Intro: E-Field Mill
You may already know that i'm addicted to any kind of sensor measurement applications. I always wanted to track down the fluctuations of earths magnetic field and i also was fascinated by measuring the ambient electric field of the earth that is maintained by charge separation processes taking place between the the clouds and the surface of the earth. Incidents like clear sky, rain or thunderstorm all have an dramatic impact on the electric field that surrounds us and new scientific findings are showing us that our health depends heavily on the surrounding electric fields.
So, that's the reason why i wanted to make myself a suitable measuring device for static electric fields. There is already one pretty good design existing, also called electric field mill that is widely used. This device uses an effect called Electrostatic Induction. This always happens when you're exposing a conductive material to an electric field. The field attracts or repels the free electrons in the material. If it is connected to ground(earth potential) charge carriers are flowing in or out of the material. After disconnection of the ground a charge remains on the material even if the electric field vanishes. This charge can be measured with a voltmeter. This is very roughly the principle of measuring static electric fields.
A few years ago i built a field mill according to plans and schematics i found in the internet. Principally it consists of a rotor with some kind of propeller on it. The propeller is a twin set of metal segments that are grounded. The rotor turns around a set of induction plates that are electrically covered and uncovered by the rotor. Every time they are uncovered the electrostatic induction of the ambient electric field causes a flow of charge carriers. This flow is reversed when the rotor covers again the induction plates. What you get is an alternating more or less sinusoidal current which amplitude is a representation of the strength of the measured field. This is the first flaw. You don't get a static voltage showing the field strength but have to take the amplitude of an alternating signal that has to be rectified first. The second issue is even more tedious. The field mill works pretty well in an undisturbed environment -lets say on the dark side of the moon when you're far away from power line hum and all this abundant electric mist that is penetrating our environment everywhere we are. Especially the 50Hz or 60Hz power line hum interferes directly with the desired signal. To tackle this problem the field mill uses a second set of inductions plates with another amplifier that takes the same signal with a 90° phase shift. In an additional operational amplifier both signals are subtracted from each other. Because they're out of phase a remainder of the desired signal remains and the interference, which is equal in both signals, is cancelled out theoretically. How good this works depends on the equality of the interference in both measurement circuits, the CMRR of the amplifier and on the question if the amplifier gets overdriven or not. What makes the situation even more uncomfortable is that you roughly doubled the amount of hardware just to get rid of the interference.
Last year i had an idea to overcome those problem with my own design. It's a little more work on the mechanic but simple in question of electronics. As always this is not a detailed step by step replication of complete device. i will show you the working principles on my design and you may change it in different ways and tailor it to your own needs. After showing you how to build it i will explain how it works and show you the result of my first measurements.
When i got the idea for this device i was proud to the bones but as you know arrogance is preceding any downfall. Yes, it was my own idea. I developed it on my own. But as always there was someone before me. The separation of charges by induction and amplification by using the capacitor effect was used in nearly every electrostatic generator design during the last 150 years. So there's nothing special about my design despite the fact that i was the first one who thought about applying those concepts for measuring weak electrostatic fields. I still hope one day i will be famous.
Step 1: List of Materials and Tools
The following list shows roughly which materials you will need. You may change and tailor those as much as you want.
- Sheets of 4mm plywood
- timber beams 10x10mm
- 8mm aluminum tube
- 6mm aluminum rod
- 8mm plexiglass rod
- 120x160mm single side copper plated PCB
- brass or copper wire 0.2mm
- a piece of 0.2mm copper sheet
- 3mm screws and nuts
- A 4mm test socket
- conductive rubber tube (Inner diameter 2mm) I got mine from amazon
- Electronic parts according to the schematic(download section)
- A 100nF styroflex capacitor as a collector for the charges. You may change this value in wide ways.
- A capstan motor for 6V DC. These are motors that were especially designed for disc players and tape recorders. Their rpm is regulated! You still can find them on Ebay.
- A 6V/1A power supply.
These are the tools you need
- Soldering iron
- Arduino development environment on your PC/Notebook
- USB-A to B cable
- file or better a lathe
- electric drill
- small buzz saw or hand saw
- wire cutter
Step 2: Making the Mechanics
In the first picture you can see the whole design is based on two sheets of plywood 210mm x 140mm in dimension. They are mounted above each other, connected by 4 pieces of timber beams that keeps them 50mm in distance. Between both sheets the motor and the wiring is contained. The motor is mounted with two M3 screws fitting in two 3mm holes drilled through the upper plywood sheet. A sheet of PCB material is working as a shield against the ambient electric field. It is mounted 85mm above the upper plywood sheet and its inner edge is just ending about the motor shaft.
