It is time to bring together a lot of things we have learned in our earlier lessons to create a Height-O-Meter, which will plot how high our BMP180 pressure sensor is above the floor. For this lesson we make simplifying assumption of constant temperature. When we use the sensor for our space probe or other high altitude experiments we will need to derive the equation again to take into account changing temperature. We went through the math of calculating height from changing pressure in LESSON 12.
In this lesson, we start with the software we developed in LESSON 11 for measuring, streaming, and plotting pressure and temperature data from the BMP180 sensor.
Remember, we connect the sensor to the Arduino as follows:
Connecting Up the BMP180 Pressure and Temperature Sensor | |
| BMP180 Pin | Arduino Pin |
| Vin | 5V |
| GND | GND |
| SCL | A5 |
| SDA | A4 |
The software we are using on the arduino side is shown below, from LESSON 11.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 | #include "Wire.h" // imports the wire library for talking over I2C #include "Adafruit_BMP085.h" // import the Pressure Sensor Library Adafruit_BMP085 mySensor; // create sensor object called mySensor float tempC; // Variable for holding temp in C float tempF; // Variable for holding temp in F float pressure; //Variable for holding pressure reading void setup(){ Serial.begin(115200); //turn on serial monitor mySensor.begin(); //initialize mySensor } void loop() { tempC = mySensor.readTemperature(); // Be sure to declare your variables tempF = tempC*1.8 + 32.; // Convert degrees C to F pressure=mySensor.readPressure(); //Read Pressure Serial.print(tempF); Serial.print(" , "); Serial.println(pressure); delay(250); //Pause between readings. } |
We modify the Python code from LESSON 11 as explained in the video above to get this code for the Python side.
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 76 77 78 79 | import serial # import Serial Library import numpy as np # Import numpy import matplotlib.pyplot as plt #import matplotlib library from drawnow import * import time tempF= [] #Array for our Temperature dat pressure=[] #Array for our Pressure Data ht=[] #Array for our calculated Heights arduinoData = serial.Serial('com11', 115200) #Creating our serial object named arduinoData plt.ion() #Tell matplotlib you want interactive mode to plot live data cnt=0 def makeFig(): #Create a function that makes our desired plot plt.ylim(0,10) #Set y min and max values plt.title('My SuperCool Height-O-Meter') #Plot the title plt.grid(True) #Turn the grid on plt.ylabel('Height in Feet') #Set ylabels plt.plot(ht, 'ro-', label='Height') #plot the height array plt.legend(loc='upper left') #plot the legend plt2=plt.twinx() #Create a second y axis plt.ylim(75,85) #Set limits of second y axis- adjust to readings you are getting plt2.plot(tempF, 'b^-', label='tempF') #plot temperature array plt2.set_ylabel('temp F') #label second y axis plt2.ticklabel_format(useOffset=False) #Force matplotlib to NOT autoscale y axis plt2.legend(loc='upper right') #plot the legend tempBucket = 0 #Create bucket to hold sum of temperature readings PBucket=0 #Create bucket to hold sum of pressure readings print "PLease put Sensor Circuit on Ground for Calibration" #Calibrate sensor for readings on floor, P0 print "5" #Give user time to put sensor on floor time.sleep(1) print "4" time.sleep(1) print "3" time.sleep(1) print "2" time.sleep(1) print "1" time.sleep(1) print "Calibrating Sensor . . ." for i in np.arange(1,11,1): #Loop ten times to take ten measurements while (arduinoData.inWaiting()==0): #Wait here until there is data pass #do nothing arduinoString = arduinoData.readline() #read the line of text from the serial port dataArray = arduinoString.split(',') #Split it into an array called dataArray temp = float( dataArray[0]) #Convert first element to floating number and put in temp P = float( dataArray[1]) #Convert second element to floating number and put in P print "P = ",P, " , Temp= ", temp tempBucket=tempBucket + temp PBucket = PBucket + P P0 = PBucket/10 #Calcualte average pressure on floor tempK = ((tempBucket/10)-32)/1.8 +273.15 #Calculate average temperature on floor in K print "Baseline Temp in K is: ", tempK #Print resulrs print "Baseline Pressure inPa. : ", P0 while True: # While loop that loops forever while (arduinoData.inWaiting()==0): #Wait here until there is data pass #do nothing arduinoString = arduinoData.readline() #read the line of text from the serial port dataArray = arduinoString.split(',') #Split it into an array called dataArray temp = float( dataArray[0]) #Convert first element to floating number and put in temp P = float( dataArray[1]) #Convert second element to floating number and put in P h=98.57*tempK*np.log(P0/P) ht.append(h) tempF.append(temp) #Build our tempF array by appending temp readings pressure.append(P) #Building our pressure array by appending P readings drawnow(makeFig) #Call drawnow to update our live graph plt.pause(.000001) #Pause Briefly. Important to keep drawnow from crashing cnt=cnt+1 if(cnt>50): #If you have 50 or more points, delete the first one from the array tempF.pop(0) #This allows us to just see the last 50 data points pressure.pop(0) ht.pop(0) |
Please go through video for complete description of this software. Remember this is only valid for small changes in height over which temperature is constant.