Overview

This page details MiRo's geometry, dimensions and conventions, which are used in translating between MiRo's signal interfaces and its physical components.

The values of physical parameters described on this page are given in miro_constants.h or miro_constants.py.
Parameters in miro_constants.h are provided as preprocessor #defines of the form MIRO_<constant_name>.
Parameters in miro_constants.py drop the MIRO_ prefix and are available at miro.constants.<constant_name> (using import miro2 as miro).
Software support is offered for wrangling the geometry of the robot—see the bottom of this page for some examples.

Coordinate system

MiRo uses a right-handed coordinate system, as shown in the image. Units are metres and Radians.

Kinematic chain

MiRo has three internal "kinematic"[1] degrees-of-freedom (DOF), which are denoted LIFT, YAW and PITCH. These three DOFs connect together MiRo's four physical links (and frames of reference), denoted BODY, NECK, GIMBAL and HEAD. Together, these links and DOFs form the internal kinematic chain.

BODY, in addition, is tilted with respect to the horizontal, by a fixed amount defined by the height of MiRo's rear slider; the corresponding untilted frame is denoted FOOT (footprint), and the fixed "DOF" between them is denoted TILT.

Finally, FOOT is positioned relative to the world frame, WORLD, by the robot's POSE, so that the complete kinematic chain takes the form shown alongside, which has frames (which have a one-to-one correspondence with links) in dark grey and inter-frame relationships in light grey. The table below summarises the fixed parameters of the chain.

Parameter Description
LOC_TILT_*Location of tilt joint in FOOT (rotational axis is y).
LOC_LIFT_*Location of lift joint in BODY (rotational axis is y).
LOC_YAW_*Location of yaw joint in NECK (rotational axis is z).
LOC_PITCH_*Location of pitch joint in GIMBAL (rotational axis is y).
The location of each joint defines the origin of its descendant frame of reference.

[1] These are known as the "kinematic" DOFs solely to distinguish them from the "cosmetic" DOFs, specifying the position of the cosmetic mobile components (eyelids, ears, and tail).

Configuration and Pose

The immediate state of the kinematic chain is denoted as "configuration" and "pose". Configuration comprises the angles of the four angular relationships and can be represented as a tuple (TILT, LIFT, YAW, PITCH). Pose is the relationship between FOOT and WORLD in the 2D plane, and can be represented as a tuple (x, y, theta).

To determine the state of the chain, the configuration can for some applications be read directly from the corresponding sensor topic sensors/kinematic_joints. Pose, however, is not available from the robot. Estimating pose can be approached in different ways, and these have been the subject of huge research effort in robotics. One way, for example, is to integrate the robot's odometry which is available at sensors/odom.

Main wheels

The table below summarises the geometric parameters of the wheels.

Parameter Description
WHEEL_DIAMETER_MDiameter of main wheels (m).
WHEEL_TRACK_MTrack of main wheels (m).
WHEEL_MAX_SPEED_M_PER_SMaximum forward/backward speed (m/sec).
WHEEL_M_PER_COUNTRatio of metres per count of the optical encoders.

Cameras

The table below summarises the geometric parameters of the cameras.

Parameter Description
LOC_EYE_*Location of left eye in HEAD (right eye is symmetrically placed).
CAM_ELEVATIONElevation of both cameras above horizontal plane.
CAM_DIVERGENCEDivergence of each camera from looking "straight ahead".
CAM_HORI_HALF_FOVThe horizontal angular half-field-of-view (radians).

The installed position of the left-hand camera assembly is obtained as follows.

  1. Start with a camera assembly with its optical centre at the origin of HEAD facing along the +ve x-axis with upright vector along the +ve z-axis.
  2. Rotate assembly by CAM_DIVERGENCE around z-axis to "look more left".
  3. Rotate assembly by -CAM_ELEVATION around y-axis to "look more up".
  4. Translate optical centre to LOC_EYE_*.

Thus, the rotation corresponding to divergence is around an axis normal to the plane including the optical axes of both cameras, both before and after the rotation, and the upright vector of the two cameras are parallel when installed. Accordingly, a vertical (before lens distortion) line in the left camera image is vertical (before lens distortion) also in the right camera image.

The stereo overlap region at the centreline is equal to 2 × (CAM_HORI_HALF_FOV - CAM_DIVERGENCE). The total horizontal field of view provided by the two cameras is equal to 2 × (CAM_HORI_HALF_FOV + CAM_DIVERGENCE).

Microphones

The table below summarises the geometric parameters of the microphones.

Parameter Description
LOC_EAR_*Location of left ear (right ear is symmetrically placed).
LOC_TAIL_MIC_*Location of tail mic.

The sensor topic sensors/mics provides data from four microphones. The third microphone is located inside the head and is intended to be used for mitigation of self-generated noise. The fourth is located in the base of the tail and can be used to help localise sounds in the forward/backward direction.

Software support

To transform between the various frames of reference you can take advantage of the tools provided with the MDK.

Example ~/mdk/bin/shared/client_map.py

This example uses the tools to map from a pixel in image space to a location in WORLD, using sensors/kinematic_joints and sensors/body_vel to configure the mapping. Using camera_model is particularly helpful since it accounts for the particular distortion introduced by the lenses of MiRo's cameras.

Example ~/mdk/bin/shared/example_kc.py

This example illustrates simple use of the kc tool to move positions and directions around from frame to frame.