> For the complete documentation index, see [llms.txt](https://docs.hello-robot.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.hello-robot.com/stretch4_docs/working-with-stretch/common_tasks/base-frame-convention-and-wheel-odom.md). # Base Frame Conventions ## Wheel Numbering Wheel numbering must increase counter-clockwise. Wheel 0 must be to the left of the forward direction. Based on these rules, here’s how the wheels should be numbered on Basquiat and Calder.
## Forward Direction We want to go from wheel numbering to a ROS frame convention where X+ is forward and Y+ is to the left, forming a right handed coordinate frame.
We need 1 number, called “gamma”, that is the angle between wheel 0 and X+.
Gamma lives in Stretch Body II, inside the [H0\_from\_driving\_dir()](https://github.com/hello-robot/stretch_body_ii/blob/8b281a316574ed511d06fcf8a5e5ed33352cd612/src/stretch_body_ii/core/hello_utils.py#L964) function. More details on H0 can be found in the [ROS docs](https://control.ros.org/rolling/doc/ros2_controllers/doc/mobile_robot_kinematics.html#omnidirectional-wheeled-mobile-robots).
## Gamepad Teleop Here is the convention for the joysticks on the gamepad controller. Each axis ranges from \[-1, 1]
Two things to note: First, we have to use the gamepad's Y axis for the robot’s X axis. Second, since the robot’s left is Y+ and the gamepad’s left is X-, we need to multiply the X range by -1. This explains the confusing negative sign in the gamepad teleop [code](https://github.com/hello-robot/stretch_body_ii/blob/7f3e2f17a41bbd0a6124ba3b349177c92d6991ac/src/stretch_body_ii/core/gamepad_teleop.py#L157):
## Wheel Odometry H0 maps base velocity to wheel velocities. H0 inverse or `H0_inv` maps wheel velocities to base frame twist. Computing wheel odometry includes: 1. Counting magnetic encoder ticks in firmware and converting to motor space rad/s in Stretch Body II 2. Using `H0_inv` to calculate base frame twist \* ```
``` 3. Scaling base frame twist (Vb) by the frequency update\_odom() is called \* ```
``` 4. Rotating scaled base frame twist (Sb) into odom frame \* ```
``` 5. Add odom change to running odom frame \* ```
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