Without seeing your C# code or a more detailed description of the problem,  I cannot help you as I do not understand what the problem is.

Set your bad rate to 115200, and remove the delay command.  That will run the loop as fast as it can and output to serial at the highest data rate supported.

I had an old Stack data acquisition unit for my kart.  Thing is probably 10 years old or older now.  It had an accelerometer in it which it used to derive track maps.  This was a long time ago, and it didn't use a gyro or magnetometer, just an accelerometer.  The track maps visually looked very accurate, although I can't say exactly how accurate, and to scale they were, but visually it did a very good job.  Accelerometer data is very noisy, so if you are just integrating the raw data, I would imagine the results to be awful.  You are going to want to use some form of complimentary filter or Kalman filter to integrate to determine your position.

Reasonable track maps can be derived from accelerometer AND a wheel speed sensor. However, remember this is also on a track and typically defined over several lap iterations.

My kart didn't have a wheel speed sensor, and it generated a map EVERY lap.  The software allowed you to choose which lap to use to generate the map, they all looked pretty similar though.  I would doubt the accuracy of the maps was very good, but for a visual representation of a lap it was quite good.  I the original poster seems to not need great accuracy, but wants a general position and good visual representation of path.  I think it should be quite possible with just an accelerometer for his purposes.  I am working on something similar, only I want to use GPS + accelerometer data and fuse it together.  Civilian GPS units generally only update @ 1-20hz, while many accelerometers can easily run at 100+ hz.

It depends on what you want it to do.  Technically, you can do it with just an accelerometer.  Or just a gyro if you wanted.  Generally, the best way to go for an accurate yaw/pitch/roll measurement you would want all 3, accel, gyro, & mag.  Without a magnetmometer, your measurements will be subject to yaw drift.  Without a gyro, your yaw/pitch/roll measurements will not be very accurate, especially when the sensor is subjected to bumpiness. Your yaw/pitch/roll readings would only be valid  if the accelerometer were rotating smoothly and slowly, or steady-state positioning. 

You might also look into hall effect sensors if you want your inverted pendulum to run for long periods of time, and don't want to worry about a potentiometer wearing out.

I had an old Stack data acquisition unit for my kart.  Thing is probably 10 years old or older now.  It had an accelerometer in it which it used to derive track maps.  This was a long time ago, and it didn't use a gyro or magnetometer, just an accelerometer.  The track maps visually looked very accurate, although I can't say exactly how accurate, and to scale they were, but visually it did a very good job.  Accelerometer data is very noisy, so if you are just integrating the raw data, I would imagine the results to be awful.  You are going to want to use some form of complimentary filter or Kalman filter to integrate to determine your position.

Seems like you could just a potentiometer at fulcrum instead of using a gyro or accelerometer.  Much simpler and probably more accurate.

Alright, here is the finished sensor calibration jig.  Ended up being about $25 for all the PVC and hardware.  The gimbals are 1/2" CPVC, the base is 3/4" CPVC, and the rest is various nylon locking nuts, bolts, and spacers.  The long piece through the middle of the sensor base is 1/4"-20 all-thread.  I bought a 12" piece and used a hacksaw to shorten it to about 6".  Now I just need to modify the FreeIMU calibration GUI tool and bit to accept inputs from my BlueTooth software serial port and I should be good to go. It stands just over 12" tall.

Sounds like it, are the results repeatable?  If so, I would guess the orientation of the sensor is such that the y-axis is your heading axis.  It is Z for mine. 

I sure will. Cost me less than $20 in 1/2" CPVC and the necessary hardware.  I should have pics up tomorrow.  It's about 12" tall by design.  I designed the base, and was thinking of using plywood or 2x4"s, but ended up buying 3/4" CPVC with a 4-way for the middle part and 2x90 deg elbows for the arms.

I verified using my iPhone compass, found magnetic north, and then faced my sensor in the same orientation (make sure it is horizontal so you don't have to deal with tilt compensation).  Your magnetometer should show 0, at magnetic north (or at least is how my my HMC5883L is setup when reading raw values).  If it doesn't, you can create an offset so that it is 0 at north.  Then rotate it 180 degrees and make sure the raw value goes to max, and then returns to 0 when back at magnetic north.  If for your application the sensor will never be tilted, this may be all you have to do if everything checks out.  If the sensor will be at other orientations other than horizontal, you will need to look into tilt compensation.

Here is the design I came up for a yaw/pitch/roll 3 gimbal jig. I should be getting the rest of the hardware tonight and starting on it tomorrow.  Should cost less than $20.  If that helps me calibrate my sensor, then it will be money well worth it.  If not, well it will be a fun little project anyway. :-)

Nothing specifically about my project, I just have not been successful in calibrating my sensors when turning them by hand 360 degrees around X,Y,Z axis.  Every time I attempt to calibrate I get very different values.  My setup uses BlueTooth, so it should be fairly easy to set it in a gimbal rig and smoothly control the motion about each axis to collect better samples to use in the calibration algorithm.  I need the sensors to to determine rotation, and translation in all axes, so I would like them to be calibrated as accurately as possible.

Thanks for the description of your calibration method, interesting stuff indeed.  I purchased components to make my 3 gimbal calibration device already. I am using 1/2 CPVC with 90deg elbows.  Actually ended up being really cheap, about $14 for everything.  I think I am going to write my own calibration program, even if I can get my sensors calibrated with the FreeIMU calibration tool, just so I can gain a better understanding of these sensors.  And because, I find the FreeIMU calibration tool cumbersome to use.   

I am wanting my sensors to give me a full 6 DOF, or I guess 9 DOF with the magnetometer, so I assume I need to properly calibrate all 3 axis of the acc and mag.  I have tried calibrating by rotating 360 degrees around each axis, and then translating in a circle in all 3 planes. 

I am using Fabio Varesano's (may his soul rest in peace) calibration tool which is in Python and uses an ellipsoid into sphere algorithm.

http://www.varesano.net/blog/fabio/ellipsoid-sphere-optimization-using-numpy-and-linalg

Maybe the algorithm used is more sensitive than your Matlab code with respect to needing smooth data in all planes and axes of rotation.

I think I will probably create a gimbal like I mentioned above, it will be fun simple project anyway, and if I can't calibrate my sensors with that, than I will probably look into other algorithms or maybe writing my own to calibration program for my sensors. 

Thanks for the help!