Servo - January 2013

Part 1

Using Digital Sensors With VEX

By Daniel Ramirez

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Compasses have been around for over a thousand years. Loadstone (a mineral with magnetic properties) was used to magnetize a metal pin and has been used in compasses for hundreds of years.

Magnetic compasses were invented during the Chinese Han Dynasty between the second century BC and first century AD, and were used for navigation by the 11th century. The compass was introduced to medieval Europe 150 years later, where the dry compass was invented around 1300.This was supplanted in the early 20th century by liquid-filled magnetic compasses.

Figure 1 shows a modern magnetic compass used for navigation, travel, or hiking. Notice the red needle that is pointing north. By aligning the graduated ring with the red end of the needle, we can read the direction in degrees, where north is zero degrees. Unfortunately, the Earth's magnetic poles do not coincide with the rotational poles, and the positions of the magnetic poles change over time on a scale that is not extremely long by human standards. Significant movements happen in a few years. (Over millions of years, the directions of the true poles also shift because of continental drift.)

For an observer at any point on the Earth's surface, there is an angle — called the magnetic declination (or magnetic variation) — between the directions of magnetic north and true north. The magnetic declination varies at different points on the Earth, and also changes with time. Close to the equator, the magnetic declination is no more than a few degrees, but in arctic and Antarctic latitudes, it can be much greater. Some magnetic compasses include means to compensate for the magnetic declination, so that the compass shows true directions relative to the Earth's rotational poles.

ELECTRONIC COMPASSES An electronic compass detects the direction of the Earth’s magnetic field electronically, so it can obtain a direction while it is stationary or moving. Electronic compasses are currently being used in iPhones, iPads, and Android tablets to provide the user with map directions while traveling by car or on hikes for navigational purposes. Electronic compasses are low cost and can be a very useful sensor for robotics. Robot navigation algorithms can take advantage of an onboard electronic compass to let the robot know which direction it is heading in while running autonomously. It can also assist the operator if the robot is being maneuvered by remote control. Of course, GPS can also provide a vehicle’s current heading if the robot is moving, but it is more expensive.

Thanks to technical advances in new GPS technologies, however, we now have low-cost digital electronic compasses as well, such as the one based on the Honeywell HMC6352 shown in Figure 2. (You can pick these up at SparkFun.com for around $35 dollars.) These electronic compasses can be used