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Compasses: from magnetite to digital Compasses have not changed much since they were first invented. All compasses react to the Earth’s low level magnetic field and it is only now that electronic compasses are able to properly discriminate between the horizontal and vertical compon­ents of that field using technology developed by Precision Navi­gation, Inc. Compasses have helped guide people over the land and oceans for thousands of years. Historians date the first vehicle compass to 2634 BC, when a Chinese inventor suspended a piece of mag­netite from a thread to guide his chariot. The problem is, most compasses available today, whether mechanical or electronic, are not a great improvement upon the original. They still bounce over bumps, get thrown off course by magnetic interference and are adversely affected by factors such as vibration, tilt and acceleration. Only recently has technology been employed to improve on the original concept. The basic mechanical compass is still just a magnetised needle suspended on a jewelled bearing. The biggest innovation in mechanical compasses within the last few thousand years has been to envelop the magnetic needle in a viscous damping fluid. This allows the compass needle to settle more quickly after the com­pass 14  Silicon Chip has been moved and greatly reduces needle oscillation. Electronic magnetometers were developed decades ago but it wasn’t until the 1970s that there were any real production ver­sions of electronic compasses available to the general public. Most of these compasses were based upon flux-gate magnetometers, a technology first invented in the 1930s. All had mechanical gimbaling in order to eliminate errors due to tilt and were fairly limited for navigation. Most found application on sailboats. Designing a compass The most fundamental step in designing any compass is to have a device which reacts to the direction of the Earth’s low-level DC magnetic field. The mechanical compass magnetised needle has done this task fairly well for thousands of years. The elec­tronic compass, however, requires some sort of electrical trans­ducer to measure this low level field, which can then be trans­formed into a heading for display. A common approach used in the past combines a magnetised card which is optically encoded and a photodiode pair which can decode the position of this card. The magnetised card then acts as a normal mechanical compass and the optical electronics pro­vide input to a microprocessor which allows the heading informa­ t ion to be processed and displayed. Unfortunately, this approach has all the same weaknesses as any mechanical compass. To obtain a really improved compass, a different approach is required. Magnetic compass variables The Earth’s magnetic field is three-dimensional, having two horizontal components (X and Y axes) and one vertical component (Z axis). The closer you travel towards the Earth’s north or south magnetic poles, the Fig.1: block diagram of a digital compass based on the Precision Navigation variable permeability magnetometer. stronger becomes the Z component of the total magnetic field. For example, at the latitude of San Fran­cisco, the Z-component accounts for almost 70% of the Earth’s total magnetic vector. This creates a problem when a compass with fixed magnetome­ters for its X and Y axes is tilted. The relatively large Z component of the field gets mapped into the X-Y plane and is subsequently translated into a heading error. Depending upon the orientation of the compass and the latitude, this tilt error typically translates into two to five degrees of heading error for each degree of tilt from level. Tilt compensation There are three solutions to this problem. The first and most obvious is to ensure that the compass always remains level which is not always practical. The second is mechanical gimbaling of the magnetic sensors to ensure that they remain level when pitch and roll are present. The third method is electronic tilt compensation. This requires measurement of the Z component of the magnetic field via a third magnetometer and the measurement of pitch and roll of the system with some sort of tilt sensor. Tilt compensation is then taken care of mathematically via a microprocessor. In applications where the system remains level, fixed two-axis magnetic compasses are quite accurate and are less expensive than tilt-compensated systems. On rolling platforms requiring continuous accuracy, mechan­ical gimbal­ ing is the most common solution. A 2-axis magnetic sensor is attached to a pendulum (gimbal) which is encased in a viscous damping fluid to reduce oscillations. Typical pendulum designs accommodate tilts from ±20 