipped with feedback sensors placed at the ionizer outlet, controlling the balance between amounts of positive and negative ions leaving the ionizer. The ion balance monitor was placed on the test surface 60 cm directly below the center fan. This sensor records an offset voltage resulting from the balance between the amount of positive and negative
ARTICLE IN PRESS M.A. Noras, D. Pritchard / Journal of Electrostatics 64 (2006) 310–315
Fig. 1. Ion balance vs. the ambient temperature dependence.
311
Fig. 3. Ion balance vs. the ambient temperature dependence in the temperature-controlled environment.
ionizers ionizers
ion balance monitor ion balance monitor
60 cm d
grounded dissipative mat Fig. 2. Simple setup for the ion balance test.
isolated, conducting plate
grounded dissipative mat
Fig. 4. Setup for the ion balance test with multiple ionizers.
ions impinging on the sensing element of the monitor. The balance of the ionizer was manually adjusted at the beginning of the test, using appropriate balancing potentiometers at the output of the ionizer. It had been found that the offset voltage was influenced by temperature variations (relative humidity was kept constant at 60% RH), even though the ion balance was continually being adjusted by the factory-installed feedback sensors mounted at the outlet of the ionizer. When the temperature and humidity were held constant (20 1C and 60% RH, respectively), the ionizer’s balance (Fig. 3) was held fairly closely to zero within 1 V offset voltage variation. Air ionizers are effective only when they can deliver sufficient and equal amounts of positive and negative ions to the region where the ESD-protected elements are located. It has been mentioned that ion balance within the protected work area is influenced by many factors. One of not fully investigated aspects is the
effect of a charged object introduced into an ESD protected area on the ion distribution within that area, and this is the topic of the study presented in this paper.
2. Test setup and measurements Two separate sets of experiments were carried out. The first group of tests was performed to investigate the influence of a DC-voltage powered conductor on the ion distribution within the space controlled by a corona discharge ionizer. The second set of experiments was carried out with a pre-charged, floating conductor bearing a limited amount of charge. The setup for these experiments is shown in Fig. 4. A 15 cm 15 cm isolated metal plate was placed on the work surface (a grounded,
ARTICLE IN PRESS M.A. Noras, D. Pritchard / Journal of Electrostatics 64 (2006) 310–315
312
static dissipative mat) where an ion balance monitor (also called a charged plate monitor (CPM)) was present. During tests, the ion balance monitor sensor was positioned at various distances from the floating, conducting plate. The temperature and humidity were kept constant at 20 1C and 60% RH, respectively.
of the CPM monitor was recorded for all the voltage levels and various sensor-to-plate distances. The same test was conducted for the negative DC voltages applied to the plate.
2.1. Experiments with a DC voltage applied to a conducting plate
The second group of experiments was conducted with an isolated plate of 15 cm 15 cm, having a 20 pF plateto-ground capacitance, as measured with an LCR meter (SR715, Stanford Research System). The plate was connected to a second charged plate monitor. The second CPM instrument delivered the charging voltage to the plate, floated the plate and provided a discharge time measurement capability. The discharge time in this case was the time necessary to bring the potential of the floating plate from the pre-defined electric potential level to either 10% or 1% of the initial potential value. The plate was therefore charged and then floated, and the voltage vs. time characteristic of the plate was recorded. Tests were performed for ambient (without ionizer) conditions and for the ionizer-controlled environment, with various values and polarities of the initial voltage applied to the plate.
