Magnetic compensation circuit and method for compensating the output of a magnetic sensor, responding to changes in a first magnetic field
A solution for compensating a magnetic field sensor to permit detection of a small magnetic field in the presence of a large magnetic field is disclosed. A magnetic field sensor detects the magnetic field which produces an analog signal then encoded by an analog to digital converter (ADC) into a digital stream. A controller operating on the digital stream incorporates additional sensor data to create a compensation signal which is sent to a digital to analog (DAC) converter. This compensation signal then modifies the output of the magnetic field sensor before entering the ADC. Compensation is software controlled, and is thus adaptable to numerous conditions requiring compensation. Apart from being easily tunable, the compensation may respond dynamically to changing conditions. The invention has particular application to airborne electromagnetic surveying where small fields scattered from the Earth are measured in the presence of a large transmitted field.
1. A feedback magnetic compensation circuit, comprising at least one sensor package which outputs a first digital signal, and a controller that receives said first digital signal, computes a digital compensation signal, and then sends said digital compensation signal to said at least one sensor package, wherein said at least one sensor package comprises:
a magnetic sensor which senses a sum of a first magnetic field and a compensating magnetic field created by a feedback coil to create a first analogue signal;
a feedback coil through which a compensation current flows to form said compensating magnetic field over a volume occupied by said magnetic sensor;
an analog-to-digital converter (ADC) which converts said first analogue signal to said first digital signal;
a digital-to-analog converter (DAC) which converts said digital compensation signal to a second analogue signal; and
a voltage to current converter which receives said second analogue signal and energizes said feedback coil with said compensation current to create said compensating magnetic field.
2. A magnetic compensator comprising:
a feedback magnetic compensation circuit comprising:
at least one sensor package that outputs a first digital signal, and
a controller that receives said first digital signal, computes a digital compensation signal, and then sends said digital compensation signal to said at least one sensor package; and
a computer which forms a digitally predicted model of the magnetic field,
wherein, for said feedback magnetic compensation circuit, said at least one sensor package comprises:
a second magnetic sensor that senses a sum of a second magnetic field and a compensating magnetic field created by a feedback coil to create a fourth analogue signal;
a feedback coil through which a compensation current flows to form said compensating magnetic field over the volume occupied by said magnetic sensor;
a second analog-to-digital converter (ADC) that converts said fourth analogue signal to said first digital signal;
a second digital-to-analog converter (DAC) which converts said digital compensation signal to a fifth analogue signal;
a voltage to current converter which receives said fifth analogue signal and energizes said feedback coil with said compensation current to create said compensating magnetic field.
3. The magnetic compensator of claim 2 , wherein the controller in said feedback magnetic compensation circuit imports said first digital signal and said digitally predicted model of the magnetic field, and outputs said digital compensation signal to said compensation circuit.
4. The magnetic compensator of claim 2 , further comprising:
a storage medium that retains said first digital signal, said digital compensation signal, and auxiliary sensor input data.
5. The magnetic compensator of claim 2 , wherein said digitally predicted model of the magnetic field is computed in real time.
6. The magnetic compensator of claim 2 , wherein said digitally predicted model of the magnetic field is computed from inputs to said computer selected from the group of:
a first digital signal output by said feedback magnetic compensation circuit,
a digital compensation signal output by the controller in said feedback magnetic compensation circuit,
AHRS device outputs,
global positioning device outputs,
accelerometer outputs,
tilt meter outputs,
angular rate transducer outputs,
current monitor outputs,
static field magnetometer outputs,
timer outputs, and
combinations thereof.
7. An active electromagnetic prospecting system compensator comprising the magnetic compensator of claim 2 , wherein a current waveform of a transmitter in the electromagnetic prospecting system is measured by a current-to-voltage converter to produce an output analogue signal, said output analogue signal being digitized by the first or the second ADC for input to the controller of said magnetic compensator.
8. The active electromagnetic system compensator of claim 7 , wherein the said magnetic compensator compensates for the primary magnetic field of said transmitter.
9. The active electromagnetic system compensator of claim 7 , wherein the controller in said magnetic compensator sends waveform information to said transmitter.
10. A compensated EM receiver comprising the magnetic compensator of claim 2 , wherein said magnetic compensator is mounted in a transportable housing with a power supply.
11. The compensated EM receiver of claim 10 , wherein the current waveform of the transmitter in an electromagnetic prospecting system is measured by a current-to-voltage converter to produce an output analogue signal, said analogue signal being digitized by the first or the second ADC for input to the controller of said magnetic compensator.
12. A method for compensating the output of a magnetic sensor, responding to changes in a first magnetic field, comprising the steps of:
A) creating a first analogue signal, which is formed by sensing a component of the superpositions of a first magnetic field and a compensating magnetic field of a feedback coil;
B) converting said first analogue signal into a first digital signal;
C) inputting said first digital signal in a controller via digital link;
D) computing an output digital signal with said controller, sending said output digital compensation signal via digital link;
E) converting said output digital compensation signal into a second analogue signal by a DAC, outputting a second analogue signal;
F) converting said second analogue signal into a compensation current by a voltage to current converter; and
G) sending said compensation current into said feedback coil so as to create said compensating magnetic field which opposes said first magnetic field where it is measured in step A.
13. The method of claim 12 , wherein the transmission of digital signals occurs via methods selected from the group of:
a direct electrical connection,
an optical connection,
an infrared connection,
a wireless connection and
combinations thereof.
14. The method of claim 12 , wherein in step D a computer converts said first digital signal and any auxiliary sensor input data into a digitally predicted model of the magnetic field.
15. The method of claim 14 , wherein the controller imports said first digital signal, said digitally predicted model of the magnetic field, and outputs said output digital compensation signal.
16. The method of claim 14 , wherein said first digital signal is stored after step D in a storage medium which retains said first digital signal, said output digital compensation signal and said auxiliary sensor input data.