CANbus Communication Specifications

Data Link Specifications

  • 1Mbaud CANbus
  • 8 time quanta per bit
  • 75% sampling point
  • Sync jump width = 1 time quanta (TQ)
  • 11-bit MsgID (standard CAN)
  • Proprietary protocol, not DeviceNet or CANopen
  • Recommended reading: Controller Area Network by Konrad Etschberger

CANbus Timing

  • 75μS to ask for position
  • 75μS per puck to respond with the positions
  • 125μS to send a packed torque to the lower 4DOF
  • 125μS to send a packed torque to the wrist
  • Control-side processing time on PC

For the 4DOF, it is: 75+(4*75)+125+PC = 500μS + PC For the 7DOF, it is: 75+(7*75)+(2*125)+PC = 850μS + PC

These numbers are limited by the 1 Mbps CANbus. Each message has a 47-bit frame (47μS), plus payload data (3 bytes, 24μS typ). CANbus transceivers are not rated above 1 Mbps due to slew-rate limitations.

ID Specifications

Message IDs

  • [GFFFFFTTTTT] (11 bits, binary)
  • G: Group, 0 = Directed message, 1 = Group broadcast
  • F: From ID, Host = 00000, Motor N = N
  • T: To ID or group

Examples:

  • 00000000011 => Directed message from host to motor 3 (3 = 00011, binary)
  • 10001100100 => Group broadcast from motor 3 to group 4

Motor IDs and Groups

Each motor in the robot has a unique communication ID, can be a part of any three groups (GRPA, GRPB, GRPC), and will listen for and process messages bound for its ID or any of its groups. There are a total of 32 possible groups (from 00000 to 11111). Motors 1 to 4 belong to groups 0, 1, and 4, motors 5 to 7 belong to groups 0, 2, and 5, and the host belongs to groups 3 and 6 by default. The default groups are:

  • 0 = All actuators
  • 1 = Lower arm torques (motors 1-4)
  • 2 = Upper arm torques (motors 5-7)
  • 3 = Position feedback
  • 4 = Lower arm property
  • 5 = Upper arm property
  • 6 = Property feedback

CANbus Frame Data Payload

Standard CANbus Message Format

CAN specifies a maximum of 8 bytes/frame payload – our typical payload consists of 4-6 bytes: [APPPPPPP] [00000000] [LLLLLLLL] [mmmmmmmm] [MMMMMMMM] [HHHHHHHH]

  • A: Action, 0 = Get property, 1 = Set property
  • P: Property (128 possible values, 0..127, 0000000..1111111), see Motor Controller Properties and Safety Module Properties below.
  • 0: Second byte (almost) always set to zero (see exceptions below)
  • L: Low byte of data value
  • m: mid-low byte of data value
  • If sending a 16-bit integer value, the following are not used:
  • M: Mid-high byte of data value
  • H: High byte of data value

The CAN frame data length code (DLC) is set to the number of bytes being transmitted.

Exceptions

  1. The Position property (P) is a 22-bit, 2's complement number. It is packed into a 3-byte frame payload [00MMMMMM] [mmmmmmmm] [LLLLLLLL]. It is always sent to Group 3.
  1. Command torque can be sent as a set of four 14-bit, 2's complement numbers. It is sent to the motor controllers in 8 bytes (max):

01234567
APPPPPPPBBBBBBbbbbbbbbCCCCCCccccccccDDDDDDddddddddEEEEEEeeeeeeee

  • A = Action
  • P = Property
  • B = Upper 6 bits of first value
  • b = Lower 8 bits of first value
  • C = Upper 6 bits of second value
  • c = Lower 8 bits of second value
  • D = Upper 6 bits of third value
  • d = Lower 8 bits of third value
  • E = Upper 6 bits of fourth value
  • e = Lower 8 bits of fourth value

Each motor has a property (PIDX: 1-4), which tells it which torque to use from the set of 4

Full Communication Example

This example contains:

  • 3 motors with IDs of 5, 6, and 7
  • A host with an ID of 0

Host sends:

  • MsgID [10000000000] → Group 0
  • Data [10000101] [00000000] [00000010] [0000000] → Set property 5 (STAT) to 2 (STATUS_READY)
  • The motors start up with STAT = 0 (STATUS_RESET)
  • Setting STAT to READY gets the motors ready to receive additional data
  • Motors will only respond to STAT and VERS commands while in RESET (for safety)

Host sends:

  • MsgID [10000000000] → Group 0
  • Data [10001000] [00000000] [00000010] [00000000] → Set property 8 (MODE) to 2 (MODE_TORQUE)
  • The motors default to MODE = 0 (MODE_IDLE)
  • Setting MODE to MODE_TORQUE tells the motors to apply any torque sent to them
  • When MODE = MODE_IDLE, motors will ignore any torque commands sent and apply braking
  • When using a WAM, the safety system will set the MODE when you press the IDLE/ACTIVATE buttons
  • Do not try to bypass the WAM’s safety system by setting the MODE directly, this will cause undesired operation.

Host sends:

Motors send:

Host uses these positions to calculate a torque.

Host sends:

Last modified 14 years ago Last modified on May 13, 2011, 2:09:20 PM