|The magnetic field experiment on the IMP-8 spacecraft utilizes a tri-axial fluxgate (saturable inductor) magnetometer. The instrument originally had three, automatically determined, ranges, ±12 nT, ±36 nT, and ±108 nT, full scale. Because of a range-change circuit failure occurring in early July 1975, the experiment was commanded into a fixed ±36 nT range on July 11, 1975 at 12:55:09 UT and has been in that range ever since. The measurements are A-to-D converted onboard, to an 8-bit resolution, yielding ±0.14 nT quantization sensitivity, which is larger than the intrinsic sensor noise level of 0.025 nT RMS.
| The fluxgate magnetometer is a tri-axial instrument developed and manufactured by the Schonstedt Instrument Company.
All fluxgate magnetometers have in common a ferromagnetic core(s) which is excited by driving, or gating, a magnetic field generated by current in a coil which contains the core. The magnetic flux induced in the core by the gating field is modified by an external magnetic field which generates even harmonics on the output winding whose amplitude depends on the magnitude of the external field.
The Heliflux sensor is a cross between a parallel and orthogonal gated core. When the AC current is applied to the primary winding, the magnetizing field has components both parallel and transverse to the core strips. The entire core is cylindrically saturated by the gating field to minimize the remanent magnetization, or core memory. Thus, the coupling between the gating field and core output is minimized by the physical orientation of the gating and output windings.
The electronics unit is comprised of a single oscillator-driver, and a preamplifier, phase detector, voltage bias and output driver for each channel. Since the output channels are the same, only one channel will be discussed.
| The oscillator generates an AC signal of 24 kHz which is power amplified and fed through the primary windings of the sensors to cyclically drive the magnetic cores of the sensors into saturation.
The presence of an external magnetic field long the axis of the sensor results in the generation of even harmonics in the secondary winding of the sensor. The amplitude of the second-harmonic voltage is proportional to the magnitude of the magnetic field and the phase depends upon the direction of the magnetic field when the direction is reversed. The second-harmonic signal is amplified by a tuned amplifier. the tuned pre-amplifier is temperature compensated.
The sensitivity is controlled by the negative feedback, and the desired range is obtained by changing the feedback elements. The operating range is determined by 2 binary bits generated by two relays. The range is selected by range change commands generated in the Magnetometer Processor; 4 command lines are required.
The analog to digital conversion is done by a double ramp (RC charge-discharge) A/D converter. The converter works by charging the RC circuit with the voltage to be measured during a precise time period, discharging the circuit into a negative reference voltage, and then measuring the discharge time with a crystal controlled clock. Most component induced errors are canceled out due to charge and discharge of the same RC components.
There are three sample-and-hold circuits and three A/D converters (one per axis) per system. For the total of two systems there are six sample-and-hold circuits and A/D converters. The three A/D converters per system have been used to provide redundancy so that the system is not lost in the case of a single converter failure.
Because the memory for the digital filter is dynamic, it is kept running all of the time. After sampling is done and the data stored in a sample-and hold circuit, processing and A/D conversion of the data are held up until the MOS memory is in the proper position with respect to the processing timing. The data processing sequence is divided into 8 equal time periods and one variable time period. Period 0 is the variable time period between the time when the sample-and-hold takes the data and the time that the MOS memory is in its desired starting state. During periods 1 and 2 the previous sample of X-axis data is processed in the digital filter. In time period 2 the A/D charge takes place. The A/D discharge (for all axes) takes place during time periods 3,4,5 and 6. In time period 3 the X-axis data which has been processed by the digital filter is processed by the delta modulator. During time periods 3 and 4 the Y-axis data is processed by the digital filter; this data is also from the previous sample time. In period 5 the filtered Y-axis data is processed in the delta modulator. In time period 7 the Z-axis data from the previous sample is processed by the delta modulator. In time period 8 of every eighth sample period the Z-axis is separately processed for both high and low bit rates.