How to record data from foreign sensors? - RC Groups
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Jun 08, 2009, 01:14 PM
Registered User

How to record data from foreign sensors?

Hi all,

i'm looking for some information how to get external sensors to the ETC Flight Data Recorder.

Especially i need to know, in which data format the signal is to be recorded. I2C, RS 232?

For my diploma thesis i have to record data from a compass module. To decide which one to buy i need this information, because there are offered several types: the ones mentioned above, analog voltage, analog voltage length, UART, SPI, ...

As a mechanical engineer i don't know that much about that and ask you for help! =)

I need to record with a resolution range of 3600 steps, so the version using the a/d expander is not reaching the goal.

Can you help?

Greetings from Bavaria... =)
Last edited by Soulkitchen; Jun 09, 2009 at 01:21 AM.
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Jun 09, 2009, 08:20 AM
Registered User
Eagletree only offers one foreign sensor interface at this time, and that is their A2D interface. It has a 15 bit resolution, or 32768 steps it will divide an analog signal into.
I'm not seeing your issue.
Jun 09, 2009, 12:02 PM
Registered User
hi dshares,

okay, i'm trying to specify my problem:
i need a compass module, which values are to be recorded by the elogger. the point is, that i cannot use the a/d expander for recording pulses, because it is to sensitive for interferences. Remember i need a solution of 3600 steps. with the range of the expander of 4 volts this is nearly 1/100 Volt per step! so this configuration is definitely too sensitive for external interferences like servomovements an so on.

digital values are not as vulnerable as analog values...

the solution is: buying a compass module, which offers the data digitalized, that means ready to record. The data have not to be converted, they have already the right data format.

But the market is great, every module uses different protocols, like mentioned SPI, I2C, RS232, and so on.

To know, which module i have to buy, i need to know first, which protocol the elogger is using! so that these two units fit to each other!

i hope my problem is now a little bit more clear... if you need some more information, just tell me!
thanks a lot for your help!! =)
Jun 09, 2009, 12:27 PM
billpa's Avatar
Hi Soulkitchen,

We have plans to offer a compass module, but I'm not sure when that will be ready.

I2C is the interface we would prefer. If you can locate an i2c compass module, we may be able to do a quick firmware change to try to support it. I'm not sure if we will be successful, though.


Bill, for Eagle Tree
Jun 09, 2009, 04:19 PM
Registered User
Wow thank you!

equusine, one point to the last module in your list..: protocol is named "TTL". Is that I2C..?
Jun 09, 2009, 04:47 PM
Registered User

here's a list of the modules i could use. Which would you prefer?

it has to be tilt compensated, resolution 1 degree (best would be 0.1 degree), accuracy 0.5 degree

the second and third mentioned by equusine (here also: TTL is I2C? True or false?)


What do you think? i have to rely on somebody, who is well versed with that =)
Jun 09, 2009, 06:02 PM
Registered User
Opps wrong one! Got carried away. The last one, the 4000 uses full RS232 voltages. In some sort of flying vehicle, the readings could be thrown off because the compass is tilt compensated using accelerometers, not gyro's!!
Jun 10, 2009, 01:21 AM
Registered User
that sounds as interesting as important:

What makes the difference using gyros or accelerometeres for tilt compensation?
Jun 10, 2009, 05:00 AM
Registered User
There may be a better (more correct!) answer to that question but my understanding and the short answer is that gyroscopes measure the angular velocity. Accelerometers measure the linear acceleration.

To put that into the real world! With gyros; This can be thought of as the ability of a blindfolded passenger in a car to feel the car turn left and right or tilt up and down as the car ascends or descends hills. Based on this information alone, he knows what direction the car is facing but not how fast or slow it is moving, or whether it is sliding sideways.

With accellerometers, the blindfolded passenger feels himself pressed back into his seat as the vehicle accelerates forward or pulled forward as it slows down etc.

Based on this information alone, he knows how the vehicle is moving relative to itself, that is, whether it is going forward, backward, left, right, up (toward the car's ceiling), or down (toward the car's floor) measured relative to the car, but not the direction relative to the Earth, since he did not know what direction the car was facing relative to the Earth when he felt the accelerations.

However, if you track both the current angular velocity and the current linear acceleration of the system measured relative to the moving system, it is possible to determine the linear acceleration of the system in the inertial reference frame. Performing integration on the inertial accelerations (using the original velocity as the initial conditions) using the correct kinematic equations yields the inertial velocities of the system, and integration again (using the original position as the initial condition) yields the inertial position. In our example, if the blindfolded passenger knew how the car was pointed and what its velocity was before he was blindfolded, and he is able to keep track of both how the car has turned and how it has accelerated and decelerated since, he can accurately know the current orientation, position and velocity of the car at any time...
Jun 10, 2009, 06:11 AM
Registered User
okay, i understood.
so for my purposes the HMC6343 would be the best option for me i think. it offers magnetic as also acc-sensors, tiltcompensated an so on.
I called also the honeywell distributor and he also recommended this module

Thank you very much for your help!
i will tell you about my steps forward!

@bill: you told me it was possible to modify the firmware of the elogger when you know the module type. Can you do that for the HMC6343-eval?
see also

Please tell me about that issue
Jun 10, 2009, 07:52 PM
billpa's Avatar
Hi Soulkitchen,

Thanks for looking into this! Wow, that's an expensive part. It's $175 per chip in single unit quantities!

This is not a good choice, I think. I hope we can locate something that would be more affordable to hobbyists, in the $5/chip range or less in volume.

I have not done any research (yet) but hope there's a part out there that is adequate for your purposes, but much closer to the lower price range.


Bill, for Eagle Tree
Jun 10, 2009, 08:32 PM
Registered User
Is the cost because it's got a PIC core chewing on the 3 accels and the magnetometer?

What about a nice IDG-300 Dual axis gyro??
3.3V leadless IC
Integrated X- and Y-axis gyro on a single chip
Factory set full scale range
18x18 mm size...
$25 or there abouts

(yes please Bill, I've got the perfect vehicle to test it on!)
Jun 11, 2009, 07:50 AM
Registered User
The costs are not the problem, it will be payed by the university... Remember, this module is not for me, but for my diploma thesis. If you think this module HMC 6343 is compatible to the elogger, i will take it.

below you can see the test rig where it should be placed in. I can only choose another module, if its accuracy and so on are better... =)

And also one important thing: it suits into the rig!
Last edited by Soulkitchen; Jun 11, 2009 at 08:12 AM.
Jun 11, 2009, 05:37 PM
billpa's Avatar
Soulkitchen, the problem is that it will take considerable engineering/development time and cost to integrate the sensor. If it were a sensor that we could offer widely to other customers, these costs would be justified. But, these costs could not be recouped with that sensor, I suspect.

If it would meet your needs, we can probably do a custom paid project to support that sensor. If this is desirable, email me at

Max, I wish we could use gyros! the problem is that they drift, and require paired linear accelerometers for drift compensation, temperature compensation, and lots of firmware (including a kalman filter) to get anything meaningful out of them. The hardware alone for "5DOFs" & the equivalent comes to hundreds of $.

This is the beauty of GPS, and thermopile sensors. They give direction and attitude at very low cost, with relatively simple software.


Bill, for Eagle Tree

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