Finding the max and min value of sensor reading

robtillaart · July 23, 2013, 9:42am

so when there is no strain deltaMax and deltaMin will be zero.

I expect that not to be true as there will probably noise. At least your sketch should be able to cope with that.

OK I understand the problem better.

A solution is to make a reference measurement in the setup() function and use that to determine UP/DOWN in the loop()
another solution is to hard code such reference values
another solution is to determine up/down direction from the array of 10 measurements.

there are many (heuristic) ways to do this e.g. compare 1st and last measurement is a first order indication.

system · July 23, 2013, 1:33pm

Hi again,

Well I defined a variable average_offset in void offset()...so now i have a fixed average offset value form first 10 readings.

void setup() 
{
  Serial.begin(9600);
  pinMode(10, OUTPUT); 
  pinMode(11, OUTPUT);
  offset();
}
void offset(){
  for (int i = 0; i< numReadings; i++)
  {
    total_offset-=readings[i];
    readings[index] =analogRead(A0);
    total_offset+=readings[i];
    index++;
  }
  average_offset=total_offset/numReadings;
}

Then when i apply the strain in up /down direction i can see that the led for up and down turns on..However I do have a small problem for the case of no strain.
In case of no strain ; average,max,min values are nearly same [+-5 acceptable].

 if(sensorMax-sensorMin <=3 && deltaMax<=10 &&deltaMin>=-10){       //nostrain
    digitalWrite(10,LOW);
    digitalWrite(11,LOW);
  }

 if(average<average_offset){                                        //DOWN
    digitalWrite(11,LOW);
    digitalWrite(10,readings[index]);
  }
 if(average>average_offset){                                        //UP
    digitalWrite(10,LOW);
    digitalWrite(11,readings[index]);
  }

this is the serial uutput of no strain case [DMAX=sensorMax-average ,DMIN=sensorMin-average A_OFF:average_offset AVG=running average ]

SENS: 579 DMAX: 0.40 DMIN: -0.60 A_OFF: 568.00 MIN: 578 MAX: 579 AVG: 578.4
SENS: 578 DMAX: 0.60 DMIN: -0.40 A_OFF: 568.00 MIN: 578 MAX: 579 AVG: 578.4
SENS: 579 DMAX: 0.60 DMIN: -0.40 A_OFF: 568.00 MIN: 577 MAX: 579 AVG: 578.0
SENS: 579 DMAX: 1.00 DMIN: -1.00 A_OFF: 568.00 MIN: 577 MAX: 578 AVG: 577.6
SENS: 578 DMAX: 0.40 DMIN: -0.60 A_OFF: 568.00 MIN: 577 MAX: 578 AVG: 577.4

but UP led is blinking because average_offset < average..therefore for no strain case LEDs dont show correctly...

2nd part ; I want to return the new average_offset each eg-.5 minutes is it possible? i should maybe take a look at time interrupts.

thanks in advance

robtillaart · July 23, 2013, 5:36pm

keep it simple.

void offset()
{
  total_offset = 0;
  for (int i = 0; i< numReadings; i++)
  {
    total_offset += analogRead(A0);
  }
  average_offset = total_offset/numReadings;
}

try this variation // and please please add spaces: InEnglishYouUseSpacesTooForReadabilitySoWhyNotInC++?

#define LED_DOWN 10
#define LED_UP 11
if (sensorMax-sensorMin <= 3 && deltaMax <= 10 &&deltaMin >= -10) {       //nostrain
    digitalWrite(LED_DOWN , LOW);
    digitalWrite(LED_UP , LOW);
}
else if (average < average_offset) {                                        // DOWN
    digitalWrite(LED_DOWN , readings[index]);
    digitalWrite(LED_UP , LOW);
  }
else if(average > average_offset ) {                                        //UP
    digitalWrite(LED_DOWN , LOW);
    digitalWrite(LED_UP, readings[index]);
  }

2nd part ; I want to return the new average_offset each eg-.5 minutes is it possible? i should maybe take a look at time interrupts.

have a look at blink without delay example how you can schedule a certain action with time. However as you want to be sure that there is no strain at the moment of recalibration you should consider a user action - e.g. make a pin high - and then read the new calibration until the pin is low.

system · July 24, 2013, 1:53pm

Hello again,

if (sensorMax-sensorMin <= 3 && deltaMax <= 10 &&deltaMin >= -10) {       //nostrain
    digitalWrite(LED_DOWN , LOW);
    digitalWrite(LED_UP , LOW);
}
else if (average < average_offset) {                                        // DOWN
    digitalWrite(LED_DOWN , readings[index]);
    digitalWrite(LED_UP , LOW);
  }
else if(average > average_offset ) {                                        //UP
    digitalWrite(LED_DOWN , LOW);
    digitalWrite(LED_UP, readings[index]);
  }