First, a set of tests was carried out for a work space not controlled by any ionizers (ambient conditions). Positive DC voltages of 100, 200, 500 and 1000 V, consecutively, were applied to the isolated metal plate and the response of the monitor was recorded. The same tests were performed for negative DC voltages of the same magnitudes applied to the plate. Measurements were repeated at various CPM sensor-to-plate distances. The next experiments were done with the ionizer controlling the work space. The ion balance was managed by a three-fan DC corona discharge ionizer unit, designed to maintain the voltage offset on the work surface within 3 V. Each of three fans of the ionizer unit contained a separate ionizer. These ionizers were manually balanced, using an adjustment potentiometers provided by the manufacturer (the metal plate was grounded during the balancing process), until the charged plate monitor was indicating 0 V offset voltage (accuracy of the monitor was 0.2%, which translates to 0:2 V). The zero voltage offset was verified at various locations over the entire work surface. Next, a positive DC voltage was applied to the metal plate and response
2.2. Discharge of a floating, conducting plate
3. Results 3.1. DC voltage tests Tables 1, 2 and 3 present results of tests conducted with a DC voltage on the plate. The first experiment was
Table 1 Voltage detected by the CPM vs. distance from the floating plate for various voltages applied to the floating plate, without ionizer d; mm
Floating plate voltage, V 100
25 250 500
200
500
1000
100
200
500
1000
1.5 2.9 0.3 0.6 0.3 0.3 CPM voltage, V
3.6 0.8 0.5
5.5 0.8 0.7
1.7 0.1 0
1.8 0.2 0.1
4.5 0.3 0.1
8.8 0.4 0.1
Table 2 Voltage detected by the CPM vs. distance from the floating plate for various voltages applied to the floating plate, with 32 l/s ð68 ft3 = minÞ air flow from each of three fans of the ionizer d; mm
Floating plate voltage, V 100
25 250 500
200
500
1000
100
200
500
1000
17.4 34.7 1.1 2.5 0.3 0.8 CPM voltage, V
79.7 3.1 1
100 at 95 mm 4.9 1.4
14 0.4 0.3
31.1 0.6 0.4
77 3.6 0.8
100 at 80 mm 7.7 1.9
ARTICLE IN PRESS M.A. Noras, D. Pritchard / Journal of Electrostatics 64 (2006) 310–315
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Table 3 Voltage detected by the CPM vs. distance from the floating plate for various voltages applied to the floating plate, with 49.6 l/s ð105 ft3 = minÞ air flow from each of three fans of the ionizer d; mm
Floating plate voltage, V 100
25 250 500
200
26.1 50.5 1.8 2.6 0.5 0.5 CPM voltage, V
500
1000
100
200
500
1000
100 at 125 mm 5.6 0.8
100 at 150 mm 9.7 2.5
17.6 1.1 0
51.9 2 0.4
100 at 130 mm 4.7 1.6
100 at 158 mm 8.3 3.5
conducted with no ionizer present in the vicinity of the floating plate and the ion balance monitor. For the monitor sensor placed in a close proximity of the floating plate powered with a DC voltage, the CPM unit was sensing the floating plate potential through a capacitive coupling (Table 1). The same tests were carried out with a three-fan ionizer producing an air flow of 32 l/s ð68 ft3 = minÞ per fan, placed 60 cm above the surface on which both the floating and the charged plate monitor plates were located. The voltage offsets recorded by the CPM at d ¼ 25 mm distance from the floating plate increased approximately 10 times for each of the voltages applied to the plate as compared with the values observed during the test without the ionizer (Table 2). When the air flow of the ionizer was adjusted to 49.6 l/s ð105 ft3 = minÞ the voltage offset on the charged plate monitor increased even further. This increase in voltage offsets was observed for both polarities of the voltages applied to the floating plate. Voltage measurement capability of the charged plate monitor was limited to 100 V. There were some measurements for which it was impossible to determine the voltage on the CPM at the given distance d between the floating plate and the CPM plate. In those cases the CPM sensor was placed at the distance from the floating plate for which the monitor was indicating the maximum measurable offset voltage. For example, (Table 2) when þ1 kV is applied to the floating conductor, the CPM displayed þ100 V maximum voltage at d ¼ 95 mm distance between the plates.
Fig. 5. Results of discharge tests without an ionizer, plate charged to þ200 V.
3.2. Discharge tests Figs. 5 and 6 show discharge characteristics, as measured by the second CPM, for the plate charged to þ200 V and 200 V, respectively. With no air ionizer the plate retains its charge for a very long time. For example, it took 90 min for a floating plate to discharge from þ200 V potential level to þ100 V. At the same time the potential of the first CPM sensor was monitored. The sensor was placed first at 25 mm, then at 250 mm and at 500 mm from the plate, consecutively. When the sensor was in close proximity to the floating conductor
Fig. 6. Results of discharge tests without an ionizer, plate charged to 200 V.
