Combined Functionality: What do you lose?

There is a trend towards combining hardware components and even processors into single chips. This is certainly true for inertial sensors:

  • 3-axis: accelerometer only
  • 6-axis: accelerometer, gyro
  • 9-axis: accelerometer, gyro, magnetometer

where each sensor measures three-orthogonal axes, hence the counting by threes. Even the combined measurement of three axes for a single sensor is noteworthy. Surely there is directionality to the physical sensing mechanism and so can all three axes crammed into a flat chip have equal sensing performance?

That is a topic for another discussion. Here we focus on packing multiple sensors into the same chip. What happens when multiple sensing technologies are crammed into the same swiss army sensor?

Sensing performance suffers.

The plot below is from a compilation of commercial sensors from 3 manufacturers, all with an unpackaged chip form factor. Accelerometer bias is plotted against the single-quantity price.

Plot of accelerometer bias vs price data for variety of 3, 6, and 9-aix inertial sensors

Notice how the accelerometer sensor bias increases as more sensors are combined into the same chip. Given more data, it looks as if there will be three vertically-shifted performance curves.

A few notes are in order:

  1. The data is from manufacturer data sheets and not independent testing
  2. This data was compiled for another project and this conclusion was simply a by-product of visualizing that data
  3. Sensor bias is not a complete picture of sensing performance, but is a fair representative
  4. Even ignoring price, the average accelerometer bias for the three classes supports this finding

Sensor Selection

Comparing and picking sensor performance across a variety of manufacturers can be a real headache. The choice is often driven by factors other than sensing performance. Things like:

  • price
  • power consumption
  • combined functionality
  • communication protocol
  • comfort level with manufacturer products
  • past projects

In fact, by the time sensing performance has been tested and evaluated for the application, the project momentum can be too large to go back and try other options. Therefore, it helps to have a feel for the trade-offs involved in picking sensors.

This comparison informs one element of that trade-off. It can still make sense to combine functionality, just be aware of the price paid.