Tutorial 1: First robot

Files for this tutorial are available in ​py4bot/examples/tutorial_1/. But don't just copy them here: write them from scratch, following the tutorial.

First, we create a new empty dir in our home dir:

$ mkdir tutorial_1
$ cd tutorial_1

All usefull classes are available from the api.py module. So, we just have to import things from here. Let's create a hexapod.py file to put our code, and import what we are going to use in this tutorial:

# -*- coding: utf-8 -*-

from py4bot.api import *

Now, let's create the robot! A multi-legs robot is mainly build from a body, and several legs.

We also need to create an actuator pool, which contains all legs joints actuators, in order to move them synchronized. In our example, actuators are standard servos, and we use the Veyron board, from DFRobot, to drive them:

import settings


class Hexapod(Robot):
    def _createBody(self):
        return Body(settings.LEGS_ORIGIN)

    def _createLegs(self):
        legs = {}
        legIk = {}
        for legIndex in settings.LEGS_INDEX:
            legs[legIndex] = Leg3Dof(legIndex, {'coxa': Coxa(), 'femur': Femur(), 'tibia': Tibia()}, settings.FEET_NEUTRAL[legIndex])
            legIk[legIndex] = Leg3DofIk(settings.LEGS_GEOMETRY[legIndex])

        return legs, legIk

    def _createActuatorPool(self):
        driver = Veyron()
        pool = ServoPool(driver)

        for leg in self._legs.values():

            # Create joints actuators
            num = settings.LEGS_SERVOS_MAPPING[leg.index]['coxa']
            servo = Servo(leg.coxa, num, **settings.SERVOS_CALIBRATION[num])
            pool.add(servo)

            num = settings.LEGS_SERVOS_MAPPING[leg.index]['femur']
            servo = Servo(leg.femur, num, **settings.SERVOS_CALIBRATION[num])
            pool.add(servo)

            num = settings.LEGS_SERVOS_MAPPING[leg.index]['tibia']
            servo = Servo(leg.tibia, num, **settings.SERVOS_CALIBRATION[num])
            pool.add(servo)

        return pool

As you can see, we just implemented 3 virtual methods, _createBody(), _createLegs() and _createActuatorPool().

Also note that we used some values from the settings.py module. This module is just a simple way to centralise the configuration of our hexapod. We will describe this module later.

To drive a robot, we need a remote control. In this tutorial, we are going to use an old gamepad, a ​Thrustmaster Firestorm Dual Analog 3. As I own such gamepad, I already added support in Py4bot:

class Gamepad(RemoteControl):
    def _createFrontend(self):
        return Thrustmaster(settings.THRUSTMASTER_PATH)

    def _buildComponents(self):
        self._addConfig("walk")
        self._addConfig("body")

        self._addComponent(Button, command=self.selectNextConfig, key="button_008", trigger="hold")
        self._addComponent(Button, command=self.selectPreviousConfig, key="button_009", trigger="hold")

        self._addComponent(Button, command=self.robot.incBodyPosition, key="button_004", mapper=MapperSetValue(dz=+5))
        self._addComponent(Button, command=self.robot.incBodyPosition, key="button_005", mapper=MapperSetValue(dz=-5))
        self._addComponent(Joystick, configs="walk", command=GaitSequencer().walk, keys=("analog_02", "analog_03", "analog_00"), mapper=MapperWalk())

        self._addComponent(Axis, configs="body", command=self.robot.setBodyExtraPosition, key="analog_00", mapper=MapperSetMultiply('yaw', coef=-15))
        self._addComponent(Axis, configs="body", command=self.robot.setBodyExtraPosition, key="analog_03", mapper=MapperSetMultiply('pitch', coef=-15))
        self._addComponent(Axis, configs="body", command=self.robot.setBodyExtraPosition, key="analog_02", mapper=MapperSetMultiply('roll', coef=15))

We won't deep into details, here; refer to the documentation and tutorial 5 for more informations about remote controls. This is an important part of the framework, and you can do very smart things, like having several configurations, using several remotes for the same robot...

