using fscanf command for distance sensor

I am connecting an ultrasonic distance sensor to matlab with the 'serial' command, using the code below. When trying to get the distances as an output with a for loop I get an error in the line where I use the command 'fscanf'. Matlab tells me:

Error using fscanf
Invalid file identifier. Use fopen to generate a valid file identifier.

Error in subsonicsensor (line 12)
  y(i)=fscanf('arduino','%cm');

Can anybody tell me how I can adjust my for loop, or rest of the code to get the right distances from the code?

I also tried using fprintf instead of fscanf. This gave me the value 7cm for al 100 measurements. Can anybody explain?

Here is my code:

clear all
clc

arduino=serial('COM3','BaudRate',9600);

x=linspace(1,100);
L = length(x);

for i=1:L
    fopen('arduino'); 
  y(i)=fscanf('arduino','%cm');
    pause(0.05);
end
  
fclose(arduino);
disp('making plot..')
plot(x,y);

Answer 1

[Walter Roberson]

Move the

      fopen('arduino');

to before the loop, and change it to

      fopen(arduino);

Change the

  y(i)=fscanf('arduino','%cm');

to

  y(i)=fscanf(arduino,'%fm');

[Robin Johannes Terstappen]

Thank you for answering. Adjusting my code this way gave me a succesfull plot one time. After running the code a second time I got the following error and no plot.

Error using serial/fopen (line 72)
Open failed: Port: COM3 is not available. No ports are available.
Use INSTRFIND to determine if other instrument objects are connected to the requested device.

Error in subsonicsensor (line 8)
fopen(arduino);

Right now the code looks like this:

clear all
clc

arduino=serial('COM3','BaudRate',9600);

x=linspace(1,100);
L = length(x);
fopen(arduino);

for i=1:L

  y(i)=fscanf(arduino,'%fm');
    pause(0.05);
end
  
fclose(arduino);
disp('making plot..')
plot(x,y);

[Robin Johannes Terstappen]

After reconnecting the usb I get the following faillure

Warning: Unsuccessful read: Matching failure in format.. Subscripted assignment dimension mismatch.

Error in subsonicsensor (line 12)
  y(i)=fscanf(arduino,'%fm');

[Walter Roberson]

You are not sending the arduino a command, so we can guess that your arduino is sending a continuous stream of results to MATLAB. At the time you connect to the arduino, it might be in the middle of sending something, so the first character available in the buffer might be 'm', which is not something that can be matched by the '%fm' format.

The quick fix: before the "for" loop, add

   fgetl(arduino);     %flush out any partial line in the buffer

[Robin Johannes Terstappen]

I think I'm almost there. I fixed the fopen error, which refused to change my port status from closed to open by adding the command delete(instrfindall); to the start of my script. The port status is now open but when running the code I get the message that my dimensions mismatch.

Warning: Unsuccessful read: Matching failure in format.. 
Subscripted assignment dimension mismatch.

Error in subsonicsensor (line 17)
  y(i)=fscanf(arduino,'%fm');

My code looks like this at the moment:

clear all
clc

delete(instrfindall);
arduino=serial('COM3');
set(arduino,'BaudRate',9600);
fopen(arduino);

x=linspace(1,100);
L = length(x);

fgetl(arduino);     %flush out any partial line in the buffer

for i=1:L

  y(i)=fscanf(arduino,'%fm');
    pause(0.05);
end
  
fclose(arduino);
disp('making plot..')
plot(x,y);

This is the arduino code that I uploaded to the Arduino UNO board by the way:

#define trigPin 12
#define echoPin 13
void setup() {
  // put your setup code here, to run once:
Serial.begin(9600);
pinMode(trigPin, OUTPUT);
pinMode(echoPin, INPUT);
}

void loop() {
  // put your main code here, to run repeatedly:
long duration, distance;
digitalWrite(trigPin, LOW);
delayMicroseconds(2);
digitalWrite(trigPin, HIGH);
delayMicroseconds(10);
digitalWrite(trigPin, LOW);
duration = pulseIn(echoPin, HIGH);
distance = (duration/2) / 29.1;

if(distance >= 400 || distance <= 2){
  Serial.println("Out of Range");

}
else {
  Serial.print(distance);
  Serial.println("cm");
}

delay(500);
}

Again what I'm trying to achieve is to get a constant output of my ultrasonic distance sensor displayed in Matlab. Thank you for helping

[Walter Roberson]

Change

  y(i)=fscanf(arduino,'%fm');

to

        thisline = fgetl(arduino);
        if strcmpi(thisline, 'Out of Range')
          fprintf('Out of Range reported at point #%d\n', i)
          y(i) = nan;
        else
          thisdist = sscanf(thisline, '%f');
          if isempty(thisdist)
            fprintf('Unexpected response at point #%d; line was: "%s"\n', i, thisline);
           y(i) = nan;
          else
            if length(thisdist) > 1
              fprintf('Multiple distances reported for point #%d, using first; line was "%s"\n', i, thisline);
            end
            y(i) = thisdist(1);
          end
        end

[Robin Johannes Terstappen]

Thanks Walter for helping us out. Matlab after running the code now shows us the step numbers at which the distance sensor gives: 'Out of Range', and a plot of the measured distances that are not out of range. However, what we would like to achieve is a non-stop real-time display of the measured distance values. How do we get matlab to print the measured values in real time without stopping?