The magnetic shield provides a low field, noise free environment for testing magnetometer sensors. The system contains coils for calibrations and stimulation of the magnetometers.

The shield is constructed of two layers of high permeability metal. The metal chosen is 4% Mo, 79% Ni, 17% Fe normally called 4-79 Molly Permalloy. The metal used comes with the trade name Hipernom. The construction details are shown in Figure 1. After assembly, the metal cans are annealed in a pure hydrogen atmosphere. The shield is soaked for several hours at a temperature above the curie point and then slowly allowed to cool below the curie temperature. This annealing produces the highest permeability for effective shielding.

The metal cans are protected by urethane foam in a rigid case. The cans and lids should be handled carefully. To bend, drill, hammer or work the metal in any way can work harden the metal. This reduces the shielding and may leave areas that are transparent to magnetic fields.

Inside the shield is a solenoid coil (Figure 2) for producing calibrated fields parallel to the longitudinal (Z-axis) of the shield. Two additional coils have been added near the center of the solenoid to provide cross axis (X,Y) stimulation. The Z axis coil produces an adequately uniform field that can be used for calibrations. The X,Y coils should only be used for stimulation (i.e. spinning fields) since they are not as uniform.

Each coil has a connection where it can be driven directly with accurate currents to calibrate the magnetometers. A precision DC current generator is typically used to drive these inputs.

In addition there is a second connector for each coil that has a series resistor which provides a scale factor of 1000 nT / Volt. This connection is especially useful in stimulating the magnetometers with spin synchronized fields generated by DAC’s in the ground support equipment.

The Magnetic shield provides an environment for magnetic sensors that can be used for calibration, controlled stimulation, noise and offset determination at room temperature. Because of the fragile nature of the shielding provided by the metal cans, care must be used in handling and use of the shields. Extreme mechanical or thermal shocks can significantly affect the shielding properties of the metal.

The initial calibration of the shield was done at room temperature (22 degree C) using a magnetometer that was cross calibrated with our local systems.

In operation, the sensor to be tested is placed on the platform which centers the sensor in the stimulus coil. To calibrate / stimulate the sensor it must be oriented with the coils. Calibration should be done with the solenoid coil (Z axis) only because of the linear field produced by this coil.

Calibration:

Calibration of a sensor requires mounting the sensor on the platform and aligning the sensor with the Z axis coil. Additional brackets and fixtures may be required to align each sensor axis parallel to this calibration coil (Z).

Scale Factor:

The scale factor can then be determined by applying calibrated DC currents. In order to compensate for the offsets of the system, the calibration field should be applied in both directions, the readings should be subtracted, and the results divided by twice the value of the applied field.

Dynamic response:

The AC response of the system is most easily determined by getting the impulse response of the system. Some care needs to be exercised to insure that the impulse used does not saturate the Magnetometer. Sine wave response can also be determine the response curves but especially at the spin period of the spacecraft.

Noise Levels:

For noise tests, the sensor can be placed in any position on the platform the lids for the cans are installed and the noise levels are measured. Since the shielding is not complete, movement of large objects ( equipment chairs etc.) should be avoided during the data collection. We have found that the noise levels measured during the evening hours are significantly lower than those observed during the normal working hours.

Offset determination:

The offsets for each axis are determined by placing the sensor in the center of the platform perpendicular to the Z axis solenoid and rotating it 180 degrees. The resulting readings are subtracted which cancels any residual field in the shield and provides the sensor offset values. We usually rotate the sensor a number of times and average the readings to determine the offsets.

Spin Simulation:

To simulate a spinning spacecraft the cross field coils (X and Y) should be driven with sine wave signals. The two signals should be phase shifted by 90 degrees. A two channel synthesizer such as the HP3326A is capable of producing these signals. To fully simulate the signals you would expect from a spinning spacecraft, you will need to phase lock the sine waves with the spacecraft Ground Support Equipment that is generating the other spin synchronization signals. ( Exceeding 10Vrms into the Voltage output will stress the installed 1/4 Watt precision resistors)

Z - axis (Longitudinal Calibration Coil)

Resistance: 1.6 Ohms

Number Turns: 176 turns #16 Gauge wire ( 8 turns / inch)

Inductance: 2.22 mHy

Current Sensitivity: 395 nT/mA

Voltage Sensitivity: 1000 nT/Volt (Rs = 393 Ohms 1/4W)

X - axis (Cross Axis Stimulation Coil)

Resistance: 3.8 Ohms

Number Turns: 100 turns #22 Gauge wire ( 50X2 turns)

Inductance: 4.47 mHy

Current Sensitivity: 495 nT/mA

Voltage Sensitivity: 1000 nT/Volt (Rs = 443 Ohms 1/4W)

Y - axis (Cross Axis Stimulation Coil)

Resistance: 3.7 Ohms

Number Turns: 100 turns #22 Gauge wire ( 50X2 turns)

Inductance: 3.95 mHy

Current Sensitivity: 441 nT/mA

Voltage Sensitivity: 1000 nT/Volt (Rs = 437 Ohms 1/4W)

Figure 1
Shield Can Sketch

Figure 2
Stimulus Coil

Stimulus Coil Connections

For more information, contact Robert J. Strangeway, strangeway@igpp.ucla.edu.