CAN Protocol¶

Introduction¶

The CAN protocol describes hardware nodes communicating with each other through CAN messages. This page documents the different CAN messages being sent throughout our system.

This page is adapted from ODRI’s page on CAN protocol. Many of the messages remain unchanged, but we added several additional messages. The original page is as follows:

ODRI CAN Protocol

Our CAN network consists of 3 nodes: The TI LAUNCHXL board that controls the motors of the Robotic Leg, the TI LAUNCHXL board that reads the encoders on the Central Pivot, and a computer which interfaces with the CAN network through a CAN interface.

Illustration of the CAN network¶

Messages sent from the Robotic Leg Board¶

ID	Meaning	Size [Bytes]	Structure of the data
0x010	Status	1	see Status Message
0x020	Current \(I_q\) (both motors)	8	MDL: \(I_q\) of motor 1 MDH: \(I_q\) of motor 2
0x030	Encoder Position (both motors)	8	MDL: Position of motor 1 MDH: Position of motor 2
0x040	Velocity (both motors)	8	MDL: Velocity of motor 1 MDH: Velocity of motor 2
0x060	Encoder Index Position This is not sent with a fixed rate but whenever the encoder index is detected.	5	MDL (4 Bytes): Motor position at the index tick MDH (1 Byte): ID of the motor
0x070	Acceleration (both motors)	8	MDL: Acceleration of motor 1 MDH: Acceleration of motor 2

Important

The motor data (current, position, velocity) is send as Q24 values. Divide by 2²⁴ to get the float representation.

Status Message¶

The status message contains one byte of data which is structured as follows:

Bit 5-7	Bit 4	Bit 3	Bit 2	Bit 1	Bit 0
Error Code	Flag motor 2 ready	Flag motor 2 enabled	Flag motor 1 ready	Flag motor 1 enabled	Flag system enabled

A motor is “ready” when the alignment process is finished.

Error Codes¶

Error Code	Meaning
0	No Error
1	Motor 1 Encoder Error
2	CAN Receive Timeout
3	Critical Motor Temperature (currently unused)
4	Some error in the Position Converter module
5	Position Rollover (position exceeded max value) This error is by default disabled. To enable it, send a ENABLE_POS_ROLLOVER_ERROR=1 command.
7	Motor 2 Encoder Error

Note that the error code for critical motor temperature won’t be used for now, as we don’t have any temperature sensing. As this might be added in the future, I already reserve an error code for this, though.

Messages sent to the Robotic Leg Board¶

ID	Name	Meaning [Unit]	Structure of the data
0x000	CAN_ID_COMMANDS	Command	MDL: value MDH: command code (see Command Codes)
0x005	CAN_ID_IqRef	Current \(I_q\) Reference \([A]\)	MDL: IqRef for motor 1 MDH: IqRef for motor 2
0x006	CAN_ID_KP	Gains for P controller \([\frac{A}{mrev}]\)	MDL: P gain for motor 1 MDH: P gain for motor 2
0x007	CAN_ID_KD	Gains for D controller \([\frac{A}{kmrev / min}]\)	MDL: D gain for motor 1 MDH: D gain for motor 2
0x008	CAN_ID_POS_REF	Reference for P controller \([mrev]\)	MDL: Reference position for motor 1 MDH: Reference position for motor 2
0x009	CAN_ID_VEL_REF	Reference for D controller \([\frac{kmrev}{min}]\)	MDL: Reference velocity for motor 1 MDH: Reference velocity for motor 2

\(mrev\) = motor revolutions. E.g. 0.5 is half a revolution, 1.0 a full revolution, 2.0 two revolutions, etc.

Important

Please note that current reference values have to be send as Q24 values. To convert float to Q24 multiply by 2²⁴ and round to an integer.

Command Codes¶

A command message (ID = 0) consists of two parts. The high bytes (MDH) contain a code that is associated with a specific parameter (see table below) while the lower bytes (MDL) contain the value that is to be assigned to the parameter.

Code	Name	Meaning [Unit]	Value	Default
0x01	ENABLE_SYS	Enable the system.	0/1	1
0x02	ENABLE_MTR1	Enable Motor 1	0/1	0
0x03	ENABLE_MTR2	Enable Motor 2	0/1	0
0x04	ENABLE_VSPRING1	Enable virtual spring mode for motor 1	0/1	0
0x05	ENABLE_VSPRING2	Enable virtual spring mode for motor 2	0/1	0
0x0C	SEND_CURRENT	Send motor currents via CAN	0/1	0
0x0D	SEND_POSITION	Send encoder positions via CAN	0/1	0
0x0E	SEND_VELOCITY	Send motor velocities via CAN	0/1	0
0x0F	SEND_ADC6	Send ADC inputs A6/B6 via CAN	0/1	0
0x14	SEND_ALL	Disable/Enable all of the configurable CAN messages	0/1	0
0x1E	SET_CAN_RECV_TIMEOUT	Set CAN Receive Timeout in milliseconds. Set to zero to disable timeout.	uint32	0
0x1F	ENABLE_POS_ROLLOVER_ERROR	Enable the position rollover error	0/1	0
0x28	CAN_CMD_P_CONTROLLER_LIMIT_IQ_MTR1	Set the current limit for the P controller of motor 1 [A]	IQ24	0
0x29	CAN_CMD_P_CONTROLLER_LIMIT_IQ_MTR2	Set the current limit for the P controller of motor 2 [A]	IQ24	0
0x2A	CAN_CMD_D_CONTROLLER_LIMIT_IQ_MTR1	Set the current limit for the D controller of motor 1 [A]	IQ24	0
0x2B	CAN_CMD_D_CONTROLLER_LIMIT_IQ_MTR2	Set the current limit for the D controller of motor 2 [A]	IQ24	0

Example: To enable motor 1, set MDH = 2 and MDL = 1.

Nomenclature: When refering to sending commands in this documentation, the following nomenclature is used: NAME=value. Example: ENABLE_SYS=1 to enable the system.

Messages sent from the Central Pivot Board¶

ID	Meaning	Size [Bytes]	Structure of the data
0x010	Status	1	see Status Message
0x031	Encoder Position (both Central Pivot Encoders)	8	MDL: Angular Position of Encoder 1 MDH: Angular Position of Encoder 2
0x041	Velocity (both Central Pivot Encoders)	8	MDL: Angular Velocity of Encoder 1 MDH: Angular Velocity of Encoder 2
0x071	Acceleration (both Central Pivot Encoders)	8	MDL: Angular Acceleration of Encoder 1 MDH: Angular Acceleration of Encoder 2

Important

The motor data (current, position, velocity) is send as Q24 values. Divide by 2²⁴ to get the float representation.

CAN Receive Timeout¶

The embedded software on the board provides a security feature that disables the motors in case the CAN connection is interrupted or the controller on the PC exits without properly shutting down the system. This is done by simply checking the time since the last current \(I_q\) reference was received and raising an error if it exceeds a specified timeout.

Note that by default, this feature is disabled! If you want to use it, you have to enable it by specifying a timeout duration greater than zero (see Command Codes above). There are a few consequences that have to be kept in mind:

Before enabling the motors, set the current references to zero, otherwise the timeout may be trigger immediately when enabled. Note that this is good practice anyway as it clears potentially dangerous previous reference values.
Current references have to be sent in a loop, even if the values do not change.

The timeout is only checked when motors are enabled and current references are not zero. This means that, as long as the current reference is zero, it is okay to enable the timeout during intialization even if current commands are not send immediately.

When the timeout is triggered, an error is set and the system is disabled. You can simply reenable it by sending (in this order) enable system command, a current=0 command and enable motor commands.