Co-located Antenna and IMU

We sell a significant number of IMUs. The IMU04 came out last year and it’s popular because integration with GPS gives stability to the velocity output when conditions are sub-optimal – like under trees. And we’ve worked really hard on this product so that the pitch and roll accuracies are now down to 0.06 degrees (RMS).

All well and good but there is an element in using one that can be a bit of a pain. You have to measure, very accurately, the distance between the GPS antenna and the IMU. Sounds simple enough but it can be really tricky, especially if there’s a significant curve to the car roof.

Why does this need to be measured? Well there are a couple of reasons. Firstly, the performance of the Kalman Filter is optimised when the correct distance is known. And secondly, because of Lever Arm Correction: a vehicle’s roof will initially travel faster than its centre of gravity when the brakes are applied. And where’s the antenna? On the roof… so integrating the IMU with the GPS data allows for this overshoot to be adjusted; but you’ve got to know how far away they are from each other and this measurement introduces the potential for human error.

Lever arm correction in braking

So here’s the solution: co-locate the IMU and antenna. This rather handy little unit allows the IMU to fit snugly inside it, with the antenna mounted on top. With the data sources coming from exactly the same place, the overshoot is eliminated.

“But” you say “with the IMU on the roof, it will pitch forward further than it would do if was mounted inside the car at the COG.” Yes, it will, but this can be translated back to where you’d normally fit it, and that’s much easier than measuring between the antenna and IMU. It also means that you can swap the setup between vehicles far more quickly.

Co-located GPS antenna and IMU.

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