Up Side Down

Published on 2016-02-14 in Henk the Hexapod.

Today I worked a little bit more with the robot. It’s actually quite convenient to have it sit on a box (so that the legs are in the air) next to my computer, connected over the USB cable.

As you can see, the OpenMV camera is actually installed up-side-down. That’s because it was more convenient for me, and it looks a little bit like a snout. I figured it wouldn’t matter, as I would simply reverse the y coordinate of any results I get from the camera. It turns out that sometimes it does matter – in particular, the face detection algorithms don’t detect up-side-down faces. I looked for a way to flip the image, and after I couldn’t find anything, I asked on the OpenMV forum, where the authors immediately helped me . Turns out the sensor has a setting which you can enable to make it flip the image in hardware. Here’s a photo from the eyes of the robot:

The quality is not that great, because it’s pretty dark in my room. I’m using a wide-angle lens, so you can see the two front legs of the robot in there.

Next I went on working on the walking code (again). As usual, starting with the Servo class:

import pyb
import ustruct
import math


class Servo:
    min_pos = 600
    max_pos = 2400
    pos_range = 180

    def __init__(self, servos, index, reverse=False, trim=0):
        self.servos = servos
        self.index = index
        self.reverse = reverse
        self.trim = trim

    def move(self, radians=None, degrees=None):
        if degrees is None:
            degrees = math.degrees(radians)
        degrees += self.trim
        if self.reverse:
            degrees = self.pos_range - degrees
        position = self.min_pos + (
            degrees * (self.max_pos - self.min_pos) / self.pos_range)
        position = min(self.max_pos, max(self.min_pos, position))
        self.servos[self.index] = position


class Servos:
    ADDRESS = 9

    def __init__(self):
        self.bus = pyb.I2C(2, pyb.I2C.MASTER)
        self.servos = [0] * 18

    def __setitem__(self, index, value):
        self.servos[index] = value

    def __getitem__(self, index):
        return Servo(self, index)

    def update(self, servos=None):
        if servos is None:
            servos = range(18)
        for servo in servos:
            try:
                pyb.disable_irq()
                self.bus.mem_write(
                    ustruct.pack('<' + 'H', self.servos[servo]),
                    self.ADDRESS,
                    servo,
                )
            finally:
                pyb.enable_irq()

Now I can move each of the joints of the robot individually. Next up, leg (practically the same as in Tote ) and then body (the hard part) inverse kinematics.