58. Exploring the Pi B+'s Ports IV - Reading the MPU-6050 using Python

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The last post described the 3D wireframe demonstration of the MPU-6050's rotations (ie the GY-521 breakout board) about the x-, y- and z-axes, and other scripts which displayed the readout of quaternions etc.  Those test programs were written in the C++ programming language. 

This time test programs are written in Python, giving me a better chance of understanding what's going on!  The circuitry is identical to that used in the last post, and of course, the I²C interface has previously been enabled.

smbus is a Python module which allows SMBus (system management bus - a sub-set of the I²C protocol) access through the I²C interface on Linux hosts. This is the key module for Python access.

Reading Accelerometer Data

In this first program, in addition to reading the data from the accelerometer, the output values are sent to a web page, just for fun!  The address of the web page depends on the Pi's IP address, which can be found using the terminal command

ifconfig

and appending :8080 to the end.   For example,

http://ip-address-of-your-pi:8080

Here's the Python code for server.py:

	#!/usr/bin/python
	#This python program serves a web page which can be accessed via
	#local wifi network, giving x and y rotations of the MPU-6050
	# Written by Andrew Birkett
	# (http://blog.bitify.co.uk/2013/11/3d-opengl-visualisation-of-data-from.html)
	# and modified by S&S Dec 2014
	import web
	import smbus
	import math
	urls = (
	'/', 'index'
	)
	# Power management registers
	power_mgmt_1 = 0x6b
	power_mgmt_2 = 0x6c
	bus = smbus.SMBus(1) # or bus = smbus.SMBus(0) for pre-Revision 2 boards
	address = 0x68 # This is the address value read via the i2cdetect command
	def read_byte(adr):
	return bus.read_byte_data(address, adr)
	def read_word(adr):
	high = bus.read_byte_data(address, adr)
	low = bus.read_byte_data(address, adr+1)
	val = (high << 8) + low
	return val
	def read_word_2c(adr):
	val = read_word(adr)
	if (val >= 0x8000):
	return -((65535 - val) + 1)
	else:
	return val
	def dist(a,b):
	return math.sqrt((a*a)+(b*b))
	def get_y_rotation(x,y,z):
	radians = math.atan2(x, dist(y,z))
	return -math.degrees(radians)
	def get_x_rotation(x,y,z):
	radians = math.atan2(y, dist(x,z))
	return math.degrees(radians)
	class index:
	def GET(self):
	accel_xout = read_word_2c(0x3b)
	accel_yout = read_word_2c(0x3d)
	accel_zout = read_word_2c(0x3f)
	accel_xout_scaled = accel_xout / 16384.0
	accel_yout_scaled = accel_yout / 16384.0
	accel_zout_scaled = accel_zout / 16384.0
	xrot = get_x_rotation(accel_xout_scaled, accel_yout_scaled, accel_zout_scaled)
	yrot = get_y_rotation(accel_xout_scaled, accel_yout_scaled, accel_zout_scaled)
	# Return angle values to 1 decimal place
	return "x-rotation: %.1f" %xrot+" degrees. y-rotation: %.1f" %yrot+" degrees"
	if __name__ == "__main__":
	# Now wake the 6050 up as it starts in sleep mode
	bus.write_byte_data(address, power_mgmt_1, 0)
	app = web.application(urls, globals())
	app.run()

view raw server.py hosted with ❤ by GitHub

Andrew Birkett's really neat code reads the accelerometer part of the MPU-6050 and prints out the x- and y-axis rotations which are calculated by the program.  The output via the web page looks like this:

x-rotation: 0.2 degrees.   y-rotation: -2.2 degrees.

The values of course change as the MPU-6050 is rotated about its x- and y-axes. for example:

x-rotation: 50.8 degrees.   y-rotation: 18.4 degrees.

The web page has to be refreshed to see new values.

Reading Gyro & Accelerometer Data

This program, Reading6050.py, also written by Andrew Birkett, looks like this:

	#!/usr/bin/python
	import smbus
	import math
	# Power management registers
	power_mgmt_1 = 0x6b
	power_mgmt_2 = 0x6c
	def read_byte(adr):
	return bus.read_byte_data(address, adr)
	def read_word(adr):
	high = bus.read_byte_data(address, adr)
	low = bus.read_byte_data(address, adr+1)
	val = (high << 8) + low
	return val
	def read_word_2c(adr):
	val = read_word(adr)
	if (val >= 0x8000):
	return -((65535 - val) + 1)
	else:
	return val
	def dist(a,b):
	return math.sqrt((a*a)+(b*b))
	def get_y_rotation(x,y,z):
	radians = math.atan2(x, dist(y,z))
	return -math.degrees(radians)
	def get_x_rotation(x,y,z):
	radians = math.atan2(y, dist(x,z))
	return math.degrees(radians)
	bus = smbus.SMBus(1) # or bus = smbus.SMBus(1) for Revision 2 boards
	address = 0x68 # This is the address value read via the i2cdetect command
	# Now wake the 6050 up as it starts in sleep mode
	bus.write_byte_data(address, power_mgmt_1, 0)
	print "gyro data"
	print "---------"
	gyro_xout = read_word_2c(0x43)
	gyro_yout = read_word_2c(0x45)
	gyro_zout = read_word_2c(0x47)
	print "gyro_xout: ", gyro_xout, " scaled: ", (gyro_xout / 131)
	print "gyro_yout: ", gyro_yout, " scaled: ", (gyro_yout / 131)
	print "gyro_zout: ", gyro_zout, " scaled: ", (gyro_zout / 131)
	print
	print "accelerometer data"
	print "------------------"
	accel_xout = read_word_2c(0x3b)
	accel_yout = read_word_2c(0x3d)
	accel_zout = read_word_2c(0x3f)
	accel_xout_scaled = accel_xout / 16384.0
	accel_yout_scaled = accel_yout / 16384.0
	accel_zout_scaled = accel_zout / 16384.0
	print "accel_xout: ", accel_xout, " scaled: ", accel_xout_scaled
	print "accel_yout: ", accel_yout, " scaled: ", accel_yout_scaled
	print "accel_zout: ", accel_zout, " scaled: ", accel_zout_scaled
	print "x rotation: " , get_x_rotation(accel_xout_scaled, accel_yout_scaled, accel_zout_scaled)
	print "y rotation: " , get_y_rotation(accel_xout_scaled, accel_yout_scaled, accel_zout_scaled)

view raw Reading6050.py hosted with ❤ by GitHub

The program produces results like this:

pi@raspberrypi ~ $ sudo python Reading6050.py



gyro data



---------



gyro_xout:  -202  scaled:  -2



gyro_yout:  -191  scaled:  -2



gyro_zout:  11  scaled:  0








accelerometer data



------------------



accel_xout:  176  scaled:  0.0107421875



accel_yout:  160  scaled:  0.009765625



accel_zout:  15364  scaled:  0.937744140625



x rotation:  0.596614942501



y rotation:  -0.656278927449



pi@raspberrypi ~ $ 

So what does the program do and what do these results mean?