Digital flux gate magnetometer (992980), страница 2
Текст из файла (страница 2)
по крайней мере один феррозондовый датчик, генератор возбуждающего сигнала, позволяющий делить этот сигнал на два, в результате получается сигнал с разделенной частотой и сигнал с неразделенной частотой;
средство для применения сигнала с поделенной частотой в феррозондовом датчике;
средство для контроля инверсии сигнала в одном из двух феррозондов;
средство для фильтрации транслированного сигнала для получения выходной величины.
2. Прибор из п.1, где вышеописанный трансляционное средство содержит трансляционную схему, соединенную, по крайней мере, с одним феррозондом, чтобы обеспечить измерение характеристик внешнего магнитного поля, основанный на сигнале от, по крайней мере, одного датчика.
3. Прибор из п.2, в котором трансляционная схема содержит средство измерения длительности сигнала феррозонда.
4. Прибор из п.1, в котором трансляционное средство содержит средство оптимизации выходного сигнала для исключения влияния шума.
5. Прибор из п.1, в котором трансляционное средство подсоединено, по крайней мере, к одному датчику, чтобы измерять выходные характеристики внешнего магнитного поля, основываясь на сигнале, по крайней мере, от одного датчика.
DETAILED DESCRIPTION OF THE EMBODIMENTS
FIG. 1 shows a general block diagram of a flux-gate magnetometer device 10 permitting external magnetic fields, especially small magnetic fields, to be measured in a high noise environment. As used herein, small magnetic fields refer preferably to magnetic fields of about 10@-7 gauss to about 1000 gauss, more preferably about 10@-5 gauss to about 100 gauss.
Generally, the flux-gate magnetometer device 10 includes a sensor portion 12 which is driven in and out of saturation using a drive signal 13 generated by drive signal generator 14 according to the present invention. The magnetometer device 10 further includes translate circuitry 16 for receiving a sensor output signal 15 from sensor portion 12. Translate circuitry 16 provides for detection and/or signal conditioning of sensor output signal 15 to provide a device output 17 representative of the magnitude of an external magnetic field present at the sensor portion 12. As used herein, an external magnetic field is defined as any magnetic field produced externally to the magnetometer device as opposed to a field generated by the coil(s) in the magnetometer device, e.g., any DC magnetic field or any slowly varying magnetic field.
The sensor portion 12 may include one or more flux-gate sensors. For example, in the use of differential circuitry according to the present invention, sensor portion 12 may include two flux-gate sensors positioned with respect to the magnetic field to be measured such that one sensor output is inverted with respect to the other.
Various configurations of flux-gate sensors are known to one skilled in the art. The present invention is not limited to any particular flux-gate sensor, nor is it limited to any particular flux-gate sensor configuration. Generally, each flux-gate sensor of sensor portion 12 has a core that is made of a ferromagnetic material. Wound on the core is a sense coil driven by the drive signal 13, e.g., a current waveform, generated by drive signal generator 14 for use in driving the core in and out of magnetic saturation. The sense coil, i.e., a pick-up coil, of each sensor detects the changes in the magnetic permeability of the core when there is an external magnetic field present. An output 15 of the sensor, e.g., voltage across the sense coil of each sensor, is indicative of the external magnetic field present at each sensor. One skilled in the art will recognize that other flux-gate magnetometer sensors are known, such as those having a drive coil and a sense coil wound around a core. Further, it will be recognized by one skilled in the art that the sensors of the sensor portion may be located on chip with the other circuitry of the magnetometer device or off chip.
The flux-gate magnetometer device 10, therefore, senses a external magnetic field by stimulation of one or more sensors of sensor portion 12 with a known drive signal 13 generated by drive signal generator 14. Generally, the nonlinear magnetic properties of the core of the sensor cause harmonics of the drive signal frequency to be generated. Preferably, the second harmonic is used as a measure of the external magnetic field. As such, the external magnetic field to be measured is proportional to the second harmonic signal generated at the sensor portion 12.
Generally, conventional drive signals or stimulus signals for flux-gate magnetometer sensors have been drive signals having a stable frequency. As referred to herein, a stable frequency drive signal refers to a repetitive periodic signal such as a constant frequency triangular waveform or a constant frequency square wave signal (e.g., a square wave signal having a constant duty cycle). With the use of a stable frequency drive signal, EMI that is present at twice that of the stimulus stable frequency drive signal is sensed by the one or more sensors of the flux-gate magnetometer and may be undesirably interpreted as at least a part of an external magnetic field. As such, the sensitivity of such flux-gate magnetometer devices using stable frequency drive signals is problematic, particularly from an immunity to EMI standpoint.
According to the present invention, drive signal generator 14 provides a drive signal 13 wherein the drive signal 13 has a time varying characteristic. As used herein, a drive signal having a time varying characteristic is defined as a drive signal having a time varying characteristic which makes it very unlikely that EMI could track or mimic the drive signal in a way such as to coincide with the second harmonic signal of that particular drive signal. Therefore, EMI is unlikely to be interpreted as the second harmonic signal which may be undesirably associated with a nonexistent external magnetic field.
As described in various embodiments herein, the time varying characteristic of the drive signal may take one of many different forms. However, such a time varying characteristic of the drive signal should make it unlikely that EMI could follow the second harmonic of the drive signal. Some characteristics which may be varied over time include duty cycle, frequency, period, phase shift, slew rate, and the like.
Further, depending upon the randomness of the time varying characteristic, EMI may be extremely unlikely to vary in the same manner as the drive signal. On the other hand, with regard to other time varying characteristics, EMI may be more likely to mimic some time varying characteristics. For example, the drive signal according to the present invention may be a drive signal whose frequency is varied over time in a pseudo-random manner or in a non-random manner such as according to a sinusoidal function. As one skilled in the art will recognize, the sinusoidal frequency varying drive signal will be more likely to be followed by EMI than a pseudo-random frequency varying drive signal.
Translate circuitry 16 may take one of many different configurations. Any circuitry which provides translation, signal conditioning, detection or any other function necessary to provide a device output 17 representative of the external magnetic field at sensor portion 12 based on sensor output 15 may be used in accordance with the present invention. As will be apparent from the more detailed description of embodiments of such circuitry provided herein, translate circuitry may include differential circuitry for cancellation of common mode noise at dual sensors, low pass filtering for filtering the sensor output and other functional circuitry, e.g., controlled inverters, multiplexers, latches, etc.
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