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Trouble writing PWM signal from PID controller

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Chris Hubbard
Chris Hubbard on 3 Dec 2019
Commented: Nikhil Rajkumar on 29 Jul 2020
I am working on a project to use an Arduino Uno with MATLAB PID control, a moisture sensor, and water pump to keep a plant at a specified moisture level. I've posted my code below, everything works perfect until the if/ifelse statements where I can't seem to get an output of anything but 0 or 1 duty cycle even when I confirmed that PID was outputting a value between 0 and 1. Any help would be greatly appreciated, thank you.
%%
End_time_CS= 10;
moisture_Threshold= 20;
tic
Kp = .05;
Ki = 0.01;
Kd = 0.01;
dt = 0.25;
i=1;
while toc <= End_time_CS
V_moisture=readVoltage(a,'A0');
moisture_f=(4.688.*exp(0.2801.*V_moisture))+((1.03.*(10.^(-6))).*exp(14.4.*V_moisture));
rec_temp_CS(i+1) = moisture_f;
rec_time_CS(i+1) = toc;
Error(i+1) = moisture_Threshold - rec_temp_CS(i);
Prop(i+1) = Error(i+1);
Der(i+1) = (Error(i+1) - Error(i))/dt;
Int(i+1) = (Error(i+1) + Error(i))*dt/2;
I(i+1) = sum(Int);
PID(i+1) = Kp*Prop(i) + Ki*I(i+1)+ Kd*Der(i);
i=i+1;
pause(.25)
%this is where the problem is I can only get it to output 0 or 1 duty
%cycle, not the PID value even when PID is between 0 and 1
if PID(i) > 1
writePWMDutyCycle(a,'D9',1)
elseif PID(i) < 0
writePWMDutyCycle(a,'D9',0)
elseif 0 < PID(i) < 1
writePWMDutyCycle(a,'D9',PID(i))
end
%PWM_log=readVoltage(a,'A5')
end
display('Control System has stopped')
  3 Comments
Nikhil Rajkumar
Nikhil Rajkumar on 29 Jul 2020
Hey! I am working on a similar project that requires a PID controllor to operate LED brightness. I was wondering if you explain what Error(i+1)/Error(i), Prop(i+1), Der(i+1), Int(i+1) and PID(i+1) do. Are they functions that you have defined? If so, please briefly explain what each of them do. I would really appreciate some help as I am new to MATLAB. Thank you so much!

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Answers (1)

Pranav Murali
Pranav Murali on 16 Mar 2020
Hi Chris,
I understand that you are receiving the correct value of PID but you are unable to control the output duty cycle of your system. The problem here is MATLAB does not support testing whether PID(i) is in between 0 and 1 with this syntax. Instead, MATLAB interprets 0 < PID(i) < 1 as (0 < PID(i)) < 1, comparing the logical result of 0 < PID(i) (either 0 or 1) to the value 1.
I would suggest you rather try with the syntax below:
elseif ((0 < PID(i)) && (PID(i) < 1))
That should solve your problem.

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