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Научни трудове на Съюза на учените в България - Пловдив. Серия В. Техника и технологии. Том XVII, ISSN 1311 -9419 (Print); ISSN 2534-9384 (Online), 2019. Scientific Works of the Union of Scientists in Bulgaria - Plovdiv. Series C. Technics and Technologies. Vol. XVII., ISSN 1311 -9419 (Print); ISSN 2534-9384 (Online), 2019
A NON-CONTACT VOLTAGE MEASUREMENT SYSTEM USING
SURFACE POTENTIAL SENSOR Dimitar Tokmakov, Sotir Sotirov , Nikolay Vakrilov, Raicho Minchev University of Plovdiv "Paisii Hilendarski", Plovdiv, Bulgaria
Abstract: This work presents the results of a new design of a complete non-contact voltage acquisition system, based on surface potential sensor EFS-22D by TDK. The system is capable to measure DC voltages from 0-950V with distance between the surface potential sensor and the probe within 1 - 3.5mm. The signal from EFS-22D sensor which is in the range from 04.5 V dc is converted from the built in analog to digital converter in ATMEGA328 microcontroller. The results from the measurements are send to LabVIEW application using Bluetooth module. The LabVIEW application provides interface for further data acquisition and visualization.
Keywords: LabVIEW non-contact voltage measurement, surface potential sensor, ESD measurement and control;
Introduction: There is a wide range of devices that can measure the electrical charge or potential of dielectric or conductive surfaces without electrical contact with them.
Electrostatic voltmeters belong to the category of the most popular devices of this type. This paper focuses on the design and construction of a microprocessor-controlled electrostatic voltmeter using a surface potential sensor. Non-contact measurement methods give a quantitative estimate of the electric field, surface potential, and surface charge distribution. The main advantage of this type of measurement is that it does not change the status of the object under investigation [1]. The absence of an electrical contact during the measurement excludes the possibility of a charge transfer between the measuring instrument and the surface being investigated. These methods are extremely important and widespread in measurements where a very high input impedance of the meter is required [2].
Materials and methods:
The EFS-22D surface potential sensors from TDK [5] is used and are featuring better than ±0.05V detector output variation noise. This voltage sensor is constructed using TDK's unique, high precision, highly stable detection circuit which feeds the measured surface potential back to the electrical field density control chopper and probe shield cover. Output of this probe is highly precise and quite stable, almost unaffected by temperature fluctuations or probe spacing (probe positioning). The adverse effects of electrical disturbances between the measured surface and the sensor electrode are greatly reduced. [5]
Figure 1 shows the construction of EFS-22D sensor. It consists from sensor electrode, electric field line blocking shutter, chopper, and board ground plane and shield case with window which forms the measuring aperture. [5]
The chopper is vibrating sinusoidal at 700Hz by the use of piezo element. The EFS-22D mechanism increases and decreases the number of electric field lines (electrical-mechanical energy conversion) by utilizing flexure of the oscillating shutter to periodically increase-decrease the effective area of a voltage element placed near the oscillating shutter. [5]
Figure 2 shows the architecture of the measurement system for non-contact voltage measurement using surface potential sensor EFS-22D by TDK. The Kelvin type vibrating electrodes sensor is placed in fixed block holder at distance between 0.5 and 3.5 mm from the measuring surface. The surface potential, measured by the probe of the sensor, is converted into analogue voltage, varying from 0 to 4.5 V, which is then converted into a digital value by built in 10 bit ADC in microcontroller ATMEGA328. The results from measurements are send to LabVIEW application to a PC via Bluetooth module HC-05.
A DC power supply module of 24V is used to power the surface potential sensor converter.
Figure 3 shows the user interface of the virtual instrument developed with LabVIEW 2018 evaluation version. For connection between the electrostatic voltmeter and the virtual instrument, the serial port is used with the NI-VISA driver.
The built-in microcontroller firmware for ATMEGA328 performs an analogue-to-digital conversion of the input signal from the EFS-22D sensor to the analogue input A0. ATMEGA328 receives the number of measurements per second on a serial port from the virtual instrument, this parameter ranging from 1 to 20. The internal measurement cycle is then started and, depending on the amount of measurements taken after the end, the average value is calculated and sent to the virtual instrument via serial port for visualization and post-processing.
The LabVIEW virtual instrument provides digital and analog visualization of measurement data as well as graphic printing through a dedicated object waveform chart that visualizes the amplitude of the measured electrostatic voltage over time or number of digital samples sent by the microcontroller ATMEGA328.
Microcontroller BlLetooth ATMEGA3Z3 module HCflB
Fig.2 Architecture of non-contact voltage measurement system using EFS-22D surface potential sensor and LabVIEW application
Fig.3 LabVIEW Virtual instrument user interface
Results and discussion: Fig. 3 shows the developed LabVIEW virtual instrument which allows user to start and stop the measurement and to visualize voltage curves over number of samples/time. The use of LabVIEW as a development environment for non-contact voltage measurement allows the system to be used as a control equipment for electrostatic discharge in a production environment.
Although the EFS-22D sensor has a linear output voltage vs. detector voltage characteristics [5] , our measurements showed that at the beginning of the 0-50V interval there is some non-linearity that can be compensated by the software of the virtual instrument. The non-linearity test of the electrostatic voltmeter was done by a series of 20 measurements of different sample voltage values ranging from 0-950 V, using Keithley 2015 (6 % digits) for the reference voltmeter.
Based on the measurements a calibration curve is built, which is approximated with the linear equation y = 1.0032x + 2.1944 with coefficient of correlation R2=0.998.
The resulting calibration equation is used by the virtual instrument measurement software for linearization of the electrostatic voltmeter in the 0-950V range.
Conclusion: The measurement system presented in this paper measures electrostatic voltage in the range from 0-950V within 0.5-3.5mm distance between the measuring probe and the measured surface.
Achieved accuracy in the range of 0-950 V is 500 mv, which allows the measuring system to be used for various applications as well in university education in physics and electronics.
The voltmeter proposed in this paper can be used for studying various types of dielectric materials, as well as for investigating voltage sources with ultra-high internal resistance.
The use of LabVIEW virtual instrument software allows the system to be used in industrial production environments for express ESD measurement and control.
ACNOWLEDGMENT
This work was funded by the University of Plovdiv "Paisii Hilendarski" science fund NPD reference No.MU17-FF-015
References:
[1]. M.A. Noras, A.Pandey, "Surface charge density measurements. Uses and limitations of kelvin probe based instruments," IEEE Industry Applications Magazine, vol. 4, pp.41-47, July-Aug. 2010 10772618/10/$26.00©2010IEEE
[2] M.A. Noras, "Non-contact surface charge/voltage measurements. Field meter and voltmeter methods," Trek Application note Number 3002. [Online].
[3] F Rossi, G. I. Opat, and A. Cimmino. Modified Kelvin technique for measuring straininduced contact potentials. Rev. Sci. Instrum., 63(7) pp3736-3743, 1992.
[4] D. M. Zacher. Feedback-based field meter eliminates need for HV source. EE Eval.Eng., pages S43-S45, November 1995.
[5] Surface potential sensors EFS series, TDK Application note.
