Why a motionless accelerometer reads 9.8 m/s² (g)
In a session of a course I am taking on making small satellites, or CubeSat’s, we were briefly told about the IMU (Inertial measurement unit) our CubeSat’s might use, which is an Adafruit breakout board. These IMU’s feature a very important component for any spacecraft, which is an accelerometer. Everything from the phone in your pocket to your Camera Drone has one of these suckers, and although they appear simple, there feature some interesting quirks.
Here’s how an online source (linked below) defines an accelerometer:
“An accelerometer is a device that measures acceleration, which is the change in an object’s velocity per second. Since velocity is measured in meters per second (m/s), acceleration is measured in meters per second per second, which is written as meters per second squared (m/s2). Technically, an accelerometer measures proper acceleration, which is not the same as coordinate acceleration. This means that the accelerometer can be used to detect the direction of gravity. This resource will explain what this means in more detail”
Using a raspberry pi 4, I went ahead and wired up the IMU, and downloaded the necessary libraries onto my pi to receive data from the sensor. I noticed something interesting. Even though the object was still on my table, it was reading values close to gravity’s acceleration in the Z-axis. I then wrote the script below to confirm my suspicions.
In the 3 second wait time, I reoriented the accelerometer and set it down onto the table. Here are the outputs.
What’s evident from this data is two things:
1) This accelerometer has a lack of precision
2) The motionless accelerometer reads an acceleration that is fairly close to 9.81 m/s², the acceleration of gravity
The first is not a large issue for me, but understanding the latter is very important. After some googling, I came across the reasoning, which was explained well with an analogy using a ruler.
Basically, the article said that if one was to hold a ruler by the end, it would bend downward because of its own weight. If one were to hold a ruler straight up, it would bend over if the ruler was moved side to side, due to a change in velocity, or acceleration. What this shows is that gravity, or change in velocity, which are both accelerations, can cause this bending or deformation. However, it’s important to notice that a ruler that has been dropped and is in free-fall has no bending.
The article furthers that what an accelerometer really does is measure these deformations or “bending”, meaning that a motionless accelerometer on earth would be affected by gravity.
The simple understanding is this: Accelerometers measure relative to an object in free-fall. So although the still accelerometer is not accelerating relative to a human sitting in a chair, it is accelerating upwards compared to an object in free fall. That’s why the acceleration read-outs are positive, not negative.