Timer doesn't display only once

Hello! I am a begginer in Matlab. I am trying to control a wheelchair by eye tracking. I need to stop and start the system by forced blinking.

The code displays 'Blink' if the blink takes longer than 3 seconds, but I need to display it only once, no matter how much the 3 second deadline is exceeded! I don't know how to do this. If I blink for 5-6 seconds, for example, the program displays 'Blink' continuously for 3-4 times.

                 if SkinPerc >=90
                  
                    t = timer;
                     t.StartDelay = 3;
                     t.ExecutionMode='singleShot';
                     t.TasksToExecute=1; 
                     t.TimerFcn = @(myTimerObj, thisEvent)disp('Blink');
                     start(t)
                     
             

[Ameer Hamza]

The question description is not clear. Are you saying that the TimerFcn runs multiple times, even though the ExecutionMode is singleShot?

[Delia Dragusin]

Yes, that is what I am saying. I need it to run only once.

[Ameer Hamza]

That is not the documented behavior of singleShot ExecutionMode. Maybe you can try the code in my answers, which explicitly deletes the timer after execution.

Answer 1

[Ameer Hamza]

You can write you timer callback such that it will automatically delete the timer object after it runs once.

t = timer;
t.StartDelay = 3;
t.ExecutionMode='singleShot';
t.TimerFcn = @timerCB;
start(t)

function timerCB(myTimerObj, thisEvent)
    disp('Blink')
    stop(myTimerObj);
    delete(myTimerObj);
end

[Delia Dragusin]

I get this error:

Error: File: combinatie.m Line: 84 Column: 1

Function definition not supported in this context. Create functions in code file.

[Ameer Hamza]

How have you defined timerCB? The easiest way is to include the function definition at the very end of your file. The other way is to create a file named timerCB.m and place the function definition

function timerCB(myTimerObj, thisEvent)
    disp('Blink')
    stop(myTimerObj);
    delete(myTimerObj);
end

inside the file.

[Delia Dragusin]

It it still not working, unfortunately, but I really appreciate your help!

[Ameer Hamza]

If this solution is not working, then I guess the Walter pointed you in the right direction. The condition SkinPerc >=90 is true in several loop iterations, which creates a new timer object in each iteration.

Answer 2

[Walter Roberson]

Your code is starting a new 3 second timer every time it finds SkinPerc >=90 . It is not the case that each individual timer is repeating multiple times: instead you have multiple timers going.

I suggest that you store a variable in an accessible location, and when you find SkinPerc >=90, check to see if the variable is already set; if it is, then skip creating a timer. If the variable is clear, then set the variable true and create the timer and start it. One of the things the timer should do is clear the variable. For information on how to share variables see

http://matlab.wikia.com/wiki/FAQ#How_can_I_share_data_between_callback_functions_in_my_GUI.28s.29.3F

However, it looks to me as if you are approaching this wrong. Instead, I would suggest that you use a structure such as

in_blink = false;
blink_recorded = false;
blink_start_time = tic;

...

if SkinPerc >=90
    if in_blink
        if ~blink_recorded & toc(blink_start_time) > 3
            disp('Blink')
            blink_recorded = true;
        end
    else
        in_blink = true;
        blink_recorded = false;
        blink_start_time = tic;
    end
else
    in_blink = false;
end

[Delia Dragusin]

I tried this way, but now it doesn’t even display ‘Blink’

[Walter Roberson]

This code expects that you are looping reading values that get converted into SkinPerc readings, and assumes that SkinPerc >= 90 is the indicator for "blink started", and that it will continue to be true continuously for several iterations of the loop, including for more than 3 real-time seconds.

These are all guesses on my part as to how your code really functions.

For example, my code is not designed to (for example) measure eyelid height with the idea that some particular pattern of heights for a particular time is "blinking". The code assumes that SkinPerc (whatever that is) is continually true for 3 seconds or more in order as the case to display Blink.

[Delia Dragusin]

Yes, this is how my code want to function, but when I try your method, it doesn't display 'Blink' at all.

Can I send you the code on the email to look at it, please? Timers and tic, toc really confuse me so much. I am stuck on this part for more than one week...

[Walter Roberson]

Sorry, I am too busy with volunteering to take on a private consulting contract at this time (code sent to me privately is a private consulting contract). If you post the code publicly here I will look at it later (after I sleep.)

[Delia Dragusin]

I would like to post it here, but I use the code for my graduation thesys. And the problem is that the antiplagiarism system will find my code online and will say it is plagiated. The code does this: I have a loop that captures images from my webcam. It detects my face, then the eye pair and lastly, the biggest eye. If the skin percent is bigger than 90, it means that I blinked. If not, it means that I have my eyes OPEN and detects the direction of my Eye movement (forward, left, right). I want to use the blink to stop and start my mini-wheelchair (which is in fact an arduino robot kit 2wd). That’s why I am interested to display only once the blink... to avoid starting and stoping continuously the system. But I tried several methods with tic and toc or with timer, but nothing seems to work. I really don’t know what to do...

I have this part:

code

if SkinPer>=90

here I should display only once 'Blink' if the eye blink exceeds 3 seconds

else

the rest of the code

[Steven Lord]

You could try creating the timer (but leaving it stopped) well before you run whatever code you have that loops and polls for the blink. When you detect a blink, start the timer. If the blink ends before the timer's StartDelay finishes, stop it. Otherwise let it execute its TimerFcn and return to being stopped.

This would avoid the need to check if the timer exists, since it always will exist. It just may not be actively running.

[Walter Roberson]

What is the difference between a blink that lasts more than 3 seconds, compared to closing the eyes for more than 3 seconds?

"Blink" is usually considered a fast process, 100 ms typically and up to 400 ms. Someone could deliberately hold their eyes closed for 3 seconds, or someone could deliberately repeatedly blink for 3 seconds, but the repeated blink option involves a pattern of SkinPerc going above and below 90.

The code I posted already deals with the "deliberately closed for 3 seconds" possibility. If you want the "blink repeatedly" possibility, then you need to define a time period with no blink that is considered to stop the pattern. People blink roughly every 3 to 4 seconds normally, so just noticing two blinks within a 3 second interval is probably not reliable enough.

If the pattern is a certain number of blinks within a particular time (3 seconds) then should the action be registered when the last of that particular number of blinks happens, or should the action be deferred until the timer interval expires? Deferring the action would allow for the possibility of defining more blinks as meaning something different, or the possibility of a cancel action (such as closing the eyes for a particular time) being defined.

[per isakson]

Before trying to fix the problems see: Debug a MATLAB Program

After the line

if SkinPer>=90 

add the statement

disp('A timer will be created')

and run the code