Connecting via CAN

Hardware

The motor board for which this firmware is written supports the CAN 2.0B standard. In theory any CAN device following the standard should be able to communicate with the board. In practice we are successfully using “PCAN-PCI Express” dual-channel cards for classic CAN by PEAK System. However, we are having problems with PEAK’s CAN FD devices, so we recommend to stick with the “non-FD” devices for now. We have not tested with hardware of other manufacturers, so we cannot say anything about those.

Linux Drivers

For the PEAK devices, SocketCAN drivers are already included in the standard Linux kernel, so there is no need to manually install any driver. In the following, it is assumed that SocketCAN is used.

Connection Setup

When using SocketCAN, there is an interface for each available channel, called “can0”, “can1”, “can2”, …

Each interface has to be configured and enabled in order to be usable. This firmware uses 1 Mbit/s and a sample point at 86.7%. To configure the interface accordingly, run

sudo ip link set can0 type can bitrate 1000000 sample-point 0.867

and to enable it run

sudo ip link set up can0

This has to be done for each interface that you want to use (replace “can0” accordingly).

Determine Names of Interfaces

The names are assigned automatically to the interfaces and the order may not seem logical from outside. So you first need to find out which physical port “can0”, “can1”, etc. correspond to.

There may be nicer ways to do this but the following precedure works well for us:

1. Disconnect any devices from the CAN ports.

2. Configure and enable all interfaces as described above.

3. Open a terminal and run watch netstat -i. It should show an output like this:

   Kernel Interface table
   Iface   MTU Met   RX-OK RX-ERR RX-DRP RX-OVR    TX-OK TX-ERR TX-DRP TX-OVR Flg
   can0         16 0         0      0      0 0             0      0      0      0 ORU
   can1         16 0         0      0      0 0             0      0      0      0 ORU
   can2         16 0         0      0      0 0             0      0      0      0 ORU
   ...
   

4. Use a motor board that has been flashed (or any other CAN device that sends messages), power it on and connect it to the first CAN port and watch the output in the terminal. For one of the listed interfaces, the RX-OK value should start increasing. This is the interface you are currently connected to.

5. Disconnect the board and connect it to the next port, repeat until all ports are identified. You may want to put labels on them.

Note: As long as the hardware configuration of the computer is not changed, the interface to port mapping should be fixed. However, when adding more CAN cards or removing some of the existing ones, the order of the others may change. In this case, the identification procedure needs to be repeated.

Testing the Connection

For a first basic test if the CAN communication is working, the console applications candump and cansend can be used. On Ubuntu, they can be installed via the can-utils package.

In the following it is assumed that interface “can0” is used. Adjust the commands accordingly when using a different interface.

1. Connect the motor board via CAN to the computer and power it on.

2. Open a terminal and run

   candump -e can0
   

   It should display a message with ID 010 once every second (the status message that is send by the board). If you don’t see this message, something is wrong!

3. If you see the status messages, you can proceed by sending the commands to enable the motors. Open a separate terminal (keep the candump running!) and execute the following commands in this order:

   cansend can0 005#0000000000000000  # set target current to zero
   cansend can0 000#000000000000001E  # disable CAN receive timeout
   cansend can0 000#0000000100000001  # enable the system
   cansend can0 000#0000000100000014  # enable sending measurements
   cansend can0 000#0000000100000002  # enable motor 1
   cansend can0 000#0000000100000003  # enable motor 2
   

   After the the command to enable sending measurements, the output of candump should start sending messages with IDs 020, 030, 040 and 050 at high frequency.

   When enabling a motor the corresponding motor should jitter a bit and then be held in place for a few seconds (assuming there actually is a motor connected). This is an initial calibration procedure that is automatically performed when the motor is enabled for the first time after power up, see Motor Alignment Calibration.

If everything behaves as described, this means that the CAN communication is generally working. As an additional test, you may want to check if the communication rate is stable. For this, keep the board on after step 3 and run the following command:

candump -t d can0,040:FFF

This will print only messages with ID 040. The first value in each row is the time passed between the current message and the previous one. This value should be close to 0.001 (= 1 ms) with only little deviation. If you see larger deviations here, this means some messages are delayed, indicating an instable connection. Whether this is a problem depends on the application but in general delays mean that the controller will be less stable.