Servo - January 2013

www.servomagazine.com/index.php?/magazine/article/january2013_Ramirez

for VEX robotic applications as we will demonstrate in this article. The Honeywell HMC6352 electronic compass provides headings from 0° to 360° within a 1° resolution (if the compass is level). This is excellent resolution and more than adequate for most robotic applications considering the modest price of the sensor. There are some sources of interference that can reduce the theoretical resolution somewhat; these will be covered below.

More expensive versions of these electronic compasses can even compensate for tilts so that they don’t need to be level to give good readings.

GETTING YOUR BEARINGS STRAIGHT

Depending on where the compass is located on the surface of the Earth, the angle between true north and magnetic north can vary widely as mentioned, increasing the farther one is from the prime meridian of the Earth’s magnetic field. The local magnetic declination is given on most maps to allow the map to be oriented with a compass parallel to true north. Users of such a compass have to know the local value of the magnetic declination, and adjust the compass accordingly. For this adjustment, the moving ring on the compass is used to compensate for declination from true north by adding or subtracting the declination of the user’s geographic location (latitude and longitude).

While these abnormalities (mostly caused by the Earth’s molten core) can cause both magnetic and electronic compasses problems, they can be adjusted for using online declination compensation calculators provided by the National Ocean and Atmospheric Administration (NOAA)/National Geophysical Data Center (NGDC).

Their online magnetic declination calculator ( www.ngdc. noaa.gov/geomag/declination.shtml) allows you to enter your location (or zip code for the USA) and get the declination value. Remember when using the declination values that east declination is positive, west is negative.

How can we correct a compass bearing to a true bearing? We can compute the true bearing from a magnetic bearing by simply adding the magnetic declination to the magnetic bearing. This works as long as you follow the convention of degrees that west is negative (i.e., a magnetic declination of 10 degrees west is - 10 and a bearing of 45 degrees west is - 45). This correction is applied to both magnetic and electronic compasses, since they use the Earth’s magnetic field to determine the direction where magnetic north is located.

Another cause of error in compass readings is taking them when the compass itself is not level. This source of error can be reduced by insuring that the compass is gimbaled.

Having thoroughly covered using analog sensors with

FIGURE 2.

Due to technical advances in new GPS technologies, we now have low-cost digital electronic compasses such as the one shown here.

the VEX microcontroller in previous issues of SERVO, in this installment we will investigate new digital sensors that are starting to show up in many new consumer products including the iPhone, iPad, and automobiles (mentioned previously). We will also show you how you can take advantage of these exciting new sensors for VEX robotic applications.

First, let’s cover some new sensors that are not quite available yet from Innovation First, Inc. (IFI), although the new VEX Cortex and VEX ARM9 microcontrollers do support them in hardware through their I2C interface. As of this writing, there is not yet support for them from the compiler vendors (RobotC and Easy C Pro), although I’m sure this is on their radar since these sensors are becoming very popular for robotics, consumer, and industrial applications.

JUST WHAT IS I2C?

I²C (“eye-squared cee;” Inter-Integrated Circuit; generically referred to as a “two-wire interface”) is a multi-master serial single-ended computer bus invented by Philips that is used to attach low-speed peripherals to a motherboard, embedded system, cellphone, or other electronic device. From the mid 1990s, several competitors (e.g., Siemens AG, Intel Mobile Communications, NEC, Texas Instruments, STMicroelectronics, Motorola/Freescale, Intersil, Microchip, etc.) have brought I²C products to market which are fully compatible with the NXP (formerly Philips’s semiconductor division) I²C system.