6

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()
  • Did you need to raise the voltage levels to 5v or 9v, or were you able to control it via 3v3 directly from the pi? – user2813274 Jul 16 '15 at 22:32
8

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.
  • Thank you very much Philo. Just another question from someone who has very limited physics knowledge. What exactly do you mean by "0 to 100% duty cycle"? Would you be so kind and write a very simple algorithm? Thank you! – user4343 Dec 29 '14 at 17:27
  • The duty cycle percentage is the ratio between powered time over cycle period. Eg. here, a 30% duty cycle at 1200 Hz means 250µs on / 583µs off. – Philo Dec 30 '14 at 15:20
  • Hi Philo. Thanks for your replay. I just created a code in Phyton based on your explanation (see the edited question) but unfortunately it still does not work. Would you help me by reviewing it and making the necessary adjustments? I would be very thankful for that. – user4343 Dec 30 '14 at 20:11
  • Problem is that I am not Python savvy... Might be a physical interface problem too? – Philo Jan 1 '15 at 18:05

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