Geometrical instruments – Indicator of direction of force traversing natural media – Magnetic field responsive
Reexamination Certificate
1998-04-24
2001-09-04
Hirshfeld, Andrew H. (Department: 2859)
Geometrical instruments
Indicator of direction of force traversing natural media
Magnetic field responsive
C033S356000, C324S253000
Reexamination Certificate
active
06282803
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, in general, to the field of electronics circuits for surveying instruments. More particularly, the present invention relates to a self compensation circuit for variations in reference levels for an electronic compass module in a laser based surveying instrument.
2. Description of the Related Art
A conventional laser based surveying apparatus developed and currently marketed by Laser Technologies, Inc. of Englewood, Colo. is the Criterion™ series of surveying instruments. The Criterion™ instrument includes, among other things, a laser transmitter and receiver, a microprocessor, a numeric keypad for data entry and instrument control commands, an internal fluxgate compass for bearing/azimuth measurement, a tilt angle sensor for inclination measurements, an aiming or spotting telescope mounted on the housing, a data collector, and serial port for data transfer. This instrument measures and computes heights, horizontal distances, vertical distances, slope distances, inclinations, coordinates, bearings or azimuths, and multiple point traverse survey data. The instrument may also be coupled to a global positioning system (GPS) receiver to pinpoint the location from which measurements are being taken.
A new modularized surveying instrument system has been developed which has all of the features of the Criterion instrument and overcomes some of the operational limitations of the Criterion instrument. This modular system uses a fluxgate compass sensor in a separable compass module. The modular fluxgate compass in a separate module can be held level while the other modules are moved as necessary to take requisite measurements.
The fluxgate compass sensor must periodically be “zeroed” or corrected for variations in the inherent physical characteristics of the sensor. This must be done periodically as the internal characteristics of the sensor change over a period of time and exposure to stray magnetic fields and other interference effects interfere with true readings. The corrections are typically manually implemented calibration steps utilizing a known reference heading.
The fluxgate compass is basically a toroidal electromagnet core driven by a drive winding coil. A pair of secondary windings coils are positioned at 90 degrees apart around the toroidal core. The basic principle utilized in the fluxgate compass is to compare the drive coil current needed to saturate the core in one direction as opposed to the opposite direction. The difference between the saturation currents is due to the external field (the earth's magnetic field). The excitation is provided by a drive coil which periodically saturates the core. Variations in the core flux due to external magnetic field effects are then detected via the induced voltage in the secondary sensing coils. Since the secondary coils are spaced apart 90 degrees, one coil output is the “X” axis component output and the other coil output is the “Y” axis component output. These outputs, when appropriately demodulated, are proportional to x and y components of the magnetic field sensed.
A commercially available fluxgate sensor is available through KVH Industries, of Middletown, R.I. Their technique is to take these secondary signals and convert them to a time value through an integrator approach and then determine the x and y components of the sensed magnetic field signal strength from an integrating converter. This approach is complex and requires a great deal of circuitry to accomplish. A simpler approach is needed to determine the direction and magnitude of an external magnetic field utilizing a fluxgate sensor.
In order to conventionally zero calibrate the KVH fluxgate compass instrument the instrument may be placed at a known location in a known orientation and then its accuracy corrected to this known location. The instrument must then be periodically re-calibrated in a similar manner to correct for zero drift of the electronics due to component aging and other long term effects. This process is time consuming and somewhat complex. Accordingly there is a need for an instrument which automatically compensates itself for sensor circuit drift and other long term effects.
SUMMARY OF THE INVENTION
The present invention is a fluxgate compass drive and sensing circuit which continuously and automatically compensates for internal sensor circuit characteristic drifts and external background effects without having to manually perform a compensation routine. Fluxgate compass self calibration in accordance with the present invention essentially involves using the output signal from the secondary coils of the two axis fluxgate magnetometer, when there is no primary coil drive signal present, as the zero reference signal level for the fluxgate magnetometer. This is accomplished by driving the primary or drive coil only part of the time and sensing secondary (x and y) signals 100% of the time. The output sensed when the drive coil is not being driven is then used automatically as the zero reference signal by the microprocessor for the next period in which the drive coil is driven. In this way, the fluxgate compass is continuously zero calibrated.
The zero calibration method in accordance with the present invention basically comprises the steps of 1) providing a plurality of sensing coils spaced around portions of a ferromagnetic toroidal core; 2) providing a drive coil positioned around the toroidal core operable to pass a drive signal from a microprocessor through the drive coil for a first period of time and a zero drive signal for a second period of time; 3) sensing a first sensor signal during said first period of time through each of said sensing coils; 4) sensing a second sensor signal from said sensing coils during said second period of time; and 5) using said second sensor signal as a zero reference signal in said microprocessor.
The self calibration drive circuit in accordance with the present invention for operating a fluxgate compass sensor via a microprocessor, wherein the fluxgate compass sensor has a toroidal core, a drive winding and at least two secondary sensing windings, comprises an oscillator connected to a first square wave generator operably connected to a frequency divider and to a plurality of anti-phase demodulator switches, the secondary sensing windings being connected to the anti-phase switches, the frequency divider being connected to the drive winding, wherein the microprocessor controls the frequency divider to provide a driver output signal to the drive winding during a first time period and a zero driver output signal during a second time period, the demodulator switches operably passing a first sensor signal during said first and second sensor signal during said second time periods to a microprocessor. The second sensor signal is then subtracted from the first sensor signal in the microprocessor so that the zero reference signal changes are automatically compensated for during each measurement.
An alternative preferred embodiment of the present invention also includes an automatic dual mode sensitivity control for the fluxgate compass core to compensate for operating changes which occur due to operation of the instrument in regions of reduced horizontal component such as with operation above the arctic circle. In these polar regions the horizontal component of the magnetic field is greatly reduced compared to the vertical or z component of the field.
This alternative embodiment also includes circuitry to compensate for the local environment of operation of the instrument, such as in an automotive vehicle. In this case the vehicle's metal will distort the magnetic readings and thus introduce an offset. The alternative embodiment senses this offset and introduces a compensating offset to negate the magnetic effects of the vehicle on the instrument.
This alternative embodiment also includes a dithering signal which is fed to the demodulation circuit to improve the accuracy of measurement transmitted to the microprocessor. Finally, t
Hirshfeld Andrew H.
Hogan & Hartson LLP
Kubida William J.
Langley Stuart T.
Laser Technology, Inc.
LandOfFree
Self calibration circuit for determining an accurate zero... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Self calibration circuit for determining an accurate zero..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Self calibration circuit for determining an accurate zero... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2470468