User guide (git)

This documentation tries to follow the developement version of Py4bot. It may or may not be up-to-date.

For installation instructions, see here.

Usage overview

As Py4bot is a framework, you will find here a description of the main things you need to understand to write your own applications.

Together with this page, have a look at the complete API.

Architecture

The main parts of the framwork are:

• the robot itself;
• the gait sequencer;
• some remote controllers.

Geometry definition

There are several coordinates systems involved when computing maths in order to make a robot walk; it is important to understand how they are defined. All these systems are cartesian.

Ground coordinates system

This coordinates system is attached to the ground; it is a fixed referential.

• the origin is somewhere on the ground
• X and Y axes define a plane parallel to the ground
• Z axis points up

Robot coordinates system

This coordinates system is attached to the robot itself.

• the origin is at the center of the robot
• X and Y axes define a plane parallel to the ground, so that Y axis is the back-to-front axis of the robot
• Z axis points up

In the ground coordinates system, the robot coordinates system is just translated in the X/Y plane, and rotated arround Z.

Body coordinates system

This coordinates system is attached to the body. It is defined in the ground coordinates system.

• the origin is at the center of the body
• X and Y axes define a plane parallel to the body, so that Y axis is the back-to-front axis of the body
• Z axis is orthogonal to X/Y

This system is used to define the position of the body in the robot coordinates system, allowing it to move along the 6 DoF (3 translations, 3 rotations).

Legs coordinates systems

This coordinates systems are attached to the legs, so each leg has its own coordinates system. They are defined in the body coordinates system.

• the origin is at the origin of the leg (usually the coxa joint)
• X axis is along the coxa of the leg
• X and Y axes are in the same plane as the one defined by the body X/Y axes
• Z axis is orthogonal to X/Y

To summurize, in the body coordinate system, the legs coordinates systems are just translated in the X/Y plane, and rotated arround Z.

The origin of a leg is defined as following:

Todo: improve drawing

Inverse Kinematic

The Inverse kinematic is the the kinematic equations of a robot to determine the joints parameters that provide a desired position of leg (foot).

Here are the angles involved.

3 DoF legs

• gamma: angle of the leg in the the body X/Y plane (angle between body X axis and leg X axis)
• alpha: angle of the femur in the leg X/Z plane (angle between coxa and femur)
• beta: angle of the tibia in the leg X/Z plan (angle between femur and tibia)

4 DoF leg

• tars: angle of the tars in the leg X/Z plan (angle between tibia and tars)

Robot

Drivers

Servos calibration

You can use the py4bot-gui-servocal.py to fine tune your servos calibration, and generate the SERVOS_CALIBRATION dict.

Gaits

Controllers

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