The Difference Between IMU, AHRS, and INS
Shopping around for an inertial sensor, people think of INS, aka inertial navigation system. Maybe you need an IMU, which is super simple. Understanding the difference between IMU, AHRS, and INS is going to help pick the right product for your specific application. Be sure to watch the video below as Morgan explains the difference between IMU and AHRS, as well as a complete breakdown of INS.
What Does IMU Stand for?
The term IMU stands for “Inertial Measurement Unit”. An IMU only consists of a magnetometer, accelerometer, and gyro. Sometimes magnetometer is not included in the package. It will not have a smart-system.
What Does AHRS Stand for?
AHRS stands for attitude heading reference system and includes a GPS but no Kalman filter. It has all the benefits of the IMU, plus GPS position. This is a good fit if there is already a filter in mind to be designed or already incorporated.
What Does INS Mean?
INS is an inertial navigation system. It takes all the sensors, fusing them into one system. It knows exactly where it is in the world based on just that output. An inertial navigation system is not a GPS. It has a Kalman filter in it, which is how the sensor itself fuses all the individual parts into one and gives navigation output with everything incorporated. It has a sensor fusion built into the device, giving a more accurate output, and includes a gyro, magnetometer, accelerometer, and GPS. This allows your robot to understand where it is in the world.
What Is a Kalman Filter & RTK?
The Kalman filter is a software package. The Kalman filter gives a fused output that the rest of your robot can run on. RTK, real-time kinematics, is a process where GPS position is taken from a base station, and output corrections are sent to your rover. The value of using an RTK base is if you work off any type of triangulation base, you can get a position. With the two GPS system combination, you can get centimeter-level accuracy. This is only available in the inertial navigation system. Another feature only accessible with INS is compassing. Based on the distance between the two GPS positions, you can derive heading, giving accurate heading dimensions to your regular navigation system.
The be-all solution that everyone really wants is, can I get great compassing heading and have position accuracy too with RTK? The answer is yes!
The Difference Between IMU, AHRS, and INS
When customers visit our website or they’re shopping around for a sensor, an inertial sensor for their product, they’re going to often think of an INS and that’s all. They just heard INS. that’s not really the case. Maybe they actually need an IMU, which is actually super simple. And they’re looking at a completely different price point. Understanding that there is a difference between an IMU, an AHRS, and INS is going to help pick the right product for your specific application.
The biggest misconception that people have about an INS is that it’s just a GPS. It’s not a GPS. It has a Kalman filter in it, it has a sensor fusion built into the device. It gives you a much more accurate output. It’s a very high-quality device generally. So, an INS is going to include a Kalman filter, which is going to consist of how the sensor itself fuses all of the individual parts into one and gives you navigation output with everything incorporated. This also includes gyro, magnetometer, and accelerometer. And in the INS solution, it also includes GPS. So it’ll take those things and give you a navigation output that your robot can then use to understand where it is in the world.
What Is An IMU?
When you’re looking to implement an IMU, it is important to recognize that it is just going to be magnetometer, accelerometer, and gyro. Sometimes magnetometer won’t be included in that package. And it will just be the random number that those little devices will end up spitting out. And it won’t have any smarts in it. It’s just going to pump out numbers. This would actually be very beneficial to use is in a camera, a stationary camera, that all you care about is just knowing whether it’s pointing at the ground or pointing at the ceiling. It’s very basic, not a whole lot to it. But again, there’s no position or anything else to it.
A lot of people, when they’re looking for a solution for their given application they’re gonna start at an INS level, which includes the magnetometer, gyro, and accelerometer. But it also includes a GPS, and it will fuse all of those sensors together to give you a robust navigation output that your system can actually use to drive around the world, and knows exactly where it’s at.
So an IMU actually stands for inertial measurement unit. And that’s all it is. It’s just a simple, basic unit.
How Is AHRS Beneficial?
The next step up from there is AHRS, which stands for attitude heading reference system, and that is going to actually include GPS. While it doesn’t actually include a Kalman filter, it does give you all the benefits of the IMU, plus the GPS position.
An AHRS unit would be a good fit for the people that already have a filter in mind that they’ve designed, or already incorporated, and they just want more sensor data pumped into it.
Is INS For Me?
And then the next step up from the AHRS is the INS, that everybody is familiar with the term INS. And that one, it stands for inertial navigation system. And as the name implies, it is a smart system. It will take all of those sensors, it will fuse them into an output that your system doesn’t have to do anything more with. It knows exactly where it is in the world based on just that output. You don’t need to send it to anything else.
The INS has what’s called the Kalman filter, and that is what fuses in the various sensors used in the device. The Kalman filter is actually a software package. most of the time that the sensor outputs are fed into. And then the Kalman filter gives you a fused output that the rest of your robot can actually run on. Some of the extra features that you can have that are exclusive to an INS is being able to use RTK. RTK, real-time kinematics is a process where you take a GPS position from a base station, and you output corrections over to your rover, which is generally your robot that is moving around. And in doing this combination of two GPS systems, you’re able to get centimeter-level accuracy. And this is only, you can only do this with an INS system.
Another feature that can only be used with INS is actually compassing. This is a newer technology, but it’s still only usable in an INS solution. And what this is is where you take GPS one and you add a GPS two. You have two GPS’s for a single device. But based off the distance between those two GPS positions, you can then derive heading between the two. And that gives you a very accurate heading dimension to your regular navigation system.
The Importance of Compassing
The reason why compassing is such a great improvement for an INS solution is because you’re able to get down to a much smaller accuracy than say, a magnetometer. And a magnetometer is based off of an earth magnetic field, and whether you’re pointing North, it’s a compass. But when you’re using GPS, it doesn’t care where North is really. Because it will always be able to derive your heading based off of those two GPS’s. And it’s not going to be affected by a piece of metal going over it, it won’t be affected by your cell phone. It’ll just always work as long as you have a GPS position.
In The End…
So one of the end-all, be-all solutions that everybody really wants is, well I can get great compassing heading, well can I have position accuracy too with RTK? And the answer is yes. You can actually put them together. You just end up kind of doing a merge of the two. You’ll have your base station, and then you’ll have a rover that has two GPS. and it works the exact same way as RTK. You get the base position information sent over to the rover, and then it does its position fix, and then it also can take the heading information off of the two GPS positions, and really give you a precise heading and position solution for your application. The compassing actually works off of the similar system to RTK. It actually runs the exact same library as the RTK solution. But it’s using that information as a local base instead of a remote base. Where an RTK system, you have a base station off somewhere else, now you have a moving base that makes and consist of compassing. And it’s those two base positions working together in an RTK solution that actually gives you compassing heading.
The value of using an RTK base is that if you work any type of triangulation based off of that base, you’re able to get that position. Triangulation works on three points. In this case, you have an RTK base, you have a satellite, and then you also have your rover. And based off of the dimensions of that triangle that you then draw, is how we’re able to actually get an RTK position down to the centimeter-level of accuracy.
An INS with compassing still suffers from the basic GPS position. GPS in general is hard to get better than one meter of accuracy. And compassing is going to still suffer from that. While we are able to derive an accurate heading of two GPS positions on our rover, we cannot derive a more accurate physical position information out of those two. You can average them, but that won’t give you any more accuracy.