CAN Bus Communication for Multiple VESCs

[kyleoptimotive]

Abstract

I want to expose vesc hw interfaces to ROS for one master VESC and 'n' other VESCs connected via CAN bus. Do you have any suggestions for how I might start this?

Purpose

To expose vesc hw interfaces in ROS for multiple vesc devices where only one is connected to the host PC via USB and the rest are connected to that vesc over CAN bus. This allows multiple vescs to be used without needing each one to be plugged in via USB to the host PC.

Implementation Details

I am using ROS Humble and Ubuntu 22.04.

[Jha1005]

I am also using also ros2 humble.Can you suggest me i have 3 vesc and i have to run it on canbus so what changes or includation i have to do ??

[kyleoptimotive]

I've done some experimentation and worked out a way to use vesc_hw_interface to talk to all VESCs as if they are one device. This isn't what I initially wanted but I figured I would at least document my findings if it helps future developers.

To enable CAN communication, I had to structure VESC packets in the format required to forward them. Once I figured out how to format them, the rest was choosing how to integrate that with the other ROS components.

My Technical Changes in Brief
In vesc_packet I created set methods to do the same thing as the existing non-CAN methods with the addition of packing can_id and command_id in the first two bytes followed by the original payload information and a CRC checksum. This was done by inheriting VescPacket with the COMM_FORWARD_CAN command id as specified in data_map.hpp. It would be cleaner if changes were made to existing methods instead of creating a set of new ones but I didn't have the time to explore that. The important thing was I needed to get the VESC packet to be in the required format for forwarding.

Example VESC packet.
[start] [length] [forward_command] [can_id] [command_to_forward] [data...] [CRC] [end]
[0x3] [0x07] [0x21] [0x0] [COMM_SET_DUTY] [data...] [CRC] [0x3]

Example code using VescPacket and new method VescPacketCANSetDuty. Notice the payload size argument is the original payload size plus two for the can_id and command_to_forward bytes.

VescPacketCANSetDuty::VescPacketCANSetDuty(double duty, uint8_t can_id) : VescPacket("CANSetDuty", 7, COMM_FORWARD_CAN)
{
  // Set up CAN forward packet structure
  *(payload_end_.first + 1) = can_id;
  *(payload_end_.first + 2) = COMM_SET_DUTY;

  // ...

  // Pack the duty value into the payload
  const int32_t v = static_cast<int32_t>(duty * 100000.0);
  *(payload_end_.first + 3) = static_cast<uint8_t>((v >> 24) & 0xFF);
  *(payload_end_.first + 4) = static_cast<uint8_t>((v >> 16) & 0xFF);
  *(payload_end_.first + 5) = static_cast<uint8_t>((v >> 8) & 0xFF);
  *(payload_end_.first + 6) = static_cast<uint8_t>(v & 0xFF);

  appendCRC();
}

start, end and length are handled by VescPacket.

I don't know if this kind of functionality is in the scope of this repository but I hope it helps others to understand VESC packet forwarding better.