Sensor Fusion or Sensor Blending

Sensors are what provides feedback to a closed loop system.  Sometime you can’t get the sensor characteristics you need.  This happens a lot in the aerospace industry.

When any one sensor cannot provide the necessary feedack then it is time for sensor fusion or sensor blending.  The simplest form of sensor fusion is a matter of two or more sensors which are filtered so that their strengths (good responsivity and low noise) are used while their weaknesses are filtered out.

Often times sensor fusion is nothing more than simple second order low pass or high pass filters with their outputs added together.  This simple fusion allows for two sensors to provide the desired output.

Simple Example of Sensor Fusion

The most simple sensor fusion that I’ve come across is the combination of two angular rate gyroscopes.  The low frequency gyro was good out to a frequency of approximately 20 Hz.  The high frequency gyro was good between 1 and 1000 Hz.  Unfortunately this system was sensitive to frequencies around 5 Hz.

Normally the blending frequency of the sensor fusion would have happened between 1 Hz and 20 Hz based on an analysis of each sensor’s noise and responsivity.  This example system was sensitive to frequencies around 5 Hz which meant that we needed to avoid frequencies between 0.5 Hz and 50 Hz.

The main weakness of the high frequency sensor was phase loss below 1 Hz.  So we designed a filter to extend the low end of the high frequency sensor down to 0.5 Hz.  More difficult to implement than to conceptualize but it takes some practice to do it correctly.

Ideal Sensor vs. Real Sensor

The ideal sensor is typically modeled with a second order system that has a natural frequency equal to the spec bandwidth and a damping of 0.707 or 1.  I default to 0.707.  This leads to a nice flat, unity response for the sensor below the bandwidth.  Real sensors are non-unity below the bandwidth – i.e. the magnitude has some ripple to it.  Sensor ripple around the blending frequency can be very problematic and must be assessed based on the system needs.

Introduction to Sensor Fusion on the Wiki

Here is an article on the wiki on Sensor Fusion.  It is currently a small, simple article that I hope to expand and encourage anyone interested in Sensor Fusion to help me expand.