Compensation system for electronic compass

• US 20020083605A1
• Filed: 01/11/2001
• Published: 07/04/2002
• Est. Priority Date: 12/20/1991
• Status: Active Grant

• Current Assignee: Howard J. Haselhuhn Jr, Kenneth L. Schierbeek, Kenneth Schofield, Richard J. Gahan, Rodney K. Blank
• Original Assignee: Howard J. Haselhuhn Jr, Kenneth L. Schierbeek, Kenneth Schofield, Richard J. Gahan, Rodney K. Blank
• Inventors: Blank, Rodney K., Gahan, Richard J., Haselhuhn, Howard J. Jr., Schofield, Kenneth, Schierbeek, Kenneth L.
• Granted Patent: US 6,427,349 B1
• Time in Patent Office: Days
• US Class Current: 33/356
• CPC Class Codes: B60K 35/00 Instruments specially adapted for vehicles; Arrangement of instruments in or on vehicles
• CPC Class Codes: B60R 1/088 using a cell of electrically changeable optical characteristic, e.g. liquid-crystal or electrochromic mirrors
• CPC Class Codes: B60R 1/12 Mirror assemblies combined with other articles, e.g. clocks
• CPC Class Codes: B60R 1/1207 with lamps; with turn indicators
• CPC Class Codes: B60R 2001/1215 with information displays
• CPC Class Codes: B60R 2001/1223 with sensors or transducers
• CPC Class Codes: B60R 2001/123 with thermometers
• CPC Class Codes: B60R 2001/1246 with clocks
• CPC Class Codes: B60R 2001/1253 with cameras, video cameras or video screens
• CPC Class Codes: B60R 2001/1284 with communication systems other than radio-receivers, e.g. keyless entry systems, navigation systems; with anti-collision systems
• CPC Class Codes: G01C 17/28 Electromagnetic compasses with north seeking magnetic elements and having electric pick-offs G01C17/26
• CPC Class Codes: G01C 17/38 Testing, calibrating, or compensating of compasses
• CPC Class Codes: G01R 33/025 Compensating stray fields G01R33/0017 takes precedence

Abstract

An electronic compass is described for use in vehicles. The compass employs a magnetoresistive sensor for sensing the earth magnetic field and the sensor is operated in alternate set/reset bias modes. In a first embodiment, the compass is provided with deviation compensation by a closed loop system including measurement of the sensor output signals and an offset current strap for nullifying the vehicle deviation field. In a second embodiment, deviation compensation is provided by operation in an initial calibration mode and by operation in a normal compensation mode to adjust compensation, as needed, on a long term basis during normal operation of the compass. In the initial calibration mode, while the vehicle is being driven, the signal peak values are adjusted to a nominal earth field level by changing the offset current. Then, compensating signal reference values for each axis are determined as each peak for that axis is determined. The system automatically exits the initial calibration mode when certain criteria have been met. In the normal compensation mode, The signal reference value for each axis is adjusted at least once during the time interval between turn-on and turn-off of the vehicle ignition switch.

Claims

1. A method for compensating for the effect of a deviating magnetic field in a vehicle compass, said compass comprising:

first and second magnetoresistive sensors responsive to an external magnetic field for developing electronic signals representative of said external magnetic field, said external magnetic field being a combination of the earth magnetic field and a deviating field of the vehicle,

said first and second sensors being oriented in a predetermined angular relation with each other, and being aligned in a predetermined angular relation with respective axes of said vehicle,

each of said sensors including means for nullifying at least part of a deviating magnetic field which impinges on it, said method comprising the steps of;

determining-whether the value of the sensor signal of each sensor is different from a nominal value which corresponds approximately to the value of the earth magnetic field,

if it is, using said nullifying means for adjusting the magnetic field impinging on each sensor to adjust the sensor signal to a value approximately equal to the nominal value,

determining the maximum and minimum peak values of each sensor signal,

determining the average of said peak values of each sensor signal,

and mathematically compensating the actual value of each sensor signal in accordance with the respective average value.