de­grees up to ±45 degrees. Should the compass tilt beyond that range, the gimbaling is no longer effective and the accuracy is greatly reduced. This approach suffers from weaknesses such as gimbal lock, large size, fragility and the relative movement of the sensor with respect to the reference frame of the system. The third approach is a so-called “strapped down” solution. By using a triaxial magnetometer to measure the X, Y and Z axes of the magnetic field and including the input of the inclin­ ome­ter, errors generated by tilting the compass module are mathemat­ically corrected by the module’s microprocessor. The inclinome­ ter’s angular evaluation also can be displayed to the user or output to a host system. Dynamic environments Tilt-compensated magnetic compasses are vulnerable to vary­ing levels of vibration and acceleration. The limiting factor is not the magnetic sensors but the tilt-compensating mechanism, be it mechanical gimbaling or inclination sensors. Mechanically gimbaled compasses are the most susceptible to “sloshing” and slow response time on rolling or rumbling platforms. Liquid inclinometers are also compromised where there is rapid accelera­ tion. Varying the viscosity of the liquid can diminish this problem. For very dynamic platforms – military aircraft, for in­stance – accelerometers and gyroscopes, combined with magnetome­ ters provide the highest In applications where the system remains level, fixed two-axis magnetic compasses are quite accurate and can be catered for by Precision Navigation’s Vector-2X compass module, shown on the right. Where tilt compensation is required as well, the Precision TCM2 module on the left is available with ±20 de­grees, ±50 degrees and ±80 degrees of compensation. January 1998  15 Silicon Chip Binders REAL VALUE AT $12.95 PLUS P &P These binders will protect your copies of SILICON CHIP. They feature heavy-board covers & are made from a dis­ tinctive 2-tone green vinyl. They hold up to 14 issues & will look great on your bookshelf. ★  Hold up to 14 issues ★  80mm internal width ★ SILICON CHIP logo printed in gold-coloured lettering on spine & cover Price: $A12.95 plus $A5 p&p Available only in Australia Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or fax (02) 9979 6503; or ring (02) 9979 5644 & quote your credit card number. Use this handy form Enclosed is my cheque/money order for $________ or please debit my ❏ Bankcard  ❏  Visa   ❏ Mastercard Card No: ________________________________ Card Expiry Date ____/____ Signature ________________________ Name ___________________________ Address__________________________ __________________ P/code_______ 16  Silicon Chip Made by Precision Navigation, Inc, this handheld digital compass has inbuilt tilt compensation up to ±15 degrees and many features that were undreamt of years ago. It can be referenced to true or magnetic north, has red and green lights to allow a fixed course to be maintained at night and can store up to 10 bearings and even multi-leg courses with a heading and time for each leg. It even alerts you to magnetic interference from nearby metallic objects, power lines, etc. reliability but at a substantially higher price. Magnetic distortion corrections All compasses can perform well in a controlled environment, where the ambient magnetic field consists solely of the Earth’s field. In most practical applications however, an electronic compass module will be mounted in a host system such as a vehi­cle, and this will contain large sources of magnetic fields such as steel chassis, transformer cores, electrical currents and permanent magnets in electric motors. This “hard iron” magnetism remains relatively stable over time and therefore can be measured and calibrated out of compass readings. Calibration typically involves rotating the vehicle through 360 degrees and storing several magnetic readings. Howev­ er, once the local magnetic fields which cause the distortion errors have been measured, the magnetic sensors must stay fixed in relative position to that local distortion field. This is a serious limitation of mechanically gimbaled com­passes. The sensors are mounted on the end of a pendulum and therefore change their relative position within the distortion field and this can degrade compass accuracies. Precision Navigations’s TCM2 module has fixed magnetometers that never move with respect to its host system, thus calibration data is valid through its full tilt range. This calibration data is stored in the device’s non-volatile EEPROM so that it is preserved during power-down. “Soft iron” magnetism is a more difficult local distortion which varies in strength and direction – ie, it can add or sub­tract to the Earth’s magnetic field within a vehicle