My first encounter with a GPS device left me wondering why it didn't provide a compass heading when I pointed it in different directions. Then I remembered that the system pinpoints location, and I couldn't get a heading without moving to a new position. What I wanted was a sensor that would detect the Earth's magnetic field. Though it's easy to envision such sensors in handheld devices, I can see many applications in autonomous robots and vehicles that need GPS coordinates and magnetic-heading information.
You can buy magnetometers, but they're not always easy to use, because to the math needed to convert sensor outputs into useful information and the effects of "soft iron" and "hard iron" on magnetometer response. To help simplify magnetometer use for equipment designers, Freescale Semiconductor offers Xtrinsic eCompass software that works with the company's accelerometers and magnetometer to provide applications with accurate magnetic-bearing information. The eCompass hardware incorporates a three-axis accelerometer and a three-axis magnetometer. The accelerometer measures the orthogonal components of the earth's gravity, and the magnetometer measures components of Earth's magnetic field.
Because a design would place both sensors on a circuit board, their outputs change depending on the board's orientation and its local physical environment. According to Freescale, if the PCB remains flat, a processor could compute a compass heading from the arctangent of the ratio of the two horizontal magnetic field components. In most cases, though, a PCB will have an arbitrary orientation, so the compass heading becomes a function of all three accelerometer readings and all three magnetometer readings. For more technical information and the math involved, download the application note, "Implementing a Tilt-Compensated eCompass using Accelerometer and Magnetometer Sensors," Freescale document AN4248.
Although the math looks complicated at first glance -- how's your vector algebra these days? -- Freescale has done most of the work and made it available as free eCompass code.
A four-element model supports hard iron interference correction, and Freescale recommends it for designers who will use the Xtrinsic MAG3110 sensor in applications with minimal soft iron distortion and small amounts of processor RAM.
Jon, these new sensors make it much easier to include new functionality in a device. I have been impressed with the Xtrinsic series since I first worked with it. Good article.
One recent contract design I did for a military contract placed an Ecompass on the main board of a portable GPS device.True, that multiple subsequent GPS fixes will provide a compass heading, the need was for a foot soldier who may be hunkered down in a foxhole, and needed direction.From the designer's perspective, many things can affect the Ecompass performance as Jon pointed out: shields, capacitors, and any Fe based components.
But two surprises were the screws and the Stainless steel shields. First, the screws were expected to wreak havoc with the Ecompass, but careful placement of the SMD to be equidistant between two screws proved sufficient .Test readings still came thru perfectly.
The second surprise was the SS shields, and we went to great lengths to requalify new, NiAg shields as engineered replacements.Halfway into the effort to requalify NiAg, one of the techs put a white-board magnet on the rev 1 stainless steel shield being phased out.It didn't stick.Foolishly, I took for granted without actually checking the metallurgical properties of the grade of SS shields, that they in fact did not possess any iron and did not affect the Ecompass performance any differently that the newer Nickel Silver shields.Didn't do my homework and it resulted in a lot of extra work for no reason.
Thanks for this @Jon. With the increased availability of compact, integrated sensors and low-power computing, I expect the boom into "sensor fusion" is just starting. I'm delighted to read that Freescale did all of the difficult calibration analysis and created an on-board algorithm. The application note you linked even includes the source code for the calibration algorithm they developed. Kudos to Freescale for making their code open so it can be reviewed, modified, improved, and adapted by the sensor community. I hope they can be an example for other instrumentation manufacturers.
Rotating the chip itself in relation to hard/soft iron can dramatically improve performance. We have a design which uses a compass in close proximity to a brushless motor. It isn't intended to provide a true heading, but does monitor rotation.
Because semiconductor manufacturers make complicated devices and want designers to use them, they create development tools and provide source code that illustrates how to use these devices. I'd say almost all MCU and programmable-sensor companies offer hardware and software to help engineers and product designers get off to a good start. In most cases, open-source code provides a way to start with a core of working software. Of course, the code might run optimally only on the MCU-maker's chips.
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