Experiments with ROBOLAB (Claude Baumann, director of the boarding school “Convict Episcopal” Luxembourg / Europe) ROBOLAB isn’t only an excellent programming environment for the RCX. It also offers prodigious features for data collection and analysis that predestinate the RCX as an instrument for measurements in educational physics, chemistry, biology, geography and even history. This paper presents an outstanding example of a study-report that has been realized with the ROBOLAB Investigator level. The subject is a mixture of electronics, geo-physics and archeology. Teachers often have problems to gain the student’s attention for the topics they are teaching. What follows could stimulate them to introduce John Dewey’s often cited “learning by doing” concept into their classes. The project description has a didactical purpose to demonstrate what can be done with ROBOLAB and doesn’t pretend to be an archeological scientific study. This experiment tries to verify, if one of our compass-sensors introduced at http://www.convict.lu/Jeunes/CompassSensor.htm and based on the Philips KMZ51 magneto-resistive sensor may be used as a magnetometer for archeometry applications. It is known for quite a long time that rocks or buried buildings disturb the Earth magnetic field. The idea is to have a human carry the sensor and the LEGO RCX as the datalogger while walking over a terrain, where foundations of an ancient Gallo-roman villa are known to be localized. These foundations should generate anomalies of the measured vertical component of the magnetic field. The sensor is very sensitive to the Earth magnetic field and has already been successfully studied, if it could be used as an inclinometer (http://www.convict.lu/Jeunes/vertical_magnetic_fieldllb_201001_1527/index.htm). The sensor is capable of reacting on weak magnetic influences. The major qualities of the device are the fact that it is insensitive to acceleration, the small size and the low price. Two important things have to be observed: - due to the high sensitivity, it should be made sure that the sensor is manipulated in a constant direction (in fact we prefer NORTH) AND absolutely orthogonal to Earth. - since the bulk of this project is to move over an uneven terrain, it will be impossible to guarantee both conditions. After datalogging the data must probably be filtered to eliminated swinging effects that are due to the stepping over ground.

Picture 1: The Earth magnetic field is composed of a horizontal and a vertical component. Only the vertical part is measured in this experiment.

1. Earth magnetic field sensor

The earth's magnetic field can be represented as a dipole situated about 440 km off center and inclined 11 degrees to the planet's rotation axis. The background Earth magnetic field strength is averaged at 53 nT, which is quite small. There exist only a few kinds of magnetometers able to read such a weak magnetic field strength:

• mechanical magnetic compasses • fluxgate compasses • hall-effect compasses • magneto-elastic compasses • magneto-resistive compasses …

The last category is the one we are trying to realize using the magneto-resistive sensor KMZ51 from Philips. This electronic device is a highly sensitive magnetic field sensor released in 1996. It employs the magneto-resistive effect of thin-film permalloy. The sensor contains a Wheatstone bridge and integrated compensation and set/reset coils. The package is held in SMD-technology.

A short current pulse on the flip coil is necessary to recover the sensor after exposure to strong disturbing magnetic fields. The difference between two voltages +Vo (Pin7) and -Vo (Pin6) can be expressed as:

dV = Vmax*cos(Theta).

Picture 2: This device, based on the magneto-resistive Philips sensor KMZ51, is extremely sensitive to variations of the Earth magnetic field. It is able to detect very weak changes of the field in the range of 1nT. (copyright: Claude Baumann) On an interesting web-site http://www.cr.nps.gov/mwac/hopewell/v6n2/four.htm we may read the following explanation:

"Magnetometry was the only passive geophysical method employed in this study. This method measures the relative strength of the earth’s magnetic field. The magnetic properties of a soil depend on the concentration of iron compounds like hematite, magnetite, and maghaemite (Weymouth 1986). Undisturbed earth will yield a uniform magnetic field, whereas buried ditches, etc., will tend to be more or less magnetic than the surrounding matrix. Magnetometers are generally most sensitive to metals and fired materials. Metals will appear in magnetic data as paired strong positive and negative values which is referred to as a dipole. When a material is heated to a certain point the heat will “reset” the material’s magnetic clock. This process is called thermo-remanent magnetism (Weymouth 1986). The atoms of the substance will align themselves to magnetic north at the time of cooling. For this reason burned features, such as hearths, kilns, fired bricks, and burned house floors, are readily visible in magnetic data. Typically, most anomalies will range between ± 5 nanotesla (nT). It is not uncommon, however, for a feature to fall within 0.1 nT of the background magnetic strength."

Picture 3: The sensor prototype. Only a few parts are needed. If the sensor is exposed to a strong magnetic field, its own magnetization is changed. To reset the initial magnetization, the button must be pressed once.

Pictures 4 & 5: The sensor is screwed to a wooden stick. During preliminary experiments, it became clear that a vertical position was the easiest to control. The LEGO RCX is fixed with cable binders.

Picture 6: Screen-shot of the RCX program that has been made with Ultimate ROBOLAB to allow datalogging at a frequency >10Hz (in this particular case: 33Hz). The sensor is "powered" as a light-sensor, but should return values from 0 to 1024. The datalogging is started for 30 seconds after a power-up time and an acoustical signal telling the operator to start stepping over ground. A short beep closes the logging. The final eternal loop is necessary to allow the data to be uploaded to the PC. 2. Ruins of an ancient Gallo-roman villa The rests of a Gallo-roman villa are localized in Bigelbach / Luxembourg at latitude 90°10' longitude 103°55'. They are the subject for at least two archeological studies. Unfortunately none of the studies have been finished due to local political problems making the published data scarce.