2. A method for compensating for the effect of a deviating magnetic field in a vehicle compass, said compass comprising:

first and second magnetoresistive sensors responsive to an external magnetic field for developing electronic signals representative of said external magnetic field, said external magnetic field being a combination of the earth magnetic field and a deviating field of the vehicle,

said first -and second sensors being oriented in a predetermined angular relation with each other, and being aligned in a predetermined angular relation with respective axes of said vehicle,

each of said sensors including means for nullifying at least part of a deviating magnetic field which impinges on it,

a digital electronic circuit including means for measuring said signals when said signals are within a predetermined measurement range,

said method comprising the steps of;

determining whether the value of the sensor signal of each sensor is within said measurement range,

if it is not, using said nullifying means for nullifying at least part of magnetic field impinging on said sensor to reduce the sensor signal to a value within said range.

3. A method as defined in claim 2 including the steps of:

determining whether the value of the sensor signal of each sensor is greater than a nominal value which is approximately equal to the earth magnetic field,

and if it is, using said nullifying means for adjusting at least part of the magnetic field impinging on each sensor to adjust the sensor signal to a value approximately equal to the nominal value.

4. A method as defined in claim 3 including the steps of:

determining the maximum and minimum peak values of each sensor signal,

determining a reference value for each sensor signal equal to the average of said peak values of each sensor signal,

and mathematically compensating the actual value of each sensor signal in accordance with the respective reference value.

5. A method for updating the deviation compensation of a vehicle compass comprising:

first and second sensors responsive to an external magnetic field for developing electronic signals representative of said external magnetic field, said external magnetic field being a combination of the earth magnetic field and a deviating field of the vehicle,

said first and second sensors being oriented in a predetermined angular relation with each other, and being aligned in a predetermined angular relation with respective axes of said vehicle,

and a memory which stores a reference value for each sensor signal, said reference value for each sensor being equal to an offset value for providing deviation compensation for the respective sensor at a previous time,

said method comprising the steps of;

detecting the occurrence of a peak in the sensor signal of one of the sensors,

measuring the value of the sensor signal of the other sensor which occurs at the same time as the occurrence of said peak,

and if there is a difference between the measured value and said reference value, adjusting the reference value in accordance with the difference.

6. A method as defined in claim 5 wherein said sensors are magnetoresistive sensors.

7. A method as defined in claim 5 wherein said compass is installed in a vehicle having a switch which is in one of two switch states when the vehicle is being driven;

and wherein said step of adjusting is executed a predetermined number of times during a time-interval that said switch is in said one state without being switched to another state.

8. A method of determining an offset calibration value for a magnetoresistive sensor in a magnetic compass, said compass including an electronic circuit and a microcomputer for processing an output signal from said sensor, said sensor being alternately biased between a reset mode and a set mode,

said method comprising the steps of:

determining whether both reset signal and the set signal are equal to or greater than zero at the same time,

if they are, calculate a sensor offset calibration value.

9. A method as defined in claim 8 wherein said step of calculating includes adding one-fourth of set signal value plus the reset signal value to one-half of a previous sensor offset calibration value which was calculated in the same way the last time the calibration value was calculated.

10. A method for compensating an electronic compass in a vehicle of the type comprising first and second sensors oriented in a predetermined angular relation with each other and with respective axes of the vehicle,

said method comprising the steps of:

determining a compensating reference signal value for each sensor,

detecting the occurrence of a peak in the sensor signal of one of the sensors,

measuring the value of the sensor signal of the other sensor which occurs at the same time as the occurrence of said peaks,

and, if there is a difference between the measured value and the reference value, adjusting the reference value in accordance with the difference.

11. In combination, for use in a vehicle,

an electronic compass comprising a magnetoresistive sensor for detecting the magnetic field of the earth,

an electronic circuit coupled with said sensor for developing signals indicative of the direction of said vehicle,

and electronic display coupled with said circuit for displaying said direction,

a rear view mirror,

said electronic display being disposed on said mirror for viewing by a driver of said vehicle.

12. The invention as defined by claim 11 wherein said sensor and said electronic circuit are mounted on said mirror.