Now, let's create our robot:

def main():
    def addGait(gaitClass, gaitName):
        group = settings.GAIT_LEGS_GROUPS[gaitName]
        params = settings.GAIT_PARAMS['default']
        for key1, value1 in settings.GAIT_PARAMS[gaitName].items():
            for key2, value2 in value1.items():
                params[key1][key2] = value2
        gait = gaitClass(gaitName, group, params)
        GaitManager().add(gait)

    addGait(GaitTripod, "tripod")
    addGait(GaitTetrapod, "tetrapod")
    addGait(GaitRiple, "riple")
    addGait(GaitWave, "metachronal")
    addGait(GaitWave, "wave")
    GaitManager().select("riple")

    robot = Hexapod()

    remote = Gamepad(robot)

    GaitSequencer().start()
    remote.start()

    robot.setBodyPosition(z=30)

    robot.mainLoop()

    remote.stop()
    remote.join()
    GaitSequencer().stop()
    GaitSequencer().join()


if __name__ == "__main__":
    main()

First, we add a few pre-defined gaits, in order to make it walk. Then, we create the robot itself, the remote control, and start all this little world!

Settings

Let's discuss about settings used in the previous part. See the user guide for the body/legs geometry conventions used.

LEGS_INDEX = ('RF', 'RM', 'RR', 'LR', 'LM', 'LF')

LEGS_INDEX contains the names used to define legs; they can be freely chosen, but these values must be used as keys for other params.

LEGS_GEOMETRY = {
    'RM': {'coxa': 25, 'femur': 45, 'tibia': 65},
    'RF': {'coxa': 25, 'femur': 45, 'tibia': 65},
    'LF': {'coxa': 25, 'femur': 45, 'tibia': 65},
    'LM': {'coxa': 25, 'femur': 45, 'tibia': 65},
    'LR': {'coxa': 25, 'femur': 45, 'tibia': 65},
    'RR': {'coxa': 25, 'femur': 45, 'tibia': 65}
}

LEGS_GEOMETRY dict contains the lengths of the different parts of the legs, in mm.

LEGS_ORIGIN = {
    'RM': {'x':  50., 'y':   0., 'gamma0' :   0.},
    'RF': {'x':  35., 'y':  80., 'gamma0' :  30.},
    'LF': {'x': -35., 'y':  80., 'gamma0' : 150.},
    'LM': {'x': -50., 'y':   0., 'gamma0' : 180.},
    'LR': {'x': -35., 'y': -80., 'gamma0' : 210.},
    'RR': {'x':  35., 'y': -80., 'gamma0' : 330.},
}

LEGS_ORIGIN dict contains the positions and orientation of the origin of the legs: (x, y) defines the center of rotation of coxa joint, and gamma0 is the angle of the legs at neutral position.

FEET_NEUTRAL = {
    'RM': LEGS_GEOMETRY['RM']['coxa'] + LEGS_GEOMETRY['RM']['femur'],
    'RF': LEGS_GEOMETRY['RF']['coxa'] + LEGS_GEOMETRY['RF']['femur'],
    'LF': LEGS_GEOMETRY['LF']['coxa'] + LEGS_GEOMETRY['LF']['femur'],
    'LM': LEGS_GEOMETRY['LM']['coxa'] + LEGS_GEOMETRY['LM']['femur'],
    'LR': LEGS_GEOMETRY['LR']['coxa'] + LEGS_GEOMETRY['LR']['femur'],
    'RR': LEGS_GEOMETRY['RR']['coxa'] + LEGS_GEOMETRY['RR']['femur'],
}

FEET_NEUTRAL dict contains the feet neutral positions of all legs. This is just the distance from the legs origins, in the ground plane, along leg X axis.

LEGS_SERVOS_MAPPING = {
    'RF': {'coxa':  0, 'femur':  1, 'tibia':  2},
    'RM': {'coxa':  4, 'femur':  5, 'tibia':  6},
    'RR': {'coxa':  8, 'femur':  9, 'tibia': 10},
    'LR': {'coxa': 15, 'femur': 14, 'tibia': 13},
    'LM': {'coxa': 19, 'femur': 18, 'tibia': 17},
    'LF': {'coxa': 23, 'femur': 22, 'tibia': 21}
}

LEGS_SERVOS_MAPPING is a table to map all joints to actuators nums.

