THM1176: Typical Application Scenarios

01.02.13

Here is a short description of how to set up the THM1176 v. 4 software for typical application scenarios.

Standard “Gaussmeter”

The default measurement parameters are designed for the exploration, with a probe in hand, of a static magnetic field. These parameters provide:

•  Units of Tesla. Use the Units pull-down menu, available on all the panels, to select more appropriate units.
•  Auto-ranging. Use the Range pull-down menu on the Setup panel to select a fixed field range. This will enable a thermometer field-strength display on the Numeric and Setup panels.
•  Continuous timed triggers at 100 Hz, with a 10 Hz screen-update rate. (On the PDA, it’s a 25 Hz measurement rate with a 1 Hz update rate.)
•  Averaging factor of 10 acquisitions/measurement.

To restore the default parameters, select the Setup tab and press the Reset button.

Before starting your measurements, we recommend performing the zero-offset correction procedure, using the zero-gauss chamber and Offset Zero button on the Setup panel. (ATTENTION: do NOT perform a zero-offset on a TFM1186 Fluxgate probe! If, by accident, you do, see the related news article for how to restore the factory settings.)

Other common things you can do:

•  Press the Hold button to temporarily freeze the measurements.
•  Select the Max checkboxes on the Numeric panel to find the max field in a volume you scan out.
•  Use the Vector display on the Numeric panel to help orient the probe along the direction of the field.
•  Use the Plot panel to display the measurements graphically. Use the “B”, “Bx”, etc. buttons to hide certain traces, thus zooming in on the remaining ones. (This last feature is not available on the PDA.)
•  Use the Graph Palette and right-clicks to manipulate the plot: horizontal and/or vertical zoom, fix the scale, enable cursors, copy the graph, clear the chart, etc. Usually it’s better to press Hold first. (Not available on PDA.)
•  Record and play back data using the controls on the File panel.
•  Optimize the measurement parameters using the Measure Setup button on the Setup panel.
•  Enable audio and/or visual alarms using the Alarm Setup button on the Setup panel.
•  Save and recall settings using the Save/Recall/Reset buttons on the Setup panel.
•  For further explanations of all these functions, please see the manual, or the on-line help by pressing the question mark in the top-right corner of the window.

“Mapping” mode

To measure and record the field at given locations in space, set up the THM1176 as follows:

•  Press the Measure Setup button on the Setup panel to set up Immediate trigger mode, with a block size of 1. You may also want to increase the averaging factor; if you increase it a lot, be sure to also increase the timeout value.
•  Set up a recording file by pressing the “…” button on the File panel, and enable recording by pressing the Record button.
•  Place the probe in the first location.
•  Optionally, enter a Record Comment, for example describing the measurement location.
•  Press the Hold button to take the measurement. For each measurement, the THM1176 beeps and blinks the Record light. The Hold button is automatically re-enabled afterwards.
•  Repeat the previous three steps until you’re done.
•  Press the Record button again to disable recording.
•  The measurements can be found in the ASCII recording file.

“AC measurement” mode

The THM1176 software provides sophisticated tools to measure and analyze rapidly varying fields. To use these tools effectively, you need to have a clear idea of what the system is doing: the THM1176 probe acquires (Bx,By,Bz) samples at a high rate, storing blocks of data in its local RAM. Using a double-buffering scheme, each block is transferred to the host via USB while the acquisition continues. The THM1176 software then processes these blocks of data, for example computing an FFT.

To get started, set up the measurement parameters for AC measurements by pressing the Measure Setup button in the Setup panel:

•  Usually one would use Continuous timed trigger (the default).
•  A typical value for the Timer rate is 2048 Hz, with a Block rate of 1 Hz (resulting in a block size of 2048 measurements). The Timer rate in continuous timed trigger mode is limited to 2.3 kHz. The block size is limited to 4096 measurements.
•  Set Averaging to 1.
•  Ensure that the timeout value is significantly larger than the time per block (= 1 / Block rate). With the proposed 1 Hz block rate, the default 5000 ms timeout is fine.
•  Press OK in the Measurement Setup dialog to start acquisition.

