What Is an Electrostatic Instrument?
An Electrostatic Instrument is device that measures the voltage of static electricity generated on the surface of an object.
This instrument is equipped with a surface potential sensor that enables non-contact measurement when pointed at the object being measured. Electrostatic Instruments are also called Surface Potentiometers or Electrostatic Potential Meters, and are mainly used in production processes in the manufacturing industry.
Uses of Electrostatic Instruments
Electrostatic Instruments play a crucial role in addressing issues caused by static electricity during the production process. Accurate measurement of static electricity is the first step in implementing preventive measures and evaluating their effectiveness. Depending on the process, these instruments may also be used for continuous monitoring of static electricity generation.
1. Foreign Matter Adhesion
Trouble occurs when charged foreign matter, such as dust, adheres to a charged product. For example, in the painting process, this can cause unevenness in the coating.
2. Electrostatic Destruction (ESD Destruction)
Semiconductor components such as integrated circuits can be destroyed by electrostatic discharge.
3.Malfunction
Devices that operate with small currents and voltages, like electronic balances, weight checkers, and metal detectors, may be affected by electromagnetic noise when static electricity is discharged.
4.Discharge to the Human Body
Electrostatic discharge from a charged object to the human body not only causes pain and discomfort to the human body, but can also cause equipment malfunction and ignition of combustible materials.
Principle of Electrostatic Instrument
When the surface of an object is charged and static electricity is generated, an electric field is generated around the object. Electrostatic Instruments measure the strength of this electric field and calculate the electrostatic voltage. The principle of a typical Electrostatic Instrument, such as Surface Potentiometer, is as follows:
The surface potential sensor uses the electrostatic induction phenomenon. When the sensing electrode receives an electrostatic field intensity Eo (proportional to the charging voltage Vo) from a charged object, an induced charge q is accumulated on the surface of the sensing electrode. When a shield plate of a size that covers the entire detection electrode is placed between the detection electrode and the object and rotated at a constant speed, the induced charge q accumulated on the detection electrode is discharged at the moment the shield plate covers it, and when the shield plate passes by, the induced charge q is accumulated again. This periodic movement of charge q, that is, the magnitude of the alternating current Is, depends on the electric field strength, so the charging voltage Vo of the object surface can be obtained by measuring the current Is.
However, in the this measurement method, the measured value depends greatly on the distance between the surface potential sensor and the object to be measured. The further away the object is from the sensor, the weaker the electric field becomes, so it is inevitable that the measured value will appear smaller. Therefore, it is necessary to keep the distance between the surface potential sensor and the object to be measured at the specified distance.
To address this distance-dependent measurement issue, another device is the voltage feedback-type surface potential meter. In this system, a high-voltage power supply is connected to the sensing electrode, and the voltage output of the high-voltage power supply is adjusted so that the alternating current Is becomes zero. Since the current Is stops flowing when the voltage of the object to be measured and the voltage of the sensor are the same, the output voltage of the high-voltage power supply at that time can be said to be equal to the charged voltage of the object to be measured.
How to Use Electrostatic Instruments
To measure the electrostatic charge of a charged object with a typical Electrostatic Instrument, the procedure is as follows:
1. Place the surface potential sensor at the distance specified by Electrostatic Instrument, with the detection electrode of the surface potential sensor parallel to the surface to be measured.
2. Set the measuring range higher than the expected voltage and start measurement. When a rough measurement value is obtained, adjust the measuring range and adopt the measured value.
When measuring with a voltage feedback type surface potential meter, the detection electrode of the surface potential sensor is placed parallel to the surface of the object to be measured, but the distance from the object does not need to be strictly defined. The voltage of the high-voltage power supply is gradually increased to find the point where the alternating current flowing to the detection electrode becomes zero. The output voltage of the high-voltage power supply at that point becomes the charged voltage of the object to be measured, and the unit of measurement value is V or KV. It is important to select an appropriate instrument based on the assumption of the maximum static electricity voltage.
Other Information on Electrostatic Instruments
Causes and Prevention of Static Electricity
The following mechanisms are known to generate static electricity:
1. Peeling Charge
This occurs when overlapping materials are peeled off, such as when a protective film is pulled off a plastic sheet.
2. Friction Charging
Occurs when objects rub against each other, such as when mixing things, taking off clothes, or when motors rotate.
3. Other Charging
Electrostatic Instrument is used to measure the electrostatic charge of objects exposed to such conditions, such as grinding and powder charging. In particular, semiconductor components are highly likely to fail due to Electrostatic Instrument discharge, so it is necessary to check the inside of the process with Electrostatic Instrument on a regular basis.
The following are specific examples of measurement targets in the production process:
- Trays containing semiconductors, other electronic components, and their parts, their protective films, and storage shelves.
- Work clothes, work shoes, work process desks, chairs, and floor surfaces.
- Grounding bands and grounding attachments.
- Manufacturing equipment, such as production equipment, inspection equipment, jigs and tools, soldering irons, etc.
- Work standards and their protective plastic cases.
- Display parts of monitors.
- Various types of purchased films.
When the generation of static electricity is unavoidable, ionizers and other static eliminators are installed to actively eliminate static electricity, but measurement using Electrostatic Instruments is essential to ensure the effectiveness of such measures.