Does ISM3D have magnetic immunity to steel structures?

AHRS

Does ISM3D have magnetic immunity to steel structures?

Ben Grant, Managing Director of Impact Subsea answers questions from users of our ISM3D - Does it have magnetic immunity to steel structures?

Does ISM3D have magnetic immunity to steel structures? – Transcript

We get a number of questions on the ISM3D one of the most common ones is ‘How good is it when it comes to Magnetic Immunity?’ If you’re on a vehicle and you’re working around steel structures, how well does it keep the heading?

I guess to answer that I have to tell you a little bit about how the sensor works.

The ISM3D has two modes of operation; the first mode is basically magnetic heading so it follows it like most mems based magnetic sensors and inside the sensor it’s got three magnetometers, three gyroscopes and three accelerometers.

The accelerometers and gyroscopes are relatively high grade so it gets the high grade attitude readings. When you’re working on magnetic heading the magnetometers find North on initial power up and then they’ll follow magnetic North and they’ll be slaved to the gyros and accelerometers.

The gyros will help prevent any temporary magnetic interference without effecting the heading. If you have a temporary magnetic interference come alongside it the magnetometers will quickly spin off and point towards it or be moved by it but the gyros will realise there’s not been any physical movement, it’ll hold that heading. If the magnetic presence sits there for a long time heading will slowly creep round and offset to it.

That’s the basic mode of operation however it’s got a more advanced mode of operation which is probably the number one reason that people purchase the ROV / AUV AHRS, ISM3D is its inertial mode.

Magnetic Immunity – Inertial Mode

In inertial mode when you power up the sensor and on initial power up for the first 10 seconds or so it uses the magnetometers to locate North then after 10 seconds the Heading update gets handed over to the gyroscopes and accelerometers and so after 10 seconds of operation it will have a heading and you can bring it alongside a magnetic structure, even a strong magnet right next to it and the heading won’t move at all, (Magnetic Immunity) even if the magnet’s left there for hours and hours.

The Heading won’t adjust at all because it practically ignores the magnetometers. It just changes the heading based on the gyroscopes. This gives you a really reliable heading which is ideal for operation around steel structures or any other structure that has a magnetic element to it.

In terms of drifts, usually you don’t want to do that kind of setup with a low-grade mems sensor because what you get is drift over time and there’s nothing to correct the error. With the ISM3D you can sit it on a desk and don’t move it for 24 hours before you get about one to two degree drift just from a static installation.

Where you get error building up in the heading is from dynamic motion when working in inertial mode and that’s due to integration error of which gyros.

If you’re on a large work-class vehicle which many of the ROV / AUV AHRS ISM3Ds are, you can maintain operations for many hours because the dynamic motion usually is relatively slow or if you put this onto a small vehicle, such as a small Videoray or a vehicle that kind of size, you’ll find if you move very quickly and rapidly error will build up on the heading.

At any point you can move away from the steel structure, click re-align to magnetic North or you can power cycle it and it will re-align magnetic North and give you a good base to start from again.

Does ISM3D have magnetic immunity to steel structures? – Just one of the questions we hope we’ve answered. If you want any more information on the ISM3D you’ll find the latest data sheet, manuals, demonstration videos and more technical information on our ISM3D product Page

or you can drop us an email at support@impactsubsea.co.uk we’ll answer any technical questions that you have.