The Excavation Electronics Challenge
Jameco Customers Lend a Helping HandLong time Jameco Customer Greg Miller wrote to us asking for some help finding a piece of re-bar buried in his yard so he could identify his property line. A traditional metal detector wasn't an option because there is "too much junk in the ground, and too many minerals, and the metal detector has a rather wide target area." We threw this design challenge to our community of experts, and like always you came through with some really fantastic tips and ideas!
Many of you provided a simple tip that doesn't use electronics; it's a method referred to as "dowsing" or "witching." Bend two re-bars (or clothes hangers, etc.) to 90 degrees. Walk, holding one in each hand. They will cross when they encounter a metal object or water pipe (due to being in a state of unstable equilibrium). This method has been called a superstition, but many of you attested to its validity and even suggested YouTube videos to view the method in action.
Others recommended apps like the iPhone SensorKinetics app or Metal Sniffer or Metal Detector apps for Android.
Here are the best of the best tips and information regarding this design challenge.
Richard Gabric: No design ideas here, but a brief summary of what I have come across over the years.
The two most common steel detecting systems with good sensitivity are the flux gate and proton precession magnetometers. I worked on both in my capacity as a geophysics electronics technician some years ago. We typically used the proton magnetometer in looking for steel objects, one involved looking for the bucket off a digger, which had fallen to the bottom of a lake.
The proton magnetometer usually has a slower sampling rate than the flux gate, since it's in its simplest form, and operates in a discontinuous mode. There are, I am sure, circuits already out there which could be used.
The proton magnetometer is an absolute total force device, is not dependent on head orientation, and is very rugged, so it is often for that reason used in survey work, despite the quite large sensing head. The head is often filled with kerosene or water; you have to watch the ambient temperature depending on the head contents. The flux gate needs three heads, copper wire wound around a core material of ferrite or Permalloy, with an appropriate BH curve. They are oriented in the X, Y, and Z planes, if you want it to behave like the proton magnetometer, and it is not absolute.
Neither type is cheap or easy to build if you want good sensitivity. In all cases, the earth's field is present as an "ambient" field; what the sensors detect is the local disturbance to the ambient field brought about by the steel, or "magnetic anomaly."
William Morgan: I have been trying to design such a thing for a while. My background is in geology, and before the fluxgate magnetometer, the magnetic surveys were done by a magnetic dip meter. Most valuable minerals had magnetite with them, so when a magnetic dip compass came near the deposit, the compass needle would deflect.
My design calls for an Arduino, a 2x16 character LCD display shield, a GPS, and a BN055 sensor, but for a magnetic locator the GPS would not be needed. Most the work is done by the BN055. The Arduino and shield are mounted on a board with a bull's eye level. The BN055 sensor is mounted on a boom from the main board (try 12") to minimize hard iron effects.
To use the magnetic locator, the instrument is leveled by the bull's eye and then a straight transit is walked by heading towards a distance point. Over a strong magnetic object, the Euler vector angles will change.
Len Poma: A simple non-powered magnetic locator can be as simple as a neodymium magnet suspended from a string. With the magnet suspended within an inch over the pavement where the re-bar is believed to be. If the magnet deflects away from what should be gravity (straight down), it is pointing in the direction of iron.
Jim Weir: Sometimes electronics is the SECOND best way to do something. A dollar store kid's compass, held close to the ground, is a wonderful "manual magnetometer;" it does not react to anything except ferrous metal. As you get closer and closer to the buried re-bar, instead of the compass tracking north, it will begin to track the re-bar. As you get closer and closer to the actual re-bar, the effect will get stronger and stronger. When you get VERY close, the compass needle will be pulled away from its horizontal swing and begin to dip towards the re-bar. Now map the area, and where the dip is the strongest in both the X and Y directions; dig.
Now go down to Uncle Joe's Surplus Store and see if you can find a watch with a radium dial. Or break a dead smoke detector, or get some "depression green glass" that is colored with uranium. Bury the radioactive parts of any of these alongside the concrete re-bar when you cover it back over. Next time you can use a Geiger counter (which IS electronic and CAN be home-made from scratch) to find the re-bar.
John Ferreira: I'd use an old AM radio as a metal detector, it's cheap if not free (if you have stuff laying around). I built one when I was a kid and it worked great!
1) Build a Colpitts oscillator attached to an old AM radio loop antenna pulled from and old AM radio.
2) Nail or screw the antenna to the bottom of 1" x 2" 4 foot stick or board.
3) On the other end use an elastic band from an old AM transistor radio, push the radio down the stick (board) about 8", that distance becomes the handle.
4) Tune the AM radio to a blank station.
5) Tune the oscillator to "beat" against the radio, such that a low frequency tone is heard.
6) Now you're ready to go metal locating. As you move the search coil (loop antenna) near metal, the Colpitts oscillator will shift frequency and a tone or whistle will be heard on the radio!
There's a plethora of Google target matches for "AM Radio Metal Detector.” When I built mine, in the late 60's, all the parts were available for free! I found lots of buried stuff, Reynolds wrap, soda pull tabs, gallon cans, include stuff so deep, I got tired of digging for it!
David Lewis: The military applies 3-D fluxgate magnetometers to find magnetic anomalies. They work like this: The deployed sensor pod includes an inertial reference frame. The detector has to be aware of its physical orientation. The systems brain sets up an internal solution correlating the outputs from the three fluxgate axes with the inertial reference frame. As the system traverses the search area, any change in the 3-D fluxgate magnetometer output which is not attributable to change in position triggers an alarm output.
Ferrous and to some very limited extent non-ferrous materials distort the earth's magnetic field. In a simple and limited case a single axis of detection may be useful. A non-magnetized ferrous bar may not distort the earths flux field. For these cases, a probe field may be generated to scan near space. Now the controller function has to do live offsets for the probe field. Probe magnetics may be modulated to provide complex magnetic vector rotations to assist in detecting objects with complex characteristics. This is non-simple.
These were the best suggestions we received, but we have to mention that we also enjoyed Chuck Reed's tip. He pointed out that, "You obviously just need a flux capacitor and a DeLorean, then you can travel back in time and see where it is. Really, what kind of geeks are we if I had to point this out?"
Paul Sisneros also sent along this handy (and amusing) diagram.
Thanks to everyone who stepped up to provide tips, designs, jokes and other information. Greg is following up with research on a few of the answers, especially those regarding flux sensors, and he’ll let us know if he is able to locate the re-bar.