3
\$\begingroup\$

I am building a payload for a high-altitude Balloon launch in the club I started which will be measuring DC voltages (electric field) at different altitudes using a wire dipole or two metal plate formation.

I obviously need a low noise amplifier to even register any data in the logger, and we are not worried about polarity, only magnitude.

To do this, I am thinking an instrumentation amplifier fed from the metal plates to the data-logging device, but I am facing a problem, how can I assure the DC amplifier is only measuring positive values? Even some good diodes have a few hundred millivolt drop, which would not be suitable for the experiment. A transformer would not work because this is only DC we are working with (even if it was AC, the high input impedance requirement would not be fulfilled).

Any ideas on rectification or ways to amplify the small DC signals which we can't be 100 percent sure the polarity is constant.

\$\endgroup\$
  • \$\begingroup\$ Precision bridge rectifier. \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 18 '15 at 5:32
  • 2
    \$\begingroup\$ Alternatively, you can offset up the output of the InAmp such that the negative side would not clip. \$\endgroup\$ – Nick Alexeev Dec 18 '15 at 6:09
6
\$\begingroup\$

That won't work. It may detect ion flows and AC fields above a certain frequency, but it can't detect DC fields. After all, the plates constitute a coupling capacitor in series with the fields you want to detect. It's a high-pass filter.

The usual instrument for this is called a "field mill," where the detector plate is placed close behind a grounded metal propeller or rotating "sector disk." The rotating disk chops any DC e-fields into low-freq AC, which is easily amplified/rectified/A-toD'd etc. Perhaps even use low-pass filtering and a synchronous detector to reject any unwanted AC.

DIY e-field sensor field mill from Scientific American:

Antenna without the field mill: if your detector plate is 10 picofarad, and the amplifier's input is MOSFET with 100 giga-ohm Z-inp, then you've formed a high-pass filter which rejects all DC below ~1Hz (since RC is 1e11ohm * 1e-11farad) It's possible to use a much higher-Z amplifier, plus a periodic shorting relay that "resets" the detector's input before taking data. But then usually you'll run into unknown humidity leakage across plastic insulators, and a (drifting) high-pass period of ?tens? of seconds. Better to just add a field mill, and measure true DC fields.

(edit) PS. Here's something I've not tried: just build a non-fieldmill e-field antenna with well-known RC time constant of perhaps a few seconds, use high-res A/D input, then massage the numbers to cancel out the RC and restore the DC value. Of course this includes a stage of integration, so any slight errors in zero-adjust would produce a constantly increasing drift. The amplifier's DC-zero would need to be extremely stable across the environmental temperature changes involved.

|improve this answer|||||
\$\endgroup\$
  • \$\begingroup\$ Is there an alternative to a field mill without spinning parts? \$\endgroup\$ – skyler Dec 18 '15 at 12:56
  • \$\begingroup\$ @skyler alternative to a field mill? Micro-size field probes use piezo-driven vibrating arms, vibrating fibers, "wobbulator" capacitors with a vibrating plate. To convert the DC e-field into AC, either the sensor-plate must move or vibrate, or a nearby ground-plate must move or vibrate. Or another approach: PLASMA CONTACTOR, which spews out a large conductive plume while measuring the potential on that plume. \$\endgroup\$ – wbeaty Dec 18 '15 at 23:07
  • \$\begingroup\$ Here's a rotating-rod e-field sensor:clasp-research.engin.umich.edu/e-field/sensor/prototype.html Also a MEMS-based vibrating electrode: sciencedirect.com/science/article/pii/S0304388601000481 For much stronger e-fields, Pockels/Kerr sensors "IOES" can detect e-fields on optical fibers made from nonlinear materials. I've detected e-fields by liquid spray, where the microamps coming out of the sprayer tip is proportional to the environmental e-field (so, the moving object is liquid droplets.) \$\endgroup\$ – wbeaty Dec 18 '15 at 23:30
0
\$\begingroup\$

I don't know anything about field measurement, but in case someone stumbles here looking to rectify and measure peak of small voltages, here's my suggestion:

schematic

simulate this circuit – Schematic created using CircuitLab

Note 1: Peak detection speed of response will be limited by R3 and R8 and also depend on the size of C1.

Note 2: Detected peak will decay with characteristic time (tau) = 5050(ohm) x 100(uF) = 0.505(seconds) in this example. (5050 is parallel combination of serial combinations of (R1+R2) || (R6+R7)), 100uF is C1. Hope this helps non field measuring folk.

BTW V1 is also amplified 100 X.

If you want to minimize error due to the input bias current add a 99 ohm resistor in series to the positive inputs of OA1 and OA4. (or for different amplifications add R1||R2).

|improve this answer|||||
\$\endgroup\$
0
\$\begingroup\$

Use an INA116, it's a great electrometer if you make sure your dynamic range of the sensor is within the rails. One problem you will have is charging of the payload. This means that your payload chassis will not have a net charge of 0. You can either, measure the charging or develop a way to keep the charge of the payload at 0.

|improve this answer|||||
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.