AHRS types - the differences between MGL SP-7 AHRS and SP-9 AHRS

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Hello everyone,

My main instrument panel consist of (2x) MGL XTREME EFIS units + SP-7 Compass + SP-9 AHRS. I know for most of you having AHRS is not really relevant but for me personally is what makes my airplane that much more “real” and as a safety add on just in case of some severe unlikely emergency scenario where I would have to cut through the intense rain or cloud (especially at night time settings if I was to fly at) just to make it out alive - this could be literally a life saving sensor suite. I have flow with a “hood” on with CFI in next seat in rather turbulent weather and using my internal human “sensors and vertigo” I could “swear” I was flying level when AHRS indicated 35 degree bank… well compass was alive to indicate the turn but that’s not much to go by to level the airplane in turbulence and “holly sh.t” conditions… I have read too many spooky stories…

Here is my story:

Initially I bought SP-6 compass (mounted inside of fuselage behind seats on top of one of the turtle deck cross members) and SP-7 AHRS. Compass was fine however SP-7 mounted without any cushioning under passengers seat close to the middle did not work so well… during take off acceleration bank drift was somewhere in 30 degree range… just because of forward acceleration (and most likely due to complete lack of vibration dampening - I must emphasize I did not get a chance to try it in “vibration dampened” setup) was drifting so bad it was very disappointing… so I bit the bullet so to say and bought SP-9 AHRS with Ring Gyros (or Ring Laser Gyros?) and ORION class accelerometers… also it is important to mention that this time I mounted this sensor on 1/8" thickness SS “heavy plate” slightly larger than sensor itself to absorb any residual vibrations and also sandwiched it between 2 “foam” pieces - top and bottom. The precision of this Ring Gyro sensor was very impressive right out of the box! Drifting of any kind at any phase of the flight was gone! Sensor is very precise to any change in any attitude. I suppose I have to give at least some credit to foam and “thick and heavy” SS plate vibration insulation combo. I am super happy with this sensor. Expensive but well worth it to my opinion.

According to web - MGL SP-9 is close to FOG AHRS sensor ability.

To me personally SP-7 with MEMS stuff seems like a “kids toy” compared to the real deal SP-9 with Ring Gyros and ORION accelerometers…

To my HUGE surprise even Dynon does not sell Ring Gyro quality/capability AHRS and offer MEMS stuff only… too expensive to manufacture or whatever…

I obviously do not advertise MGL products in any way but on the other hand I feel grateful that they offer rather pricey but very capable option for those who really need or want it.

Hope this helps to someone.

Minde

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MGL is coming out with a better AHRS soon, according to @JEFlatt53 at Legit Avionics. We plan to incorporate it, but will go with the internal AHRS for now.

In a former life I did experimental helicopter FBW and I used Honeywell stuff. Accelerometers and ring laser gyros. Awesome stuff. At powerup the system would figure out which way was up based on the accels, then sense the earth’s rotation to figure out where true north was.

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The SP-9 uses a Silicon Sensing Vibrating Ring Gyro. The cost and performance of these devices is way beyond the simple budget devices in the SP-7. Another important distinction is the sample rate. The SP-7 is around 40 Hz and SP-9 is 100 Hz. This make a big difference in vibration performance as well. SP-7 was also calibrated in-house at MGL- a very tricky task…

The new generation AHRS is getting factory calibrated gyros and accelerometers. It will run at 500 Hz and now uses double precision math. Should be much more robust than the old SP-7. I’m quite excited to see how it works.

The SP-7 will work okay in the right setup. It is just particularly sensitive. The vibration damper I designed has helped a few Sonex folks get good results. 12 - 30 Khz is where it really counts. That’s the operating frequency of the MEMS system.

I’ve learned a few different ways how vibration is no good on MEMS IMUs. Even if it’s a $20,000 inertial navigation system, it must be isolated from vibration. Fly an RC “drone” miniquad with hard vs soft flight controller mounting and you can feel a big difference. Remote sensing airborne data cannot be post-processed and reconstructed properly without proper vibration isolation for the IMU. This is especially true when you have engines, propellers, motors, etc.

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Hi Evan,

The MX-1 & 2 have an internal AHRS, should they be mounted with vibration dampers?

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There’s not a whole lot you can do for vibration when using the internal AHRS. I have heard that an appropriate density foam gasket around the mounting hole helps, but it’s a hard thing to mitigate.

Once the new AHRS is in production, all the Lite systems will use it as well.

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