MartinSant.net

Talk about fast, I ordered the servo controller off the Pololu web site on Friday afternoon and it was in my mailbox Monday! From Utah to Virginia in three days! Can’t beat that.

Well, hooked it all up and a few minutes of messing around and I find my pyusb examples don’t work out of the box. Go figure.

Tried again, this time with just some simple serial commands. Far, far simpler. Here is a quick Class to set the position of the servos and trigger a script.

import serial
import time

class ServoController:
    def __init__(self):
        usbPort = '/dev/ttyACM0'
        self.sc = serial.Serial(usbPort)

    def closeServo(self):
        self.sc.close()

    def setAngle(self, n, angle):
        if angle > 180 or angle <0:
           angle=90
        byteone=int(254*angle/180)
        bud=chr(0xFF)+chr(n)+chr(byteone)
        self.sc.write(bud)

    def setPosition(self, servo, position):
        position = position * 4
        poslo = (position & 0x7f)
        poshi = (position >> 7) & 0x7f
        chan  = servo &0x7f
        data =  chr(0xaa) + chr(0x0c) + chr(0x04) + chr(chan) + chr(poslo) + chr(poshi)
        self.sc.write(data)

    def getPosition(self, servo):
        chan  = servo &0x7f
        data =  chr(0xaa) + chr(0x0c) + chr(0x10) + chr(chan)
        self.sc.write(data)
        w1 = ord(self.sc.read())
        w2 = ord(self.sc.read())
        return w1, w2

    def getErrors(self):
        data =  chr(0xaa) + chr(0x0c) + chr(0x21)
        self.sc.write(data)
        w1 = ord(self.sc.read())
        w2 = ord(self.sc.read())
        return w1, w2

    def triggerScript(self, subNumber):
        data =  chr(0xaa) + chr(0x0c) + chr(0x21) + chr(0x27) + chr(subNumber)
        self.sc.write(data)

# MAIN
         
Maestro = ServoController()

iterations = 2

while(iterations > 0):
    for chan in range(0,3):
        for pos in range(0, 180):
            Maestro.setAngle(chan, pos)
            time.sleep(.002)
            #print Maestro.getPosition(chan), Maestro.getErrors()

    for chan in range(0,3):
        for pos in range(180, 0, -1):
            Maestro.setAngle(chan, pos)
            time.sleep(.002)
            #print Maestro.getPosition(chan)

    e1, e2 = Maestro.getErrors()
    if e1 != 0 or e2 != 0:
       print e1, e2
       break

    iterations = iterations - 1

    print "Trigger Script"
    Maestro.triggerScript(0)
    
time.sleep(2)
Maestro.closeServo()

The port name (ttyACM0) is mentioned in the Pololu docs and you can also see it come and go in the /dev/ directory on the RPi.

Adding the other commands listed in the Pololu documentation is left as an exercise for the reader 😉