APPLICATION NOTE Non-contact surface charge/voltage measurements Fieldmeter and voltmeter methods Dr. Maciej A. Noras Abstract Methods of measurements of surface electric charges and potentials using electrostatic fieldmeters and voltmeters are discussed. The differences and similarities between those methods are presented. The AC-feedback voltmeter is also described as an unique method that combines advantages of both fieldmeters and voltmeters. 1 Introduction D1 is the amplitude of vibrations, [m], There is a broad variety of instruments that can ! is the circular frequency of vibrations, ! = 2πf measure an electric charge and/or voltage on [rad/s], where f is a frequency in [Hz], a dielectric or conducting surface. Electrostatic fieldmeters and voltmeters belong to the cate- A is the surface area of the sensing electrode, 2 gory of the most popular devices. This paper [m ], focuses particularly on the differences and simi- larities between electrostatic voltmeter and elec- is the relative electric permittivity of the mate- rial between the electrode and the surface trostatic fieldmeter methods. Both measurement ≈ techniques come with many variations due to an under test, 1 for air, extraordinary effort put into developing of low- is the electric permittivity of vacuum, = 8.85 · cost, accurate devices [1–10]. Which method is 0 10-12 [F/m]. better? Hopefully, the answer can be found in this application note. The current signal I is amplified and demodu- In order to make the comparison easier, consider lated using a phase-sensitive demodulator circuit a voltmeter and a fieldmeter, both using the Kelvin (Figures 1 and 2) to produce a voltage Vp di- vibrating capacitive sensor as the detecting ele- rectly proportional to the amplitude of the current. ment. Assume that the sensor and the surface Electrostatic voltmeters and fieldmeters utilize this under test can be modelled as a parallel-plate ca- method of detecting and conditioning of the sig- pacitor. In this configuration an electric current I nal. The difference is in the way the processed is being induced in the sinusoidally vibrating sen- signal Vp is utilized. sor [11, 12]. This current is proportional to the value of the electric potential present on the sur- face under test [12]: 2 Electrostatic fieldmeters dC I = U · dt Figure 1 presents an electrostatic fieldmeter. A · d 0A fraction of the detected and processed voltage V = U · ! = p dt D0 + D1 sin( t) is inverted and fed back to a screening electrode. ! ! · · D1 cos( t) At this point the sensing electrode is influenced = -U 0A 2 (1) [D0 + D1 sin(!t)] by two electric fields: one created by the tested surface and one generated by the screen. There- U is the difference of potentials between the fore, the greater the surface voltage, the greater tested surface and the vibrating probe, [V], the inverted voltage on the screen. Fields cre- D0 is a constant representing the separation ated by these two voltages cancel each other. Po- between electrode and the tested surface tentiometer P is used to establish a constant ratio when the electrode is not vibrating, [m], between Vs and the measured voltage Vp. When Non-contact surface charge/voltage measurements Fieldmeter and voltmeter methods the sum of the two fields equals zero, the stabil- of the Kelvin sensor oscillations: ity of the signal detected by the vibrating sensor 0 dV is greatly enhanced. However, the potential dif- I = C · t (2) dt ference between the surface and the sensor can lead to the discharge and damage of the equip- Therefore, when currents I and I' cancel each ment if spacing D0 becomes too small. The value other, of measured Vs is also sensitive to the changes of 0 the distance D0. I = I dC dV U · = C · t (3) dt dt 3 Electrostatic voltmeters As both I and I' currents are inversely proportional to spacing D0, the ratio of the amplitude of Vt to U (the DC test surface voltage) remains constant An example of the electrostatic voltmeter circuit over the large range of D0. As shown in Figure is shown in Figure 2. In this voltage-following 3, the Vt signal is obtained by amplification of the device the output of the integrator drives a high current I converted to a voltage at the preampli- voltage amplifier circuit to replicate the voltage on fier. At high gain the current I is being cancelled the tested surface. The amplified voltage is then to a very small value. applied to the sensor thus nullifying the electric field between the tested surface and the sens- ing electrode. Potential on the electrode "follows" 5 Summary the potential on the surface. In this case there is no threat of the eventual discharge between Figure 4 presents a comparison of measurement the probe and the surface under test, even at errors for a standard fieldmeter and the Trek close spacing. This ability of following the volt- model 520 electrostatic voltmeter. The data indi- age makes the electrostatic voltmeter measure- cate that it is important