The core component of this device is a disk. It has a diameter of 110mm and is made out of single side copper coated PCB material. I used a mill to cut out a round disk of of the PCB. I also used a mill to cut the copper coating into four segments that are electrically insulated. It is also very important to cut a ring around the middle of the disk where to motor shaft will go through. Otherwise it would electrically ground the segments! On my lathe i cut a small piece of 6mm aluminum rod in a way that it takes a 3mm hole at the bottom with two rectangular 2,5mm holes that have M3 threads cut in. The other end i cut down to a small 3mm shaft to fit in the middle hole of the disk. The adapter then was super-glued to the bottom of the disk. The disk assembly could then be screwed to the motor shaft.
Then you see another important component. A segment of the size of the ones on the disk, made out of 0,2mm copper sheet This segment is mounted on two sheets of plywood. When the disk is mounted this segment is very narrowly under the rotating disk. the distance is just about 1mm. It's important to keep this distance as small as possible!
The next important things are the ground whisker and the charge pick-up. Both are made of aluminum tube and rods with cut in threads to mount them all together. You can do any type of variation you like here. You just need something conductive running over the surface of the disk. For the whiskers i tried a lot materials. Most of them were damaging the disk segments after a while. Finally i found a hint in a book about electrostatic devices. Use conductive rubber tubing! It's not damaging the copper coating and wears and wears...
The ground whisker is placed on a location in a way that it loses contact to the underlying disk segment when it is starting to uncover the ground plate. The charge pick-up is placed such that it takes the segment in the middle when it's at maximum distance from the ground plate. See that the charge pick-up is mounted on a piece of plexiglass rod. This is important because we need a good insulation here. Otherwise we would have a loss of charges!
Then you see that the 4mm test socket is placed in the "basement" of the assembly. I provided this connection because i was not sure if i would need a real "ground" connection or not. Under normal conditions we are dealing with such low currents that we have an intrinsic grounding anyway. But maybe there will be a test setup in the future where we might need it, who knows?
Step 3: The Wiring
Now you have to electrically interconnect everything so that it works properly. Use the brass wire and solder together the following parts.
- The 4mm test plug
- The ground whisker
- The shield
- one wire of the charge collect capacitor
Solder the 2nd wire of the capacitor to the charge pick-up.
Step 4: Making the Electronics
Follow the schematic to place the electronic components on a piece of perfboard. I soldered pin headers to the edges of the board to connect it with the Arduino Uno. The circuit is damned simple. The collected charge is picked-up at the capacitor and fed into an high-impedance amplifier which boosts the signal by 100. The signal is low-pass filtered and then routed into one input of the arduino's analog-to-digital converter inputs. A MOSFET is used for the Arduino to switch on/off the disk motor.
It's very important to connect the ground of the mechanic assembly to the virtual ground of the electronic circuit which is where R1/R2/C1/C2 meet! This is also the ground of the charge collecting capacitor.
Step 5: The Software
There isn't much to say about the Software. It's written very straightforward. The application knows some commands to get configured properly. You can access the arduino if you have the Arduino IDE installed on your system because you need the virtual comport drivers. Then plug an USB cable to the arduino and your PC/Notebook and use a terminal program like HTerm to connect the arduino via the emulated comport with 9600 bauds, no parity and 1 stopbit and CR-LF on enter.
- "setdate dd-mm-yy" sets the date of the RTC-module connected to the arduino
- "settime hh:mm:ss" sets the time of the RTC-module connected to the arduino
- "getdate" prints date and time
- "setintervall 10...3600" Sets the sampling intervall in seconds from 10s to 1h
- "start" starts the measurement session after syncing to the upcoming full minute
- "sync" does the same but waits for the upcoming full hour
- "stop" stops the measurement session
After receiving "start" or "sync" and doing the synchronization stuff the application first takes a sample to see where the zero-point or bias is. Then it starts the motor and waits 8s for the rpm to stabilize. Then the sample is taken. Generally there is a software averaging algorithm that continuously averages the samples over the last 10 samples to avoid glitches. The previously taken zero-value is now subtracted from the measurement and the result sent over the comport together with date and time of the measurement. A example of a measurement session looks like this:
03-10-18 11:00:08 -99
03-10-18 11:10:08 -95
03-10-18 11:20:08 -94
03-10-18 11:30:08 -102
03-10-18 11:40:08 -103
03-10-18 11:50:08 -101
03-10-18 12:00:08 -101
So, the measurements are shown as deflections from zero measured in digits which can be positive ore negative depending on the spatial direction of the electric flux. Of course there's a reason why i decided to format the data in columns of date, time and measurement values. This is the perfect format to visualize the data with the famous "gnuplot" program!
