Is it possible to transmit a deterministic signal on a CAN bus?

I'm using the Vehicle Network Toolbox and a NI-9860 in a cDAQ

I'm able to trasmit a signal value to the CAN bus like so:

canch1 = canChannel('NI','CAN3');

canch1.Database = canDatabase('example_database.dbc'); %apply a database

start(canch1)

message.Signals.example_signal = 1;

transmit(canch1,message)

stop(canch1)

What I want to do is transmit a sine wave signal to the CAN bus with a time step of 1ms. I tried to implement this using a for loop, however the signal ends up being deformed by the execution time of the code.

t = linspace(0,5,5000); % 5 seconds at 1000 Samples/s

f = 0.5; % sine frequency hz

sine = 0.7 * sin(2*pi*f*t); % create sine

start(canch1)

message = canMessage(canch1.Database,'example_message');

for i = 1:length(sine)

message.Signals.example_signal = sine(i);

pause(1/1000)

transmit(canch1,message)

end

Is there a way to transmit the sine signal onto the CAN bus in a deterministic way?

Can I somehow store the signal in a buffer on the NI hardware and trigger the transmit? Do I need to create an array of CAN messages and transmit that?

It seems to me that if I can create an array of CAN messages and use the trasmit function, the attribute of the CAN database 'GenMsgCycleTime' should define the cycle time of a message in ms. In my database GenMsgCycleTime == 1.

Answer 1

[Harimurali]

Hi Adrian,

To transmit messages with precise timing, you need a real-time system or hardware support for buffering and scheduling messages. The "NI-9860" module in a "cDAQ" chassis does have some capabilities for deterministic timing, but this usually requires using NI's software tools, like LabVIEW with the NI-XNET driver, which is designed for real-time applications.

The timing of your MATLAB script by using a more precise timer object or by adjusting the priorities of MATLAB's execution, but this will not guarantee real-time performance.

Here's an example MATLAB script of how to use a timer object to transmit CAN messages at a more consistent interval:

% Load the DBC file

candb = canDatabase("example_dbcfile.dbc");

message = canMessage(candb, 'example_message');

canch = canChannel('NI','CAN3');

start(canch)

t = linspace(0, 5, 5000); % 5 seconds at 1000 Samples/s

f = 0.5; % Sine frequency Hz

sine = 0.7 * sin(2 * pi * f * t); % Create sine

% Start transmitting the sine wave

transmitSineWave(canch, message, sine, 1000); % 1000 Hz transmission rate

function transmitSineWave(canch, message, sine, frequency)

% Create a timer object

t = timer;

t.ExecutionMode = 'fixedRate'; % Execute the timer repeatedly at a fixed rate

t.Period = 1 / frequency; % Set the period of the timer based on the desired frequency

t.TasksToExecute = length(sine); % Set the number of times to execute the timer

t.TimerFcn = @(~, ~) transmitOneSample(canch, message, sine); % Timer function to call

t.StopFcn = @(~, ~) delete(t); % Clean up the timer after execution

% Start the timer

start(t);

end

function transmitOneSample(canch, message, sine)

persistent index;

if isempty(index)

index = 1;

end

% Update the signal value

message.Signals.example_signal = sine(index);

% Transmit the CAN message

transmit(canch, message);

% Update the index or reset if it reaches the end of the sine array

index = index + 1;

if index > length(sine)

index = 1;

end

end