(25 mm), the first CPM was able to detect the voltage induced on the sensor plate and monitor it. When the distance between the floating plate and the sensor was increased to 250 and 500 mm, the voltage recorded by the first CPM was negligible (very close to zero). The
ARTICLE IN PRESS 314
M.A. Noras, D. Pritchard / Journal of Electrostatics 64 (2006) 310–315 Table 4 Discharge times (100% to 10% and 100% to 1% of initial voltage value) for 32 l/s ð68 ft3 = minÞ air volume flow rate
Fig. 7. Results of discharge tests with a 3-fan ð32 l=s ð68 ft3 = minÞ each fan), plate charged to þ200 V.
Fig. 8. Results of discharge tests with a 3-fan ð32 l=s ð68 ft3 = minÞ each fan), plate charged to 200 V.
Discharge voltage (V)
Discharge time (s)
þ1000–þ10 þ1000–þ1 1000–10 1000–1
11.0 14.3 12.8 14.7
ionizer
Fig. 9. Results of discharge tests with a 3-fan ð49:6 l=s ð105 ft3 = minÞ each fan), plate charged to þ200 V.
ionizer
Fig. 10. Results of discharge tests with a 3-fan ð49:6 l=s ð105 ft3 = minÞ each fan), plate charged to 200 V.
ionizer
ionizer
next two sets of results, with an ionizer supplying air ions into the space where the isolated, floating plate and the first CPM sensor are located, are presented in Figs. 7 and 8. In the ionizer controlled space the discharge time of the plate decreased very significantly (Table 4). The air flow produced by each of 3 fans of the ionizer was 32 l/s ð68 ft3 = minÞ. Figs. 9–10 present discharge characteristics of the plate with the air flow of 49.6 l/s ð105 ft3 = minÞ per fan, and the discharge times are presented in Table 5.
4. Discussion When a DC voltage was applied to the isolated plate with no ionizer present in the vicinity of the plate, the
Table 5 Discharge times for 49.6 l/s ð105 ft3 = minÞ air volume flow rate Discharge voltage (V)
Discharge time (s)
þ1000–þ10 þ1000–þ1 1000–10 1000–1
6.5 7.6 7.7 9.1
ARTICLE IN PRESS M.A. Noras, D. Pritchard / Journal of Electrostatics 64 (2006) 310–315
charged plate monitor sensor detected only the potential induced by the plate. After a balanced ionizer started delivering air ions into the space where the tests were conducted, motion of the ions were influenced by the electric field created by the plate. For example, the conducting plate with a negative potential applied to it was attracting the positive ions, repelling the negative ones at the same time. As a result, the space around the plate had excess of negative airborne ions. This effect was immediately detected by the CPM sensor as a negative voltage offset, signaling that the ionizer was out of balance. Note that ionizers equipped with an internal feedback only (from the outlet of the ionizer), normally would not react to such air ions imbalance, since the imbalance had been created on the work surface, not at the output of the ionizer. In order to bring the amount of positive and negative ions reaching the CPM sensor (and the ionizer-controlled surface area on which that sensor is located) back to balance, it would be necessary to use the signal from the sensor, not from the ionizer outlet, as a feedback to the ionizer. It had been observed that the voltage offset recorded by the CPM became larger when the air volume flow rate from the ionizer was increased. It is important to distinguish between air ion imbalance created by an isolated, charged object and by an object that carries a permanent DC voltage applied to it. A floating, charged item usually gets neutralized by an ionizer relatively quickly, depending on the size and initial charge of the object, as well as on the ionizer used for the task of charge neutralization and environmental factors such as humidity, temperature, etc. For example, a square, 15 cm by 15 cm metal isolated plate, having plate-to-ground capacitance of 20 pF ð2 pFÞ and charged to 1000 V would discharge to the voltage of 10 V under an ionizer within a couple of seconds (see Tables 4 and 5). This means that the distribution of airborne ions in the space around the charged object was disturbed by the electric field from the plate for the same, relatively short, amount of time. A DC-powered object disturbs the air ion field permanently, leading to possibility of an electric charge accumulation in the vicinity of that object. While observing discharge characteristics (Figs. 5–10) of the isolated, floating plate, it could be noticed that the CPM sensor in close proximity (25 mm) to the plate displayed an offset voltage overshoot when the potential of the floating plate dropped to zero. One possible explanation of this phenomenon is that the CPM sensor, bearing the capacitance of 20 pF, becomes charged due to excess of ions of either polarity. At the moment when the potential of the floating plate is close to zero and it
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does not affect the air ion distribution anymore, the sensor is still charged and it attracts charges of the opposite polarity until its potential reaches zero. At that instance, in close proximity of the sensor, there are still ions heading toward the sensor’s surface, and they cause the observed ‘‘overshoot’’ effect.
