cubemars_hardware

This package contains a ros2_control hardware interface to control the CubeMars AK series motors with a SystemInterface. It was tested on two AK70-10.

Motor Setup

The package assumes that the motor is setup in servo mode.

To adjust the motor settings you connect the motor with the R-link and use the "Upper Computer" program. More information is available on the CubeMars Technical Support and Download page.

Make sure that you enable "Send status over CAN" (called send_can_status in AppParams). Otherwise the state interfaces will not work properly. Also, the upload frequency of the actuators should be at least as fast as the update rate of the controller manager. If no CAN message is received during one update loop you will get a warning in the read function.

Example motor settings can be found in our main repo.

SocketCAN

SocketCAN is used to handle the communication between the motors and the Linux system. You need a working CAN interface for this to work. The exact setup will depend on your hardware but you will at least have to intitialize the interface.

sudo ip link set can0 type can bitrate 1000000
sudo ip link set can0 up

When typing ifconfig you should then see the CAN interface. You can use can-utils to read all incoming messages.

sudo apt install can-utils
candump can0

Hardware Interface

The following command interfaces are published:

• position: Position(-Speed) Loop Mode
• velocity: Speed Loop Mode
• effort: Current Loop Mode

Your ros2_control controller can only claim one of the command interfaces. Claiming multiple command interfaces at the same time is not possible.

The following state interfaces are published:

• position
• velocity
• effort
• temperature

The hardware interfaces can also be listed by starting the controller manager and running the following command.

ros2 control list_hardware_interfaces

ros2_control Parameters

An example ros2_control URDF config with this hardware interface can be found in our main repo.

hardware tag:

• can_interface: name of the Linux CAN interface, e. g. can0

joint tag:

• can_id: CAN ID of the actuator
• pole_pairs: Pole pairs. Used for unit conversion
• gear_ratio: Gear ratio. Used for unit conversion
• kt: Torque constant. Used to convert current to torque
• trq_limit: OPTIONAL. Torque limit. If the limit is reached the motor is disabled and the hardware interface is shutdown.
• enc_off: OPTIONAL. Encoder offset in $\text{rad}$. (see explanation below)
• pos_limit_min: OPTIONAL. Upper position limit in $\text{rad}$. (see explanation below)
• pos_limit_max: OPTIONAL. Lower position limit in $\text{rad}$. (see explanation below)
• vel_limit: OPTIONAL. Velocity limit in $\text{rad}/\text{s}$. (see explanation below)
• acc_limit: OPTIONAL. Acceleration limit in $\text{rad}/\text{s}^2$. (see explanation below)
• read_only: OPTIONAL. If set to 1, the current position is logged and no commands are sent to the motors.

Encoder Offset

For single-encoder motors there might be an offset between your desired origin and the zero position of the motor on startup. You can compensate for this offset by using enc_off. Note that for this to work properly, the motor has to be close to the origin on startup, otherwise the encoder value will wrap around.

Models with an output encoder can remember a permanent zero position. It can be set using the following command (replace XX with the hexadecimal representation of the CAN ID).

cansend can0 000005XX#01

Position Limits

The pos_limit_min and pos_limit_max specify the hardware limits of joint position. If they are set, any commands witch actuate the motor into the direction to violate the limits are ignored. Note that the motor is not disabled and can be actuated into the opposite direction to go back to the range.

Velocity and Acceleration Limits

The position command interface will by default use the Position Mode (servo mode 4) where the motor runs to the specified position at maximum speed and acceleration. If you want to use the Position-Speed Loop Mode (servo mode 6) you have to specify BOTH vel_limit and acc_limit. This will limit the maximum acceleration and velocity of the motor (trajectory planning). This does not work well together with a joint_trajectory_controller.

Note that these limits have no effect if only one is set or if you don't use the position command interface.