SERVOS_CALIBRATION = {
     0: {'offset':   0, 'neutral': 1500, 'ratio':  0.090},  # coxa  leg RF
     1: {'offset': 180, 'neutral': 1500, 'ratio':  0.090},  # femur leg RF
     2: {'offset':  90, 'neutral': 1500, 'ratio':  0.090},  # tibia leg RF

     4: {'offset':   0, 'neutral': 1500, 'ratio':  0.090},  # coxa  leg RM
     5: {'offset': 180, 'neutral': 1500, 'ratio':  0.090},  # femur leg RM
     6: {'offset':  90, 'neutral': 1500, 'ratio':  0.090},  # tibia leg RM

     8: {'offset':   0, 'neutral': 1500, 'ratio':  0.090},  # coxa  leg RR
     9: {'offset': 180, 'neutral': 1500, 'ratio':  0.090},  # femur leg RR
    10: {'offset':  90, 'neutral': 1500, 'ratio':  0.090},  # tibia leg RR

    15: {'offset':   0, 'neutral': 1500, 'ratio':  0.090},  # coxa  leg LR
    14: {'offset': 180, 'neutral': 1500, 'ratio': -0.090},  # femur leg LR
    13: {'offset':  90, 'neutral': 1500, 'ratio': -0.090},  # tibia leg LR

    19: {'offset':   0, 'neutral': 1500, 'ratio':  0.090},  # coxa  leg LM
    18: {'offset': 180, 'neutral': 1500, 'ratio': -0.090},  # femur leg LM
    17: {'offset':  90, 'neutral': 1500, 'ratio': -0.090},  # tibia leg LM

    23: {'offset':   0, 'neutral': 1500, 'ratio':  0.090},  # coxa  leg LF
    22: {'offset': 180, 'neutral': 1500, 'ratio': -0.090},  # femur leg LF
    21: {'offset':  90, 'neutral': 1500, 'ratio': -0.090},  # tibia leg LF
}

SERVOS_CALIBRATION contains some calibration:

• offset is the angle between the servo reference and the real angle. See the FAQ for the real angles definition;
• neutral is the pulse value for the neutral position of the servo;
• ratio is angle per pulse width.

Offsets may vary, depending how you mount the servos. Same, ratio sign may have to be inverted on one side, if you have a symmetrical design; all angle are always computed using trigonometric direction (CCW).

See the user guide how to fine tune servos calibration.

Finally:

GAIT_LEGS_GROUPS = {
    'tripod':      (('RM', 'LF', 'LR'), ('RF', 'LM', 'RR')),
    'tetrapod':    (('RR', 'LM'), ('RF', 'LR'), ('RM', 'LF')),
    'riple':       (('RR',), ('LM',), ('RF',), ('LR',), ('RM',), ('LF',)),
    'metachronal': (('RR',), ('LM',), ('RF',), ('LR',), ('RM',), ('LF',)),
    'wave':        (('RR',), ('RM',), ('RF',), ('LR',), ('LM',), ('LF',))
}

GAIT_PARAMS = {
    'default': {
        'length': {
            'min': 5.,
            'max': 30.
        },
        'angle': {
            'min': 0.5,
            'max': 5.
        },
        'height': {
            'min': 20.,
            'max': 40.
        },
        'speed': {
            'min': 25.,
            'max': 250.
        }
    },
    'tripod': {
        'length': {
            'max': 40.
        },
        'angle': {
            'max': 10.
        }
    },
    'tetrapod': {},
    'riple': {},
    'metachronal': {},
    'wave': {}
}

GAIT_LEGS_GROUPS contains the legs grouped together and controlled at the same time during the gait usage.

Note that we need to define sequences, so don't forget the comma to define a tuple with a unique element.

GAIT_PARAMS contains some additional gaits params:

• length is the distance (range) each leg will move for an entire cycle when the robot translate at full speed;
• angle is the angle (range) each leg will turn for an entire cycle when the robot rotate at full speed;
• height is the height (range) the leg will lift up;
• speed is the speed (range).

These values must be adjusted for each robot, to avoid legs collisions.

THRUSTMASTER_PATH = "/dev/input/by-id/usb-Mega_World_Thrustmaster_dual_analog_3.2-event-joystick"

No need for further explanations.

In the next tutorial, we will create a virtual robot, using the 3D simulator classes.