5. Conclusions Effect of the electrically charged objects on the ion balance of the corona ionizer has been investigated. It has been found out that when the conducting item is connected to a constant DC power source, its presence disturbs airborne ion distribution which in turn may lead to charge accumulation within the direct surrounding of the object. If the charged item carries a limited charge (is floated), the air ion distribution is perturbed only for the time necessary to neutralize the object. In either case, the ion balance in the vicinity of the charged object is altered. In order for the air ionizer to account for and correct the ion distribution disturbance at the protected area, it would be desirable to control air ion production based on the feedback information from the protected space, not from the outlet of the ionizer.
References [1] J. Chang, A. Berezin, Neutralization of static surface charges by a flow stabilized corona discharge ionizer in a nitrogen environment, J. Electrostat. 51–52 (2001) 64–70. [2] J. Chang, A. Berezin, Neutralization of static surface charges by an AC ionizer in a nitrogen and dry air environment, J. Appl. Phys. 91 (3) (2002) 1020–1025. [3] C. Noll, Balanced static elimination in variable ion mobility environments, J. Electrostat. 49 (2000) 169–194. [4] A. Steinman, Best practices for applying air ionization, EOS/ESD Symposium, 1995, pp. 245–252. [5] N. Jonassen, The physics of charge neutralization by air ions, EOS/ ESD Symposium, 1986, pp. 35–40. [6] D.M. Fehrenbach, R.T. Tsao, An evaluation of air ionizers for static charge reduction and particle emission, EOS/ESD Symposium, 1992, pp. 19–25. [7] L. Levit, A. Wallash, Measurement of the effects of ionizer imbalance and proximity to ground in MR head handling, EOS/ ESD Symposium, 1998, pp. 375–382. [8] EOS/ESD Association Standard for Protection of Electronic Discharge Susceptible Items—Ionization, ANSI ESD STM 3.12000, ESD Association, 2000. [9] D. Pritchard, Improving ionizer balance using external feedback sensors at the work surface, Trek Application Note No. 1003 available at: ohttp://www.trekinc.com/Download/1003FeedbackSensors.PDF4.
Journal of Electrostatics 64 (2006) 310–315 www.elsevier.com/locate/elstat
Ion imbalances on an ionizer-controlled work surface M.A. Noras, D. Pritchard Trek, Inc., 11601 Maple Ridge Rd., Medina, NY 14103, United States Received 18 December 2003; received in revised form 5 July 2005; accepted 7 July 2005 Available online 6 September 2005
Abstract This paper presents results of investigations carried out to test the influence of charged objects introduced into the vicinity of an ionizer-controlled work surface. The qualitative and quantitative distribution of ions on the ionizer-controlled surface was the main focus of the experiment. Tests were conducted using an ionizer equipped with a feedback control signal loop from a sensor at the output of the ionizer (factory installed internal feedback). r 2005 Elsevier B.V. All rights reserved. Keywords: Ionizer balance; Ion distribution; ESD; Air flow; Charge decay
1. Introduction In many electronic production and assembly environments, there exists a problem of static electricity. Static charges present within the work area, on tools, parts, etc., produce a risk of damage to sensitive materials, components and equipment, therefore every effort is being made to avoid this type of hazard [1–8]. Among several methods for static charge elimination, neutralization of static charges with ionizers has become one of the most popular. There are many kinds of ionizers: radioisotope, X-ray, ultraviolet, corona discharge—all those devices have been shown to be effective as static electricity eliminators. The corona discharge ionizer is frequently preferred over other ionizers, because it is relatively easy to control the balance between amounts of positive and negative ions being produced. This is usually done by adjusting the voltage applied to electrodes that produce the corona discharge. Ion balance is very important, and if not established properly and maintained, excess of ions of either polarity may lead to charge accumulation at various Corresponding author. Tel.: +1 585 798 3140; fax: +1 585 798 5033. E-mail address: [email protected] (M.A. Noras).
