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ELEC2645 Project - Week 20

Former Member

ELEC2645 - Embedded Systems Project - 'Sensor-matic!'

 

The aim of the project is to create an interface between a distance and a temperature sensor and an LCD screen.

 

Noise Cancellation In The SRF02 Sensor

 

Purpose:

 

  • The SRF02 Distance Sensor is subject to interference when taking readings.
  • The readings can fluctuate massively - even when the sensor is held in one position against a stationary solid object (i.e. a wall).
  • Typically, the fluctuates jumped up by 200+ cm.
  • This effect was observed at multiple distances suggesting the interference is random as opposed to systematic.

 

Method:

 

  • The problem can be tackled in a number of different approaches.
  • The easiest method is to simply calculate an average.
  • This method requires that 'X' amount of readings are taken every iteration and summed together.
  • The total sum is divided by the 'X'.
  • This reduces the significance of random errors (although doesn't eliminate them).

 

Code:

 

  • The initial total distance (int totalDistance) is set to equal zero.
  • The number of readings (previously 'X') was selected to be 10.
  • A 'For Loop' counts through 10 readings per iteration and each time.
  • The value of the distance at each stage is added to the totalDistance variable.
  • The final total is divided by 10 to give an output distance.

 

// Cancelling Out Sensor Noise Fluctuations

 

            int totalDistance = 0; // Create a Total Function

 

            for (int iteration = 0; iteration < 10; iteration++) { // Loop Through 10 Iterations

 

                    totalDistance = totalDistance + srf02.getDistanceCm();

                   

                lcd.refresh();

            }

 

Results:

 

  • The output is shown on the Nokia LCD screen.
  • The output is less susceptible to interference - the variations are much closer to the truth value.
  • A very high value would still cause an inaccurate reading - the code can be improved to solve this problem.

 

Improvements:

 

  • Cumulative/Rolling Average - Rather than 10 readings being taken and then the average found. A cumulative average continuously calculates the average based on the current ten numbers in an array/sample space. On the eleventh reading, the first one is neglected and a new average is found. The process repeats by discarding the 'firsts' element in the array and entering in the 'tenth' and recalculating.

 

  • Current Method inc. Rejection - The current method of determining the average can be implemented however to determine more accurate values, extra code could be implemented which rejects a distance which is above/below by 30% of the previous. In order to make sure the average is unaffected, the rejected distance would need to be replaced with another value - perhaps the distance of the previous reading.
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