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Analyzing Sensor Data on PSLab

PSLab Android App and Desktop app have the functionality of reading data from the sensors. The raw sensor data received is in the form of a long string and needs to parsed to understand what the data actually conveys.

The sensor data is unique in terms of volume of data sent, the units of measurement of the data etc., however none of this is reflected in the raw data. The blog describes how the sensor data received by the Android/Desktop app is parsed, interpreted and finally presented to the user for viewing.

The image below displays the raw data sent by the sensors

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Fig: Raw Sensor data displayed below the Get Raw button

  • In order to understand the data sent from the sensor, we need to understand what the sensor does.
    • For example, HMC5883L is a 3-axis magnetometer and it returns the value of the magnetic field in the x, y & z axes in the order of nanoTeslas.
    • Similarly, the DAC of PSLab – MCP4728 can also be used like other sensors, it returns the values of channels in millivolts.
    • The sensor MPU6050 being 3-axes accelerometer & gyroscope which returns the values of acceleration & angular momentum of the x, y & z axes in their SI units respectively.
  • Each sensor has a sensitivity value. The sensitivity of the sensor can be modified to adjust the accuracy of the data received. For PSLab, the data returned is a float number with each data point having 4 bytes of memory with the highest sensitivity. Although sensitivity is not a reliable indicator of the accuracy of the data. Each value received has a lot of trailing values after the decimal and it is evident that no sensor can possibly achieve accuracy that high, so the data after 2-3 decimal places is garbage and not taken into consideration.
  • Some sensors are configurable up to a great extent like MPU6050 where limits can also be set on the range of data, volume of data sent etc. whereas some are not configurable and are just meant for sending the data at regular intervals.
  • In order to parse the above data, if the sensor returns a single value, then the data is ready to be used. However, in most cases like above where the sensors return multiple values, the data stream can be divided into equal parts since each value occupies equal space and each value can be stored in different variables.
  • The stored data has to be presented to the user in a better understandable format where it is clear that what each value represents. For example, in case of the 3 axes sensors, the data of each axis must be distinctly represented to the user.

Shown below are the mock-ups of the sensor UIs in which each value has been distinctly represented.

         

Fig: Mock-ups for the sensor UIs (a) – HMC5883L (b) – MPU6050

Each UI has a card to display those values. These values are updated in real time and there are additional options to plot the data received in real time and in some cases also configure the sensor. In addition to that there are features for data logging where the data is recorded for a given time interval specified by the user and on completion of recording, calculations like the mean, standard deviation etc. are presented to the user.

Additional Resources

  1. Analyzing sensor data using Arduino, similar to method for PSLab – http://tronixstuff.com/2014/01/21/online-data-analysis-arduino-plotly/
  2. YouTube video to understand analysis of data from MPU6050 in Arduino – https://www.youtube.com/watch?v=taZHl4Mr-Pk