Journal notes, More Polarizing Power Supply, a Minor Storm
GELLER Labs "Backyard Science"
Thoughts on a proton precession magnetometer design - a Proton Magnetometer Project. Build an Earth's field magnetometer.
The goal of this project is a low cost high performance proton magnetometer (a digital magnetometer) kit for amateur scientists to be able to accurately measure and monitor changes in the Earth's total magnetic F field and to observe geomagnetic storms. There is a regular daily (diurnal) variation in the Earth's magnetic field. During events related to solar activity, there can be sudden changes in the field (such as a sudden impulse) as well as large excursions in the field which can be more than ten times the regular diurnal variation caused by magnetic storms.
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Sunday, May 30, 2010
Overnight: PDF, TXT, USGS PDF, two day view of minor G1 storm as observed here in upstate NY PDF, NOAA 7 day K index PDF. While the excursions in the field are still a little larger than seen during very quiet geomagnetic periods, overnight there was relatively little scatter in the short term (2 minute) measurements. While difficult to quantify, it is evident that the FDM magnetometer instrument noise is now at an acceptable level for use as a geomagnetic observatory.
While drawing block and schematic diagrams, writing the project articles, and designing the PC boards, I will also begin to build a rough budget for various versions of a FDM magnetometer, perhaps on an Excel spread sheet. This will no doubt change quite a bit in the months to follow. Here is an Excel work sheet of a first look at a cost estimate for the project (showing various equipment purchase approaches, low cost to higher cost). Since many experimenters will have many of the items on hand, it is possible the cost could be less, however for those buying everything, right now it looks like around about $350 at best. The estimate does not include the cost of a computer, since the FDM magnetometer can run on a regular existing desktop while still being used for other day to day uses. Also, as we have discussed, some might use other digitizers (in place of the NI USB 6008) and write their own software for the digitizer (our compiled LV program will only run with the NI USB 6008). The cost is still way below the existing $3000 to >$10,000 alternatives.
Afternoon: I noticed a sudden increase in the field around 2 pm EST PDF. I also noticed about that time that the NOAA Costello Geomagnetic Activity Index page was predicting a rise in the K index PDF. Then, just before I left on some errands, it looked like the field was becoming more stable PDF. Checking back around 5:49 EST (21:49 UTC), sure enough there has been another G1 minor magnetic storm PDF. A field change on the order of 10 nT to 20 nT over 5 to 8 measurements (10 to 16 minutes) might be a good indication of the onset of a storm.
Evening: Here is how the G1 minor storm played out PDF.
Automatic geomagnetic storm indication? Fortunately, changes caused by a nearby parked vehicle are step functions over one or two measurements, and even the garbage truck, which stops for a time here and there, only leaves one to four distorted points, so maybe there is a relatively easy automatic software alarm here? The rate of change of onset might be somewhat dependent on latitude. For example field changes at the Alaska monitoring stations tend to be larger than field changes here in upstate NY.
Monday, May 31, 2010
Very early morning: 12 hours later, this looks like a significant excursion in the field ( PDF, here is the alert PDF. The disturbance is K 4, one level below a minor G1 storm. afternote: afternote: This activity did settle down and did not lead to a G1 storm. Perhaps an automatic magnetic storm alarm should look at not only rate of change over 5 to 8 measurements, but also rate of change between each measurement.
Overnight: PDF, TXT, USGS PDF, several day view PDF including past G1 geomagnetic storms. The spike is the garbage truck stipped during a measurement within about 100 feet of the sensor (if had been moving, auto-retry and the FOM filter would have rejected it. Note the different scales, 10 nT per division and 5 nT per division, as well as the number of divisions. The first overnight graph is the small less active region on the right side of the several day view. Today's overnight graph spans 53,750 nT to 53,800 nT (50 nT) at 5 nT per division. The several day view spans 53,720 nT to 53,840 nT (120 nT) at 10 nT per division.
