How to program an Arduino Uno to collect data using a timer interrupt and analyze that data in matlab?

Question

I working on a project where I want to collect sensor data (from a sensor) and I want to analyze the first 20 data sample (e.g., find the range, trendline e.t.c.). I know how to compile a timer interrupt in Arduino IDE (using an ISR timer), but how would I be able to do this something similar (ISR timer) in MATLAB. The reason being, I want to collect my sensor data at exactly 20 Hz and using a delay function can prove to be inaccurate. Also, when I run a sample program that collects data in MATLAB, my sample frequencies change from 15Hz to even 24Hz (as shown below) . Does anyone have any suggestion on how I can do this? If the question is not clear enough I can elaborate more in the comments.

This is my MATLAB code:

clear all;
a = arduino('com4', 'uno');
i = 0;
v1 = zeros(1e4,1);
v2 = zeros(1e4,1);
x = zeros(1e4,1);
t = timer('TimerFcn', 'stat=false; disp(''Timer!'')',... 
                 'StartDelay',5,'Period', 1);
start(t)

stat=true;
tic
while(stat==true)
   %while i<60
    i = i + 1;
    v1(i) = readVoltage(a, 'A0');
    v2(i) = readVoltage(a, 'A1');
    x(i) = i;
    disp(x(i));
    pause(.00001);
   
  % end
end
toc


% %Plot voltage versus time
v1 = v1(1:i);
v2 = v2(1:i);
x = x(1:i);

p = v1;
n = v2;
%Plot voltage versus time
figure(1);
plot(x,p,'-',x,n,'-')
xlabel('Elapsed time (sec)')
ylabel('v')
title('Ten Seconds of voltage Data')
%set(gca,'xlim',[t(1) t(i)])
legend('v1', 'v2');

%
delete(t) % Always delete timer objects after using them.

Output is

>> Timer
     1

     2

     3

     4

     5

     6

     7

     8

     9

    10

    11

    12

    13

    14

    15

    16

    17

    18

    19

    20

    21

    22

    23

    24

    25

    26

    27

    28

    29

    30

    31

    32

    33

    34

    35

    36

    37

    38

    39

    40

    41

    42

    43

    44

    45

    46

    47

    48

    49

    50

    51

    52

    53

    54

    55

    56

    57

    58

    59

    60

    61

    62

    63

    64

    65

    66

    67

    68

    69

    70

    71

    72

    73

    74

    75

Timer!
    76

Elapsed time is 5.032553 seconds.
>> 

I apologize for the long output. But as you can see, I specified a pause time of .00001, which should give me well over a 20Hz sampling frequency. But I only get 76 samples/5 seconds = 15.2Hz. Also, sometime when I run this the program may collect 79sample/5secons, or even 121sample/5seconds. It just is not consistent. I was hoping to solve this with a timer interrupt that will be hard coded in MATLAB to collect data at exactly 20Hz. Please let me know what y'all think. Thank you.

Answer 1

I've configured the timer differently making use of its UserData property. The timer will update the data every fixed period for a specified number of samples.

close all;
clear all;

% Some useful constants.
NUM_SAMPLES = 20;
PERIOD = 0.05;

global a;
a = arduino('com4', 'uno');

% Initialise data. It will be updated by the timer.
data.i = 0;
data.v1 = zeros(NUM_SAMPLES, 1);
data.v2 = zeros(NUM_SAMPLES, 1);
data.x = (1:NUM_SAMPLES) * PERIOD;

% Initialise timer for a fixed period for a specific number of samples.
t = timer;
t.ExecutionMode = 'fixedRate';
t.Period = PERIOD;
t.TasksToExecute = NUM_SAMPLES;
t.TimerFcn = @TimerCallback;
t.UserData = data;

% Run the timer and wait for it to complete.
start(t);
wait(t);
stop(t);
data = t.UserData;  % Retrieve the updated data.
delete(t);

% Plot voltage versus time.
figure(1);
plot(data.x, data.v1, '-', data.x, data.v2, '-');
xlabel('Elapsed time (s)');
ylabel('Voltage (V)');
title('Voltage Data');
st = sprintf("%0.3f seconds", NUM_SAMPLES * PERIOD);
subtitle(st);
legend('v1', 'v2');

function TimerCallback(obj, event)
    global a;
    obj.UserData.i = obj.UserData.i + 1;
    obj.UserData.v1(obj.UserData.i) = readVoltage(a, 'A0');
    obj.UserData.v2(obj.UserData.i) = readVoltage(a, 'A1');
end

Answer 2

Each communication with Arduino takes 20ms, you have two in the loop so in the ideal conditions you could get 25 readings/second. See here the source

This also depends on the size of the data you request.

I think this is what you are looking for but I cant give you any advice since I don't have Matlab and Octave is not having this feature.