Magnetometer Design

Designs

Design Restraints

Magnetometer must:
• operate at temperatures down to 50 mK
• operate using less than 20 micro watts of power
• measure magnetic fields around 1/3 of a Gauss
• be no larger than 1" by 1" by .25"

 

Magneto Inductive Sensors (Primary)

Magneto inductive magnetometers are a single winding coil on a ferromagnetic core that changes permeability within the Earth's field. The coil is the inductance element in a L/R relaxation oscillator. The oscillator's frequency is proportional to the field being measured. A static DC current is used to bias the coil in a linear region of operation. As the sensor is rotated 90º from the applied magnetic field, the observed frequency shift can be as much as 100%. The oscillator frequency can be monitored by a microprocessor's capture/compare port to determine field values. These magnetometers are simple in design, inexpensive, and have low power requirements. Their temperature range is -20ºC to 70ºC, and they are repeatable to within 4 mG.

Parts

After looking at a number of companies and fabrication requirements we decided to buy magneto inductors and test them at lower temperatures. We purchased two types of magneto inductors and a driver from PNI corporation.

For a micro controller we are using Basic Stamp 2 on a Board of Education from Parallax Inc.

Liquid Nitrogen Tests

Since the sensors we purchased were not extensively tested at very low temperatures, we decided to do some testing of our own. Both sensors were submerged in liquid nitrogen (boiling point of 77.35 K) to determine the rate that the resistance decreases with temperature and to see if the casing could withstand low temperatures.

The sensors functioned at 77K and the casing remained intact. The resistance decreases with temperature, and therefore the power decreases. The table below outlines the resistance and power consumption of the two sensors at room temperature and liquid nitrogen temperatures.

 
Temp (K)
Sen-L Sen-S65
Resistance (Ohms)
293
114.1 38.0
77
14.5 4.6
Power (mW/ms)
293
0.0938 0.0625
77
0.0125 0.0063

 

Fluxgate Magnetometer (Secondary)

A basic fluxgate magnetometer incorporates two coils, a primary and a secondary, wrapped around a common high-permeability ferromagnetic core. The core's magnetic induction changes in the presence of an external magnetic field. A drive signal applied to the primary winding causes the core to oscillate between saturation points. The secondary winding outputs a signal that is coupled through the core from the primary winding. This signal is affected by changes in the core's permeability and appears as an amplitude variation in the sensing coil's output. The signal can be demodulated with a phase-sensitive detector and low pass filtered to retrieve the magnetic field value. Another way of looking at the flux-gate operating principle is to sense the ease of or resistance to core saturation caused by the change in its magnetic flux. The difference is due to the external magnetic field.

From all that we have researched, a fluxgate magnetometer can work at cryogenic temperatures and consume low enough power. Most models used a nonmagnetic bobbin wound with ferromagnetic tape. We were planning to use three Amorphous E (cobalt based Metglas) tape-wound cores each with an outer diameter less than 0.25" in order to make a square probe that would fit the size requirements. For fabrication, we were planning on sending the design drawing to the Coil Winding Specialist (CWS) company and having them wind the three cores and possibly the cores together in a frame.

The fluxgate magnetometer was put on hold because the magneto inductor had made much more progress, the fluxgate would need to be fabricated rather than bought, and time was becoming an increasingly restrictive factor.

© Olin College NASA Study 2004