Bots-hitechnic-compass

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HiTechnic compass

Copy the hicompass library into the directory of the rest of your NXT libraries

• Detailed design and usage information for the HiTechnic compass sensor
  • Note: The description of how heading is stored in the sensor is incorrect. 0x44 points to the low byte and 0x45 points to the high bit. In other words, the formula for getting heading is (0x44) + 255*(0x45)

Functions

Expect more functions in the near future

• CompassSensor(bot, port) - declares a HiTechnic compass object
• get_manufacturer() - returns the eight-character string in the "manufacturer" block of the device (should be HiTechnc)
• get_type() - returns the eight-character string in the "Type" block of the device (should be "Compass ")
  • Note the extra space after Compass
• get_sample(tries=5) - returns the heading measurement of the sensor in degrees clockwise from due North.
  • Although bus errors wit the hicompass are relatively rare, get_sample() automatically tries again if it fails to get a heading.
    • You may specify the number of tries before it fails as the first argument, but the default is five
• get_relative_heading(target=0) - returns the heading relative to target. Returns a value between -180 and 180.
• is_in_range(min,max) - returns whether the compass heading is within the specified range.
  • To get a range of the opposite side of the circle, reverse max and min
• calibrate_mode() - sets the compass to calibrate mode to correct for magnetic interference.
  • see Calibrating the Compass below
• measure_mode() - sets the compass to measure mode. The compass is in this mode by default

Examples

Plug your compass into port four and place the following code somewhere in a program that already works with your robot

import nxt.hicompass
bot = (whatever variable you use to refer to your bot)
compass = hicompass.CompassSensor(bot,PORT_4)
print compass.get_manufacturer()
heading = compass.get_sample()
while heading > 10:
        heading = compass.get_sample()
        print 'Heading: ', heading

This will tell the sensor to collect headings until it is within ten degrees East of due North.

Calibrating the Compass

1. Assemble a mobile robot that can drive in a circle
   • The size and precision of the circle is unimportant as long as the robot can make a full revolution
2. Set your compass sensor to calibrate mode and instruct your robot to drive in a circle
   • Your program should tell the robot to make between one and a half and two revolutions in about twenty seconds and stop
3. Be sure to set your compass sensor back to measure mode when you are done!

Notes

The compass sensor is a very sensitive device. Here are some tips to make sure you get accurate readings

• Keep the sensor level
• Make sure the sensor is steady. The more it bounces around the less accurate your readings will be
• Keep the sensor at least four inches from your NXT brick and six from your motors. They generate magnetic fields that will result in inaccurate readings.
• Writing to the sensor (changing modes) takes a finite amount of time. Writing multiple times in succession or writing and immediately trying to get a sample is likely to return a bus error.
  • Multiple consecutive reads are not an issue.