0304-3886/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2005.07.004
places within the electrostatic discharge (ESD) protected area, defeating the purpose of the ionizer. In order to maintain control over the ion balance, corona discharge ionizers are frequently equipped with an internal feedback system. Sensors mounted at the ionizer output are supposed to detect all imbalances and enforce appropriate corrections. Unfortunately the balance is controlled not at the ESD protected work surface, but at the source (output of the ionizer). For example, an overhead bench ionizer is usually placed 0.5–1 m above the work surface. As a result, by the time when ions reach the protected surface, the ion balance is often distorted. Factors such as air flow patterns, air flow obstructions, ambient humidity and temperature may heavily influence ion balance at the ESD protected area [9]. Fig. 1, for example, shows the influence of temperature on the ion balance recorded by an ion balance monitor. This measurement was performed with a commercial threefan ionizer placed 60 cm above the test surface (Fig. 2). The ionizer was factory-equipped with feedback sensors placed at the ionizer outlet, controlling the balance between amounts of positive and negative ions leaving the ionizer. The ion balance monitor was placed on the test surface 60 cm directly below the center fan. This sensor records an offset voltage resulting from the balance between the amount of positive and negative
ARTICLE IN PRESS M.A. Noras, D. Pritchard / Journal of Electrostatics 64 (2006) 310–315
Fig. 1. Ion balance vs. the ambient temperature dependence.
311
Fig. 3. Ion balance vs. the ambient temperature dependence in the temperature-controlled environment.
ionizers ionizers
ion balance monitor ion balance monitor
60 cm d
grounded dissipative mat Fig. 2. Simple setup for the ion balance test.
isolated, conducting plate
grounded dissipative mat
Fig. 4. Setup for the ion balance test with multiple ionizers.
ions impinging on the sensing element of the monitor. The balance of the ionizer was manually adjusted at the beginning of the test, using appropriate balancing potentiometers at the output of the ionizer. It had been found that the offset voltage was influenced by temperature variations (relative humidity was kept constant at 60% RH), even though the ion balance was continually being adjusted by the factory-installed feedback sensors mounted at the outlet of the ionizer. When the temperature and humidity were held constant (20 1C and 60% RH, respectively), the ionizer’s balance (Fig. 3) was held fairly closely to zero within 1 V offset voltage variation. Air ionizers are effective only when they can deliver sufficient and equal amounts of positive and negative ions to the region where the ESD-protected elements are located. It has been mentioned that ion balance within the protected work area is influenced by many factors. One of not fully investigated aspects is the
effect of a charged object introduced into an ESD protected area on the ion distribution within that area, and this is the topic of the study presented in this paper.
2. Test setup and measurements Two separate sets of experiments were carried out. The first group of tests was performed to investigate the influence of a DC-voltage powered conductor on the ion distribution within the space controlled by a corona discharge ionizer. The second set of experiments was carried out with a pre-charged, floating conductor bearing a limited amount of charge. The setup for these experiments is shown in Fig. 4. A 15 cm 15 cm isolated metal plate was placed on the work surface (a grounded,
ARTICLE IN PRESS M.A. Noras, D. Pritchard / Journal of Electrostatics 64 (2006) 310–315
312
static dissipative mat) where an ion balance monitor (also called a charged plate monitor (CPM)) was present. During tests, the ion balance monitor sensor was positioned at various distances from the floating, conducting plate. The temperature and humidity were kept constant at 20 1C and 60% RH, respectively.
of the CPM monitor was recorded for all the voltage levels and various sensor-to-plate distances. The same test was conducted for the negative DC voltages applied to the plate.