Picture 7: The location of the Gallo-roman villa.

Picture 8: Exact localization of the villa and an annexe; the latter was discovered only in 1994. (Source: Laurent Willkomm)

Picture 9: ROBOLAB Investigator allows easy reports.

Picture 10: A few trees in the middle of a meadow suggest that the area must hide many stones, otherwise farmers would have ploughed here already long time ago. Bibliography concerning the archeological site: - Ternes, 1967, 131; FMRL I 40; FMRL II 27; PSH 102, 1986, 159 et 169; (FMRL I-V Weiller 1972-1996; PSH Publications de la Section Historique de l'Institut Grand-Ducal) - Laurent Willkomm, Raymond Linden, unpublished maps concerning the last study from 1994 - Michel Polfer, Habitat rural et occupation du sol dans la partie occidentale de la civitas treverorum - L'évolution au Bas-Empire (fin IIIe - Ve siècle), on-line paper http://www.proarchaeologia.org/OnlinePaper/MPLawir3/

Picture 11: A heap of stones give evidence of ancient human dwellings. It is supposed that the villa was occupied until the area of the Constantine dynasty (3d to 5th c.). 3. Measuring the vertical Earth magnetic field along a line In order to test the sensor-device, a part of the villa was chosen that presented some helpful characteristics: even ground, visible foundations, possible passages over more than one wall for a single path.

Picture 12: The planned path over the foundations. In ancient times the villa must have been quite impressive. (Source: Laurent Willkomm) For the reason that North direction yields the strongest vertical field (due to the 66° inclination for Luxembourg), the path was chosen in that direction.

Picture 13: The thin red guide line and the measuring tape are fixed.

Picture 14: A field of ruins.

Picture 15: A sketch of the visible foundations along the path.

Picture 16: Stepping along the red guide line. As can be seen on the picture, it is not obvious to maintain the sensor in a perfect orthogonal position.

Picture 17: The data is transferred to the PC via infrared channel with the ROBOLAB upload feature. The tested distance was 10.5m. Since the red guide line was fixed to a metallic bar, the sensor-value drastically decreases at the end of the walk. The flat end-line is the result of a motionless period while waiting for datalogging completion. The walking speed can be calculated to 10.5 / 21 =0.5m/s. As supposed, the resulting curve presents oscillations that must be eliminated by a digital filtering method.

Picture 18: Details of the disturbing oscillations. These oscillations present an approximate frequency of 0.5Hz. They are the result of steps at 1Hz rate. Observing the sensor during the walk reveals that the left leg step produces an impulse that makes the sensor shortly turn to the right, then return to the initial position and vice versa. The highly directionally sensitive device registers the swinging. The cut-off frequency for the low-pass data filter should therefore be fixed to 0.5Hz.

Picture 19: ROBOLAB Investigator compute tools allow advanced data manipulations. This graph is obtained by operating successively the fast Fourier transform, the digital filtering and finally the inverse Fourier transform.

Picture 20: The LabVIEW G-code program-diagram of the filter procedure. (Note that the FFT vis are located in …Robolab 2.5.4\vi.lib\Analysis\2dsp.llb.)

Picture 21: The front panel of the filtering procedure. Increasing the cut-off frequency adds high frequency components to the data.

Picture 22: Overlaying the sketch and the resulting curve, reveals the correct sensor response. The result of the low-pass filtering is a curve with significant peaks and valleys, where the valleys represent passages over villa-foundations that cause anomalies to the Earth magnetic field proving that the sensor may well be used for archeometry applications. 4. Stabilizing the sensor through a gyroscope Even if adequate filtering methods may extract the correct data from the mixture of valuable, noisy and disturbing frequencies, it could be a good idea to add a physical stabilization to the sensor device. An improvised LEGO gyroscope can contribute to the elimination at source of the swing effects.

Picture 23: The effect of precession applies a considerable force against any position changing of a gyroscope. Get more information about gyroscopes at http://science.howstuffworks.com/gyroscope1.htm or http://www.geocities.com/CapeCanaveral/2188/main.html

Picture 24: A LEGO 9V ungeared motor that rotates large wheels at high speed produces an exploitable gyroscopic effect that helps maintaining the sensor in its direction.

Picture 25: CAD details of the LEGO gyroscope.

Picture 26: The comparison of two passages over the same area with and without active gyroscope shows that the physical device stabilizes the sensor. The additional experiment proves that the gyroscope damps oscillations that are generated through stepping.

5. Conclusion From this first serious test with the device we can conclude: 1. The KMZ51 based compass-sensor may successfully be used as a magnetometer for archeometry applications. 2. Using a vertical pole as the sensor support is acceptable with the restriction that swinging effects that are generated through the human walking should be damped by at least one additional gyroscope. 3. Swinging effects and noise must be filtered out. 4. The same path could be passed over several times to allow data averaging. 5. The walking speed should be constant to allow perfect filtering. The RCX could give acoustical signals as beats. 6. The terrain should be scanned in parallel paths. After each path the data is then uploaded to a Laptop. The following data-manipulation should produce a 3D graph showing a pattern of the terrain that could reveal hidden foundations of ancient human dwellings. 7. The KMZ51 sensor has been described as submitted to a certain drift that should be eliminated through an alternate change of the sensor magnetization. But for the purpose this is not necessary, since only relative magnetic field variations are measured. Absolute ratings are irrelevant here.