Plotting in a parallel while reading serial

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Steven Brace 2019-6-7

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https://ww2.mathworks.cn/matlabcentral/answers/465995-plotting-in-a-parallel-while-reading-serial

评论： Adam 2019-6-10

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Hi there,

I have the following code (which I have simplified a bit to post).

It captures the position and force from a test rig, which I am printing out over serial as fast as possible and reading into matlab.

It grabs the most recent reading from each serial device and stores it along with the current time (using tic toc).

I then plot this data on a graph.

My problem is that while matlab is executing the plot I miss some new incoming serial data and build up a back log.

I have improved the situation by only plotting every 50 times i collect data, but this still allows a buildup to occur during plotting. Can i carry out the plot in a parallel thread to avoid affecting the serial reads? How would i structure this?

Thanks!

positioncounter=0;
forcecounter=0;
tic
        
    while running==1
              
        if rig.bytesavailable>0 %grab rig position and time
            positioncounter=positioncounter+1;
            position = fscanf(rig);
            ResultsPosition(positioncounter,1)= toc;
            ResultsPosition(positioncounter,2)=str2double(position(1:end-2));
            
        end
        
        if s.bytesavailable>0 %grab force and time
            forcecounter=forcecounter+1;
            force = fscanf(s);
            ResultsForce(forcecounter,2)=9.81*str2double(force(1:end-2));
            ResultsForce(forcecounter,1)= toc;
        end
        
        try
            yyaxis left
            plot(ResultsForce(:,1),ResultsForce(:,2));
            yyaxis right
            plot(ResultsPosition(:,1),ResultsPosition(:,2));
        end
    
    running =get(handles.BTN_CAPTURE,'Value'); %is button still toggled on
    drawnow();
    end

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Walter Roberson 2019-6-8

Are you using true serial, or are you using serial over USB ?

There is an arduino-like device that sends data direct to USB instead of buffering it, and so can achieve higher data rates. Unfortunately I keep losing track of which device it is.

Adam 2019-6-10

在 MATLAB Online 中打开

Do you need or want every point to be its own plot, as is in your code included in the question? This is, in general, an expensive way to plot data. Much more efficient is to create one plot and then edit its XData and YData properties to add your new data.

e.g.

yyaxis left
hPlotForce = plot(ResultsForce(:,1),ResultsForce(:,2));
yyaxis right
hPlotPosition = plot(ResultsPosition(:,1),ResultsPosition(:,2));

for the first point, then

set( hPlotForce, 'XData', [ hPlotForce.XData, ResultsForce(:,1) ], 'YData', [ hPlotForce.YData, ResultsForce(:,2) ] );
set( hPlotPosition, 'XData', [ hPlotPosition.XData, ResultsPosition(:,1) ], 'YData', [ hPlotPosition.YData, ResultsPosition(:,2) ] );

on each subsequent pass.

Normally I do this by storing a handle to the initially empty plot e.g.

hPlotForce = [];

then in my code I have an if else

if isempty( hPlotForce )
   ...
else
   ...
end

where the first code above goes in the if and the second block of code in the else - i.e. if the plot hasn't been created yet then call plot, if it has then edit the existing plot.

Sometimes simply speeding up the way you plot solves the problems itself without you needing to come up with complex parallelisation. It may not in this case, I don't know, but calls to plot are expensive and create a new graphics object every time and the more individual graphics objects you have the slower your code is generally.

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