This part is not consistent..Well when there is no strain I get

SENS: 621 DMAX: 0.80 DMIN: -0.20 A_OFF: 621.80 MIN: 621 MAX: 622 AVG: 621.2
SENS: 622 DMAX: 0.80 DMIN: -0.20 A_OFF: 621.80 MIN: 621 MAX: 621 AVG: 621.0
SENS: 621 DMAX: 0.00 DMIN: 0.00 A_OFF: 621.80 MIN: 621 MAX: 621 AVG: 621.0
SENS: 621 DMAX: 0.00 DMIN: 0.00 A_OFF: 621.80 MIN: 621 MAX: 621 AVG: 621.0

which fullfills all cases for no stain..However when I apply strain in up or down dsirection and hold it for a second the max and min values come close therefore the boths leds getting off like there is no strain..I need to find a specific condition for no strain case..

Additionally, reagarding coming back to void offsett in order to define new offset value every 5 minutes i tried this ;

int updateCounter = 0;

 if (updateCounter++ >20) {
    offset();
    updateCounter = 0;
  }

after e.g 20 loops [around ]1 minute it goes back to void offset() and defines the average_offsett again and yeap it works at least now i am able to see my max min average and average offset value..but the only problem is led condition in no strain case .

system · July 27, 2013, 11:57pm

Hello again,

Regarding my enquiry, I have progressed a bit more now there is only one problem left!..

I am now able to read sensor,take average the sensor readings, finding max and min value of sensor readings and now i also can see the standard deviation of sensor readings..Moreover,I can also save a average_offset value from 1st reading and every 5 minutes this value is renewed in order to find the direction of strain...The only problem left : I can not determine the case for no strain!

when there is no strain ;
Sensor reading ,max,min,average_offset and average values are nearly same--standard deviation is nearly zero..
Up Direction ;
average value decreases so if (average_offset > average ) then its UP
Down Direction ;
average value increases so if (average_offset < average ) then its DOWN

but for both UP and DOWN cases when i hold it for a while; Sensor reading ,max,min, and average values are coming closer so I dont have any option to define a condition for no stain case...

any idea?

thanks in advance..

robtillaart · July 28, 2013, 12:19pm

you should only do the calibration if there is no strain.
If you do it while in the up position that would become the no strain.

 if ((abs(average_offset - average) < smallThresshold)  && ((millis() - lastTime) > 300000UL)) calibrate();

system · October 18, 2013, 9:38am

...hi, i'm new to arduino and programming. i want to find the maximum reading of a sensor. i made it worked but the problem is i want to get the maximum of all the values and not only of certain number of reading. can you help?thanks in advance..

robtillaart · October 18, 2013, 11:22am

...want to get the maximum of all the values...

use code similar as proposed in answer 2/3 - Finding the max and min value of sensor reading - #3 by anatolyk69_gmail.c - Sensors - Arduino Forum -
create a variable that holds the max value so far.
initialize it with the first reading and
update it when needed after each new reading of the sensor.

system · October 18, 2013, 6:08pm

...hi again!!
sorry, i'm really new to this. isn't the "INBUF" in answer 2 represents the number of readings to be compared?
suppose i have this very simple one and i want to serial.print the maximum reading and the current reading at the same time. thank you very much!

const int ledPin = 13;      // led connected to digital pin 13
const int sensorPin = A0; // the piezo is connected to analog pin 0

// these variables will change:
int sensorReading = 0;      // variable to store the value read from the sensor pin

void setup() {
 pinMode(ledPin, OUTPUT); // declare the ledPin as as OUTPUT
 Serial.begin(9600);       // use the serial port
}

void loop() {
  // read the sensor and store it in the variable sensorReading:
  sensorReading = analogRead(sensorPin);
  
  Serial.println(sensorReading);         
  delay(100);  // delay to avoid overloading the serial port buffer
}

robtillaart · October 18, 2013, 6:38pm

try this, I removed all the comments so you only need to read the code.

const int ledPin = 13;     
const int sensorPin = A0;
int sensorReading = 0; 
int maxReading = 0;

void setup() 
{
 pinMode(ledPin, OUTPUT);
 Serial.begin(9600);
}

void loop() 
{
  sensorReading = analogRead(sensorPin);
  if (sensorReading > maxReading) maxReading = sensorReading;
  
  Serial.print(maxReading);         
  Serial.print('\t');         
  Serial.println(sensorReading);
  
  delay(100); 
}

system · October 18, 2013, 6:48pm

wow..thank you very much for the help!!!stupid me, it was just that simple.

thanks again!!!

robtillaart · October 18, 2013, 8:55pm

sometimes you have to walk long and winding roads to come to a simple solution - I know it I've been there (even recently;)

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Finding the max and min value of sensor reading

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