The FDM proton magnetometer instrument design (based on a Professor Mandelshtam's FDM method (harmonic inversion of time signals, see Jan 23 entry)) has settled down to where about 60 measurements per hour yield about 30 plotted measurements per hour which are plotted on the published graphs (measurements where the figure of merit (FOM) is 2e-6 or better (0.1 nT). Over short periods the yield can be as low as 30% to 40%. As best as I can determine to date, this success rate is more a function of the change in field with time (dB/dt) and/or the field gradient across the sensor during a 1.1 second digitization interval. Many of the discards can be attributed to passing vehicles. Others might be related to nearby frequency spurs caused by spurious magnetic fields (i.e. electromagnetic interference (EMI)) including power line related noise (such as momentary high currents, as related to AC motors). The FDM magnetometer is believed to be less sensitive to VHF/UHF/microwave interference because of the input common mode RF filter (see end of May 29 entry).
Spaceweather.com (no affiliation) continues to be an excellent source of solar related geomagnetic reports for amateur scientists as well as an accurate short term (several days) predictor of geomagnetic events. Once there is onset or near-onset of large field changes, NOAA's Costello Geomagnetic Activity Index one day graph gives an accurate indication of where event are headed.
Monday, June 1, 2010
Overnight: PDF, TXT, USGS PDF (We are in Upstate, NY; roughly compare to Corbin, VA and Stennis, MS) Several Day View PDF. The field has been quieting down since the 28th.
More on the software filter: As seen in the three measurement lines copied below, a bad data point was taken, but fortunately failed for high figure of merit of 2e-4 (FOM > 2e-6 (0.1 nT)):
5/31/2010 7:04:02 PM 53810.46 2290.348 0.976 0.000002000 14.0
5/31/2010 7:06:02 PM 52378.84 2229.393 1.377 0.000200000 -5.7
5/31/2010 7:06:11 PM 53809.92 2290.325 1.188 0.000080000 29.1
Almost as expected with selection of the wrong frequency component (highest amplitude component in the FDM spectra), the FDM S/N ratio was very poor. In fact 20 log times the ratio of the highest amplitude frequency to the sqrt of the sum of the squares of all of the other amplitude components within the FDM bandwidth (300 Hz) yielded a negative number (more energy in the noise spurs compared to the energy at the fundamental frequency). While, this measurement calculation was properly rejected for high FOM, the danger is that an anomolously high amplitude spur calculated to high accuracy might have made it through to the graph. The FDM S/N has proven less useful, however, now it seems that an additional filter of something like FDM S/N must be > +5 dB might reject cases like this one. I added the FDM S/N ratio filter to the Labview program. As described earlier, I call it the FDM S/N ratio, since only noise over the relatively narrow FDM spectra is considered (currently 300 Hz or +/- 3,524 nT).
Tuesday, June 2, 2010
Overnight: PDF, TXT, USGS PDF. Quiet overnight, note the vertical span is 50 nT today, with the field excursion last 12 hours on the order of 20 to 25 nT (seemingly a normal quiet period diurnal variation for this area). Small spikes and offsets are from parked vehicles and trucks (spikes are often caused by drop-offs or pickups). 602 of 1070 measurements from about 6/1/10 5 pm (more measurements than are shown on the PDF of the graph above), were rejected by the software filter, almost all for high FOM > 2e-6 (0.1 nT). This number also seems pretty typical by now. Of the 14 or so measurements rejected for high FDM SNR, 13 appear to have been good measurements, many with exceptionally good FOM values. The FDM SNR filter appears to be on the conservative side, although I suppose it might prove useful in relatively high EMI noise conditions.
Still running on the low cost HY-1803D polarziation power supply at 1.5 A polarization (two second pulse). Received the Agilent E3615A today. Spent the afternoon cleaning it up and calibrating it, then switched over to it this evening to see if there any noticable difference in the FDM magnetometer rejected measurement rate. I set the output voltage to 10.75 V and the constant current (cc) mode to 1.5 A. At this evening's outdoor temperature, the voltage drops to about 9.43 V during a polarization cycle. While certainly not needed for this experiment, it is a very solidly built piece of equipment. It is somewhat longer in depth than the hp 6237B, about 2" deeper, and weighs in around 12 pounds. The panel voltmeter and ammeter both calibrated easily. okay, so we will see what the data looks like in the morning.
All indications to date are that the measurement rejection rate is now dominated by dB/dt and/or gradient across the counter-wound coils during the 1.1 second measurement of the proton precession signal. Will need to make a count of rejected vs. total measurements in the morning. At first glance, I do not see much difference in the performance of the FDM magnetometer using the new E3615A power supply.
