martinmagal@fablabil

week 13, apr 17

weekly assignment: add an output device to a microcontroller board and program it to do something

the tools

hardware:

raspberry pi, (aka: raspi)

servo motor

software:

the operating system for the raspberry pi: raspbian, a debian linux for raspi's

a kernel module developed by adafruit designed to control a servo thru the raspi's GPIO pins

putty, a tool to connect to the raspi thru your laptop without the need for a separate keayboard & screen

python

the idea

in my final project interactive installation i intend to use a raspberry pi to collect tweets and output sound and voices and maybe lights/video.

so i wanted to test the raspi and begin to set up my "system" while doing the output device's assignment.
i decided to try controlling a servo motor with the raspi's GPIO pins (see picture above).

the program

the raspi has a set of input and output pons called GPIO (general purpose input output).
the only GPIO pin that can control a servo is pin 18, also called PWM, for Pulse Width Modulation

the kernel module i installed (mentioned above) uses a file type of interface, where you control what the output pin 18 and therefore the servo is doing, by reading and writing to special files, see here

the python program used to control the servo is quite quite short and straightforward, and allows you to control angle and speed.

# Servo Control
import time
def set(property, value):
try:
f = open("/sys/class/rpi-pwm/pwm0/" + property, 'w')
f.write(value)
f.close()
except:
print("Error writing to: " + property + " value: " + value)

def setServo(angle):
set("servo", str(angle))

set("delayed", "0")
set("mode", "servo")
set("servo_max", "180")
set("active", "1")

delay_period = 0.01

while True:
for angle in range(0, 180):
setServo(angle)
time.sleep(delay_period)
for angle in range(0, 180):
setServo(180 - angle)
time.sleep(delay_period)