to keep the appropriate ment independent of the distance D0 - at least spacing between the fieldmeter sensor and the within a certain range of D0. If the span between tested surface in order to consider the measure- the surface and sensor is too big, the probe be- ment reliable. Table shows a brief comparison be- comes influenced by other electric fields present tween fieldmeter, electrostatic voltmeter and AC- in the vicinity. feedback electrostatic voltmeter. Because of their principle of operation, the electrostatic fieldmeters are suitable for measurements conducted on rel- atively large areas. They are also not as accurate 4 AC-feedback voltmeter as electrostatic voltmeters. Since the results pro- vided by the fieldmeters depend strongly on the The AC-feedback voltmeter uses a different tech- probe-to-surface distance D0, it is more conve- nique to achieve spacing independent surface nient to read them as the electric field intensity voltage/charge measurements [6]. Rather than values (thus the name, fieldmeter). Magnitude of cancelling the Kelvin current I by use of a feed- fields measured this way is usually high, therefore back DC voltage which follows the surface test there is a risk of discharges between the probe voltage to produce zero electric field, the AC feed- and the tested surface. Fieldmeters are less ex- back method utilizes a nullifying current I' to zero pensive than electrostatic voltmeters, since they the Kelvin current I. The current I' is produced by do not require high voltage circuitry to produce external generator circuit tuned to the frequency proper feedback to the sensor. There are also 2 advancedenergy.com Non-contact surface charge/voltage measurements Fieldmeter and voltmeter methods oscillator integrator bootstrapped pre-amplifier power supply amplifier vibrating phase sensitive V Kelvin demodulator sensor P surface under test Figure 1: Electrostatic fieldmeter [13]. oscillator integrator bootstrapped pre-amplifier power supply amplifier vibrating phase sensitive V Kelvin demodulator sensor circuit common surface under test Figure 2: Electrostatic voltmeter (voltage follower) [13]. oscillator pre-amplifier amplifier 1 amplifier 2 - + vibrating Kelvin sensor V V t surface under test Figure 3: AC-feedback electrostatic voltmeter [6, 10]. advancedenergy.com 3 Non-contact surface charge/voltage measurements Fieldmeter and voltmeter methods 200 Trek Model 520 ESVM fieldmeter 180 160 140 120 tested with 1 [kV] applied to the 310 [cm2] circular plate 100 error, [%] 80 60 40 20 0 5 10 15 20 25 distance from the surface under test, [mm] Figure 4: Comparison test between electrostatic voltmeter and fieldmeter. 4 advancedenergy.com Non-contact surface charge/voltage measurements Fieldmeter and voltmeter methods other types of fieldmeters available, for example charge. However, the person conducting mea- radioactive fieldmeters, rotating vane units. Even surement has to be aware of the high voltage though their construction and principle of opera- present on the probe and proceed with caution. tion are relatively simple, they suffer from disad- AC-feedback voltmeter is a low-cost alternative for vantages such as presence of the radioactive ma- the voltage follower type voltmeter. It does not terial, poor accuracy and high power consumption have high voltage circuitry and is accurate within by the drive motor of the rotating vane device. a certain specified range of distances D0. For ex- Electrostatic voltmeters, particularly the voltage ample Trek's model 520 holds the 5% accuracy followers, can be employed for tests of relatively over the distance between 3 and 30 [mm] [10,14]. small charged areas - they have much better res- There is a risk of discharges between the probe olution than fieldmeters. Voltmeters are also very and the tested surface, so the resolution of the accurate over a certain range of distances D0. AC-feedback voltmeter is limited by the distance Since the potential on the sensor during the mea- D0. Table 1 summarizes features and disadvan- surement is theoretically equal to the potential tages of the electrostatic fieldmeters and DC and of the tested surface, there is no hazard of dis- AC-feedback voltmeters. Electrostatic DC-feedback ESVM AC-feedback ESVM fieldmeter general recommendation for tests of large large and small large and small surfaces surfaces surfaces measured variable electric field voltage voltage intensity cost low high medium spatial resolution poor very good good accuracy good at the large excellent at the very good within the probe-to-surface small specified distance probe-to-surface probe-to-surface distance distance probe potential ground (possibility potential of the ground (possibility of arcing) tested surface of arcing) distance independent no within a certain, within a broad range specified range (depends on the (depends on the probe type) probe type) Table 1: Overview of features References 3525936, 1970. [1] R.