What the THM1176 software displays depends on the display mode selected with the pull-down menu on the Numeric panel:

•  Raw: The Numeric panel displays the last acquisition in each block, while the Plot panel displays the entire block of data.
•  sDev, P-P: The Numeric and Plot panels both display the standard deviation or peak-to-peak value, respectively, of each block of data.
•  FFT, FFTx, FFTy, FFTz: The Numeric panel displays the magnitude B of the last measurement, as well as the amplitude and frequency of the dominant spectral peak. If you want the amplitude of another peak, enter its frequency in the Target F field. The Plot panel displays the FFT, with the amplitude in the selected units on the vertical axis. (On the PDA, you may have to change the units to get a good FFT plot.)

Other things you can do:

•  If, instead of continuous measurements, you want to do a “one-shot” measurement every time you press the Hold button, select Timed trigger. This trigger mode provides lower timing jitter, resulting in better spectral resolution. The UIF does not limit the sample rate (the Rate light will illuminate if the trigger rate is too high), but in practice the rate is not much higher than in Continuous timed trigger mode.
•  If you want the highest possible sample rate, use the Immediate trigger. This is a free-running trigger (i.e. no timer), but in practice the trigger rate is very stable. The rate is indicated in the Timer rate field.
•  You can increase the block size to increase the spectral resolution.
•  Most of the things you can do in “normal gaussmeter” mode.

Note that higher sample rates give higher max frequency (Nyquist), and bigger block sizes give slower update rates but better frequency resolution. The default parameters proposed above provide a Nyquist frequency of 2048 Hz / 2 = 1024 Hz, 2048 samples / 2 = 1024 FFT bins, resolution of 1024 Hz / 1024 bins = 1 Hz, and an update rate of 2048 Hz / 2048 samples = 1 Hz.

The amplitude scale of the FFT corresponds to the amplitude of the corresponding frequencies in a Fourier expansion of the signal. For example, the signal in the screen shot (below) consists of a DC component with a 50 Hz ripple, plus some high frequency junk. Looking at the Raw Plot display, we can estimate the amplitude of the DC component at around 47.66 uT (earth's field), and that of the 50 Hz noise at around (47.74 - 47.58) / 2 = 0.08 uT. On the FFT display, we see peaks at DC and 50 Hz, at roughly those amplitudes; in addition, one discovers that the "higher frequency junk" is mostly 150 and 250 Hz – i.e. the odd harmonics that turn a 50 Hz sine wave into a triangle wave.

thm1176_typ_appl_news1 thm1176_typ_appl_news2

In this example, we note that the amplitude of the 50 Hz peak is not quite 0.08 uT, but more like 0.035 uT. There are several reasons for this:

•  The Raw and FFT screen shots came from two different easurement blocks. If we watch the FFT, we see that the peaks "dance".
•  Our guesstimate of the 50 Hz ripple amplitude is very rough, and probably includes a bunch of the 150 and 250 Hz components.
•  More fundamentally, the energy of the 50 Hz peak is spread over several FFT bins, and therefore results in a low amplitude estimate. This is clearly visible in the zoomed version of the FFT. The Numeric display uses another computational method that provides a better measurement of the amplitude at a particular frequency; in this example, we got a fairly consistent 0.063 uT at 50 Hz. The last screen shot shows the amplitude of the 150 Hz peak.

thm1176_typ_appl_news2 thm1176_typ_appl_news4
thm1176_typ_appl_news5

Acquisition synchronized with an external trigger

Synchronizing the THM1176 acquisition with an external trigger – for example a rotational or linear encoder – requires writing some LabVIEW code. However, the amount of code is minimal, providing immediate access to the standard features of the THM1176 software.

You just have to modify one function, WaitForBusTrigger.vi. The supplied version of this VI waits for a button press; you must modify it to wait until the next interrupt arrives from your external trigger source. Then you rebuild the THM1176 software using the supplied Build Specification, and off you go! Click on Measure Setup in the Setup panel, select Bus Trigger, and the THM1176 software takes care of the rest.