How to control a 88004 Servo Motor with a Raspberry Pi?

Question

I am able to control a simple servo motor such as SG90 by using pulse-width modulation. However when it comes to controlling a 88004 LEGO Servo Motor it is simply a nightmare.

By using PWM all I managed was bringing it to its normal position. It seems the pulse length doesn't play a role at all - I've tried several ones...

Another difference between a SG90 and a 88004 is that the first has only three wires (PWR, GND and Control) whereas the LEGO servo has 4 (PWR, C1, C2 and GND).

• Which of those wires (C1 and C2) should be used with the PWM?
• How long should the pulses be to move the motor through its 15 positions?
• Is there another way to control a 88004 directly from the Raspberry Pi such as I2C?

EDITED

It works!

#Importing the necessary library
import RPi.GPIO as GPIO
from time import sleep

#Reference by GPIOs IDs
GPIO.setmode(GPIO.BCM)

#Assigning the GPIOs to the Lego's command wires
C1 = 21
C2 = 26
moveFactor = 14.285714286

#Setting up the necessary GPIOs
GPIO.setup(C1, GPIO.OUT)
GPIO.setup(C2, GPIO.OUT)

#Setting up the PWMs - (GPIO ID, Frequency)
pwm1 = GPIO.PWM(C1, 1200)
pwm2 = GPIO.PWM(C2, 1200)


def setPosition(position):
    pwm1.stop()
    pwm2.stop()
    i = round(position * moveFactor, 2)

    if position >= 0:
        print 'Position: ' + str(position) + ' at a Duty Cycle of ' + str(i)
        pwm1.start(i)
    else:
        print 'Position: ' + str(position) + ' at a Duty Cycle of ' + str(i*-1)
        pwm2.start(i*-1)

#Testing the commands with a LED
try:
    setPosition(-7)   #180 Degrees
    sleep(2)
    setPosition(7)    #0 Degree
    sleep(1)
except:
    pass

pwm1.stop()     # Back to the normal position (90 degrees)
pwm2.stop()
GPIO.cleanup()

Answer 1

LEGO "Servo Motor" has little to do with a RC servomotor. To drive it, you need to:

• Power it at 9V (probably works at 5 volts, but with less torque) through PWR/GND terminals
• To move in one direction, send a PWM signal (1200 Hz, 0 to 100% duty cycle) on C1 and keep C2 at GND level. As duty cycle varies, servo motor will move along 7 positions on one side. See this video.
• To move in the other direction and reach the 7 other positions, send PWM to C2 and keep C1 at ground level.