Raspberry Pi Model B+ - GPIO18 Servo Control with PWM

servo.rb

#! /usr/bin/ruby

# Servo Rotation

# Wiring:
#     From +5V (PIN2) to Positive on Servo (Red)
#     From Ground (PIN6) to Ground on Servo (Brown)
#     From GPIO18 to 1kOhm Resistor to Signal on Servo (Orange)

# Requires Ruby (`sudo apt-get install ruby ruby1.9.1-dev`)
# Requires WiringPi (You may need to `sudo gem install wiringpi` first)
require 'wiringpi'

# Set variable io to interact with GPIO using the GPIO Pin Layout
# The Raspberry Pi B+ model has only one hardware implementation of
# PWM which means that you will need to use GPIO18 (Pin 1).
io = WiringPi::GPIO.new(WPI_MODE_GPIO)
pin = 18
io.mode pin, PWM_OUTPUT

# We will use an instance variable to keep track of the last position.
# This will be helpful so that when making small iterations from the
# previous servo position, we will not bump the ends.
@last_position = 0

# Pulser method will take the frequency and duration (20ms)
# This method will put HIGH load for freq miliseconds and pause for
# freq minus duration (20ms) for a total of 20ms execution time.

# For my crappy servo, I have the following rates which yours may differ:
#   0 Degrees - 0.5ms HIGH / 19.5ms LOW
#  90 Degrees - 1.5ms HIGH / 18.5ms LOW
# 180 Degrees - 2.5ms HIGH / 17.5ms LOW
def pulser(io,pin,freq,dur)
  io.pwmWrite pin, 900
  sleep (freq/1000)
  io.pwmWrite pin, 0
  sleep ((dur-freq)/1000)
end

# Main method to call for rotating the servo. Using the io and pin
# that was declared at the beginning of this script.
# iter - Calculates the difference in the last position to determine
#        how many times to pulse the servo. A full range of motion
#        from 0 to 180 degrees does a good job in about 25 loops
# degrees - Converts the degrees into miliseconds for pulsing
def servo_rotate(io,pin,deg,override = false)
  iter = (((deg - @last_position).abs.to_f / 180) * 24).to_i + 1
  iter = 25 if override
  iter.times do
    degrees = 0.5 + ((deg.to_f/180)*2)
    pulser(io,pin,degrees,20.0)
  end
  @last_position = deg
end

# Set the servo back to 0 degrees. Notice the override is being passed
# as true which will override the iter variable in the server_rotate
# method.
puts "Resetting servo to 0 degrees..."
servo_rotate(io,pin,0,true)

# Continuous loop to prompt for degrees input
loop do
  puts "Enter Degrees (0..180)"
  degrees = gets.chomp.downcase
  case degrees.to_i
  when (0..180)
    servo_rotate(io,pin,degrees.to_i)
  else
    puts "Please enter a value between 0 and 180"
  end
end