To understand more about IMUs and IRS, I have chosen the Honeywell 6DF-1N6-C2-HWL Inertial Measurement Unit as an example:

  • I don't see any mention of the output of the IMU in the linked datasheet - have I got it wrong? And how to convert the output into meaningful data? How do I convert raw accelerometer data into Speed in knots, for example? Is there a specific formula to calculate the speed (velocity might be apt) while also taking into account the movements in other directions as well?
  • Is the raw data from this specific IMU accurate enough for use in a GA aircraft / Light Sport Aircraft?
  • Am I right in saying that this IMU doesn't need Pitot / Static sources?

While most details might be about this specific IMU, please feel free to also give answers that apply to all IMUs as a whole.

EDIT: I do not want any positional data. I prefer an AHRS with Airspeed and Altitude sensing capabilities.

  • $\begingroup$ Please confirm that you want to use it as IMU only (for rates and attitude) and not as a position and airspeed source. The question reads so at the moment. $\endgroup$ – Gürkan Çetin Oct 24 '17 at 16:46
  • $\begingroup$ I have edited the question - I don't want position, but I do want Airspeed... Can't the IMU's built in accelerometer be used for calculating airspeed? $\endgroup$ – user18035 Oct 25 '17 at 13:50
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    $\begingroup$ Not possible. For example, Consider a parachute or a kite, you stay stable (no accelerations no inertial change) but the airspeed can be 20-30 knots. You'd certainly need some air measuring device (a pitot probe) to estimate airspeed. $\endgroup$ – Gürkan Çetin Oct 25 '17 at 13:53
  • $\begingroup$ Airspeed and altitude have to come from a pitot/static source. The accelerometer would only be able to give you velocity, which isn't the same thing as @GürkanÇetin stated $\endgroup$ – selectstriker2 Oct 25 '17 at 13:53
  • $\begingroup$ @GürkanÇetin: While that is correct, I think we can integrate the accelerations together - if the current acceleration is 0, then the velocity will remain the same... right? $\endgroup$ – user18035 Oct 25 '17 at 14:33

Regarding the Specified IMU

The referenced document describes the output protocol.

SAEJ1939 CAN 29 bit identifier communication output — the standard for the transportation segment — allows more data to be transmitted than an RS-485 output

You would need to use SAE J1939 to decode the actual meaning of the output data. It is possible that Honeywell could provide you more information as part of purchasing the product.

To use this to develop an AHRS solution you would still need pitot & static inputs to derive pressure altitude and airspeed, as you won't be able to get those from the provided sensors.

acceleration range up to ±6 g, rotation rates up to ±75 deg/s,inclination angles up to ±50 deg and a sensor update rate from 1 Hz to 100 Hz

I think the biggest hurdle you have here is that it will only measure inclination angles up to 50 deg. Aircraft can find themselves in > 50 deg pitch attitudes, which won't be registered by the IMU.

This particular product (Honeywell 6DF-1N6-C2-HWL) seems to be designed for ground vehicle applications rather than aviation.

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    $\begingroup$ Or OP could use a GPS signal to null the integration errors, instead of the pitot & static. $\endgroup$ – Koyovis Oct 24 '17 at 15:18
  • $\begingroup$ The SAEJ1939 CAN 29 bit is the protocol of output right? I saw that part... But do you mean to say that they haven't given the actual data that is produced by the IMU? $\endgroup$ – user18035 Oct 25 '17 at 13:47
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    $\begingroup$ Not having used the SAEJ1939 standard myself, I can't say for sure, but I expect that the standard itself identifies a message ID and format for the various output data. Otherwise you would need to either observe the messages coming out or contact Honeywell for their output data. $\endgroup$ – selectstriker2 Oct 25 '17 at 13:52
  • $\begingroup$ OK... Is it possible to integrate the accelerometer data to produce airspeed - I think the IMU is a little more accurate and less prone to errors - especially if we integrate the output more frequently, say 500 Hz or 1000 Hz? $\endgroup$ – user18035 Oct 25 '17 at 14:40
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    $\begingroup$ No, as airspeed and velocity are not the same thing. Also the IMU only reads from the sensors at a maximum rate of 100 Hz. Integrating at a faster rate wouldn't make the data any more accurate. Honestly it might be worth while to look at other IMUs, as you still have the issue with a maximum of +/- 50 deg pitch $\endgroup$ – selectstriker2 Oct 25 '17 at 14:43

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