or system. Only a few electronic compass modules can handle soft iron anoma­lies. and can be made quite small. Because they work inductively, they draw a fraction of the current of flux-gate sensors, typically 2-3mA instead of 40-60mA. Flux gate technology Magnetoresistive (MR) Flux gate sensors typically comprise a low-coercivity core surrounded by drive and sense coils. The core is saturated with an AC current in the drive coil, inducing an AC voltage in the sense coil which includes the drive frequency and its second and higher order harmonics. The presence of an external magnetic field will cause a shift of the core’s hysteresis loop, creating a second harmonic which can be correlated to the strength of the external magnetic field. Most flux gate magnetometers are biaxial; ie, they only sense the Earth’s horizontal (X and Y) magnetic field. Accurate sensing of the vertical (Z axis) magnetic field component is critical when a compass is electronically gimbaled. Some flux gate compass manufacturers do offer electronically gimbaled modules. These are typically coupled biaxial sensors with one redundant axis combined with a tilt sensor. Permalloy and other materials exhibit a variation of their ohmic resistance when subjected to varying external magnetic fields. Magnetoresistors are typically fabricated by depositing thin film or nickel-iron (NiFe) onto a silicon substrate as a standalone magnetoresistive bridge, or integrated with signal processing circuitry. A magnetic field rotates the internal magnetisation vector in the film and the varying angle of this vector with the current flow alters the resistance. MR sensors are relatively inexpensive to manufacture but like fluxgates, their analog output needs to be converted through A/D circuitry for many applications, which increases costs and complexity. Magneto-inductive Precision Navigation’s magnetoinductive sensors were pat­ented in 1989. Each single-axis sensing coil is wound on an elongated strip of high direct-current permeable magnetic materi­ al and is self-biasing. Each sensor provides an oscillation signal that varies in frequency when oriented at different angles with respect to the Earth’s magnetic field. A microprocessor can then receive sensor information in frequency form, which is converted into an orientation with respect to the Earth’s magnet­ic field. The frequency of the oscillating signal at the output of the sensing circuit varies substantially (eg, by about 100%) as the sensing coil is moving from a parallel to an antiparallel orientation, with respect to the Earth’s magnetic field. These substantial frequency differences mean that a very accurate digital readout of angle between the sensing coil orientation and magnetic North is obtained from the microprocessor. Due to the simplicity of design and materials, magneto-inductive sensors are very inexpensive to manufacture SILICON CHIP This advertisment is out of date and has been removed to prevent confusion. Hall effect Hall effect sensors are at the low end of the sensitivity spectrum. They are fabricated with monolithic integrated circuit processes and are thus small and inexpensive. However, they are largely impractical for measuring the Earth’s field because they suffer from drift, instability and poor sensitivity. In the future, we can expect to see this technology in con­sumer products ranging from hand-held GPS receivers with built-in compassing to toys and mobile communications equipment. SMART ® FASTCHARGERS Brings you advanced technology at affordable prices Availability For further information on the Precision Navigation Vector-2X and TCM2 modules, contact Sphere Communications, PO Box 380, Darlinghurst, NSW 2010. Phone (02) 9344 9111; fax (02) 9349 5774. For information on the Precision Navigation Outback-ES digital compass, contact Sphere Communications or Av-Comm Pty Ltd, phone (02) 9949 7417 (see their 32-page catalog elsewhere in this issue). Acknowledgement: this article was adapted from an ar ticle entitled “Magnet­ ic Compass­ ing” by George Hsu of Precision Navigation originally published by Measurements & Control, SC September 1995. As featured in ‘Silicon Chip’ Jan. ’96 This REFLEX® charger charges single cells or battery packs from 1.2V to 13.2V and 110mAh to 7Ah. VERY FAST CHARGING. Standard batteries in maximum 1 hour, fast charge batteries in max. 15 minutes AVOID THE WELL KNOWN MEMORY EFFECT. NO NEED TO DISCHARGE. Just top up. This saves time and also extends the life of the batteries. SAVE MONEY. Restore most Nicads with memory effect to remaining capacity and rejuvenate many 0V worn-out Nicads EXTEND THE LIFE OF YOUR BATTERIES Recharge them up to 3000 times. 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