Thursday, June 3, 2010
Overnight: PDF, TXT, USGS PDF, sample FDM spectra s1, s2, log s2. It was a relatively quiet overnight, with some periods of slight activity. 597 (~55%) of 1088 measurements were rejected for high FOM, a typical number by now falling in a range of about 40% to 60%. The rejection rate corresponds to 64 measurements per hour (over an ideal rate of 30 per hour if all measurements had a figure of merit (FOM) of 2e-6 (0.1 nT) or better).
This measurement rejection rate, while using the Agilent E3615A polarization power supply overnight, appears to support the premise that better regulation or lower ripple (to a point) is not particluarly important in this FDM magnetometer design. The very low cost import HY1803D provided the same performance.
A fixed voltage supply without the automatic crossover to a constant current mode (an active cc limit, also referred to as automatic cross-over) should work fine too. However, at least for now, it is helpful to have a fixed polarization current as one more constant while varying other parameters of the experiment. The load resistance of the twisted pair feed cable to the powered coil as well as the resistance of the powered coil of the counter-wound pair varies as expected with outdoor temperature by about the resistance-temperature coefficient of copper. For example last evening, the outdoor temperature was about 67 F (19.4c). The voltage at cc = 1.5A was 9.26 V for a load R of 6.17 ohms. This morning, the outdoor temperature is about 70 F (21.1c). The voltage at cc = 1.5A is 9.33 V for a load R of 6.22 ohms. The temperature was taken from an non-precision wall mounted thermometer in a different location, and the sensor is presently under a black plastic rain cover (it heats in the sun), so the temperature values are very rough to just to show that there are changes in load R that are compensated for by the cc mode of the polarization power supply.
Friday, June 4, 2010
Very early: PDF1, TXT, PDF2, another minor G1 geomagnetic storm.
Spent yesterday afternoon cleaning (be sure to get the huge dust balls off all of the analog circuitry, or they might reak havoc as parallel resistors on very humid days; there is no filter system), calibrating and switching over to an Agilent 6632B power supply as the polarization power source. It has a fast response loop setting for inductive loads and at over $2,500 new, it is truly the LEXUS (or as my dad used to say, a Cadillac) of low power bench supplies. Will see if there is any difference in magnetometer performance overnight ... (afternote: While testing one of our voltage reference products, we noted that the wide band noise output of the 6632B is quite a bit higher (~50% higher to 100 kHz, ~10 times for a 25 MHz bw) than our E3631A bench supply, less of an issue for the magnetometer, however another issue to consider when selecting and buying new or used power supplies.)
Overnight: PDF, TXT, USGS PDF. The magnetic storm caused relatively small excursions here in upstate, NY, however the geomagnetic field was certainly active compared to the quiet diurnal cycle. It was a K5 (G1 minor storm) at Boulder, Co PDF.
There was no noticeable improvement in instrument performance using the Agilent 6632B as the polarization power supply overnight. Of 725 measurements, 401 were rejected for high FOM, a pretty typical number by now. The 6632B is just a little noisy (fan noise) and probably inefficient at no load most of the time and a small load during the polarization cycle. The exhaust air is noticably warm. The fan speed automatically adjusts with power delivered to the load, so each two second polarization cycle, the fan speed ramps up as if it is somehow is involved in the FDM computation. I can see why companies with racks of these might be surplusing them to reduce air conditioning bills. Nevertheless, especially with its internal 4 1/2 digit voltmeter and ammeter, it is one of the highest quality supplies I have seen to date.
Here is a snap shot of the prototype with the three polarization power supplies that we have been discussing (HY1803D, Agilent E3615A, Agilent 6632B) JPG. The hp 6237B is supplying the +/- 6 V analog rails. It is expected that all of these supplies could be simple low cost surplus open frame linears. In fact, while we have been testing the FDM magnetometer with some relatively high end test equipment, it seemingly could run on extreme basics such as inexpensive surplus shielded cable, plain filtered tap water, and batteries or simple rectifier power supplies. It is beginning to look like it might be a workhorse well suited for an amateur geomagnetic observatory.
Project Documentation (very early stages)
QUESTIONS/COMMENTS/notice of typos, etc. send email to joegeller @ gellerlabs dot com
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