November 27, 2021
November 27, 2021
In the dry season, we often get electric shock from the metal objects, especially door knobs. This is due to the static electricity charged in the human body. Any object can be electrificated and, normally, the electric potential of the object reaches several kilovolts or higher. The static electricity often causes malfunction or damage to the electronic devices.
I have wanted to know how much static electricity is charged in the object or human body, so that I built a surface potential meter to measure the electric potential of the electrificated objects.
Static electricity is an electric charge in/on the object as shwon in Fig.1a. An object gets electric charge is termed electrification. The electric potential of object is proportional to the amount of electric charge from the formula Q = CV, where Q is the amount of charge, C is the capacitance of object and V is the electric potential. The unwanted electrification occures mainly by triboelectric effect, in two different materials contact and separate, rather than charging by a DC supply showin in this figure. Note that the positive charges are illustrated as moving charged particle in the figure, but the electron, a negative charge, is the only charged particle which can move in the solid conductive materials, so that the positive charge in the figure is positive imbalance of electric charges due to less electrons.
Unlike generic voltage measurements, the static electricity needs to be measured in non-contact, because the potential of electrificated object is relatively high, in kV order, and the electric charge immediately drains away from the object when a contact probe is attached as shwon in Fig.1b. The charge drained out will able to be measured, but the voltage cannot be determined in only amount of charge and the measurement is destructive, it changes electric charge of the object. Also the contact probe cannot be attached in many case, because the electrification occures not on only conductive object but on the surface of non-conductive materials, such as plastic. Well then, how we can measure the surface potential of the electrificated object?
Electric charge produces an electric field surrounding the charge as shwon in Fig.2. The field's strength and direction in the electric field are expressed by density and direction of the electric field line. The field's strength produced by a point charge is proportional to the amount of charge and inversely proportional to the square of distance from the charge. Thus the electric potential proportional to the amount of charge can be determined by detecting the field's strength nearby the object at the certain conditions.
There are many methods to measure the field's strength. To measure the static field, a mechanism as shown in Fig.3 is usually used. The detector electrode tied to the ground is placed in the shieleded box and the box has a window that opened/closed repeatedly on the detector electrode. When the window is closed (a), the electric field from the object is shielded and does not come in the case. When the window opens to the electrificated object (b), the detector electrode is exposed to the electric field get into the case, as the result charges attracted by the electric field move into the detector electrode from the ground. When the window closes again and the electric field in the case is lost, the charges in the electrode drain to the gorund by repulsion force each other.
By chopping the electric field with some mechanism, charges move in and out the detector electrode. The amount of charge moved by an electric field is proportional to the field's strength and the capacitance of electrode. The movement of electric chages is just electric current and the field's strength can be output as a DC voltage by processing the current to the detector electrode as shown in Fig.4. This configuration works as chopper stabilized amplifier that can detect small AC signals stably.
Fig.5 shows the schematic of the built surface potential meter. The charges moving in/out to the detector electrode is converted into the voltage by a transimpedance amplifier. The detected voltage is captured by an A-D converter of the MCU and then processed in software and displayed into the 3-digit 7-segment display.
The detector electrode needs to be held at a certain distance form the surface being measured. To set it easy, the surface is lit by two crossing laser beams. The distance at the laser spots get overlaped is the right distance, 40 mm for this meter, and the center of measureing circle, about φ40 mm in this case.
There are some mechanisms being used to chop the electric field. Real implementations tend to use a tuning fork type chopper. In this project, a simple rotary shutter, rotating disc with apertures, is used. The grounded shutter disk is driven by a CD spindle motor at 500-1000 RPM and the aperture timing is detected with a photo reflective sensor.
A plastic case SW-100 from Takachi is used for the body. The case is painted with nickel based conductive paint to make it electric shelded box. At the beginning, I painted only inside of the case but the outer surface is easily electrified and it causes an offset error in the reading, so that both side of the case needs to be painted. When use a metal case, it will need to be in bare metal inside and outside. A banana jack on the body is for grounding.
The A-D conversion is triggered in 5 kHz by TC1.OC1B. The measurement process is driven by the A-D converson complete interrupt and the process advances by shutter rotation. Measurements are done and the reading is updated every 6 rotations, the fisrt rotation for auto-null, following 4 rotations for measurement and the last rotation for battery monitoring.
To get a correct reading, the measurement needs to be done under the certain conditions in the measuring object as follows:
Reading is the relative electric potential of the object to the meter body. The measurement range is within the voltage that does not saturate amplifire output, ±50 kV in this case. If the amplifire saturated, it reads "Err" to indicate the overscale. To measure the absolute electric potential, the meter body needs to be grounded. When measure it in hand-held without the gorunding of meter body and human body, the reading is relative electric potential to the human body. Measureing a gounrded surface will read electric potential of the human body in reverse polarity.
This is the same way as normal measurement but a jumper setting. The calibration mode is enabled by setting the PB3 pin low level. On power-on, detector window is covered by a grounded conductive object to get a zero-level. After zero-level calibration, "CAL" is displayed for next step. Set the meter as measuring a metal plate with +5.00 kV of reference voltage applied and turn power off. A gain calibration factor is calculated to read it +5.00 kV and stored into the EEPROM on power off.
For the calibration process of this meter, I built a High Voltage Reference Generator prior to this project.