2.1. Experiments with a DC voltage applied to a conducting plate
The second group of experiments was conducted with an isolated plate of 15 cm 15 cm, having a 20 pF plateto-ground capacitance, as measured with an LCR meter (SR715, Stanford Research System). The plate was connected to a second charged plate monitor. The second CPM instrument delivered the charging voltage to the plate, floated the plate and provided a discharge time measurement capability. The discharge time in this case was the time necessary to bring the potential of the floating plate from the pre-defined electric potential level to either 10% or 1% of the initial potential value. The plate was therefore charged and then floated, and the voltage vs. time characteristic of the plate was recorded. Tests were performed for ambient (without ionizer) conditions and for the ionizer-controlled environment, with various values and polarities of the initial voltage applied to the plate.
First, a set of tests was carried out for a work space not controlled by any ionizers (ambient conditions). Positive DC voltages of 100, 200, 500 and 1000 V, consecutively, were applied to the isolated metal plate and the response of the monitor was recorded. The same tests were performed for negative DC voltages of the same magnitudes applied to the plate. Measurements were repeated at various CPM sensor-to-plate distances. The next experiments were done with the ionizer controlling the work space. The ion balance was managed by a three-fan DC corona discharge ionizer unit, designed to maintain the voltage offset on the work surface within 3 V. Each of three fans of the ionizer unit contained a separate ionizer. These ionizers were manually balanced, using an adjustment potentiometers provided by the manufacturer (the metal plate was grounded during the balancing process), until the charged plate monitor was indicating 0 V offset voltage (accuracy of the monitor was 0.2%, which translates to 0:2 V). The zero voltage offset was verified at various locations over the entire work surface. Next, a positive DC voltage was applied to the metal plate and response
2.2. Discharge of a floating, conducting plate
3. Results 3.1. DC voltage tests Tables 1, 2 and 3 present results of tests conducted with a DC voltage on the plate. The first experiment was
Table 1 Voltage detected by the CPM vs. distance from the floating plate for various voltages applied to the floating plate, without ionizer d; mm
Floating plate voltage, V 100
25 250 500
200
500
1000
100
200
500
1000
1.5 2.9 0.3 0.6 0.3 0.3 CPM voltage, V
3.6 0.8 0.5
5.5 0.8 0.7
1.7 0.1 0
1.8 0.2 0.1
4.5 0.3 0.1
8.8 0.4 0.1
Table 2 Voltage detected by the CPM vs. distance from the floating plate for various voltages applied to the floating plate, with 32 l/s ð68 ft3 = minÞ air flow from each of three fans of the ionizer d; mm
Floating plate voltage, V 100
25 250 500
200
500
1000
100
200
500
1000
17.4 34.7 1.1 2.5 0.3 0.8 CPM voltage, V
79.7 3.1 1
100 at 95 mm 4.9 1.4
14 0.4 0.3
31.1 0.6 0.4
77 3.6 0.8
100 at 80 mm 7.7 1.9
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Table 3 Voltage detected by the CPM vs. distance from the floating plate for various voltages applied to the floating plate, with 49.6 l/s ð105 ft3 = minÞ air flow from each of three fans of the ionizer d; mm
Floating plate voltage, V 100
25 250 500
200
26.1 50.5 1.8 2.6 0.5 0.5 CPM voltage, V
500
1000
100
200
500
1000
100 at 125 mm 5.6 0.8
100 at 150 mm 9.7 2.5
17.6 1.1 0
51.9 2 0.4
100 at 130 mm 4.7 1.6
100 at 158 mm 8.3 3.5
conducted with no ionizer present in the vicinity of the floating plate and the ion balance monitor. For the monitor sensor placed in a close proximity of the floating plate powered with a DC voltage, the CPM unit was sensing the floating plate potential through a capacitive coupling (Table 1). The same tests were carried out with a three-fan ionizer producing an air flow of 32 l/s ð68 ft3 = minÞ per fan, placed 60 cm above the surface on which both the floating and the charged plate monitor plates were located. The voltage offsets recorded by the CPM at d ¼ 25 mm distance from the floating plate increased approximately 10 times for each of the voltages applied to the plate as compared with the values observed during the test without the ionizer (Table 2). When the air flow of the ionizer was adjusted to 49.6 l/s ð105 ft3 = minÞ the voltage offset on the charged plate monitor increased even further. This increase in voltage offsets was observed for both polarities of the voltages applied to the floating plate. Voltage measurement capability of the charged plate monitor was limited to 100 V. There were some measurements for which it was impossible to determine the voltage on the CPM at the given distance d between the floating plate and the CPM plate. In those cases the CPM sensor was placed at the distance from the floating plate for which the monitor was indicating the maximum measurable offset voltage. For example, (Table 2) when þ1 kV is applied to the floating conductor, the CPM displayed þ100 V maximum voltage at d ¼ 95 mm distance between the plates.
