Why does the Pixhawk have 2 IMUs

Question

I was looking at the Pixhawk specs and noticed that it has 2 different IMUs- Invensense and STM. Is it for redundancy or does it have any other higher purpose?

Answer 1

Generally engineers implement dual, triple or more sensors with the same function for one or more of the following reasons:

1. Reliability: the system should be reliable. Several values can be fetched from several sensor. A voter decides the output(final) value. Boing 777 has 6 sensors for each function.

2. Safety and critical systems: if one sensor fails, there should be another one which can replace it.

3. Power consumption: some sensor consume more power because of their higher precision. Using two sensor with different precisions can make a big difference on power consumption. The sensor with high precision is turned on whenever high precision sampling is needed.

4. Frequency analysis: if you sample from a sensor at 1200Hz and another external frequency with same value is applied then you will get a dc-offset because of the two frequencies interfering with each other.

In case of Pixhawk, I think the main reason is frequency analysis. There are strong vibrations during flight.

Answer 2

This was on rcgroups:

Reply by Rob_Lefebvre on December 31, 2014 at 7:02am

Here is a brief history:

1. The APM class boards used the MPU6000 gyro/accel chip.

2. The Pixhawk was originally designed to use the LSM303D chip, as it was supposed to be better.

3. Initial prototype testing of Pixhawk boards revealed problems with the LSM chip. To avoid further delays of Pixhawk production, it was decided to add the MPU chip back onto the board, so now there would be 2.

4. At some point shortly after the MPU was added, the LSM problem was solved, but it was decided to leave both chips on the board anyway, as it allows us to take advantage of redundant sensors.

5. Up until 3.2, the MPU chip was the only one being used. The LSM was basically ignored. If it failed, the code did not care.

6. Starting with 3.2, we wanted to start taking advantage of the sensor redundancy, so we began using the data from the LSM. Generally this is fine, except for cases where the LSM has a problem. Or the MPU has a problem. We did not forsee this failure rate until the code was released. The problem is, either chip can fail, and the code doesn't know which is good and which is bad. It just knows they're different. In this early stage, using two sensors does not really offer redundancy and increased reliability. It actually doubles our chance of having a problem.

Answer 3

The particulars you are asking about are;

ST Micro L3GD20 3-axis 16-bit gyroscope ST Micro LSM303D 3-axis 14-bit accelerometer / magnetometer

Invensense MPU 6000 3-axis accelerometer/gyroscope

The MPU 600 provides a stand alone I2C bus for attaching a remote magnetometer, to get it away from current and metal. But it also requires external clocking for accuracy.

Why someone would choose to use 2 different brands, I am not sure. It may be that one is used for INS and the other to transmit telemetry data at a different rate. The MPU 6000/6050 is known for its ease of use as it can do math processing on board, i.e. Output ready to use quaternions.

Answer 4

Why someone would choose to use 2 different brands, I am not sure.

It is VERY common and indeed common sense to use different brands of a critical component for security.

This is because the chances are the the same brand component is likely to have the same point of failure. Therefor two different brands are used as they are unlikely to have the same point of failure. In aerospace systems this is the MINIMUM requirement i.e. several IMUs all of different brands.