Timer object is triggering much faster than specified?

Hello, I have written some code to read modbus registers at a frequency of 2 Hz using a timer object, but when I examine the sampling intervals recorded, they vary from 0.05 to 0.27 s, with a median of 0.08s. Could you please help me to understand why and how to fix this? If there is a cleaner way to write this functionality, I would appreciate your comments about this too.

I have included the whole code below, but think these 3 lines are the most important for this issue:

samplingPeriod=0.5; %sampling interval in seconds
tim=timer('ExecutionMode','fixedRate','BusyMode','drop','Period',samplingPeriod,'TasksToExecute',numReadings,'StopFcn',@deltim,'TimerFcn',{@getreading},'Name','mTimer');
start(tim)

%To stop part way through, type stop(tim) into the command line

%% User definable variables

IP1='192.168.1.1'; %IP address of 1st instrument to interogate

FileNameO = "ABB_31AUG23_NoT2corr_2.csv";

measParamNamesO = ["meas1","T1","T2","Out2", "P1","Out3"];%parameter names to measure

measParamAddressO = [1, 401, 451, 453, 501, 458]; %parameter modbus addresses. Must be input register type.

runTime=4*24*60*60; %Total measuring time in seconds

samplingPeriod=0.5; %sampling interval in seconds

numReadings=round(runTime/samplingPeriod +1); %number of readings to take

%% define preset variables

format longG;

global reading

global k

global measParamNames

global measParamNum

global measParamAddress

global m1

global measParamTypes

global FileName

FileName = FileNameO;

measParamNames = measParamNamesO;

measParamAddress = measParamAddressO;

measParamNum = length(measParamNames);

measParamTypes = strings(1,measParamNum);

measParamTypes(:,:) = "single";

reading=table('VariableNames',["time","sTime",measParamNames],'VariableTypes',["double","double",measParamTypes],'Size',[numReadings,2 + measParamNum]);

k=1;

%% setup Modbus connection

% Create a Modbus Object.

m1 = modbus('tcpip', IP1);

m1.Timeout = 1;

%% Setup timer

tim=timer('ExecutionMode','fixedRate','BusyMode','drop','Period',samplingPeriod,'TasksToExecute',numReadings,'StopFcn',@deltim,'TimerFcn',{@getreading},'Name','mTimer');

start(tim)

function getreading(mTimer,~)

global reading

global k

global measParamNum

global measParamAddress

global m1

try

reading.time(k)=now;

for i=1:measParamNum

reading{k,i+2} = read(m1,'inputregs', measParamAddress(i), 1, 1, 'single');

end

k=k+1;

catch

fprintf('Error in getreading function %s\n', datestr(now,'HH:MM:SS.FFF'));

end

end

function deltim(mTimer,~)

%% Finishing func

global reading

global k

global FileName

reading(k:end,:)=[];

reading.sTime=(reading.time(1:end)-reading.time(1))*24*3600;

writetable(reading,FileName);

hold off

hold on

plot(reading.sTime,reading.meas1)

xlabel("Time (s)");

ylabel("Measured Value (%)")

hold off

figure()

histogram(reading.sTime(2:end)-reading.sTime(1:end-1))

title('Sampling period histogram');

xlabel('Sampling period (s)');

ylabel('Number of instances');

delete(mTimer);%clear timer object

clear m1 k runTime period numReadings IP1 measParamTypes measParamNum measParamNamesO measParamNames measParamAddressO measParamAddress % Clear the Modbus Object created.

disp('Logging has finished')

end

Answer 1

[Varun]

Hi Chirs,

I understand that you are attempting to read “modbus” registers at a frequency of 2 Hz using a timer object. However, you mentioned that the recorded sampling intervals vary and are not consistent with the desired sampling period of 0.5 seconds.

The MATLAB timer object is implemented using a combination of MATLAB's event-driven architecture and the underlying operating system's timer mechanisms. It internally utilizes the operating system's timer mechanisms to trigger timer events. However, the actual timing may be affected by factors such as system load, other tasks running concurrently, handling timer queuing conflicts, and the underlying hardware capabilities. Avoid using timer objects for real-time applications.

To address this issue and achieve more accurate timing, you can consider the following suggestions:

1. Use the “tic” and “toc” functions: Instead of relying solely on the timer object's fixed rate, you can use the tic and toc functions to measure the elapsed time within each iteration. Internally, tic and toc do not rely on the operating system for timing. Instead, they use the high-resolution performance counter of the CPU to measure the elapsed time. It typically has a much higher resolution than the system clock used by the operating system. Here's an example:

desiredPeriod = 0.5; % desired sampling interval in seconds 
numReadings = 10; % number of readings to take 
tic; 
for i = 1:numReadings 
    % Perform your modbus register reading here 
    
    % Calculate the elapsed time 
    elapsedTime = toc; 
    
    % Wait for the remaining time to achieve the desired period 
    remainingTime = desiredPeriod - elapsedTime; 
    if remainingTime > 0 
        pause(remainingTime); 
    end 
    
    % Reset the timer 
    tic; 
end 

2. Consider using the timer object's “StartDelay” property: You can set the “StartDelay” property of the timer object to delay the start of the timer execution, allowing for a more accurate initial timing. This can help compensate for any initialization or setup delays.

Please refer the following pages to learn more on the Limitations of “timer” object and Handling Timer Queuing Conflicts:

• https://www.mathworks.com/help/matlab/ref/timer.html?searchHighlight=timer&s_tid=srchtitle_support_results_1_timer
• https://www.mathworks.com/help/matlab/matlab_prog/handling-timer-queuing-conflicts.html

[Chris Georgiou]

Thank you Varun

Restarting MATLAB eliminiated the error. I'm not sure what was going on.

If there were queing conflicts or issues with system load, wouldn't timer frequency decrease? What I have observed is a large increase.

Your suggestion to use the tic toc and pause functions is an interesting 1, I quite like that. Do you think that this would be able to keep time over days? For instance if I set such a function running at 2Hz after exactly 24 hours would we have 2*60*60*24 samples, give or take a couple?