Fig. 5. Results of discharge tests without an ionizer, plate charged to þ200 V.
3.2. Discharge tests Figs. 5 and 6 show discharge characteristics, as measured by the second CPM, for the plate charged to þ200 V and 200 V, respectively. With no air ionizer the plate retains its charge for a very long time. For example, it took 90 min for a floating plate to discharge from þ200 V potential level to þ100 V. At the same time the potential of the first CPM sensor was monitored. The sensor was placed first at 25 mm, then at 250 mm and at 500 mm from the plate, consecutively. When the sensor was in close proximity to the floating conductor
Fig. 6. Results of discharge tests without an ionizer, plate charged to 200 V.
(25 mm), the first CPM was able to detect the voltage induced on the sensor plate and monitor it. When the distance between the floating plate and the sensor was increased to 250 and 500 mm, the voltage recorded by the first CPM was negligible (very close to zero). The
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M.A. Noras, D. Pritchard / Journal of Electrostatics 64 (2006) 310–315 Table 4 Discharge times (100% to 10% and 100% to 1% of initial voltage value) for 32 l/s ð68 ft3 = minÞ air volume flow rate
Fig. 7. Results of discharge tests with a 3-fan ð32 l=s ð68 ft3 = minÞ each fan), plate charged to þ200 V.
Fig. 8. Results of discharge tests with a 3-fan ð32 l=s ð68 ft3 = minÞ each fan), plate charged to 200 V.
Discharge voltage (V)
Discharge time (s)
þ1000–þ10 þ1000–þ1 1000–10 1000–1
11.0 14.3 12.8 14.7
ionizer
Fig. 9. Results of discharge tests with a 3-fan ð49:6 l=s ð105 ft3 = minÞ each fan), plate charged to þ200 V.
ionizer
Fig. 10. Results of discharge tests with a 3-fan ð49:6 l=s ð105 ft3 = minÞ each fan), plate charged to 200 V.
ionizer
ionizer
next two sets of results, with an ionizer supplying air ions into the space where the isolated, floating plate and the first CPM sensor are located, are presented in Figs. 7 and 8. In the ionizer controlled space the discharge time of the plate decreased very significantly (Table 4). The air flow produced by each of 3 fans of the ionizer was 32 l/s ð68 ft3 = minÞ. Figs. 9–10 present discharge characteristics of the plate with the air flow of 49.6 l/s ð105 ft3 = minÞ per fan, and the discharge times are presented in Table 5.
