Design Feature: March 14, 1996

The familiar needle-in-a-bubble magnetic compass is a marvel of simplicity and utility, but it's subject to errors from external sources such as vibration, tilt, acceleration, and extraneous magnetic fields. Also, the traditional compass is not adaptable to electronic readout or computer interface. Other approaches are available, but have limitations (Reference 1), hence, the development of electromagnetic devices to replace the ancient magnetized-needle technology.

Picture 1

Electronic compasses not only enhance accuracy, but also provide ease of calibration. An automobile, for example, can generate magnetic fields of 150,000 to 250,000 nanoteslas (nT). The earth's field is about 18,000 nT. It's possible to compensate for the external field the car causes by strategically placing field-cancellation magnets near a bubble compass, but it's much easier to compensate electronically.

Various techniques are available for configuring static (no moving parts) magnetic detectors. Table 1 (derived from Reference 2) gives the relative attributes of the four most widely used technologies: Hall effect, magnetoinductive, flux-gate, and magnetoresistive.

Hall-effect devices are the smallest and least expensive and provide the lowest sensitivity of the four. Their low cost and small size stem from their adaptability to monolithic-IC processing.

The operating theory of Hall-effect devices is simple (Figure 1). If a magnetic field (B) impinges perpendicularly on a thin film carrying a current (I), a voltage (V) develops across the sides of the film. These devices have a useful detection range of about 10⁶ to 100 teslas (T). The earth's horizontal magnetic field measures about 18×10⁶T. These figures show that the sensitivity of a Hall-effect detector is too low to provide any useful resolution in a compass, as any significant move away from true north-south would reduce the vector to an undetectable level.

Dinsmore Instrument Co has developed a way to magnify the earth's field to bring it within the range of Hall-effect devices. The result is the company's 1490 sensor ($12), which indicates the four cardinal directions (north, south, east, west), and the four intermediate directions (northeast, southeast, southwest, northwest). For greater resolution (18 increments), Dinsmore offers two $35 models that provide sine and cosine analog outputs.

Greater resolution demands magnetic sensors that have much greater sensitivity than 10⁶T. Flux-gate sensors provide 10¹⁰T sensitivity, which is ample for measuring the earth's 18×10⁶T field. (For further explanation of flux-gate theory see Reference 3.) An external magnetic field (Figure 2) adds to the ac magnetic field generated by drive coils wound in opposition on permeable cores. When the external magnetic field is zero, the output of the pickup coil is zero. When the external field is parallel to the cores, an output appears on the pickup coil at the second harmonic of the drive frequency. The amplitude of the output is proportional to the magnitude of the external field.

Figure 3 (derived from Reference 4 ) gives another possible configuration for a flux-gate detector. This topology uses one toroidal core. The use of one pickup coil (1A and 1B) would yield the same operation as the configuration in Figure 2. Because the output is proportional to the sine of the angle of impingement of the earth's field, any heading reading would be ambiguous, however. The addition of a second, orthogonal, winding (2A and 2B) yields a cosine output that removes the ambiguity.

Magnetometers incorporating flux-gate sensors are available from Applied Physics Systems. The $2000 Model APS533 is a three-axis system containing three orthogonal flux-gate magnetometers that produce ±4-V/G (Gauss) analog outputs. KVH Industries uses flux-gate technology in a broad spectrum of electronic compasses. Some of KVH's compass systems use motion-stabilizing devices or gyros to provide stability.

Several materials, including Permalloy, exhibit a variation of their ohmic resistance in the presence of varying magnetic fields. Magnetoresistive sensors exploit this trait. Their fabrication entails depositing a thin-film bridge on a silicon substrate. This bridge can be either a stand-alone magnetoresistive bridge, or it can interface with signal-processing circuitry on the chip. A magnetic field rotates the internal magnetization vector in the film. The varying angle of this vector, along with the current flow, alters the resistance.

A variety of magnetometer systems incorporating Permalloy-based magnetoresistive sensors are available from Honeywell Inc. The systems range from single sensors to three-axis sensor hybrids, to complete three-axis "smart" magnetometers. Figure 4 shows a suggested application circuit using the HMC2003 three-axis hybrid. With the 12-bit A/D converter (ADC) shown, the least-significant bit represents about 1 mG (10⁷T) for a full-scale range of ±2G.

The remaining magnetic-sensor type, magnetoinductive (Table 1), is the only type that yields a direct digital output. For all the others, you need A/D conversion to effect an interface with digital systems. Precision Navigation Inc uses magnetoinductive technology, and incorporates it in a line of compass modules.

Figure 5 (derived from Reference 2) shows a typical three-axis system. Each single-axis, self-biasing sensing coil is wound on an elongated strip of permeable magnetic material. Each sensor provides an oscillation signal whose frequency varies with the coil's orientation to the earth's magnetic field. A microprocessor can convert the sensor frequency to a digital orientation value. The frequency of the oscillation at the sensor output varies substantially (by approximately 100%) as the sensing coil moves from a parallel position to being perpendicular to the earth's magnetic field.

The magnetic sensors described here often receive assistance from other devices. For example, some flux-gate compass systems use gyro stabilization for positional stability. KVH, for example, offers modules incorporating both rate gyros that compensate for errors from acceleration, as well as inclinometers that provide accurate readings of heading, pitch, and roll. Applied Physics Systems produces an angular-orientation sensor that incorporates a three-axis flux-gate magnetometer and a three-axis accelerometer. Precision Navigation uses "electronic gimbaling" (a three-axis magnetometer with a two-axis inclinometer) in its TCM2 compass module.

A variety of magnetic sensors offers alternatives to the traditional compass. Magnetic devices feature reasonable cost, excellent accuracy, and easy calibration procedures.

1. "Overview of Compass Technology," KVH Industries Inc, Middletown, RI, June 1994.

2. Hsu, George, "Magnetic Compassing," Precision Navigation Inc, reprint from Measurements & Control, September 1995.

3. Wan, H, Pant, B, and Krahn, D, "A Brief Study of Magnetism and Magnetic Sensors," Honeywell Inc, presented at Sensors Expo September 1995.