Engineering 100-950

Lab 3: Group B Portion

Adding the BME680 4-in-1 sensor, temperature sensor, and accelerometer.

Contents

• Lab 3: Group B Portion
  • Contents
  • Materials
  • Procedure
  • 1. Adding the BME680 Temperature, Pressure, Humidity, VOC Sensor
  • 2. Adding the TMP36
  • 3. Adding the Accelerometer
  • 4. Calibrating the Accelerometer

Materials

• 1 Arduino Nano
• 1 Breadboard
• 1 Programming Cable (and adapters if necessary)
• 1 TMP36 Temperature Sensors
• 1 BME680 Temperature, Pressure, Humidity, VOC Sensor
• 1 Accelerometer
• A handful of jumper wires
• A computer with the Arduino IDE installed and setup.
• ENGR100-950 Arduino Library

Procedure

1. Adding the BME680 Temperature, Pressure, Humidity, VOC Sensor

• Link to BME680 Adafruit Breakout Spec Sheet
• Link to BME680 Sensor Spec Sheet

In Lab 2, you wired up the BME680. If you do not remember how to wire it up, open up the breakout spec sheet and look at the section tittled SPI Wiring.

To make sure that the BME680 is working you can again use the BME680_Example_Code by navigating to File → Examples → ENGR100-950 → BME680_Example_Code. Check all of the outputs and ensure they are plausible before proceeding. If they seem extraordinarily different from expected values, ask an instructor for help.

2. Adding the TMP36

• Link to TMP36 Spec Sheet

Just like we did in the last lab, we now need to plug in the TMP36 to an analog pin on the Arduino, and read it using analogRead() and Serial.println().

Here is the wiring diagram again for your reference:

Go to File → Examples → Basics → AnalogReadSerial and change the analogRead() function called in loop() to be the pin you plugged your TMP36 into. You know on the Arduino Nano this value will be between 0-1023, and that your max voltage, as reported by the Arduino, is 5V. Convert your raw value to the voltage outputted by the TMP36 by dividing it by 1023 and multiplying it by 5V. This is the voltage your Arduino recorded.

Once you have your TMP36 connected, it’s time to make a calibration curve. You can do this in the same manner as in the last lab using the cold chamber or going outside. Enter these calibration curves into your Arduino code by modifying the temperature variables with a slope-intercept equation, and verify that the serial monitor is producing realistic temperature values. Save these calibration curves (slope and intercept values) somewhere for later use! It may be less annoying to calibrate all of the sensors at the end once you have all the sensors connected but before you have the data logger plugged in. This is up to you! In whatever case, make sure to clearly record your slope and intercept values from the curves.

3. Adding the Accelerometer

• Link to ADXL335 Spec Sheet

Begin by skimming over the provided spec sheet and become familiar with the pin layout. Connect the sensor to the Arduino, based on the pin-out provided and using the 3.3V pin as the power supply. Each of the axes (x, y, and z) will be connected to its own analog pin. You will not have anything connected to the ST pin.

Add code to the file used for measuring the TMP36 to store the voltage values from each of the three axes in comma-separated values (CSV) format.

(Hint: You can use Serial.print() to print values without a newline character between them, which may help you print CSV integers to the serial monitor for testing. Then you can just use Serial.print(","); to add a comma between the values. The last line you print, which will be the last sensor column of your CSV matrix, should use Serial.println() in order to make a new line for the next data sample). Convert these raw values to voltages using the equations used in previous labs, then print the voltages in the aforementioned format.

// The "|" represents where you are after printing to the serial monitor
Serial.print("firstVal");
//firstVal|

Serial.print(",");
//firstVal,|

Serial.println("secondVal");
//firstVal,secondVal
//|

Serial.print("I am in a new line because of println");  
//firstVal,secondVal
//I am in a new line because of println|

4. Calibrating The Accelerometer

To calibrate your accelerometer, you will have to go through the same procedure that you did in lab 2, for all 3 axes of the accelerometer. This means that for each axis, you will need at least 2 known values of acceleration that you can subject the sensor to while reading the voltage.

In order to find 2 known values of acceleration, we must understand what exactly is it that the accelerometer measures. The name “accelerometer” is kind of misleading in this case, since it doesn’t actually measure total acceleration. Internally, the sensor has a tiny object (usually silicon) that is connected to the rest of the sensor via a spring-like connection. This connection has electrical properties that change based on the tension that the connection experiences. This is a simplified explanation, but the actual sensors don’t differ too much.

If a force is applied to the outside of the sensor, the tension in the connection increases to keep the small internal object accelerating at the same rate as the sensor. This means that the sensor can only detect acceleration from forces that are applied only to the outside of the package, since if a force was applied equally to the internal object, the tension in the connection would not have to increase to keep the object accelerating with the rest of the sensor. Usually this isn’t a problem, since most forces we’re concerned with are fall into this category, however, there is one major force that doesn’t: gravity. Recall from physics that gravity acts in a way that makes all objects accelerate at the same rate (\(9.8\,\frac{m}{s^2}\)). This is the exact type of force that accelerometers cannot measure, since the internal connection does not have to apply any force to the internal object to keep it accelerating with the sensor - in the absence of external forces, everything accelerates at \(9.8\,\frac{m}{s^2}\).

Now let’s try to figure out what the accelerometer would measure in 2 different situations: in free-fall, and sitting stationary on a lab table. Start by drawing a free-body diagram for each situation, and then remove the gravitational force. The acceleration from the remaining forces is what the sensor would measure. In the situation of free-fall, since gravity is the only force (in the absence of air resistance), the accelerometer measures nothing. In the situation of the sensor sitting stationary, the normal force opposes gravity and is exactly equal to the force from gravity. The accelerometer will only measure the acceleration from the normal force, and since the normal force is equal to gravity in this case, it will measure \(9.8\,\frac{m}{s^2}\), or 1 G, upwards. You will use this fact to calibrate the accelerometer.

In this orientation, the Z-axis is straight up, and from our previous findings it should measure 1G. X and Y are both perpendicular to the force of gravity and would be recording 0Gs. If you turn your board upside down, the Z axis should read -1G. Rotate your sensor around as needed so that each axis has at least two data-points where it is (anti-)parallel to the force of gravity and make sure to take note of the voltage value for both points.

With this calibration process completed, each axis will have a calibration value of 1G and -1G. Record the voltage values in your spreadsheet, and calculate the calibration curve for each axis.

Now update the code to print the new calibrated values in the same CSV format as before. Save these calibration curves (slope and intercept values)!

• Groups A & B Lab Manual - Finishing Steps