William,

Welcome! Your board and especially your documentation are VERY impressive. Would you like to do another one of our Q&A interviews here to explain to everyone your thinking behind developing the board and some of the things people have done with it?

Best,

Chris

Bill, I'm interested in using your older 2-axis board for an application. Do you know of any discussions dedicated to it? Awesome documentation, BTW. Having thought myself for long hours on how to do all of that in PIC assembly, I know what a monumental task it must have been.Thanks.

Hi Bill,
I am very impressed with the work you have accomplished. My own project is in many ways an effort to duplicate your results (cheap IMU as an AHRS for micro-UAV control). I am currently tackling the problem of initial alignment of the AHRS. I have skimmed through the DCM paper and did not see anything there. I haven't looked through your code yet either. Could you give me a quick idea of how you did the initial alignment?
Thanks.

Tom

Thanks for getting back to me with this. This would be great news if it pans out. Let me know.

BTW, would that be 6 HW PWM outputs, or would some of those be in s/w?

- Roy

Hi Bill,

My apologies...this has been my fault all along. I assumed I had the wires connected to the board correctly becaues I was measuring a positive voltage between the outside pin and the middle pin. I didn't think I would be able to detect a voltage from the signal.

In your previous comment when you said that I should measure a voltage with respect to ground for the signal I went back and did some futher testing and realized I simply had them plugged in backward.

The RollPitchYaw demo is working great now! I'll load up John's heli code and give that a test as well with the receiver properly connected and I bet it will work just fine.

Thanks again!

Justin

Hi Bill,

No I do not mind (publishing), but I appreciate your asking - as you can tell, I am very ignorant in this area, and humbled when i read through all the guys out there that are so knowledgeable, but doing my best to learn...I appreciate your patience...

You know, I thought after reading your paper on DCM that I was right back to where I was in regards to the gravity thing and the accelerometers, but since you addressed that aspect regarding using them anyway to correct the gyro drift that maybe I once again just didn't understand - I thought somehow since you were collecting continuous data that you were able to integrate the accelerometer only (no appreciable gravity influence) data into the gyro data - yikes, WRONG, again!

I would appreciate any thoughts from you or readers...this thing is really starting to bug me for a solution (reasonable one!) and I am willing to invest as long as I know there is some chance of success, particularly in regards to the COS since all I need there is data output - obviously the "canarderon" thing will take some mechanical magic as well...

I really do appreciate your time and thoughts, Frank

Hi Bill...
I have a graph taken from one of the simulation programs we use to design the rockets (RockSim); it represents a typical rocket flight from liftoff to apogee...thought it might help you get a sense of the forces involved...

Curious as to your thinking that the GPS velocity may be useful in some way - my understanding of the accuracy of the GPS position would not lend itself to a rocket which is changing its postion so quickly, but you indicate it might do quite well - is the designed-in inaccuracy of the GPS signals their absolute position, but the position change is quite accurate? Accurate enough to track something going 1.5+ mach?

It seems as though, based upon what I have learned so far, and your coments, that none of the typically available sensors are free from the need to reference/sensing gravity except the IR, and maybe the magnetometers?

Could we use a magnetometer for the basic reference? Model rockets typically do not utilize a lot of ferrous material, and other than their time sitting on a big metal launch pad/rail, their environment is pretty metal free...could calibrate away from the pad for the baseline, then once it leaves, it is "free" once again...

How do the "big boys" get around the gravity issue?

Thanks, Frank

Hi Bill,
I am re-reading your paper "Direction Cosine Matrix IMU" ([pdf] Draft 5/17/2009). I think there is a typo in Eqn. 20 (or there is a chance that I am not getting something). Vector notation, for Z row of the matrix, is represented as follows [rxx, rxy,rxz], and I think that it's a typo as Z row , as I understand it, should be [rzx,rzy,rzz]. What do you think?

Hi Bill,

Thanks for the further thoughts...I follow you until you get to the magnetometer and the issue of the single vector availability...so even though you have two or three magnetometers available you still only get one real ref - I guess that is due to the resolution capability of the sensors...

I ran across a guy who told me about a project at Utah State that if you have not seen you might be interested in as it relates to the rocket thing:
https://chimaera.usu.edu/

If you go to the documentation page, there are some papers descibing their work (see Attitude Control, p.95 in the Flight Readiness paper)...they used a MicroStrain commercial IMU for attitude reference - no GPS...one excerpt from the Navigation section particularly interests me:
A SINS (strap-down INS) works by measuring the accelerations and angular rotation rates of the rocket using an Inertial Measurement Unit (IMU), and integrating these measurements directly to find the velocity and position of the rocket. The IMU measures the sum of all specific surface forces applied to the rocket, including motor thrust and drag, but cannot measure the gravitational force. To account for this, the equations of motion for the rocket
must include the gravitational force before the SINS can integrate and find the rocket velocity and position. SINS is a particularly good measurement device for the thrusting phase of the rocket’s flight, when the motor thrust
varies unpredictably.

A Kalman filter is a mathematically optimal state estimator. It takes multiple measurements, along with their statistical characteristics, and outputs the most probable state of the rocket and its statistical variance. It is unique, in that it is a time-domain filter that requires no system to store previous state data. These qualities make it ideal for inclusion in the navigational algorithm, so that the rocket state is not dependent on a single measurement device.

Sounds to me they just use an estimate for gravity? Thoughts...

Regards, Frank

Hi Bill…

The initial goal is to provide a signal (based upon degrees of tilt of rocket tip from vertical) to either inhibit, or possibly trigger, second-stage (sustainer) motor ignition after burnout of the first-stage (booster) motor. The effort is to maximize the coast period between burns. [I attached a flight simulation graph to an earlier email to you to help paint the picture, but I think it may have been reduced to useless by your blog program. If there is a direct email address you can release to me I can resend that graph.]

During booster burn, we expect acceleration along the rocket’s vertical axis to range between 0 and <18 g. After burnout, during coast, deceleration along the rocket’s axis will range from about negative 1.5 g to about 0 g. All other accelerations (both linear and angular) are likely to be quite small since rocket the does not spin much and tips over very slowly.

Booster (first-stage) burn would be between 4 and 12 seconds. The coast period may be from 10-15 seconds. Therefore, the total duration of the sustainer upward flight prior to ignition is less than 15-25 seconds, so indeed long term drift of the sensors may be a minor concern.

In regards to using a magnetometer, the launch rail is typically 10-14’ high and made of steel. I suppose you could calibrate the sensor(s) away from the pad, then mount the rocket, then launch it – once away, there are negligible iron effects. The lull between placing the rocket on the rail and launch could be up to 10 minutes or more.

Further use might include adaptation of the IMU into a vertical-trajectory/vertical-stabilization system to control flight all the way to apogee, which could be as much as one minute flight time.

Regards, Frank