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CAN Communication Guide

This document describes the CAN/CANFD communication protocol of the BXI actuator in MIT control mode.

1. Communication Overview

The motor uses CANFD communication and is compatible with classic CAN. The motor replies using the same frame format as the received CANFD/CAN frame, so frame type and variable bitrate usually do not need separate configuration.

  • Max CANFD bitrate: 1M + 5M
  • Bitrate without variable bitrate: 1M

By function, frames are divided into:

  • Master control frame: control commands sent from upper controller to motor
  • Motor response frame: motor status feedback sent from motor to upper controller

2. MIT Mode Master Control Frame (8 Bytes)

2.1 Data Layout

Control Frame D[0] D[1] D[2] D[3] D[4] D[5] D[6] D[7]
ID p_des[15:8] p_des[7:0] v_des[11:4] v_des[3:0] \| Kp[11:8] Kp[7:0] Kd[11:4] Kd[3:0] \| t_ff[11:8] t_ff[7:0]

2.2 Field Description

  • Frame ID: equals target motor can_id, default 1; broadcast frame uses 0x7FF
  • p_des: position command value
  • v_des: velocity command value
  • Kp: position proportional gain
  • Kd: position differential gain
  • t_ff: torque feedforward value

2.3 Default Parameter Ranges

Kp / Kd ranges (configurable via upper computer):

  • Kp: [0, 500]
  • Kd:
  • MOTOR_50: [0, 5]
  • MOTOR_50_L: [0, 5]
  • MOTOR_70: [0, 5]
  • MOTOR_85: [0, 20]
  • MOTOR_J3505: [0, 5]
  • MOTOR_J3510: [0, 5]

p_des / v_des / t_ff ranges (configurable via upper computer):

  • p_des: [0, 12.5]
  • v_des: [0, 45.0]
  • t_ff:
  • MOTOR_50: [0, 40]
  • MOTOR_50_L: [0, 40]
  • MOTOR_70: [0, 80]
  • MOTOR_85: [0, 160]
  • MOTOR_J3505: [0, 18]
  • MOTOR_J3510: [0, 18]

Temperature communication range: [-30.0, 150.0] ℃.

2.4 Bit Width Description

A standard CAN frame has only 8 bytes. The MIT control command packs 5 parameters into 8 bytes:

  • Position: 16 bits (2 bytes)
  • Velocity: 12 bits
  • Kp: 12 bits
  • Kd: 12 bits
  • Torque: 12 bits

3. CANFD Broadcast Multi-Motor Control

A CANFD frame can carry up to 64 bytes. In multi-motor control:

  • Frame ID is fixed to 0x7FF
  • Data for motor id occupies its 8-byte block: (id - 1) * 8 to id * 8 - 1

That is, each motor uses a full 8-byte MIT control block, concatenated in motor-ID order in one frame.

Example (first 16 bytes, controlling M[1] and M[2]):

Control Frame (ID=0x7FF) D[0] D[1] D[2] D[3] D[4] D[5] D[6] D[7]
M[1] p_des[15:8] p_des[7:0] v_des[11:4] v_des[3:0] \| Kp[11:8] Kp[7:0] Kd[11:4] Kd[3:0] \| t_ff[11:8] t_ff[7:0]
Control Frame (ID=0x7FF) D[8] D[9] D[10] D[11] D[12] D[13] D[14] D[15]
M[2] p_des[15:8] p_des[7:0] v_des[11:4] v_des[3:0] \| Kp[11:8] Kp[7:0] Kd[11:4] Kd[3:0] \| t_ff[11:8] t_ff[7:0]

Subsequent motors follow in 8-byte steps.

For example, motor 3 uses D[16]~D[23], motor 4 uses D[24]~D[31], and so on, up to 64 bytes.

4. Special Control Frames (8 Bytes)

All commands below are fixed 8-byte values:

  • Motor enable: 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFC
  • Motor disable: 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFD
  • Save zero position: 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFE
  • Enable terminal output: 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFB
  • Disable terminal output: 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFA

5. Motor Response Frame

The response frame ID can be configured via upper computer, default 0x11 (internal field: master_id).

Main feedback includes position, velocity, torque, driver temperature, and motor temperature.

5.1 Data Layout

Feedback Frame D[0] D[1] D[2] D[3] D[4] D[5] D[6] D[7]
MST_ID ID POS[15:8] POS[7:0] VEL[11:4] VEL[3:0] \| T[11:8] T[7:0] T_MOS T_Motor

5.2 Field Description

  • ID: motor ID
  • POS: position data
  • VEL: velocity data
  • T: torque data
  • T_MOS: MOS temperature on driver (℃)
  • T_Motor: internal winding temperature (℃)

Position, velocity, and torque use linear mapping to convert floating-point values to signed fixed-point values:

  • Position: 16 bits
  • Velocity: 12 bits
  • Torque: 12 bits

5.3 Relationship Between master_id and can_id

  • Default relation: master_id = can_id | 0x010
  • Modifying can_id will synchronize update of master_id
  • master_id can also be written independently via upper computer