Step 6: How It Works
I just told you that the working principle of this device is electrostatic induction. So how it works in detail? Lets assume for moment we would be one of those segments on the disc. We are rotating at a constant speed continuously being exposed to the ambient electric field and then hiding again from the flux under the protection of the shield. Imagine we actually would get out of the shadow into the field. We would get in contact with the grounding whisker. The electric field would act on our free electrons and lets say the field would repel them. Because we are grounded there would be an amount of electrons fleeing from us and vanishing in the earth.
Now, while the turning of the disk continues at some point we would lose contact to the ground whisker. Now no more charge can flee from us but the way back for the charges already gone is also closed. So we are left behind with a lack of electrons. If we like it or not, we are charged now! And our charge is proportional to the strength of the electric flux.
How much charge do we have?
During the time we got exposed to the electric field we lost some electrons. How much have we lost? Well, with every electron we lost, our charge climbed up. This charge generates an rising electric field of its own between us and the ground. This field is opposite to the ambient one which generated the induction. So the loss of electrons continues up to the point where both field are equal and cancel out each other! After we lost contact with ground we still have our own electric field against the grounded plate which has earth potential. You know how we call two conductive plates with an electric field in between? This is a capacitor! We are part of charged capacitor.
We are a capacitor now!
You know the relation between charge and voltage on a capacitor? Let me tell you, it is U=Q/C where U is the voltage, Q is the charge and C the capacity. The capacity of a capacitor is reversely proportional to the distance of its plates! That means the wider the distance the lower the capacity. Now what happens while we keep on turning on the wheel with no contact to ground? We are increasing the distance to the ground plate. While we're doing this our capacity falls dramatically. Now look again at U=Q/C. If Q is constant and C is declining, what happens? Yes, the voltage is rising! This is a very clever way to amplify the voltage by just applying mechanical means. You don't need an operational amplifier, noise filtering and statistical computing here. It's just clever and plain physics that boosts our signal up to a level where signal processing with electronics just becomes a boring tasks. All the cleverness of this device relies on electrostatic induction and the capacitor effect!
What does it mean?
But what exactly did we boost in this way? Do we have more electrons now? No! Do we have more charge anyway? No! What we boosted is the ENERGY of the electrons and this is what enables us to use simpler electronic circuits and less filtering. Now we reached the aphel of our trajectory and finally the charge pick-up takes our energized electrons and collects them into the charge collector capacitor.
Immunity against interference
When you take a look at the video you'll see that despite the usual interference in my home the output signal of the device is steady and practically noise free. How is this possible? Well i think its because signal and interference are not going separate way up to the amplifier as in the classic field mill. In my design the interference affects the collected charge right from the moment on the connection to ground is lost . That means every sample is affected in some way by interference. But because this interference has no DC component as long as it's symmetrically, the interference result is always averaged-out in the charge collector capacitor. After enough disc turns and samples fed in the charge collector the average of the interference is zero. I think that's the trick!
Step 7: Testing
After some testing, debugging and improving i installed the field mill together with my old win-xp notebook in my attic and did a test run over approximated one day. The results were visualized with gnuplot. See the attached data file "e-field-data.dat" and the gnuplot configuration file "e-field.gp". To view the results just start gnuplot on your target system and type at the prompt >load "e-field.gp"
See the picture showing the results. It's quite remarkable. I started the measurement on 2018-10-03 when we had fine weather and blue sky. See that the electric field was pretty strong and negative, while we have to take care because whats "negative" and whats "positive" currently is not reasonable specified. We would need a calibration of our device to align with real physics. But anyway, you can see that over the measurement cycles the field strength went down alongside with the weather starting to deteriorate and becoming cloudy and rainy. I was somehow amazed about those findings but still have to check if these correlate with physics.
Now it's your turn. Go on and make your own electric field mill and explore the secrets of our planet on your own quest! Have fun!