4. Discussion When a DC voltage was applied to the isolated plate with no ionizer present in the vicinity of the plate, the
Table 5 Discharge times for 49.6 l/s ð105 ft3 = minÞ air volume flow rate Discharge voltage (V)
Discharge time (s)
þ1000–þ10 þ1000–þ1 1000–10 1000–1
6.5 7.6 7.7 9.1
ARTICLE IN PRESS M.A. Noras, D. Pritchard / Journal of Electrostatics 64 (2006) 310–315
charged plate monitor sensor detected only the potential induced by the plate. After a balanced ionizer started delivering air ions into the space where the tests were conducted, motion of the ions were influenced by the electric field created by the plate. For example, the conducting plate with a negative potential applied to it was attracting the positive ions, repelling the negative ones at the same time. As a result, the space around the plate had excess of negative airborne ions. This effect was immediately detected by the CPM sensor as a negative voltage offset, signaling that the ionizer was out of balance. Note that ionizers equipped with an internal feedback only (from the outlet of the ionizer), normally would not react to such air ions imbalance, since the imbalance had been created on the work surface, not at the output of the ionizer. In order to bring the amount of positive and negative ions reaching the CPM sensor (and the ionizer-controlled surface area on which that sensor is located) back to balance, it would be necessary to use the signal from the sensor, not from the ionizer outlet, as a feedback to the ionizer. It had been observed that the voltage offset recorded by the CPM became larger when the air volume flow rate from the ionizer was increased. It is important to distinguish between air ion imbalance created by an isolated, charged object and by an object that carries a permanent DC voltage applied to it. A floating, charged item usually gets neutralized by an ionizer relatively quickly, depending on the size and initial charge of the object, as well as on the ionizer used for the task of charge neutralization and environmental factors such as humidity, temperature, etc. For example, a square, 15 cm by 15 cm metal isolated plate, having plate-to-ground capacitance of 20 pF ð2 pFÞ and charged to 1000 V would discharge to the voltage of 10 V under an ionizer within a couple of seconds (see Tables 4 and 5). This means that the distribution of airborne ions in the space around the charged object was disturbed by the electric field from the plate for the same, relatively short, amount of time. A DC-powered object disturbs the air ion field permanently, leading to possibility of an electric charge accumulation in the vicinity of that object. While observing discharge characteristics (Figs. 5–10) of the isolated, floating plate, it could be noticed that the CPM sensor in close proximity (25 mm) to the plate displayed an offset voltage overshoot when the potential of the floating plate dropped to zero. One possible explanation of this phenomenon is that the CPM sensor, bearing the capacitance of 20 pF, becomes charged due to excess of ions of either polarity. At the moment when the potential of the floating plate is close to zero and it
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does not affect the air ion distribution anymore, the sensor is still charged and it attracts charges of the opposite polarity until its potential reaches zero. At that instance, in close proximity of the sensor, there are still ions heading toward the sensor’s surface, and they cause the observed ‘‘overshoot’’ effect.
5. Conclusions Effect of the electrically charged objects on the ion balance of the corona ionizer has been investigated. It has been found out that when the conducting item is connected to a constant DC power source, its presence disturbs airborne ion distribution which in turn may lead to charge accumulation within the direct surrounding of the object. If the charged item carries a limited charge (is floated), the air ion distribution is perturbed only for the time necessary to neutralize the object. In either case, the ion balance in the vicinity of the charged object is altered. In order for the air ionizer to account for and correct the ion distribution disturbance at the protected area, it would be desirable to control air ion production based on the feedback information from the protected space, not from the outlet of the ionizer.
References [1] J. Chang, A. Berezin, Neutralization of static surface charges by a flow stabilized corona discharge ionizer in a nitrogen environment, J. Electrostat. 51–52 (2001) 64–70. [2] J. Chang, A. Berezin, Neutralization of static surface charges by an AC ionizer in a nitrogen and dry air environment, J. Appl. Phys. 91 (3) (2002) 1020–1025. [3] C. Noll, Balanced static elimination in variable ion mobility environments, J. Electrostat. 49 (2000) 169–194. [4] A. Steinman, Best practices for applying air ionization, EOS/ESD Symposium, 1995, pp. 245–252. [5] N. Jonassen, The physics of charge neutralization by air ions, EOS/ ESD Symposium, 1986, pp. 35–40. [6] D.M. Fehrenbach, R.T. Tsao, An evaluation of air ionizers for static charge reduction and particle emission, EOS/ESD Symposium, 1992, pp. 19–25. [7] L. Levit, A. Wallash, Measurement of the effects of ionizer imbalance and proximity to ground in MR head handling, EOS/ ESD Symposium, 1998, pp. 375–382. [8] EOS/ESD Association Standard for Protection of Electronic Discharge Susceptible Items—Ionization, ANSI ESD STM 3.12000, ESD Association, 2000. [9] D. Pritchard, Improving ionizer balance using external feedback sensors at the work surface, Trek Application Note No. 1003 available at: ohttp://www.trekinc.com/Download/1003FeedbackSensors.PDF4.